Low-threshold mechanisms and cortical activation pathways.

In chronic experiments with alert cats, stimulation with electrical current of moderate strength at various points on the lateral surface of the cerebral hemispheres leads to activation of various brain regions. In addition to high-threshold cortical points, low-threshold points have been discovered which are located in the sensorimotor region and in the Ep field of the auditory zone. The latter possess the same low thresholds for evoking an activation response as do points in the mesencephalic RF and the thalamic CM, VPL, and GM. Connections have been discovered (in the morphological part of the study) between the auditory Ep field and the intralaminar nuclei of the thalamus and the brain-stem part of the RF; the major projections run from the dorsal part of the Ep field into the lateral zone of the tegmentum. It is proposed that the role of the cortical low-threshold foci could involve the triggering of the nonspecific activation apparatus in accord with the biological significance of the signals being analyzed.