Distribution of tactile learning and its neural basis

The brain's sensory processing systems are modified during perceptual learn ing. To learn more about the spatial organization of learning-related modif ications, we trained rats to utilize the sensory signal from a single intac t whisker to carry out a behavioral task. Once a rat had mastered the task, we clipped its "trained" whisker and attached a "prosthetic" one to a diff erent whisker stub. We then tested the rat to determine how quickly it coul d relearn the task by using the new whisker. We observed that rats were imm ediately able to use the prosthetic whisker if it were attached to the stub of the trained whisker but not if it were attached to a different stub. In deed, the greater the distance between the trained and prosthetic whisker, the more trials were needed to relearn the task. We hypothesized that this "transfer" of learning between whiskers might depend on how much the repres entations of individual whiskers overlap in primary somatosensory cortex. T esting this hypothesis by using 100-electrode cortical recordings, we found that the overlap between the cortical response patterns of two whiskers ac counted well for the transfer of learning between them: The correlation bet ween the electrophysiological and behavioral data was very high (r = 0.98), These findings suggest that a topographically distributed memory trace for sensory-perceptual learning may reside in primary sensory cortex.