Lecture 23 Flashcards
plasticity
the degree to which your body map can change based on experience
slow adapting
mechanoreceptors that keep firing when there’s pressure on them
rapid adapting
only fire when there’s a transition: initial pressure and releasing pressure
if you touch one it fires and then turns off and if you pull away it fires and then turns off
merkel responsive to
fine detail
closest to the surface
meissner cells
rapid adapting: going very quickly
respond when something is fluttering: if you’re gripping something and it’s slipping
tactile acuity
how fine of detail can you detect in the environment
two point threshold
taking two probes and apply them on different parts of the skin and asking: do you feel one point or two points? (detecting the difference of locations of those two points)
the closer the two points while still being able to distinguish them defines how fine the detail is
grating acuity
two different orientation of gratings (vertical or horizontal) but you can use diff sizes of gratings
and if you can detect whether it’s vertical or horizontal defines the level of detail that you can detect
Which mechanoreceptor do you expect to have the greatest
acuity in the somatosensory system?
merkel disk
furthest out
Recording from cells during tactile acuity tasks for gratings
- There is a high density of Merkel disk receptors in the fingertips.
- The firing of the Merkel receptors reflects the pattern in the grooves in the grating acuity test, while the Pacinian corpuscle does not (pattern is erratic and has lots of “noise”).
- Both two-point thresholds and grating acuity studies show these results.
If we map Merkel cell
spacing (density) against
tactile acuity, we see…
….a strong correlation with areas
of the hand associated with
detecting detail.
density of these cells determine how much sensitivity to you have to detail
as density decreases sensitivity decreases (fingertips -> palm)
Pacinian corpuscle
is primarily responsible for sensing vibration.
determined by layers of skin on the end of the receptor: if you get a rapid vibration on the skin those little vibration:
mechanistic process: the info getting through is based purely on the structure
– Nerve fibers associated
with PCs respond best to high rates of vibration.
They are rapid adapting cells.
– The structure of the PC is responsible for the response to vibration. Fibers with the PC covering removed will respond to continuous pressure (proving it's based purely on the structure outside of the cell).
David Katz (1925) proposed that perception of texture (tactile patterns over a large area) depends on two cues:
– Spatial cues that are determined by the size, shape, and distribution of surface elements. (SA?)
– Temporal cues that are determined by the rate of vibration (rate of change) as skin is moved across finely textured surfaces. (RA?)
two different ways of getting spatial and temporal cues
duplex theory of texture perception
Two receptors acting together may be responsible for this perception of texture
to tell the diff in textures you need the coincidence of these cues
merkel cells and Pacinian corpuscle
Research by Hollins and Reisner
(2000) shows support for the role of temporal cues.
– In order to detect differences between fine and very fine textures, participants needed to move their fingers across the surface. This suggests the need for a rapid adapting receptor.
but if it was moving, people were very accurate (temporal, feeling for vibrations = important psychophysical evidence)
Selective adaptation experiment to determine which rapid adapting cell is involved in texture.
Hollins
In Hollins et al. (2001), participants’ skin was adapted (fatigured)with either:
– 10-Hz stimulus for six minutes to selectively adapt the Meissner corpuscle.
– 250-Hz stimulus for six minutes to selectively adapt the Pacinian corpuscle.
– Subjects then ran their fingers over two textures and judged which was ‘finer.’
• Results showed that…
- no adaption = right ~78%
- after adaption =
results of Hollins
evidence for duplex theory of texture perception
• Results showed that only the
adaptation to the 250-Hz stimulus
affected the perception of fine
textures.
• Together, these studies show that (1) movement is necessary for fine texture detection (temporal cue) and (2) this involves Merkel disks and Pacinian corpuscles.
from skin to cortex
Nerve fibers travel in
bundles (peripheral nerves)
along the dorsal root to the
spinal cord, and two major pathways in the
spinal cord: spinothalamic and medial-lemniscal pathway
Paths cross to other side of body and synapse in the thalamus.
Spinothalamic pathway
consists of smaller diameter fibers
that carry temperature and
pain information.
signals a little slower
Medial lemniscal pathway
consists of large fibers that
carry proprioceptive and
touch information.
touch signals a little faster
In the cases studies of Ian Waterman and Charles Freed, they appear to have lost input to which of the sensory pathways?
Medial lemniscal pathway
they still get pain and temperature
dissociation!
Both pathways pass
through the
ventral posterolateral nucleus in the thalamus (like almost all sensory info).
somatotopic or topographic represenation
sensory info nearby on the skin are represented as nearby in the thalamus: spatially representing what’s happening on the periphery at the skin
As we saw in vision, the
thalamus maintains the
organization from the receptors.
neurons at VPN
also have center-surround
organization.
