Biomedical Engineering Reference
In-Depth Information
1.25
summed response
detector 1
detector 2
detector 3
1.00
0.75
0.50
0.25
0.00
-0.25
-0.50
0
1
2
3
4
5
time [sec]
Figure 14.8
Dynamic response profile of the minimal correlation model shown in
Figure 14.2
in
response to the on- (at time = 1 sec) and off-set (at time = 4sec) of constant pattern
motion. Shown are the summated output signals (in black) of an array of 8 such
motion detectors (sampling base = 2 deg) together covering one spatial wavelength
(
16 deg) of the pattern, together with the output signals of three neighboring
individual detector signals (in red, green and blue, respectively). The pattern was
moving at 32 deg/sec resulting in a temporal frequency of 2 Hz.
λ
=
notable things here.
First of all, the spatially summated response changes over time in the initial pe-
riod of the stimulation. Before assuming its steady-state response given by Equation
(14.2), the response is transiently ringing at the temporal frequency of the stimulus.
The ringing is damped and settles over time to the steady-state value. Indeed, after
being postulated based on modelling [4], such ringing behavior has been observed
experimentally in fly motion sensitive neurons [27, 66]. Beside the velocity step,
sinusoidal velocity modulation represents another example case where the dynamic
responses of correlation detectors have been investigated [26]. Again, the spatially
integrated response was found to deviate from being proportional to the velocity sig-
nal when either the amplitude or the frequency of the velocity modulation exceeded
certain values. In summary, thus, the spatially integrated motion detector output fol-
lows the dynamics of the velocity input within certain limits; beyond that, significant
distortions may occur. Those limits are in general determined by the time-course of
the intrinsic filters of the motion detection system.
Another feature that can be seen in Figure 14.8 pertains to the individual detector
signals. In contrast to the overall response that exhibits a ringing only initially, the
individual detector output signals continue to oscillate at the temporal frequency of
Search WWH ::
Custom Search