Static Visual Acuity

Static visual acuity (SVA) is “the ability to see a nonmoving target at a fixed distance.” SVA is commonly measured at far (6 m) or simulated distance viewing and is a standard part of the vision evaluation of athletes. ^, SVA has been measured in athletes both monocularly and binocularly with the Keystone telebinocular ( www.keystoneview.com ), standard Snellen charts, logMAR charts, and tumbling E and Landolt C optotypes. The author prefers to measure both monocular and binocular SVAs with Landolt C optotypes or a logMAR chart (Bailey-Lovie or ETDRS [Early Treatment Diabetic Retinopathy Study]) ^, at a distance of 3 m or further.

The athlete should be evaluated with the habitual correction used for sports participation. Interestingly, a large percentage of athletes have been reported to have a significant uncorrected refractive error; those athletes who do wear refractive correction either do not wear that correction for sports or they are undercorrected. ^{, ,} A measurement of SVA should also be performed with the best optical correction in place to assess any change in visual acuity produced by that correction. Recommendations for the use of, or change in, vision correction during sports participation are discussed in Chapter 6 .

The measurement of SVA is an essential element of any vision evaluation because degraded visual acuity can have a detrimental effect on many other aspects of visual performance. Reduced visual acuity has been shown to affect dynamic visual acuity (DVA), depth perception, ^, and accommodative accuracy. Some studies have found better SVA in athletes than in nonathletes ^{, , , , , , ,} and some have found no difference. ^{, , ,} Some athletes may perform at a high level despite having deficient visual acuity. ^{, , , ,} Several studies in which visual acuity was degraded with plus addition lenses did not find a detrimental effect of defocus. However, review of the study protocols reveals that subjects were assessed on predictable, repetitive motor tasks.

The expected level of visual acuity most likely depends on the visual task demands of each sport situation because studies of different athlete populations have found differing visual acuity results. For example, the average visual acuity of professional baseball players has been found to be significantly better than the general population average, ^, whereas this has not consistently been found in soccer. ^{, ,} The method used to measure visual acuity also has an influence on the research results. When SVA is assessed using chart systems with 20/20 (6/6) as the best acuity measurable, there is no statistically significant difference between the visual ability of athletes and that of nonathletes. ^, Even when a best acuity demand of 20/15 (6/4.5) is presented, Laby et al. found that 81% of professional baseball players could achieve that level. Laby et al. subsequently modified their assessment method to achieve acuity demands down to 20/7.5, reporting overall mean SVAs of approximately 20/13, with several athletes attaining SVAs of 20/9.2 or better. Considering all these differences, mixed results, and the demands of the sport, it is suggested that for many athletes an SVA of at least 20/15 (6/4.5) OD (oculus dexter), OS (oculus sinister), and OU (oculus unitas) is a desired standard for competitive athletes.

Dynamic Visual Acuity

Many of the visual demands in sports require discrimination of information that is moving, such as judging the speed and trajectory of a tennis serve. Traditional SVA measurements do not fully address the visual demands encountered in some types of sports, especially those in which judgments about rapidly moving objects are important. A significant amount of early research investigated the physiologic parameters of resolving visual targets in motion, referred to as dynamic visual acuity (DVA). ^, Ludvigh and Miller ^, were the first to use the term DVA, and DVA has been defined as the ability to resolve detail when relative movement exists between the observer and the test object.

Many variables in the stimulus parameters can affect DVA, including target luminance, angular velocity, and the time exposure of the target. ^, Human attributes that can affect DVA include the resolving power of the retina, peripheral awareness, oculomotor abilities, and psychologic functions that affect interpretation of visual information. ^{, ,} From the research, several conclusions have been made concerning DVA: (1) visual acuity for a moving target is reduced compared with that of a stationary target, and acuity becomes progressively more reduced with increasing velocity of the target; (2) the correlation between SVA and DVA decreases with increasing target velocity; (3) a progressive decline in acuity occurs with advancing age that accelerates in older age groups and is more pronounced with DVA than SVA; and (4) males, in general, perform consistently better than females on DVA tasks. ^{, , ,} Enhancement of either the target parameters or physiologic abilities of the subject can improve DVA abilities. ^{, ,}

DVA research was stimulated by the theory that, for many activities, discrimination of moving objects (or stationary objects while the person is in motion) is a critical element of human performance. This concept applies to sport situations as well as common daily tasks such as driving. ^{, , ,} Several attempts have been made to determine the relation between DVA and visual task performance; however, further investigation is necessary to correlate clinical measurements of DVA with visual task performance. ^a

a References: , , , , , , , , , , , ,

DVA is a widely recommended visual function to evaluate in athletes. ^b

b References: , , , , , , , , , , , , .

Few DVA measurement systems are currently available, and older methods use a predictable rotator device, with some adding a portable laser ( Fig. 4.1 ). ^, Rotators are available from Bernell ( www.bernell.com ) and JW Engineering ( www.jtac.com ) as well as other retailers. There are formal and informal assessments of DVA advocated for assessment of vestibuloocular response in brain injury, but these assessments are not suitable for differentiation of performance in athletes. The instruments used to measure DVA and kinetic visual acuity are available from Kowa (Tokyo, Japan) as the dynamic vision analyzer HI-10 and kinetic visual acuity meter AS-4, respectively. The inVision package from NeuroCom was a system that showed promise as an effective diagnostic tool in a study to establish preliminary normative data and demonstrated acceptable test-retest reliability, but it is no longer commercially available. More recently, computer software has been designed by Centro de Optometria Internacional (COI-SV at www.coi-sl.es ) and DynVA (also referred to as DinVA 3.0) to assess DVA, ^{, ,} and the DinVA 3.0 has been shown to be a valid and reliable measure of DVA. The Nike SPARQ Sensory Station included a test of DVA called Target Capture that demonstrated good test-retest reliability and is currently available from Senaptec ( www.senaptec.com ). The RightEye Dynamic Visual Acuity Tests ( www.righteye.com ; see Fig. 4.2 ) include measures of DVA with head still while the object is moving, head moving while the object is still, and both head and object moving. All three measures of DVA with RightEye have shown good test-retest reliability and have the added benefit of incorporating information collected from a remote eye tracker. A study of collegiate baseball players in Japan found significantly better DVA compared with nonathletes when subjects were allowed to track the target rather than hold steady fixation. The authors of this study suggested that the better DVA of athletes was primarily due to an improved ability to track moving targets with their eyes rather than due to improved perception of moving images on the retina.

