Orientation sensitivity of ganglion cells in primate retina

Morphology and physiology of primate M- and P-cells

Catarrhines and platyrrhines, the so-called Old- and New-World anthropoids, have different cone photopigments. Postreceptoral mechanisms must have co-evolved with the receptors to provide trichromatic color vision, and so it is important to compare postreceptoral processes in these two primate groups, both from anatomical and physiological perspectives. The morphology of ganglion cells has been studied in the retina of catarrhines such as the diurnal and trichromatic Macaca, as well as platyrrhines such as the diurnal, di- or trichromatic Cebus, and the nocturnal, monochromatic Aotus. Diurnal platyrrhines, both di- and trichromats, have ganglion cell classes very similar to those found in catarrhines: M (parasol), P (midget), small-field bistratified, and several classes of wide-field ganglion cells. In the fovea of all diurnal anthropoids, P-cell dendritic trees contact single midget bipolars, which contact single cones. The Aotus retina has far fewer cones than diurnal species, but M- and P-cells are similar to those in diurnal primates although of larger size. As in diurnal anthropoids, in the Aotus, the majority of midget bipolar cells, found in the central 2 mm of eccentricity, receive input from a single cone and the sizes of their axon terminals match the sizes of P-cell dendritic fields in the same region. The visual responses of retinal ganglion cells of these species have been studied using single-unit electrophysiological recordings. Recordings from retinal ganglion cells in Cebus and Aotus showed that they have very similar properties as those in the macaque, except that P-cells of mono- and dichromatic animals lack cone opponency. Whatever the original role of the M- and P-cells was, they are likely to have evolved prior to the divergence of catarrhines and platyrrhines. M- and P-cell systems thus appear to be strongly conserved in the various primate species. The reasons for this may lie in the roles of these systems for both achromatic and chromatic vision.

Meridional anisotropy in visual processing: implications for the neural site of the oblique effect

The contention is examined that the oblique effect, i.e., the well-known performance deficit in detecting orientation difference in oblique lines as compared to vertical and horizontal ones, has its origin in a relative deficiency of neurons with obliquely-oriented receptive fields in the primary visual cortex. Psychophysical observations demonstrate a prominent oblique effect also in visual tasks involving widely-separated elements and other stimuli that would elicit little or no response in oriented neurons in the visual cortex. Conversely, some tasks, e.g. position discrimination, exhibit no oblique effect even with short, high-contrast lines. When the comparison with the reference can be accomplished during a single brief exposure rather than sequential ones, thresholds for orientation differences between adjacent contours in oblique meridians are also elevated compared to those in the vertical and horizontal, but to a lesser extent. In one particular texture discrimination task some but not all observers have a conspicuous oblique effect. The discrimination only of the direction of streaming random dots, not of their speed, is poorer for motions in oblique meridians. The findings imply that the neural locus for the oblique effect is more central than the primary visual cortex.

The distribution of preferred orientations in the peripheral visual field

Orientation discrimination was measured for line targets in the fovea and in locations along four meridians (vertical, horizontal, 45 degrees and 135 degrees ) in the peripheral visual field. In all locations, the performance for vertical and horizontal contours was on average almost twice as good as for oblique ones. In addition, especially in peripheral locations along oblique meridians, thresholds tend to be better for radially-oriented contours compared to tangential ones. These effects are robust to length of the test lines and to small changes in position and cannot therefore be due to cortical singularities (pinwheels). The results suggest that the advantage of horizontal and vertical contours, as well as possibly of radial ones, is a manifestation of a characteristic structural property of the organization of the visual pathway.