The Sense of Touch
Jusepe de Ribera
c. 1615-16
Norton Simon Museum
Pasadena CA
Four classes of touch receptors
Touch fibers innervate small regions of skin
Central projections of touch fibers in the spinal cord
Touch is Relayed in a Distinct Pathway
Somatosensory Areas of the Cortex
Major sulci of the parietal lobe
CS: Central sulcus
PCS: Postcentral sulcus
IPS: Intraparietal sulcus
CS
PCS
LF: Lateral fissure
IPS
LF
Cortical Cytoarchitecture
Anatomical Projections to and from S-I Cortex
Cortical receptive fields enlarge at each synapse
Convergence Increases Receptive Fields
A
B
• Convergence from same receptor class (modality)
• Convergence from neighboring skin areas (stimulated together
wire together)
• Arrangement selects features (e.g., edge orientation)
Orientation selective neuron (Area 2)
Minicolumns of neurons span the cerebral cortex
Jones E G PNAS 2000;97:5019-5021
Information is processed in columns of cortical neurons
Neurons in all 6 layers share common inputs
Jones E G PNAS 2000;97:5019-5021
Receptive fields in a cortical column overlap
Cortical layers differ in projection targets
Orderly maps of the body in the brain (homunculus)
Dermatomes represent the body in the spinal cord
Somatotopy of the thalamic ventral posterior nuclei
Brain maps reflect innervation density
Optical imaging of brain maps in monkeys
Median nerve cut silences part of S-I cortex
Merzenich MM et al. Neuroscience 10: 639-665, 1983
New receptive fields 4 hours after nerve cut
Merzenich MM et al. Neuroscience 10: 639-665, 1983
New hand representation in S-I after median nerve cut
Merzenich MM et al.
Neurosci 10: 635-665,
1983b
New receptive fields after digit amputation
Merzenich MM et al. J Comp Neurol 224: 591-605, 1984
Dorsal rhizotomy (C5-C8) alters hand maps in S-I
Darian-Smith C, Ciferri MM. J Comp Neurol
491: 27-45. 2005.
Dorsal rhizotomy (C5-C8) impairs hand function
Darian-Smith C, Ciferri MM.
J Comp Neurol 491: 27-45.
2005.
Loss of hand function correlates with lesion size
Darian-Smith C, Ciferri MM.
J Comp Neurol 491: 27-45.
2005.
Cortical maps are altered by experience
Pons TP, et al. Science 252: 1857-60 (1991))
Amputees experience phantom sensations
Cortex reorganizes in amputees
Trigeminal fibers sprout after a dorsal column lesion
Jain N et al. PNAS 2000;97:5546-5550
Learning alters brain maps
Larger finger representation of left hand of violinists
Elbert T, et al. Science 270: 305-307 (1995)
Training focuses and enhances cortical activity
M. Lotze et al. NeuroImage 20:1817–1829 (2003)
Parietal Cortex Includes S-I, S-II and PPC
Lateral view
Anatomical Projections to and from S-I Cortex
S-I neurons receive inputs from specific modalities
Spatiotemporal receptive fields in S-I
Sripati AP et al. J Neurosci 26: 2101–2114, 2006
Inhibition extracts important features
A
B
• Winner-take-all sharpening of strongest inputs (feedback inhibition)
• Order of receptor stimulation determines output (lateral inhibition)
• Higher areas can control ascending output (descending inhibition)
Direction sensitive neurons (Area 2)
Convergence patterns enable feature detection
Information Processing in Cerebral Cortex
• Bottom up: Receptors encode stimuli
1. What: the sensory modality
2. Where: position in space or body
3. Intensity: stimulus amplitude
4. Duration: persistence or adaptation
• Top down: Information interpreted in context
1. Memory: previous encounters and learning
2. Attention: selection of features relevant to task
3. Circuitry: structure of pathways and synapses
4. Motivation: emotional connotation
Inhibition modifies receptive fields
Mechanoreceptors Sense Hand Actions
Cortical circuits
for grasping
PFC
Vision
F5
AIP
(PMv)
F2
(PMd)
7b
PRR
Arm
5d/5v
Hand
Hand
Arm
Arm
Hand
M-I
S-I
Hand
DCN/VPL
MNs
Hand
Object
S-II/PV
Similar posterior parietal areas in humans and monkeys
PPC and motor control of the hand
•
•
•
•
•
Represents body position in space
Integrates somatosensory and visual inputs
Motor planning of reach and grasp
Projects to premotor cortex
Lesions cause dysmetrias in contralateral
field and clumsy movements
– Reach undershoot
– Poor hand preshaping and wrist orientation
PPC neurons signal direction of reaching
PPC neurons respond before touch
.
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Unit H17094-131-3.2
N = 224 trials
Previous
Hold
Approach
Low er
Contact
Relax
Grasp
Release
Lift
Next
10
0
-1000
-750
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-250
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250
500
750
1000 1250
Time re contact (ms)
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1750
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2750
3000
Area 5 responses reflect approach style
Area AIP (BA7) neurons code hand posture
Posterior Parietal Cortex and frontal motor areas
IP (unfolded)
A
P
Somatosensory Areas of the Cortex
fMRI responses to passive and active touch
Attention modulates S-II responses
S-II integrates sensory and learned responses
S-I Cortical Lesions Impair Touch
• Loss of touch sensation in lesioned area
– Impaired two-point discrimination
– Poor texture discrimination
– Loss of stereognosis
– Localized to specific body region in the map
• Impaired motor function especially hand
– Clumsy grasping
– Poor motor coordination
Neurological tests of tactile sensation
• Simple somatosensory
– Touch threshold
– Two-point discrimination
– Vibration sense
– Position sense of index finger
• Complex tactile recognition
– Texture discrimination
– Form recognition (stereognosis)
– Size discrimination
Parietal lobe sensorimotor deficits in 9 patients
Area 2 inactivation produces clumsy grasp
Neurological tests of active touch
• Hand position and force control
– Grip force
– Rhythmic tapping
– Reach to a target
• Exploratory and skilled movements
– Insertion of pegs in slots
– Precision (pincer) grip of small objects
– Exploratory movements when palpating objects
Parietal lobe sensorimotor deficits in 9 patients
Neglect of contralateral space (optic ataxia)
Lesion of Right
Posterior Parietal
Cortex
Cognitive Touch: Object Identification
• Stereognosis: identification by touch
• Integration of features detected by receptors
• Template matching: compare features to
previous examples
• Naming
– Declarative memory
– Template storage and modification (learning)
– Ventral stream (hippocampus)