Physiological properties of single units were investigated in the suprageniculate nucleus (SG) and in the cerebral cortex along the anterior ectosylvian sulcus (AES), including the insular cortex. The recording was performed with the aid of carbon-filled glass micropipettes in barbiturate-anesthetized cats. The main findings of the study can be summarized as follows. 1. The physiological properties of the cells in the suprageniculate nucleus and in the AES/insular cortex exhibited striking similarities in a series of aspects: (a) The frequencies of occurrence of uni-, bi- and trimodal cells were similar. (b) The majority of the unimodal cells (75% in the AES/insular region and 65% in the SG) has visual sensitivity in both structures. The bimodal and trimodal cells were also dominated by visual sensitivity. (c) The somatosensory and auditory modalities were similarly present in both structures, although less frequently than the visual one. (d) No systematic topological organization was found in either structure. (e) The visual, somatosensory and auditory receptive fields were uniform and covered a fairly large proportion of the personal space. 2. Statistical comparison of some physiological properties of cells situated deep in the AES with those of cells in the insular cortex revealed differences as follows: (a) The insular cortex contained significantly more bi- and trimodal cells than the sulcal areas. (b) Cells in the insular cortex preferred significantly lower stimulus velocities and larger stimuli than cells in the depths of the AES. These results seem to support the notion of a suprageniculate-AES/insular thalamo-cortical multisensory entity. Additionally, the physiological differences between the sulcal AES cortex and gyral insula are in agreement with the morphological differences found earlier in the afferentation of these areas (Norita et al., 1986, 1991).
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