Visual pathways and psychophysical channels in the primate

Abstract

The main cell systems of the retina that provide input to the striate cortex are now well described, although certain aspects of their anatomy and physiology remain contentious. Under simple stimulus conditions and in a threshold context psychophysical performance can often be assigned to one or other of these systems, and an identification of psychophysical channels with afferent pathways is justifiable. However, results from psychophysical studies using more complex stimulus conditions are more difficult to relate to 'front end' channels, and it is more difficult to separate the physiological contributions of afferent pathways from those of cortical mechanisms, in particular the separation of dorsal and ventral streams.

Figure 1. A standard view of primate retinal structure

A, a view of the eye; the sketch of primate retinal wiring in C appears complex but is made up of standard retinal elements: receptors, bipolar and ganglion cells as indicated. The lamina structure of the primate retina is clearly delineated in B (scale bar 50 μm). C shows a sketch of primate retinal wiring. Each pathway conforms to the standard pattern (A), with receptors connected to bipolar cells to ganglion cells. The three main pathways sketched (parasol, midget and blue-yellow) are those providing the major input to striate cortex. Photomicrograph kindly provided by U. Grünert.

Figure 2. Three examples in which retinal signals are not perceptually utilized

A, temporal frequency tuning for red–green modulation in psychophysics and PC pathway retinal ganglion cells, replotted from Lee et al. (2007 with permission from ARVO as the copyright holder). Psychophysical sensitivity (derived from Swanson et al. (1987)) and converted to cone contrast sensitivity. Physiological data are based on cell firing rates required to reach a response criterion. B, psychophysical and PC pathway sensitivity as a function of retinal eccentricity. Replotted from Martin et al. (2001). C, PC cell (+M-L cell, 4.8 deg eccentricity) responsivity to luminance (40% contrast) and red–green chromatic (100% modulation contrast) gratings (2 Hz drift rate) as a function of spatial frequency. Visual resolution is similar for both conditions.