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.
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