Responses of the H1 Neuron of the Fly to Jumped Edges

Directional motion detection was measured as the response of the H1 neuron of the fly. The stimulus was the jump of a single black-white edge or a single bar through an angle of 1.5°, which is similar to the angle between adjacent receptor axes. An edge that advances by one receptor causes the same change in that receptor whichever way it moves, but the response is to one direction only. Therefore the steady state of the receptors before the stimulus jump is available to the directional motion perception mechanism no matter how long the stimulus has been at rest. This short-term memory of the previous state of the receptors persists even though the bar disappears briefly during its jump. Similarly, the response to a bar is directional although a black bar that jumps one way causes the same changes in a photoreceptor pair as a white bar that jumps the other way. Responses to ‘off9 are distinguished from directional responses to motion. If the contrast of the bar is reversed at the jump, the directionality is lost, showing that algebraic multiplication does not occur when the stimulus is a narrow bar. Motion is inferred by interaction of the nearest edge with the former position of an edge having the same orientation. Black-white edges therefore do not interact with white—black edges to produce a directional response. The results are discussed in terms of the template model, which is a Boolean representation of spatio-temporal fields of expectant neurons in parallel behind each visual axis.