Touch

1. Touch

2. The Skin Functions: Provides tactile information. Warns us of damaging stimuli. Contains body fluids and organs. Protects against bacteria. Regulates body temperature.

3. Touch Touch helps us identify objects and provides unique information (e.g., texture) Touch is important in development and social interaction (e.g., Harlow’s monkeys).

4. The skin on the hand contains thousands of pressure-sensitive mechanoreceptors. Touch acuity is best when fingers move over the object of interest.

5. Measuring sensitivity and acuity of touch Pressure sensitivity is greatest for fingertips and lips (the lightest touch). The back is least sensitive to pressure. Females more sensitive than males. Vibrotactile sensitivity is greatest for palms. Females are more sensitive than males.

6. Measuring sensitivity and acuity of touch Touch acuity (pattern acuity) is measured using the two-point threshold test. Tactile contrast sensitivity. Sensitivity Linked to centre/surround receptive fields in the thalamus. Braille. Spatial frequency

7. Neural Processing for Touch Receptive field sizes of the different mechanoreceptors determines our ability to discriminate fine details. Sensitive body parts have higher density of RA1 and SA1 fibers.

8. Receptive Fields Receptive fields in the thalamus have centre-surround organization. Cortical receptive fields (left) are smaller in the fingers and larger on the hand and forearm.

9. Neural Processing for Touch Two-point threshold:the smallest discriminable distance between two points

10. Localization of tactile stimulation Localization ability enables judgment of where stimulus has been applied to the skin. Tactile judgments of relative position are highly accurate.

11. Perception of surface texture Surfaces have unique “texture signatures” (e.g., coarse vs. fine). Gratings of different spatial frequencies measure tactile acuity.

12. Tactile sensitivity (temperature) Touch temperature is the perception of surface temperature. Objects differ in thermal conductivity. Touch temperature is based on temperature gradient between object and skin.

13. Thermoreceptors Located just below the skin. Continuous nerve impulses at a certain temperature. Small receptive fields (less than 1mm2). There are spaces between receptive fields (“blind spots”). Two classes: (1) cold receptors (2) warm receptors Responds to CHANGES in temperature!!!! Perceived temperature depends on the state of the receptors.

14. Thermoreceptors • Warm Fibers: • increased responding with increasing temperature- sustained firing- decreased firing when temperature decreases- do not respond to mechanical stimulation • Cold Fibers: • increased responding with decreasing temperature- sustained firing- decreased firing when temperature increases- do not respond to mechanical stimulation

15. Mental set and tactile sensitivity Uncertainty makes tactile discrimination more difficult. Advance information improves the identification of a tactile stimulus. Practice also improves tactile discrimination.

16. Touch Fibers Each nerve fiber signals touch to a specific area of the skin (the fiber’s receptive field).

17. Temporal properties Slowly adapting (SA) fibers respond to initial stimulation and continue responding (perception of light, uniform pressure). Rapidly adapting (RA) fibers respond only to start and stop points of stimulation (perception of buzzing vibration).

18. Spatial properties Punctate fibers have small receptive fields with sharply defined boundaries. Diffuse fibers have large receptive fields with fuzzy boundaries.

19. The four-channel model: Mechanoreceptors Spatial Property Punctate Diffuse Meissner corpuscles (transient stimulation) Pacinian corpuscles (very sensitive with large RFs) RA SA Temporal Property Merkel disks (steady pressure of small object) Ruffini endings (steady pressure and stretching)

20. Properties of Mechanoreceptors

22. Ascending pathways for touch Reflex (OUCH!!!!) Fibers (receptors) spinal cord interneurons muscle Sensory Analysis Fibers (receptors) spinal cord lemniscal neurons brainstem

23. Somatosensory Cortex Damage to somatosensory cortex destroys ability to recognize objects by touch.

24. The body is mapped topographically onto somatosensory cortex, but body parts are not represented equally. Homoculus

25. Homunculi

26. Tactile Object Recognition Passive Touch vs. Active Touch Haptic perception: exploration of 3D objects with the hand Exploratory procedures (Lederman & Klatzky)

27. Exploratory Procedures http://psyc.queensu.ca/~cheryl/labpage.html

28. Visual and Haptic Object Recognition Haptic perception: involves coordination of motor, sensory, and cognitive systems Amedi, Jacobson, Hendler, Malach and Zohary (2002). Cerebral Cortex, 12(11), 1202-1212

29. Somatosensory Cortex Certain cortical neurons respond selectively to orientation and direction.

30. Disorders related to somatosensory cortex A person with unilateral neglect denies ownership of limbs on one side of the body. A person with a phantom limb experiences sensation from a limb that no longer exists.

31. Phantom Limbs Amputees often report that they can still feel their missing limb, and sometimes this is painful! Referred sensation: stimulation of one part of the body results in a sensation on another part of the body (i.e. the phantom limb). The amount of functional cortical reorganization is positively correlated with the degree of phantom limb pain. Flor, Elbert, Wienbruch, Pantev, Knecht, Birbaumer, Larbig & Taub (1995).

32. Pain perception Nociceptors respond to painful stimuli. Two categories: (1) mechanical receptor - severe pressure on skin - tearing (2) thermal receptor - responds to very high and very low temperatures

33. http://www.sfn.org/content/Publications/BrainBriefings/pain.htmlhttp://www.sfn.org/content/Publications/BrainBriefings/pain.html Pain Nociceptors: receptors in the skin that respond to intense pressure, extreme temperature, or burning chemicals. The perception of pain can be modulated by cognitive factors:expectation, placebo, shifting attention, emotional distraction, individual differences

34. Endorphins, Opiates and Pain Relief The “reward pathway” contains opioid receptors for exogenous and endogenous substances. - neurotransmitters (dopamine) - opiate drugs (morphine, heroin, cocaine) - endorphins

35. Somatosensory cortex plasticity Cortical reorganization occurs in monkeys when fingers are surgically connected. PET studies reveal differences in the brains of musicians that suggest somatosensory cortical changes occur in humans.

36. Cortical Plasticity String instrument players have larger representation in primary sensory cortex for their left hands than normal controls. The amount of cortical magnification was correlated with the age at which the person began to play. Elbert, Pantev, Wienbruch, Rockstroh & Taub (1995)

37. Cross-Modal Plasticity People who have been blind from a very young age show activity in visual cortex during Braille reading. A TMS pulse to the visual cortex impaired Braille reading. Evidence of functional reorganization of the brain! Cohen, Celnik, Pascual-Leone, Corwell, Faiz, Dambrosia, Honda, Sadato, Gerloff, Catala & Hallett (1997).

38. Summary • The homuculus describes the amount of cortex devoted to processing sensory and motor information from the different parts of the body. • Haptic perception results from active touch and exploratory hand movements. • Haptic and visual object recognition share an overlapping region in the ventral “what” visual stream. • Nociceptors provide information about painful stimuli.

39. Central neural mechanisms, such as emotional state and drugs, modulate our perception of pain. • The somatosensory system can undergo substantial reorganization after intensive practice and injury.