Discussion, speculation, and research concerning the role of vision and visual information processing in sports have a long history. There is little debate that vision is a critical factor in sports performance or that visual information is the dominant sensory system when performing practically any perceptual motor task such as those tasks encountered in sports. The visual physiologic attributes of athletes have been extensively studied and compared with nonathletes, novices, and other athletes of varying skill levels. Some believe that the literature supports the opinion that athletes possess superior visual systems that allow them to see and process critical visual information better than their peers. Others contend that the literature does not support the opinion that their visual system physiology is superior but that elite athletes are able to use available visual information more efficiently and effectively than novices. The results of research attempting to address these contentions have been equivocal, thereby allowing both sides of the debate to claim support for their assertions.
Many factors influence sport performance, such as biomechanical factors, strength and conditioning factors, visual factors, and cognitive factors. Experts in any of the relevant fields would be expected to find support for the importance of factors in their area of expertise because they evaluate aspects of performance from the perspective of their expertise. Definitive answers to global questions are rarely ascertained by isolating factors without consideration of the complete process. The suggestion that superior visual skills are of little consequence compared with the cognitive processing of the visual information overlooks the role that visual information plays in cognition. This argument would contend that it is not the golfer’s ability to see the details of the green that is critical when attempting a putt but rather the ability of the golfer to interpret that visual information in order to select the appropriate direction and distance for the putt. This is certainly a cogent argument when comparing a novice with an expert golfer with similar visual abilities; the two may see the same information, but the experience of the expert allows interpretation of the contours of the green and judgment of the distance to the hole better than the novice. However, when comparing golfers with similar skills and experience, the golfer with poor contrast sensitivity will be at a disadvantage in reading the contours of the green when compared with the golfer with excellent contrast sensitivity. This visual disadvantage is present notwithstanding any perceptual adaptations that have developed in the golfer with poor contrast sensitivity in order to succeed; the golfer has deficient ability to use contrast information to judge the contours of the green. No matter how well developed the cognitive processing of visual information becomes, poor visual information creates an impediment to peak performance. Peak sports performance cannot be expected without both adequate visual information and the cognitive abilities to use the visual information. Superior capacity in either vision abilities or cognitive proficiency would logically offer an advantage to the athlete over a peer with less-developed skills.
Information Processing Model for Sports Performance
Sports performance generally requires the athlete to process visual information and execute an appropriate motor response. Many elaborate information processing models have been developed to understand the exact nature of the processes occurring in skilled motor performance. A traditional information processing model of skilled motor performance first proposed by Welford, and later modified by others, , , is presented here because it provides a useful framework for understanding the relevant aspects of sports performance.
The information processing model in Fig. 3.1 proposes that skilled motor performance is the result of three central processing mechanisms: the perceptual mechanism, the decision mechanism, and the effector mechanism. These three mechanisms are proposed to operate sequentially; however, significant consideration is given to the effects of both intrinsic and extrinsic feedback, as well as the contributions of experiential memory. This process is also referred to as the perception-action cycle, with visuomotor integration guiding the process in this discussion.
The perceptual mechanism receives an incredible amount of information from a wide variety of sensory receptors (e.g., vision, vestibular, tactile, and auditory receptors). Sensory channel capacity limits the amount of information that can be thoroughly processed, requiring that the current input information be selected for processing that has immediate relevance for executing the required task. This also requires that irrelevant sensory information be filtered out by similar neurologic mechanisms. The athlete’s experience and ability to control attention are suggested to guide this selection and filtering process. , For example, attentional focus and distraction can produce different size estimates, accuracy, and putting errors in golf. The perceptual mechanism is then responsible for organizing and interpreting the processed information in an approach that facilitates optimal performance.
