Responses of the H1 neuron of the fly to contrast and moving bars

Abstract

The responses of the H1 neuron of the fly were measured as a function of contrast. The stimuli were either transient jumps of edges, bar and regular stripes, or smooth movements of these patterns. The directional motion-sensitive system saturates with respect to contrast at low contrast levels, showing that it is adapted to detecting small contrasts rather than measuring them over the whole range, and suggesting that linear calculations cannot be based on responses to sine-wave stimulation. Below saturation the response is not related to the square of the contrast, suggesting that multiplication of the stimulus intensity at neighbouring receptors is not a significant component of the motion processing mechanism. The short-term memory of the previous state, that is essential for motion detection, takes longer to build up at lower contrast and the effects of contrast are independent of pattern. The same conclusion is also suggested by the effect of the width of a smoothly moving bar upon the responses of the H1 neuron as a function of angular velocity. The results are interpreted in terms of the (as yet unknown) spatio-temporal fields of unit motion-detecting neurons, along the lines of the template model for transient stimuli (Horridge 1990) rather than in terms of correlation of the visual image with itself. Show more