Bernell Rotator with visual acuity chart for dynamic visual acuity testing.

RightEye system to assess dynamic visual acuity.

Courtesy RightEye, LLC.

Contrast Sensitivity

Contrast sensitivity measures the visual system’s ability to process spatial or temporal information about objects and their backgrounds under varying lighting conditions. Measurement of contrast sensitivity function (CSF) has been recommended in athletes because many sports involve visual discrimination tasks in suboptimal lighting due to environmental variability. ^{, ,} Snellen-type visual acuity measurements may not be sensitive to the subtle visual discrimination tasks inherent in many sports because the acuity task is usually performed only under high-contrast conditions. Consider the decreased contrast effect of a white, cloudy sky as a background when judging the trajectory of a fly ball in baseball when the ball is also predominantly white. Ginsburg ^, has suggested that lower spatial frequencies provide spatial localization information about objects and that higher spatial frequencies are the first to be affected by illumination, movement, and increased viewing distance. In sports that require the athlete to process visual information from an object in motion (e.g., a baseball pitch), evaluation of CSF at high spatial frequencies may provide vital diagnostic information.

Several investigations have compared CSF in athletes by using gratings of varying spatial frequency. ^{, , , , ,} The general results from these studies demonstrate elevated CSF across all spatial frequencies for athletes, and a study of Olympic athletes in a variety of sports demonstrated some differences by sport. Specifically, softball athletes performed significantly better than track-and-field athletes on both Landolt C and grating acuity at 18 cycles per degree. Athletes may have a positive benefit in CSF with aerobic excercise. Contrast sensitivity also may be degraded in contact lens wearers if the lenses are not optimal, even when visual acuity appears acceptable. Interestingly, reducing CSF seemed to affect rifle shooting performance more than reducing visual acuity in a study to determine minimum impairment criteria for vision-impaired shooting. Therefore contrast sensitivity measurement is an essential part of the evaluation of athletes who wear contact lenses during sports participation.

Many systems are commercially available to measure CSF. Most use grating patterns that vary in spatial frequency and contrast level; however, fixed-chart and computer-generated symbols of varying contrast levels are also available. Contrast sensitivity measurements usually involve detection of a threshold contrast level at each spatial frequency, with the resultant CSF plotted on a graph. The principal instruments used to measure CSF in athletes are the Vistech Contrast Test System ( www.vistechconsultants.com ) and Vector Vision contrast sensitivity test ( www.vectorvision.com ). These systems were primarily chosen for the speed of test administration and portability of the tests. ^{, , , , , ,} The Vistech system is externally lit and requires a light meter to calibrate, whereas the Vector Vision test is internally illuminated and somewhat self-calibrating ( Fig. 4.3 ). Major concerns have been reported regarding the reliability and repeatability of the Vistech Contrast Test System for measuring CSF, although other studies have demonstrated adequate reliability and repeatability. Letter-based CSF charts, such as the Pelli-Robson chart or Mars Letter Contrast Sensitivity Test, have shown better repeatability, ^, while the Mars charts may be more practical for sports vision use due to better portability and durability. Computer-based measures of CSF have become widely available as computer-based acuity chart systems have become ubiquitous, offering CSF assessment using a variety of optotpyes or grating patterns. The Nike SPARQ Sensory Station, and the similar Senaptec Sensory Station, use a concentric ring pattern rather than a vertically oriented grating to assess CSF. The use of a concentric ring target is thought to minimize the influence on astigmatism on CSF and has been found to have good test-retest reliability. Both the Pelli-Robson and Senaptec Sensory Station measure CSF at the height of the CSF (approximately 6 cycles/degree) and the high spatial frequency cutoff (typically 18 cycles/degree).

The CSV-1000E contrast sensitivity chart.

Whatever CSF test system is chosen, evaluation is often recommended for athletes and should be performed binocularly with habitual sports correction in place. ^c

c References: , , , , , , , , , ,

Refractive Status

Assessment of refractive status is an essential element of the visual evaluation of the athlete. Interestingly, it is such a basic element of a vision evaluation that it is rarely directly discussed in the literature describing evaluation procedures for athletes. Ample data are available concerning visual acuity performance, which has obvious implications for uncorrected refractive errors, but refractive status of the athletes is presented infrequently. ^d

d References: , , , , , , , , , , , , ,

Only a few reports in the literature concern the percentage of athletes who use vision correction (spectacles or contact lenses). ^{, , ,} Studies of athletes participating in the Amateur Athletic Union Junior Olympic Games found that approximately 20%–35% of athletes had refractive error greater than ±0.75 D. ^, The incidence of refractive error (myopia, hyperopia, and astigmatism) found in these studies is similar to that found in the general population, dispelling the perception that athletes have a lower incidence of refractive error. A similar study of teenaged athletes found a similar range and mean refractive error in teenaged athletes as in age-matched nonathletes. A study of professional football (soccer) players found that approximately 16% were myopic and 20% were hyperopic, while only 25% used vision correction. Interestingly, a study found a higher rate of refractive error in professional baseball players than in the general population. In contrast, professional baseball players were not found to have a less high-order aberrations compared with a control population; the lower amounts of trefoil were considered clinically insignificant.