Traditionally, the perceptual mechanism for visual information is conceptualized as a bottom-up process with a neural chain of visual signals from the retina traveling through the lateral geniculate nucleus to the primary visual cortex. From these basic responses, the neural signal is then fed forward through increasing complex visual processing regions that are tuned to respond to specific properties. The visual signals then diverge into two neural streams that provide additional processing of the signals: a dorsal pathway that provides information about spatial properties (called the “where” pathway) and a ventral pathway through the inferior temporal cortex that provides further information about object details and identification. These two neural streams converge in areas of the prefrontal cortex (PFC) and posterior parietal cortex (PPC) providing significant information to assist with decision making. This traditional, bottom-up sequential processing model has been modified to suggest a more dynamic model that also demonstrates top-down processing. In these models of neural processing, prior experience and attentional focus direct a process of “perceptual binding” that selectively processes critically relevant visual information. Perceptual binding guides visual processing of important details in order to overcome the neural limitations of processing all the incoming visual information and thereby improves efficiency and shortens reaction time (RT) to visual signals. The PFC and PPC appear to direct this process by the development of predictive models and stimulus-response mapping in order to rapidly identify critical visual information. For disruptive visual information (e.g., the appearance of an immediate threat), the process is primarily bottom-up, whereas goal-directed attention (e.g., looking for specific details such as the release of a pitch in batting) is top-down and directed by the PFC and PPC. A study comparing team handball athletes to athletes in nonteam sports and nonathletes found that sports expertise did not produce differences in basic attention tasks (attentional breadth, tracking performance, and inattentional blindness), suggesting that any differences in attention skills may be task-specific. Studies of gaze behaviors and visual search patterns during skilled sports performance by elite athletes compared to near-elite athletes show that fixations are typically clustered on features that provide a significant amount of information about the task being viewed. More thorough reviews of the role of visual attention in the perception-action cycle and its application in sports are available. ,
The sensory information that has been processed is conveyed to a decision mechanism. The purpose of the decision mechanism is to determine the appropriate motor response strategies for the sensory information, which may also include the repression of a motor response in some sport situations. The athlete’s sport knowledge and past experience obviously exert substantial influence on the effectiveness of decision processing.
The motor response selected by the decision mechanism is transmitted to the effector mechanism. The neural commands necessary to produce the desired response at the correct time are organized and sent to the appropriate brain centers for execution of the action. The motor response is both initiated and controlled by the effector mechanism. Both internal and external information is continually processed by the perception and decision mechanisms, allowing both control and adjustment of the motor response to occur when sufficient time exists to alter the response.
This model is both simple and practical for understanding the processes occurring when an athlete must react and respond to sensory information in a sport situation. Comparisons have been made between this model and the functioning of a computer; the computer receives information input, processes that information in the manner that it was programmed, and produces the planned output result. Although this analogy of computer hardware and software has been challenged, and other factors are not clearly revealed by this model, it remains a useful global method for understanding the processes involved in sports performance. The computer analogy also clarifies the importance of both superior hardware and software for achieving peak human performance.
Perceptual Mechanism
In the information processing model presented, the perceptual mechanism stage of processing is the most directly related to the realm of the vision care provider. It is the basic role of the vision care provider to ensure that the sensory receptors for visual information are functioning adequately. A comprehensive vision evaluation should evaluate the patency of the basic functions of the visual apparatus and identify any deficits that may limit performance potential. A secondary level of vision care is the evaluation of visual performance skills that relate to sports performance, yet it has been challenging to develop visual performance evaluation procedures that appropriately measure relevant vision skills that are directly related to sports tasks. Recommendations for procedures to assess visual performance abilities in athletes are discussed in Chapter 4 . Four general areas of visual information processing in the perceptual stage have been defined, each of which is richly supported by intrinsic and extrinsic feedback and experience: visual resolution, depth judgment, eye movements, and peripheral vision.
Visual Resolution
The first aspect of the information processing model is sensory reception of the information. The ability to resolve subtle details can be a factor in perceptual processing if the details contain relevant visual information. Static visual acuity has been found to be better in athletes than in nonathletes in some studies, whereas some have found no difference. The variance in results is most likely a consequence of the visual acuity testing methods used and the differences in the visual task demands of the variety of sports assessed. The ability to resolve detail when movement between the observer and the test object is induced, referred to as dynamic visual acuity (DVA), has arguably more relevance in many sports than static measurements. Most of the research has attempted to determine the relation between DVA and visual task performance; however, studies have compared athletes and nonathletes. , , Despite significant differences in methods for measuring DVA, all but one study found better DVA in athletes. A significant amount of human and stimulus variables can affect DVA, including the resolving power of the retina, peripheral awareness, oculomotor abilities, target luminance, angular velocity, the time exposure of the target, and psychologic functions that affect interpretation of visual information. In addition to DVA, measurement of contrast sensitivity function (CSF) has been recommended in athletes because athletes often must perform visual discrimination tasks with suboptimal lighting because of environmental variability. The common conclusions from investigations comparing CSF in athletes indicate elevated CSF in athletes. , , , , Therefore the consensus of studies evaluating the resolution capacities of athletes indicates that although athletes can still perform with suboptimal visual acuities, superior visual resolution capabilities are expected.
