Sensory Systems: Auditory, Tactile, Proprioceptive

Sensory  systems  are  the  peripheral  parts  of  the nervous system responsible for the transformation of  physical  stimuli  into  a  neural  code.  Receptors of  each  sensory  system  are  sensitive  to  a  distinct kind of energy, like the hair cells of the inner ear to  sound  energy  and  the  mechanoreceptors  of the  tactile  system  to  mechanical  energy  or  the visual  receptors  to  electromagnetic  energy.  Each system encodes four essential features of a stimulus. Besides stimulus modality, these are intensity, duration,  and  spatial  location.  Vision,  hearing, kinesthesia, and touch serve as initial functions for generating information within the central nervous system  (CNS),  which  in  turn  contributes  sensations and bottom-up input to perception and up to higher cognition. Vision is discussed more in depth in  a  separate  entry,  whereas  the  roles  of  hearing, kinesthesia, and touch are discussed more fully in this entry.

Nearly  all  kinds  of  goal-directed  actions  in sports  rely  on  currently  updated  information about the environment as well as about one’s own posture  and  kinesthesia.  Though  vision  is  often understood as the most important sensory modality concerning the regulation of actions in sports, it is also crucial to realize that vision only can work in relation to the perceived self. Information about the physical self from different modalities—tactile, proprioceptive  and  vestibular  information—is integrated   within   a   multisensory   supramodal coherent  representation,  the  body  scheme.  This body representation is broadly involved into spatial sensorimotor processing of skilled action and it  is  continuously  updated  with  ongoing  movements.  Patrick  Haggard  and  Daniel  Wolpert  distinguished  between  a  body  scheme  and  a  body image  much  more  dependent  on  conscious  visual representations.

The  organization  of  the  somatosensory  representation—as a main element of the body scheme— is  related to the localization of  a  distinct  area  on the body surface. The region of skin from which a tactile stimulus will alter the firing rate of a receptor  neuron  is  called  the  receptive  field.  Receptive fields have been found for neurons of the somatosensory,  the  visual,  and  the  auditory  systems. Neighboring  cutaneous  receptive  fields  from  the human skin are represented also in adjacent cortical fields, leading to a somatotopic organization of the somatosensory cortex in the form of a homunculus.  topographic  organization  of  the  cortical projection pattern can also be found for the visual system  in  the  primary  visual  cortex  as  well  as  a tonotopy  pattern  in  the  primary  auditory  cortex, altogether  known  as  sensory  maps.  Nevertheless visual input is integrated into the body scheme in a mostly unconscious manner, as shown by Michael Graziano  and  coauthors  at  the  end  of  the  20th and  beginning  of  the  21st  century,  and  is  further modulated  by  auditory  input.  Multisensory  integration  is  particularly  crucial  for  the  emergence of  a  coherent  body  scheme,  and  some  empirical findings  illustrate  the  continuous  impact  of  intersensory adjustments. Examples include the rubber hand illusion, reported by Matthew Botvinick and Jonathan Cohen, and an altering of perceived size of a single body segment evoked by the vibration of related tendons, reported by Jim Lackner.

Sensation Versus Perception

In contrast to perception, which explicitly includes the  level  of  awareness,  sensory  sensations  do  not usually  reach  the  level  of  conscious  awareness. Sensation  is  the  detection  of  basic  features  of  a stimulus.  It  is  understood  as  the  first  stage  of  an activation  of  a  sensory  system,  as  a  peripheral internal  representation  of  distal  stimuli,  clearly dominated  by  bottom-up  processes.  Nevertheless sensory  information  can  directly  influence  motor behavior  and  ascending  sensory  pathways  have several  linkages  to  descending  motor  functions. Besides such physiological indications for sensory motor  circuits,  there  are  supplemental  references from the field of applied kinesiology. Here, special training procedures as well as scientific approaches in  fields  like  sensorimotor  control,  sensorimotor training, or sensorimotor adaptation can be found; they involve certain subdomains of motor behavior with  a  particular  low  impact  of  conscious  attention such as the regulation of balance or posture, as well as speech articulation. On the other hand, perception is based on higher brain functions and the  bottom-up  fractions  are  supplemented  much more   by   top-down   fractions   like   experiences and  expectations  when  interpreting  objects  and events in the world. However, as the title of James Jerome  Gibson’s  book,  The  Senses  Considered  as Perceptual  Systems,  suggests,  it  is  utterly  impossible  to  definitively  separate  levels  and  functions of the sensory system from that of the perceptual system.