Standardized examination procedures to evaluate refractive status are recommended for athletes, and prescribing recommendations are addressed in Chapter 6 . The need for cycloplegic examination is left to the discretion of the practitioner and is usually based on concerns regarding latent hyperopia in the athlete. Of note, most young athletes are also students and vision conditions that can affect school performance should be addressed as well. A study found that the SVOne autorefractor provides an acceptable measure of refractive error in baseball players.

Eye/Hand/Foot Preference

The phenomenon of ocular dominance was first described by Giovanni Battista della Porta in 1593, and interest in the relation between ocular dominance and performance has been active ever since. Dominance has been defined as any sort of physiologic preeminence, priority, or preference by one member of any bilateral pair of structures in the body when performing various tasks. Many studies have attempted to determine the best method for assessing the types of eye dominance but tests of sighting preference are the most frequently used. Tests that allow binocular viewing under more natural viewing conditions than a traditional sighting test, which forces the athlete to choose one eye over the other, may provide more useful information for sports applications. ^,

Many studies have investigated the relation between hand and foot preference and eye preference. ^{, , , ,} The preferred eye does not always correspond to the preferred hand or foot, and when they are different the condition is referred to as crossed dominance. Duke-Elder reported that 33% of right-hand dominant people are left-eye dominant, 50% of left-hand dominant people are right-eye dominant, and an estimated 20%–40% of the general population is crossed eye and hand dominant. Athlete populations appear to have a similar distribution of crossed and uncrossed eye-hand preference as found in the general population. ^{, , , , , ,} Entangled in the issue of testing methods to determine eye preference are reports of dominance switching with hand used when sighting and issues of central dominance in which neither eye is aligned with the sighted target (also sometimes referred to as ambiocular). ^{, ,}

Many theories have proposed advantages or disadvantages of having crossed eye and hand dominance in sports performance. ^{, , , , , , , ,} These speculations were further inspired by the findings of Coren and Porac, who found that information from the dominant eye is processed approximately 14 ms faster than information from the nondominant eye. Functional magnetic resonance imaging studies further demonstrate a larger activation area in the primary visual cortex of the dominant eye. The effect of the dominant eye on batting in baseball has received considerable scrutiny. Although the findings have been somewhat contradictory, the preponderance of evidence indicates no relation between eye preference patterns and batting performance. ^{, , , , , , ,} In a study with golfers, ocular dominance in putting stance was found to be different than in primary gaze; however, the magnitude of dominance in putting stance was not associated with increased putting success. The only sports in which eye dominance appears to be important are “sighting” sports such as target shooting. ^{, , , , , , ,} In these sports, ipsilateral (same side) dominance offers advantages to acquiring the skills required for success.

Although the role of ocular preference in sports success is inconclusive, evaluation of a preferred eye, hand, and/or foot has been included in many sports vision evaluations. ^{, , , , , , , ,} When assessment of ocular preference in athletes is important, a pointing test paradigm that preserves natural binocular viewing conditions is recommended. Preference can be qualified as strong, partial, none, or mixed. ^{, ,}

Ocular Alignment

The precise alignment of the two eyes, triggered by retinal image disparity, is responsible for providing a significant amount of information regarding object location. The amount of innervation exerted by each of the six extraocular muscles in each eye to align on a target or object provides some of the information necessary to judge depth and is logically a critical feature of sports performance when precise depth judgments are necessary for success. Studies have demonstrated that extraocular muscle tonus changes produced by altering the amount of heterophoria result in changes in perceived distance. If a shift occurs in a relatively esophoric direction, perceived distances are increased; exophoric shifts induce a shortening of perceived distances. Athletes are commonly agreed to have better ocular alignment, especially when viewing at a far distance, than nonathletes; however, results of studies comparing athletes with nonathletes have not been conclusive. ^{, , , ,} Early studies suggested that athletes have lower amounts of heterophoria ^, but more recent studies have not confirmed these results. ^{, ,} Because changes in heterophoria produce changes in perceived distance, stability of vergence posture may be a more critical factor in spatial localization than the amount of heterophoria.

The measurement of ocular alignment is a common aspect of a sports vision evaluation; however, the methods used to assess this function vary considerably. ^e

e References: , , , , , , , , , , , , ,

Depth Perception

The perception of depth has generated a considerable amount of interest relative to visual performance. Wheatstone was the first to describe and demonstrate the illusion of depth by inducing retinal image disparity, and the psychophysical mechanisms and parameters of stereoscopic vision have been scrutinized extensively. Early research studied static stereoscopic vision, whereas later research investigated the perception of depth in motion (dynamic stereopsis). These factors are discussed more extensively in Chapter 3 .

The relation between depth perception abilities and athletic performance was a logical correlation to explore because many sport tasks require judgments of spatial localization. Several studies have demonstrated that binocular vision can improve performance on certain tasks compared with performance by individuals using only one eye. ^{, ,} However, the research comparing performance on tests of static stereopsis with athletic populations have had mixed results. ^f

f References: , , , , , , , , , , , , ,

An assessment of depth perception is an almost universal element to a sports vision evaluation. ^g

g References: , , , , , , , , , , , , ,

A homemade Howard-Dolman device for assessment of real-space depth perception.

Vergence Function

An assessment of vergence subsystem function is frequently recommended for athletes. ^{, , , , , , ,} The underlying premise is that strength and flexibility in vergence function provide better stability of visual information to the athlete, particularly when the athlete must deal with excessive fatigue and psychologic stress. A correlation between stability of vergence information and spatial judgment consistency has been assumed.

Only three studies have reported on vergence range measurements in athletes. Coffey and Reichow reported narrower vergence range findings at 6 m compared with published norms and discussed speculation that narrower vergence ranges relate to more precise spatial judgment ability. Hughes et al. found no statistically significant difference between the vergence ranges at 6 m on elite, intermediate, and novice table tennis competitors. Omar et al. found better vergence break values in teenaged nonathletes compared with athletes for both base-in and base-out ranges at 6 m, but only significantly better on recovery values in the base-in direction. Therefore, measurement of vergence ranges at 6 m may only be valuable as an assessment of motor compensation ability when an athlete has a large heterophoria at 6 m.

