Lecture 5 - Plasticity (motor and sensory cortex) Flashcards
SENSORIMOTOR CORTEX
▪ In ancient times, people thought a
“little person” (homunculus) was
controlling you.
▪ Sensory and Motor cortex are
cortically structured (known as
motor- and sensory homunculus).
This is similar to retinopy, yet
instead of the outside world, the
mapping happens from within.
▪ Representations cross over (left body
side represented in right hemisphere
and vice versa).
▪ Representations are topographically
organised, which means
neighbouring body areas are nearby
(e.g. hand and arm).
▪ Representational size does not match body size (e.g. your tongue has a larger representational
size than your elbow)
o Cortical magnification (relative to density of tactile receptors → the density of
receptors in your hand is far greater than the density of receptors in your back. This
density is based on the amount of incoming information)
o Afferent magnification (relative to frequency/significance of usage → you use your
hands and tongue more than you use your elbow)
▪ Idea of a homunculus is just a guide/schema, representations are overlapping (correlated
experience - e.g. single fingers used frequently together tend to show increased commonality
of representations within S1).14 | P A G E
▪ Transcranial magnetic stimulation (TMS) can be used to estimate the size of cortical
representations. It sends an electromagnetic pulse to your brain to generate activity in the
motor cortex, which will lead to a twitch in some part of your body, depending on what area
was activated.
o If you stimulate the motor cortex, you get a motion response (twitch in body)
o If you stimulate the sensory cortex, you get a tactile response (feeling of being
touched)
PLASTICITY WITH ANIMAL TESTING
▪ The somatosensory cortex in mice represents whiskers in the so-called barrel cortex. It is a
great example of plasticity. The barrel cortex adapts to the arrangement of distal sensors.
o Barrels form a topographic map with a 1:1 correspondence to whiskers. This is a great
experimental advantage.
o If the whiskers are removed, the barrels
disappear.
o If one of the whiskers is removed, the
other whiskers take over the ‘silent space’
and their mapping grows. This way, no
part of the brain is left unused.
▪ Sensorimotor deprivation occurs when a body
part gets amputated. Over time, due to plasticity,
the adjacent regions take over the ‘silent’
cortical region. For e.g. finger amputations,
extensive cortical reorganisation occurs and the
deprived area is taken over by the other fingers.
o An unproven theory is that this even
increases the sensitivity of the
remaining fingers.
o The adjacent regions do not necessarily have
to be of similar functions as the ‘silent’
cortical region. In a monkey, it was tested
that if you remove part of the hand, the
adjacent region for sensory facial
information would take over.
▪ Somatosensory plasticity due to
sensory deprivation (by e.g. the cutting
of the medial nerve) can lead to
immediate unmasking, which means
that a specific area would always
respond to e.g. touch on both sides of
your finger, but was ruled by the
strongest side and if this strongest side
goes silent, the other side will take over,
as the silent side cannot overrule the
other side anymore.
o The immediate unmasking occurs immediately, and over time the adjacent areas start
to fully take over the silent area.
▪ Motor cortex plasticity after peripheral nerve damage
(damage to motor and not sensory fibers) leads to changes in
motor maps in already a few hours, as intervention on active
sensing changes motor representations.15 | P A G E
▪ In case of a lesion (infarct) in the motor
cortex, the cells in parts of the motor cortex
can get destroyed, which will lead to a loss
in function. This is called the primary
effect of the infarct. After a period of time
however, a secondary effect can occur, as
the area for the lost functionality will be
reduced in neighbouring regions as well. A
secondary loss aggravates the primary
infarct.
o This can be caused by, for example, the lessened use of
an arm with decreased functionality, because of a lesion.
This also tells you something about experience and how
experience shapes the representations in your brain.
o Rehabilitation prevents a secondary loss of cortical
representations, as the area of the decreased
functionality can reclaim areas of less important
functionalities. It is a constant battle of what functions
are more important.
o Rehabilitation can be done by, for example, tying the unaffected arm, so the subject is
forced to use the arm with decreased functionality.
o The positive effect of rehabilitation is long-lasting.
▪ Plasticity can also be experience-dependent
o If you sew the skin of two
fingers of a monkey
together, the sensory
experience for both fingers
becomes very similar. The
effects of the
somatosensory cortex
would be that the
boundary between the representation of the fingers becomes blurred and the receptive
fields come to encompass both fingers.
▪ This is NOT an effect of nerve-cells growing across the two fingers, but rather
cortical reorganization.
o Asking a monkey to repeatedly use their index finger to
retrieve food, would, over time lead to a growth in
representation of the fingertip, as the monkey gained more
experience. This is rearrangement without damage.
