We used cross-correlation analysis to characterize the incidence and strength of stimulus-induced neuronal synchronization in different layers of SI barrel cortex and as a function of neuronal location in different barrel columns. To reduce the possibility of evoking responses that were coordinated by simultaneous whisker movements, multiple whiskers were sequentially stimulated with airjets that moved back-and-forth across the peripheral whisker pad. From a sample of 627 neurons, we characterized 1,182 neuron pairs and found that 687 (58.1%) of these displayed significant peaks of synchronized activity that exceeded the 99.9% confidence limits. Whereas 88% of the infragranular neuron pairs were synchronized during whisker stimulation, only 30% of the neuron pairs in the granular or supra-granular layers displayed synchronized responses. The strength of synchronization, as measured by the correlation coefficient, was significantly higher in the infragranular layers than in the other layers. These results indicate that synchronized outputs from the infragranular layers do not depend on synchronized inputs from the upper cortical layers. We also found that synchronization varies with the spatial configuration of the neurons and is strongest for neuron pairs residing in the same row. Given the dense local projections between neighboring barrel columns in the same row, our results indicate that neuronal synchronization is greatest when stimuli simultaneously activate those peripheral receptors whose cortical representations are most densely interconnected. Finally, we compared the present results with synchronized responses in somatosensory (SI) barrel cortex that were evoked by controlled, pulsatile whisker movements in a previous study. We conclude that highly-controlled whisker stimulation increases stimulus coordination and may exaggerate the incidence and strength of synchronization among neurons in the granular or supragranular layers.
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