Eye Movement

The eyes perceive spatial information with full acuity when light falls on a small region at the back of the  retina  called  the  fovea.  Because  of  the  small area of the fovea, the area over which we are able to see clearly is only about 2° to 3° of visual angle. Visual angle indicates the size of an image on the retina. It is usually determined by extending lines from  the  edges  of  the  object  as  viewed  in  space through the lens to the retina. One way to estimate the  size  of  this  area  is  to  hold  your  thumb  up  at arm’s length on object in space. The width of your thumb subtends 2° to 3° of visual angle projected into  space.  Gaze  control  is  the  process  of  directing the eyes to objects or events within a scene in real time and in the service of the ongoing perceptual  and  cognitive  processes.  The  reason  the  eyes move is to keep objects of interest projected on the high-acuity  fovea.  Usually  the  eyes  move  before the head, localizing a target first. Head movement follows  because  of  the  greater  size  and  inertia  of the head compared to the eyes. The movement of the eyes and head to the target is normally smooth, with  the  processing  of  visual  information  occurring  within  100  ms  of  the  eyes  stabilizing  on  the new location.

Eye  movements  were  first  studied  with  the head  fixed  as  even  the  slightest  movement  of  the head could cause large errors in the eye data being collected.  Today,  with  the  advent  of  light  mobile eye  trackers,  it  is  possible  to  record  accurate  eye movements  as  the  participant  moves  naturally throughout  the  world,  making  head  movements. Eye-tracking  methods  permit  the  measurement of  the  location  of  the  gaze  in  space  or  the  eye

movements  relative  to  the  head.  Most  eye  trackers  used  in  sport  are  corneal-reflection  systems that  consist  of  two  video  cameras  mounted  on  a headband or glasses. One camera records a video of the eye and the other the scene in front of the performer. The location of the gaze is determined using the center of the pupil and the corneal reflection, which is artificially placed on the eye (using infrared  light).  Data  are  collected  digitally  in  x/y coordinates,  as  well  as  the  location  of  the  gaze being  shown  on  the  scene  video  as  a  cursor  or other  marker  every  frame  or  field  of  video.  The accuracy  of  most  corneal-reflections  systems  is about 1 degree of visual angle. The most common National  Television  System  Committee  (NTSC) data collection is 30 Hz, or 30 frames per second (33.33  ms  per  frame);  or  60  Hz,  or  60  fields  per second (16.67 per field).

Types of Eye Movements and Gaze

Most  eye-tracking  systems  record  fixations,  pursuit tracking, and saccadic eye movements. A fixation occurs when the gaze is held on an object or location  within  3°  of  visual  angle  for  100  ms  or longer. The 100 ms threshold is approximately the minimum amount of time the brain needs to recognize or become aware of what is being viewed. Pursuit-tracking  eye  movements  are  similar  to fixations, except the eyes follow a moving object, such as a ball or a person. The 100-ms threshold still applies for the same reason as it is used for fixations. Saccades occur when the eyes move quickly from  one  fixated  or  tracked  location  to  another. Saccades  are  among  the  fastest  movements  that humans can make, exceeding 900 degrees per second.  Saccades  are  ballistic  eye  movements  that bring the point of maximal visual acuity onto the fovea  so  that  an  object  can  be  seen  with  clarity. During saccades, information is suppressed, meaning the information between two fixated locations is  not  consciously  perceived.  Instead,  the  information  gained  during  fixations  is  maintained  in memory thereby ensuring a stable, coherent scene. Saccadic  suppression  prevents  us  from  seeing  a world that is blurry and therefore difficult to comprehend.

The  fixated  or  tracked  information  falling upon the fovea constitutes focal vision. The focal system  is  used  when  fixated  or  tracked  information  falls  on  the  fovea  and  aspects  of  an  object or  location  are  viewed  with  full  acuity  or  detail.

In  contrast,  information  that  falls  off  the  fovea becomes  increasingly  blurry  (of  low  resolution) and is viewed using ambient or peripheral vision. The ambient or peripheral system is specialized for motion  and  low  light  conditions.  Both  the  focal and ambient systems are needed for the visual system to function properly.