An assessment of the near point of convergence (NPC) has been studied as a visual factor in athletes. Christenson and Winkelstein found athletes performed better on the NPC test than did nonathletes, whereas other studies found better performance in nonathletes. ^, The theory is that NPC testing is a dynamic procedure that requires simultaneous performance of oculomotor skills and vergence function and therefore it may assess vergence function more globally than other procedures that isolate aspects of vergence function. Falkowitz and Mendel found that the better batters in a cohort of baseball players had better NPC findings than those whose batting performance was poorest. However, Hughes et al. found no statistically significant difference between athletes at different skill levels and 27% of Junior Olympic athletes did not meet minimum criteria for passing this test. These results suggest that measurement of NPC in athletes may not provide clinically useful information regarding sports performance because the test does not directly simulate the visual tasks found in most sports.

An evaluation of vergence facility has been recommended because the visual demands of many sports involve the ability to adjust vergence posture rapidly. Two methods have been used to measure vergence facility in athletes: the use of prisms to alter the vergence demands at a fixed distance ^{, ,} and the use of charts at two different distances, with fixation being rapidly alternated between the two charts. Christenson and Winkelstein found athletes performed better on a vergence facility test using 8 ^Δ base out and 4 ^Δ base in at 6 m than nonathletes, and Omar et al. also found better performance using 12 ^Δ base out and 3 ^Δ base in. However, Hughes et al. did not find significant differences among elite, intermediate, and novice table tennis competitors using 10 ^Δ base out and 4 ^Δ base in. Coffey and Reichow advocate use of the Haynes distance rock test ^, ( Fig. 4.5 ), theorizing that it more closely simulates real-world accommodative-vergence facility. Most athletes need to look between far, intermediate, and near distances quickly, requiring rapid accommodative-vergence responses. When a prism is introduced, the vergence system must adjust ocular alignment to regain image fusion; however, the accommodative system must remain focused close to the plane of the target. This separation of accommodation and vergence is a standard method to assess relative vergence facility at near in patients with asthenopia during near work, but it is generally not a factor in the visual task demands of sports. Therefore use of a near-to-far alternating fixation procedure is recommended for assessing vergence facility in athletes. The performance results of a procedure such as the Haynes distance rock test are influenced by limitations in visual acuity, accommodative skills, and oculomotor skills (fixation and saccadic eye movements), and the procedure will not reveal suppression tendencies. Near-Far Quickness on the Nike Sensory Station and Senaptec Sensory Station has been associated with better ability at avoiding strikeouts in professional baseball.

The Haynes distance rock test.

Accommodative Function

An assessment of accommodation subsystem function is frequently recommended for athletes. ^h

h References: , , , , , , ,

The earliest study reporting accommodative function in athletes was part of a series of Russian studies. Normal accommodative amplitudes were found in 100 “well-trained” athletes. A study found statistically better performance on amplitude of accommodation in teenaged athletes compared to nonathletes, although the difference may not be clinically significant. Although accommodative amplitude is a common procedure for assessing accommodative function, the task of clearing letters at a very near distance does not reproduce the typical visual task demands encountered in sports. The approximately 0.6-s latency of the accommodative response has been suggested to preclude it as a factor in many rapid reactive sports.

More recent studies have evaluated accommodative facility to more closely simulate the visual demands of many sports that involve the ability to adjust focus rapidly for a variety of distances. Two methods have been used to measure accommodative facility in athletes: lenses to alter the accommodative demands at a fixed distance ^, and charts at two different distances, with fixation rapidly alternated between the two charts. Three studies that used lenses (at near and far distances) to assess accommodative facility in athletes found no significant difference in performance compared to nonathletes. ^{, ,} However, accommodative facility using lenses was found to be better in intermediate and advanced volleyball players compared to beginners and nonplayers. The study using the Haynes distance rock test presented normative data for a population of elite athletes and therefore did not compare performance with that of nonathletes. A study of athletes in interceptive sports found slightly better performance on a near-far accommodative facility test in the athletes. When a lens is introduced, the accommodative system must adjust ciliary muscle tonus to regain image clarity; however, the vergence system must remain aligned with the plane of the target to prevent diplopia. This separation of accommodation and vergence is a standard method to assess relative accommodative facility binocularly at near in patients with asthenopia during near work, but it is not generally a factor in the visual task demands of sports. Therefore the use of an alternating near-to-far fixation procedure is recommended for assessing accommodative facility in athletes. The performance results of a procedure such as the Haynes distance rock test are influenced by limitations in visual acuity, vergence skills, and oculomotor skills (fixation and saccadic eye movements) and the procedure will not reveal suppression tendencies. Near-Far Quickness on the Nike Sensory Station and Senaptec Sensory Station has been associated with better ability at avoiding strikeouts in professional baseball.

Oculomotor Function

One of the most commonly heard coaching imperatives is “keep your eyes on the ball (puck, opponent, target, etc.).” The ability to maintain fixation of a rapidly moving object is frequently a critical aspect for allowing visual processing of crucial information in sports. The ability to change fixation from one location to another rapidly and accurately is also an essential aspect of many sports tasks. In nondynamic sports such as precision target shooting, the ability to maintain steady fixation is a vital aspect of successful performance. Therefore the assessment of oculomotor function can include evaluation of pursuit eye movements, saccadic eye movements, and steadiness of fixation. Another aspect of oculomotor function is searching eye movements, as discussed in Chapter 3 . However, this function is much more difficult to assess in clinical practice.

