Perceptual learning of contrast discrimination and its neural correlates in macaque V4 and V1

Abstract

We make frequent evaluations of subtle contrast differences in our visual environment, and often under challenging illumination conditions, whether photopic, scotopic or mesopic. Our contrast discrimination abilities are rigorously honed from an early age, and we continue to carry out these fine perceptual judgments throughout our lifetimes. Thus, the issue of whether substantial improvement in contrast discrimination is possible during later periods in life, such as during adulthood- and the circumstances that allow this- has sometimes come under discussion. Our adult macaque subjects underwent extensive training on a contrast discrimination task, in which stimuli were positioned at a variety of peripheral and parafoveal locations. We present clear evidence of contrast perceptual learning at the behavioural level and show that these changes have neuronal correlates primarily in V4, rather than in V1. Learning was specific to stimulus location and spatial frequency, but was transferable across orientations; it took place to a limited degree under stimulus roving conditions, and could be either facilitated or impeded by the addition of flanker stimuli, depending on the subject. Upon removal of flankers, levels of psychometric and neurometric performance returned to their pre-flanker state. In V4, learning-induced changes encompassed a shift in the point of neurometric equality and the semi-saturation constant (C50) towards the trained contrast; a decrease in noise correlations across channels; and an increase in choice probability. In V1, enhancements in performance were characterised by an increase in spike discriminability; a shift in the point of neurometric equality and the C50 towards the trained contrast(s); and a widening in the range and a steepening of the contrast response function, during the early phase of training. Deteriorations in performance were accompanied by the reverse effects on V1 activity; furthermore, a general decrease in V1 firing rates occurred when training was carried out over an extended period of time, after performance had reached its peak.