MECHANISM OF THALAMOCORTICAL AUGMENTATION AND REPETITION

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Abstract
Investigation of the mechanism underlying the production of repetitive and augmenting sensory responses has revealed the following: Decortication reduced but did not abolish the repetitive response recorded from the thalamus. Enhancement of the cortical response by local application of drugs increased the thalamic effect. Removal of the thalamus abolished the repetitive and augmented responses evoked by capsular stimulation. These and other expts. show that extensive thalamic afferents are derived from the internal capsule as well as the medial lemniscus. It is concluded that in this system there is a mutual reinforcement of cortical and thalamic effects, but that the thalamus alone is necessary for the occurrence of repetitiveness and augmentation. Augmentation induced by a conditioning shock, and revealed by a second test shock, extends over several hundred milliseconds. Furthermore, once the process is started, it proceeds at a rate which is not influenced by subsequent stimulation. Repetitive sensory potentials induced by the conditioning shock destroy augmentation. Following the repetitive effect, the process of augmentation again builds up to a maximum just before the next repetitive response. The timing of the repetitive rhythm bears a highly specialized relationship to the afferent stimuli. The rhythm is timed by the last stimulus, whether it falls during the repetitive train or ahead of the first repetitive potential. Likewise, the rhythm is set by the last shock both when the number of thalamocortical elements fired is small and large. The data presented above imply certain properties of the thalamocortical system which are summarized and discussed with reference to a circuit diagram whose elements are to be regarded as symbolizing such physiological phenomena as the excitatory state, detonator action and repetitiveness.

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