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The motor memory we use everyday-for sport, playing a musical
instrument and even typing-is acquired through repeated practice
and stored in the brain. New motor skills can be learned through
practice, but often those skills can be all but lost by the following
day. This loss of motor skills can be attributed to the storage of
newly learned motor skills in short-term memory. By repeating
the exercises on a daily basis, however, these skills become stored in
long-term memory. In 2006, a team led by Soichi Nagao, head of the
Laboratory for Motor Learning Control at the RIKEN Brain Science
Institute (BSI), discovered that short-term motor memory is transferred
to a different site to become long-term memory, and that brain activity
while the body is at rest is important for creating long-term memory-
more so than the brain activity while the body is working. With this
discovery, research into the mechanism of motor memory is now
breaking new ground. A 40-year debate
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The mechanism of memory was originally attributed to changes in the transmission
efficiency of synapses in the brain. Around 1970, it was proposed that motor memory
specifically was created within the neural network of the cerebellum. One of the
advocates of this theory was Masao Ito, senior advisor to the BSI. In 1982, Ito
experimentally discovered the phenomenon of 'long-term depression', by which
the transmission efficiency in the cerebellum is depressed over a long period of
time.
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In 1983, Nagao started work as a laboratory lecturer for Ito, who was then in the Faculty
of Medicine at The University of Tokyo. "Since then, I have continued my studies to
verify and develop the theoretical hypothesis proposed by Dr Ito and other researchers,"
says Nagao.
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The process of eye movement is a good illustration of how the hypothesis proposed by Ito
works). Command signals that control eye movement are transmitted to the vestibular
nucleus in the medulla oblongata over two routes: one direct and the other indirect
passway via the parallel fibers and Purkinje cells of the cerebellar cortex. The vestibular
nucleus processes the command signals transmitted over both routes and outputs the
processed signals to the motor nerves, which in turn, control the movement of the eye.
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A single Purkinje cell is connected to up to 200,000 parallel fibers, through which
ommand signals to control movements are transmitted to the Purkinje cell. A single
climbing fiber is connected to the Purkinje cell, and when an eye fails to move properly,
error signals are transmitted from the eye to the Purkinje cell through an inferior olivary
nucleus and the climbing fiber. This process triggers long-term depression, suppressing
the transmission efficiency of the command signals at the synapses connecting the parallel
fibers to the Purkinje cell. When the eye moves properly, no long-term depression is
triggered because the error signals are low. In this way, command signals from the
Purkinje cell to the vestibular nucleus can change dynamically because of the long-term
depression induced in response to eye movement. "Command signals from the vestibular
nucleus to the motor nerves are controlled so that the eye can move properly," says Nagao.
"In other words, Dr Ito's theory states that motor memory is created in Purkinje cells in the
cerebellar cortex through long-term depression."
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Purkinje cells in the evolutionarily oldest region of the cerebellar cortex-the vestibulocerebellum-
have axons that extend to the vestibular nuclei, whereas Purkinje cells in the newer region-the
archicerebellum and neocerebellum-have axons that extend to the deep cerebellar nuclei to
form neural circuits.
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"Doctor Ito's hypothesis is controversial and has been debated for about 40 years. A
typical argument runs as follows: it is true that Purkinje cells are capable of transmitting
the information necessary for motor memory, but the memory is created in the vestibular
or cerebellar nuclei."
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The long-term motor memory
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In 2006, Nagao published an article that finally settled the dispute over where motor
memory is created: in Purkinje cells in the cerebellar cortex, or in the vestibular or
cerebellar nuclei? The achievement came just two years after he established the
Laboratory for Motor Learning Control at the RIKEN BSI in 2004. Nagao and his
laboratory members developed a new experimental system using mice to examine the
memory of eye movement. One of the mechanisms of eye movement allows us to track
objects reflexively, like when watching the passing view from a train window. If this
mechanism, known as optokinetic eye movement response, is working properly, the
view is clear and we can see passing objects. The memory function of the optokinetic
eye movement response can be evaluated by placing a checkered screen in front of a
mouse and moving it in a wave-like manner. "The mouse will get used to the
movement in about an hour of exercise and will be able to follow the movement of
the checkered screen by moving its eyes in a wave-like motion. This is because
motor memory has been created, and the eye movement efficiency of the mouse
has improved. However, when the mouse is kept in a dark room after the exercise,
and when the eye movement efficiency is examined the next day, you will find the
efficiency has returned to the initial state."
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Even when motor memory has been established after one day of exercise, most
memory is lost within several hours. This is because only short-term memory is
created in the first instance, but this can be improved. "A mouse can gradually
improve its eye movement efficiency when we get the mouse to do exercises on
a daily basis. A mouse that has done such exercises for one week can maintain
its eye movement efficiency at a higher level than before it started, even after the
exercises have been terminated. This means that long-term memory has been
established."
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However, it was still unknown whether these short- and long-term motor memories
are created in Purkinje cells in the cerebellar cortex or somewhere else. "To stop the
function of the cerebellar cortex, we administered local anesthesia to the cerebellar
cortex of a mouse that had repeated specific exercises for four days and examined
the eye movement efficiency of the mouse. We found that the efficiency remained
at the same level as when the exercise was started on the fourth day. In other
words, the previous three days had contributed to improving the eye movement
efficiency and the efficiency then remained at that level, indicating that long-term
memory was created at a site other than in the cerebellar cortex." The exercise on
the fourth and subsequent days, on the other hand, did not add to improving the
eye movement efficiency, indicating that short-term memory was created in Purkinje
cells in the cerebellar cortex.
