Diffuse thalamic projection system in monkey.

IN 1942, Dempsey and Morison (4-6, 24) discovered in cats a thalamic system which upon direct repetitive stimulation evoked recruiting wave responses in broad areas of the cortex. Evidence was presented that the physiological counterpart of this response to stimulation was burst or spindle activity recorded from the cortex of cats under surgical barbiturate anesthesia. Interest in the mechanism of the recruiting response has been heightened by proposals concerning its functional significance, the most discussed being Jasper's suggestion that it represents the basis for petit ma1 epilepsy (15, 17, 18). In the cat, sites of origin for the diffuse thalamic projection system have been found to be the centre median and intralaminar nuclei (24), as well as the ventralis anterior and rostra1 pole of the reticular nucleus (33). These component nuclei act as a functional unit for, on repetitive stimulation of any one of them, recruiting waves can be recorded froh all (33). The mediation of recruiting to the cortex has been suggested to occur either through the reticular nucleus throughout its extent (17, 30, 31) or by way of the rhinencephalon (31). It has been shown (33), however, that the principal transmission of recruitment to the cortex occurs through connections with the thalamic associational nuclei, although possibly some direct connections also exist between the recruiting nuclei of origin and the cortex. In accordance with these findings, the cortical responses were found to be limited to areas having projections from these association nuclei, with identical cortical localization irrespective of which recruiting nucleus was stimulated (33). The foregoing evidence in the cat suggested that the diffuse projection system is organized for mass thalamic influence on associational cortex (33). However, because of the relatively poor differentiation of the cat's associational nuclei and cortex, in the present study more conclusive evidence was sought in the macaque, which, with its higher order of both cortical and subcortical development, more closely simulates the human brain.