Abstract
The extraction of directional motion information from
changing retinal images is one of the earliest and most
important processing steps in any visual system. In the fly
optic lobe, two parallel processing streams have been
anatomically described, leading from two first-order
interneurons, L1 and L2, via T4 and T5 cells onto large,
wide-field motion-sensitive interneurons of the lobula
plate. Therefore, T4 and T5 cells are thought to have a
pivotal role in motion processing; however, owing to their
small size, it is difficult to obtain electrical recordings of
T4 and T5 cells, leaving their visual response properties
largely unknown. We circumvent this problem by means of
optical recording from these cells in Drosophila, using the
genetically encoded calcium indicator GCaMP5 (ref. 2). Here
we find that specific subpopulations of T4 and T5 cells are
directionally tuned to one of the four cardinal directions;
that is, front-to-back, back-to-front, upwards and
downwards. Depending on their preferred direction, T4 and T5
cells terminate in specific sublayers of the lobula plate. T4
and T5 functionally segregate with respect to contrast
polarity: whereas T4 cells selectively respond to moving
brightness increments (ON edges), T5 cells only respond to
moving brightness decrements (OFF edges). When the output
from T4 or T5 cells is blocked, the responses of postsynaptic
lobula plate neurons to moving ON (T4 block) or OFF edges
(T5 block) are selectively compromised. The same effects are
seen in turning responses of tethered walking flies. Thus,
starting with L1 and L2, the visual input is split into
separate ON and OFF pathways, and motion along all four
cardinal directions is computed separately within each
pathway. The output of these eight different motion detectors
is then sorted such that ON (T4) and OFF (T5) motion
detectors with the same directional tuning converge in the
same layer of the lobula plate, jointly providing the input to
downstream circuits and motion-driven behaviours.
Users
Please
log in to take part in the discussion (add own reviews or comments).