Categorization is fundamental to our
perception and understanding of the environment. However, little is known about the
neural bases underlying the categorization of
sounds. Using human
functional magnetic resonance imaging (
fMRI) we compared
the brain responses to a category discrimination task with an auditory discrimination task using identical sets of
sounds. Our stimuli differed along two dimensions: a speech-nonspeech dimension and a fast-slow
temporal dynamics dimension. All stimuli activated regions in the primary and nonprimary auditory cortices in the
temporal cortex and in the parietal and frontal cortices for the two tasks. When comparing the
activation patterns for the category discrimination task to those for the auditory discrimination task, the results show that a core group of regions beyond the auditory cortices, including
inferior and middle frontal
gyri, dorsomedial frontal
gyrus, and
intraparietal sulcus, were preferentially activated for familiar speech categories and for novel nonspeech categories. These regions have been shown to play a role in
working memory tasks by a number of studies. Additionally, the categorization of nonspeech
sounds activated left
middle frontal gyrus and right
parietal cortex to a greater extent than did the categorization of speech
sounds. Processing the
temporal aspects of the stimuli had a greater impact on the left
lateralization of the categorization network than did other factors, particularly in the
inferior frontal gyrus, suggesting that there is no inherent
left hemisphere advantage in the categorical processing of speech stimuli, or for the categorization task itself.
