How Memory SpeaksRoundup
... Researchers divide memory into categories, the most familiar to us being “declarative,” memory that is consciously recalled. It is the type of memory that was set off by the article in The New York Times [read this morning, that mentionedSvoboda, the far-right Ukrainian political party]. How was my memory of the meaning of svoboda initially acquired and then stored?
I first learned that svoboda means “freedom” in 1988, when I studied Russian prior to a scientific visit to the USSR. Within hours of acquiring that fact there was growth of new synaptic connections between neurons, as well as restructuring of existing synaptic connections, in the part of my brain called the hippocampus (“seahorse” in Greek, due to its shape). These synaptic changes were linked to a series of biochemical changes: release of messenger molecules within the neurons, as well as switching on genes that led to the production of new proteins.
The rapid process of initial memory stabilization in the hippocampus could have been easily disrupted. Perhaps my beeper would have buzzed, alerting me to a medical emergency, diverting my attention away from the new vocabulary. Such diversions can interfere with the biochemical cascade and block new memory formation. Twenty-six years later, I would not have registered the meaning, and thus the irony, of the political party Svoboda.
How was the fact stored? It turns out that our records of life experiences are gradually transformed into a more permanent form in which they are relatively stable. Neuroscientists use the term “consolidation” to describe these post-experience processes of memory stabilization. Consolidation involves reorganization of the brain both at the level of synapses—the connections between nerve cells—and at the level of brain regions. Within hours, consolidation at synapses is complete, with changes in localized neurocircuits. Regional consolidation is a more prolonged process and involves the gradual reorganization of areas of the brain that support memory. Such remodeling occurred not only for the declarative memory that svoboda means “freedom”: it also occurred for the recurring dream that I first had in college, and for the memory that the woman next to me in bed is my wife.
A second category of memory is termed “nondeclarative.” These memories are not conscious but essentially reflexive, yet also involve synaptic changes and reorganization at the regional level. Classical examples include swinging a tennis racket or riding a bike, and on that Sunday using a fork and knife, and deftly scooping coffee.
Neuroscientists have gained considerable insight into memory by studying certain people who have lost it. One such person is known by the initials HM. He suffered from severe epilepsy that could not be controlled with medication. In 1953, his medial temporal lobes, parts of the brain roughly at the level of the sideburns, were removed in an experimental surgery. Although the operation succeeded in reducing the frequency and severity of HM’s seizures, it left his memory profoundly impaired.
Over the ensuing five decades, nearly one hundred studies have been conducted on him, both at the Montreal Neurological Institute and at the Massachusetts Institute of Technology.1 In initial studies, his neurosurgeon, William Beecher Scoville, and the psychologist Brenda Milner observed that HM could not remember articles on the front of the day’s newspaper. This inability to form new memories is called anterograde amnesia.
HM’s deficit included memories not only of new facts (semantic memory) but also of events (episodic memory). Yet he could retain a number or a visual image for a short period of time after learning it. He also remembered events from his childhood. Scoville and Milner posited that the medial temporal lobes were needed to generate recent, but not remote, memories. In later experiments, such long-term memories were discovered to be stored in the cortex, including areas that originally processed the information.
In 1962, Milner conducted a now famous experiment that yielded surprising results: despite all his deficits, HM was able to learn new motor skills. Milner asked HM to trace the outline of a star shown in a mirror. He could see only his hand, because the pencil and star were reflected in the mirror, with left and right reversed. Over the course of several days, HM performed this mirror-tracing task, and on each sequential attempt, he reduced the time it took and made fewer errors. Yet each time he began to trace the star once again, he told Milner that he had never done it before. Though he lacked “declarative memory” (that is, awareness of prior attempts), HM was able to retain his level of improved performance for as long as a year. Milner concluded that such skills, which depend on visual perception and motor ability, appear to involve brain regions outside of the medial temporal lobes....
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