Fig. 1. Experimental setup for presenting looming stimuli. (A) Left rear view of
the experimental setup. The rear plate and left side panel of the base were
removed from the figure to permit a clear view of the position of the locust
in the setup. Computer-generated looming stimuli were projected onto a rear
projection dome screen using a LCD projector. The locust was held in place on
the inside of the dome with a rigid tether. A synchronization pulse from the
stimulus was sent to the TTL input channel of the multichannel
neurophysiological recording system. (B) Magnified view of area enclosed by
the broken box in A showing the position of the multichannel probes below the
tethered locust (see text for details of recording techniques). (C) Scaled
images of the `locust' (top) and `bird' (bottom) looming stimuli. The `locust'
was designed such that the two sets of wings rotated about the joint with the
body in antiphase at 25 beats s-1, which emulated the flapping of
real locust wings. The `bird' was designed with fixed wings to emulate a real
bird during a gliding approach. For one sequence of approaches to each
experimental animal the `bird' rotated ±45° about its longitudinal
axis (arrows) at 1 roll s-1 to emulate internal object motion. (D)
The angle of the experimental locust's field of view subtended by components
of the looming stimuli identified in C. For both types of stimuli the object
stopped 37 ms before collision. Lb, `locust' body:
Lw, maximum width of `locust' wings;
Bb, `bird' body; Bw, `bird' wing.