Functional gradients of auditory sensitivity along the anterior ectosylvian sulcus of the cat

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.