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Robotic Brain Universe

Touch and the parietal lobe

How fingertips, the somatosensory map and the parietal cortex turn touch and space into action, and how touch is being restored artificially.

Intermediate · about 9 min · updated 2026-10-02 · awaiting clinical review

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Illustrative simulation excitatory inhibitory

Touch receptors and their densities, the postcentral somatosensory map and its plasticity, the posterior parietal cortex as a centre for sensory–motor integration and intentions, the precuneus; optimal combination of vision and touch, gain fields and coordinate transformations; sensory loss, spatial neglect and phantom-limb pain; and artificial touch through nerve and cortical stimulation and parietal brain–computer interfaces.

Contents
  1. The feeling brain
  2. What the somatosensory and parietal cortex are
  3. Why touch and space go together
  4. How touch becomes action
  5. When the body map changes
  6. When touch and space fail
  7. The mathematics of touch and space
  8. Technology: artificial touch
  9. Milestones
  10. Frontiers
  11. Check yourself

The feeling brain

Your fingertips are among the most finely instrumented surfaces in nature. Recording single nerve fibres in awake volunteers, Johansson and Vallbo estimated that the skin of a fingertip carries about 241 touch-sensing units per square centimetre, against 58 in the palm.[1]

Those signals build a map of the body in the postcentral gyrus, first charted by Penfield's stimulation of patients' brains, and the parietal lobe behind it turns touch, sight and body position into plans for action. The map is not fixed: after an amputation it reorganises, and the more it reorganises, the more phantom-limb pain people report.[2,3,4]

Touch can now be written back into the brain. Stimulating the sensory cortex of a person with a spinal cord injury evoked sensations felt on the hand, and in a later study, feeding such sensations back to a person controlling a robotic arm halved the time needed for a clinical test.[5,6]

What the somatosensory and parietal cortex are

The postcentral gyrus contains the primary somatosensory cortex, with a map of the opposite side of the body that parallels the motor map just in front of it. Excisions of the postcentral gyrus raise the thresholds for detecting touch on the opposite side of the body.[2,7]

The skin of the hand holds four kinds of touch receptor unit, distinguished by how quickly they adapt and the size of their receptive fields: RA, PC, SA I and SA II. The RA and SA I units, with small, sharply defined fields, become far denser towards the fingertips and are thought to account for fine spatial acuity.[1]

Behind the postcentral gyrus, the posterior parietal cortex (including the supramarginal gyrus and the superior parietal lobule) was long thought of as purely sensory but is now seen as a centre for sensory–motor integration, with subregions for planning eye, reaching and grasping movements. On the medial surface, the precuneus takes part in visuospatial imagery, recalling episodes and taking a first-person perspective.[3,8]

Key numbers

Touch-sensing units per cm² at the fingertip
about 241[1]
In the palm
about 58[1]
Relative density, palm : finger : fingertip
1 : 1.6 : 4.2[1]
Correlation between cortical reorganisation and phantom-limb pain
r = 0.93[4]
Robotic-arm test time without and with touch feedback (median)
20.9 s and 10.2 s[6]

Why touch and space go together

Touch is essential for using our hands. When we pick something up, the brain sets up controllers for each phase of the action that use predictions and signals from the fingertip receptors to tailor force to the object's properties, encoding contact events to check that each phase went as planned.[9]

The parietal cortex also combines senses. Ernst and Banks measured how variable people's visual and haptic judgements of an object's height were, and found that people combined the two in a statistically optimal way, giving more weight to the more reliable sense; vision dominates only when it is the more precise.[10]

And it links where something is to how to act on it. Neurons in parietal area 7a respond to both where a stimulus falls on the retina and where the eyes are pointing, and by combining the two represent the location of objects in space.[3,11]

How touch becomes action

Columns. Mountcastle's recordings in the somatosensory cortex showed that neurons stacked vertically through the cortex respond to the same kind of stimulus from the same place on the body, the first evidence for cortical columns.[12]

Maps that move. After amputation of one or two fingers in adult monkeys, the cortical representations of neighbouring fingers and the palm expanded within months to take over most or all of the territory that had served the missing digits, and were represented in finer grain.[13]

Plans, not just percepts. Andersen and Buneo describe a map of intentions in the posterior parietal cortex, with separate subregions for planning eye movements, reaches and grasps, using shared spatial representations to convert sensory input into motor output.[3]

