It is often assumed that the ultimate goal of a
motion-detection system is to faithfully represent the
time-dependent velocity of a moving stimulus. This assumption,
however, may be an arbitrary standard since the requirements
for a motion-detection system depend on the task that is
to be solved. In the context of optomotor course stabilization,
the performance of a motion-sensitive neuron in the fly's
optomotor pathway and of a hypothetical velocity sensor
are compared for stimuli as are characteristic of a normal
behavioral situation in which the actions and reactions
of the animal directly affect its visual input. On average,
tethered flies flying in a flight simulator are able to
compensate to a large extent the retinal image displacements
as are induced by an external disturbance of their flight
course. The retinal image motion experienced by the fly
under these behavioral closed-loop conditions was replayed
in subsequent electrophysiological experiments to the animal
while the activity of an identified neuron in the motion
pathway was recorded. The velocity fluctuations as well
as the corresponding neuronal signals were analyzed with
a statistical approach taken from signal-detection theory.
An observer scrutinizing either signal performs almost
equally well in detecting the external disturbance.