Biomedical Engineering Reference
In-Depth Information
Figure 14.16
The same as
Figure 14.15,
except for the fact that this time there is only large-field
motion of the background (see xt-plot in top row). Under these circumstances, inhibi-
tion of the motion signals by the CH-cell dendrite leads to an annulment of excitation
on the FD-cell dendrite (See color insert.).
detection are settled at a different level than those of the spatially integrating stages,
i.e., the tangential cells of the lobula plate. While the properties of local motion de-
tectors have been inferred from a black box analyses using visual motion stimuli as
input and, in most cases, their spatially and/or temporally integrated signals as out-
put, as manifested either in the tangential cells or in optomotor response behavior,
spatial processing of local motion information was studied directly by electrophys-
iological recordings of the neural elements. This has to do with the trivial fact that
the columnar elements in the medulla where local motion detection is likely to take
place [2, 3, 18, 33] are much smaller than the large tangential cells of the lobula plate
and, thus, are much harder to record from intracellularly. As a consequence, the
motion detection system as simulated today represents a hybrid model with purely
algorithmic processing stages at the level of local motion detection (low-pass filter-
ing, multiplication) and biophysically realistic compartmental models of the neurons
thereafter.
Thus, a major thrust of present research efforts is to uncover the cellular identities
of the neurons constituting the local motion detector and, hopefully, the biophysi-
cal processes underlying their elementary operations using e.g., genetically encoded
indicators of neural activity [69]. However, independent of that, our understanding
of the functional properties of fly motion vision will be tremendously improved by
considering the fact that the circuits which usually are tested in the laboratory under
open-loop conditions, operate naturally under closed-loop conditions [94]. Further-
more, the characteristics of the fly's flight maneuvers [91, 92, 93] together with the
spatial frequency content in natural scenes [32] is likely to shape the input to the
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