Lecture 23

Question 1

plasticity

Answer 1

the degree to which your body map can change based on experience

Question 2

slow adapting

Answer 2

mechanoreceptors that keep firing when there’s pressure on them

Question 3

rapid adapting

Answer 3

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

Question 4

merkel responsive to

Answer 4

fine detail

closest to the surface

Question 5

meissner cells

Answer 5

rapid adapting: going very quickly

respond when something is fluttering: if you’re gripping something and it’s slipping

Question 6

tactile acuity

Answer 6

how fine of detail can you detect in the environment

Question 7

two point threshold

Answer 7

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

Question 8

grating acuity

Answer 8

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

Question 9

Which mechanoreceptor do you expect to have the greatest

acuity in the somatosensory system?

Answer 9

merkel disk

furthest out

Question 10

Recording from cells during tactile acuity tasks for gratings

Answer 10

• 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.

Question 11

If we map Merkel cell
spacing (density) against
tactile acuity, we see…

Answer 11

….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)

Question 12

Pacinian corpuscle

Answer 12

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).

Question 13

David Katz (1925) proposed that
perception of texture (tactile patterns over a large area) depends on two cues:

Answer 13

– 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

Question 14

duplex theory of texture perception

Answer 14

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

Question 15

Research by Hollins and Reisner

(2000) shows support for the role of temporal cues.

Answer 15

– 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)

Question 16

Selective adaptation experiment to determine which rapid adapting cell is involved in texture.

Hollins

Answer 16

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 =

Question 17

results of Hollins

evidence for duplex theory of texture perception

Answer 17

• 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.

Question 18

from skin to cortex

Answer 18

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.

Question 19

Spinothalamic pathway

Answer 19

consists of smaller diameter fibers
that carry temperature and
pain information.

signals a little slower

Question 20

Medial lemniscal pathway

Answer 20

consists of large fibers that
carry proprioceptive and
touch information.

touch signals a little faster

Question 21

In the cases studies of Ian Waterman and Charles Freed, they appear to have lost input to which of the sensory pathways?

Answer 21

Medial lemniscal pathway

they still get pain and temperature

dissociation!

Question 22

Both pathways pass

through the

Answer 22

ventral posterolateral nucleus in
the thalamus (like almost all sensory info).

Question 23

somatotopic or topographic represenation

Answer 23

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.

Question 24

neurons at VPN

Answer 24

also have center-surround

organization.

Question 25

Signals travel from the thalamus

to the…. in the?

Answer 25

… primary somatosensory
receiving area (S1) and the
secondary receiving area (S2) in
the parietal lobe.

Question 26

Body maps (homunculi) on the
cortex in S1 and S2 show more

Answer 26

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

Question 27

areas that have a greater density of merkle cells (fingertips)

Answer 27

have the greatest representation in cortex

Question 28

Plasticity in neural functioning

leads to

Answer 28

multiple homunculi and
changes in how cortical cells are
allocated to body parts.

cortical rep may change depending on experiences over a lifetime

Question 29

cortical magnification

Answer 29

highest density receptors are over-represented in the cortex (fingertips)

Question 30

Wilder Penfield and the
Montreal procedure

how do we know the homunculus represents the body in these ways?

= The sensory homunculus

Answer 30

• 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

Question 31

Body areas with high acuity have larger areas of

Answer 31

cortical tissue devoted to them.

This parallels the cortical magnification factor seen in the visual cortex for the cones in the fovea.

Question 32

Areas with higher acuity also have smaller

Answer 32

receptive fields on the skin.

Question 33

The motor homunculus

Answer 33

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

Question 34

Somatosensory map

Answer 34

what your body is like

how it’s organized

how it’s shaped

Question 35

motor map

Answer 35

matches closely to the somatosensory map

all about where the muscles are and what they’re doing: things that underly the sense of touch

Question 36

Is the sensory homunculus a module?

Answer 36

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).

Question 37

Experience-dependent plasticity

study to see whether the sensory homunculus was a module

Answer 37

• 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.

Question 38

What about maps in humans?

Over-representation of
fingers in musicians

Answer 38

• Over the course of years
of practice, plasticity
increases the size of the
neural representation of
the fingers beginning in somatosensory cortex 

• focal dystonia.

Question 39

focal dystonia

Answer 39

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

Question 40

How do we feel objects in the environment?

Answer 40

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).

Question 41

Haptic perception

Answer 41

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

Question 42

Psychophysical research
shows that people can
identify objects haptically in

Answer 42

one to two seconds.

Question 43

• Klatzky et al. (1985) have
shown that people use
exploratory procedures
(EPs):

Answer 43

– 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

Question 44

ruffini cylinders

Answer 44

stretching of the skin

degree to which the tension of the skin changes as you’re grabbing or reaching your arm out

Question 45

rapid adapting cells

Answer 45

meissner cells (lower frequency) and Pacinian corpuscle (higher frequency)

Question 46

what’s responsible for differences in acuity during Two-point threshold measurement of tactile acuity?

Answer 46

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