CAPLAB
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Welcome to the CAP Lab
The Consciousness, Attention, and Perception lab is part of the Perception and Attention theme at the School of Psychology, University of Aberdeen. We are interested in the neural processes underlying various aspects of visual perception, such as awareness, attention, short-term memory, and object recognition. We utilize a combination of psychophysics, EEG and computational techniques to address these questions.
Opportunities
If you are interested in the research we do here, you are encouraged to join our lab. There are many ways to do so.
Current Opportunities
- We currently have one or two openings in the lab: We are looking for an enthusiastic and committed undergraduate to work on projects relating to Time Perception, Object Recognition and Numbers. The intern will be involved in designing, collecting, and analysing data on several experiments. This internship is on a voluntary basis. It provides an excellent opportunity to learn state-of-the-art scientific methods. If you are interested, please contact Rama (see contact details below). Please include a note about your interests and your CV or current academic achievements.
Short term research
- You could do paid internships (usually summer internships right after Level 3, although there are some opportunites during the 2nd year as well).
- You could also volunteer at anytime of the year.
Long term research
- You could apply for the Masters (Research) programme.
- You can also apply for the PhD programme.
- Finally, if you already have a PhD, you can join the lab as a post-doctoral fellow.
For further information and specific inquiries please contact Rama at the details below.
Contact
Address
William Guild Building, Room S7
University of Aberdeen
Kings College
Old Aberdeen
AB24 3FX
The United Kingdom
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+44 1224 272243 -
rama@abdn.ac.uk
Research
A crucial question in vision science is how do we recognize objects? It seems trivial, easy, and self-evident, but taking a cursory look at today's artificial intelligence research (or at face detection software in cameras) is enough to suggest how incredibly complex the process is. At our lab, we have been working on uncovering the mechanisms underlying object recognition, using visual crowding as a tool. Currently, we are working on determining the neural processes underlying attention, object recognition and visual short term memory (VSTM) using a combination of psychophysics and EEG.
Object recognition
Object recognition takes place in two steps. First, the features (orientation, color, etc) of an object are detected independently. Then these features are put together to form the representation of that object. Crowding is a breakdown of the second step. When a target object is flanked closely by other objects, the features of the target and the flankersget mixed up leading to a jumbled percept. This is crowding. It offers a direct window into how feature integration occurs, and hence serves as a handy tool in investigating object recognition.
Crowding is illustrated below. Fixate (keep your eyes focused) on the black square in the center. If you try to read the central letter of the triplet on the right, you will find it very hard. The same letter at the same distance on the left is extremely easy to identify. It is the presence of the the flanking letters on the right that makes the target letter unidentifiable.
Attention
Visual attention directs the limited resources of the visual system to the currently relevant input. We are interested in how attention is deployed in space in order to select objects. Currently, We are applying a variety of techniques including Steady State Visual Evoked Potentials, Multivariate Classifiers and Representational Similarity Analysis to EEG data obtained in attentional tasks to determine the shifts in spatial attention as a function of time. Similar techniques are also being applied to determine how the brain represents various features of objects, such as location and motion.
Current Research
- Neural oscillations in attention, awareness and VSTM
- Neural basis of crowding
- Subitization
- Time perception
People
Current Members
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Rama Chakravarthi
Lecturer
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Danai Papadaki
Doctoral student
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Crystal Silver
Doctoral student
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Justin Claydon
Doctoral student
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Andy Nordqvist
Doctoral student
Rama Chakravarthi
Lecturer and PI
I am interested in the neural processes underlying various aspects of visual perception, such as awareness, attention, short-term memory, and object recognition. I utilize a combination of psychophysics, EEG and computational techniques to address these questions.
You can view my CV or download it here. You can contact me by email.Danai Papadaki
Doctoral student
Co-supervised with Dr Søren Andersen
I am interested in attention, crowding and the underlying neural mechanisms of motion perception.
My phd project integrates aspects of cognitive psychology and visual perception to investigate how the presence of irrelevant and potentially conflicting information affects our ability to process and respond to relevant information.
You can contact me by email.
Crystal Silver
Doctoral student
Co-supervised with Dr Bert Timmermans
I have interdisciplinary research interests spanning social and cognitive psychology, and neuroscience.
My PhD investigates the mechanisms of social agency, the sense of agency experienced in social interactions where the "other" is an independent agent. I utilise behavioural, eye-tracking and neuroscience (EEG) techniques within my research.
You can contact me by email.
Justin Claydon
Doctoral student
I'm interested in the optimality with decision making. The focus of my current work is to determine when people make optimal decisions and when they don't, using neural (EEG) and eye-tracking measures. I have also worked on a project which aimed to determine the presence and characteristics of periodicities in rapid visual categorisation.
You can contact me by email.
Andy Nordqvist
Doctoral student
I am interested in aesthetics and its underlying neural processes. I will be using a combination of behavioural and EEG techniques to study these mechanisms.
You can contact me by email.
Current Undergraduate Members
- Eleanor Paterson
- Elyshia Mistry
- Imke Eickholt
Former Postgraduate Members
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Josephine Reuther
Post-Doctoral Fellow
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Leili Soo
Doctoral student
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Plamen Antonov
Doctoral student
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Oana Iosif
Doctoral student
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Nicholas Jeerakun
MRes
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Kyle Ferris
MRes
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Nicholas Hall
MRes
Josephine Reuther
Post-Doc
I am interested in visual recognition
I investigate the influence of higher-level object properties on object recognition and especially, the influence of object category and meaning on visual crowding
You can contact me by email
Leili Soo
Doctoral student
Co-supervised with Dr Søren Andersen
I completed my PhD at the University of Aberdeen. My project focused on discovering the underlying mechanisms of visual crowding. During this project, we challenged the traditional measure of crowding (read more in our article). We also measured how targets and flankers are processed in the visual cortex in different target-flanker distances using Steady-State Visual Evoked potentials.
My doctoral thesis is available online .
Plamen Antonov
Doctoral student
Co-supervised with Dr Søren Andersen
I am curious about the temporal dynamics of attentional selection. I've investigated how we focus on relevant information while avoiding distractions during spatial and feature-based attention by using behavioural measures, and EEG recordings of steady-state visual evoked potentials (SSVEPs) and alpha waves.
You can contact me by email or check out my twitter profile.
Oana Iosif
Doctoral student
I am interested in consciousness
I am exploring the neural mechanisms underlying visual awareness by using visual illusions and awareness suppression techniques such as Continuous Flash Suppression and combining these with EEG recording.
Nicholas Jeerakun
MRes
I am fascinated by the study of consciousness and the mind. I am interested in using eletrophysiological tools together with cognitive neuroscientific and philosophical theories to understand the mechanisms behind the emergence of consciousness (the so called 'hard problem'). My current focus is on elaborating a novel paradigm to investigate indirectly conscious awareness in decision making within the framework of the free will debate. Contact me via email or visit my website .
Kyle Ferris
MRes
Given my background in both psychology and philosophy, I'm interested in consciousness, awareness, sense of self and agency, and the evolutionary history of the human mind.
In my Masters project I looked at the mechanisms that underlie our tendency to attribute ‘agency’ to an action based on post-hoc feedback, or in other words, the idea that people feel that they caused an action based on feedback received after the action is completed.