The resolution of visual details also requires the athlete to be proficient at adjusting focus for a variety of distances. Studies that have evaluated accommodative facility in athletes compared with nonathletes have had mixed results, primarily because of the method of testing used. The use of lenses to manipulate accommodative demand does not simulate the visual task demands encountered in sports, and studies using this method have found no difference in athlete performance. , , The use of resolution threshold demand targets at two different distances with fixation being rapidly alternated between the two charts may better represent the accommodative task demands of sport. , ,
Depth Judgment
Discrimination of distance information and judgments of spatial localization are commonly encountered in sports. The results of research comparing performance on tests of static stereopsis with a variety of testing procedures in athletic populations have had mixed results; some have found better stereopsis , , , , , and others have found no difference. , , , The lack of athlete differences in many studies has been suggested to be due to several factors, including (1) many stereopsis assessments are conducted at near distances rather than at far viewing conditions encountered in most sports tasks; (2) the maximum level of stereopsis measured (typically between 20 and 40 arc seconds) is not a threshold level for many competitive athletes; (3) the stereopsis assessments simulate depth by artificially creating disparity with filters, which may produce different thresholds and different results than real image/object depth judgments ; and (4) the static nature of the testing may not measure depth perception abilities used in sports, and testing of dynamic stereopsis may discriminate sport-related visual abilities better. Many studies have used near stereotests because these are commonly used for clinical assessment, and many assert (without evidence) that stereopsis is nonfunctional beyond 1–2 m. A study of stereopsis assessment using separated LED lights at distances of 20 and 40 m and further found the ability to make depth estimates was present under binocular viewing but not monocular. Better ocular alignment can contribute to better depth perception; however, early findings of lower amounts of heterophoria in athletes , have not been confirmed in recent studies. , Various evaluation methods for assessing vergence function have been conducted with athletes and all but one study found better performance in athletes than in nonathletes. , , , , Although vergence responses are a type of eye movement used for tracking the trajectory of something moving toward or away from the observer, in addition to a compensating system for ocular misalignment, they are considered in this text because of the direct impact of vergence information on depth localization. Again, the consensus of studies evaluating the binocular abilities of athletes indicates that although athletes can still make distance judgments by using the abundance of monocular cues to depth that are often present, superior binocular depth perception and robust vergence function are expected.
Eye Movements
Oculomotor function is another aspect of the perceptual mechanism in information processing and can include evaluation of pursuit eye movements, saccadic eye movements, and steadiness of fixation. The ability to initiate a pursuit eye movement to maintain fixation of a moving object can be a critical aspect for allowing visual processing of crucial information in sports. The ability to initiate an accurate saccadic eye movement to shift fixation from one location to another is also an essential aspect of many sports tasks. Athletes have not demonstrated shorter latencies for the initiation of pursuit or saccadic eye movements , , ; although if a target trajectory is predictable, shorter latency periods can be learned for these eye movements. , The quality of pursuit and saccadic eye movements in athletes, however, has been found to be better than in nonathletes. , , , In precision sports such as target shooting, skilled athletes demonstrate better ability to maintain steady fixation despite distractions, which is a vital aspect of successful performance. The use of fixations and eye movements to search for critical information efficiently and effectively may be a more sensitive discriminator of expert skill than the traditional measurements of eye movements performed by vision care providers.
The visual search patterns of experts compared with novices during specific sports demands have been the focus of many studies. The study paradigms typically used attempt to discriminate differences in the number of fixations to determine the amount of information assessed by the observer and differences in the duration of fixations to determine the amount of time expended to collect the visual information from each specific fixation. Most studies have found that experts have a lower number of fixations 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. Research that has investigated more open-field viewing conditions, or that use photographic or video displays and do not require physical movement by the subjects, has found the opposite—experts have a greater number of fixations on more peripheral aspects of the action. Irregardless of the visual search pattern, the accuracy of object localization and motor response depend on the type and accuracy of the eye movements used. , A universal finding in all studies of visual search patterns is that the fixations are typically clustered on features that provide a significant amount of information about the task being viewed. Additionally, novices are much less skilled at determining these informational locations and distributing their fixations in a manner most efficient for processing the information within the time constraints of the action. Visual search strategies have been found to vary among individuals at all expertise levels, which can affect results and conclusions in the studies, as can the method for determining expert and novice or near-expert status. The distribution of attention to central and peripheral visual information is also not measured by these systems; therefore determining the exact nature of the visual information being processed during a fixation is difficult. The optimal visual search pattern for specific sports tasks has yet to be determined, if such an optimal pattern exists, because of an inability to demonstrate objectively the features containing the premium information and the most advantageous temporal distribution of fixations for attending to those premium features.
The process of visual search patterns during critical sport situations appears to represent continual cycles of the information processing model presented, in which visual fixation information is processed by the perceptual mechanism, the decision mechanism determines the next appropriate eye movement response (based on experience), and the effector mechanism organizes and controls the signals delivered to the extraocular muscles. A considerable amount of perceptual, motor, and cognitive feedback is present in this process, leading to speculations about the relative value of each portion of the process. As previously mentioned, evidence suggests that all aspects of information processing must be operating optimally to potentiate peak performance.