From fingertip to actionFingertip receptorsRA, PC, SA I, SA II unitsPeripheral nervemedian and ulnar nerves for thehandThalamusa stroke here can cause sensorylossPostcentral gyrusprimary somatosensory mapPosterior parietal cortexcombines touch, vision, eyeposition; plansMotor and premotor cortexactionintentionsgrip and contact
From fingertip to action. Touch signals reach the somatosensory map and are combined in the posterior parietal cortex with vision and body position to plan actions; every grip then generates new touch signals that check the plan.[1,2,3,9,14,15]
Text version of the diagram
  1. Fingertip receptors: RA, PC, SA I, SA II units. Leads to Peripheral nerve.
  2. Peripheral nerve: median and ulnar nerves for the hand. Leads to Thalamus.
  3. Thalamus: a stroke here can cause sensory loss. Leads to Postcentral gyrus.
  4. Postcentral gyrus: primary somatosensory map. Leads to Posterior parietal cortex.
  5. Posterior parietal cortex: combines touch, vision, eye position; plans. Leads to Motor and premotor cortex (intentions).
  6. Motor and premotor cortex: action. Leads to Fingertip receptors (grip and contact).

When the body map changes

The somatosensory map stays adjustable in adulthood. In Merzenich's monkeys, mapped repeatedly 2 to 8 months after digit amputation, the boundaries between finger representations shifted, often by hundreds of micrometres, and new sharp borders formed where formerly separate fingers met.[13]

The sense of owning a body part can change in minutes. In the rubber-hand illusion, people who watched a rubber hand being stroked while their own hidden hand was stroked in synchrony came to feel the touch where they saw it, on the rubber hand.[16]

When touch and space fail

Sensory loss. Removing the postcentral gyrus raises touch thresholds on the opposite side of the body, and thalamic infarcts can cause sensory loss depending on the arterial territory involved.[7,15]

Neglect. Unilateral spatial neglect, in which a patient ignores one side of space, is common after injury to the right hemisphere. Corbetta and Shulman argue it is better explained by dysfunction of distributed attention networks, a ventral one damaged directly and a dorsal one that controls spatial attention, than by damage to one specific region.[17]

Phantom pain. In people who had lost an arm, magnetoencephalography showed a very strong relationship (r = 0.93) between how much the somatosensory map had reorganised and how much phantom-limb pain they had, but not other phantom sensations.[4]

The mathematics of touch and space

Two simple ideas explain much of what the parietal cortex does with touch: weighting each sense by how reliable it is, and combining signals multiplicatively to change coordinate frames.[10,11]

Optimal combination of vision and touch[10]
s^=wV s^V+wH s^H,wV=1/σV21/σV2+1/σH2,wH=1−wV\hat s = w_V\,\hat s_V + w_H\,\hat s_H, \qquad w_V = \frac{1/\sigma_V^2}{1/\sigma_V^2 + 1/\sigma_H^2}, \quad w_H = 1 - w_V

The maximum-likelihood estimate of a property seen and felt: each sense's estimate is weighted by its reliability (the inverse of its variance). Ernst and Banks found that people's judgements of an object's height followed this rule.

Symbols in Optimal combination of vision and touch
SymbolMeaningUnit
s^V,s^H\hat s_V, \hat s_Hthe visual and haptic estimates—
σV2,σH2\sigma_V^2, \sigma_H^2their variances (noise)—
wV,wHw_V, w_Hweights given to vision and touch—
Variance of the combined estimate[10]
σVH2=σV2 σH2σV2+σH2  ≤  min⁡ ⁣(σV2, σH2)\sigma_{VH}^2 = \frac{\sigma_V^2\,\sigma_H^2}{\sigma_V^2 + \sigma_H^2} \;\le\; \min\!\left(\sigma_V^2,\, \sigma_H^2\right)

Combining two senses this way gives a more precise estimate than either alone, which is the benefit the brain gains from integration.

Symbols in Variance of the combined estimate
SymbolMeaningUnit
σVH2\sigma_{VH}^2variance of the combined estimate—
Gain field[3,11]
r(x,e)=f(x)⋅g(e),xhead=x+er(x, e) = f(x)\cdot g(e), \qquad x_{\text{head}} = x + e

A parietal neuron's response depends on where a stimulus falls on the retina (ff), scaled by where the eyes are pointing (gg). A population of such neurons implicitly carries the stimulus position relative to the head; Zipser and Andersen showed that a network trained with backpropagation develops units with the same properties.

Symbols in Gain field
SymbolMeaningUnit
xxposition of the stimulus on the retina°
eeeye position in the head°
f,gf, gretinal tuning and eye-position gain—
Spacing of touch units[1]
d≈1ρd \approx \frac{1}{\sqrt{\rho}}

With ρ\rho units per unit area, the typical distance between neighbouring units is about the inverse square root of the density. At 241 units per cm² on the fingertip this is about 0.6 mm; at 58 per cm² in the palm, about 1.3 mm, consistent with the finer spatial acuity of the fingertips.