Signals travel from the thalamus
to the…. in the?
… primary somatosensory
receiving area (S1) and the
secondary receiving area (S2) in
the parietal lobe.
Body maps (homunculi) on the cortex in S1 and S2 show more
cortical space allocated to parts
of the body that are responsible
for detail.
two of them: one representing the stuff that happens on the left and on the right
areas that have a greater density of merkle cells (fingertips)
have the greatest representation in cortex
Plasticity in neural functioning
leads to
multiple homunculi and
changes in how cortical cells are
allocated to body parts.
cortical rep may change depending on experiences over a lifetime
cortical magnification
highest density receptors are over-represented in the cortex (fingertips)
Wilder Penfield and the
Montreal procedure
how do we know the homunculus represents the body in these ways?
= The sensory homunculus
• Prior to epilepsy surgery (cutting out chunks of the brain to stop seizures), Penfield probed the brain with electrodes and asked subjects to report their sensations.
• This procedure gave us
detailed motor and sensory
maps for the human body = topographic maps highly consistent across people.
• Cortical representation of the
body are now called the
Penfield Homunculus
Body areas with high acuity have larger areas of
cortical tissue devoted to them.
This parallels the cortical magnification factor seen in the visual cortex for the cones in the fovea.
Areas with higher acuity also have smaller
receptive fields on the skin.
The motor homunculus
On the other (anterior/rostral) side
of the central sulcus, in the frontal lobe, we have a map similar to the sensory homunculus that is involved with motor control.
gross movements of the body and finer and finer movements
motor map is close to the somatosensory map: want to have the smallest amount of transition time between knowing where the body is and its movements
Somatosensory map
what your body is like
how it’s organized
how it’s shaped
motor map
matches closely to the somatosensory map
all about where the muscles are and what they’re doing: things that underly the sense of touch
Is the sensory homunculus a module?
cause it’s so closely matched between individuals : theoretically you don’t want it to change that greatly
It was originally thought that the sensory and motor maps were static and would not change.
However, a number of interesting experiments and clinical observations (nature’s experiments) now show that the maps can change in response to sensory input (study’s with ADULTS).
Experience-dependent plasticity
study to see whether the sensory homunculus was a module
- In one experiment, monkey’s were trained to do a task that stimulated the finger tips (prolonged acuity task).
- Cortical representation of finger tip (blue area) in somatosensory cortex was measured before and after.
• Greater representation was
found for the stimulated area
after training (3 months later).
= still enough intrinsic connectivity that if you change the experience you can change the representation: IT’S HOW THEY’RE BEING USED
• This was suggested as a
means by which people may
recover function following a
stroke.
This demonstrated that the cortical representation of
the body can change over time in response to environmental stimuli.
• This is another example of neural plasticity – the
selective alteration of neural properties over time –
into adulthood.
What about maps in humans?
Over-representation of
fingers in musicians
• Over the course of years of practice, plasticity increases the size of the neural representation of the fingers beginning in somatosensory cortex
• focal dystonia.
focal dystonia
Areas responsible for
sensing and controlling
fingers can become fused
(in violinists).
the cortex that’s representing the fingers expand and overlap and fuse
motor problem
making it hard to control the hand
This interferes with motor
control for the fingers, but
the cause seems to be in
the somatosensory cortex.
because the somatosensory and motor cortexes are working together so much
How do we feel objects in the environment?
Humans use active rather than passive touch to interact with the
environment.
• Haptic perception is the active exploration of 3-D objects with the hand(s).
Haptic perception
how you’re going to use the info coming in and the characteristic movements to ID objects
It uses three distinct systems:
- Sensory system
- Motor system
- Cognitive system
Psychophysical research
shows that people can
identify objects haptically in
one to two seconds.
• Klatzky et al. (1985) have
shown that people use
exploratory procedures
(EPs):
– Lateral motion (picking up texture)
– Pressure (getting how hard)
– Enclosure (getting the shape)
– Contour following (getting the whole outline of the object)
hold true across individuals
ruffini cylinders
stretching of the skin
degree to which the tension of the skin changes as you’re grabbing or reaching your arm out
rapid adapting cells
meissner cells (lower frequency) and Pacinian corpuscle (higher frequency)
what’s responsible for differences in acuity during Two-point threshold measurement of tactile acuity?
where the merkel cells are denser, like the finger tips, you have the probes on two different receptor fields, where as where they are larger and less dense (palm) you have a higher possibility that the two probes will be on the same receptor site and therefore not be able to tell the difference between the two points
