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Somatosensation

• Chris Rorden• Cortical & subcortical pathways• Layout of the motor strip• Plasticity, reorganization• Phantom limb syndrome

www.mricro.com

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Somatosensation

>20 types of receptors in skin: touch, temperature, stretch, etc

2 pathways to brain– Dorsal columns

Precise touch, joint angle, etc. Crosses side at medulla

– Antero-lateral Tract Coarse information regarding

pain and temperature Convergence of information Crosses side at entry in spinal

column

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Cerebral Cortex

Thalmus

Spinal cordLight Touch (Dorsal)Pain (Ventrolateral)

Medula

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Can help distinguish between psychiatric and neurological injury.

– Psychiatric conversion disorder: often glove/stocking anesthesia

– Neurological disorder: follows spinal innervation (dermatomes)

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Emotion and cognition

Kolb suggests emotionality decreases after spinal cord injury. Degree of change dependent on amount of cord severed.

This is not a well-replicated finding. See Nicotra (2006) for review, Cobos et al (2002)

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Primary Somatosensory Strip (S1)

S1 Primary somatosensory strip– Receives input from body– Topographic map of body

Distorted: Face and hands over represented

– Most anterior portion of parietal cortex

M1 Primary Motor Strip M1 sends outputs to muscles Cortical map corresponds to S1 Motor strip most posterior portion of

frontal cortex Topic of future lecture

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S1

Between Central sulcus (Rolandic)Postcentral sulcus.

Local sulcal landmarksPrecentral sulcusPrecentral sulcusLateral fissure (Sylvian)Intraparietal SulcusSuperior Frontal Sulcus

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Somatosensory cortex

Primary S1 = BAs 1,2,3 Secondary S2 and tertiary regions

located in parietal BAs 5 and 7 (Parietal Association Areas)– These receive input from S1– Also, receive direct anterolateral

projections– Combine information from both sides– Combine information from different

senses

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2nd or 3rd Somatosensory injury

Patients with damage to S2 or tertiary somatosensory cortex show higher level deficits.– Basic sensitivity unimpaired– Fine discrimination impaired, e.g. agnosia

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Astereognosis

Astereognosis is the inability to discriminate between objects of different shapes, textures, weights, and size based on touch alone.

It can be caused either by damage to sensory nerve pathways or, where there is little or no sensory loss, by lesions in the parietal lobe of the brain. Where there is parietal lobe damage the asterognosis will be on the opposite (contralateral) side of the body to the lesion.

Astereognosis is tested by asking the patient to close his/her eyes and then placing familiar objects such as a key or coin in his/her hand and asking him/her to identify it.

A similar condition also caused by lesions to the sensory pathways or the parietal lobe is agraphesthesia. This is the inability to recognise letters or numbers drawn on the patient's hand with a semi-sharp object.

Patients who cannot recognize an object by touch may still be able to draw the object and recognize the object pictured in the drawing.

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Plasticity of somatosensory cortex

Woolsey and Wann (1976) examined plasticity of somatosensory cortex in mice.– Normally, cortical barrels

topographic map of space.– If whiskers removed, mapping of

remaining whiskers grows

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Plasticity of S1

Plasticity in primates (finger areas).– Qi et al. (2000) Reorganization of primary motor

cortex in adult macaque monkeys with longstanding amputations. J. Neurophysiol. 84, 2133-2147.

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Phantom Limbs

MEG offers evidence of reorganization.– Patient lost one arm– When face is brushed, he experiences his old arm is touched. – Consistent spatial mapping of face to lost limb.– MEG reveals that arm and face encroach hand area

Figure below: arm hand and face regions in normal locations contralateral to intact arm, but arm and face representation have grown together contralateral to lost limb.

– For review Ramachandran and Hirstein (2000), Brain, 121, 1603-1630

ArmHandFace

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Is plasticity reversible?

Sirigu et al. (Nature Neuroscience, 4, 691-692).– CD lost both hands in 1996– Bilateral hand transplantation in 2000– Both M1 and S1 show elbow activity had taken over

hand area before graft.– After graft: hand area enlarges and elbow

representation shrinks.

