Is the McCollough Effect Coded in Disparity-Sensitive Units?

Abstract

Previous studies have demonstrated that the McCullough effect (ME) is coded by both binocular and monocular units [see for example Vidyasagar, 1976 Nature (London)261 39]. At ECVP95, McLoughlin and Savoy (1995 Perception24 Supplement, 94) presented a contiguity-learning based model capable of accounting for a large dataset of both binocular and monocular ME variants. One issue not directly addressed by this model concerns the site of binocular ME adaptation. Specifically, are the binocular ME ‘units’ disparity selective? The methods used were developed from those of McLoughlin and Savoy. Subjects were exposed to alternating coloured gratings at various crossed and uncrossed disparities (relative to fixation points). A colour cancellation paradigm was used to record the strength of the ME for a series of binocularly disparate composite achromatic test gratings both before and after adaptation. These test gratings had a small random amount of colour added to them so that the subjects could not assume that the patterns were achromatic before making adjustments. Subjects waited 5 min in the dark after adaptation before being tested. This removes any possible confounds associated with retinal afterimages. Preliminary results suggest, in agreement with previous studies, that the main component of the ME is monocular. However, the binocular component appears to be sensitive to disparity. A second series of experiments wherein two opponent MEs were induced, one at crossed and the other at uncrossed disparities, supports this finding. As in the Vidyasagar variant, the magnitude of the ME is extremely weak in this case.