Thalamo-caudate-cortical relationships in synchronized activity. II. Further differentiation between spindle systems by cooling and lesions in the mesencephalon

Abstract

Electrolytic lesions or cryogenic blockage of the mesencephalic reticular formation facilitated spontaneous spindle (‘synchronized’) activity. In addition, spindle thresholds were markedly reduced at spindle eliciting sites and spindles were elicited to stimulation of some previously ineffective sites in the diencephalon and mesencephalon. In the lesioned animal, one group of nuclei elicited spindles at early latencies on the anterior sigmoid gyrus and at delayed latencies on the suprasylvian gyrus. In contrast, stimulation of another group of nuclei gave the opposite results, spindles at early latencies on the suprasylvian gyrus and at delayed latencies on the sigmoid gyrus. Thus, electrical stimulation of these nuclei replicated a frequent observation that some spontaneous spindles appear earlier on one or the other of these two gyri. These findings supported the hypothesis that the two groups of nuclei may be the source of relatively independent synchronizing influences on the sigmoid and suprasylvian gyri. The results were discussed in terms of postulated ‘ventral’ and ‘dorsal’ spindle systems, the development of spindle activity within each system and between the two systems, and the competitive effect of reticular arousal influences on synchronized activity.