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Auditory cortex in the grey squirrel: Tonotopic organization and architectonic fields

The representation of sound frequency within auditory cortex has been investigated with microelectrode mapping techniques in grey squirrels. The cyto‐ and myeloarchitecture of mapped cortical surfaces was examined and related to recording data. Among the results were the following: (1) A primary field can be defined on the basis of both physiological and anatomical criteria. It occupies nearly half of the auditory responsive area in the squirrel. (2) The best frequency of neurons is relatively constant within vertical penetrations across the active middle cortical layer(s) of the primary field. (3) Best frequencies ranging from 190 Hz to 33 kHz were defined within the primary field. There is an orderly representation of sound frequency (and of the cochlear partition) within the field, with lowest frequencies represented rostrally and highest frequencies represented caudally. A given frequency band (sector of the basilar partition) is represented across a roughly straight belt of cortex of approximately constant width that crosses the mediol‐lateral dimension of the primary field. There is a proportionately greater surface area of representation of higher frequency octaves within the field. (4) In different squirrels, there is significant variation in the location of the primary field and in the orientation of the frequency representation within it. (5) Auditory responses were recorded within at least two secondary auditory fields. The total region responsive to tonal stimulation approximately corresponded with the anterior temporal field, as defined by Kaas et al. ('72).