HPHR Paper™ · No. 29 · HPHR-P-029
Does the Brain Have a Map of the Body?
Companion Paper to HPHR Academy™ Episode 29 HPHR Insight: HPHR-I-035
- Author
- Dr Emmanuel Frantzis, DC, MSc, IFMCP, DACNB, DO (MP)
- Level
- Beginner
- Topic
- Pain Neuroscience
Author
Founder, High Performance Health Restoration™
Executive Summary
The brain maintains organized neural representations of the body, but these representations should not be understood as rigid anatomical pictures. They are dynamic systems constructed from cutaneous, proprioceptive, visual, vestibular and motor information. Within the primary somatosensory cortex, different body regions are represented somatotopically, with neighbouring regions of the body generally represented within neighbouring cortical territories.[1,2] The amount of cortex devoted to a body region does not correspond directly to its physical size. Areas requiring greater sensory discrimination, including the fingers and lips, occupy proportionately larger cortical territories.[3] Multiple maps exist within the primary somatosensory cortex, and their subdivisions process different combinations of tactile and proprioceptive information.[1] These representations help the nervous system determine the location and condition of the body, guide movement and compare anticipated sensory consequences with the feedback that follows an action. Their capacity for modification is an expression of neuroplasticity. This adaptability supports learning and recovery, but it also means that altered sensory input, injury, pain and changes in movement may influence how the nervous system represents the body.
THE PRIMARY SOMATOSENSORY CORTEX
The primary somatosensory cortex, first written as S1, is located within the anterior parietal cortex and includes Brodmann areas 3a, 3b, 1 and 2.[1,4] Ascending somatosensory information reaches these cortical regions through the thalamus after undergoing processing within spinal and brainstem pathways.[1,4]
The subdivisions of S1 do not process identical information. Proprioceptive input is strongly represented within area 3a, while cutaneous input is prominent within area 3b.[1] Area 1 is particularly involved in cutaneous processing and texture discrimination. Area 2 integrates cutaneous and proprioceptive information important to the recognition and manipulation of objects.[1]
Each of these cortical areas contains an organized representation of the contralateral side of the body.[1] The foot is represented closer to the cerebral midline, while the face and tongue are represented more laterally. The familiar sensory homunculus illustrates this general arrangement, but it should not be interpreted as a literal miniature body embedded within the cortex.
CORTICAL MAGNIFICATION
Some areas of the body occupy substantially more cortical territory than others. Sur et al. (1980) demonstrated that the amount of somatosensory cortex devoted to a body region reflects factors such as receptor density and receptive-field organization rather than the region’s physical dimensions.[3] The hands, lips and face receive disproportionately large representations because they require high levels of sensory discrimination.
Cortical magnification demonstrates that the nervous system allocates processing resources according to functional demand. The hand occupies relatively little physical space compared with the trunk, but its capacity for object manipulation, tactile exploration and fine discrimination requires considerably more detailed cortical processing.
REPRESENTATION AS A DYNAMIC PROCESS
The word “map” may create the impression of a fixed anatomical structure. Neural representations are more accurately understood as patterns of connectivity and activity that are continually informed by sensory input and behaviour.
In my thesis, I discussed literature demonstrating that the somatosensory cortex participates not only in perception, but also in error processing, motor adaptation and learning.[5,6] Mathis et al. (2017) found that somatosensory cortical function was essential to adapting forelimb movements under altered sensory conditions.[5] Kumar et al. (2019) subsequently demonstrated that the somatosensory cortex participates in the consolidation of human motor memory.[6]
These findings indicate that somatosensory representations do more than identify where a sensation occurred. They help establish and preserve the sensory states used to guide subsequent movement.
BODY REPRESENTATION AND MOVEMENT
Accurate movement requires the nervous system to estimate the body’s position before an action begins, predict the sensory consequences of that action and compare those predictions with incoming feedback. Proprioceptive information from muscles and joints, cutaneous information from the skin, vestibular information concerning head movement and orientation, and visual information concerning the body and environment all contribute to this process.
Somatosensory and motor representations also communicate extensively. Areas 1 and 2 of the somatosensory cortex have reciprocal connections with the primary motor cortex. Parietal regions also communicate with premotor, motor, thalamic and cerebellar systems.[1] The brain’s representation of the body is therefore inseparable from the networks used to plan and control action.
PAIN AND THE REPRESENTED BODY
Pain may influence how attention, sensation and movement are organized around a body region. Altered afferent input, protective movement, reduced use and changes in cortical excitability may all affect the neural activity through which the body is represented.
