A quantitative study of the projection area of the central and the paracentral visual field in area 17 of the cat

Abstract

Cells in cat's area 17 respond optimally if elongated contrasts are presented at a certain angle of orientation with respect to the retina, or to the visual field, respectively (Hubel and Wiesel, 1962). The preferred orientation and the range of orientation sensitivity of cells in close proximity to one another have been determined in order to investigate the spatial arrangement of the orientation domain in area 17. A slight overrepresentation of vertical and horizontal orientations is seen in cells with complex receptive fields, whereas in cells with simple fields all orientations are represented to an equal degree. The orientation selectivity, defined as the halfwidth of tuning curves constructed from the cells response to a moving stimulus, is less than 60 degrees in more than 80% of all cells investigated, and is on the average 20–30 degrees smaller in cells with simple than in cells with complex receptive fields. In 80% of all cases considered the difference in the preferred orientation between two cells less than 200 μm horizontally distant in area 17 is less than 30 degrees, which is of the order of an individual cells orientation selectivity. Each cell, therefore, will respond to some extent to that orientation which is preferred by the cells in the immediate surroundings. Sequential changes in the preferred orientation between cells successively recorded are observed as the postlateral gyrus is explored from anterior to posterior and from medial to lateral. On these general trends a random variation in the preferred orientation between neighbouring cells of the order of 5–10 degrees is superimposed. One orientation sequence (180 degrees) occupies 700–1200 μm, so that on the average a change in the preferred orientation of the order of 10 degrees is complete after 50 μm distance in the cortex measured parallel to the pial surface. Assuming that 18 different orientations (± 5 degrees) functionally represent one complete orientation sequence it is found that ‘all’ orientations are functionally represented by the cells contained in a cortical cylinder of 300–700 μm in diameter. Cells having the same preferred orientation are grouped together in cortical regions which appear in crossection as a band or a spot. These regions have been termed iso-orientation bands or spots. The diameter of the spots and the small diameter of the bands do not exceed 100 μm. Taking an average orientation selectivity of 40 degrees for cells vertically aligned in area 17 it is calculated that cells situated 100 μm to either side of an iso-orientation band or around an iso-orientation spot still respond with 50% of the discharge to their own optimal orientation. The functional subunit of the orientation domain, the orientation subunit, consists of that cells which respond at all to a particular orientation. These cells are vertically aligned through all cortical layers (Hubel and Wiesel, 1963) and are located on the average 200 μm (range 25–450 μm) in a horizontal direction to either side from the center iso-orientation band or spot. The sensitivity to the orientation functionally represented by the subunit decreases with increasing distance from the center band, and from the center spot, respectively. The spatial properties of the subunit imply, that each subunit has indeterminate boundaries and that it shares cells with its immediate neighbours. From this it is concluded, that in most parts of area 17 of the cat there is a continuous orientation representation.