Change in pattern of ongoing cortical activity with auditory category learning

Humans are able to classify novel items correctly by category(1,2); some other animals have also been shown to do this(3-7). During category learning, humans group perceptual stimuli by abstracting qualities from similarity relationships of their physical properties(1,2,8). Forming categories is fundamental to cognition(9) and can be independent of a 'memory store' of information about the items or a prototype(10). The neurophysiological mechanisms underlying the formation of categories are unknown. Using an animal model of category learning(6), in which frequency-modulated tones are distinguished into the categories of 'rising' and 'falling' modulation, we demonstrate here that the sorting of stimuli into these categories emerges as a sudden change in an animal's learning strategy. Electro-corticographical recording from the auditory cortex(11) shows that the transition is accompanied by a change in the dynamics of cortical stimulus representation. We suggest that this dynamic change represents a mechanism underlying the recognition of the abstract quality (or qualities) that defines the categories.