Memory

How do our brains allow us to recall our memories?

memory

Memories are big sellers in American culture. Think of all the songs about memories, the movies, the stories, the books. In 2007, for the first time in history, book publishers bought more debut manuscripts called memoir than they bought debut novels. For new writers, memoirs are now a more likely entrance into the world of publishing rather than the traditional novel or autobiographical novel.

"It's not that memoirs don't have autobiographical material or meaning," Daniel Menaker, executive editor of Random House in 2007, told Robert J. Hughes of the Wall Street Journal. "It's just that they're more selective and more concentrated on single things -- the memory of a parent, the memory of a childhood, the memory of a great professor ..."

Memoirs just don’t compare to the “just-the-facts expectations of a biography or autobiography,” Menaker recounted in the WSJ article “Publisher’s Solution To Slow Sales: My Story.”

So what is it about “memory” that captures the collective attention of entire cultures and civilizations? Finding all the answers to that question would take this article beyond its scope, but let’s say unequivocally that the mystifying aspects of memory that philosophers and artists have written about for centuries are as compelling today as ever before.

And those in the sciences, specifically cognitive neuropsychologists and cognitive neuroscientists, are as eager to probe the depths of memory as artists. But their interests center on scientific explanations, desiring to know how memory lives and works – or fails to live and work – on an anatomical and cellular level. They want to know the mechanics of memory, the “styles” or “types” of memory, and how memory affects learning and motivation.

Memories get an update

Memory researchers received huge momentum in the 1960s with the introduction of neuroimaging technologies. Before that decade, researchers tested memory through the use of neuropsychological assessments – tests specifically designed to measure or rate types of recall and recognition. (see article on Neuropsychological Assessments.)

In many research studies, assessments were given to individuals with brain injuries and deficits, identified as having specific brain impairments, and then comparing the results to normal participants without brain injuries. Researchers also used long-term observational studies on those with brain damage and deficits.

Neuropsychological assessments and observations still have a place in research and clinical practice, but now researchers visually observe the human brain as it undertakes different types of memory tasks. Scanning participants’ brains allows researchers to map the areas that “light up” as tasks are completed. (For more information, see article on Neuroimaging.) Many of these studies show activated areas spread across both brain hemispheres and in diverse regions – a relatively new discovery in brain research and memory.

To further support these findings, research starting in the 1970s and 1980s began focusing on the brain’s networks of sensory nerve cells called neurons, and how these networks connect with neural regions widely distributed across the brain. These studies along with the neuroimaging research overturned some long-held assumptions that memory involved only specific brain areas or regions, making it much more plausible that memory actually involves a set of processes (rather than systems) involving several neural structures.

Short-term Memory - Getting a new name

In 1974, Alan Baddeley, a professor and researcher at the University of York, proposed a change in the prevailing theory of memory, which basically thought that memory had “three storage” areas in the brain. This prevailing theory stated that the brain had a storage area for brief sensory input; a storage area for short-term memory; and a storage area for long-term memory.

But using the new technologies, scientists soon recognized the oversimplification of this theory. Based on Baddeley’s research, scientists began to understand memory, specifically short-term memory, as having multiple “subsystems,” changing the name from short-term to “working memory.”

Working memory refers to sensory inputs that remain active for a short period, such as that required to repeat and dial a telephone number just heard.

Baddeley’s model proposes that working memory has a central processor or central executive that coordinates the activity of two sub-systems. This central processor is located in the brain’s frontal lobe.

(For information on the brain’s anatomy and structures mentioned throughout this article, reference the article The Basics of Brain Structure.)

The central processor connects to and controls data sent between two subsystems, one responsible for phonological memory, and one responsible for visual-spatial memory, also called visuospatial memory.

The phonological memory storage area stores and processes verbal auditory information and sounds, which brain imaging studies show localized in the brain’s left hemisphere. The visuospatial memory area appears to be located in the brain’s right hemisphere, specifically in the occipital lobes, which are largely responsible for visual processing.

In addition to the central processor and the two main subsystems, brain imaging studies show activation of even more “sub-areas” of the prefrontal cortex. These sub-areas of the prefrontal cortex are activated when working memory has to process more difficult or complex concepts.

In total, research shows that the brain’s processes for working memory involve a number of sub-sections or regions, each responsible for a specific short-term memory task. And, scientists agree, these sub-sections are probably distributed across several neural regions.

Types of long-term memory

Extensive research on both humans and animals has shown that different types of long-term memory exist, and are also represented by different neural regions and structures within the brain.

Two types of long-term memory identified by neuroscientists are explicit (declarative memories ) and implicit (non-declarative memories).

  • Explicit (declarative) memory is the conscious recollection of data. Declarative memory stores facts, events, and information about the world, such as snow is cold, fire is hot, and in December many people, including “my family members,” celebrate a holiday called Christmas.
  • Implicit (non-declarative) memory is memory that does not have to be consciously “recalled.” So for example, the motor skills required to drive a car or ride a bike would be considered implicit memory. These skills and knowledge are so ingrained that individuals are no longer aware of them.

More on declarative memory

Through neuropsychological studies on patients with brain damage, many researchers believe declarative or explicit memory can be broken down into other subcategories.

