How memory speaks

The human brain works in mysterious ways. Our memory is divided into sections that correspond to long-term and short-term memories, writes Jerome Groopman

The role of memory in virtually every activity of our day is put in sharp focus when it is lost. Su Meck, in his book, “I Forgot to Remember,” pieces together a fascinating tale of life after suffering head trauma as a young mother.

A ceiling fan fell and struck her head. “You might wonder how it feels to wake up one morning and not know who you are. I don’t know. The accident didn’t just wipe out all my memories; it hindered me from making new ones for quite some time. I awoke each day to a house full of strangers.

And this wasn’t just a few days. It was weeks before I recognised my boys when they toddled into the room, months before I knew my own telephone number, years before I was able to find my way home from anywhere. I have no more memory of those first several years after the accident than my own kids have of their first years of life.”

A computed tomography scan of Meck’s brain showed swelling over the right frontal area. But neurologists were at a loss to explain the genesis of her amnesia. 

Science of the brain

Memory does not exist in a single site or region of the central nervous system. There are estimated to be 10 to 100 billion neurons in the human brain, each neuron making about one thousand connections to other neurons at the junctions termed synapses.

Learning, and then storing what we learn through life, involve intricate changes in the nature and number of these trillions of neuronal connections. But memory is made not only via alterations at the synaptic level; it also involves regional remodelling of parts of our cortex. Our brain is constantly changing in its elaborate circuitry and to some degree, configuration.

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. 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 Soviet Union. 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 stabilisation 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 stabilisation.

Consolidation involves reorganisation 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 localised neurocircuits. 

Regional consolidation is a more prolonged process and involves the gradual reorganisation of areas of the brain that support memory. Such remodelling 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.

Parts of memory

A second category of memory is termed “nondeclarative.” These memories are not conscious but essentially reflexive, yet also involve synaptic changes and reorganisation at the regional level. Classical examples include swinging a tennis racket or riding a bike.

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.

Fascinating results

Over the ensuing five decades, nearly 100 studies have been conducted on him, both at the Montreal Neurological Institute and at the Massachusetts Institute of Technology.

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 1962, Milner conducted a now famous experiment that yielded surprising results. Milner asked HM to trace the outline of a star shown in a mirror. 

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.