Mechanism of Thalamocortical Augmentation and Repetition

In another paper two types of cortical activity elicited by stimulation of the sensory path have been described. Evidence was presented that these phenomena, the so-called augmented and repetitive sensory responses, share a single neuronal mechanism, but may, on the other hand, be segregated both from the simple “primary” sensory response and the generalized 8 to 12 per second “spontaneous” bursts (Dempsey and Morison, 1943). The present study was directed toward a more thorough investigation of the properties of the system responsible for the augmented and repeating effects. METHODS. The methods employed were entirely similar to those employed in the preceding papers and need not be repeated. RESULTS. Site of repetition and augmentation. Since repetition is not a feature of sensory responses recorded from the lemniscus (Dempsey and Morison, 1943), attention was directed to the thalamus and cortex. Adrian (1941) has demonstrated that the thalamus deprived of its cortex still exhibits repetitive responses. In the present investigation records taken from the thalamus before and after cutting of the internal capsule showed that the repetitive response of the thalamus, though still present after the procedure, was always depressed. In order to facilitate cutting of the capsule in this and other types of experiments, the ventricle was carefully unroofed before any observations were made, so that the cutting of the capsule could be carried out with a minimum of additional operative trauma. Since depression of the thalamic response might still be attributable to some nonspecific effects of the operation, however, the influence of the cortex on the thalamic response was tested in another way. Cortical repetition was enhanced by local application of acetylcholine (Chatfield and Dempsey, 1942). The thalamic repetitive response was enhanced pari passu (fig. 1). The question still remained as to whether the cortex alone may exhibit repetitive or augmented sensory responses. In order to solve this question, stimulating electrodes were introduced into the capsule and the cortical responses recorded. Division of the capsular fibers caudal to the electrodes abolished both the repetitive and augmented (fig. 2) responses. Even though acetylcholine was applied to the cortex in the hope of compensating for the loss of an asynchronous thalamic bombardment (cf. its facilitating effect in intact animals above), the abolished repetitive response did not return. The primary response induced by capsular stimulation invariably was smaller and its second and third component declined more rapidly with frequency before section of the thalamic radiations. As the augmenting response developed (fig. 2A), a decline in the primary response occurred. Figure 2B shows the larger