Keck Center for Integrative Neuroscience, University of California at San Francisco
The Cortical Representation of Sound
What is the right stimulus for a neuron in the auditory cortex? While the understanding of the functional role of different classes of neurons in the awake primary visual cortex has been extensively studied since the early experiments of Hubel and Weisel over thirty years ago, our understanding of the basic response selectivity and functional role of neurons in the primary auditory cortex is much farther from complete. In particular, while moving bars are now well-recognized as optimal for many neurons in the primary visual cortex, for an auditory cortical neuron it is not clear how to select the most appropriate stimulus. Our understanding of auditory cortex physiology results almost exclusively from work done in anesthetized animals, and the vast majority of studies have used single stimuli, typically pure tones, to characterize neuronal responsiveness along a single continuum, such as pure tone frequency or sound source location, although this has recently begun to change. We recorded from neurons in the auditory cortex of the awake or anesthetized primate, and used a novel reverse correlation technique to compute the receptive fields (or preferred stimuli) of auditory cortical neurons, encompassing both multiple frequency components and ongoing time. These spectrotemporal receptive fields make clear that neurons in the primary auditory cortex, as in the primary visual cortex, typically show considerable structure, sometimes including multiple excitatory and inhibitory regions in their receptive fields. These features demonstrate that auditory cortical neurons can be sensitive to stimulus edges in frequency composition or in time, and sensitive to stimulus transitions such as changes in frequency. They provide insights into the mechanisms of sound feature decomposition in the cerebral cortex. In addition, auditory cortical neurons can often be driven to high rates of firing by using custom-designed stimuli, made to fit their predicted pattern of selectivity.