Proprioception: Kinesthesia, Touch, and the Vestibular System

Proprioception can be subdivided into two or three subsystems describing the tactile system as part of proprioception.

  1. Kinesthesia (literally,  sense  of  movement) generates   information   about   the   positions   and movements  of  the  limbs  and  other  body  parts including strength sensation. Based on kinesthesia, humans possess information about the rate and the direction  of  limb  movements.  This  information can  be  self-generated  or  externally  imposed  and can  be  encoded  by  different  kinds  of  mechanoreceptors  in  joints  and  capsules  (slowly  adapting Ruffini  endings),  muscles  (muscle  spindle  receptors),  tendons  (Golgi  tendon  organs)  and  ligaments  (slowly  adapting  Golgi  receptors).  Muscle spindles  signal  limb  position  and  movement  by encoding   muscle   length;   Golgi   tendon   organs encode muscle tension. Subtlety of kinesthesia can be determined by measuring the just notable difference or detection threshold for a joint, the threshold displacement in degrees as well as the smallest angular  excursions  or  the  smallest  rotation  angle of a joint. Reported values vary between joints and can  be  related  to  the  initial  joint  angle,  muscle loading, active versus passive effectuation, and age of the participants.
  1. The tactile system, as well as parts of kinesthesia, is  based  on  mechanoreceptors  in  the  skin. Besides  detection  and  specification  of  mechanical deformation of the skin (force, pressure), about 12 different   kinds   of   cutaneous   mechanoreceptors play  a  significant  role  in  kinesthesia.  Cutaneous mechanoreceptors  can  be  subdivided  into  rapidly responding (A) vs. slowly adapting receptors (B).
  • A. Receptors of the first subcategory are well suited for the specification of rapid movements and fine motor skills. The loss of skin sensation of the hand—as an example of a body part with a high degree of tactile resolution and a large receptive field in the cerebral cortex—causes severe deficits in fine motor hand skills.
  • B. Slowly adapting receptors signal static position information as well as information about slow displacements. Several functions of the tactile system rely on such slowly adaptive proprioceptors, as shown by Type 1 and Type 2 receptors, which are associated with Merkel cell complexes and Ruffini endings and continue discharge with maintained deformation of the skin.

Kinesthetic information and tactile information are  essential  parts  of  the  somatosensory  system. Tactile  information  is  entangled  closely  to  kinesthetic  information,  as  demonstrated  by  the  bending  of  any  joint  that  stretches  a  related  region  of skin  around  the  joint  and  relaxes  another  area proportionately.

  1. The vestibular system is located in the inner ear as  three  semicircular  converging  canals—the superior, the posterior, and the horizontal canal— which  indicate  rotational  accelerations;  they  are supplemented by the otolith organs in the saccule and  utricle,  which  indicate  linear  accelerations. Besides balance control, the vestibular system contributes  to  spatial  orientation  and,  via  mediating structures, posture regulation, as well as control of eye  movements.  Balance  regulation,  as  a  well-known example of sensorimotor regulation, is not solely  governed  by  the  vestibular  system;  kinesthetic information and visual information are integrated seamlessly.

