sensation and perception so ma to sensation

8/4/2019 Sensation and Perception So Ma to Sensation

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Sensation

&

Perception


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17 Somatosensation

Touch

Heat/Cold

Pain


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Administrative stuff


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The final

Friday, December 10th, 10.30 am -12.30 pm

In HGS 101

Comprehensive, with an emphasis on things

that havent been covered in previous exams.

Like the midterms. Interpreting graphs,

drawing diagrams, multiple choice and short

written answers.


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The early final

For graduating seniors

Their grades have to be in by Dec the 3rd.

Format is similar as in regular final

This leaves only one day: Dec the 2nd.

But when on the 2nd? Someone has a mandatory

class from 11-12, someone else has another finalfrom 2.30.

What about really early (911 am) or really late

(5-7 pm)?


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The Review session

When?

Where?


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Auditory system

PostScript


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Young adults are most sensitive for

frequencies corresponding to human speech


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Range of audible frequencies of several species at 60 dB

SPL. No species covers the whole range.


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SomatosensationTouch

Temperature

Pain


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Organ: The skin


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Temperature

Pain


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The skin receptors (transducers)


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a) Merkel b) Meissner

c) Ruffini d) Pacinian


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Meissner corpuscle, shallow, stack of cells

Merkel disk, shallow, disk-shaped

Ruffini endings, deep, branched cylinder

Pacinian corpuscle, very deep, onion-like


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Spatiotemporal fiber properties

Time constants

short long

RF

structure

Small, Sharp

boundaries

Large, fuzzy

boundaries

Rapidly adapting,

punctate

(RA-P)

Slowly adapting,

punctate

(SA-P)

Rapidly adapting,

diffuse

(RA-D)

Slowly adapting,

diffuse

(SA-D)


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Meissner corpuscle: RA-P

Merkel disk: SA-P Ruffini endings: SA-D

Pacinian corpuscle: RA-D


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Spatiotemporal fiber properties

Time constants

short long

RF

structure

Small, Sharp

boundaries

Large, fuzzy

boundaries

(RA-P) (SA-P)

(RA-D) (SA-D)

Meissner

Corpuscle

Merkel

Disk

Pacinian

Corpuscle

Ruffini

Ending


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Meissner corpuscle. Hooked up to RA-P fibers.

Optimal stimulus: Pressure. Perception: Flutter.

Merkel disk. Hooked up to SA-P fibers.

Optimal stimulus: Light tapping. Perception: Pressure.

Ruffini endings. Hooked up to SA-D fibers.

Optimal stimulus: Lateral Stretching. Perception: Stretch.

Pacinian corpuscle. Hooked up to RA-D fibers.

Optimal stimulus: Vibration. Perception: Vibration.


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One last word on skin receptors

Merkel disks = Merkel discs = Merkel receptors

Ruffini endings = Ruffini cylinders

SA-P = SA-1, SA-D = SA2

RA-P = RA-1, RA-D = RA2


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On receptive fields on the skin

The vision analogy holds

Somatosensory receptive fields have a

antagonistic center-surround structure

For the following experiment, have two

sharp (but relatively blunt) objects like

toothpicks or pen-points ready.

If you hurt yourself, dont sue anyone.


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Excitatory

region

Inhibitory

region


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1 stimulus in

excitatory

region:

Feels like 1

strong point


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2 stimuli inexcitatory

regions, far

apart:

Feels like 2separate strong

points


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regions, close

together (both

inhibiting andexciting each

other)

Feels like 1 point


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regions,

intermediate

distance (eachinhibits the

other)

Feels like 2 weak

points


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Relative sensitivity of body parts

This rationale (measuring the two-point-threshold) was used to elicit the absolute

sensitivity of different regions of skin. It allows to infer the spacing of receptive

fields andin turn the spatial resolution ofa given skin region for a given body part.

This was done very early (as early as 1880),using a compass.


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2-point Thresholds on skin


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Pathways for touch

Afferent touch fibers enter the spinal cord in the dorsalroot.

Some of the fibers synapse onto local inter-neurons whichin turn synapse onto motor neurons, implementing a spinalreflex arc (like a withdrawal reflex)


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Ascending pathways for touch

Other fibers travel upwardto the brain, carryingtouch-information to thebrain-stem.

They are in the lemniscalpathway

After synapsing in thebrainstem, fibers cross tothe opposite side of thebrain.

After synapsing in thethalamus, these neuronsproject to cortical areas.


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Cortical processing of touch

The projection area is

the somatosensory

cortex (S1, S2) It is situated

immediately posterior

of the central sulcus.

Both S1 and S2

contain a full sensory

MAP of the body.


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The sensory homunculus

Literally
http://www.cs.uta.fi/~jh/homunculus.htmlhttp://www.cs.uta.fi/~jh/homunculus.html


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Threshold/Homunculus

Correspondence The lower the 2-point

threshold for a given skin

region (= higher spatialresolution), the more

relativespace is devoted to

cortical represenation/

processing of that region insomatosensory cortex.

Roughly an inverse

correlation.


