Learning and the Septohippocampal System

The largest subcortical limbic structure is the hippocampal formation. The hippocampus appears to be involved in the processing of memory and, in collaboration with other limbic structures, various affective and cognitive functions. Damage to the hippocampus results in an animal's inability to store recent memory but does not interfere with memories already consolidated before damage occurred. The hippocampus in conjunction with the septum appears to play an important role in response inhibition and habitua-tion. It also serves important sensory processing functions. One sensory function it performs is the detection of novelty and familiarity. This attentional feature of the hippocampus may represent a significant factor in the hippocampal-lesioned animal's inability to form certain memories. Some theories suggest that an attentional/contextualizing interference may cause the hippocampus to "attend" inaccurately to significant stimuli.

The hippocampus together with other prominent structures belonging to the Papez circuit (hypothalamic mammillary body, the anterior thalamic nuclei, and the cingulate gyrus) appears to play important interactive roles in the elaboration of emotional experience and expression (Steinmetz, 1994) (Fig. 3.3). According to the Papez circuit theory, emotional experience is generated when inputs from the hypothalamus are projected from the anterior thalamus into the cingulate cortex—the site where "environmental events are endowed with an emotional consciousness." Fibers from the cingulate cortex subsequently converge on the hippocampus, from where the loop is closed as the processed input is relayed back to the hypothalamus. Steinmetz summarizes the basic functions of this circuit:

Each of these loops seems to serve a specific function that is associated with limbic system activity such as timing (septal loops), response processing (cingulate gyrus), processing of sensory stimuli (trisynaptic loop) and so on. The loop structure that is associated with the septo-hippocampal system provides sophisticated circuitry for information processing such as the processing that is necessary for generating emotional responses. Indeed, the neural processes that are involved in generating and regulating emotional responses require the integration of much information such as assessing the organism's internal and external environments, matching present experiences with past experiences, and selecting responses (both autonomic and somatic) that are appropriate for the situation. A relatively complicated circuitry, such as the limbic system with its variety of structures and interconnections, is likely at the heart of generating and regulating emotional states.

An important correlation appears to exist between the hippocampus and the septal area in their joint inhibitory functions. Under conditions of arousal and septal-hippocampal inhibitory control over ongoing behavior, the hippocampus exhibits a steady theta brain wave in contrast to surrounding desynchro-nized activity occurring elsewhere in the brain. Theta waves are produced in the hippocampus by novelty, pain, and frustration. Lesions of various brain sites (medial nucleus of the septum and certain nuclei of the thalamus), as well as the effects of various drugs (especially barbiturates), abolish or disrupt

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  • guglielmo
    Why is the hippocampus important to dogs?
    6 years ago

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