An important aspect of oculomotor function is the physiologic time required for initiation of the requisite eye movement for the visual task. The average latency for initiation of a pursuit eye movement is typically 125 ms, and the average latency for initiation of a saccadic eye movement is 200 ms. Studies have found mixed results when comparing pursuit and saccadic eye movement latencies; some have not shown that athletes have shorter latencies for the initiation of pursuit or saccadic eye movements, ^{, ,} whereas some have. ^, However, if a target trajectory is predictable, reduction of the latency period for pursuit or saccadic eye movements can be learned. ^{, ,}

A study of racquet-sport athletes found quicker saccadic responses to positive positional errors compared with nonathletes. Elite shooters have shorter saccadic latencies on both simple reaction to a sudden target appearance and discrimination between targets and distractors, ^, and shorter latencies for the first saccade were found to distinguish good from poor cricket batsmen. Further study found elite cricket batsmen used two predictive saccades to anticipate a pitch: one to the location of the ball bounce and then to the location of the bat-ball contact point. These predictive saccades were coupled with head movements to help maintain pursuit eye movement of the ball. Studies with college and professional baseball players found similar head tracking of pitches from a ball machine or live pitcher that assist the maintenance of gaze position close to the ball. A study comparing saccade speeds into cardinal gaze positions using RightEye measurements found the professional baseball players had significantly faster speeds in several directions than amateur prospects and nonathletes. Similar differences in head movement have been noted with tennis strokes between experienced and inexperienced players. Positive positional errors are created by displacing the saccadic target in the same direction as the saccadic movement after initiation of the saccade. These results open questions of innate skill versus motor learning through experience and the modulation of attention with eye movement performance. Interestingly, in a study comparing experts in ball sports to controls, athletes did not show shorter latencies in the prosaccade condition (eye movements toward a suddenly appearing stimulus), whereas antisaccades (eye movements away from a suddenly appearing stimulus) showed significantly shorter latencies. Saccade accuracy has been found to be better in fencers and tennis players compared with other athletes and nonathletes.

Pursuit eye movements have the capacity to follow targets at speeds of up to 40 degrees/second (or faster if the trajectory is predictable), and saccadic eye movements have the capacity for speeds up to 1000 degrees/second. Studies comparing the speed of pursuit eye movements in athletes and nonathletes have found mixed results, depending on the sports studied. ^{, ,} Pursuit eye movements have been shown to correlate to expertise in gymnasts of different levels and better performance compared to nonathletes. Accuracy of smooth pursuit eye movements has also been correlated with better batting performance in professional baseball.

The quality of pursuit and saccadic eye movements has been studied with clinical assessment procedures and results suggest that athletes have better eye movement skills. ^{, ,} The quality of eye movement skills has been correlated to batting performance in two studies; however, the subjectivity of the eye movement assessments brings the reliability and validity of these studies into question. ^, One study evaluated the quality of saccadic eye movements objectively at 3 m by projecting the King-Devick Test (a test designed for clinical assessment of saccadic fixation eye movement at 40 cm) on a screen; even though results indicated superior performance by athletes compared with nonathletes, the test does not control for visual-verbal automaticity differences. Similarly, measurement of saccadic eye movement speed using optotype naming at 6 m found better performance in intermediate and advanced volleyball players than in beginners and nonplayers.

An assessment of oculomotor function is a common element in a sports vision evaluation. ⁱ

i References: , , , , , , , , ,

The gaze patterns of expert athletes have become an important area of study in the field of sports vision. This area has been largely ignored in clinical practice because these systems are generally prohibitively expensive ^, ; however, the advent of lightweight portable eye tracking technology has allowed for evaluation and feedback of eye movements in sporting activities that are carried out in natural settings. These systems typically consist of two cameras mounted on an eyeglass-type frame: one to monitor eye position and one to monitor the scene (point-of-view) with an external camera that is positioned to monitor motor performance characteristics. The data collected by the mobile eye tracker is then synchronized with elements of motor performance using software programs that can operate in real time. Studies with such mobile eye trackers have typically found that experts have a lower number of fixations that occur for longer durations than do novices during the viewing of specific sport situations, especially when the subjects are required to move while gaze behaviors are recorded. Across a variety of aiming and interceptive tasks, experts typically demonstrate longer fixation durations before initiation of the motor performance. Similarly, these patterns are also found on successful, relative to unsuccessful trials. This long-duration fixation that occurs just prior to motor response has been called the quiet eye (QE) period. The exact neural mechanisms that direct gaze behavior during QE are still under investigation; however, it appears to show an advantageous period of cognitive processing allowing for computation of force, direction, and velocity that guide and fine-tune the motor response.

There are several companies that market complete mobile eye tracking systems for use with sport applications, such as SensoMotoric Instruments ( www.smivision.com/en/gaze-and-eye-tracking-systems/applications/sports-professional-training-education.html ), Tobii Pro ( www.tobiipro.com/fields-of-use/human-performance/ ), and Arrington Research ( www.arringtonresearch.com ), but these systems tend to be quite expensive. There are also some recent eye tracking systems that are monitor-based rather than eyeglass-based, such as RightEye ( www.righteye.com/sport ), Tobii EyeX ( www.tobii.com/xperience/products/ ), and the Eye Tribe Tracker Pro ( www.theeyetribe.com/ ), and offer more affordable options. RightEye includes a monitor-based assessment of pursuit and saccadic eye movements that provides a performance score for sports applications (see Fig. 4.2 ). These platforms, however, limit the natural environment applications of eyeglass-based systems and have not typically been used for sport-related research.

Peripheral Awareness

The visual field is the entire extent of the external world that can be seen without a change in fixation. In many sports situations, especially team sports, processing of information from the peripheral visual fields is a beneficial element to successful performance. The factors involved in assessment of peripheral vision include the extent of the visual fields, the sensitivity of the visual fields, the visual response speed to peripheral information, and spatial localization accuracy of peripheral stimuli.