Peripheral Vision
Processing of information from the peripheral visual fields is a universally beneficial element to successful sports performance, whether the task is to monitor teammates and opponents or maintain steady balance. Results indicate that athletes have a larger extent of horizontal and vertical visual fields than nonathletes , , and that athletes have better form recognition at more peripheral locations. , , , The restriction of peripheral vision has also been found to increase the latency and accuracy of head movements during eye movement localization tasks as well as significantly degrade balance ability. Therefore peripheral sensitivity also appears to be enhanced in athletes.
The evidence concerning visual resolution, depth judgment, eye movements, and peripheral vision supports the role of excellent visual information as a vital element for the function of the perceptual mechanism in the model of information processing for skilled performance. The evidence also converges on the general conclusion that athletes should, and typically do, possess superior visual skills. The results do, however, clearly illuminate the need for visual evaluation procedures sensitive to the visual task demands required in specific situations of sports, especially if these procedures are to be used to predict performance capabilities of an athlete. For example, one study that reported no difference between the visual skills of experts and novices used an accommodative facility test with a +1.00/−1.00 D flipper at 50 cm. Not only has the use of lenses to assess accommodative facility been found to be nondiscriminatory with athletes but also the use of +1.00 and −1.00 D is such a low accommodative demand at 50 cm that virtually all subjects would perform at maximal capacity, generating a superthreshold response that would predictably result in a lack of discrimination between subject categories. Additional consequences result when considering specific visual skills in isolation without the substantial dynamics of both visual skill interaction and other forms of intrinsic and extrinsic feedback essential in the information processing model.
Decision Mechanism
The critical role that the decision mechanism performs in the information processing model is undeniable. The ability to rapidly select the stimulus-response choice most compatible with the sport situation provides the biomechanical elements of performance with the opportunity for success. This mechanism requires the athlete to know where crucial visual information exists, be able to direct attention to those crucial elements, select the best information from all that is available, organize and interpret the information in the most appropriate manner based on experience and memory of similar situations and information, and select the most accurate response with consideration of an anticipated action plan.
In many sport situations, this process must occur in a time interval that approaches the limits of human capacity. In some sport situations, the time factors exceed the human capacity to process the important visual information before initiating a motor response, so the motor response must be initiated with the anticipation of the most likely scenario that will unfold. For example, in a penalty shot in hockey, the puck can cross the goal line within approximately 100 ms of being struck. The typical simple visual RT is approximately 150–200 ms; therefore the goaltender cannot wait until the puck has been hit to predict the trajectory of the shot. Use of advance cues for anticipation of sports action has been studied to address this aspect of performance, which can mean the difference between a smooth, efficient response and an embarrassing misplay.
An athletes’ ability to process visual information rapidly from a competition situation and structure that information into a useful composition to facilitate performance decisions is a critical ability acquired as expertise improves. Sports researchers have adopted a study paradigm used with chess in which experts were able to recall more structured chess positions from brief exposures than nonexperts. However, the experts did not exhibit superior recall when presented with unstructured chess positions, suggesting that the superior recall of experts was the result of task-specific experience rather than exceptional memory abilities. The same results have been found when speed of recognition has been evaluated in athletes using numerical stimuli rather than sport-specific stimuli. , These same structured versus unstructured recall differences between experts and novices have been found with athletes in various team sports, including basketball, , field hockey, , and football. The organization of common sport situations into a knowledge architecture clearly is a common development with sport expertise. This knowledge architecture offers many advantages, including the ability to process larger quantities of information in a short amount of time and the possibility of priming the perceptual and effector mechanisms for subsequent information.
The superior ability to structure and recall sport-related perceptual information should improve an expert’s ability to make more accurate decisions in a shorter amount of time or make those decisions earlier in the time course of the action. Study results have been mixed. Expert field hockey players demonstrated better accuracy than lower level players, but the tactical decisions were not made any faster. Studies of soccer, , , volleyball, ice hockey, baseball, , and motorsports found faster decisions with equivalent accuracy. The crux of the difference in expert performance was proposed to be the ability to use advance visual cues to anticipate the location of subsequent critical information and use that information to improve performance.
The common paradigm for assessing the use of advanced cues for anticipation of sport action has been the use of film occlusion. Subjects watch a brief video of a sports situation and the footage is occluded at various intervals. This occlusion requires the subjects to predict the outcome of the interrupted action (e.g., where the tennis ball will land). This is a form of temporal occlusion that indicates how expertise affects the minimal time interval and the type of information that benefits the experienced athlete.
