Biomedical Engineering Reference
In-Depth Information
Figure 7
. (
A,B
) Weights of the transient units onto the Hebb Layers with delays of 0, 0.5, 1.0,
and 1.5 ms. White indicates excitatory weights; black inhibitory. Inputs in visual space (-60 to
60() are represented along rows; the different Hebb layer units along columns. (
C,D
) Responses
as a function of time. Spikes of sustained and transient visual units, and the two Hebb Layers in
response to a stimulus rotating from right to left (C) and from left to right (D) through the entire
visual field. Waves in the lower half show the unit's internal potentials.
movements while looking at stimulus objects in space. The sources of the in-
structive inputs are proprioceptors that signal the vehicle's own turning move-
ments. The instructive inputs could also be derived from motor command
signals. In the network of Figure 4, proprioceptors for leftward or rightward
movement fire a burst of spikes to all the units of their corresponding Hebb
layer. Thus, the convergence of transient and proprioceptive information onto
Hebb layer units should result in their acquiring selectivity to one direction of
motion, but without regard to position in the visual field. Extracting depth from
parallax, however, likely requires that the spatial arrangement of different veloc-
ity vectors be preserved. As we have seen, the topography was established in
mapping of the sustained units over the Hebb layers, so if the sustained inputs
also bias the Hebb units to fire, along with the proprioceptive inputs, direction-
ally selective maps with topography should be formed.
