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Somatosensation
• Chris Rorden
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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
Medula
Light Touch (Dorsal)
Pain (Ventrolateral)
Spinal cord
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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 landmarks
Precentral sulcus
Lateral fissure (Sylvian)
Intraparietal Sulcus
Superior 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.
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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
Arm
Hand
Face
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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?
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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.