▪ The same goes for the fingers of humans that
learned to play instruments at an early age.
o More experience induces larger representations.
o Experience alone is not enough. Training needs to be complex to lead to largest
change. Training needs to be outside your cortical comfort zone.16 | P A G E
▪ To draw appropriate conclusions about causation and or correlation, we need good
baselines/control experiments. Plus: Always (!) check for alternative explanations for
experimental effects.
▪ Blakemore and Cooper conducted selective rearing
experiments on cats, in which cats would be exposed
only to vertical or horizontal lines for many hours a
day. This then showed plasticity in the cat visual
cortex, as the cats could not detect contours that were
opposite to their rearing environment.
Electrophysiology of visual cortex mirrors this
finding.
▪ Matteucci and Zoccolan found that, after exposing rats to only static scenes and images (by
degrading the temporal continuity of visual experiences), there was a reduction of the number
of complex cells, while the development of simple cells was spared.
▪ Plasticity has its limits. Sometimes the damage or
changes are so dominant, that it cannot be
recovered.
o This was seen in the frog-eye experiment,
in which a frog’s eye was rotated 180
degrees, causing the frog to always jump
into the wrong direction, as the frog’s preycatching behaviour was inverted (and does
not recover).
THE HUMAN BRAIN AND PLASTICITY
▪ Phantom limbs are limbs that are lost, but
subjectively perceived. They can often be
mentally “moved” too.
▪ Sensory input loss does not necessarily result in
replacement of the original representation.
o Hard to tell because a lost limb cannot be
stimulated anymore.
▪ “Phantom movement” elicits activity patterns
comparable to two-handed controls. This is based on residual activity for the lost limb.
o If the subject moves other parts of their body, then they would probably give a
stronger response in this region than for the fantasized movement. The phantom
movement just says that the activity is not fully gone.
▪ Training can lead to structural changes in the human brain, which can
for example be seen in the brains of qualified taxi drivers in London
after completing their training.
▪ Cortical plasticity is based on different processes:
o Fast changes in cortical representations. Immediate, but
transient enlargement, that shrinks over time again.
o Slow changes in cortical representations. Slower, persistent
(structural) changes that remain.
o Mental practice can have comparable effects as physical practice (e.g. in piano play
practice).
▪ The Visual Word Form Area starts to be selective for words, the moment you learn how to
read. It is reliably activated by the visual display of words, independently of size, font,
position, etc. Words and faces have in common that they require details to be recognised.17 | P A G E
o Neuronal Recycling Theory: VWFA is where it is because of matching prior
selectivity and connectivity. Your brain used an area that already roughly did what
was required to read (such as facial recognition), and then used it for something else
(in this case: reading). The assumptions for this are:
▪ The organization of the human brain is subject to anatomical constraints from
evolution and thus is not infinitely plastic. Therefore, you cannot just put the
new functionality at a random location in the brain. It needs to be at a place
already somewhat specialised in the desired functionality.
▪ Cultural tools like reading and writing are not present in the brain at birth, but
rather must find a neuronal niche in the brain whose circuit is set up to
perform a similar function and is sufficiently plastic to reorient itself enough
to accommodate this novel use.
▪ The original organization of the cerebral cortex is never fully erased once
these cultural tools invade the cortical areas. Instead, these initial neural
constraints exert a powerful influence on
what can be learned.
o Labile states are regions that are selective for
something, but it is not quite established what
this feature will be selective for. After schooling,
such a region can be taken over for reading.
Without schooling, other regions would take over
these labile states.
o Faces have mirror symmetry, but words do not
(as they have a reading direction). Because the
area for writing and reading has inherited some selectivity of facial recognition, this
can result in Spontaneous Mirror Writing, as children that just learned how to read
and write have not unlearnt this selectivity. This also explains early confusions of ‘d
vs b’ and ‘p vs q’.
BLIND PEOPLE AND PLASTICITY
▪ Blind people can learn to read using tactile information (Braille).
o This can lead to Cross-Modal Plasticity: early visual cortex is recruited for detailed
spatio-tactile tasks. This is also called sensory rerouting.
▪ Blind participants show reliable (category-unspecific) activation in the occipital lobe.
▪ Highly selective ‘visual’ areas, such as the EBA, are recruited by auditory signals in the blind.
▪ What happens for blind people (activations of
“visual” areas for auditory signals), happens
also in reverse for deaf participants (auditory
cortex activation for visual input).
▪ Face selectivity for tactile information in blind
subjects. Long range connectivity determines
selectivity, speaking against foveal-bias, visual
expertise, and visual experience for faceselectivity
