Tactile perception

Mariacarla MemeoRobotics, Brain and Cognitive Science(RBCS)

Multisensory mapping groupIstituto Italiano di Tecnologia (IIT), Italy

● Perceptual process● Human sense of touch

1. Somatosensory system

2. Mechanoreceptors

3. Tactile acuity

4. Receptive field

● Electric metaphor of mechanoreceptors

Outline

Perceptual process

Skin

Skin - heaviest organ in the body

Functions:

Provides tactile information.

Warns us of damaging stimuli.

Contains body fluids and organs.

Protects against bacteria.

Regulates body temperature.

Why touch is important?

Haptic senses—or “touch”—is the most basic of senses; we learn this before vision and smell.

Everyday Tasks:

● Grasping objects;

● Dialing a phone;

● Playing a guitar or piano;

● Finding a light switch;

● Feeling your pulse.

Touch is complex (e.g. tying a shoelace): bi-directional communication channel – both input & output

Human sense of Touch

Touch helps us identify objects and provides unique information (e.g., texture, friction)

Skin structure and mechanoreceptorsu

Somatosensory system

Human Sense of Touch

Somatosensory system

The 'somatosensory system' is a sensory that detects experiences labelled as touch or pressure, temperature(warm or cold), pain(including itch and tickle) and those that belong to proprioception(muscle movement and joint position).

Somatosensory Cortex

Damage to somatosensory cortex destroys ability to recognize objects by touch.

The body is mapped topographically onto somatosensory cortex, but body parts are not represented equally.

Somatosensory Cortex

ANTEROLATERAL SYSTEM

This system is involved in the perception of touch, temperature, and sharp pain. Next instead is selectively involved in the perception of deep, chronic pain.

Somatosensory Pathway

Mechanoreceptors

These are thetissue “filter”between stimulusand nerve ending .

Each type encodesa different aspectof deformation.

(Stimulus)Mechanical

filter

Transducer(mechanical energy →bioelectric signals)

Encoder

Ruffini cylinder

Pacini corpuscole

Meissner corpuscole

Merkel receptors

Two types located close to surface of the skin

Merkel receptor fires continuously while stimulus

is present.

Responsible for sensing fine details

Meissner corpuscle fires only when a stimulus is

first applied and when it is removed.

Responsible for controlling hand-grip

Mechanoreceptors

Mechanoreceptors

Two types located deeper in the skin

Ruffini cylinder fires continuously to stimulation

Associated with perceiving stretching of the skin

Pacinian corpuscle fires only when a stimulus is first applied and when it is removed.

Associated with sensing rapid vibrations and fine texture

Mechanoreceptors

Mechanoreceptors

Mechanoreceptors – temporal features

Mechanoreceptors – spatial features

Tactile acuity thresholds are determined by Merkel receptors (SA1).

● Measuring tactile acuity

● Two-point threshold - minimum separation needed between two points to perceive them as two units

Tactile acuity

● Measuring tactile acuity

● Grating acuity - placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating

Tactile acuity

Tactile acuity

Merkel receptors are densely packed on the fingertips - similar to cones in the fovea.

Both two-point thresholds and grating acuity studies show these results

The receptive field of an individual sensory neuron is the particular part of the body surface in which a stimulus will trigger the firing of that neuron.

The two-point threshold for any part of the body is determined by the size of the receptive fields and the extent of overlap.

Receptive Field

Tactile acuity is determined by how close the mechanoreceptors are to each other and by the size of the receptive field

Receptive Field

Inhibitory region

Excitatory

region

Receptive FieldMechanoreceptors found in areas of the body with less tactile acuity tend to have larger receptive fields.

Electrical metaphor

Ruffini cylinder

Pacini corpuscole

Meissner corpuscole

Merkel receptors

● Perceptual process (continue)● Meaning

1. Psychophysics

2. Absolute threshold

3. Differential threshold (Just Noticeable Difference)

● Tactile feedback

● Tactile technologies

● Simulate tactile perception (our robot)

Outline

Perceptual process

Science that focuses on how the physical environment is integrated into our personal, subjective world.

