Functions of the Hippocampus

Functions of the Hippocampus

The hippocampus is the elongated portion of the cere-bral cortex that folds inward to form the ventral surface of much of the inside of the lateral ventricle. One end of the hippocampus abuts the amygdaloid nuclei, and along its lateral border it fuses with the parahippocampal gyrus, which is the cerebral cortex on the ventromedial outside surface of the temporal lobe.

The hippocampus (and its adjacent temporal and parietal lobe structures, all together called the hip-pocampal formation) has numerous but mainly indi-rect connections with many portions of the cerebral cortex as well as with the basal structures of the limbic system—the amygdala, the hypothalamus, the septum, and the mamillary bodies. Almost any type of sensory experience causes activation of at least some part of the hippocampus, and the hippocampus in turn dis-tributes many outgoing signals to the anterior thala-mus, hypothalamus, and other parts of the limbic system, especially through the fornix, a major commu-nicating pathway. Thus, the hippocampus is an addi-tional channel through which incoming sensory signals can initiate behavioral reactions for different pur-poses. As in other limbic structures, stimulation of dif-ferent areas in the hippocampus can cause almost any of the different behavioral patterns such as pleasure, rage, passivity, or excess sex drive.

Another feature of the hippocampus is that it can become hyperexcitable. For instance, weak electrical stimuli can cause focal epileptic seizures in small areas of the hippocampi. These often persist for many seconds after the stimulation is over, suggesting that the hippocampi can perhaps give off prolonged output signals even under normal functioning conditions. During hippocampal seizures, the person experiences various psychomotor effects, including olfactory, visual, auditory, tactile, and other types of hallucina-tions that cannot be suppressed as long as the seizure persists even though the person has not lost con-sciousness and knows these hallucinations to be unreal. Probably one of the reasons for this hyper-excitability of the hippocampi is that they have a dif-ferent type of cortex from that elsewhere in the cerebrum, having only three nerve cell layers in some of its areas instead of the six layers found elsewhere.

Role of the Hippocampus in Learning Effect of Bilateral Removal of the Hippocampi—Inability to Learn. Portions of the hippocampi have been surgicallyremoved bilaterally in a few human beings for treat-ment of epilepsy. These people can recall most previ-ously learned memories satisfactorily. However, they often can learn essentially no new information that is based on verbal symbolism. In fact, they often cannot even learn the names of people with whom they come in contact every day. Yet they can remember for a moment or so what transpires during the course of their activities. Thus, they are capable of short-term memory for seconds up to a minute or two, although their ability to establish memories lasting longer than a few minutes is either completely or almost com-pletely abolished. This is the phenomenon called anterograde amnesia.

Theoretical Function of the Hippocampus in Learning. Thehippocampus originated as part of the olfactory cortex. In many lower animals, this cortex plays essen-tial roles in determining whether the animal will eat a particular food, whether the smell of a particular object suggests danger, or whether the odor is sexually inviting, thus making decisions that are of life-or-death importance.Very early in evolutionary development of the brain, the hippocampus presumably became a crit-ical decision-making neuronal mechanism, determin-ing the importance of the incoming sensory signals. Once this critical decision-making capability had been established, presumably the remainder of the brain also began to call on the hippocampus for decision making. Therefore, if the hippocampus signals that a neuronal input is important, the information is likely to be committed to memory.

Thus, a person rapidly becomes habituated to indif-ferent stimuli but learns assiduously any sensory expe-rience that causes either pleasure or pain. But what is the mechanism by which this occurs? It has been sug-gested that the hippocampus provides the drive that causes translation of short-term memory into long-term memory—that is, the hippocampus transmits some signal or signals that seem to make the mind rehearse over and over the new information until per-manent storage takes place. Whatever the mechanism, without the hippocampi, consolidation of long-term memories of the verbal or symbolic thinking type is poor or does not take place.