Nicholas Hall
MRes
Co-supervised with Prof Arash Sahraie
I completed a Masters Degree at the University of Aberdeen conducting a study into the neural correlates of face perception, looking at the role of attention in this process. I am interested in consciousness and hope to uncover the neural correlates of consciousness, using patients displaying the blindsight phenomenon. I also studied for my undergraduate Psychology degree at the University of Aberdeen and completed a thesis on temporal visual processing in schizotypy.
Former Undergraduate Members
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Sid Henriksen
Honours student
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Diana-Maria Marosi
Honours student
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Robert Ainsley
Summer intern
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Ben Lonnqvist
Undergraduate student
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Jirko Rubruck
Undergraduate student
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Maira Riga
Undergraduate student
Sindre Henriksen
Honours student
Sid did a summer internship (supported by the Developing scientists fund) at our lab in his third year. He then completed his honours thesis on the temporal dynamics of awareness using Motion-Induced-Blindness as a tool. He was awared the Alan B. Milne Prize for the best Psychology thesis for the academic year 2012-13.
Sid completed his PhD under the joint Supervision of Profs Jenny Read (University of Newcastle) and Bruce Cumming (NIH, USA) on computational and neuroscientific underpinnings of depth perception.
Diana-Maria Marosi
Honours student
Diana conducted her honours thesis at the lab. She worked on the effect of social interactions on time perception, for which she received the Alan B. Milne Prize for the best Psychology thesis for the academic year 2013-14.
She completed her M.Phil at the University of Cambridge. Her Masters thesis was on the interrelationships between different kinds of music, personal musical preferences and aggressive behaviour.
Robert Ainsley
Summer intern
Robert's summer internship was supported by the Developing Scientisits fund, during the summer of 2014. He worked on whether visual crowding could be alleviated by priming.
Robert is a tennis enthusiast and runs his own tennis training service. He recently graduated from the University of Aberdeen.
Ben Lonnqvist
Undergraduate (Economics)
I am interested in computational vision and statistical models
I am interested in using computational models to help expand our understanding of cognitive processes – particularly object recognition and attention. My research investigates the plausibility of deep convolutional neural networks as compelling models of human visual processing.
You can contact me by email.
Jirko Rubruck
Undergraduate student
I am interested in visual recognition, particularly object recognition.
I am fascinated by the relative invariance of core object recognition and how computational models might be used to understand and emulate underlying neural processes.
You can contact me by email.
Maira Riga
Undergraduate student
I am a third year student at the University of Aberdeen. I am particularly interested in biological psychology, notably vision science.
I also enjoy cognitive and affective neuroscience, french movies and progressive rock.
Publications*
- Silver, C.A., Tatler, B.W., Chakravarthi, R., and Timmermans, B. (2024). The time course of Temporal Binding in social and nonsocial interactions, (2024) Psychonomic Bulletin & Review. 22 (2): 7 abstract pdf doi
- Almeida, J., Fracasso, A., Kristensen, S., Valério, D., Bergstrom, F., Chakravarthi, R., Tal, Z., and Walbrin, J. (2023). Neural and behavioral signatures of the multidimensionality of manipulable object processing, Communications Biology. 6: 940 abstract pdf doi
- Chakravarthi, R., Papadaki, D. and Krajnik, J. (2022). Visual field asymmetries in numerosity processing, Attention, Perception and Psychophysics. 84 (8): 2607-2622 abstract pdf doi Covered in Nature Reviews Psychology Highlight
- Reuther, J., Chakravarthi, R., Martinovic, J. (2022). Masking, crowding and grouping: Connecting low and mid-level vision, Journal of Vision. 22 (2): 7 abstract pdf doi
- Lee, R. J., Reuther, J., Chakravarthi, R., Martinovic, J. (2021). Emergence of crowding: the role of contrast and orientation salience, Journal of Vision. abstract pdf doi
- § Chakravarthi, R., Rubruck, J., Kipling, N., and Clarke, A. D. F. (2021). Characterising the in-out asymmetry in visual crowding, Journal of Vision. 21 (11): 10 abstract pdf doi
- Silver, C. A., Tatler, B., Chakravarthi, R. , and Timmermans, B. (2021). Social Agency as a Continuum, Psychonomic Bulletin and Review, 28: 434 - 453. abstract pdf doi
- Poncet, M. and Chakravarthi, R. (2021). Subitizing object parts reveals a second stage of individuation, Psychonomic Bulletin and Review, 28: 476 - 486. abstract pdf SI doi
- Poncet, M., Fabre-Thorpe, M. and Chakravarthi, R. (2020). A simple rule to describe interactions between visual categories, European Journal of Neuroscience, 52: 4639 - 4666. abstract pdf SI doi
- Lonnqvist, B., Clarke, A., and Chakravarthi, R. (2020). Crowding in humans is unlike that in convolutional neural networks, Neural Networks, 126: 262 - 274. abstract pdf doi arXiv
- Antonov, P. A., Chakravarthi, R. , and Andersen, S. K. (2020). Too little, too late, and in the wrong place: Alpha band activity does not reflect an active mechanism of selective attention, Neuroimage, 219: 117006. abstract pdf doi
- Reuther, J. and Chakravarthi, R. (2020). Response selection modulates crowding: a cautionary tale for invoking top-down explanations, Attention, Perception and Psychophysics, 82: 1763 - 1778. abstract pdf doi
- Reuther, J., Chakravarthi, R., and Hunt, A. R. (2020). The eye that binds: Feature integration is not disrupted by saccadic eye movements, Attention, Perception and Psychophysics, 82: 533 - 549. abstract pdf doi
- Chakravarthi, R. and Bertamini, M. (2020). Clustering leads to underestimation of numerosity, but crowding is not the cause, Cognition, 104195. abstract pdf SI doi
- Chakravarthi, R. and Herbert, A. (2019). Two’s company, three’s a crowd: Individuation is necessary for object recognition, Cognition, 184: 69 - 82. abstract pdf doi
- Soo, L., Chakravarthi, R., and Andersen, S. K. (2018). Critical resolution: a superior measure of crowding, Vision Research, 153: 13 - 23. abstract pdf doi
- § Reuther, J. and Chakravarthi, R. (2017). Does self-prioritization affect perceptual processes? Visual Cognition, 1 - 18. abstract pdf doi
- Jennings, B. J., Tsattalios, K., Chakravarthi, R., and Martinovic, J. (2016). Combining S-cone and luminance signals adversely affects discrimination of objects within backgrounds, Scientific Reports, 6 (20504): 1-10. abstract pdf doi
- Costa, S. L., Gonçalves, O. F., DeLuca, J. Chiaravalloti, N., Chakravarthi, R., and Almeida, J. (2015). The temporal dynamics of visual processing in Multiple Sclerosis, Applied Neuropsychology: Adult, 23 (2): 133 - 140. abstract pdf doi
- Rosen, S., Chakravarthi, R., and Pelli, D. G. (2014). The Bouma law of crowding, revised: Critical spacing is equal across parts, not objects, Journal of Vision, 14 (6): 10, 1-15. abstract pdf doi Covered in a blog.
