Formation of category representations in superior temporal sulcus.

The human brain contains cortical areas specialized in representing object categories. Visual experience is known to change the responses in these category-selective areas of the brain. However, little is known about how category training specifically affects cortical category selectivity. Here, we investigated the experience-dependent formation of object categories using an fMRI adaptation paradigm. Outside the scanner, subjects were trained to categorize artificial bird types into arbitrary categories (jungle birds and desert birds). After training, neuronal populations in the occipito-temporal cortex, such as the fusiform and the lateral occipital gyrus, were highly sensitive to perceptual stimulus differences. This sensitivity was not present for novel birds, indicating experience-related changes in neuronal representations. Neurons in STS showed category selectivity. A release from adaptation in STS was only observed when two birds in a pair crossed the category boundary. This dissociation could not be explained by perceptual similarities because the physical difference between birds from the same side of the category boundary and between birds from opposite sides of the category boundary was equal. Together, the occipito-temporal cortex and the STS have the properties suitable for a system that can both generalize across stimuli and discriminate between them.