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Gerstmann Syndrome

Gerstmann (1924) noted association of 4 deficits:– Finger agnosia: inability to recognize, identify, differentiate, or orient

the fingers of either hand (both patient’s hands as well as examiners), finger autotopagnosia.

To test: patient closes eyes and indicates which of their fingers has just been touched. Number the fingers from 1 to 5 and have the patient call out a number corresponding to the finger which was touched, or the patient can point to the corresponding finger with the opposite hand.

– Agraphia: writing disability– Right–left disorientation: Difficulty recognizing the right or left sides of

the body, both his own and that of the examiner facing him– Dyscalculia: Calculation impairment

78% of patients with Gerstmann’s syndrome have left hemisphere damage (Heimburger et al., 1964).

– Often aphasic

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Gerstmann Syndrome

Clearly, dexterity of fingers required for writing.

Also, fingers used to learn arithmetic, perhaps explaining dyscalculia.

Benton (1961, 1977) critic– individual symptoms found in isolation.

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Gerstmann Anatomy

Mayer et al. (1999) report single patient with pure Gerstmann Syndrome.– IQ intact– Language comprehension and speech production intact– Toe agnosia (i.e. finger agnosia of feet)

White matter beneath the bottom of the left angular gyrus.

braininfo.rprc.washington.edu

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Body Schema Disturbance

Lesions of the Parietal lobes may produce characteristic disturbances of the cognitive model representing knowledge about the arrangement of body parts and their spatial relationship to objects in the environment. Many aspects of this model are implicit and patients with disturbance of the body schema are often unaware of their impairment.

See also Lecture 9 - anosognosia

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Autotopagnosia

Many left hemisphere patients do have autotopagnosia without other components of Gerstmann’s syndrome.

Autotopagnosia is the inability to name body parts or to identify body parts Deficits in body awareness are assessed by having the patient point to

their own body parts and those on others (e.g. "Show me your left hand".), or by asking the patient to perform simple actions with parts of the body (e.g. "Touch your right ear with your left hand"). Assessment should be made of both common and difficult body parts, and should include responses to both command and imitation.

Contiguity errors common – patient points to area near designated region Semantic errors – Similar categories, e.g points to elbow when asked to

point to knee. Many patients also have problems pointing to other components – e.g.

‘point to the chain on this bicycle’

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Keeping track of where our body is

We are aware of where our limbs are. This awareness is based on integrating new

outputs (motor commands) and inputs (sensory feedback) with previous information of limb position.

What happens if this system is disrupted?

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A: Vanishing arm

Wolpert et al. (1998) Nature Neuroscience 529-533– PJ 50 yo woman with large left hemisphere cyst– Tactile extinction: miss right touch when simultaneously touched on left

(vision fine).– Visual fields intact, no neglect– Reaching to visual items: fine at fovea, poor in periphery.– Astereognosis (unable to identify items with touch) on right hand.– Reports her right limbs vanish after several seconds without vision.

Can only voluntarily ‘regain’ limb through vision If limb is touched, she becomes aware of position

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B: Results

PJ asked to grip force transducer and apply constant squeeze.

With visual feedback (shown grip force on visual display) she can maintain grip with either hand.

Without visual feedback, grip force of right hand decays over several seconds.

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C: Results

How long does it take PJ’s arm to vanish?– PJ places right arm behind back (unseen)– Weight placed in hand.– Asked to report when she can no longer feel weight.– Heavier weights require longer to vanish.

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D: implications

We talk as if sensation, motor control and attention are independent processes. E.G. Structure of lectures in this course. However, these are tightly coupled. Wolpert et al (1998) demonstrate that somatosensation (feelings one’s limb) is updated from visual (seeing the limb) as well as

tactile inputs (something touching the limb). Figure from Fuster (2000) illustrates this.

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Key Readings

Phantom limbs– Franz and Ramachandran (1998) Nature Neuroscience, 1,

443-444.

Pure case of Gerstmann syndrome– Mayer et al. (1999) Brain, 122, 1107-1120.

Evidence of a vanishing arm– Wolpert et al. (2000) Nature Neuroscience, 1, 529-533.