- Chakravarthi, R., Carlson, T. A., Chaffin, J., Turret, J., and VanRullen, R. (2014). The temporal evolution of coarse location coding of objects: Evidence for feedback. Journal of Cognitive Neuroscience, 26 (10): 2370 - 2384. abstract pdf doi
- Reuther, J. and Chakravarthi, R. (2014). Categorical membership modulates crowding: evidence from characters, Journal of Vision, 14 (6): 5, 1-13. abstract pdf doi
- Van Vugt, M. K., Chakravarthi, R., and Lachaux, J. P. (2014). For whom the bell tolls: periodic reactivation of sensory cortex in the gamma band as a substrate of visual working memory maintenance, Frontiers in Human Neuroscience, 8: 696. abstract pdf doi
- Chakravarthi, R. and VanRullen, R. (2012). Conscious updating is a rhythmic process, Proceedings of the National Academy of Sciences, 109 (26): 10599-10604. abstract pdf doi
- Freeman, J.,Chakravarthi, R., and Pelli, D. G. (2012). Substitution and pooling in crowding, Attention, Perception, & Psychophysics, 72 (4): 379 - 396 abstract pdf doi
- Chakravarthi, R. and Pelli, D. G. (2011). The same binding in contour integration and crowding, Journal of Vision, 11 (8): 10, 1-12. abstract pdf doi
- Chakravarthi, R. and VanRullen, R. (2011). Bullet trains and steam engines: Exogenous attention zips but endogenous attention chugs along, Journal of Vision, 11 (4):12, 1-12. abstract pdf doi
- Chakravarthi, R. and Cavanagh, P. (2009). Recovery of a crowded object by masking the distracters: Determining the locus of feature integration, Journal of Vision, 9 (10):4, 1-9. abstract pdf doi
- Chakravarthi, R. and Cavanagh, P. (2009). Bilateral field advantage in visual crowding, Vision Research, 49 (13): 1638 – 1646. abstract pdf doi
- Vickery, T. J., Shim, W. M., Chakravarthi, R., Jiang, Y. V., and Luedeman, R. (2009). Supercrowding: Weakly masking a target expands the range of crowding, Journal of Vision, 9 (2):12, 1-15. abstract pdf doi
- Chakravarthi, R. and Cavanagh, P. (2007). Temporal properties of the polarity advantage effect in crowding, Journal of Vision, 7 (11): 1 – 12. abstract pdf doi
- Ramakrishna, C. (2002). Real latencies and facilitation, Consciousness and Cognition, 11(2): 300 – 303. pdf doi
§ Preregistered/Presubmitted Study
* Copyright notice: The publishers of these articles hold the copyright. The pdf version of the articles has been provided for download only as a means to ensure timely dissemination of scholarly and technical work. You may not distribute them, make them available for download by others, or use them for any profit-making enterprise. The articles may not be re-posted without the explicit permission of the copyright holder
Rama's CV
You can download a pdf of my CV here.
Education and Employment
| 2012 – Present: | Lecturer, School of Psychology University of Aberdeen, Scotland UK |
| 2009 – 2012: | Post Doctoral Fellow; Advisor: Dr. Rufin VanRullen Centre de Recherche Cerveau et Cognition CNRS, Toulouse, France, |
| 2007 – 2009: | Post Doctoral Fellow in Psychology and Neural Science; Advisor: Prof. Denis Pelli New York University, New York NY |
| 2002 – 2007: | PhD in Psychology: Cognition, Brain and Behavior; Advisor: Prof. Patrick Cavanagh Harvard University, Cambridge MA |
| 1999 – 2001: | M.S. in Consciousness Studies; Advisor: Dr Shantanu Nagarkatti Birla Institute of Technology and Science, Pilani, India |
| 1993 – 1999: | M.B., B.S. (Bachelor of Medicine, Bachelor of Surgery) Manipal Academy of Higher Education, Manipal, India |
Grants and Funding
| 2019 – 2020: | Experiencing myself through you: Self-agency in social interactions, Carnegie Research Incentive Grant awarded to Bert Timmermans and me, £9,914 |
| 2019 – 2021: | Investigating the microstructure of human visual fields and generating low-vision applications, BBSRC International Partnering award granted to Dr Jasna Martinovic, Dr Daniel Coates (Houston), Dr Josephine Reuther (post-doc) and me, £10,000 |
| 2018 – 2021: | Neural mechanisms of long-range spatial vision: an investigation of perceptive, integrative and association fields across the lifespan, BBSRC grant awarded to Dr Jasna Martinovic (PI) and me (co-PI), £344,000 |
| 2006 – 2007: | Graduate Society Dissertation Completion Fellowship, Harvard University, $18,000 |
| 2007: | Stimson travel grant for presenting at VSS conference, 2007, $500 |
| 2006: | McMasters travel grant for presenting at VSS conference, 2006, $500 |
| 2005: | Graduate Society Fellowship Summer Award, Harvard University, $3,000 |
| 2004: | Mind, Brain and Behavior Graduate Student Award, Harvard University, $5,000 |
| 2003 – 2004: | Harvard University Graduate Summer Awards, $3,000/year |
| 2002 – 2004: | Harvard University GSAS Merit Fellowship, $70,000 |
| 2000 – 2001: | Sir Ratan Tata Trust Scholarship for the M.S. Program, INR 15,000 |
Teaching and Research supervision
Teaching
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| Level 2: |
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| Masters: | Introduction to Bayesian Statistics |
Research Supervision
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Admin Responsibilities
- Levels 3 and 4 Cognitive Neuroscience Course-coordinator
- Director of Communications Team
- Equality and Diversity Committee
- Personal tutor (non-academic mentorship) for 20-25 students per year
- Organising Perception and Attention group meetings
Neural and behavioral signatures of the multidimensionality of manipulable object processing
Almeida, J., Fracasso, A., Kristensen, S., Valério, D., Bergstrom, F., Chakravarthi, R., Tal, Z., & Walbrin, J. (2023)
Understanding how we recognize objects requires unravelling the variables that govern the way we think about objects and the neural organization of object representations. A tenable hypothesis is that the organization of object knowledge follows key object-related dimensions. Here, we explored, behaviorally and neurally, the multidimensionality of object processing. We focused on within-domain object information as a proxy for the decisions we typically engage in our daily lives – e.g., identifying a hammer in the context of other tools. We extracted object-related dimensions from subjective human judgments on a set of manipulable objects. We show that the extracted dimensions are cognitively interpretable and relevant – i.e., participants are able to consistently label them, and these dimensions can guide object categorization; and are important for the neural organization of knowledge – i.e., they predict neural signals elicited by manipulable objects. This shows that multidimensionality is a hallmark of the organization of manipulable object knowledge.