The neuroscientist Endel Tulving, professor emeritus at the University of Toronto, breaks down the category of declarative, explicit memories into semantic and episodic memories.

Semantic memory. Semantic memory consists of facts about the world without personal reference to the “self.” This type of data includes data such as capitals of countries, statistics on almost any topic, important historical dates, etc.

Episodic memory. Episodic memory is unique to the person recollecting information. An example of episodic memory would be the vacation one spent last summer at the beach, or the party one went to last evening, or the breakfast food one enjoyed this morning with one’s best friend. It could be said that one “owns” his or her episodic memory.

Tulving proposes that it’s not the type of information stored in semantic and episodic memory systems that necessarily differentiates the two sub-categories, but the subjective experience of the individual at encoding and retrieving the information. He states that it’s possible to be aware of one’s body in space, traits, and characteristics – even autobiographical facts – without a feeling of having “relived the past experience.”

In other words, Tulving would probably agree that the difference between many  “autobiographies” that simply recall the facts, dates, and objective experiences of a life, and “memoirs” could be described as the difference between semantic and episodic memories.

Hippocampus and declarative memory

Researchers have extensively studied the hippocampus in both people and animals, and they know that this inner brain structure plays a major part in declarative (explicit) memory.

For instance, after attending a party and meeting new people, an individual retains memories of different faces, the foods eaten, the music played. All of this sensory input gets processed in the brain’s regions for visual, olfactory, and auditory inputs. But all of these inputs connect together thanks to the hippocampus, which forms an “episode” of the evening rather than separating the sensory inputs into separate memories.

The hippocampus mediates the ability for the individual to recall the party weeks, months, even years later. Yet, after the passing of time, the party’s events become so intricately linked that the hippocampus is no longer needed to act as the control center. So just smelling a grilled steak that was served that night could bring back the entire party, and all its sensory inputs. Scientists call this the encoding in long-term memory.

Spatial memory appears also to be controlled by the hippocampus – more specifically to the right region of hippocampus. Researchers believe that this right hippocampal area maps the spatial layouts of specific places and locations.

Semantic memory, on the other hand, fires up the frontal and temporal cortexes, and many scientists believe the hippocampus is diverted altogether.

A Case Study Regarding the Hippocampus

Scientists know that the hippocampus is important for declarative or explicit memory, thanks in part to one groundbreaking study in the 1950s.

Psychologist Brenda Milner, Ph.D., of McGill University in Canada, observed a patient called HM after brain surgery. HM had severe epileptic seizures that began in the hippocampus or nearby in the temporal lobes of the cerebral cortex. So HM’s doctors decided to remove the hippocampus and cortex of the temporal lobes – a surgery done back then for the most problematic and potentially harmful seizures.

The surgery did reduce the frequency of the seizures. His IQ stayed above normal, and he could talk normally, solve puzzles, and keep a string of digits in his mind – such as a telephone number – very successfully if allowed to mentally rehearse the numbers over and over.

However, Milner and her colleagues observed something odd about HM’s memory. He couldn’t remember an event even minutes after it happened. Over the course of several months, he did well on tasks that he could practice or repeat, but he could not form new memories of events or people. He reread magazines and newspapers over and over because he couldn’t remember reading them, and on several occasions, HM went back to his old house at a previous address having no memory of moving to a new house.

HM suffered from anterograde amnesia, a deficit that wouldn’t allow him to form new memories. He was mentally “stuck” in his memories that occurred prior to the operation.

Milner and other scientists eventually concluded that the hippocampus and temporal structures are greatly involved and essential for declarative memory. HM’s problems have since been observed in a number of patients after him who have suffered strokes and brain injuries that have damaged these same brain structures.

More on implicit memory

Procedural memory such as knowing how to drive or ride a bike does not appear to link to or activate the hippocampus. Instead, the structures involved with motor control are actively involved in this type of memory: the cerebellum, the basal ganglia, and the motor cortex.

Other types of memory

Researchers have identified still other forms of memories. Emotional memories seem controlled by the brain’s limbic system in addition to the hippocampus. In particular, the amygdala seems to control the emotion of fear. And further research has now shown that other structures of the limbic system play an active role in encoding long-term memories.

More research needed

Taken altogether, memory involves many components using many different parts of the human brain. How all these components fit and operate together has spawned many hypotheses and theories. One could say the scientific discovery of how memory works in the brain is still in its first chapter, even though philosophers, psychologists, and scientists from many disciplines have studied the concept of memory for centuries.

Even many memoir writers claim to not really know what is their own “fiction” concerning their lives, and what, in truth, actually occurred. They acknowledge that memories are subjectively filtered through their own brain’s processes – which may in fact differ dramatically from other’s stored memories.

Which gives neuropsychologists and neuroscientists more fuel – and another area of study for their research studies: how does the remembrance of an event that two people attended differ so dramatically?

Careers in Memory Research

If memory intrigues you on a scientific level, from the acquiring and processing of it in the briefest of periods, to the acquired and stored memory of a lifetime, you should consider a research career in neuropsychology or neuroscience. Usually a Ph.D. is required for this type of research, and a strong background in psychology is essential. Contact schools offering degrees in psychology for more information.