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Nagao and his team then conducted experiments to confirm whether long-term memory
is created in the vestibular nucleus. "We discovered that no motor memory is created in
either the cerebellar cortex or the vestibular nucleus alone, but that short- and long-term
memories are created at different sites: short-term memory in Purkinje cells in the
cerebellar cortex and long-term memory in the vestibular nucleus. Around the time we
published these experimental results, other research groups also reported similar
results using different models. That is how we succeeded in settling the 40-year debate."
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Neither short- nor long-term memory is created when the functioning of the cerebellar
cortex is impeded. "Motor memory is initiated by short-term memory by long-term depression
in Purkinje cells in the cerebellar cortex. Short-term memory becomes long-term memory
when it is transferred in one way or another to the vestibular nucleus, into which axons
extend from the Purkinje cells. Long-term memory is considered to be created in neural
circuits in the vestibular or cerebellar nuclei, into which axons extend from the Purkinje
cells.
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Not made for sophisticated motion
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It is thought that information that can be expressed verbally, such as experienced events
and the spelling of English words, is stored in the hippocampus, deep in the cerebrum, as
short-term memory, and is later transferred to the cerebral cortex to become long-term
memory. "The short-term memory of effable information is not immediately transferred
to the cerebral cortex, but stays in the hippocampus for a few weeks. We have clarified
that short-term motor memory created in Purkinje cells in the cerebellar cortex is trans
ferred to the vestibular or cerebellar nuclei in as little as several hours or as long as
several days."
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However, it has become clear that not all information in short-term motor memory becomes
long-term motor memory. Nagao and his team used monkeys to investigate how
sophisticated motion is memorized. In the experiment, the speed of a moving ball is
suddenly increased for a fifth to a tenth of a second. "In this experiment, monkeys are
required to follow the complex motion of the ball, called 'smooth-pursuit eye movement'.
Monkeys usually get used to the motion only after the exercise has been repeated a few
dozen times, moving their eyes based on their prediction of the change in the speed of
the ball. However, sophisticated motor memory of this kind is easily lost in just ten to 15
minutes. In other words, the memory of sophisticated motion is stored quickly but also lost
quickly. One major advantage in terms of short-time memory in the cerebellar cortex is that
sophisticated motion can be memorized very quickly." For example, a baseball batter will
be able to hit a curve ball after the movement of the ball has been observed several
times and memorized. Unfortunately, however, it has become clear that sophisticated
motor memory of this kind hardly ever becomes long-term memory because only the
broad information in short-time memory is transferred to long-term memory.
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"It is said that skills that are learned physically cannot really be forgotten. For example,
once you have learned how to ride a bicycle, you will be able to ride a bicycle again with
a little practice even after a long period without riding. We can consider long-term motor
memory to be robust and difficult to forget. However, we should respect the importance of
'a little practice' because long-term motor memory only holds rough or broad information.
We need to activate the functions of the cerebellar cortex by memorizing sophisticated
movement to get the 'feel' for the movement. I do not think that any professional musicians
would be able to show their abilities at a concert without practicing beforehand."
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There are evolutionary reasons why short- and long-term memories are created at different
sites in the brain. "Keeping specific sophisticated motor information in the cerebellar cortex
for a long time would make it difficult to adapt movements quickly in response to
environmental changes. I think that transferring only basic motor information to the vestibular
or cerebellar nuclei as long-term memory ensures that sophisticated movements will always
be stored in the cerebellar cortex and remain adaptable in response to changing situations."
All vertebrate animals, from fish to mammals, have the same mechanism of short- and
long-term motor memory storage at different sites. Invertebrate animals, however, are known
to store short- and long-term motor memories at the same site in their neural circuits.
"This difference is one of the features that differentiate vertebrate animals from invertebrates."
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Interval training for long-term memory
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The mechanism of how short-term memory in the cerebellar cortex is transferred to the
vestibular or cerebellar nuclei to become long-term memory is still open for debate. "Making
use of a variety of techniques, we are working towards answering the question." Recently,
Nagao and his team discovered that short-term memory is transferred efficiently to long-term
memory when experiments on motor memorization using animals are conducted with
intervals between the exercises rather than conducting the experiment continuously.
"For example, a one-hour experiment can be divided into four short 15-minute exercises
with intervals of 30 minutes between them for higher transfer efficiency. We also discovered
that no long-term memory is created irrespective of the number of repeated exercises if the
functions of the cerebellar cortex are deactivated during those intervals."
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These findings suggest that the activities of the cerebellar cortex during periods of inactivity
are important in creating long-term memory. This presents the question of what happens in the
cerebellar cortex during the intervals to cause long-term memory to be created in the vestibular
or cerebellar nuclei. "We have not yet obtained experimental data that can lead us to a
conclusion. However, we are beginning to collect important data that suggest the importance
of 'long-term potentiation', which enhances the transmission efficiency of synapses, for
long-term memory creation."
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Coachly advice
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Nagao's research provides some valuable pointers as to how to improve sporting and
performance skills. "The key is to repeat exercises, with intervals, on a daily basis. Top
athletes and musicians will be more talented than ordinary people in creating short-term
and long-term memories. Motor memory, however, is not created without repeated exercise.
I think even someone who is thought of as a genius needs to continue daily exercises
because motor information on sophisticated movements is difficult to establish as
long-term memory."
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