Close windowVisual field asymmetries in numerosity processing
Chakravarthi, R., Papadaki, D. and Krajnik, J. (2022)
A small number of objects can be rapidly and accurately enumerated, whereas a larger number of objects can only be approximately enumerated. These subitizing and estimation abilities, respectively, are both spatial processes relying on extracting information across spatial locations. Nevertheless, whether and how these processes vary across visual field locations remains unknown. Here, we examined if enumeration displays asymmetries around the visual field. Experiment 1 tested small number (1–6) enumeration at cardinal and non-cardinal peripheral locations while manipulating the spacing among the objects. Experiment 2 examined enumeration at cardinal locations in more detail while minimising crowding. Both experiments demonstrated a Horizontal-Vertical Asymmetry (HVA) where performance was better along the horizontal axis relative to the vertical. Experiment 1 found that this effect was modulated by spacing with stronger asymmetry at closer spacing. Experiment 2 revealed further asymmetries: a Vertical Meridian Asymmetry (VMA) with better enumeration on the lower vertical meridian than on the upper and a Horizontal Meridian Asymmetry (HMA) with better enumeration along the left horizontal meridian than along the right. All three asymmetries were evident for both subitizing and estimation. HVA and VMA have been observed in a range of visual tasks, indicating that they might be inherited from early visual constraints. However, HMA is observed primarily in mid-level tasks, often involving attention. These results suggest that while enumeration processes can be argued to inherit low-level visual constraints, the findings are, parsimoniously, consistent with visual attention playing a role in both subitizing and estimation.
Close windowMasking, crowding and grouping: Connecting low and mid-level vision
Reuther, J., Chakravarthi, R., and Martinovic, J. (2022)
An important task for vision science is to build a unitary framework of low and mid-level vision. As a step on this way, our study examined differences and commonalities between masking, crowding and grouping - three processes that occur through spatial interactions between neighbouring elements. We measured contrast thresholds as functions of inter-element spacing and eccentricity for Gabor detection, discrimination, contour integration, using a common stimulus grid consisting of 9 Gabor elements. From these thresholds, we derived a) the baseline contrast necessary to perform each task and b) the spatial extent over which task performance was stable. This spatial window can be taken as an indicator of field size, where elements that fall within a putative field are readily combined. We found that contrast thresholds were universally modulated by inter-element distance, with a shallower and inverted effect for grouping compared to masking and crowding. Baseline contrasts for detecting stimuli and discriminating their properties were positively linked across the tested retinal locations (parafovea and near periphery), whereas those for integrating elements and discriminating their properties were negatively linked. Meanwhile, masking and crowding spatial windows remained uncorrelated across eccentricity, although they were correlated across participants. This suggests that the computation performed by each type of visual field operates over different distances that co-varies across observers, but not across retinal locations. Contrast-processing units may thus lie at the core of the shared idiosyncrasies across tasks reported in many previous studies, despite the fundamental differences in the extent of their spatial windows.
Close windowEmergence of crowding: the role of contrast and orientation salience
Lee, R. J., Reuther, J., Chakravarthi, R., Martinovic, J. (2021)
Crowding causes difficulties in judging attributes of an object surrounded by other objects. We investigated crowding for stimuli that isolated either S-cone or luminance mechanisms or combined them. By targeting different retinogeniculate mechanisms with contrast-matched stimuli, we aim to determine the earliest site at which crowding emerges. Discrimination was measured in an orientation judgement task where Gabor targets were presented parafoveally among flankers. In the first experiment, we assessed flanked and unflanked orientation discrimination thresholds for pure S-cone and achromatic stimuli and their combinations. In the second experiment, to capture individual differences, we measured unflanked detection and orientation sensitivity, along with performance under flanker-interference for stimuli containing luminance only or combined with S-cone contrast. We confirmed that orientation sensitivity was lower for unflanked S-cone stimuli. When flanked, the pattern of results for S-cone stimuli was the same as for achromatic stimuli with comparable (i.e., low) contrast levels. We also found that flanker interference exhibited a genuine signature of crowding only when orientation discrimination threshold was reliably surpassed. Crowding, therefore, emerges at a stage that operates on signals representing task-relevant featural (here, orientation) information. Since luminance and S-cone mechanisms have very different spatial tuning properties, it is most parsimonious to conclude that crowding takes place at a neural processing stage after they have been combined.
Close windowCharacterising the in-out asymmetry in visual crowding
Chakravarthi, R., Rubruck, J., Kipling, N., and Clarke, A. D. F. (2021)
An object's processing is impaired by the presence of nearby clutter. Several distinct mechanisms, such as masking and visual crowding, are thought to contribute to such flanker8 induced interference. It is therefore important to determine which mechanism is operational in any given situation. Previous studies have proposed that the in-out asymmetry (IOA), where a peripheral flanker interferes with the target more than a foveal flanker, is diagnostic of crowding. However, several studies have documented inconsistencies in the occurrence of this asymmetry, particularly at locations beyond the horizontal meridian, casting doubt on its ability to delineate crowding. In this study, to determine if IOA is diagnostic of crowding, we extensively charted its properties. We asked a relatively large set of participants (n=38) to identify a briefly presented peripheral letter flanked by a single inward or outward letter at one of four locations. We also manipulated target location uncertainty and attentional allocation by blocking, randomising or precueing the target location. Using multi-level Bayesian regression analysis, we found robust IOA at all locations, although its strength was modulated by target location, location uncertainty and attentional allocation. Our findings suggest that IOA can be an excellent marker of crowding, to the extent that it is not observed in other flanker-interference mechanisms such as masking.
Close windowSocial Agency as a Continuum
Silver, C. A., Tatler, B., Chakravarthi, R., and Timmermans, B. (2021)
Sense of Agency, the phenomenology associated with causing one’s own actions and corresponding effects, is a cornerstone of human experience. Social Agency can be defined as the Sense of Agency experienced in any situation in which the effects of our actions are related to a conspecific. This can be implemented as the other’s reactions being caused by our action, joint action modulating our Sense of Agency, or the other’s mere social presence influencing our Sense of Agency. It is currently an open question how such Social Agency can be conceptualised and how it relates to its non-social variant. This is because, compared to non-social Sense of Agency, the concept of Social Agency has remained over-simplified and under-researched, with disparate empirical paradigms yielding divergent results. Reviewing the empirical evidence and the commonalities and differences between different instantiations of Social Agency, we propose that Social Agency can be conceptualised as a continuum, in which the degree of cooperation is the key dimension that determines our Sense of Agency, and how it relates to non-social Sense of Agency. Taking this perspective, we review how the different factors that typically influence Sense of Agency affect Social Agency, and in the process highlight outstanding empirical questions within the field. Finally, concepts from wider research areas are discussed in relation to the ecological validity of Social Agency paradigms, and we provide recommendations for future methodology.
Close windowSubitizing object parts reveals a second stage of individuation
Poncet, M. and Chakravarthi, R. (2021)
Humans can efficiently individuate a small number of objects. This subitizing ability is thought to be a consequence of limited attentional resources. However, how and what is selected during the individuation process remain outstanding questions. We investigated these in four experiments by examining if parts of objects are enumerated as efficiently as distinct objects in the presence and absence of distractor objects. We found that distractor presence reduced subitizing efficiency. Crucially, parts connected to multiple objects were enumerated less efficiently than independent objects or parts connected to a single object. These results argue against direct individuation of parts and show that objecthood plays a fundamental role in individuation. Objects are selected first and their components are selected in subsequent steps. This reveals that individuation operates sequentially over multiple levels.