Hearing and the Emergence of Acoustic Events

Although it is obvious that vision is the dominating  sense  for  many  sports  as  well  as  for  motor learning, which is frequently based on visual models,  it  might  be  surprising  that  hearing  is  a  sense that  is  exclusively  sensitive  to  the  perception  of motion.  Sound  is  an  acoustic  consequence  of  a kinetic  event—the  existence  of  a  kinetic  event  is essential  for  generating  a  sound  event.  If  there  is no movement in the surroundings, there is nothing to hear. The air or other media such as water and ground  must  be  set  into  vibration  by  motion  to generate and to transmit audible sounds within the hearing range of the human ear. Only sound events within the frequency range of about 20 hertz (Hz) and  up  to  20,000  Hz  can  be  transformed  within the hair cells located in the cochlea. Additionally, a minimal pressure of the sound wave—also dependent on frequency—is necessary to elicit an auditory  sensation.  As  noted  by  Bertram  Scharf  and Søren Buus, a sound of 2 kilohertz (kHz) is audible at a sound pressure level (SPL) of 0 decibels (dB), a sound of 3.8 kHz at about –5 dB, as has been measured on young adults; both indicate the extreme sensitivity of the human ear. In comparison to the optic  nerve,  consisting  of  about  1,000,000  nerve fibers,  the  auditory  nerve  subsumes  only  about 30,000 fibers; though the number and interpretation  is  arguable,  this  may  indicate  that  the  ear  is much more designed to analyze temporal features contrary to vision, which is more sophisticated in analyzing spatial features.

When  setting  the  frequency  range  of  human movements  in  relation  to  the  hearing  range,  it is  evident  that  humans  are  unable  to  hear  their own  movements  directly.  Highest  motion  frequencies  have  not  been  observed  in  sportsmen but  in  world-class  pianists  performing  trills  at about 16 Hz and thereby below the threshold of the  hearing  range  at  20  Hz.  Only  the  impact  of human  movements  on  ambient  surfaces,  such  as hitting  the  tennis  ball  with  the  racket  or  stemming  the  skis  into  an  icy  snow,  induces  audible sounds.  But  once  a  kinetic  event  generates  an audible  sound,  there  is  much  kinetic  information  coded  acoustically.  On  a  wooden  surface,  a bouncing  Ping-Pong  ball  sounds  different  from  a tennis  ball.  Kinetic  features  as  well  as  properties of  involved  materials  (balls,  wood)  specify  the sound  parameters  systematically,  which  can  be subdivided into two essential categories: material induced sound features versus kinetically induced sound features.

The  material  category  specifies  sound  features such as spectral composition and sound envelope, which  includes  the  components  attack,  sustain, and decay. These parameters are specified by the physical  parameters  of  the  related  materials  and the related medium, the air. When hitting the tennis  ball,  the  density  of  the  racket  as  well  as  the tension  of  the  racket  string  specify  the  sound  in interaction  with  the  special  features  of  the  tennis  ball  (material,  pressure).  Further  features  of the  sound  are  assigned  to  the  kinetic  category, like amplitude and duration of sound, which are specified  by  kinematic  and  dynamic  parameters. The  kinetic  energy  of  the  approaching  tennis ball  as  well  as  direction  and  spin  specify  loudness,  hardness,  timbre,  and  sound  duration;  the selected  technique  also  influences  these  characteristics: A slice sound exhibits a longer duration and  is  of  less  hardness,  so  it  sounds  smoother than  a  straight  backhand.  There  is  much  information about the kinetics encoded in sound, and motor  control  and  motor  learning  are  indeed supported  by  auditory  information.  Only  a  few experts are aware of the auditory impact on motor behavior because auditory information processing takes place widely in the background of conscious awareness.