Visual Motor Pathways

Visuomotor control is the process whereby visual information  is  used  to  direct  and  control  movements.  Visual  information  is  registered  first  on the  retina,  then  passes  through  the  optic  nerve, the  lateral  geniculate  nucleus,  and  the  optic  or striate  radiations  to  the  visual  cortex  or  occipital lobe at the back of the head. Located in the occipital  cortex  are  visual  sensors  that  begin  the  processing  of  registering  and  interpreting  what  the performer  sees,  with  specific  detectors  for  initial registration (V1), shape (V2), angles (V3), motion (V3a),  color  (V4),  V5  (motion  with  direction), depth  and  self-motion  (V6),  and  depth  of  stereo motion (V7). Once an object, person or location is registered within the visual cortex, visual information moves rapidly forward in the brain, along the dorsal  attention  network  (DAN)  and  the  ventral attention network (VAN). The DAN projects from the occipital lobe to the parietal lobe and forward to the frontal lobe in a journey that goes roughly over the top of the head to the frontal lobe. In contrast, the ventral attention network (VAN) projects forward  along  the  sides  of  the  head  through  the temporal lobes to the frontal areas. Commands are then sent from the frontal areas to the motor cortex to initiate action. With practice and the development of expertise, it is thought the DAN system sustains  attention  on  critical  spatial  locations, thereby  blocking  out  competing  anxiety-producing stimuli that may intrude from the VAN system while the movement is being performed.

Eye Tracking Methods

Two  research  methods  are  widely  used  in  sport, called the visual-search and vision-in-action paradigms. The visual-search paradigm is the older of the  two  paradigms,  dating  back  to  the  beginning of  psychology.  When  the  visual-search  method is  used,  eye  movements  are  recorded  as  the  athlete  sits  or  stands  and  views  photographs,  videotapes,  virtual  reality,  or  computer  simulations  of events  from  their  sport.  Motor  responses  usually involve a manual key press, step sensor, or pencil-and-paper  response.  Using  this  paradigm,  it  has been established that expert athletes, compared to novices,  are  superior  at  recalling  and  recognizing sport-specific patterns of plays, are faster in detecting specific cues, are more efficient in recognizing objects,  and  are  better  at  anticipating  upcoming events.

The vision-in-action paradigm differs from the visual-search  paradigm  in  recording  the  athlete’s gaze,  in  situ,  under  conditions  similar  to  those found in real world play or competition. Elite and non-elite athletes perform well-known tasks, such as  golf  putting,  basketball  free  throw,  and  soccer or  ice  hockey  goaltending,  in  experiments  that couple perception and action. Most studies require the athlete to perform the task until an equal number of successful and unsuccessful trials have been completed thus allowing the detection of gaze and perceptual-motor differences during successful and unsuccessful performance. A major finding resulting from this method is the discovery of the quiet eye,  which  is  the  final  fixation  or  tracking  gaze maintained on a specific location prior to a critical movement in the task. Quiet eye onset occurs prior to the movement and quiet eye offset occurs when  the  gaze  deviates  off  a  specific  location  for more  than  100  ms.  Numerous  quiet  eye  studies have  shown  that  elite  athletes  have  earlier  and longer quiet eye durations than non-elite; a longer duration quiet eye is also a characteristic of more successful  performance.  Quiet  eye  training  has proven  to  be  effective  in  improving  performance compared  to  traditional  methods  in  a  number  of sports and other areas.

A few studies have also tested the same athletes using  both  the  visual  search  and  vision-in-action methods,  with  different  results  reported  in  terms of  the  gaze  and  motor  performance.  When  the athletes  were  tested  using  the  vision-in-action method,  they  used  fewer  fixations  to  fewer  locations  in  the  task  environment.  Their  fixations occurred earlier and were longer in duration than when  they  were  tested  using  the  visual-search paradigm and video simulations of the same task. When  the  vision-in-action  paradigm  was  used, the  athletes’  anticipatory  actions  occurred  earlier  and  both  their  reaction  time  and  movement times  were  faster  than  when  the  visual-search method  was  used.  These  results  suggest  that  the demands of real-world performance create a tight coupling  between  perception  and  action  that  can only  be  detected  when  the  athlete  is  tested  under realistic conditions.

References:

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