Prof. Greg Francis

1PSY 310: Sensory and Perceptual Processes

Purdue University

Touch

PSY 310

Greg Francis

Lecture 33

Why is the Braille system better?

Purdue University

Touch perception Vision and audition involve perception of objects from a

distance Safe and dependent on the transfer of energy (light, air

pressure)

Touch inherently involves contact of your body withobjects in the world More dangerous and dependent on the transfer of mechanical

energy

You can recognize objects in the dark

You can explore properties of an object that cannot beidentified by vision (strength, stretch,…)

People are very sensitive about touch Higher status people are allowed to touch lower status people,

not the other way around

Prof. Greg Francis

2PSY 310: Sensory and Perceptual Processes

Purdue University

Touch perception In 1992 the Queen of England visited Australia

At the parliament, the premier of Australia, Paul Keating,touched the Queen’s back

This caused a huge uproar among the British press

Keating was labeled the “Lizard of Oz”

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Touch perception In 2000 the Queen of England visited Australia again

Something similar happened when the prime minister, JohnHoward, put his arm around the queen while introducing her tosome other people

A spokeswoman for the prime minister said:

"We firmly deny that there was any contact whatsoever."

Prof. Greg Francis

3PSY 310: Sensory and Perceptual Processes

Purdue University

Cutaneous sensations You skin is one of the largest organs of your body

It includes a variety of sensory systems

Cutaneous sensations are those that are based on stimulationof receptors in your skin

Neural system connected to the skin is called thesomatosensory system

Sensations include:

Touch: tactile sensation

Temperature

Nociception: Pain

Proprioception: position of the limbs in space

Kinesthesis: movement of the limbs

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Mechanoreceptors

Embedded in the skin

are four types of

receptors that respond to

different types of touch

Merkel receptor

Meissner corpuscle

Ruffini cylinder

Pacinian corpuscle

Prof. Greg Francis

4PSY 310: Sensory and Perceptual Processes

Purdue University

Mechanoreceptors

Embedded in the skin

are four types of

receptors that respond to

different types of touch

Merkel receptor

Meissner corpuscle

Ruffini cylinder

Pacinian corpuscle

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Pacinian corpuscle Large enough to see with the naked eye

Layered like an onion

Prof. Greg Francis

5PSY 310: Sensory and Perceptual Processes

Purdue University

Pacinian corpuscle Pressure on the connective lamina opens pathways

for exchange of potassium and sodium

This leads to a neural response

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Neural response The frequency of action potentials codes the

amount of pressure above a threshold

Prof. Greg Francis

6PSY 310: Sensory and Perceptual Processes

Purdue University

Stimulus responses Different receptors respond preferentially to different kinds of stimuli

Meissner’s are found very near the surface and respond best tochanging details, giving a perception of flutter

Merkel’s are near the surface and respond best to unchangingdetail (pressure)

Ruffini’s are found deep in the skin and respond best to unchanginggross movement (stretching)

Pacinian are found deep in the skin and muscles and respond bestto changes in gross movement (vibration)

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Temporal responses Merkel receptor – slow (~1 Hz)

pressure (top layers of skin)

Meissner corpuscle – medium fast (~10 Hz)

flutter

Ruffini cylinder – fast (~ 100 Hz)

stretching

Pacinian corpuscle – very fast (~400 Hz)

vibration

Prof. Greg Francis

7PSY 310: Sensory and Perceptual Processes

Purdue University

Receptor adaptation The fibers connected to the receptors adapt to

constant pressure. Substantial differences acrossreceptor types

Slow adapting (SA): Merkel and Ruffini

Continue to fire throughout a sustained stimulus

Rapid adapting (RA): Meissner and Pacinian

Fire only at the onset and offset of a sustained stimulus

We are mostly interested in changes in mechanicalcontact with our bodies

Change is important to identifying threats or opportunities inour environment

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Receptive field size Receptors respond differently to the spatial positions

of stimuli

Small receptive field: Meissner and Merkel

Large receptive field: Ruffini and Pacinian

Prof. Greg Francis

8PSY 310: Sensory and Perceptual Processes

Purdue University

Neural connections Different receptors have nerves that

keep the responses separated

The brain can know which receptoris responding by which neuron isfiring

Similar to place coding on the basilarmembrane

Similar to retinotopic coding on theretina

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Neural fibers Often combine the adapting properties and receptive

field sizes to label the neural fibers connected tothese receptors

RA1: Meissner (flutter)

Rapid adaptation, small receptive field

RA2: Pacinian (vibration)

Rapid adaptation, large receptive field

SA1: Merkel (pressure)

Slow adaptation, small receptive field

SA2: Ruffini (stretching)

Slow adaptation, large receptive field

Prof. Greg Francis

9PSY 310: Sensory and Perceptual Processes

Purdue University

Receptive field size The size of the receptive field is related to spatial resolution and

discrimination

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Surface detection The properties of the receptors explains why we move

our hands to feel the properties of a surface

Place the index finger of your right hand on the desktop

Place the index finger of your left hand on a piece ofpaper

If you do not move them, it is difficult to judge which ismore smooth

Now gently move your hands to lightly touch thesurface

It is now easy to tell that the paper is rougher than thedesk top

Prof. Greg Francis

10PSY 310: Sensory and Perceptual Processes

Purdue University

Surface detection Different surfaces have

different physical properties Bumps and crevices

Discriminations of thesebumps requires fine spatialresolution Meissner or Merkel

The Meissner is moresensitive than the Merkel, soit’s behavior is what limits yourability

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Surface detection But the Meissner connects to the RA1 fiber

It responds only to onset and offset of a stimulus

So, keeping the hand still will not generate anyresponse

You have to move the hand around to get the RA1 fiberto respond to the changes in the bumps

For other kinds of discriminations (e.g, rapidly vibratingstimuli), other fibers will determine your resolution

Prof. Greg Francis

11PSY 310: Sensory and Perceptual Processes

Purdue University

Braille letters Raised dots that allow blind people to read

Why not just use the standard alphabet letters?

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Braille letters Compare the roman letters A and C and their

Braille counterparts

Prof. Greg Francis

12PSY 310: Sensory and Perceptual Processes

Purdue University

Braille letters When blurred, similar to the spatial resolution of the

fingertips The letters A and C are indistinguishable

Their Braille counterparts can be discriminated

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Thermoreceptors There are also

receptors in the skinthat respond to hotand cold

Neural fibers respondto changes intemperature

Prof. Greg Francis

13PSY 310: Sensory and Perceptual Processes

Purdue University

Why so many receptors? Many perceptual experiences are combinations of

responses from several different receptors

For example, there is no receptor for wet

We experience the sensation of wetness when we geta response from Meissner: touch

Merkel: pressure

Cold fibers: temperature

It’s similar to how three cone receptors allow formillions of perceived colors

There may also be millions of different touch-relatedperceptual experiences

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Conclusions Touch

Receptors

Fibers

Adaptation

Receptive fields

resolution

Braille system

Temperature

Prof. Greg Francis

14PSY 310: Sensory and Perceptual Processes

Purdue University

Next time

Cortical representation of touch

Two-point thresholds

Sensory homunculus

Pain perception