The hippocampal formation photographic view

The brain is being shown from the dorsolateral aspect (as in Figure 14A). The left hemisphere has been dissected by removing the cortex and white matter above the corpus callosum: the lateral ventricle has been exposed from this perspective. The choroid plexus tissue has been removed from the ventricle in order to improve visualization of the structures (see Figure 20A). This dissection also shows the lateral aspect of the lenticular nucleus, the putamen, and the fibers of the internal capsule emerging between it and the thalamus (see Figure OA, Figure OL, Figure 7, Figure 25, and Figure 27).

A similar dissection has been performed in the temporal lobe, thereby exposing the inferior horn of the lateral ventricle (see Figure 20A). A large mass of tissue is found protruding into the inferior horn of this ventricle — named the hippocampus, a visible gross brain structure. In fact, the correct term now used is the hippocampal formation. In a coronal section through this region the protrusion of the hippocampus into the inferior horn of the lateral ventricle also can be seen, almost obliterating the ventricular space (shown in the next illustration; see also Figure 29, Figure 30, Figure 38, and Figure 76).

The hippocampal formation is composed of three distinct regions — the hippocampus proper (Ammon's horn), the dentate gyrus, and the subicular region, as explained in the previous diagram. The fiber bundle that arises from the visible "hippocampus," the fornix, can be seen adjacent to the hippocampus in the temporal lobe (see Figure

70A and Figure 70B), and it continues over the top of the thalamus to the septal region and mammillary nucleus (discussed with the previous illustration).

Clinical Aspect — Memory

We now know that the hippocampal formation is one of the critical structures for memory. This function of the hippocampal formation became understood because of an individual known in the literature as H.M., who has been extensively studied by neuropsychologists. H.M. had surgery several decades ago for a valid therapeutic reason — the removal of an epileptic area in the temporal lobe of one side, which was the source of intractable seizures. Most importantly, the surgeons did not know, and could not know according to the methods available at that time, that the contralateral hippocampal area was also severely damaged. This surgery occurred, unfortunately, before the functional contribution of this area to memory formation was known. Since the surgery, H.M. has not been able to form any new memory for events or facts, although he has been taught new motor skills (called procedural memory). (The full story of H.M. and his deficits is found in Kolb and Whishaw — see the Annotated Bibliography.)

We now know that bilateral damage or removal of the anterior temporal lobe structures, including the amygdala and the hippocampal formation, leads to a unique condition in which the person can no longer form new declarative or episodic memories, although older memories are intact. The individual cannot remember what occurred moments before. Therefore, the individual is unable to learn (i.e., to acquire new information) and is not able to function independently. If surgery is to be performed in this region nowadays, special testing is done to ascertain that the side contralateral to the surgery is intact and functioning.

Interhemispheric fissure

Fibers of internal capsule

Lateral surface of putamen

Fornix

Lateral ventricle (occipital horn)

Hippocampus

Interhemispheric fissure

Fibers of internal capsule

Lateral surface of putamen

Fornix

Lateral ventricle (occipital horn)

Hippocampus

Lateral ventricle (anterior horn)

F = Frontal lobe P = Parietal lobe T = Temporal lobe O = Occipital lobe

FIGURE 73: Hippocampus 3 — The Hippocampus (photograph)

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