Journal of Neuroscience, Vol 15, 5596-5611, Copyright © 1995 by Society for Neuroscience
Visual motion detection circuits in flies: peripheral motion computation by identified small-field retinotopic neurons
JK Douglass and NJ Strausfeld
Arizona Research Laboratories, University of Arizona, Tucson 85721, USA.
Giant motion-sensitive tangential neurons in the lobula plate are thought
to be cardinal elements in the oculomotor pathways of flies. However, these
large neurons do not themselves compute motion, and elementary motion
detectors have been proposed only from theory. Here we identify the forms,
projections, and responses of small-field retinotopic neurons that comprise
a well known pathway from the retina to the lobula plate. Already at the
level of the second and third synapses beneath the photoreceptor layer,
certain of these small elements show responses that distinguish motion from
flicker. At a level equivalent to the vertebrate inner plexiform layer (the
fly's outer medulla) at least one retinotopic element is directionally
selective. At the inner medulla, small retinotopic neurons with bushy
dendrites extending through a few neighboring columns leave the inner
medulla and supply inputs onto lobula plate tangentials. These medulla
relays have directionally selective responses that are indistinguishable
from those of large-field tangentials except for their amplitude and
modulation with contrast frequency. Centrifugal neurons leading back from
the inner medulla out to the lamina also show orientation-selective
responses to motion. The results suggest that specific cell types between
the lamina and inner medulla correspond to stages of the
Hassenstein-Reichardt correlation model of motion detection.