PhD ThesisWe 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.The Medical Research Council, UK