Sensory Auditory Information in Sports

The impact of music and sound on motor behavior  is  multifaceted.  Football  players  adjust  with team members to fans’ acclamation; they respond to  the  acoustical  atmosphere  within  the  arena, and  they  react  to  the  rhythm  and  the  accents  of the referee’s whistles. Sport gymnasts and dancers synchronize  their  movements  to  accompanying music  or  arrange  certain  elements  related  to  the melody  individually,  together  with  a  partner  or within an ensemble. Runners synchronize the step cadence  to  perceived  music  and  even  pace  themselves over a race by arranging a special sequence of  different  music  pieces  with  varying  tempi  for tuning their running speed over the course. Music can be used to modulate the heart rate (HR), the perceived exertion, or the general arousal as well as  the  basic  activation  of  distinct  brain  regions. An  overview  on  effects  of  music  on  sports  has recently  been  published  by  Costas  Karageorghis and David-Lee Priest.

In  most  of  these  correlations  between  music and  motor  behavior  perceptions,  higher  cognitive  functions  are  broadly  involved:  Music  is audible  only  after  auditory  stimuli  are  processed by   a   widespread   cortical   network.   However, a  more  direct,  sensory-based  impact  of  sound on  motor  behavior  occurs  as  acoustic  information  from  earlier  stages  of  auditory  processing  is integrated  into  motor  control.  Natural  motion attendant  sounds  are  used  for  controlling  and optimizing  motor  behavior:  The  performance of  a  tennis  player  decreases  if  auditory  perception  is  occluded,  and  similar  findings  have  been reported   on   table   tennis.   Synchronization   in team  rowing  is  partially  realized  by  analyzing sounds of the boat and of the swivels. In a diversity  of  sports,  the  “all-around  sense”  of  hearing directs  the  spatially  restricted  visual  attention. For  instance,  in  team  sports  the  orientation  of gaze  is  partially  affected  by  surrounding  sounds induced by the steps of teammates or opponents. Auditory  input  affects  gaze  behavior  as  well  as orientation  behavior  fundamentally  straight  on the sensory level, when integrated with visual and proprioceptive  input  in  the  colliculi  superiores. These structures located in the brain stem are on the  other  hand  emitting  motor  efferences,  which are  directly  specified  by  the  special  formation  of the current multisensory input. The multisensory motor  characteristics  of  the  colliculi  superiores have been studied intensely by Barry E. Stein and M.  Alex  Meredith.  This  region  can  be  understood  as  an  outstanding  example  for  a  straight but flexible responding sensorimotor interface. It indicates  how  seamlessly  multisensory  information is merged together already on the subcortical level—and  even  down  to  the  level  of  a  single neuron: A multisensory convergence neuron integrates  auditory,  visual,  and  proprioceptive  input, and  the  same  neuron  responds  with  descending motor output.

Further Perspectives for Sports and Exercise

There  is  no  doubt  that  skilled  action  in  sports relies   on   supramodal   representations.   It   has become  evident  that  the  integration  of  information from different sensory modalities in terms of an early multisensory integration is crucial for the emergence of a body scheme as well as for spatial representations, where visual, auditory, and body segment-related  sensory  maps  are  continuously integrated into supramodal representations. These representations  act  as  a  reference  framework  for the  regulation  of  skilled  action.  For  developing new  methods  of  training  and  exercises  prospectively, mechanisms of multisensory integration can be  focused.  Information  from  different  modalities  is  integrated  according  to  certain  rules  (spatial  convergence,  temporal  coherence).  If  one  of the  engaged  sensory  streams  can  be  shaped  in  a certain  form  or  an  additional  perceptual  stream can be created in a way that is integrated into the supramodal representation, a diversity of applications  can  be  addressed  in  the  future,  to  include enhancement  of  mental  training,  recalibration of  the  body  scheme,  or  a  directed  adaptation  of sensations  of  a  distinct  modality.  The  approach of  realtime  movement  sonification,  developed by  Alfred  Effenberg  and  currently  neurophysiologically  supported  by  Gerd  Schmitz  and  colleagues, could be a method prospectively used in many  different  applications  in  sports  and  motor rehabilitation.

References:

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