Sensorimotor Representations

In  anatomical  terms,  sensation  and  movement are each represented across distributed regions in the  human  brain.  Somatosensory  information— that  is,  information  pertaining  to  sensation  of the  body—is  gathered  from  the  environment  and transmitted  to  the  brain  via  the  spinal  cord  and via  the  thalamus,  from  where  it  is  distributed  to three  regions  of  the  parietal  cortex:  the  primary somatosensory  cortex  (located  in  the  postcentral gyrus), the secondary somatosensory area, and the posterior  parietal  area.  These  areas  are  primarily involved in the representation of touch and of limb position, particularly as it occurs in the absence of visual  feedback.  The  motor  regions  of  the  brain include  the  primary  motor  cortex,  located  in  the precentral  gyrus,  the  premotor  cortex,  and  the supplementary  motor  cortex.  These  areas  act  in concert to contribute to the organization and production of movement and are collectively referred to as the sensorimotor cortex.

First  illustrated  by  Wilder  Penfield  and  Edwin Boldrey in 1937, the sensorimotor cortex, in particular the primary somatosensory and motor cortices,  is  topographically  organized,  meaning  that particular  areas  of  the  body  map  onto  particular areas  of  the  cortex.  Furthermore,  the  size  of  the representation  is  proportional  to  the  sensitivity (primary somatosensory cortex) and dexterity (primary motor cortex) of the represented area, rather than its physical size. For example, the somatosensory representation of the thumb and the lips are each  approximately  as  large  as  that  of  the  entire leg. The reason is that the skin of the thumb and the lips are densely populated by cutaneous receptors, giving these areas a high degree of sensitivity and therefore large cortical representations, while the  skin  on  the  leg  is  far  less  densely  populated by  these  receptors  and  therefore  less  sensitive.  A similar  topographical  map  exists  in  the  primary motor cortex, in which parts of the body that perform  highly  dexterous  movements,  for  example the fingers and tongue, occupy a larger volume of the primary motor cortex with projections to the muscles of these areas, compared with other parts of the body such as the trunk and pelvis.

On  a  cellular  basis,  neurons  in  the  primary motor cortex are tuned to the direction in which a movement is performed. These cells are activated strongly  when  movements  are  performed  in  one direction  in  space  and  less  so  for  movements  in another.  It  is  unclear,  however,  whether  the  preferred  direction  for  particular  cell  populations  is represented in an intrinsic coordinate system—for example,  flexion  and  extension  of  a  joint—or  an extrinsic  coordinate  system—for  example,  movement to the left or the right.

Plasticity of Sensorimotor Representations

For  many  years,  it  was  thought  that  there  was little capacity for sensorimotor representations to be altered in the adult brain. More recently, it has been  demonstrated  extensively  that  sensorimotor representations  in  the  adult  brain  can  undergo functional reorganization, a process known as plasticity, in response to damage to peripheral or central structures, or as a consequence of experience. For  example,  imaging  studies  have  shown  that arm  amputees  activate  a  larger  than  normal  area of sensorimotor cortex when performing shoulder movements of the corresponding amputated arm, suggesting  that  the  remaining  arm  muscles  have an expanded representation. Similarly, transcranial magnetic  stimulation  (TMS),  which  can  be  used to map the area of sensorimotor cortex projecting to  specific  muscles,  has  demonstrated  that  motor practice results in an increase in the area of sensorimotor representation of the muscles involved in the practice. These observations confirm that sensorimotor  representations  are  dynamic  and  malleable, depending on circumstances.

Motor Learning as a Tool to Understanding Sensorimotor Representations

Motor learning can be considered as a form of perturbation in which behavior and neuronal activity is examined before, during, and after a period of adaptation.  The  pattern  of  behavior  during  these three  important  periods  of  adaptation  can  reveal different  properties  of  the  movement  representation  in  the  brain.  In  particular,  the  brain’s  ability  to  generalize  motor  learning  and  apply  what was  learned  to  different  contexts  gives  important insights  into  sensorimotor  representation  in  the brain.  For  example,  learning  to  hit  a  tennis  forehand  does  not  require  exposure  to  all  possible ball  trajectories,  only  a  subset.  Once  learned,  the challenge  is  to  optimize  the  generalization  of  the skill  in  the  appropriate  context.  Too  little  generalization results in improvements that are limited to  very  specific  conditions,  whereas  overgeneralization results in inappropriate actions for a given condition.  For  instance,  it  is  no  use  trying  to  hit a  forehand  when  a  backhand  stroke  is  required. The key point here is that patterns and constraints of  generalization  can  reveal  how  movements  are encoded in the brain.

Levels of Sensorimotor Representation

Complex sensorimotor skills can be represented in a  number  of  different  frames  of  reference.  In  the example  of  hitting  a  tennis  ball,  the  brain  must first  represent  the  purpose  of  the  movement  (hitting  the  ball),  the  required  end-point  result  (racquet  meeting  the  ball),  the  required  joint  torques to position and orientation of the arm, and finally the  required  muscle  activation  pattern  to  achieve the  movement  outcome.  At  each  stage,  there  is  a different  sensorimotor  representation  that  guides the building of the movement. During motor learning, the exact pattern of errors in the movements can  give  clues  about  the  neural  representation  in the brain and the frame of reference that was used during the adaptation phase.

References:

  1. Hluštik, P., Solodkin, A., Gullapalli, R. P., Noll, D. C., & Small, S. L. (2001). Somatotopy in human primary motor and somatosensory hand representations revisited. Cerebral Cortex, 11, 312–321.
  2. Donoghue, J. P. (1995). Plasticity of adult sensorimotor representations. Current Opinion in Neurobiology, 5, 749–754.
  3. Saltzman, E. (1979). Levels of sensorimotor representation. Journal of Mathematical Psychology, 20, 91–163.
  4. Soechting, J. F., & Flanders, M. (1989). Sensorimotor representations for pointing to targets in threedimensional space. Journal of Neurophysiology, 62,582–594.

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