The Visual EEG Lab

Part of the Perception and Attention research theme at the School of Psychology, University of Aberdeen

Colour and Luminance in Object Representation

Early advantage of luminance for object representation and its cross-talk with chromatic pathways in human visual scene analysis

Detection and identification of objects is the most crucial goal of human visual perception. But the ease with which humans perceive objects in the environment is deceiving. The computations of visual signals in various brain regions which underlie object vision are anything but simple.

For example, during perception, different visual properties (e.g. luminance and colour) are initially sampled and processed by separate channels and undergo several stages of transformation in the parts of the brain that process visual signals. These separate channels in the visual system processes incoming information from the eyes in parallel, through a series of rapid hierarchically organized stages. This involves a cascade of neural activity that encompasses a whole range of brain areas, from sensory regions that represent simple features such as orientation to more broadly tuned brain regions, ending with the regions that store our knowledge of the world.

Through these processes, a coherent representation of our environment segmented into objects that are layered in visual scenes is formed in less than 300 milliseconds of processing time within a range of distributed brain areas.

In this project, it is our aim to investigate in which way the brain manages to coordinate the processing of information from different channels, such as colour and luminance, along each of the transformative stages that lead to the perception of objects. Both colour and luminance channels are crucial for everyday object vision but their contributions differ, with luminance considered to be more relevant for rapid processing of lines, edges, shape and motion and colour being more relevant for segmentation of visual scenes. But the extent to which colour and luminance pathways function independently or interactively at different stages of visual processing in humans remains unknown even after many years of study, due to the difficulties in

1) Producing adequate stimuli for comparing low-, mid- and high-level visual processing along different pathways

and

2) Analysing the differences in rapidly evolving neural processes that subserve normal human vision in the initial 300 miliseconds of processing time.

The proposed project will attempt to overcome these problems through an innovative experimental approach which joins recordings of brain activity with a millisecond resolution together with the tools of colour psychophysics which can separate out different visual processing channels by precisely defining the chromatic and luminance properties of the stimuli.  We have created a large stimulus set of objects and matched 'non-objects' along the lines suggested by Sassi et al (2010, iPerception, 1, 121-142), using spatial parameters of Gabors adjusted to deliver approximate performance along colour and luminance channels (see Wuerger and Morgan, 1999, JOSA A, 16, 436-444), and are in the process of conducting a series of psychophysical experiments using these stimuli.The time-course of cortical activations and their underlying generators will be identified using EEG. Our pilot study (Martinovic, Mordal and Wuerger, 2011, Journal of Vision, 11(7):1-15) has looked at a subset of signals, we now aim to test all directions in DKL colour space and attempt to model summed signals for low, mid and high-level vision  by using stimuli that elicit excitations in single or multiple colour or luminance pathways.

The findings of this study will provide an important insight into the ways in which the human brain utilises different types of information during visual processing and will thus significantly contribute to current knowledge on the relations between parallel processing of different features such as colour or luminance along the different hierarchical stages, from identification of differently oriented lines, to identification of familiar objects. Moreover, the study will describe the neural mechanisms that allow preferential inputs of luminance information into object representation processes and thus enable it to drive everyday vision.

We're running several experiments related to this project as part of undergraduate dissertations and summer placements are available in the lab for those students interested in visual perception and electroencephalography who would like to gain further research experience.