Quasar 3 C 446

7 APRIL 1967 regular 15-minute Skinner box test sessions. It is possible, however, that at least two of the five rats included in the U group could be appropriately placed in the SS-P group if manner, rather than rate, of bar-pressing is considered. Thus one of these animals, given a prolonged 200-minute test session, pressed at an average rate of only once per minute. Nevertheless, pressing did not occur at random. From Fig. 2 it can be seen that this animal with an electrode in dorsal central grey tended to press in clusters of about four at regular intervals. Moreover, the time between presses within a cluster tended toward constancy. In some clusters the intervals between each bar-press varied from 28 to 32 seconds. During these latter intervals the animal adopted a frozen posture, as if attending to some aftereffect of the stimulus. Probably the timing of responses within a cluster was closely tied to the duration of the aftereffect. The self-stimulation behavior of such animals seems no less significant than that of the SS-P animal which pressed 1100 times in a 15-minute session. At the conclusion of testing, the animals were killed and electrode placements (Fig. 3) were ascertained from cresyl violet or cresyl violet-luxol fast blue stained brain sections. The tips of the electrodes of several SS animals were located in centralis medialis, centralis lateralis, rhomboidalis, and reuniens, and one tip was located in nucleus parafascicularis. These SS placements correspond to nuclei making up the thalamic reticular system (8). Most SS-P placements were located in periventricular and central grey brain regions. Functions proposed for the thalamic reticular system include sleep, consciousness, control of cortical rhythms, epilepsy, attention, learning, and nonspecific motivation. While such proposals are not mutually exclusive, such a long list seems compatible with our belief that there is little understanding of the functional significance of the thalamic reticular system. At the risk of adding to this list of supposed functions we feel that the present investigation indicates that the system is involved in specific motivational processes. Our contention is based on strong evidence indicating that selfstimulation always includes activation of specific motivational processes (9). Many reports (for example, 4, 6) indicate that stimulation of the periventricular fibers and central grey is noxious. Experiments and theories have been based on the belief that selfstimulation cannot be obtained from electrodes implanted within this area (6, 10). The present study indicates, however, that the self-stimulation effect is also consistently found in this region. R. M. COOPER L. H. TAYLOR Department of Psychology, University of Calgary, Calgary, Alberta, Canada