Defense reflex and motion detector responsiveness to approaching targets: The motion detector trigger to the defense reflex pathway

Summary

The latency of the crayfish,Procambarus clarki, visually evoked defense reflex varies inversely with the velocity of an approaching object (Figs. 3, 4, 5). Several lines of evidence demonstrate that the latency variations may be attributed to the time required for the target visual angle to expand by a criterion number of degrees (Table 1). The probability that a stimulus will elicit the defense reflex, increases monotonically with the velocity of target approach. Single unit analysis of optic nerve interneurons indicate that the sustaining unit response latency was ≧ reflex latency (Fig. 8). Dimming units were only weakly responsive to approaching objects. Furthermore, the dimming units exhibited very little differential responsiveness over 76% of the behaviorally relevant range of stimulus velocity (Fig. 10). Motion detectors exhibited strong responses to approaching targets (Fig. 11) and both the mean discharge rate and the number of brief interspike intervals/stimulus increased linearly with stimulus velocity (Fig. 14, 15). It is proposed that a central neuronal threshold for eliciting the defense reflex is a criterion number of motion detector spikes or brief interspike intervals.