Complementary mechanisms create direction selectivity in the fly

TLDR

Direction selectivity in visual motion is a classic neural computation problem, with two proposed mechanisms—preferred direction enhancement and null direction suppression—first manifesting in Drosophila T4 and T5 cells. By stimulating individual optic lobe columns and recording from single T4 neurons, the study shows that preferred direction enhancement and null direction suppression operate in distinct receptive‑field sub‑regions. These results explain the strong directional selectivity of fly motion‑sensing neurons and advance our understanding of elementary motion detection.

Abstract

How neurons become sensitive to the direction of visual motion represents a classic example of neural computation. Two alternative mechanisms have been discussed in the literature so far: preferred direction enhancement, by which responses are amplified when stimuli move along the preferred direction of the cell, and null direction suppression, where one signal inhibits the response to the subsequent one when stimuli move along the opposite, i.e. null direction. Along the processing chain in the Drosophila optic lobe, directional responses first appear in T4 and T5 cells. Visually stimulating sequences of individual columns in the optic lobe with a telescope while recording from single T4 neurons, we find both mechanisms at work implemented in different sub-regions of the receptive field. This finding explains the high degree of directional selectivity found already in the fly’s primary motion-sensing neurons and marks an important step in our understanding of elementary motion detection.