Dynamics of nonlinear suppression in V1 simple cells
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1
CNRS UPR2191, Unite de Neuroscience Integratives et Computationelles, France
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2
University of California, School of Optometry, United States
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3
Osaka University, Graduate School of Frontier Biosciences, Japan
The visual responses of V1 neurons are affected by several nonlinearities, acting over different timescales and having different biological substrates. Some are considered nearly instantaneous: such is the case for the motion-dependent nonlinearities, and for the fast-acting contrast gain control, which increases the neuronal gain and accelerates the response dynamics for high contrast stimuli. Another, slower contrast dependent nonlinearity, also termed contrast adaptation, adjusts the neuronal dynamic range to the contrast prevailing in the receptive field for the past few seconds. While cortical mechanisms likely participate in slow contrast adaptation, the functional origins of the fast contrast- and motion-dependent nonlinearities are still debated. Some studies suggest that they can be accounted for by a model consisting of a linear spatiotemporal filter followed by a static nonlinearity (LN model), while others suggest that additional nonlinear cortical suppression is required. It should also be noted that the time constants of fast and slow nonlinearities are not very well known; thus their effects could mix in the responses to seconds-long drifting gratings.
To clarify these issues, we measured contrast and motion interactions in V1 Simple cells with white noise analysis techniques. The stimulus was a dynamic sequence of optimal gratings whose contrast and spatial phase changed randomly every 13 ms. We also varied the distribution from which contrasts were drawn, to explore the effects of slow contrast adaptation. We reconstructed the 2nd-order kernels at low and high average contrasts, and fitted multi-LN models to the responses. None of the Simple cells we recorded conformed to a pure LN model, and most of them (79%) showed evidence of nonlinear (predominantly divisive) suppression at high ambient contrast. Suppression was often (but not always) motion-opponent; suppression lagged excitation by ~11 ms; and suppression improved the response temporal precision and thus the rate of information transfer. At low average contrast, the response was noisier and suppression was less visible. The response was dominated by excitation, whose gain increased and whose kinetics slowed down. Our findings suggest that both fast- and slow-acting nonlinearities participate in the contrast-dependent changes in temporal dynamics observed with drifting gratings. More generally we propose that contrast adaptation trades neuronal sensitivity against processing speed, by changing the balance between excitation and delayed inhibition.
Conference:
Bernstein Conference on Computational Neuroscience, Frankfurt am Main, Germany, 30 Sep - 2 Oct, 2009.
Presentation Type:
Oral Presentation
Topic:
Sensory processing
Citation:
Levy
M,
Truchard
A,
Sadoc
G,
Ohzawa
I and
Fregnac
Y
(2009). Dynamics of nonlinear suppression in V1 simple cells.
Front. Comput. Neurosci.
Conference Abstract:
Bernstein Conference on Computational Neuroscience.
doi: 10.3389/conf.neuro.10.2009.14.153
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Received:
28 Aug 2009;
Published Online:
28 Aug 2009.
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Correspondence:
Manuel Levy, CNRS UPR2191, Unite de Neuroscience Integratives et Computationelles, GIF/YVETTE, France, levym@musc.edu