Perusing Psychology

On Tuesday, December 2, 2008, Henry M., the most famous patient in modern neuroscience research and literature, passed away. He was 82. In 1953, H.M. had an experimental brain operation to try to stop his frequent seizures; his medial temporal lobes were resected bilaterally, with significant portions of his amygdalas and hippocampi in both cerebral hemispheres removed (parts of the brain are still resected in intractable epilepsy cases, however neurosurgeons do not perform that exact surgery any more because of the negative effects). His seizures stopped but immediately after the operation he had a severe anterograde amnesia. This means that from when he received the operation at age 27, he was unable to establish new memories for world events and for general information.

His amnesia became the focus of much scientific study from after his operation until the present. No one patient has been studied more in the 20th and 21st centuries than H.M. His memory impairment was also interesting because his overall intellectual abilities were still intact as was his personality. Neuropsychologists and neuroscientists will forever be grateful for the things they learned from H.M.

The New York Times has a very nice article about H.M.

Yesterday I posted a video clip about Clive Wearing. Here is the first part of a different documentary about Clive. This video goes more in-depth about his condition. Clive is sometimes referred to as the man with the shortest memory. Not only were his two hippocampi destroyed, but also surrounding areas of the his temporal lobes as well as portions of his left frontal lobe. He also remembers very little from before his illness, which is quite rare; this condition is called retrograde amnesia. Clive lives in an ever-present now, without connection to past or future. Come back for more about his case in the coming days.

Clive Wearing is a 70 year old British man who contracted herpes simplex encephalitis in 1985. The virus destroyed his hippocampi bilaterally (as well as surrounding areas). He has complete anterograde amnesia and can only remember up to about 20 seconds. He retained the ability to play the piano and conduct a choir (which he did previously to his illness); this is because this procedural memory involves different areas of the brain, including the basal ganglia and the cerebellum. I’ll revisit this case over the coming days. Meanwhile, here is a clip from a BBC production that presents part of Clive’s story.

I have to apologize for the paucity of posts on this blog. The grind of my semester got to me. After a few week break it’s time for a neuroanatomy post.

WIthin the temporal lobe of the brain is an elongated structure called the hippocampus. Some people have compared its shape to that of a seahorse (the word hippocampus comes from the Greek {hippos + campos}, which roughly means “seahorse”). This structure is special for a number of reasons. One is its role in memory encoding and consolidation.

From cytoarchitectonic standpoint, the hippocampus is special because unlike the surrounding cortex, it consists of only three layers instead of six. The hippocampus is phylogenetically an old part of the cortex, which means that it is an older branch on the evolutionary tree, whereas the rest of the cortex (more accurately called the neocortex), especially cortex of the frontal lobes, is a much newer development.

The hippocampus (to be more accurate, there are two hippocampi – one in each cerebral hemisphere) resides within the medial portion of the temporal lobe. It is continuous with the parahippocampal cortex, entorhinal cortex (the hippocampus receives its main input from this cortex), and perirhinal cortex. I’ll not write about the internal structure of the hippocampus, which becomes fairly complex, due to the brief nature of this post.

The hippocampus sends white matter tracts off its dorsal and posterior portions (the hippocampus also communicates through other tracts and pathways – this circuit is not the only output of the hippocampus). These white matter tracts are the fimbria of the hippocampus (technically, the fimbria are the “offshoots” of the alveus of the hippocampus). The fimbria proceeds upwards from the posterior portion of the hippocampus, at which point it ceases to be the fimbria and is called the fornix.

The fornices (plural of fornix) are prominent white matter tracts passing above the thalamus and medially in the brain. The fibers travel forward, then turn downward just posterior to the anterior commissure (a white matter tract that connects both hemispheres) to terminate in the mammillary bodies, two bumps on the ventral side of the brain. They are part of the hypothalamus of the brain. From there, the pathway courses upward through the mammilothalamic tract (MTT) to the anterior nucleus of the thalamus. From there axons course to the cingulate gyrus, then to the underlying cingulum (large white matter tract), and back to the hippocampus (via the parahippocampal and entorhinal cortices). This circuit is part of the limbic system and is called the Papez (pronounced “papes” – rhymes with capes) circuit. This circuit is important for emotion (and memory).

Today in Science a team of scientists (Hagar Gelbard-Sagiv, Roy Mukamel, Michal Harel, Rafael Malach, and  Itzhak Fried) at the Weizmann Institute of Science in Israel, UCLA, and Tel Aviv University published their research where they directly recorded via implanted electrodes the firing of hippocampus neurons during learning and free recall. This represents the first time in humans this has been done. Here’s the abstract from Science:

The emergence of memory, a trace of things past, into human consciousness is one of the greatest mysteries of the human mind. Whereas the neuronal basis of recognition memory can be probed experimentally in human and nonhuman primates, the study of free recall requires that the mind declare the occurrence of a recalled memory (an event intrinsic to the organism and invisible to an observer). Here, we report the activity of single neurons in the human hippocampus and surrounding areas when subjects first view television episodes consisting of audiovisual sequences and again later when they freely recall these episodes. A subset of these neurons exhibited selective firing, which often persisted throughout and following specific episodes for as long as 12 seconds. Verbal reports of memories of these specific episodes at the time of free recall were preceded by selective reactivation of the same hippocampal and entorhinal cortex neurons. We suggest that this reactivation is an internally generated neuronal correlate of the subjective experience of spontaneous emergence of human recollection. (Published Online September 4, 2008; Science DOI: 10.1126/science.1164685)

The New York Times also has an article about the research.