Body Image and Somatosensation

1. Body Image and Somatosensation Cognitive Neuroscience

2. Sensory Receptors Skin Muscles and joints Internal organs

3. Sensory Receptors General types: • Mechanoreceptors: physical distortion (stretching/bending) • Proprioreceptors: changes in body position • Thermoreceptors: temperature changes • Nocioreceptors: painful/damaging stimuli • Chemoreceptors: specific chemicals

4. Sensory receptors: Receptive Fields • Size • Location • Response properties • Receptor density

5. Sensory receptors • Response properties • Mechanoreceptors: stimulus frequencies adaptation rate (rapid/slow) • Thermoreceptors: warm/cold

7. What does this reveal about receptor density and receptive field size?

8. From sensory receptors to brain • Two primary, parallel pathways: • Touch and proprioception • Dorsal column-medial lemniscal pathway • Temperature and pain • Spinothalamic pathway

12. Somatosensory cortex • S1: primary somatosensory cortex (areas 1, 2, 3a, 3b) • S2: secondary somatosensory cortex • Posterior parietal cortex

13. S1: Somatotopy

14. S1: Cortical magnification

15. S1: Multiple maps Area 1: Texture Area 2: Size, shape

16. Posterior parietal cortex • Neurons with very complex response properties • Activity is modulated by attention • Multi-modal receptive fields • Large receptive fields

17. Multiple body representations • S1-somatopic sensory representations • Semantic representations • propositional knowledge • “a wrist is where a watch is worn” • “the eyes are above the nose” • Structural descriptions • Part of the object recognition system that allows for the visual identification of body parts across changes in orientation and appearance • Body Schema (posterior parietal, S2) • Integration of somatosensory, proprioceptive and vestibular info to provide a constantly updated representation of body part positions relative to other body parts and the environment

18. Body schema: Evidence • Studies with neurologically intact subjects • Autotopagnosia and other disorders subsequent to neural injury • Phantom limbs Conclusions • Mental representation of body and body movements • Body image is dynamic and flexible • Body image exists independent of sensory input • Contribution of vestibular information to updating body image

19. Autotopagnosia Autotopagnosia: • as a result of neural injury, inability to identify body parts of self or others • a broad category, specific cases may show different deficits; not well understood Sirigu et al. (1991) 62 year old woman (Alzheimer’s disease): • Inability to know current location of body parts • To verbal command (“point to your eye”) • Imitation (“point to the same part on your self as I am pointing to on my self) • Despite being able to: • Name body parts pointed to • Define body part functions • Point to marked locations on her body • Accurate reaching and grasping

20. Parsons (1994) • Evidence for: • the mental representation of the body schema • processes that manipulate this representation • Compare real movement times to left-right judgment times • Real movements: Ss hands were palm down on table and were instructed to move right or left hand to the position in a visually presented stimulus • Left/right judgments: same physical arrangements but subjects had to press a foot pedal to indicate if the visually presented stimulus corresponded to the right or left hand

26. Parsons (1994) Real movements: • Medial movements are faster than lateral ones • Right hand movements faster than left hand (most subjects right-handed) Left-right judgments: • Correlation between movement and left-right judgment times was .90

28. Lackner (1988): Body Image Illusions Provides evidence for: Dynamic representation of the dimensions and spatial contours of the body • Known: • E.g.,If biceps muscle is vibrated -> reflexive flexion of the forearm • If it is vibrated but the arm is restrained -> forearm is experienced as moving in extension (stretching) • By vibrating the appropriate muscles can elicit illusory motion in virtually any direction • Thus muscle afferent signals influence position sense • Question: • What happens if the stimulated arm is holding the person’s nose, chin, head, waist? • Procedure: • Ss blindfolded • Vibration 120 pulses.sec for 3 minutes • Report changes in sensation

30. Lackner (1988) • Subject grasping nose: • Biceps vibration • 10/14 Ss experienced extension; • 5/10 experienced noses elongating by as much as 30cm, in keeping with the apparent motion of the hands • 3/10 experienced fingers elongating • 2/10 experienced fingers and noses elongating • Others report increased pressure and tension • Triceps vibration • 13/14 experienced flexion • 6/13 experienced noses being pushed inside the head • 3/13 experienced fingers passing thru nose andlocated inside heads • 3/13 felt head tilted backwards • 1/13 felt head nose melt together

32. Lackner (1988) • Palm of right hand on head • Biceps vibration • 13/14 experienced extension • 8/13 felt heads elongated up to as much as 30 cm. • 2/13 felt heads rise because necks were elongated • 2/13 reported forward tilt of heads and lengthening of forearms • Triceps vibration • 13/14 experienced flexion • 4/13 felt fingers had moved down inside their heads • 5/13 felt the tops of their heads had been pushed down inside their heads by their hands • 3/13 felt heads had been tiled 20-30 degrees backward

37. Lackner (1988) Why does this happen? 1. stimulation is interpreted as extension or flexion 2. somatosensory info indicates that fingers and nose are in contact 3. the forearm can move while maintaining contact with nose only if nose is moving 4. info from head position indicates that head is stationary What is a brain to conclude??????????

38. Lackner (1988) • Body image perceptions are distorted to be compatible with perceived limb position, even if this involves physically impossible situations • Evidence of the great flexibility of body schema representations • Why might this be useful? • Body changes greatly thru development and body schema must be calibrated over time

39. Phantom Limbs • Subsequent to limb amputation many individuals report the clear and vivid sensation of the continued presence of the limb (may last for short period of time or years) • Natural phantom: the limb is of normal size, moves appropriately (it be so real that the amputee may forget and fall or injure him/herself) • Deformed phantom: too short, too long, too big; it can adopt abnormal positions or move in unnatural ways • May or may not be accompanied by pain (very difficult to treat) Evidence that body image is independent of sensory input

40. Ramachandran (1998) “After Lord Nelson lost his right arm during an unsuccessful attack on Santa Cruz de Tenerife, he experienced compelling phantom limb pains, including the unmistakable sensation of fingers digging into his phantom palm. The emergence fo these ghostly sensations in his missing limb let the sea lord to proclaim that his phantom was “direct evidence for the existence for the soul.” For if an arm can exist after it is removed, why can’t the whole person survive physical annihilation of the body. It is proof, Lord Nelson claimed, for the existence of the spirit long after it has cast off its attire”

41. Vestibular input updates body image Vestibular apparatus: fluid-filled canals of the inner with sensory cells that signal head movements; important for controlling balance and coordinating eye movements with body movements Andre et al. (2002) • Caloric stimulation: • squirt cold (or hot) water into the ear • Triggers nystagmus (vestibular-ocular) reflex (used clinically to assess brain-stem damage in unconscious patients) • 31 amputees: • 17 w/out phantoms, 10 w/ painful phantoms, 2 w/ deformed phantoms, 2 with normal phantoms • Results: • 28/31 experienced a normal phantom and for those with painful phantoms, the pain disappeared • Effects lasted a mean of 9 minutes (20 secs -2 hours)