The Production of Rhythmically Recurrent Cortical Potentials after Localized Thalamic Stimulation

During the course of experiments designed to determine the cortical representation of localized thalamic regions, it was observed that stimulation at a frequency of 5 to 15 per second was followed by a recruiting cortical response when the stimulating electrodes were placed in the region of the midline nuclei (Morison and Dempsey, 1942). Moreover, single stimuli delivered to this area frequently produced a train of cortical potentials lasting a few seconds. These trains of spikes were strikingly like the recurrent spontaneous bursts which are seen in the electrocorticograms of animals anesthetized with barbiturates (Derbyshire, Rempel, Forbes and Lambert, 1936). In view of this similarity to a component of the normal spontaneous activity, it was thought advisable to study tJhe effects of stimulation of the medial thalamic nuclei in more detail. The following sections deal with the results of such a study. MATERIAL ANDMETHODS. Cats were used. The methods employed have been discussed elsewhere (Morison and Dempsey, 1942) and need not be repeated. RESULTS. A. Description of recruiting response. In cats under relatively deep nembutal anesthesia, in which spontaneous bursts of 8 to 12 per sec. cortical potentials were relatively rare, a slow series of electrical stimuli applied to the dorsomedial regions of the thalamus was followed by recruiting potentials which were widespread in the cortex. The first shock of a series usually produced no response, but following the second or third a cortical potential developed which on successive stimulation increased rapidly in magnitude until a maximum was reached after the fifth or sixth shock. Figure 1 illustrates this recruitment response in an experiment in which the spontaneous 8 to 12 per sec. bursts had been almost abolished by deep nembutal anesthesia. These potentials bore a remarkable superficial resemblance to the spontaneously generated bursts of 8 to 12 per sec. activity (fig. 2). The recruited potential usually was monophasic and surface negative. After recruitment to its greatest height, however, it sometimes became diphasic with a positive component following the initial negativity. Diphasicity was more frequently encountered in lightly anesthetized preparations. The electrical polarity was not, however, completely constant. Recruited potentials of opposite signs were sometimes recorded from different cortical regions. The latency of the recruited response was long, varying in different experiments between 20 and 35 msec. In any single experiment the latency was less