Symbols in Spacing of touch units
SymbolMeaningUnit
ρ\rhodensity of touch unitsunits/cm²
ddtypical spacing between unitscm

Technology: artificial touch

Feeling through a prosthesis. Stimulating the median and ulnar nerves of an amputee through electrodes inside the nerve fascicles, driven by sensors in a robotic hand, gave near-natural sensory feedback in real time: without sight or sound the participant could grade grip force and identify the stiffness and shape of objects.[14]

Touch from the cortex. Microstimulation of the hand area of the somatosensory cortex evokes sensations felt on the hand, many with natural qualities such as pressure, that stay stable for months; grading the stimulus amplitude grades the perceived intensity.[5]

Reading plans. Microelectrode arrays in the posterior parietal cortex of a person with tetraplegia recorded populations from which imagined goals, trajectories and types of movement could be decoded, a source of high-level control signals for prosthetics.[20]

An early neural-network model of the brain. In 1988 Zipser and Andersen trained a network with backpropagation to compute spatial location from retinal and eye-position signals and found that the network accounted for the observed response properties of real neurons in parietal area 7a.[11]

Milestones

Mapping touch and space

  1. 1937Penfield maps sensory as well as motor representation in the human cortex.[2]
  2. 1957Mountcastle finds columns in somatosensory cortex.[12]
  3. 1970Postcentral excisions are shown to raise somatosensory thresholds.[7]
  4. 1979The density of touch receptors across the human hand is measured.[1]
  5. 1984Adult somatosensory maps reorganise after digit amputation.[13]
  6. 1988A backpropagation network reproduces parietal neurons' responses.[11]
  7. 1995Phantom-limb pain is linked to cortical reorganisation.[4]
  8. 1998The rubber-hand illusion.[16]
  9. 2002Vision and touch are shown to combine optimally; intentional maps in parietal cortex are reviewed.[3,10]
  10. 2011Spatial neglect is recast as a disorder of attention networks.[17]
  11. 2014A bidirectional hand prosthesis restores near-natural touch through the nerves.[14]
  12. 2015Motor imagery is decoded from human posterior parietal cortex.[20]
  13. 2016Cortical microstimulation evokes touch felt on the hand.[5]
  14. 2021Touch feedback halves task times with a brain-controlled robotic arm.[6]

Frontiers

Bidirectional interfaces, which both read movement intentions and write touch, are bringing brain-controlled limbs closer to natural use: in the 2021 study, faster times came mainly from spending less time trying to grasp objects, bringing performance closer to that of able-bodied people.[6]

The posterior parietal cortex is a candidate source of richer control signals than motor cortex, carrying imagined goals and trajectories rather than only muscle commands.[3,20]

Check yourself

Check yourself

  1. Which part of the cortex contains the primary somatosensory map?
    Show answer

    The postcentral gyrus, just behind the central sulcus.

  2. Which touch units account for fine spatial acuity, and why?
    Show answer

    RA and SA I units: they have small, well-defined receptive fields and are much denser at the fingertips.

  3. How does the brain combine vision and touch to judge an object?
    Show answer

    Like a maximum-likelihood estimator, weighting each sense by its reliability (inverse variance).

  4. What happens to the somatosensory map after a finger is amputated?
    Show answer

    Neighbouring fingers and palm expand into the territory of the missing digit.

  5. What did Flor and colleagues find about phantom-limb pain?
    Show answer

    Its severity correlated very strongly (r = 0.93) with the amount of somatosensory cortical reorganisation.

  6. What is spatial neglect and after which injuries is it common?
    Show answer

    Ignoring one side of space; common after right hemisphere injury.

  7. What is a gain field?
    Show answer

    A neuron's response to a stimulus position scaled by eye position, letting a population represent locations relative to the head.

Glossary[1,2,3,4,8,11,16,17]

Primary somatosensory cortex
The touch and body-position map in the postcentral gyrus.
Mechanoreceptor
A sensory ending that responds to pressure, vibration or stretch of the skin.
Posterior parietal cortex
Cortex behind the somatosensory area that integrates senses and plans actions.
Precuneus
Medial parietal cortex involved in imagery, memory retrieval and self-related processing.
Haptic
Relating to perception through active touch.
Spatial neglect
Failure to attend to one side of space after brain injury.
Phantom limb
Sensations, sometimes painful, felt in a limb that has been amputated.
Cortical reorganisation
Changes in which body parts a region of cortex represents.
Gain field
Multiplicative modulation of a neuron's response by another variable such as eye position.
Rubber-hand illusion
Feeling touch on a seen rubber hand when it is stroked in synchrony with one's own hidden hand.

