How memory works
Q: How do we store memories in our brain? How do we
recall memories? Rajeev, Bangalore, India
MRI of the brain. Image
courtesy of Scott Camazine, copyright, used with permission.
A: Your seemingly simple question leads us into an intricate maze of neurons,
which we shall explore in a
three-part story.
- How information flows through the brain.
- How neuron networks store and retrieve memories.
- How synapse molecules change to define a network path and, hence, a
pattern and a memory.
The simple overview is: Nerve network patterns store memories. We
recall a memory only when we activate that network of interconnected
neurons.
-
How information flows through the brain.
Information flows from the outside world through our sight, hearing smelling,
tasting and touch sensors. Memory is simply ways we store and recall
things sensed.
Recalling memories re-fires many of the same neural paths we originally used to
sense the experience and, therefore, almost re-creates the event. Memories
of concepts and ideas are related to
sensed experiences, since we extract the essence from sensed experiences to form
generalized concepts.
Consider Sir Isaac Newton, for example. Newton "hammered wooden
pegs" into the ground, and "cut sundials into stone" to measure the Sun's
movement through the sky, writes James Gleick in Isaac Newton.
"This meant seeing time as akin to space, duration as length. . ." Newton generalized what he
observed into a concept of time.
The cortex and its various lobes. Short-term memory activates regions in the frontal lobe (shown in blue); the parietal lobe (yellow) holds tactile sensations and maps
of the space around us. The occipital lobe (red) is a vision area; the
temporal lobe (dark yellow) contains auditory areas and the hippocampus.
Drawing from Gray's Anatomy.
We store — for fractions of a second
— sensory information in areas
located throughout the cortex. See figure. Then some data moves into short-term
memory.
Finally, some of that information goes in long-term storage in various parts of the
cortex, much of it returning to the sensory cortex areas where we originally
received it.
 Information
flow from our senses into our various memories. Drawing courtesy of Bruno Dubuc and
http://thebrain.mcgill.ca/,
modified by author.
Only the data
that catches our attention (like a police car behind us) or because we need it soon (a telephone number) goes into short-term
memory. We keep short-term data for maybe half a minute. Short-term storage is small; it
holds about seven independent items at a time, such as, 'carry' numbers when
doing arithmetic.
Finally information that may help us in the future (for instance, the
downwind smell of a
saber-tooth tiger) goes into long-term memory, where it can last a lifetime.
Long-term memory involves three processes: encoding, storage and retrieval.
- First we break new concepts into their composite parts to establish meaning.
Furthermore, we include the context around us as we learn a new concept, or
experience another episode in our life. For example, I might encode
the word 'Delicious apple' with key descriptive ideas — such as:
red color, tastes sweet, looks round, the sound of the crisp apple as I snap off a
bite — and then such contextual items
as 'I'm feeling good because it's a happy fall day and I'm picking apples.'
- Second, as we store the memory, we attach it to other related memories,
like 'similar to Granny Smith apples but sweeter', and thus, consolidate the
new concept with older memories.
- Third, we retrieve the concept, by following some of the pointers that trace the
various meaning codes and decoding the stored information to regain meaning. If
I can't remember just what
'Delicious apple' means, I might activate any of the pointer-hints, such as
'red' or 'picking apples.' Pointers connect with other pointers; so one hint
may allow me to recover the whole meaning.
How do our brains consolidate a new short-term memory, such as "Delicious
apple' and place it into long-term memory?
 The
route information travels from the hippocampus, around the areas in the limbic
system (thin blue line in the drawing) involved in complex memory, learning and
spatial memory then fans out into various regions of the cortex associated
with cognitive, motor, emotional and spatial processing (aqua areas),
before finally returning to the hippocampus. This path is called the Papez
circuit. Drawing courtesy of Bruno Dubuc and http://thebrain.mcgill.ca/.
We use the hippocampus, an ancient evolutionary part of the cortex, to
consolidate new memories. An event creates temporary links among
cortex neurons. For example, 'red' gets stored in the
visual area of the cortex, and the sound of a bitten apple gets stored in the
auditory area. When I remember the new fact, 'Delicious apple', the new memory
data converge on the hippocampus, which sends them along a path several times
to strengthen the links.
The information follows a path (called the Papez circuit and illustrated in
the figure), starting at the hippocampus, circulating through more of the limbic system
(to pick up any emotional associations, like 'happy fall day', spatial
associations 'apple orchard'), then on to
various parts of the cortex, and back to the hippocampus. Making the
information flow around the circuit many times strengthens the links enough that they "stabilize," and no longer need the hippocampus to
bring the data together, says neuroscientist
Bruno Dubuc of the Canadian Institutes of Neuroscience, Mental Health, and
Addiction. The strengthened memory paths, enhanced with environment
connections, become a part of long-term
memory.
Next week I will tell the second part in this three-part story: "How
neuron networks store and retrieve memories."
Further Reading:
How memory
works, part 2, WonderQuest
How memory
works, part 3, WonderQuest
The brain from top to bottom by Bruno Dubuc, Canadian
Institutes of
Neuroscience, Mental Health, and Addiction
Medical, Science and
Nature Images by Scott Camazine
Neuroscience for kids by Eric Chudler, University of Washington
Brain
Facts and Figures by Eric Chudler, University of Washington
Spatial
short-term memory pinpointed in human brain, National Health Institutes,
1998.
(Answered March12, 2007)
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