Electroencephalography and Clinical Neurophysiology

Repetitive stimulation in midline and intralaminar regions of the thalamus causes widespread changes in ongoing cortical activity. Contrasting effects may be obtained from the same loci by low and high frequency stimulation. Thus, stimulation at 6-12/see produces recruiting responses (Morison and Dempsey 1942), whereas high frequency stimulation at the same recruiting sites induces low voltage, fast activity, that is, desynchronization of spontaneous, ongoing activity, and blockade of recruiting responses (Moruzzi and Magoun 1949). Thalamo-cortical pathways for recruiting appear to course rostrally to the frontal pole of the thalamus, via n. ventralis anterior and n. reticularis, before entering the internal capsule (Nauta and Whitlock 1954; Nashold et al. 1955). Lesions in these nuclei have been shown to prevent or attenuate cortical recruiting responses (Hanbery et al. 1954; Eidelberg et al. 1958 ; Chow et al. 1959). Less is known regarding the anatomical course of the desynchronizing effects and blocking of recruiting by high frequency stimulation. The present study has sought to determine whether thalamieally induced desynchronization is mediated by the same pathways as those involved in recruiting.