Close windowA simple rule to describe interactions between visual categories
Poncet, M., Fabre-Thorpe, M. and Chakravarthi, R. (2020)
Humans can rapidly categorise visual objects when presented in isolation. However, in everyday life we encounter multiple objects at the same time. Far less is known about how simultaneously active object representations interact. We examined such interactions by asking participants to categorise a target object at the basic (Experiment 1) or the superordinate (Experiment 2) level while the representation of another object was still active. We found that the ‘prime’ object strongly modulated the response to the target implying that the prime’s category was rapidly and automatically accessed, influencing subsequent categorical processing. Using drift-diffusion modelling we show that a prime, whose category is different from that of the target, interferes with target processing primarily during the evidence accumulation stage. This suggests that the state of category-processing neurons is altered by an active representation and this modifies the processing of other categories. Interestingly, the strength of interference increases with the similarity between the distractor and the target category. Considering these results and previous studies, we propose a general principle that category interactions are determined by the distance from a distractor’s representation to the target’s task-relevant categorical boundary. We argue that this principle arises from the specific architectural organisation of categories in the brain.
Close windowCrowding in humans is unlike that in convolutional neural networks
Lonnqvist, B., Clarke, A. D. F. and Chakravarthi, R. (2020)
Object recognition is a primary function of the human visual system. It has recently been claimed that the highly successful ability to recognise objects in a set of emergent computer vision systems---Deep Convolutional Neural Networks (DCNNs)---can form a useful guide to recognition in humans. To test this assertion, we systematically evaluated visual crowding, a dramatic breakdown of recognition in clutter, in DCNNs and compared their performance to extant research in humans. We examined crowding in two architectures of DCNNs with the same methodology as that used among humans. We manipulated multiple stimulus factors including inter-letter spacing, letter colour, size, and flanker location to assess the extent and shape of crowding in DCNNs to establish a clear picture of crowding in DCNNs. We found that crowding followed a predictable pattern across DCNN architectures that was fundamentally different from that in humans. Some characteristic hallmarks of human crowding, such as invariance to size, the effect of target-flanker similarity, and confusions between target and flanker identities, were completely missing, minimised or even reversed in DCNNs. These data show that DCNNs, while proficient in object recognition, likely achieve this competence through a set of mechanisms that are distinct from those in humans. They are not equivalent models of human or primate object recognition and caution must be exercised when inferring mechanisms derived from their operation.
Close windowToo little, too late, and in the wrong place: Alpha band activity does not reflect an active mechanism of selective attention
Antonov, P. A., Chakravarthi, R., and Andersen, S. K. (2020)
Selective attention focuses visual processing on relevant stimuli in order to allow for adaptive behaviour despite an abundance of distracting information. It has been proposed that increases in alpha band (8 - 12 Hz) amplitude reflect an active mechanism for distractor suppression. If this were the case, increases in alpha band amplitude should be succeeded by a decrease in distractor processing. Surprisingly, this connection has not been tested directly; specifically, studies that have investigated changes in alpha band after attention-directing cues have not directly assessed the neuronal processing of distractors. We concurrently recorded alpha activity and steady-state visual evoked potentials (SSVEPs) to assess the processing of target and distractor stimuli. In two experiments, participants covertly shifted attention to one of two letter streams (left or right) to detect infrequent target letters ‘X’ while ignoring the other stream. In line with previous findings, alpha band amplitudes contralateral to the unattended location increased compared to a pre-cue baseline. However, there was no suppression of SSVEP amplitudes elicited by unattended stimuli, while there was a pronounced enhancement of SSVEPs elicited by attended stimuli. Furthermore, and crucially, changes in alpha band amplitude during attention shifts did not precede those in SSVEPs and hit rates in both experiments, indicating that changes in alpha band amplitudes are likely to be a consequence of attention shifts rather than the other way around. We conclude that these findings contradict the notion that alpha band activity reflects mechanisms that have a causal role in the allocation of selective attention.
Close windowClustering leads to underestimation of numerosity, but crowding is not the cause
Chakravarthi, R. and Bertamini R. (2020)
Humans have the remarkable ability to rapidly estimate the number of objects in a visual scene without relying on counting, something referred to as a number sense. It has been well documented that the more clustered the elements are, the lower their perceived numerosity is. A recent account of this observation is the crowding hypothesis, which posits that the perceived underestimation is driven by visual crowding: the inability to recognise objects in clutter. Crowding can impair individuation of the elements, which would explain the underestimation. Here, we tested the crowding hypothesis by assessing numerosity estimation and crowding for the same stimulus configurations in the same participants. Experiment 1 compared the two tasks when numerosity can be considered to be estimated directly by the visual system (reference patch density = 0.12 items/deg2), while Experiment 2 used high density stimuli (density = 0.88 items/deg2), where numerosity may be estimated indirectly. In both cases, we found that spacing and similarity between elements affected estimation and crowding tasks in markedly different ways. These results are incompatible with a crowding account of numerosity underestimation and point to separate mechanisms for object identification and number estimation, although grouping may play a moderating role in both cases.
Close windowResponse selection modulates crowding: a cautionary tale for invoking top-down explanations
Reuther, J. and Chakravarthi, R. (2020)
Object recognition in the periphery is limited by clutter. This phenomenon of visual crowding is ameliorated when the objects are dissimilar. This effect of inter-object similarity has been extensively studied for low-level features and is thought to reflect bottom-up processes. Recently, crowding was also found to be reduced when objects belonged to explicitly distinct groups; that is, crowding was weak when they had low group membership similarity. It has been claimed that top-down knowledge is necessary to explain this effect of group membership, implying that the effect of similarity on crowding cannot be a purely bottom-up process. We tested the claim that the effect of group membership relies on knowledge in two experiments and found that neither explicit knowledge about differences in group membership nor the possibility of acquiring knowledge about target identities is necessary to produce the effects. These results suggest that top-down processes need not be invoked to explain the effect of group membership. Instead, we suggest that differences in flanker reportability that emerge from the differences in group membership are the source of the effect. That is, when targets and flankers are sampled from distinct groups, flankers cannot be inadvertently reported, leading to fewer errors and hence weaker crowding. Further, we argue that this effect arises at the stage of response selection. This conclusion is well sup- ported by an analytical model based on these principles. We conclude that previously observed effects in crowding attributed to top-down or higher level processes might instead be due to post-perceptual response selection strategies.
Close windowThe eye that binds: feature integration is not disrupted by saccadic eyemovements
Reuther, J., Chakravarthi, R. and Hunt, A. R. (2020)
Feature integration theory proposes that visual features, such as shape and colour, can only be combined into a unified object when spatial attention is directed to their location in retinotopic maps. Eye movements cause dramatic changes on our retinae, and are associated with obligatory shifts in spatial attention. In two experiments, we measured the prevalence of \review{conjunction errors (that is, reporting an object as having an attribute that belonged to another object)}, for brief stimulus presentation before, during, and after a saccade. Planning and executing a saccade did not itself disrupt feature integration. Motion did disrupt feature integration, leading to an increase in conjunction errors. However, retinal motion of an equal extent but caused by saccadic eye movements is spared this disruption, and showed similar rates of conjunction errors as a condition with static stimuli presented to a static eye. The results suggest extra-retinal signals are able to compensate for the motion caused by saccadic eye movements, thereby preserving the integrity of objects across saccades and preventing their features from mixing or mis-binding.
