Vision and the establishment of direction-selectivity: a tale of two circuits

Abstract

Direction-selective neurons, which respond selectively to motion in one direction, have been characterized in visual circuits across many species. Recently, the development of these directional neurons has been explored in both retina and primary visual cortex (V1). The development of direction-selective cells in V1 requires visual experience. In contrast, direction-selective ganglion cells in retina are present at the age of the earliest light responses. The vision-independent signals guiding the asymmetric wiring underlying retinal direction selectivity remain unknown. The details of how retinal and cortical circuits extract motion information could explain their differing requirements for visual experience in development.

Figure 1. Two ways of implementing the correlation model of direction selectivity postulated by Barlow and Levick

A The Excitatory (or Facilitation) model hypothesizes that the preferred motion elicits excitatory responses from receptor A whose delayed output combines with the excitatory output from B (via an ‘And’ conjunction gate) to produce a response that is greater than expected from the linear sum of the responses generated by the individual receptor inputs. B: The Inhibitory model relies upon delayed inhibition arising from null direction motion to “veto” the excitatory responses of subsequent receptors. Using apparent motion stimuli consisting of two bars flashed with a time delay in adjacent spatial locations, Barlow & Levick found that the apparent motion in the null direction strongly suppressed the response to the second bar. While they also detected facilitation in the preferred direction, the strong null inhibition led Barlow and Levick to favor the Inhibitory model as the primary source of directional responses in the retina.