Modelling Fly Motion Vision - Computational Neuroscience: A Comprehensive Approach - page 411

Figure 14.12
Spatial integration properties of compartmental models of LPTCs. (a) Simulation
of a LPTC receiving input from two arrays of elementary motion detectors (EMD)
tuned to opposite directions of motion and forming excitatory (+) and inhibitory (-)
inputs onto the dendrite of the cell, respectively. A VS-cell was 3D-reconstructed
from cobalt-stained material and was simulated as having only passive membrane
properties. (b,c) Gain control in the model cell before and after blockade of inhibi-
tion. When stimulated by patterns of increasing size at two different velocities, the
axonal membrane potential saturates at different levels (gain control). After blocking
the inhibitory inputs, both velocities yield similar responses (Modified from [83]).
spatial saturation of the resulting membrane potential in the axon. If the velocity of
pattern motion is changed, a different saturation level is assumed (Figure 14.12b).
Blocking the inhibitory input still resulted in a spatial saturation but abolished the
phenomenon of gain control (Figure 14.12c): now, the same level was approximated
for increasing pattern size independent of the pattern velocity [83].
In summary, thus, gain control is produced without the need of any further net-
work interactions. These assumptions were experimentally verified by blocking the
inhibitory input with picrotoxinin [83] resulting in three observations: a) the pre-
ferred direction response grew larger and the null direction response changed its
sign from a hyper- to a depolarization, b) the change of input resistance induced by
preferred direction motion decreased showing that, before, inhibitory currents were
activated as well and c) as a final proof of the above explanation, gain control was
abolished.