References

  1. Johansson RS, Vallbo AB. Tactile sensibility in the human hand: relative and absolute densities of four types of mechanoreceptive units in glabrous skin. The Journal of Physiology 1979;286(1):283-300. doi:10.1113/jphysiol.1979.sp012619
  2. Penfield W, Boldrey E. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 1937;60(4):389-443. doi:10.1093/brain/60.4.389
  3. Andersen RA, Buneo CA. Intentional maps in posterior parietal cortex. Annual Review of Neuroscience 2002;25:189-220. doi:10.1146/annurev.neuro.25.112701.142922
  4. Flor H, Elbert T, Knecht S, Wienbruch C, Pantev C, Birbaumer N, et al.. Phantom-limb pain as a perceptual correlate of cortical reorganization following arm amputation. Nature 1995;375(6531):482-484. doi:10.1038/375482a0
  5. Flesher SN, Collinger JL, Foldes ST, Weiss JM, Downey JE, Tyler-Kabara EC, et al.. Intracortical microstimulation of human somatosensory cortex. Science Translational Medicine 2016;8(361):361ra141. doi:10.1126/scitranslmed.aaf8083
  6. Flesher SN, Downey JE, Weiss JM, Hughes CL, Herrera AJ, Tyler-Kabara EC, et al.. A brain-computer interface that evokes tactile sensations improves robotic arm control. Science 2021;372(6544):831-836. doi:10.1126/science.abd0380
  7. Corkin S. Somatosensory Thresholds. Archives of Neurology 1970;23(1):41. doi:10.1001/archneur.1970.00480250045007
  8. Cavanna AE, Trimble MR. The precuneus: a review of its functional anatomy and behavioural correlates. Brain 2006;129(3):564-583. doi:10.1093/brain/awl004
  9. Johansson RS, Flanagan JR. Coding and use of tactile signals from the fingertips in object manipulation tasks. Nature Reviews Neuroscience 2009;10(5):345-359. doi:10.1038/nrn2621
  10. Ernst MO, Banks MS. Humans integrate visual and haptic information in a statistically optimal fashion. Nature 2002;415(6870):429-433. doi:10.1038/415429a
  11. Zipser D, Andersen RA. A back-propagation programmed network that simulates response properties of a subset of posterior parietal neurons. Nature 1988;331(6158):679-684. doi:10.1038/331679a0
  12. Mountcastle VB. Modality and topographic properties of single neurons of cat's somatic sensory cortex. Journal of Neurophysiology 1957;20(4):408-434. doi:10.1152/jn.1957.20.4.408
  13. Merzenich MM, Nelson RJ, Stryker MP, Cynader MS, Schoppmann A, Zook JM. Somatosensory cortical map changes following digit amputation in adult monkeys. Journal of Comparative Neurology 1984;224(4):591-605. doi:10.1002/cne.902240408
  14. Raspopovic S, Capogrosso M, Petrini FM, Bonizzato M, Rigosa J, Di Pino G, et al.. Restoring natural sensory feedback in real-time bidirectional hand prostheses. Science Translational Medicine 2014;6(222):222ra19. doi:10.1126/scitranslmed.3006820
  15. Schmahmann JD. Vascular Syndromes of the Thalamus. Stroke 2003;34(9):2264-2278. doi:10.1161/01.STR.0000087786.38997.9E
  16. Botvinick M, Cohen J. Rubber hands 'feel' touch that eyes see. Nature 1998;391(6669):756. doi:10.1038/35784
  17. Corbetta M, Shulman GL. Spatial neglect and attention networks. Annual Review of Neuroscience 2011;34:569-599. doi:10.1146/annurev-neuro-061010-113731
  18. Rhoton AL. The Cerebrum. Neurosurgery 2002;51(suppl_4):S1-1-S1-52. doi:10.1097/00006123-200210001-00002
  19. Yousry T. Localization of the motor hand area to a knob on the precentral gyrus. A new landmark. Brain 1997;120(1):141-157. doi:10.1093/brain/120.1.141
  20. Aflalo T, Kellis S, Klaes C, Lee B, Shi Y, Pejsa K, et al.. Decoding motor imagery from the posterior parietal cortex of a tetraplegic human. Science 2015;348(6237):906-910. doi:10.1126/science.aaa5417

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