Early studies investigated the extent of visual fields in athletes compared with nonathletes. Results indicate that athletes have a larger extent of horizontal and vertical visual fields than do nonathletes ^{, , , ,} and that athletes have better form recognition at more peripheral locations. ^{, , ,} One study found no difference in the extent of visual field between varsity and junior varsity college football players, so it may not be a sensitive discriminator of skill level. Some studies have found increases in the extent of visual fields with exercise ^, ; however, these findings have been challenged more recently as being caused by the differences in visual field techniques, limited sample size, and lack of a control group in earlier studies. Recently some studies have assessed the synchronoptical ability: the ability to assess visual information occurring at two different locations simultaneously. For example, the judgment of a foot fault in tennis requires the line judge to determine if the server’s foot crossed the line before contact with the ball during the serve. This may be a relevant testing paradigm for sports in which the athlete must process both peripheral and central information simultaneously for optimal performance. Study results demonstrate that synchronoptical differentiation is difficult without support from audition but that the skill appears trainable. A study of basketball players found that skilled players showed significantly higher response accuracy and faster response times than less skilled players in three video-based viewing conditions of basketball scenarios, demonstrating superiority in information extraction when presented in either central or peripheral vision. Physical activity does not appear to increase peripheral perception performance, but it does improve central perceptual performance. Therefore the search for visual field defects in routine vision testing has a low yield in athletes yet it should be performed as part of the vision examination for sports participation for general health purposes.

Sports vision practitioners have attempted to evaluate peripheral response speed with the Wayne Peripheral Awareness Trainer (PAT) ( www.wayneengineering.com ) and some normative data have been published. ^, The PAT is no longer commercially available; however, it was composed of a central circular module that contained a central fixation light with eight clear rods extending from it with red light-emitting diodes (LEDs) at each end ( Fig. 4.6 ). The athlete holds a joystick, fixates centrally, and moves the joystick in the direction of an LED when it is perceived in peripheral vision. This system only assesses peripheral response speed, not accuracy, and fixation is not monitored in a controlled manner. The level of ambient room lighting affects performance on the Wayne PAT; performance on the instrument significantly improves as room illumination is decreased. The Vienna Test System ( www.schuhfried.com ) contains several measures of peripheral perception and has been used to evaluate performance in athlete populations. ^, Moderate to good reliability has been found with the peripheral perception subtests, indicating reasonable applications for assessment in sports. Other systems designed to assess peripheral eye-hand response, discussed later in this chapter, may also provide useful information about peripheral vision performance in athletes. Even though assessment of peripheral vision is recommended as part of a sports vision evaluation, further development is necessary to more closely simulate the visual task demands of sports. ^j

j References: , , , , , , , , , , , , , ,

The Wayne Peripheral Awareness trainer.

Courtesy Alan Berman, OD, Ridgefield, Connecticut.

Ocular Health Status

A complete assessment of ocular health is an obligatory portion of a sports vision evaluation. ^{, , , ,} A complete assessment generally includes evaluation of the external adnexa, anterior segment of the eye with biomicroscopy, pupillary responses, intraocular pressure, posterior segment through a dilated pupil, and the visual fields of each eye. The presence of any ocular abnormality should be fully charted and photographed if possible and the consequences of the abnormality explained in detail to the athlete. The potential risks of any abnormality to sports participation should be fully addressed and appropriate recommendations discussed for protection of the athlete. Chapter 6, Chapter 7 cover these issues in detail.

Speed of Recognition

The ability to process visual information rapidly has been considered an essential element for success in fast-action sports. Athletes must analyze available temporal and spatial information during sports situations relatively quickly to make accurate decisions concerning performance responses. Visual processing speed can be measured psychophysically and has been referred to as inspection time (IT). ^, Shorter ITs allow accurate decisions to be made from shorter stimulus durations than from longer ITs. Tachistoscopic procedures to evaluate speed and span of recognition have been used in research for many years. ^,

IT measurements have been shown to have a development pattern characteristic of many physical abilities, have good test-retest reliability, and correlate with measures of cognitive abilities. ^, Evoked potential findings suggest that IT may be an index of the speed of transfer of information from sensory registers to short-term memory. IT measures may provide a valid and reliable method for evaluating speed of recognition abilities in athletes.

Several studies have investigated speed of recognition abilities in athletes. Most studies have found that experienced athletes can evaluate information more rapidly than inexperienced observers; sport situations studied include baseball, cricket, volleyball, tennis, motorsports, and “fast ball” sports. ^{, ,} McLeod, however, did not find faster processing speeds in professional cricketers using film footage of cricket bowlers stopped at stages of the ball delivery, although the study was criticized for a small sample size and no statistical analysis. In addition, the study design only evaluated one of a number of important aspects concerning the movement of a ball in flight. An in-depth analysis of 252 professional baseball players using a series of Bayesian hierarchic latent variable models found that the Perception Span assessment on the Nike Sensory Station had strong associations with on-base percentage and strikeout rate.

Other studies have investigated both speed and span of recognition by evaluating the ability to recall a sequence of numbers presented tachistoscopically for 1/50 of a second and found no difference in athletes compared with nonathletes. ^{, ,} However, another study did find a significant difference in performance both for span of recognition and speed of recognition, even when distraction factors were added to the task to simulate competition conditions. Despite these differences in research results, the author concludes that the use of numerical stimuli may be the confounding factor in the assessment of speed of recognition in athletes; use of target parameters that more closely simulate the visual information processed in sport situations may yield better discrimination of IT abilities that correlate with sports performance. ^, For example, the projection of photographs of baseball pitchers shown at the moment of ball release may better assess how rapidly a baseball player can identify the type of pitch being thrown. A study using a metric combining variables of target size, contrast, and presentation time was shown to correlate with several plate discipline metrics in baseball (InzoneSwingPct, inzoneFbSwingPct, ChasePct, FbChasePct, BBperPa). Similarly, oculomotor processing speed has also been correlated with better batting performance metrics in professional baseball. The Perception Span assessment on the Nike Sensory Station and Senaptec Sensory Station uses a grid pattern composed of up to 30 circles (see Fig. 4.7 ) with a pattern of yellow-green dots flashed simultaneously for 100 ms within the grid. Athletes touch the screen to recreate the pattern of dots and the grid pattern increases in size with an increasing number of dots. The use of tachistoscopic testing for speed of visual processing is frequently recommended as part of the vision assessment of athletes. ^{, , , , , , ,}

A sample pattern briefly flashed (100 ms) during the Perception Span assessment on the Nike and Senaptec Sensory Stations.

