Body Awareness lab

December 2004

Duality of body representation

• The human body– Sensory surface

– A physical object

– “Me”

• Everywhere in the world, self begins with body(Baumeister, 1999)

General principles

• Spatially organised map

• Multimodal (touch, proprioception, vision)

• Plasticity

Body Surface Map

• Primary somatosensory cortex

• Raw sense of touch

• spatially extended

• composed of parts

• disproportionate

Penfield & Rasmussen (1950)

Body surface: Key concepts

• 1. Receptive field

Def: the part of the body that a neuron receives from

Body surface: Key concepts

• 2. Threshold

The point at which some feature of a stimulus just becomes detectable

Measure how much information the brain has about the stimulus

Absolute detection threshold: “I can just feel it”

Body surface: Key concepts

• 2. Threshold

The point at which some feature of a stimulus just becomes detectable

Measure how much information the brain has about the stimulus

Absolute detection threshold: “I can just feel it”

Motion threshold: “I can just feel that it is moving”

Spatial threshold: “I can just feel its spatial form”

2 Point Discrimination Threshold (2PDT)

Tactile spatial threshold:

- depends on density of skin receptors

- reflects somatosensory cortex homunculus

Methods:

- 2 point discrimination threshold

- measure of spatial resolution

Body surface: Key concepts

Stimulus on finger

Brain activity

Neurons with finger RFs

Body surface: Key concepts

Brain activity

Neurons with finger RFs

2 close stimuli

Close stimuli: same set of neurons: feels like ONE stimulus

Body surface: Key concepts

Brain activity

Neurons with finger RFs

2 distant stimuli

Distant stimuli: different sets of neurons: feels like TWO stimuli

2PDT

2PDT is a measure of the separation between two tactile stimuli at which the subject can just tell there are two stimuli rather than one

Corresponds to size of the receptive field of a virtual neuron in somatosensory cortex

Easy, sensitive measure of touch information

2 Point Discrimination Threshold (2PDT)

2 Point Discrimination Threshold (2PDT)

One

2 Point Discrimination Threshold (2PDT)

Er, one?

2 Point Discrimination Threshold (2PDT)

Two!

Effective RF size

2 Point Discrimination Threshold (2PDT)

One!

DUMMY TRIAL: stops subjectSaying ‘TWO’ continually

2 Point Discrimination Threshold (2PDT)

Two!

Easy dummy trial: remind what 2Feels like

2 Point Discrimination Threshold (2PDT)

Tactile spatial threshold:

- face/hands have many skin receptors, and small RFs

- reflects somatosensory cortex homunculus

Methods:

- 2 point discrimination threshold

- measure of spatial resolution

2PDT methods

• Prepare 2 point stimuli

• Range of separations appropriate for body part

• Random order

• Catch trials with 1 stimulus (25%)

– Prevents subject always responding “Two”

– Report errors, but don’t analyse

• 5-10 presentations per stimulus

2PDT methods

• Results

• 2PDT = separation at which subjects say “Two” 75% of the time

2PDT on palm

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Separation (mm)

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2PDT problems

• Watch out for:

• Training effects (give everyone same practice, ideally 5 minutes minimum)

• Vision (subject mustn’t see if it’s 2 or 1)

• Exploratory movements

Modulating the sense of touch

• Use 2PDT as a measure of tactile resolution

• What factors influence tactile resolution?

• Repeated measures, within subjects:– Visual adaptation (e.g., compare 2pdt on index finger pre-post 30

minutes viewing fingertip at high or low magnification: think of suitable visual task w. feedback to maintain visual attention).

– Haptic training (e.g., compare 2PDT pre-post 30 minutes blindfold coin sorting, with feedback)

Modulating the sense of touch

• Viewing the body improves 2PDT

• Kennett, Taylor-Clarke, Haggard (2001) Current Biology, 11, 1198-1201.

• Visual-tactile enhancement

Modulating the sense of touch

• Does visual-tactile enhancement effect extend to:

– Viewing other people’s bodies

• Dependent variable: 2PDT estimate

• (View object/view body) * +/- other factor

– (self/other)

• Look for interaction in 2x2 ANOVA

• Counterbalancing very important: training effects

• Gaze direction and gaze depth MUST be fixed: spatial setup very important

• 10 subjects normally enough

From body surface to body space

How do we perceive our own body?

• Touch subject at one of several locations

• Errors are bigger and more interesting if touch is very light (Q-tip, fine paintbrush)

• Rapp et al. (2002) Neuroreport, 13, 207-211

• Blindfold subject attempts to point to location with other arm (pens, OHP slides, graph paper, digital camera etc).

• Several repetitions, random order, calculate mean error, infer subjective body shape

• Stats: ANOVA on mean errors

From body surface to body space

Ideas:

1. Measure localisation errors using the Rapp method with:

- Variable delays 0, 2, 10 s between being stimulation and response

- An active displacement of the touched hand, passive displacement by experimenter, or no displacement between stimulation and response (keep delay constant).

2. Are localisation errors on the arm aligned along the arm like the tactile RFs?

- even when arm is rotated?

- even with 5 s delay between touch and localisation movement? (http://firmin.lyon.inserm.fr/534/534YR-Gros.pdf see p. 163)

3. Measure localisation errors on the arm and abdomen in people with/without high levels of body image concern (Think carefully: needs 40 subjects to be worth it!)

Tactile patterns

Almost all work on touch involves a single focal stimulus

How do we integrate large areas of body surface to represent patterns?

Tactile patterns

E.g.,

Collinearity judgement

“Middle one is above”

“Middle one is below”

Tactile patterns

Methods: Either:

* 2 Alternative Forced Choice (2AFC)

Move middle one up/down at random. Identify distance of middle one from line at which subjects score 75% correct (50%=chance)

OR

* Method of adjustment

Gradually move middle one from above or below until subject says “I feel the points are colinear”. Record distance from the line

Tactile patterns

Methods: 3 Simultaneous Qtip stimuli, blindfold

Body parts: palm, cheek, abdomen, forearm-upperarm

Questions: Compare geometry of judgements on same body part:

above/below horizontal line

left/right of vertical line: Is there a body axis effect?

Does making the subject lean to one side impair performance?

(Spidalieri, J Neurophysiol, 78, 545; Spidalieri, Neuroreport, 10, 2473)

How is performance changed if the line terminators span 2 body parts: is pattern integratio worse over upper+forearm than forearm alone or upper arm alone (keep arm still, keep distance constant!)

Ethical issues

Be gentle!

(avoid touching eyes, eyelids, mouth etc).

Be hygienic!

Be respectful. Respect bodies, persons and cultures!

Have fun!

Useful references

A general intoduction to the idea of body representation:Berlucchi G., Aglioti S. The body in the brain: neural bases of corporeal awareness. Trends in Neurosciences, 1997

A general introduction to the sense of touch can be found in most neuroscience textbooks, e.g.,Goldstein: Sensation and Perception,

Kandel Schwarz and Jessell, Essentials of Neural Science and Behaviour, ch 18.,

To find ideas for experiments, try the following journals:

Perception and Psychophysics, Journal of Experimental Psychology: (Human Perc and

Perf), Brain, Neuroreport, Experimental Brain Research

Try the following keywords:touch, tactile, haptic, cutaneous sensation, body representation, body schema

Useful references

This presentation is at:

http://www.psychol.ucl.ac.uk/patrick.haggard/lab_2004.ppt