The septo-hippocampal division of the limbic system plays an important role in both learned and unlearned behaviors. My colleagues and I found that lesions of this system abolish or severely impair several species-typical behaviors, such as nest building, sand digging, hoarding, wheel running, or pup retrieving. These lesions also interfere with particular kinds of learning and enhance others. For example, animals with lesions of the septum become more responsive to the reinforcing effects of food; they respond faster on a food-reinforced operant task but perform poorly when the task requires them to withhold responding.
Many laboratories have shown the septo-hippocampal system to play an important role in detecting and remembering particular contexts, both spatial and nonspatial. Some neurons in the hippocampus respond only when the animal is in particular spatial locations, or is about to move into one. The ability of the hippocampus to function properly in this respect depends on the integrity of its connections with the medial septum.
Neil R. Carlson
Role of Limbic System in
Learning and Species-typical
Behaviors.
Learning and Species-typical
Behaviors.
The effect of disruption of the septo-hippocampal pathway may be related to the impairment of species-typical behaviors that require the animal to make responses in particular places. The involvement of the septo-hippocampal system in recognition of contexts has been used to explain the fact that humans with bilateral hippocampal lesions develop a severe anterograde amnesia. Recently, neuroscientists have become interested in the ability of neurons in the hippocampal complex to change their excitability after being electrically stimulated. The increased excitability appears to be caused by structural changes and interacts with the performance of learning tasks.
My most recent work has involved investigating a phenomenon known as kindling. When parts of the brainÑespecially the amygdalaare very briefly stimulated electrically, once a day over a period of days, the animal develops full-blown grand mal seizures. Although other investigators found that electrolytic or excitotoxic lesions of the amygdala failed to abolish the seizures, my colleagues and I discovered that infusion of colchicine into the amygdala permanently abolished them without producing any evidence of structural damage there. Presently, I am trying to determine the physiological basis of this phenomenon. One possibility is that the colchicine diffused into the dentate gyrus of the hippocampal complex and damaged granuale cells, which are susceptible to this drug. If so, this finding would directly implicate the trisynaptic circuit of the hippocampal formation in seizures produced by kindling. Because kindling is the accepted animal model of generalized seizures caused by focal epilepsy, this finding would be of clinical as well as scientific interest; currently, the only treatment for focal epilepsy that cannot be controlled by medication is surgical removal of the medial temporal lobe.
Back to NSB Faculty