Abstract
A critical issue for understanding the physiological bases of cortical learning is identifying which particular representations can be altered by training. One hypothesis is that all cortical circuits are equally mutable with appropriate training. In this scheme, if specific neurons in primary visual areas provide the most appropriate signals for a task, then these neurons are modified by training to produce perceptual improvements. Two lines of evidence have supported this reasoning. First, the mutability of receptive field properties in primary visual cortex has been demonstrated by retinal lesioning experiments. Second, psychophysical experiments have shown perceptual improvements that fail to transfer across dimensions that are represented in primary visual cortex. For example, several studies of perceptual learning have reported perceptual improvements that are confined to the retinotopic location of training. Recent evidence suggests, however, that not all representations are equally mutable. In particular, single unit recording in monkeys trained in an orientation discrimination task, suggests that the representations in areas V1 and V2 are not altered by extensive training, while representations in an intermediate cortical area, V4, are. Imaging studies of the effects of orientation adaptation in humans reveal a similar pattern: little effect in primary visual areas and a robust effect in intermediate areas. These studies suggest that the hierarchy of visual areas is defined by both an elaboration of receptive field properties and an increase in the mutability of those properties.