● Ernst Weber & Gustav Fechner -- psychophysicists

Psychophysics

● Weber published “De tactu” describing the minimum amount of tactile stimulation needed to experience a sensation of touch – the absolute threshold.● Using weights he found that holding versus lifting them gave different results

(due to muscles involved).

● He used a tactile compass to study how two-point discrimination varied across the body.● On the fingertip .22 cm, on the lips .30 cm,

on the back 4.06 cm.

Absolute threshold

Aesthesiometric compass

● Weber studied how much a stimulus must change in order for a person to sense the change.● How much heavier must a weight be in order for a person to notice that it is

heavier?

● This amount is called the just noticeable difference JND

Just Noticeable Difference (JND)

1)A stimulus is presented at astarting quantity Q.

1)Once the subject is adapted tothis stimulus, the attribute isincreased …

…each time the subject is asked…

yes or no

if there is a difference in thestimulus.

Just Noticeable Difference (JND)

After repeated trials theincrement T that produces a

perceptible difference

is identified:

75% of right difference inthreshold identified.

Just Noticeable Difference (JND)

After repeated trials theincrement T that produces a

perceptible difference

is identified.

It is calculated when the

75% of right difference inthreshold identified.

Just Noticeable Difference (JND)

When the starting quantity is thestimulus zero value (completeabsence of the stimulus), thenthe first jnd value T is called

… absolute threshold Q0…

This is the minimum stimulusquantity necessary to produceany sensation.

Just Noticeable Difference (JND)

● JND can be expressed as a ratio:

where (Q + Q0) is stimulus magnitude and k is a constant and DQ means the change in R (D usually means change).

or

● Weber's law asserts that the ratio k is constant across a wide range of stimulus quantities, although the value of k is different for different sensory domains or types of stimulus.

kQQ

T

)( 0

Just Noticeable Difference (JND)

kQQ

Q

D

)( 0

● Fechner related the physical and psychological worlds using mathematics.

● Fechner (1860) said:“Psychophysics, already related to physics by name must on one hand be based on psychology, and [on] the other hand promises to give psychology a mathematical foundation.” (pp. 9-10)

● Fechner called Weber’s finding about the JND “Weber’s Law.”

Fechner Contribution

● Fechner's Law allows the estimation of the sensory intensity (S) from the stimulus quantity:

where S is sensation, k is Weber’s constant and R is the magnitude of a stimulus

● The larger the stimulus magnitude, the greater the amount of difference needed to produce a JND.

)log( 0QQkS

Fechner Contribution

Relationship of JND with Stimulus

)log( 0QQkS

● His methods are still used in psychophysics.

● Ideas from signal detection theory have been applied to a wide variety of other topics.● Threshold for criminal behavior, scenic beauty.

● Scaling techniques, including rating scales, were placed on a sound scientific basis, especially by S.S. Stevens modified Fechner’s Log

Law to a Power Function in the early 1950’s.

Fechner Contribution

Goal is to stimulate the skin in a programmable manner to create a desired set of sensations.

Tactile feedback is generated by a tactile device, sometimes called a tactile displays or tactile interfaces.

Tactile feedback aims to create a metaphor to communicate specific information.

Tactile Feedback

Pins or other mechanical vibrating elements, such as servo motors - either alone or in an array, as in devices for Braille display

● typically used for fingertip stimulation

Tactile Technologies

Wagner & Howe 2002

Kyung et al. 2006

Brayda et al. 2010

Cannella 2009

Tactile device, able to deliver simple tactile maps of virtual objects by means of heights information.

TActile MOuse (TAMO)

Cushions of air can be inflated or deflated to vary pressure on skin

Tactile Technologies

Okamura et al. 2014Chih-Hung King 2008

Siarhei Vishniakou et al. 2013

● Skin stretch to simulate moving stimuli.

● Electro-tactile devices.

Hayward et al. 2003-7Provancher et al. 2013

Kajimoto laboratory 2014

Tactile Technologies