The Location of Mechanisms for Instantaneous Colour Constancy in Human Vision

Abstract

Abstract: : Purpose: The principal aim of this study was to establish the relative contribution retinal mechanisms, the primary visual cortex and extrastriate visual areas make to instantaneous colour constancy (ICC) in human vision. Methods: Changes in the perceived colour of a central test stimulus as a result of surround illuminant changes were investigated in a number of successful binocular and dichoptic experiments. The test patch served both as "reference", when the surround was illuminated with daylight, and as matching stimulus, when the surround was illuminated with tungsten. In dichoptic experiments the test patch and the surrounding Mondrian were presented separately to each eye using a multi-mirror stereoscope. The locking of the two images in binocular fusion was achieved using a sparse, random dot stereogram embedded in the Mondrian surround. The subject's task was to alter the test target chromaticity so that its perceived colour remained invariant with the change of surround illuminant in a number of binocular, monocular and dichoptic stimulus conditions. Results: The results reveal the largest ICC index with binocular viewing. A rapid decrease in ICC with the dark border width separating the test stimulus from the adjacent surround was observed for both binocular and monocular viewing conditions, with the immediate surround (i.e., < 1.5^o separation) contributing most to the constancy effect. Dichoptic implementation of the dynamic colour matching technique yielded reduced, but significant ICC index values with more equal contribution from distant surrounds. The results reveal the importance of short-range spatial interactions and the heavy reliance on local comparison of receptor signals without knowledge of prevailing conditions. Experiments carried out in subjects with cerebral achromatopsia reveal the existence of "hidden" constancy, even when the subjects were unable to consciously perceive the result of functioning ICC mechanisms. Conclusions: The evidence obtained from all the experiments carried out in this investigation points to V1, and particularly those layers of V1 that are rich in monocularly driven neurones, as the principal neural substrate for mediation of instantaneous colour constancy.

Keywords: color vision • color appearance/constancy • color vision