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J Neurophysiol 100: 1523-1532, 2008. First published June 18, 2008; doi:10.1152/jn.00076.2008
0022-3077/08 $8.00

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Synchronization of Neuronal Responses in Primary Visual Cortex of Monkeys Viewing Natural Images

¹Centro de Neurociencias Integradas and Programa de Fisiología y Biofísica, Facultad de Medicina, Universidad de Chile, Santiago, Chile; ²Max Planck Institute for Brain Research, Frankfurt am Main; ³Neuroinformatics, Institute for Biology (Neurobiology), Free University and ⁴Bernstein Center for Computational Neuroscience, Berlin, Germany; ⁵Theoretical Neuroscience Group, RIKEN Brain Science Institute, Saitama, Japan

Submitted 22 January 2008; accepted in final form 12 June 2008

When inspecting visual scenes, primates perform on average four saccadic eye movements per second, which implies that scene segmentation, feature binding, and identification of image components is accomplished in <200 ms. Thus individual neurons can contribute only a small number of discharges for these complex computations, suggesting that information is encoded not only in the discharge rate but also in the timing of action potentials. While monkeys inspected natural scenes we registered, with multielectrodes from primary visual cortex, the discharges of simultaneously recorded neurons. Relating these signals to eye movements revealed that discharge rates peaked around 90 ms after fixation onset and then decreased to near baseline levels within 200 ms. Unitary event analysis revealed that preceding this increase in firing there was an episode of enhanced response synchronization during which discharges of spatially distributed cells coincided within 5-ms windows significantly more often than predicted by the discharge rates. This episode started 30 ms after fixation onset and ended by the time discharge rates had reached their maximum. When the animals scanned a blank screen a small change in firing rate, but no excess synchronization, was observed. The short latency of the stimulation-related synchronization phenomena suggests a fast-acting mechanism for the coordination of spike timing that may contribute to the basic operations of scene segmentation.