THE ELECTRICAL ACTIVITY OF A THALAMOCORTICAL RELAY SYSTEM

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Abstract
Stimulation of sensory elements at any level in the medial lemniscus-internal capsule relay system is followed by responses of 3 types in the sensory cortex and ventrolateral thalamic nucleus: 1, a "primary" response consisting of 2 positive and one negative deflection which follows rapid frequencies of stimulation; 2, an "augmenting sensory response" consisting of a positive and negative component which, although absent on the first stimulus, grows dramatically in size on successive stimulations; 3, a "repetitive sensory response" consisting of a train of potentials at a frequency of 8-12/sec. which follows the primary response to a single stimulus and which fatigues quickly on repeated evocation. The primary and the augmenting responses have been separated from one another on the basis of latency, anatomical localization, selective abolition by lesions and differences in physiological behavior. Similarly, the separability of the primary and the repetitive responses has been demonstrated by the failure of one to block the other. The simultaneous presence of the repetitive response blocks the appearance of augmentation. The repetitive response, although superficially similar to the spontaneous burst potentials of the cortex, may be separated from them since the burst potentials occur in different areas in the thalamus, and since the presence of burst potentials does not prevent the evocation of repetitive responses. The validity of this finding is further indicated by the fact that the repetitive response is not affected by simultaneously induced recruiting potenials or by high-frequency stimulation of intralaminar thalamic areas, although these procedures have been shown previously to interfere with the burst potentials. Similarly, the augmented response is not blocked by the burst potentials. Evidence is presented that a type of spontaneous effect, the "projection activity," results from activity in the thalamic relay nucleus and its cortical projection. The projection activity differs from the spontaneous cortical bursts in regularity, susceptibility to anesthesia, thalamic localization and dependence upon afferent impulses. Its interaction with the repetitive sensory response is complex. It is abolished by rapid augmenting stimuli, whereas augmenting responses are superimposable upon the burst potentials. It is suggested that the recognition of different types of "spontaneous" activity facilitates study of the interaction of cortical potentials with induced effects. The bearing of these results upon various contradictory statements in the literature is discussed.

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