Paper Available: Model of cat V1 nonlinear response properties

[Ken Miller]

The following paper is now available from:
http://www.keck.ucsf.edu/~ken  [click on 'publications', then
             click on 'models of neuronal integration and circuits']
or directly by ftp:
ftp://ftp.keck.ucsf.edu/pub/ken/kayser_etal.pdf   (pdf file)
ftp://ftp.keck.ucsf.edu/pub/ken/kayser_etal.ps.gz (gzipped postscript) 


"Contrast-Dependent Nonlinearities Arise Locally in a Model of
Contrast-Invariant Orientation Tuning"
	   by Andrew Kayser, Nicholas J. Priebe, and Kenneth D. Miller
	   In Press, Journal of Neurophysiology

ABSTRACT:

We study a recently proposed ``correlation-based'', push-pull model of
the circuitry of layer 4 of cat visual cortex (Troyer et al., 1998).
This model was previously shown to explain the contrast-invariance of
cortical orientation tuning.  Here we show that it can simultaneously
account for several contrast-dependent (c-d) ``nonlinearities'' in
cortical responses.  These include an advance with increasing contrast
in the temporal phase of response to a sinusoidally modulated
stimulus; a change in shape of the temporal frequency tuning curve, so
that higher temporal frequencies may give little or no response at low
contrast but reasonable responses at high contrast; and contrast
saturation that occurs at lower contrasts in cortex than in the
lateral geniculate nucleus (LGN).  In the context of the model
circuit, these properties arise from a mixture of nonlinear cellular
and synaptic mechanisms: short-term synaptic depression, spike-rate
adaptation, contrast-induced changes in cellular conductance, and the
nonzero spike threshold.  The former three mechanisms are sufficient
to explain the experimentally observed increase in c-d phase advance
in cortex relative to LGN.  The c-d changes in temporal frequency
tuning arise as a threshold effect: voltage modulations in response to
higher-frequency inputs are only slightly above threshold at lower
contrast, but become robustly suprathreshold at higher contrast.  The
other three nonlinear mechanisms also play a crucial role in this
result, allowing contrast-dependence of temporal frequency tuning to
coexist with contrast-invariance of orientation tuning.  Contrast
saturation, and the observation that responses to stimuli of
increasing temporal frequency saturate at increasingly high contrasts,
can be induced both by the model's push-pull inhibition and by
synaptic depression.  Previous proposals explained these nonlinear
response properties by assuming contrast-invariant orientation tuning
as a starting point, and adding normalization by shunting inhibition
derived equally from cells of all preferred orientations.  The present
proposal simultaneously explains both contrast-invariant orientation
tuning and these contrast-dependent nonlinearities, and requires only
processing that is local in orientation, in agreement with
intracellular measurements (Ferster, 1986; Anderson et al., 2000).


Ken
 
        Kenneth D. Miller               telephone: (415) 476-8217
	Associate Professor		fax: (415) 476-4929
        Dept. of Physiology, UCSF	internet: ken at phy.ucsf.edu
        513 Parnassus			www: http://www.keck.ucsf.edu/~ken
        San Francisco, CA 94143-0444