Multiple Object Tracking

In many dynamic, reactive team sports, athletes must track teammates and opponents while simultaneously moving in response to the game. These sports require athletes to extract the crucial visual information from a dynamically changing environment in order to make the best decision on how to respond appropriately. These visual abilities that are critical to success in many sports have been called multiple object tracking (MOT), first described by Pylyshyn and Storm. Research has demonstrated that this form of visuospatial cognition is enhanced in expert athletes. Professional athletes have been found to have better MOT scores than high-level amateur athletes and nonathletes, and a study of NBA players found that MOT motion speed thresholds measured in the preseason correlated with a number of in-game statistics, including assists, steals, and assist-to-turnover ratios, during the following season. This preliminary evidence suggests that MOT abilities are fundamental properties for optimal sports performance.

Assessment of MOT is available with the CogniSens NeuroTracker ( https://neurotracker.net ) and Senaptec Sensory Station. These platforms show a set of identical targets with a random subset of the targets initially identified at the start of a trial to be tracked. At the conclusion of each trial the targets stop and the athlete is asked to identify the targets that were to be tracked. The assessment increases the demand for the athletes by modifying the number of targets to be tracked and the speed at which the objects move. Performance in the MOT task, therefore, is often limited by the number of targets an athlete can follow. As the number of targets increases, limitations in attentional resources lead to diminished performance. In a study of basketball players, elite athletes displayed better tracking performance than the intermediate-level athletes or nonathletes when tracking three or four targets; however, no differences were observed among the three groups when tracking two targets.

Visualization and Imagery

Visualization, or mental imagery, is the act of constructing mental images of an object or event that resemble the actual appearance of the object or event. Studies have suggested that mental imagery may share the same types of neural processes as visual perception, which has significant implications in sports. Some studies have evaluated the differences between visual imagery and motor imagery and suggest they reflect different aspects of mental imagery. Elite athletes have been repeatedly shown to use imagery strategies in preparation for performance.

Many studies have suggested that mental imagery of motor skill performance shares cognitive processes with the actual performance of motor tasks. Neuroimaging studies of subjects performing motor imagery demonstrate brain activity in the areas of motor preparation and performance. ^, Studies using electromyographic recordings show that mental imagery of a motor activity produces low-level motor effects that correspond to patterns of physical action; similar muscles and motor programs are activated. ^{, ,} Motion imagery has also been shown to correspond to attention for visuospatial imformation. Expertise levels influence the amount of muscular response during skill imagery, in which higher levels of expertise produce increased electromyographic activity. Task difficulty and expertise level have also been shown to affect the time required for mental imagery more than that for physical performance. ^, Athletes with lower skill levels require more time to execute the mental imagery of a physical performance than the actual time required to perform the physical act.

Suin first described methods involving mental imagery and relaxation as a psychologic intervention to affect motor performance favorably. The use of mental imagery has become a significant factor for athletes, coaches, sports psychologists, and some sports vision specialists. ^{, , ,} Relatively few studies have investigated the effectiveness of mental imagery in actual competition; most investigate the effect on skill acquisition. ^, The results of studies in skill acquisition or skill improvement have been contradictory, although the athlete skill level and length of intervention have been confounding variables for study comparison. ^, Sports in which visual motor tasks have been studied include basketball, tennis, karate, swimming, gymnastics, racquetball, archery, golf, high jumping, and volleyball.

Although the value of visualization and mental imagery in sports performance has been acclaimed, no objective assessment of individual ability exists. Many subjective assessment procedures are used by sports psychologists, such as the Sport Imagery Ability Questionnaire (SIAQ), which have demonstrated good validity, internal and temporal reliability, invariance across gender, and an ability to distinguish among athletes of different competitive levels. However, use of these, or other, procedures to assess visualization skills is recommended, but they are rarely used by sports vision practitioners. ^{, , , , ,}

Motor Response Time

The motor response time, also referred to as the motor reaction time (RT), has been defined as the actual time required to complete a simple, predetermined motor movement. Visual motor RT is the total time required by the visual system to process a stimulus plus the time needed to complete the motor response. Motor response time is a measure of the neuromuscular processing portion of the RT reflex, separate from the visual processing portion of RT.

Little has been published concerning the measurement of motor response time. One study used the motor RT program on the Wayne Saccadic Fixator ( www.wayneengineering.com ) and found significantly faster times in athletes than in nonathletes. Another study presented normative information from a population of elite athletes using a different device. Devices for measuring motor response time are available from Lafayette Instrument ( www.lafayetteinstrument.com ), and measuring the eye-hand and eye-foot response time is a recommended procedure by some as part of a visual evaluation of athletes. ^{, , ,}

Central Visual Motor Reaction Time

Visual motor RT, also referred to as psychomotor speed, refers to the amount of time that elapses between the initiation of a visual stimulus and the completion of a motor response to the stimulus. This is the full completion of the RT reflex, including the period required for the retinal cells to detect the stimulus, the time necessary for the transmission of the retinal cell information to the visual cortex, and the time required for the neuromuscular system to send the information to the muscles that need to be stimulated to make the appropriate motor response. The time interval between the onset of the stimulus and the initiation of the response has been referred to as the reaction or premotor element of the overall RT and the rest of the response is referred to as the motor element. ^, Soccer players have shown shorter premotor RTs compared with nonathletes, although the sample was small in this study. The neurophysiologic parameters of RT are discussed extensively in Chapter 3 . Many sport situations require the athlete to make a specific motor response to visual information; therefore the speed of visual and neuromuscular processing is considered by many to be a valuable attribute for an athlete. ^{, , , , , ,}

The measure of a simple RT reflex represents the minimal amount of time required to process a visual stimulus presentation and perform a simple motor response to that stimulus. Several studies have found faster simple RTs in athletes (both eye-hand and eye-foot RTs) in various sports compared with nonathletes or as a discriminator between types of sports and expertise levels. ^{, , , ,} However, other studies have not found this correlation. ^{, ,} Visual RTs have been shown to be impaired by factors such as reduced IQ, ^{, ,} cold, fatigue, ^, exercise, and restriction of peripheral visual fields with protective eyewear. ^, Gender differences have also been reported, with men achieving faster times than women on average, but other studies have not found this difference. ^, Performance on complex visual motor tasks is discussed in the Speed of Recognition and Peripheral Eye-Hand Response sections of this chapter.