Close windowTwo’s company, three’s a crowd: Individuation is necessary for object recognition
Chakravarthi, R. and Herbert, A. (2019)
Object recognition is essential for navigating the real world. Despite decades of research on this topic, the processing steps necessary for recognition remain unclear. In this study, we examined the necessity and role of individuation, the ability to select a small number of spatially distinct objects irrespective of their identity, in the recognition process. More specifically, we tested if the ability to rapidly individuate and enumerate a small number of objects (subitizing) can be impaired by crowding. Crowding is flanker-induced interference that specifically impedes the recognition process. We found that subitizing is impaired when objects are close to each other (Experiment 1), and if the target objects are surrounded by irrelevant but perceptually similar flankers (Experiments 2–4). This impairment cannot be attributed to confusion between targets and flankers, wherein flankers are inadvertently included in or targets are excluded from enumeration (Experiments 3–4). Importantly, the flanker induced interference was comparable in both subitizing and crowding tasks (Experiment 4), suggesting that individuation and identification share a common processing pathway. We conclude that individuation is an essential stage in the object recognition pipeline and argue for a cohesive proposal that both crowding and subitizing are due to limitations of selective attention.
Close windowCritical resolution: a superior measure of crowding
Soo, L., Chakravarthi, R. and Andersen, S. K. (2018)
Visual object recognition is essential for adaptive interactions with the environment. It is fundamentally limited by crowding, a breakdown of object recognition in clutter. The spatial extent over which crowding occurs is proportional to the eccentricity of the target object, but nevertheless varies substantially depending on various stimulus factors (e.g. viewing time, contrast). However, a lack of studies jointly manipulating such factors precludes predictions of crowding in more heterogeneous scenes, such as the majority of real life situations. To establish how such co-occurring variations affect crowding, we manipulated combinations of 1) flanker contrast and backward masking, 2) flanker contrast and presentation duration, and 3) flanker preview and pop-out while measuring participants’ ability to correctly report the orientation of a target stimulus. In all three experiments, combining two manipulations consistently modulated the spatial extent of crowding in a way that could not be predicted from an additive combination. However, a simple transformation of the measurement scale completely abolished these interactions and all effects became additive. Precise quantitative predictions of the magnitude of crowding when combining multiple manipulations are thus possible when it is expressed in terms of what we label the ‘critical resolution’. Critical resolution is proportional to the inverse of the smallest flanker free area surrounding the target object necessary for its unimpaired identification. It offers a more parsimonious description of crowding than the traditionally used critical spacing and may thus constitute a measure of fundamental importance for understanding object recognition.
Close windowDoes self-prioritization affect perceptual processes?
Reuther, J. and Chakravarthi, R. (2017)
The tendency to prioritize information related to the self (or socially salient information) has been established for several cognitive tasks. However, earlier studies on this question suffered from confounds such as familiarity and intimacy. Recently, a series of studies overcame this limitation using newly learnt associations between geometric shapes and identities. Results from these studies have been argued to show that self-prioritization affects perceptual processing. In two studies, we replicated and extended the original shape-identity association paradigm to test an alternative hypothesis that self-prioritization does not affect perceptual processes but arises from potential memory differences introduced during the formation of associations. We found that induced memory differences lead to response patterns similar to those that have been attributed to changes in the perceptual domain. However, even extended learning undertaken to equate memory for various identity-based associations did not eliminate the effects of self-prioritization, leaving the question open if the differences are cognitive or perceptual in nature. The current evidence can be explained both in terms of memory differences and perceptual effects. Hence, we strongly recommend that the existence of perceptual effects of self-prioritization should be investigated directly rather than through changes in reaction times in match–non-match tasks.
Close windowCombining S-cone and luminance signals adversely affects discrimination of objects within backgrounds
Jennings, B. J., Tsattalios, K., Chakravarthi, R., and Martinovic, J. (2016)
The visual system processes objects embedded in complex scenes that vary in both luminance and colour. In such scenes, colour contributes to the segmentation of objects from backgrounds, but does it also a ect perceptual organisation of object contours which are already de ned by luminance signals, or are these processes una ected by colour’s presence? We investigated if luminance and chromatic signals comparably sustain processing of objects embedded in backgrounds, by varying contrast along the luminance dimension and along the two cone-opponent colour directions. In the rst experiment thresholds for object/non-object discrimination of Gaborised shapes were obtained in the presence and absence of background clutter. Contrast of the component Gabors was modulated along single colour/luminance dimensions or co-modulated along multiple dimensions simultaneously. Background clutter elevated discrimination thresholds only for combined S-(L + M) and L + M signals. The second experiment replicated and extended this nding by demonstrating that the e ect was dependent on the presence of relatively high S-(L + M) contrast. These results indicate that S-(L + M) signals impair spatial vision when combined with luminance. Since S-(L + M) signals are characterised by relatively large receptive elds, this is likely to be due to an increase in the size of the integration eld over which contour-de ning information is summed.
Close windowThe temporal dynamics of visual processing in multiple sclerosis
Costa, S. L., Gonçalves, O. F., DeLuca, J. Chiaravalloti, N., Chakravarthi, R., and Almeida, J. (2015)
Although the integrity of the visual system is often affected in multiple sclerosis (MS), the potential relationship between the temporal dynamics of visual processing and performance on neuropsychological tests assessing processing speed (PS) remains relatively unexplored. Here, we test if a PS deficit is related to abnormalities within the visual system, rather than impaired higher-level cognitive function. Two groups of participants with MS (1 group with PS deficits and another without) and a healthy control group, matched for age and education, were included. To explore the temporal dynamics of visual processing, we used 2 psychophysical paradigms: attention enhancement/prioritization and rapid serial visual presentation. Visual PS deficits were associated with a decreased capability to detect visual stimuli and a higher limitation in visual temporal-processing capacity. These results suggest that a latent sensorial temporal limitation of the visual system is significantly associated to PS deficits in MS.
Close windowThe Bouma law of crowding, revised: Critical spacing is equal across parts, not objects
Rosen, S., Chakravarthi, R. and Pelli, D. G. (2014)
Crowding is the inability to identify an object among flankers in the periphery. It is due to inappropriate incorporation of features from flanking objects in perception of the target. Crowding is characterized by measuring critical spacing, the minimum distance needed between a target and flankers to allow recognition. The existing Bouma law states that, at a given point and direction in the visual field, critical spacing, measured from the center of a target object to the center of a similar flanking object, is the same for all objects (Pelli & Tillman, 2008). Because flipping an object about its center preserves its center-to-center spacing to other objects, according to the Bouma law, crowding should be unaffected. However, because crowding is a result of feature combination, the location of features within an object might matter. In a series of experiments, we find that critical spacing is affected by the location of features within the flanker. For some flankers, a flip greatly reduces crowding even though it maintains target–flanker spacing and similarity. Our results suggest that the existing Bouma law applies to simple one-part objects, such as a single roman letter or a Gabor patch. Many objects consist of multiple parts; for example, a word is composed of multiple letters that crowd each other. To cope with such complex objects, we revise the Bouma law to say that critical spacing is equal across parts, rather than objects. This accounts for old and new findings.
