tactile perception according to texts of Vincent Hayward, J.J Gibson. florian wille // tactile perception // // 1 of 15

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1 tactile perception according to texts of Vincent Hayward, J.J Gibson. florian wille // tactile perception // // 1 of 15

2 tactile vs visual sense The two senses complement each other. Where as the visual sense is perfect for long range, the tactile sense helps to judge on close range. At the extreme case of a knife-edge, sharpness is more accurately judged by touch than by vision. 1 1 J.J. Gibson, 1962, p485 florian wille // tactile perception // // 2 of 15

3 tactile vs visual sense limbs are motor as well as sensing organs - where as the eyes are only sensing organs. 1 solid objects are tangible and visible (exceptions: smoke is not tangible, glass may not be visible). 2 some characterisitics are can just be judged by one or the other: colour - visible, temperature - tangible, etc. 2 1 see J.J. Gibson, 1962, p478 2 see J.J. Gibson, 1962, p488 florian wille // tactile perception // // 3 of 15

4 ...but still quite a lot in common Like with the visual sense certain constants are applying: 1 ì Unity of the phenomenal object. ì Stability of the phenomenal object. even though touched with two or more fingers the object is perceived as one. when sliding your hand over an object you realise that it is your hand that`s moving, not the object. ì Rigidity or plasticity of the phenomenal object. when sqeezing an object the joint-and-tendon receptors and the cutaneous and deep-tissue receptors, make out the rigidity of an object. ì Shape of the phenomenal object. One perceives the object-form but not the skin-form. 1 see J.J. Gibson, 1962, p481 ff florian wille // tactile perception // // 4 of 15

5 illusions ì aristoteles illusion Cross you fingers and touch your nose. You defeat the concept of unity - feeling two surfaces instead of one. It`s the other way around if you touch an inside corner. ì two boxes If you`ve got two boxes of different size but same weight - the smaller box will appear heavier. Of two boxes of sames size and weight the one labeled heavy will appear heavier. Also cold objects appear heavier than warm objects. ì parchment skin illusion A blackboard appears smoother if you use earplugs while writing on it with chalk. florian wille // tactile perception // // 5 of 15

6 experiment the rubber hand florian wille // tactile perception // // 6 of 15

7 florian wille // tactile perception // // 7 of 15

8 Is active touch a blend of two modes of sensation, kinesthesis and touch proper? According to J.J. Gibson this explaination fails to take account of the purposive character of touching and, second, it fails to emphasize the multiplicity of so-called kinesthesis. Kinesthesis J.J. Gibson suggests that the term Kinesthesis is not accurate any more. Historically the word refers to movement of the body, and originally meant the muscle sense. But it is known that there is sensitivity to the position of the body and of all its members relative to one another. This is an articular sense, not a muscle sense (...). 1 1 J.J. Gibson, 1962, p478 florian wille // tactile perception // // 8 of 15

9 ì passive touch 49% correct matches ì passive touch with rotation 72% correct matches ì active touch 95% correct mathes florian wille // tactile perception // // 9 of 15

10 your toe knows what your finger is doing. florian wille // tactile perception // // 10 of 15

11 experiment ì hit the tabletop First, hit with the st a table top to appreciate its resistance. From the oscillations, there is a clear perception of its elasticity. If the same fist rests the surface but an assistant bangs the table at some distance, then eectively the same tactile stimulus gives rise to the perception of a distant shock rather than something about the mechanics of the table. florian wille // tactile perception // // 11 of 15

12 different ways of perceiving motion of an object by touch. ì Translatory motion over the skin. distance illusion ì Friction at a single area of the skin. ì Passive rotation of the joints of the limb. point your finger at nose. ì Friction on the skin combined with active movement of the arm. We are able ot trace a path of pressure across the skin, and identify the item that touched the skin. We are able to detect movement as friction on our skin down to as speed of 1mm/sec. Also able to discriminate if we or the object is moving. If only the receptors in the joints of the limb are stimulated we can judge their absolute position. Striking an object that is moving we are able to discrminate between the objects movement and the movement of our hand (object - subject movement). florian wille // tactile perception // // 12 of 15

13 Perception of the texture, substance, and shape of an object by touch ì Curvature as against planarity of a surface ì Slant of a surface with respect to gravity ì Parallelity of two surfaces ì Distance between two surfaces ì Plane-angle, or edge of a surface ì Edge-angle, or corner of a surface Can judge curvature objects of complex geometry like saddle shaped objects. Able to judge the slant of a adjustable palm board with a precision seldom exceeding 2-3. Able to bring to adustable palm boards in parallel position with small errors. Distances can be felt acurately. We are also able to judge if the finger-span of one hand is equal or different to the other. The angle is perceptible as varying from sharp to dull - often more acurately judged by touch than by vision. The fingers appear to seek corners and edges of objects. florian wille // tactile perception // // 13 of 15

14 what? J.J. Gibson 1962 Parallelity of two surfaces. If a pair of adjustable palmboards is held between the two hands, behind a curtain, they can be adjusted to the parallel position with small errors. Vincent Hayward 2007 Judgement of orientation can also yield surprising results [46]. It can easily be demonstrated by asing a blind-folded volunteer to orient two pencils on a table in the same direction, say by sixty degrees toward the right, with one pencil in each hand. The directions in which the pencils point will be quite different. florian wille // tactile perception // // 14 of 15

15 illusions ì Distance Missjudgement Run two of your fingers over your skin. As you get closer to a sensitive region the distance between the fingers appears bigger. florian wille // tactile perception // // 15 of 15