Throughout our lives we are shaped by our experiences. They
not only change our behaviour but even how we think.These psychological changes are the result of corresponding
physical changes in the connections between the neurones in our brains.
We have a view that the structure of our brain is fixed, but
the function of our brain is to interpret the environment, discover
relationships and change our behaviour accordingly. Our brain is not a static
organ and indeed to function properly it needs to be dynamic and changing on
every level.
This changing and shaping of the connections in our brain is
known as neuroplasticity.
Hebbian plasticity:
The first person to notice this “plastic” nature of the
brain was the Canadian psychologist Donald Hebb.
In his book the organization of behaviour, he wrote his now
classic Hebb’s postulate:
“When an axon of cell
A is near enough to excite a cell B and repeatedly or persistently takes part
in firing it, some growth process of metabolic change takes place in one or
both cells such that’s As efficiency as one of the cell firing B in increased”
Simply put;
When two neurones fire at the same time, the connections
between them are strengthened, and thus they become more likely to fire again
together in the future.
When two neurones repeatedly fire in an uncoordinated
manner, the connections between them weaken, and they are more likely to act independently
in the future.
This can be simplified to the mantra:
- · Cells that fire together, wire together.
- · Cells that fire apart, wire apart.
These plastic mechanisms form the basis of the brains
ability to change in the face of the environment, to learn and remember.
Plasticity, memory and learning:
But how do these plastic changes form the basis of learning
and memory?
The type of learning plasticity has been applied to most is classical conditioning.
The most famous example of classical conditioning is
Pavlov’s dogs,
a stimulus (food) which produced a response (salivation),
was paired with a stimulus that did not produce a response (bell.)
After multiple exposures, the bell produced the same
response (salivation) even without the presence of the original stimulus (the food)
the similarities with Hebb’s postulate are easily seen.
The neurones responsible for the bell and the neurones
responsible for the salivation were repeatedly activated at the same time,
this strengthened the synaptic connections between them, and so in the future
they were activated together.
When we learn something, a set of neurones are triggered and
become connected. This now connected “assembly” of cells persists, and if this
set of neurones is triggered again, we will re-experience the event as a
memory.
The theoretical memory trace In the brain is known as an
Engram
The relationship between cell assemblies and memory was
initially investigated by Karl Lashley.
He taught a rat to complete a maze, then destroyed a different
part of the cortex each time, and would see which area affected the maze
memory.
The only relationship he found was
that the number of errors made was directly proportion to the amount of cortex
destroyed.
Lashley concluded that memory is
equally distributed in all cortical areas, through these interconnected cell
assemblies*. |
Storing information in assemblies like this can also explain
another phenomenon of memory, how only a partial cue can trigger the reactivation
of a whole memory, for example how a small detail, such as a familiar smell can
cause to us relive a detailed memory.
Activation of a single part of
the assembly will reverberate through all its connections, activating other
cells encoded at the same time.
Sources:
Mu-ming Poo Part 1: The Cellular Basis of Learning and
Memory. http://www.ibioseminars.org
Hebb, D.O. (1949). The organization of behaviour. New York: Wiley & Sons
The Brain that changes itself: Dr Norman Doidge,
The method of pawlow in animal psychology. Robert
M. Yerkes and Sergius Morgulis (1909).Harvard
University. First published in The Psychological Bulletin, 6, 257-273.
The neurobiology of
consolidations, or, how stable is the engram? Dudai Y. Annu Rev. Psychol. 2004;55:51-86.
Studies of cerebral
function in learning IX. Mass action in relation to the number of elements in
the problem to be learned†
Lashely and wiley 1933. The Journal of Comparative Neurology. Volume 57, Issue
1, pages 3–55, February 1933
*”we now know that Lashely’s experiment
did not distinguish between memory or motor areas, and so the rats impairment
in the maze may not be due to memory impairment but instead the impairment of its motor functions. The current view is that memory is indeed distributed by
not, but not equally. Some areas such as the hippocampus paly a particularly crucial
role, which we will discuss later.”