Close windowThe temporal evolution of coarse location coding of objects: Evidence for feedback
Chakravarthi, R., Carlson, T. A., Chaffin, J., Turret, J., and VanRullen, R. (2014)
Objects occupy space. How does the brain represent the spatial location of objects? Retinotopic early visual cortex has precise location information but can only segment simple objects. On the other hand, higher visual areas can resolve complex objects but only have coarse location information. Thus coarse location of complex objects might be represented by either (a) feedback from higher areas to early retinotopic areas or (b) coarse position encoding in higher areas. We tested these alternatives by presenting various kinds of first- (edge- defined) and second-order (texture) objects. We applied multi- variate classifiers to the pattern of EEG amplitudes across the scalp at a range of time points to trace the temporal dynamics of coarse location representation. For edge-defined objects, peak classification performance was high and early and thus attributable to the retinotopic layout of early visual cortex. For texture objects, it was low and late. Crucially, despite these differences in peak performance and timing, training a classifier on one object and testing it on others revealed that the topography at peak performance was the same for both first- and second-order objects. That is, the same location in- formation, encoded by early visual areas, was available for both edge-defined and texture objects at different time points. These results indicate that locations of complex objects such as textures, although not represented in the bottom–up sweep, are encoded later by neural patterns resembling the bottom–up ones. We conclude that feedback mechanisms play an important role in coarse location representation of complex objects.
Close windowCategorical membership modulates crowding: Evidence from characters
Reuther, J. and Chakravarthi, R. (2014)
Visual crowding is generally thought to affect recognition mostly or only at the level of feature combination. Calling this assertion into question, recent studies have shown that if a target object and its flankers belong to different categories crowding is weaker than if they belong to the same category. Nevertheless, these results can be explained in terms of featural differences between categories. The current study tests if category- level (i.e., high-level) interference in crowding occurs when featural differences are controlled for. First, replicating previous results, we found lower critical spacing for targets and flankers belonging to different categories. Second, we observed the same, albeit weaker, category-specific effect when objects in both categories had the exact same feature set, suggesting that category-specific effects persist even when featural differences are fully controlled for. Third, we manipulated the semantic content of the flankers while keeping their feature set constant, by using upright or rotated objects, and found that meaning modulated crowding. An exclusively feature-based account of crowding would predict no differences due to such changes in meaning. We conclude that crowding results from not only the well-documented feature-level interactions but also additional interactions at a level where objects are grouped by meaning.
Close windowFor whom the bell tolls: periodic reactivation of sensory cortex in the gamma band as a substrate of visual working memory maintenanc
Van Vugt, M. K., Chakravarthi, R. and Lachaux, J. P. (2014)
Working memory (WM) is central to human cognition as it allows information to be kept online over brief periods of time and facilitates its usage in cognitive operations (Luck and Vogel, 2013). How this information maintenance actually is implemented is still a matter of debate. Several independent theories of WM, derived, respectively, from behavioral studies and neural considerations, advance the idea that items in WM decay over time and must be periodically reactivated. In this proposal, we show how recent data from intracranial EEG and attention research naturally leads to a simple model of such reactivation in the case of sensory memories. Specifically, in our model the amplitude of high-frequency activity (>50 Hz, in the gamma-band) underlies the representation of items in high-level visual areas. This activity decreases to noise-levels within 500 ms, unless it is reactivated. We propose that top-down attention, which targets multiple sensory items in a cyclical or rhythmic fashion at around 6–10 Hz, reactivates these decaying gamma-band representations. Therefore, working memory capacity is essentially the number of representations that can simultaneously be kept active by a rhythmically sampling attentional spotlight given the known decay rate. Since attention samples at 6–10 Hz, the predicted WM capacity is 3–5 items, in agreement with empirical findings.
Close windowConscious updating is a rhythmic process
Chakravarthi, R. and VanRullen, R. (2012)
As the visual world changes, its representation in our consciousness must be constantly updated. Given that the external changes are continuous, it appears plausible that conscious updating is continuous as well. Alternatively, this updating could be periodic, if, for example, its implementation at the neural level relies on oscillatory activity. The flash-lag illusion, where a briefly presented flash in the vicinity of a moving object is misperceived to lag behind the moving object, is a useful tool for studying the dynamics of conscious up- dating. Here, we show that the trial-by-trial variability in updating, measured by the flash-lag effect (FLE), is highly correlated with the phase of spontaneous EEG oscillations in occipital (5–10 Hz) and frontocentral (12–20 Hz) cortices just around the reference event (flash onset). Further, the periodicity in each region independently influences the updating process, suggesting a two-stage periodic mechanism. We conclude that conscious updating is not continuous; rather, it follows a rhythmic pattern.
Close windowSubstitution and pooling in crowding
Freeman, J., Chakravarthi, R., and Pelli, D. G. (2012)
Unless we fixate directly on it, it is hard to see an object among other objects. This breakdown in object recognition, called crowding, severely limits peripheral vision. The effect is more severe when objects are more similar. When observers mistake the identity of a target among flanker objects, they often report a flanker. Many have taken these flanker reports as evidence of internal substitution of the target by a flanker. Here, we ask observers to identify a target presented in between one similar and one dissimilar flanker. (Simple) substitution takes only one letter, which is often the target but, by unwitting mistake, is sometimes a flanker. The opposite of substitution is pooling, which takes in more than one letter. Having taken only one letter, the substitution process knows only its identity, not its similarity to the target. Thus, it must report similar and dissimilar flankers equally often. Contrary to this prediction, the similar flanker is reported much more often than the dissimilar flanker, showing that rampant flanker substitution cannot account for most flanker reports. Mixture modeling shows that simple substitution can account for, at most, about half the trials. Pooling and nonpooling (simple substitution) together include all possible models of crowding. When observers are asked to identify a crowded object, at least half of their reports are pooled, on the basis of a combination of information from target and flankers, rather than being based on a single letter.
Close windowThe same binding in contour integration and crowding
Chakravarthi, R. and Pelli, D. G. (2011)
Binding of features helps object recognition in contour integration, but hinders it in crowding. In contour integration, aligned adjacent objects group together to form a path. In crowding, flanking objects make the target unidentifiable. But, to date, the two tasks have only been studied separately. May and Hess (2007) suggested that the same binding mediates both tasks. To test this idea, we ask observers to perform two different tasks with the same stimulus. We present oriented grating patches that form a “snake letter” in the periphery. Observers report either the identity of the whole letter (contour integration task) or the phase of one of the grating patches (crowding task). We manipulate the strength of binding between gratings by varying the alignment between them, i.e. the Gestalt goodness of continuation, measured as “wiggle”. We find that better alignment strengthens binding, which improves contour integration and worsens crowding. Observers show equal sensitivity to alignment in these two very different tasks, suggesting that the same binding mechanism underlies both phenomena. It has been claimed that grouping among flankers reduces their crowding of the target. Instead, we find that these published cases of weak crowding are due to weak binding resulting from target-flanker misalignment. We conclude that crowding is mediated solely by the grouping of flankers with the target and is independent of grouping among flankers.
