Somatosensation - Body Image and Somatosensation Cognitive...

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Unformatted text preview: Body Image and Somatosensation Cognitive Neuroscience Sensory Receptors Skin Muscles and joints Internal organs Sensory Receptors p Mechanoreceptors (the majority type) : n physical distortion (stretching/bending) p Meissners corpuscles, Pacinian corpuscles, Ruffini's endings, Merkel's disks p Thermoreceptors: n temperature p Nocioreceptors: n painful/damaging stimuli n free nerve endings p Proprioreceptors: n changes in body position n in muscles, tendons Sensory receptors: Receptive Fields Size p Location p Response properties p p Receptor density p Response property: Adaption Rate Detect Movement Detect Shape What does this reveal about receptor density and receptive field size? From sensory receptors to brain p Two primary, parallel pathways: n Touch and proprioception p Dorsal columnmedial lemniscal pathway n Temperature and pain p Spinothalamic pathway Somatosensory cortex p S1: primary somatosensory cortex (areas 1, 2, 3a, 3b) p S2: secondary somatosensory cortex p Posterior parietal cortex (5,7) S1: Somatotopy S1: Cortical magnification S1: Multiple maps Area 1: Texture Area 2: Size, shape Posterior parietal cortex p Neurons with very complex response properties p Activity is modulated by attention p Multimodal receptive fields p Large receptive fields recall: dorsal pathway's role in "perception for action" Multiple body representations (1) (2) (3) (4) S1somatopic sensory representations Semantic representations p declarative knowledge p "a wrist is where a watch is worn" p "the eyes are above the nose" Structural descriptions p Part of the visual object recognition system that allows for the visual identification of body parts across changes in orientation and appearance Body Schema (posterior parietal, S2) n Integration of: p p p p somatosensory, proprioceptive (info regarding tension in muscles, tendons and joints), vestibular visual info n to provide a constantly updated representation of body part positions and size relative to other body parts and the environment Body schema Evidence Studies with neurologically intact subjects p Autotopagnosia and other disorders subsequent to neural injury p Phantom limbs p Conclusions There is a mental representation of the body and body movements p Body image is dynamic and flexible p Body image exists independently of sensory input p Vestibular information contributes to updating body image p Autotopagnosia Autotopagnosia: p p as a result of neural injuryinability 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): p Failed on: p p Verbal command ("point to your eye") Imitation ("point to the same part on your self as I am pointing to on my self) Name body parts pointed to (which she can see) Define body part functions Point to marked locations on her body (which she can see) p Despite being able to: p p p p Authors concluded: inability to know current location of body parts Parsons (1994) p Evidence for: the mental representation of the body schema n processes that manipulate this representation n p Compare real movement times to leftright judgment times n 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, press foot pedal when done n 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 Parsons (1994) Real movements: p Medial movements are faster than lateral ones p Right hand movements faster than left hand (most subjects righthanded) Leftright judgments: p Correlation between movement and leftright judgment times was .90 Imaging mental transformations of the body image p Bonda et al. (1995) n n PET study Experimental condition: See a hand p Press a button to indicate if it is a right/left hand p n Control condition: Prior to scanning study 8 hand photos (not used in the exptl condition) p During scanning: show hand photos familiarity judgment p Bonda et al. (1995) Results: sig bilateral superior parietal lobe (SPL) activity Bonda et al. (1995) p IPL (Inferior Parietal Lobe): Left hemisphere only (perhaps associated with finding that certain autotopagnosias result almost exclusively from LH damage) Lackner (1988): Body Image Illusions Evidence of dynamic nature of the representation of the dimensions and spatial contours of the body p Known: If biceps muscle is vibrated > reflexive flexion of the forearm p If it is vibrated but the arm is restrained > forearm is experienced as extension (stretching) p By vibrating the appropriate muscles can elicit illusory motion in virtually any direction Thus muscle afferent signals influence position sense p The reverse occurs with triceps stimulation What happens if the stimulated arm is holding the person's nose, chin, head, waist? p Question: p p Procedure: Ss blindfolded p Vibration 120 pulses.sec for 3 minutes p Report changes in sensation p Lackner (1988) p Subject grasping nose: n Biceps vibration 10/14 Ss experienced sensation of 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 n Triceps vibration 13/14 experienced flexion 6/13 experienced noses being pushed inside the head 3/13 experienced fingers passing thru nose and located inside heads 3/13 felt head tilted backwards 1/13 felt head nose melt together Lackner (1988) "....the bizarre sensations have no discomfort associated with them. The subject does not feel that his nose is being painfully stretched by his hand....Instead, there is a sense of wonder as the dimensions of the body are perceived to change as one subject reported...: "Oh my gosh, my nose is a foot long! I feel like Pinocchio" (p. 284) Lackner (1988) p Palm of right hand on head n 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 n 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 2030 degrees backward 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?????????? Lackner (1988) p Body image perceptions are distorted to be compatible with perceived limb position, even if this involves physically impossible situations p Evidence of the great flexibility of body schema representations p Why might this be useful? n Body changes greatly thru development and body schema must be calibrated over time Ehrsson, et al. (2005) Compared activation in conditions: Waist reducing illusion vs. stimulation that does not result in illusion Posterior parietal activity n Relationship between BOLD response and illusion "strength" Phantom Limbs p 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) n n n 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) 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 of 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" Yang et al (1994) MEG of patient with left arm amputated below elbow Right hand area is invaded by face representation. RH SI reorganization following deafferentation SII, PPC not reorganized. Face Upper Arm Hand Stimulation of Face Produces Sensation on Phantom Hand (Same Patient) B I T P B Ball of Thumb T Thumb P Pinky I Index Finger Congenital phantom limbs p In approx 1520% of cases of congenital absence of limbs, a phantom limb is experienced "I've never had arms....and yet I've always experienced the most vivid phantom limbs, from as far back in my childhood as I can remember...as I'm talking....they are gesticulating....another interesting things about them.... Is that.... they're 6 to 8 inches too short.... When I put on my artificial arms, my phantoms are much shorter than they should be....my phantom fingers should fit into the artificial fingers like a glove by my arm is abut 6 inches too short. I find this incredibly frustrating because it doesn't feel natural. I usually end up asking the prosthestist to reduce the length of my artificial arms....." (pg. 41) Ramachandran (1998): Ramachandan concludes: "To me this was proof that Mirabelle's phantoms were not wishful thinking. If she wanted to be like other people, why would she want shorterthannormal arms? There must be something going on inside her brain that was giving rise to the vivid phantom experience" Evidence for hardwired body schemas? Are HardWired Body Schemas Source of Transsexuality ? p Ramachandran & McGeoch (2007) n n n We hypothesize that, perhaps due to a dissociation during embryological development, the brains of transsexuals are "hardwired" in manner which is opposite to that of their biological sex. Around 60% of men who have had to have their penis amputated for cancer will experience a phantom penis ... We go on to predict that maletofemale transsexuals will be much less likely to experience a phantom penis than a "normal" man who has had his penis amputated for another reason. We also predict that some femaletomale transsexuals will have a phantom penis ... Vestibular input updates body image Vestibular apparatus: fluidfilled 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) p Caloric stimulation: squirt cold (or hot) water into the ear p Triggers nystagmus (vestibularocular) reflex (used clinically to assess brainstem damage in unconscious patients) p p 31 amputees: 17 w/out phantoms, 10 w/ painful phantoms, 2 w/ deformed phantoms, 2 with normal phantoms p Applied caloric stimulation p p Results: 28/31 experienced a normal phantom and for those with painful phantoms, the pain disappeared p Effects lasted a mean of 9 minutes (20 secs 2 hours) p VisualTactile Integration Botvinick & Cohen (1998) n 20 Ss seated with left arm resting on table n Standing screen hid arm from view n Lifesized artificial rubber model of the left hand and arm placed directly in front on the table n Ss fixated the artificial arm/hand n Artificial hand and real left hand were stroked synchronously with paintbrushes for 10 minutes n Ss filled out questionnaire Botvinick & Cohen (1998) Figure 1 Questionnaire results. Subjects indicated their response on a sevenstep scale ranging from 'agree strongly' (+++) to 'disagree strongly' (). Points indicate mean responses. Bars indicate response range. The questions underlined showed a statistically significant tendency to evoke affirmative responses (P <0.002 for underlined questions, P <0.018 after correcting for multiple comparisons). VisualTactile Integration: Treating phantoms n Ramachandran et al. p Amputees placed intact arm on one side of a standing mirror p Adjusted phantom (block from "view" by the mirror) so that it lined up with the intact arm p When a subject looks in the mirror, creates the sensation that that arm blocked from view is being seen p Performed a range of actions with the intact arm, while looking in the mirror p Results for a number of individuals, over time, the phantom disappeared p Ramachandran: "probably the first example in medical history of a successful "amputation" of a phantom limb" Multiple body representations (1) (2) (3) (4) S1somatopic sensory representations Semantic representations p declarative knowledge p "a wrist is where a watch is worn" p "the eyes are above the nose" Structural descriptions p Part of the visual object recognition system that allows for the visual identification of body parts across changes in orientation and appearance Body Schema (posterior parietal, S2) n Integration of info: p p p p somatosensory, proprioceptive (info regarding tension in muscles, tendons and joints) vestibular visual n to provide a constantly updated representation of body part positions and size relative to other body parts and the environment ...
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This note was uploaded on 07/29/2008 for the course NEUROSCIEN 70 taught by Professor Whitney during the Spring '08 term at Johns Hopkins.

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