Regional effect of visual clutter

Stabilizing responses to disturbances are a critical part of the flight
control system in flies. While strongly mediated by mechanoreception, much
of the final steering response results from the wide field motion
detection system associated with vision. The accuracy of the optomotor
responses is affected when the characteristics of the visual input exceed
the dynamic range of the motion detecting system. This has been
demonstrated for stimulus parameters such as contrast, brightness,
velocity, and spatial and temporal frequencies in gratings, but less is
known about the effect of variables in more complex moving stimuli. By
presenting laterally-moving dotfields of different densities we found
that, in Drosophila melanogaster, the amplitude of the optomotor response
is significantly affected by the level of visual clutter in the stimulus.
Flies countersteer strongly within a relatively low and narrow range of
element densities. But this effect is exclusive to the ventral region of
the eye, and dorsal stimuli elicit an unaltered and stereotypical response
throughout levels of visual clutter. This highlights local specialization
of the eye and suggests the lower region may play a more critical role in
translational flight-stabilization.