Functional Gradients of Auditory Sensitivity along the Anterior Ectosylvian Sulcus of the Cat

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Abstract
Determining the spatial direction of sound sources is one of the major computations performed by the auditory system. The anterior ectosylvian sulcus ( AES) of cat cortex is known to be important for sound localization. However, there are contradicting reports as to the spatial response properties of neurons in AES: whereas some studies found narrowly tuned neurons, others reported mostly spatially widely tuned neurons. We hypothesized that this is the result of a nonhomogenous distribution of the auditory neurons in this area. To test this possibility, we recorded neuronal activity along the AES, together with a sample of neurons from primary auditory cortex ( A1) of cats in response to pure tones and to virtual acoustic space stimuli. In all areas, most neurons responded to both types of stimuli. Neurons located in posterior AES ( pAES) showed special response properties that distinguished them from neurons in A1 and from neurons in anterior AES ( aAES). The proportion of space- selective neurons among auditory neurons was significantly higher in pAES ( 82%) than in A1 ( 72%) and in aAES ( 60%). Furthermore, whereas the large majority of A1 neurons responded preferentially to contralateral sounds, neurons in pAES ( and to a lesser extent in aAES) had their spatial selectivity distributed more homogenously. In particular, 28% of the space- selective neurons in pAES had highly modulated frontal receptive fields, against 8% in A1 and 17% in aAES. We conclude that in cats, pAES contains a secondary auditory cortical field which is specialized for spatial processing, in particular for the representation of frontal space.