Body Awareness lab psychol.ucl.ac.uk/patrick.haggard/lab_2005

1. Body Awareness labhttp://www.psychol.ucl.ac.uk/patrick.haggard/lab_2005.ppt December 2005

2. Duality of body representation • The human body • Sensory surface • A physical object • “Me” • Everywhere in the world, self begins with body(Baumeister, 1999)

3. General principles • Spatially organised map • Multimodal (touch, proprioception, vision) • Plasticity

4. Body Surface Map • Primary somatosensory cortex • Raw sense of touch • spatially extended • composed of parts • disproportionate Penfield & Rasmussen (1950)

5. Body surface: Key concepts • 1. Receptive field Def: the part of the body that a neuron receives from

6. 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”

7. 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”

8. 3 basic approaches • Tactile resolution • Tactile distance • Tactile field

9. 2 Point Discrimination Threshold (2PDT). How good is sense of touch? - depends on density of skin receptors - reflects somatosensory cortex homunculus - role of psychological body factors? Method: - 2 point discrimination threshold

10. 2PDT: Definition 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

11. 2 Point Discrimination Threshold (2PDT) Two!

12. 2 Point Discrimination Threshold (2PDT) One

13. 2 Point Discrimination Threshold (2PDT) Er, one?

14. Effective RF size of virtual Somatosensory neuron 2 Point Discrimination Threshold (2PDT) Two!

15. 2 Point Discrimination Threshold (2PDT) One! DUMMY TRIAL: stops subject Saying ‘TWO’ continually

16. 2PDT methods • Prepare 2 point stimuli • Range of separations appropriate for body part • Random order • Catch trials with one tactile stimulus (25%) • Prevents subject always responding “Two” • Report errors, but don’t analyse • Start easy (big separation), make harder… • Until errors occur… make easier/harder with decreasing step size • Threshold reached after 2 (or more) reversals

17. Typical Staircases: Method of Limits Arm in Darkness For many experiments it’s best to: FIRST: find 2pdt in a baseline condition THEN: deliver a fixed set of stimuli just below and above each person’s 2pdt (e.g., on arm, 22, 27, 32 mm delivered 5 times each) Analyse %age accuracy METHOD OF CONSTANT STIMULI 60 50 40 Separation of two tactile stimuli (mm) 30 20 10 0 1 5 10 15 20 25 30 35 1 5 10 15 20 25 Trial number

18. Generic Touch experiment problems • Watch out for: • Training effects (give everyone same practice, ideally 5 minutes minimum) • Vision (subject mustn’t see whether it’s 2 or 1) • Exploratory movements

19. Modulating the sense of touch • Use 2PDT as a measure of tactile resolution • What body factors influence tactile resolution?

20. Viewing the body improves 2PDT • Kennett, Taylor-Clarke, Haggard (2001) Current Biology, 11, 1198-1201. • Visual-tactile enhancement

21. Generic tactile resolution designs • Research question: does X influence tactile resolution? • Dependent variable: 2PDT estimate • Repeated measures/within subjects • Counterbalancing very important: training effects • Ideally use 2x2 ANOVA, look for interaction • Gaze direction and gaze depth MUST be fixed: spatial setup very important • 10 subjects normally enough

22. 2pdt research questions? • Does viewing hand enhance tactile resolution on the hand? (dull replication of previous work!) • Does viewing the hand just AFTER touch enhance resolution on hand?(never been studied. But on Tactile iconic memory: Harris, et al. (2002). Transient storage of a tactile memory trace in primary somatosensory cortex. Journal of Neuroscience, 22, 8720-8725 • Vary touch-vision interval, 1s, 5s, 10s • Movable cardboard screen unmasks hand just after touch • Does viewing the face (in a mirror) enhance tactile resolution on the hand? (face and hand overlap in SI) • Always compare view hand with view neutral object (see Kennett et al., Current Biology 2001).

23. 3 basic approaches • Tactile resolution • Tactile distance • Tactile field

24. Tactile distance • Touch blindfold subject with suprathreshold distance • Verbal report “feels like 44 mm” • Distance judgement uses implicit body representation De Vignemont F, Ehrsson H & Haggard P. (2005) Bodily illusions modulate tactile perception. Current Biology, 15, 1286-1290. Taylor-Clarke M & Haggard P. (2004). Keeping the world a constant size: Cortical Magnification and object constancy in human touch. Nature Neuroscience, 7, 219-220.

25. Tactile distance: questions 1. Is tactile distance perception biased by forces acting on body? • Judge distance on arm while holding heavy object • Arm hanging down, arm extended above head, control • Predict extension/compression of distance respectively • Use range of about 5 distances, all above the 2pdt • Dependent variable: reported distance • Repeated measures: Need full dataset!

26. Tactile distance: questions 2. Is tactile distance perception biased by active force/effort? • Middle finger bent over: sustained effort, but no movement • Control condition: middle finger at rest

27. Tactile distance: questions 3. Is there a bias in interhemispheric tactile distance estimation? • Judge distances on forehead • Left forehead, Right forehead, symmetrical about body midline • Back? Abdomen? 53 mm

28. 3 basic approaches • Tactile resolution • Tactile distance • Tactile field

29. Tactile field How do we perceive our own body? • Is the space in between two touches represented as a tactile field (like the visual field)? How do we integrate large areas of body surface to represent patterns? • Filler vibrotactile stimulus + distance judgement on arm? • Filler inside/outside distance/null • No literature! Ground-breaking!

30. Ethical issues Be gentle! (avoid touching eyes, eyelids, mouth etc). Be hygienic! Be respectful. Respect bodies, persons and cultures! Have fun!

31. 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 Haggard P, Taylor-Clarke M & Kennett S (2003). Tactile perception, cortical representation and the bodily self. Current Biology, 13, R170-R173. 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, somatosensory, body schema

32. Useful references This presentation is at: http://www.psychol.ucl.ac.uk/patrick.haggard/lab_2005.ppt