Modality specificity of neuronal responses within the cat's insula

1. Electrophysiological recordings of single-unit responses, multiunit responses, and electrically evoked field potentials have been made using carbon fiber-containing micropipettes in cats anesthetized with barbiturate and immobilized with gallamine triethiodide. Recording sites sampled cortical regions throughout the insula, including zones more ventrally situated and more rostral and caudal than those described in the preceding, companion paper. One-hundred eleven cells in total were tested with a battery of different types of stimuli. 2. Stimuli were divided into two classes, according to either the intensity of the stimulus or its form. These are called physiological forms, or levels of stimulation, and nonphysiological forms or levels. The nonphysiological forms of stimuli for visual, somatosensory, and auditory modalities consisted of (for visual stimuli): 1) electrical stimulation of the optic nerve or 2) bright flashes light at 100% contrast; for somatosensory, electrical stimulation of the radial nerve by implanted cuff electrodes; and for auditory, stimulation with bursts of white noise generated at high intensities (80-100 dB) or with a loud click stimulus. Physiologically relevant levels of stimuli for these same modalities were: moving bars of light projected onto a tangent screen in front of the animal (visual); light cutaneous deformation, hair displacement, and light pressure delivered to various regions on the surface of the cat's body with hand-held probes, or delivered manually (somatosensory); and white noise generated at low intensities (ca. 40 dB) (auditory). 3. Cells situated in dorsal insular regions responded to visual stimuli when levels of sensory activation were employed using natural means, within normal, physiologically relevant limits. Responses to auditory or somatosensory stimulation were observed in this region only when very intense forms of "natural" stimulation, or when electrical stimulation (nonphysiologically relevant levels of stimulation) was delivered. In this latter case, the same cells in several instances could be made to appear polymodally responsive. With cells situated in ventral insular regions, some polymodal responses to physiologically relevant levels of stimulation were noted, although it was considerably more common to obtain unimodal responses. Nonphysiological levels of activation yielded evidence for a polymodal convergence onto the greater proportion of cells recorded. 4. Field potential recordings with microelectrodes revealed widely overlapping representations of all modalities in both dorsal and ventral regions of the insula, irrespective of the sensitivity displayed by the local neuronal r