Coincidence detection or temporal integration? What the neurons in somatosensory cortex are doing

To assess the impact of thalamic synchronization on cortical responsiveness , we used conditional cross-correlation analysis to measure the probability of neuronal discharges in somatosensory cortex as a function of the time b etween discharges in pairs of simultaneously recorded neurons in the ventro basal thalamus. Among 26 neuronal trios, we found that thalamocortical effi cacy after synchronous thalamic activity was nearly twice as large as the e fficacy rate obtained when pairs of thalamic neurons discharged asynchronou sly. Nearly half of these neuronal trios displayed cooperative effects in w hich the cortical discharge probability after synchronous thalamic events w as larger than could be predicted from the efficacy rate of individual thal amic discharges. In these cases of heterosynaptic cooperativity, thalamocor tical efficacy declined to asymptotic levels when the interspike intervals were >6-8 msec, These results indicate that thalamic synchronization has a significant impact on cortical responsiveness and suggest that neuronal syn chronization may play a critical role in the transmission of sensory inform ation from one brain region to another.