Functional specialization in non-primary auditory cortex of the cat: areal and laminar contributions to sound localization.

The purpose of this study is to: (1) examine the relative contributions of the 13 acoustically-responsive regions of the cerebral cortex to sound localization; (2) examine the laminar contributions to spatial localization behavior for each of the cortical areas identified to be critical for accurately determining the position of a sound source; and (3) synthesize the findings from sound localization studies and the underlying corticocortical and corticotectal connections to develop a processing system for sound localization information within and between the cerebral cortex and the superior colliculus. First, we examined performance on a sound localization task before, during, and after unilateral or bilateral reversible cooling deactivation of each region of acoustically-responsive cortex. Overall, unilateral deactivation of primary auditory cortex and the dorsal zone (AI/DZ), the posterior auditory field (PAF), or the auditory field of the anterior ectosylvian sulcus (AES) yielded profound sound localization deficits in the contralateral field. Bilateral deactivations of the same regions yielded bilateral sound localization deficits. Second, graded cooling of AI/DZ or PAF showed that deactivation of only the superficial layers was required to elicit sound localization deficits. However, graded cooling of AES revealed that cooling of the superficial layers alone does not cause significant sound localization deficits. Profound deficits were identified only when cooling extended through the full thickness of AES cortex. Therefore, we propose that the superficial layers of AI/DZ or PAF and the deeper layers of AES are necessary for determining the precise location of a sound source. Finally, when these results are combined with data on corticocortical and corticotectal projections, we propose that signals processed in the superficial layers of AI, DZ, or PAF feed forward to the auditory field of AES. In turn, neurons in the deeper layers of AES project to the intermediate and deeper layers of the superior colliculus. Therefore, we propose that sound localization signals processed in primary and non-primary auditory cortex are transmitted to the superior colliculus by means of the auditory field of the AES.