Barbiturate spindle activity in the thalamic lateral ventro-posterior nucleus and the second somato-sensory area of the cortex.

(1) Barbiturate spindles recorded from the second somato-sensory cortical area (SII) were similar to spindles in the primary somato-sensory area (SI) both with respect to incidence, duration of each spindle and per cent spindle time. The spindle wave amplitude was smaller in SII. The highest spindle wave amplitude was observed in the anterior part of SII which receives input from nucleus ventralis postero-lateralis (VPL). No spindle activity was observed in the posterior part of SII which receives input from the posterior nuclear group (PO) of the thalamus. (2) Barbiturate spindles recorded from a locus in VPL and its projection area in SII were cross-correlated. The analysis resulted in high cross-correlation factors, indicating that a considerable degree of spindle wave synchrony existed between the spindles. This wave synchrony was reduced by moving the cortical electrode a short distance. (3) Cortical spindles recorded from corresponding sites in SI and SII were cross-correlated, and gave a high cross-correlation coefficient. This synchrony was markedly reduced if one of the electrodes was moved a few millimetres away from the optimal point. (4) Spindles started simultaneously in corresponding sites of SI and SII. A high degree of coincidence was found also between spindles in a VPL locus and the corresponding projection site in SII. Local anaesthesia applied to or total removal of SI failed to influence the spindle activity in SII and vice versa. Similarly, the SI-SII synchrony survived a deep incision cutting all connections between the two areas. (6) It is suggested that spindles in corresponding sites of SII and SI have a common thalamic pacemaker which probably projects to both areas by axonal branching.