Close windowBullet trains and steam engines: Exogenous attention zips but endogenous attention chugs along
Chakravarthi, R. and VanRullen, R. (2011)
Analyzing a scene requires shifting attention from object to object. Although several studies have attempted to determine the speed of these attentional shifts, there are large discrepancies in their estimates. Here, we adapt a method pioneered by Carlson et al (2006) that directly measures pure attentional shift times. We also test if attentional shifts can be handled in parallel by the independent resources available in the two cortical hemispheres. We present 10 ‘clocks’, with single revolving hands, in a ring around fixation. Observers are asked to report the hand position on one of the clocks at the onset of a transient cue. The delay between the reported time and the veridical time at cue onset can be used to infer processing and attentional shift times. With this setup, we use a novel subtraction method that utilizes different combinations of exogenous and endogenous cues to determine shift times for both types of attention. In one experiment, subjects shift attention to an exogenously cued clock (baseline condition) in one block and in other blocks perform one further endogenous shift to a nearby clock (test condition). In another experiment, attention is endogenously cued to one clock (baseline condition) and on other trials an exogenous cue further shifts attention to a nearby clock (test condition). Subtracting report delays in the baseline condition from those obtained in the test condition allows us to isolate genuine attentional shift times. In agreement with previous studies, our results reveal that endogenous attention is much slower than exogenous attention (endogenous: 250ms; exogenous: 100 ms). Surprisingly, the dependence of shift time on distance is minimal for exogenous attention, whereas it is steep for endogenous attention. In the final experiment we find that endogenous shifts are faster across hemifields than within a hemifield suggesting that the two hemispheres can simultaneously process at least parts of these shifts.
Close windowRecovery of a crowded object by masking the distracters: Determining the locus of feature integration
Chakravarthi, R. and Cavanagh, P. (2009)
Object recognition is a central function of the visual system. As a first step, the features of an object are registered; these independently encoded features are then bound together to form a single representation. Here we investigate the locus of this ‘feature integration’ by examining crowding, a striking breakdown of this process. Crowding, an inability to identify a peripheral target surrounded by flankers, results from ‘excessive integration’ of target and flanker features. We presented a standard crowding display with a target C flanked by four flanker C’s in the periphery. We then masked only the flankers (but not the target) with one of three kinds of masks – noise, metacontrast, and object substitution – each of which interferes at progressively higher levels of visual processing. With noise and metacontrast masks (low-level masking), the crowded target was recovered, whereas with object substitution masks (high-level masking), it was not. This places a clear upper bound on the locus of interference in crowding suggesting that crowding is not a low-level phenomenon. We conclude that feature integration, which underlies crowding, occurs prior to the locus of object substitution masking. Further, our results indicate that the integrity of the flankers, but not their identification, is crucial for crowding to occur.
Close windowBilateral field advantage in visual crowding
Chakravarthi, R. and Cavanagh, P. (2009)
Thirty randomly oriented T’s were presented in a circle around fixation at an eccentricity of 11 degrees such that each T was crowded by its neighbors. Two locations within the same hemifield (unilateral condition) or one location in each hemifield (bilateral condition) were precued for subsequent probing. Observers were then asked to report the orientation of a target T at one of these locations. A bilateral field advantage was found: target identification was better when the two precued targets were in different hemifields than when they were within the same hemifield. This bilateral advantage was absent when only targets were presented, without any distracters. Further controls showed that this advantage could not be attributed to differences between horizontal and vertical target alignments or to visual field anisotropies. A similar bilateral advantage has been reported for multiple object tracking (Alvarez & Cavanagh, 2005) and other attentional tasks. Our results suggest that crowding also demonstrates separate attentional resources in the left and right hemifields. There was a cost to attending to two targets presented unilaterally over attending to a single target. However, this cost was reduced when the two crowded targets were in separate hemifields.
Close windowSupercrowding: Weakly masking a target expands the range of crowding
Vickery, T. J., Shim, W. M., Chakravarthi, R., Jiang, Y. V., and Luedeman, R. (2009)
Crowding is the impairment of peripheral object identification by nearby objects. Critical spacing (the minimum target-flanker distance that does not produce crowding) scales with target eccentricity and is consistently reported as roughly equal to or less than 50% of target eccentricity (0.5e). This study demonstrates that crowding occurs far beyond the typical critical spacing when the target is weakly masked by a surrounding contour or backwards pattern mask. A target was presented at a peripheral location on every trial and participants reported its orientation. Flankers appeared at target-flanker distances of 0.3–0.7e, or were absent. The target was presented with or without a mask. When flankers were absent, the masks only mildly impaired performance. When flankers were present but the mask was absent, target identification was nearly perfect at wide target-flanker distances (0.5e–0.7e). However, when flankers were present and the target was masked, performance dropped significantly, even when target-flanker distances far exceeded the typical crowding range. This phenomenon (“supercrowding”) shares critical features with standard crowding: flankers similar to the target impair performance more than dissimilar flankers, and the characteristic anisotropic profile of crowding is preserved. Supercrowding may reflect a general interaction between crowding and other forms of masking.
Close windowTemporal properties of the polarity advantage effect in crowding
Chakravarthi, R. and Cavanagh, P. (2007)
If the target in a crowding display differs from the distracters in its contrast polarity, the extent of crowding is reduced compared to the condition where all the elements in the display have the same polarity. In experiment 1 we test the temporal properties of this polarity advantage by reversing the contrast of the target and flankers at 4 frequencies between 2 and 15 Hz. In the same-polarity condition, target and distracters were all white in one frame but all black in the next. In the opposite-polarity condition, the target was white and distracters black in one frame and all reversed in the next frame. Less crowding was seen for the opposite polarity condition at lower frequencies but this advantage disappeared at 7.5 Hz and higher frequencies. In experiment 2, we test whether this result can be explained by lateral masking, using a display that matched the crowding configuration. Lateral masking did not exhibit a polarity advantage at any frequency. Hence, the polarity advantage in crowding, and its loss at 6 to 8 Hz, cannot be attributed to lateral masking. It is known that attention has a coarse temporal resolution (6 – 8 Hz). The findings of this study suggest a role for attention in crowding, as opposed to low-level mechanisms like lateral masking.
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The time course of Temporal Binding in social and nonsocial interactions
Silver, C.A., Tatler, B.W., Chakravarthi, R. and Timmermans, B. (2024)
Temporal Binding (TB) is the subjective compression of action-effect intervals. While the effects of nonsocial actions are highly predictable, it is not the case when interacting with conspecifics, who often act under their own volition, at a time of their choosing. Given the relative differences in action-effect predictability in non-social and social interactions, it is plausible that TB and its properties differ across these situations. To examine this, in two experiments, we compared the time course of TB in social and nonsocial interactions, systematically varying action-effect intervals (200–2,100 ms). Participants were told they were (a) interacting with another person via a live webcam, who was in fact a confederate (social condition), (b) interacting with pre-recorded videos (nonsocial condition), or (c) observing two pre-recorded videos (control condition; Experiment 2). Results across experiments showed greater TB for social compared to nonsocial conditions, and the difference was proportional to the action-effect intervals. Further, in Experiment 1, TB was consistently observed throughout the experiment for social interactions, whereas nonsocial TB decreased from the first to the second half of the experiment. In Experiment 2, the nonsocial condition did not differ from control, whereas the social condition did, exhibiting enhanced binding. We argue these results suggest that the sociality of an interaction modulates the ‘internal clock’ of time perception.
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