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Sidenote

Recently, the analogy between retinotopic

mapping in vision and somatotopic mapping in

somatosensation has come under fire. Critics of this model argue that there are many

more body maps in the somatosensory cortices

They also argue that the mapping is much more

distributed and not somatotopic.

Homunculus model too nice to be true.


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Another analogy to vision

Just like in the case of eye movements, where the

brain needs to know if the eyes moved or the

world moved, an efference copy is sent from themotor system to the sensory system.

In the case of the somatosensory system, this is the

somatosensory cortex.

This is the reason why one cant tickle oneself.The body knows that its not an external stimulus

and takes it into account (dismissing it).


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Somatosensory efference copy and

tickling If one is instructed to push oneself and then

push another person with the same force,

the other person experiences this as a muchstronger push (escalation)

Blakemore et al. (1998):


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By the way

Brodmann Areas 1,2,3

= Somatosensory cortex

= S1, S2

=Postcentral gyrus

=Postcentral cortex


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Temperature

Pain


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Thermoreceptors


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Thermoreceptors

Free nerve endings act as thermoreceptors

There are separate classes of thermoreceptors:

Cold fibers and Warm fibers Cold fibers respond to a decrease in

temperature, while warm fibers respond to anincrease in temperature.

An individual fiber has a preferred temperature(exhibits a temperature tuning curve just likeV1 neurons are tuned for orientation)


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Thermoreceptors

Both cold and warm fibers dont respond to

mechanical pressure.

Preferred temperatures of cold fibers range from20 C to 45 C, while preferred temperatures of

warm fibers range from 30 C to 48 C.

This response to a preferred temperature is asustained response, it doesnt adapt like the

additional increase/decrease response discussed

previously.


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Thermoreceptor tuning curves

Explains paradoxical cold

Jumping into a very hot bath transiently activatesboth hot and cold fibers.

For a short time, the bath feels both hot and cold

Cold

Hot


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Higher temperature processing

Temperature information reaches the brain via the

spinothalamic tract in the spinal cord.

Its target in the brain is both Somatosensorycortex, but also Insular cortex.


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TemperaturePain


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Pain as a special perceptual quality

A quality that has complex phenomenologicalfacets (behavioral, sensory, emotional)

Pain perception can be modulated by all kindsof factors, including behavioral states (stress,sex), cognitive states (hypnosis), mental states(trance), social norms and drugs.

Its significance for society is enormous. E.g.:Should fishing be legal?

Evolutionary significance to organism obvious.


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Pain is not a purely sensory

experience It shares components of an emotion (distress

that is associated with pain).

But it also shares sensory characteristics. Itsignals the presence of stimuli that are harmful

to the organism.


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Pain receptors

Free nerve endings in the skin

They are connected to various fibers:

A

small, myelinated. High conductance speed Csmall, unmyelinated. Slow conductance speed


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Pain receptors

Afast, sensitive to mechanical noxious stimuli.

Cslow, sensitive to many noxious stimuli

(chemical, etc.) This distinction has been used to explain the

phenomenon ofdouble-pain:

One noxious stimulus causes first a quick, sharp

pain (mediated by A fibers) and is followed by a

dull and burning pain (mediated by C fibers)


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Pain pathways

Pain information also

reaches the brain via

the spino-thalamic

tract.

There are projections

to the Somatosensory

cortex and theAnterior cingulate

cortex


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Cortical pain processing

Sensory aspects of pain seem to be processed in the

Somatosensory cortex.

Emotional distress associated with pain seems to be processed

in the Anterior Cingulate Cortex (ACC). Subjects with lesions in ACC could still accurately judge the

intensity of pain. But they were not in the least bothered by it.

On the other hand,

subjects empathy forthe pain of others only

elicits activity in

ACC, not

Somatosensory cortex.


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The Gate control theory

Devised by Melzack & Wall (1965)

Tries to explain the fact that pain intensity can becognitively (top-down) controlled.

Components of the model: Large fibers (involved in touch)

Small fibers (involved in Pain perception)

Substantia gelatinosa (the gate)

T-cells (Transmission cells, sending pain signals tothe brain)

If T-cells are sufficiently active, we experience pain.


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The Gate control theory

The large fibers are under cognitive control. If they are active, they activate

the substantia gelatonisa (sg) in the spinal cord.

If the sg is active, the activity ofT-cells is diminished.

If the T-cells dont reach their activity threshold, no pain is experienced.

If small fibers are active first, sg is inhibited, both fibers increase T-activity


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Explanatory power of the theory

Cognitive factors can diminish pain experience.

Large fibers have descending tracts.

Chemical Analgesics act to inhibit small fibers,disinhibiting the sg reducing pain.

Rubbing the skin around a hurting area reduces pain

by activating large (touch-sensitive) fibers, activating

sg reducing pain.

This effectively implements a peripheral pain control


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Central control of pain

There is a system of endogenous opiatesand corresponding receptors in the brain.

Endogenous opiates are particularlyenkephalins and endorphins.

In situations of stress, release of endorphinsreduces the expected pain centrally.

Morphine also acts on these endogenousopiate receptors.

sensation and perception so ma to sensation

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