The literature I discussed in my thesis supports the conclusion that bodily representation is plastic and that pain may influence several levels of sensory and motor processing. The extent, location and clinical significance of these changes vary across individuals and conditions. A change in body perception does not necessarily prove that large-scale remapping has occurred within the primary somatosensory cortex.
The most defensible conclusion is therefore not that every person with persistent pain has a distorted cortical map. It is that pain may alter bodily representation at several levels, including sensory processing, sensorimotor integration, attention, movement and body perception.
CLINICAL RELEVANCE
A person may have difficulty accurately locating a body region, distinguishing sensory stimuli, coordinating movement or trusting a painful area even when the region remains anatomically intact. Such findings may reflect changes at several levels of sensorimotor processing and should not automatically be attributed to continuing structural tissue damage.
Rehabilitation can provide the nervous system with repeated sensory and motor information through touch, proprioception, visual feedback and purposeful movement. This does not mean that every pain condition can be corrected by “remapping the brain.” It means that sensory discrimination and movement provide biologically meaningful information capable of participating in neural adaptation.
CURRENT EVIDENCE UPDATE
The following publications were incorporated during development of the Academy Lesson but were not included in my thesis. They are therefore confined to this Current Evidence Update.
Flor (2003) reviewed evidence that persistent neuropathic and musculoskeletal pain may be accompanied by functional reorganization within somatosensory and motor systems.[CU1] The review reported relationships among chronicity, pain intensity and representational change in certain pain conditions.
Moseley and Flor (2012) subsequently reviewed sensory, motor and cognitive interventions intended to influence altered cortical representations.[CU2] Their analysis supported the possibility that cortical plasticity may provide a rehabilitation target while demonstrating that persistent pain cannot be reduced to one cortical mechanism.
HPHR PERSPECTIVE
The nervous system does not simply monitor a pre-existing body. It continually constructs and updates a working representation from the information available to it. That representation guides perception, movement and protective behaviour.
The clinical objective is not merely to produce movement, but to provide clear and useful sensory information through movement. Accurate, tolerable and purposeful activity may help the nervous system develop a more reliable understanding of the body and its present capabilities.
HPHR PRINCIPLE™
The brain does not simply monitor the body. It continually updates its understanding of it.
CONCLUSION
The brain contains multiple organized representations of the body rather than one fixed internal map. These representations integrate tactile, proprioceptive, visual, vestibular and motor information and are continually modified through experience. Their plasticity supports sensory discrimination, learning and movement while also providing a mechanism through which injury, pain and altered behaviour may influence bodily perception.
PROVENANCE STATEMENT
The main Paper is derived exclusively from my thesis sections addressing the primary somatosensory cortex, somatotopic organization, sensorimotor integration, sensory learning and motor adaptation. Sources not contained in my thesis appear only within the Current Evidence Update and use a separate reference sequence.
REFERENCES FROM MY THESIS
Delhaye, B. P., Long, K. H., & Bensmaia, S. J. (2018). Neural basis of touch and proprioception in primate cortex. Comprehensive Physiology, 8(4), 1575–1602. https://doi.org/10.1002/cphy.c170033
Iwamura, Y., Iriki, A., & Tanaka, M. (1994). Bilateral hand representation in the postcentral somatosensory cortex. Nature, 369(6481), 554–556. https://doi.org/10.1038/369554a0
Sur, M., Merzenich, M. M., & Kaas, J. H. (1980). Magnification, receptive-field area, and hypercolumn size in areas 3b and 1 of somatosensory cortex in owl monkeys. Journal of Neurophysiology, 44(2), 295–311. https://doi.org/10.1152/jn.1980.44.2.295
Loutit, A. J., Vickery, R. M., & Potas, J. R. (2021). Functional organization and connectivity of the dorsal column nuclei complex reveals a sensorimotor integration and distribution hub. Journal of Comparative Neurology, 529(1), 187–220. https://doi.org/10.1002/cne.24942
Mathis, M. W., Mathis, A., & Uchida, N. (2017). Somatosensory cortex plays an essential role in forelimb motor adaptation in mice. Neuron, 93(6), 1493–1503.e6. https://doi.org/10.1016/j.neuron.2017.02.049
Kumar, N., Manning, T. F., & Ostry, D. J. (2019). Somatosensory cortex participates in the consolidation of human motor memory. PLOS Biology, 17(10), e3000469. https://doi.org/10.1371/journal.pbio.3000469
CURRENT EVIDENCE UPDATE REFERENCES
CU1. Flor, H. (2003). Cortical reorganisation and chronic pain: Implications for rehabilitation. Journal of Rehabilitation Medicine, Supplement 41, 66–72. https://doi.org/10.1080/16501960310010179
CU2. Moseley, G. L., & Flor, H. (2012). Targeting cortical representations in the treatment of chronic