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Body Awareness lab psychol.ucl.ac.uk/patrick.haggard/lab_2006

Body Awareness lab http://www.psychol.ucl.ac.uk/patrick.haggard/lab_2006.ppt. December 2006. 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.

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Body Awareness lab psychol.ucl.ac.uk/patrick.haggard/lab_2006

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  1. Body Awareness labhttp://www.psychol.ucl.ac.uk/patrick.haggard/lab_2006.ppt December 2006

  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. Some basic approaches • Tactile resolution • Tactile distance • Tactile pattern integration

  9. 2 Point Discrimination Threshold (2PDT). How good is sense of touch? - depends on density of skin receptors - reflects somatosensory cortex homunculus - acuity, spatial resolution - 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 measure of primary 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 (about 25%) • Prevents subject always responding “Two” • Report errors, but don’t analyse • Method of limits or Method of Constant Stimuli

  17. Modified method of limits (Staircase) Start Easy (wide separation) Reduce separation until subject says “One” REVERSAL Increase sepation in smaller steps until subject says “Two” REVERSAL Continue for as long as practical with reducing step size Average last 2 reversals Intersperse catch ‘1 tap’ trials (not analysed) 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. Method of constant stimuli FIRST: find approximate 2pdt in a baseline condition THEN: deliver a fixed set of stimuli just below and above each person’s 2pdt (e.g., on arm, 20-35 mm delivered 10 times each) Include catch 1-tap trials, but don’t analyse Compare %age 2-tap trials correctly perceived as “two” Repeat in different experimental conditions %2-tap trials correctly perceived as “Two” 2-point separation (mm) 1 5 10 15 20 25

  19. 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

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

  21. Viewing the body improves 2PDT, compared to viewing neutral objects • Kennett, Taylor-Clarke, Haggard (2001) Current Biology, 11, 1198-1201. • Visual-tactile enhancement

  22. View Other’s Hand View Object View Own Hand Social Cognitive Neuroscience of TouchHaggard, Social Neuroscience, 2006

  23. Generic tactile resolution designs • Research question: does X influence tactile resolution? • Dependent variable: 2PDT estimate/%correct • Repeated measures/within subjects • Counterbalancing very important: training effects • Gaze direction (and ideally gaze depth) MUST be fixed: spatial setup very important • 10-12 subjects normally enough

  24. 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 for 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 neutral object (see Kennett et al., Current Biology 2001). • Fingertips can be too sensitive for 2pdt: use back of fingers or palm

  25. 2pdt research questions (cont)? • Somatotopic congruence: view little finger, touch on index finger • Touch right index finger • Use a mirror so subject sees L little or L index finger at location of R index finger • Mask mirror so subject sees just one finger • Does viewing other people’s hands enhance touch on one’s own hand?(Haggard, Social Neuroscience, 2006) • Does the orientation of the other person’s hand matter? • 1st person orientation, 3rd person orientation, own hand, neutral object

  26. 3 basic themes • Tactile resolution • Tactile distance • Tactile pattern integration

  27. Tactile distance • Touch blindfold subject with suprathreshold distance • Verbal report “feels like 44 mm” • Absolute estimatation Method • Use 3-5 distances, centred on same point, • several trials of each distance, random order • Use distances slightly above 2pdt for that body part(Haggard et al, Current Biology, 13, R170-173 for values) • Within-subjects, repeated-measures design • Distance judgement depends on stimulus properties • And 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.

  28. Tactile distance: questions • Does movement across skin increase perceived distances, relative to static stimuli? Does movement speed matter? • Does tactile movement that is pleasant (velvet) increase perceived distance relative to movement that is unpleasant (sandpaper)? • Do distances between points applied with high pressure feel the same as between points applied with low pressure? • Does tactile distance perception on the lower leg depend on body height? (Control: equivalent distances on forehead. Use regression)

  29. 3 basic themes • Tactile resolution • Tactile distance • Tactile pattern integration

  30. Tactile pattern integration • How does the brain build up the overall spatial form of an object from several distinct touches? • P.J. Fitzgerald, J.W. Lane, P.H. Thakur and S.S. Hsiao, Receptive field (RF) properties of the macaque second somatosensory cortex: RF size, shape, and somatotopic organization, J. Neurosci.26 (2006), pp. 6485–6495. • P.J. Fitzgerald, J.W. Lane, P.H. Thakur and S.S. Hsiao, Receptive field properties of the macaque second somatosensory cortex: representation of orientation on different finger pads, J. Neurosci.26 (2006), pp. 6473–6484. • Haggard P. From skin to object in the somatosensory cortex. Current Biology, 16, R885-R886.

  31. Figure 1. Three forms of orientation selectivity in secondary somatosensory cortex. The lower row shows digit pads which excite (red) or inhibit (blue) the firing of a cortical neuron. (A) A neuron with both excitatory and inhibitory regions within its receptive field. The tactile shape to which the neuron optimally responds is shown in blue in the upper panel. (B) Signals from two neurons with purely excitatory and purely inhibitory responses in complementary regions of the fingerpads could be integrated by a higher order neuron to give the same selectivity for shape. (C) A neuron whose response varies with the orientation of a stimulating bar. The optimal orientation on each pad within the receptive field is shown. An optimal stimulus for this neuron would be an object with the same gross orientation as A, but with additional oriented structure, such as corrugations.

  32. Are all the orientations the same? • YES • NO

  33. Are all the orientations the same? • NO

  34. How many degrees of orientation can be just detected? • Method of constant stimuli • 50% ‘all same’, 50% ‘not all same’ • Plot probability ‘not all same’ against orientation change on target finger • Find 75% correct threshold • Does it matter which finger is the odd one out? Are the central fingers better integrated than the outlying ones? CONTROL: show this is not due to acuity/2pdt differences

  35. Is an orientation change of N degrees on one finger more or less detectable than an orientation change of N/2 on 2 fingers? • Compare 75% thresholds between conditions, within subjects • Does it matter whether the 2 target fingers are adjacent or not? • Is the Standard Dev of orientation across all 4 fingers a good predictor of orientation discrimination?

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

  37. 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

  38. Useful references This presentation is at: http://www.psychol.ucl.ac.uk/patrick.haggard/lab_2006.ppt

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