Organization and properties of neurons in a visual area within the insular cortex of the cat

Abstract

1. Extracellular recordings from 304 neurons were obtained with carbon fiber-containing multibarrel micropipettes. The cells were isolated in the insula in cats anesthetized with barbiturate and immobilized with gallamine triethiodide. Cells were tested with visual stimuli in the form of bars of light, moving edges, and square-wave, drifting grating patterns. 2. The spatial extent of the visually responsive region of insular cortex was assessed and was found to be limited to a surface area of approximately 6-8 mm2, the perimeter being delimited caudally by visually unresponsive cortex of the anterior sylvian gyrus, rostrally by the cortex surrounding the posterior third of the orbital sulcus (ventral bank), dorsally by the rostral extension of the dorsal bank of the anterior ectosylvian sulcus, and ventrally by a visually unresponsive zone bounded by a region about 2 mm2 ventrolateral from the anterior ectosylvian sulcal infolding. Furthermore, a group of unimodal, visually responsive cells often was found in the upper bank of the anterior rhinal sulcus. 3. The possibility of there being a visuotopic organization of insular neurons was examined by analyzing the distribution of receptive-field representation of neurons in sequential penetrations, as well as by searching for spatial progressions in the locations of visually responsive areas within the region. No such clear-cut organization was found among the cells of the insula. 4. Visually responsive neurons were encountered in groups, within electrode penetrations. These groupings were roughly segregated into three distinct levels within the depth of the cortex: the first between the pial surface and 600 micron, the second between 1,100 micron and 1,800 micron, and the third between 2,000 micron and 2,500 micron. 5. Neurons were classified according to their velocity sensitivity, directional preference, orientation sensitivity, length preference, modality specificity, response to electrical stimulation of extrageniculostriate regions, and response to light stimulation in the presence of microiontophoretically administered bicuculline methiodide (BMI). 6. Cells of superficial layers tended to exhibit a preference for high-velocity movements of light bars (600 degrees s-1), whereas those of deeper laminae generally preferred relatively lower velocity movements (60 degrees s-1). The clear preferences of many cells for certain directions of movement within the 360 degrees arc suggested the presence of a dynamic orientation sensitivity. 7. Proportionately more cells preferred moving bars (57%) to small moving spots (43%).(ABSTRACT TRUNCATED AT 400 WORDS)