The Binovi Touch Saccadic Fixator ( www.binovitouch.com ), Dynavision D2 ( www.info.dynavisioninternational.com ), Senaptec Sensory Station, RightEye, Cognivue Advanced ( www.cognivue.com ), and the Multi-Operational Apparatus for Reaction Time (MOART) system ( www.lafayetteinstrument.com ) are commercially available devices for measuring visual motor response time; measuring the eye-hand and eye-foot response time is a recommended procedure by some as part of a visual evaluation of athletes. ^{, ,} The Dynavision D2, Senaptec Sensory Station, and RightEye have demonstrated reliability to assess eye-hand RT. ^{, ,} Options for assessment of eye-foot RT include the MOART system using the foot switch, the RT-2S Simple Reaction Time Tester ( www.atpwork.com ), and the Quick Board ( www.thequickboard.com ).

Peripheral Eye-Hand Response

Peripheral eye-hand response assessment, also sometimes called eye-hand coordination, is the ability to make synchronized motor responses with the hands to visual stimuli. Many sports require the athlete to react with hand movements to rapidly changing visual information, such as in baseball, tennis, hockey, and football. This skill area is a repeated complex RT function for an extended period. A simple stimulus-response procedure that requires minimal cerebral processing results in a faster RT than a complex stimulus/response procedure that requires discrimination of visual information. ^,

Studies have been designed to provide normative information for athletes with available instrumentation to evaluate peripheral eye-hand response. ^{, , ,} The instrumentation designed for evaluating peripheral eye-hand response has usually been a two-dimensional panel mounted on a wall with an array of lights. The athlete is required to press a randomly lit button as rapidly as possible with one hand; then another button is lit in a random position on the instrument and the RT reflex cycle is repeated for the established period. The instruments are programmed to test in two primary modes: (1) visual proaction time refers to a self-paced mode for a set period in which each light stays lit until the button is pressed, then the next random light is lit and (2) visual reaction refers to an instrument-paced stimulus presentation in which each light stays lit for a preset amount of time (typically 0.75 s) before automatically switching to another light whether the button is pressed or not. A third option has been referred to as a Go/No-Go task. ^, The test setup is similar to other visual RT measures of peripheral eye-hand response; however, the light is either a “go” stimulus (e.g., green) or a “no-go” stimulus (e.g., red). Athletes are instructed to hit the “go” stimulus lights and to not hit the “no-go” stimulus lights. This test paradigm adds a layer of quick decision-making to the task in order to determine how effectively an athlete can make an uncomplicated decision to either generate a motor response or inhibit it. A study of baseball players found more variable RTs in a baseball-specific Go/No-Go task based on the level of experience, but this variability was not found in nonathletes or tennis and basketball players. ^, A subsequent study found that Go/No-Go RT is affected by sport-specific stimuli. A study of professional baseball players found that better eye-hand peripheral response correlated with better plate discipline batting metrics (e.g., fewer at bats before gaining a walk, swinging less often at pitches outside the strike zone, the ability to gain a walk), as well as longer playing time and increased likelihood of competing at the major-league level.

One study found better visual proaction times in youth athletes than in nonathletes ; however, another study found no difference in visual proaction time between adult athletes and nonathletes. Visual reaction has only been compared in athletes and nonathletes in one study; athletes performed better on the visual reaction setting with the Wayne Saccadic Fixator than did nonathletes. The level of ambient room lighting affects performance on the Wayne Saccadic Fixator and the AcuVision 1000 instruments; performance on the instruments significantly improves as room illumination is decreased. ^{, ,} Peripheral eye-hand response performance was shown to remain stable with perceived fatigue factors. Artificially reducing the depth perception has been shown to diminish peripheral eye-hand response accuracy in the hitting stroke of elite-level table tennis athletes. Gender differences have also been reported that are most likely related to differences in visual RTs, with men achieving faster times than women on average. ^,

The Wayne Saccadic Fixator was the original instrument developed for this type of assessment, and although it is no longer commercially available, there are many similar instruments. The Binovi Touch Saccadic Fixator, Dynavision D2, SVT ( www.sportsvision.com.au ), Vision Coach ( www.visioncoachtrainer.com ), BATAK Pro ( www.batak.com ), FitLight ( www.fitlighttraining.com ), Senaptec Sensory Station, Reflexion ( www.reflexion.co ), Sanet Vision Integrator ( www.svivision.com ), and the MOART system are commercially available devices for measuring peripheral eye-hand response; measuring peripheral eye-hand response is a recommended procedure by some as part of visual evaluation of athletes. ^{, , , , , , , , , ,} FitLight is unique in that it employs wireless LED powered lights that are controlled by a computer and can be flexibly placed at distances up to 50 yards from the controller, rather than embedded in a fixed board (see Fig. 4.8 ). The Senaptec Sensory Station, SVT, and Dynavision D2 have demonstrated reliability and standards for assessing peripheral eye-hand response. ^{, ,} The Vienna Test System contains several measures in the peripheral perception subtests that include RT measurements to peripheral targets and may provide useful information to complement other measures of peripheral eye-hand response.

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