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- P. Adorjan, C. Piepenbrock,
and K. Obermayer. Contrast Adaptation and Infomax in Visual Cortical
Neurons.
.
Rev. Neurosci., 10:181-200, 1999.
(FTP Gzipped PostScript, 31 pages, 116 kb)
In the primary visual cortex (V1) the contrast response function of
many neurons saturates at high contrast and adapts depending on the visual
stimulus. We propose that both effects - contrast saturation and adaptation -
can be explained by a fast and a slow component in the synaptic dynamics. In
our model the saturation is an effect of fast synaptic depression with a
recovery time constant of about 200 ms. Fast synaptic depression
leads to a contrast response function with a high gain for only a limited
range of contrast values. Furthermore, we propose that slow adaptation of the
transmitter release probability at the geniculocortical synapses is the
underlying neural mechanism that accounts for contrast adaptation on a time
scale of about 7 sec. For the functional role of contrast
adaptation we make the hypothesis that it serves to achieve the best visual
cortical representation of the geniculate input. This representation should
maximize the mutual information between the cortical activity and the
geniculocortical input by increasing the release probability in a low
contrast environment. We derive an adaptation rule for the transmitter
release probability based on this EMinfomax principle. We show that
changes in the transmitter release probability may compensate for changes in
the variance of the geniculate inputs--an essential requirement for contrast
adaptation. Also, we suggest that increasing the release probability in a low
contrast environment is beneficial for signal extraction, because neurons
remain sensitive only to an increase in the presynaptic activity if it is
synchronous and, therefore, likely to be stimulus related. Our hypotheses are
tested in numerical simulations of a network of integrate-and-fire neurons
for one column of V1 using fast synaptic depression and slow synaptic
adaptation. The simulations show that changing the synaptic release
probability of the geniculocortical synapses is a better model for contrast
adaptation than the adaptation of the synaptic weights: only in the case of
changing the transmitter release probability our model reproduces the
experimental finding that the average membrane potential (DC component)
adapts much stronger than the stimulus modulated component (F1 component). In
the case of changing synaptic weights, however, the average membrane
potential (DC) as well as the stimulus modulated component (F1 component)
would adapt. Furthermore, changing the release probability at the recurrent
cortical synapses cannot account for contrast adaptation, but could be
responsible for establishing oscillatory activity often observed in
recordings from visual cortical cells.
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