Body space and Sense of Touch December 2003

Nor was it without some reason that I believed that that body which, by a special right, I call mine, belonged to me more properly and closely than any other; for, in truth, I could never be separated from it as from other bodies; I felt in it, and by it, all my appetites and affections, and, finally, I was affected by the feelings of pleasure and pain in its parts, and not in the parts of other bodies which are separated from it. Descartes (1637) Sixth Meditation

Duality of body representation We experience our body both exteroceptively, as a physical object, and interoceptively, from “the inside”

Duality of body representation Exteroception: The body is experienced as spatially extended, oriented and composed of parts Penfield & Rasmussen (1950)

Duality of body representation Interoception: The body is inextricably linked to “me” Everywhere in the world, self begins with body (Baumeister, 1999)

Body representation: implications Consciousness and self-consciousness in tactile perception Body representations: - bridge between the self and the world - link between consciousness and self- consciousness

General methodology Speeded vs unspeeded perception tasks Threshold as a measure of perceptual function How good is the sense of touch? What is tactile spatial resolution?

General methodology Tactile spatial resolution: - depends on density of skin receptors - reflects somatosensory cortex homunculus Methods: - 2 point discrimination threshold - gap detection

Measuring thresholds (easy way) Staircase methods of finding thresholds: compare 2 stimuli (“test” and “reference”) along a dimension. E.g., “which is bigger?”, “which one has the gap in it?” A. start easy, reduce difference between stimuli in fixed steps by changing one stimulus (“test”) to make it more like the “reference” B. if subject makes errors: (1) note value of this “reversal” (1) increase difference to make task easier (2) reduce step size C. after 3 correct responses, reverse again, halve step size again

Typical Staircases

Measuring thresholds (easy way) Keep adjusting direction of test stimulus change (easier/harder) and step size (rate of change) until N reversals obtained Threshold (smallest difference between stimuli subject can detect) is: final value of staircase, or average of last few (e.g., N-2) reversals Note: 1. N>3 under all circumstances!!! 2. Check threshold estimate stable from one staircase to next 3. Be patient: staircase estimation can take time 4. Several more elaborate and efficient methods exist (see: e.g., Garcia-Perez MA (200). Optimal setups for forced-choice staircases with fixed step sizes. Spatial Vision, 13, )

Other features of methodology 1. Report exact details of how threshold obtained (staircase rules). 2. Report stability of threshold estimates from one staircase to next 3. Always within-subjects/ repeated-measures designs (compare each subject’s threshold in condition A to their threshold in condition B) 4. Minimum 8 subjects (hint: to make best use of time, restrict number of conditions, and focus on number of subjects) 5. Subjects should be blindfolded throughout (vision dominates touch) 6. Mark precise location of stimuli on skin with pen

Multimodal body perception Vision gives us exteroceptive body information Touch gives us interoceptive body information How do these interact? 1. Measure gap detection while viewing: - own arm, other person’s arm, neutral object Kennett S, Taylor-Clarke M & Haggard P (2001). Noninformative vision improves the spatial resolution of touch in humans. Current Biology 11: Johnson KO, PHILLIPS JR (1981) TACTILE SPATIAL-RESOLUTION.1. 2-POINT DISCRIMINATION, GAP DETECTION, GRATING RESOLUTION, AND LETTER RECOGNITION. J.Neurophysiol. 46:

Multimodal body perception 2. Measure gap detection on: - viewed finger, occluded adjacent finger - compared to neutral object in location of viewed finger, or location of occluded adjacent finger Does benefit of visual-tactile enhancement spread to adjacent fingers?

Tactile receptive fields Lateral inhibition between neurons in S1 Sense of touch impaired by interference on adjacent digits Distractor enters the tactile RF, and reduces the signal to noise ratio

Tactile receptive fields 3. Measure 2PDT or gap detection with/without constant buzzing on adjacent digits/skin areas - Does interference increase the tactile threshold? - Is effect reduced by splaying fingers? If so, then tactile RFs are modulated by external space as well as body space Ref: Tanosaki M, Suzuki A, Kimura T, Takino R, Haruta Y, Hoshi Y, Hashimoto I (2002) Contribution of primary somatosensory area 3b to somatic cognition: a neuromagnetic study. Neuroreport 13:

Body space and external space External space is geometric Body space is distorted

Cortical size illusion An object placed on the hand/lips feels larger than the same object placed on the arm/back This may reflect larger brain representation of the hand/lips than arm/back Output of the S1 homunculus is insufficiently rescaled Ref:Greene B (1982). The perception of distance and location for dual tactile pressures. Perception-and-Psychophysics Apr; Vol 31(4): Perception-and-Psychophysics

Cortical size illusion 1. Cortical plasticity: size of S1 territories varies as we use them. Get people to recognise tactile numbers traced on the unseen upper arm for 20 minutes. Does the difference in perceived size of objects on finger and trained arm diminish? pre-test, tactile training on arm, post-test Gandevia and Phegan (1999). Journal of Physiology, 514,

Cortical size illusion 2. Does viewing a body part modulate the size illusion? Vision may drive the rescaling process: should reduce the illusion even if non- informative. Compare perceived size of objects on unseen right hand and right forearm, in 3 visual conditions: - blindfold, gazing ahead - viewing untouched LEFT hand and forearm in position where right hand felt to be, using an interposed mirror (Ramachandran et al 1999., Medical Hypotheses, 52, 303-5) - viewing neutral objects in the mirror at location of right hand/arm Test ANOVA interaction between body part and viewing condition

Cortical size illusion 3. The hand and face have overlapping representations in somatosensory cortex: this is why phantom limbs can be felt when touching the face. Can touch on the face bias the interpretation of finger inputs? Test perceived size of objects on finger and arm in 2 conditions - control condition - bias condition in which a constant pressure is applied to the ipsilateral side of the face - could work better if the tested hand is spatially near the face

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 (Sorry, no page numbers) 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), Bulletin of Psychonomic Society, Brain, Somatosensory and Motor Research Try the following keywords: touch, tactile, haptic, cutaneous sensation, body representation, body schema

Ethical issues Be gentle! (avoid touching eyes, eyelids, mouth etc). Be hygienic! Be respectful! Have fun!

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