How Memory Works (and How to Preserve It)

How Are Memories Made?

Medscape: On a related note, can you summarize how memories form and are preserved?

Dr De Brigard: This is actually a difficult question, for which we only have the general shape of an answer, because many of its details are still unknown or unclear. Moreover, as it happens in neuroscience—and in all of biology, to be precise—these sorts of explanations involve many levels of description.

In the case of memory, we are talking abut changes that occur at the molecular level, the synaptic level, the cellular level, and even at the level of neural assembly and networks. That being said, when talking about memory formation and preservation, scientists usually separate three different stages: encoding, consolidation, and retrieval.

"Encoding" refers to the process in which information that is experienced becomes suitable to be stored in memory. For the case of episodic memories—ie, memories about particular personal events, the spatiotemporal contexts of which we can remember—this process requires one to rehearse in working memory the information to be stored.

The second stage is consolidation; this refers to the process by which the encoded information is stabilized, presumably into a more permanent "memory trace." Scientists normally distinguish two kinds of consolidation: synaptic consolidation, which are the changes at the synaptic level that occur in order for a particular memory to become stabilized, and systems consolidation, which is the involvement of the macrostructures of the brain that are required for the formation of memory traces.

Both kinds of consolidation interact; synaptic consolidation occurs in the neurons of the brain structures involved in system consolidation, although both processes differ in their timing. Consolidation at the systems level involves the interaction between the hippocampus and the sensory cortices of the brain where the encoded information was perceptually processed. Traditional views suggested that, somehow, the hippocampus was required to "record" the pattern of brain activation across the sensory cortex, to be reactivated at the time of retrieval.

According to the traditional view, the hippocampus was also supposed to be unnecessary for the retrieval of information, which is actually driven by the prefrontal cortex. (This helps to explain, for instance, why H.M. could remember remote episodes from his past despite having no hippocampi). Now, however, the hippocampus appears to be required at all stages—although this view remains controversial. Either way, what appears to be clear is that retrieval of stored information somehow requires the brain to reinstate the state it was in during encoding.

Medscape: What facets of memory decline with the more common neurodegenerative disorders, such as Alzheimer disease?

Dr De Brigard: Different neurodegenerative disorders have different effects on memory. Alzheimer disease, for instance, tends to manifest initially with difficulties in episodic memory—particularly difficulties in encoding and retrieving relational information, such as information that involves connections between items, or between items and their context of encoding. [Editor's note: In other words, episodic memory allows us to recall specific events in our lives.] Other neurodegenerative diseases, such as Parkinson disease, that involve the deterioration of the substantia nigra in the midbrain are associated with difficulties in learning certain associations that require immediate feedback. Certain variants of frontotemporal dementias, such as semantic dementia, tend to be associated with difficulties in retrieving words or meanings, and are marked by speech impairment.

Sadly, many of these symptoms get much worse with time, which is why it is important to visit your doctor as soon as you start noticing them.

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Cite this: How Memory Works (and How to Preserve It) - Medscape - Feb 13, 2015.