Spatial occlusion has also been used, in which strategic portions of the footage are masked (e.g., the badminton opponent’s racquet) to determine which features provide the best information to the athlete. Studies involving badminton, baseball, cricket, field and ice hockey, soccer, squash, tennis, and volleyball have all demonstrated the superior ability of experts to use advance visual cues to anticipate the outcome of the sport action. , , , , , These studies provide ample empirical evidence that the development of sport expertise produces enhanced ability to identify and use sport-specific visual cues to anticipate action outcomes. Anticipation of forthcoming action allows the athlete to shuffle the most likely scenarios to the top of the stack of possibilities effectively, thereby reducing the time needed to match stimulus-response choices as the action progresses.
The complex visual behavior of advance cue utilization allows the skilled athlete the enviable advantage of shortening visual RTs in sport-specific tasks and also establishes a proactive feedback mechanism in the information processing model. The accurate processing of cues in advance of sport action allows the athlete to direct attention to the correct locations and temporal aspects of critical features of the action, thereby reducing the athletes’ uncertainty about the impending action. Modulation of attention has been shown to be another discriminating aspect of sports performance in athletes and is developed in a manner that is task specific. Elite shooters demonstrate narrower attentional focus with less influence from visual field distractions, , whereas volleyball players have a more expansive attentional focus. The ability to modulate attention appropriately, and often split attention between multiple stimuli, is another valuable aspect of the decision mechanism in the information processing model.
As expertise is developed in a sport, the complex knowledge structures acquired facilitate expanded and enhanced use of mental imagery strategies. Mental rehearsal is the act of constructing mental images of an event, and it is commonly used by elite athletes in preparation for performance. Studies have demonstrated that mental imagery may share the same types of neural processes as visual perception, which has significant implications in sports. Mental imagery of motor skill performance shares cognitive processes with physical skill performance, and comparable brain activity in the areas of motor preparation and performance has been demonstrated. Similar muscles and motor programs are also activated during imagery, and expertise levels influence the amount of muscular response during skill imagery. , , As the use of mental rehearsal is expanded and enhanced with sport skill development, the possibility of priming the perceptual and effector mechanisms for subsequent information offers significant potential advantages if the advance visual cues are accurately located and interpreted.
The fundamental role of the decision mechanism in achieving peak sports performance is indisputable. The ability of the elite athlete to find and use critical features in sport situations quickly and convert that information rapidly into effective response strategies through anticipation and response priming characterizes the elite athlete as “intelligent.”
Effector Mechanism
The effector mechanism is responsible for converting information processed by the perceptual mechanisms and decision mechanisms into appropriate motor response signals. The organization and control of the motor response signals must be sufficiently accurate to allow the proper biomechanical action sequence to occur with precise timing for optimal performance. Coordination of hand reactions, foot reactions, body reactions, and balance adjustments must be directed with efficient precision within the time constraints of the specific sport situation. The information from the perceptual and decision mechanisms concerning the space-time behavior of critical factors in fast-action sports should contain the vital information necessary for the motor responses to occur at the proper time and location. For successful performance, the motor responses must also be sufficiently adjustable to allow modification on the basis of continuing input from the perceptual and decision mechanisms as the sport action continues.
Visual-motor RT refers to the amount of time that elapses between the initiation of a visual stimulus and the completion of a motor response to that stimulus. The effector mechanism is responsible for translating the processed information to the neuromuscular system, which sends the information to the muscles that need to be stimulated to make the appropriate motor response. 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. The assessment of simple RT may be the most direct method of evaluating the effector mechanism because the requirements of the perceptual and decision mechanisms are minimal. Several studies have found faster simple RTs in athletes (both eye-hand and eye-foot RTs) in various sports compared with nonathletes, and it has been demonstrated to be a discriminator between expertise levels. , ,
A simple stimulus-response procedure that requires minimal cerebral processing will result in a faster RT than a complex stimulus-response procedure that requires discrimination of visual information. , Peripheral eye-hand or eye-foot response, also called eye-hand and eye-foot coordination, is a repeated complex RT function for an extended period in which synchronized motor responses with the hands or feet must be made in response to unpredictably changing visual stimuli. The studies designed to provide normative information for athletes using available instrumentation to evaluate peripheral eye-hand response have had mixed results depending on the testing paradigm. , , , , A test paradigm that 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 is called Go/No Go. , 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. , In a study of executive functioning, high-level youth soccer players outperformed youth amateur players in suppressing ongoing motor responses and in the ability to attain and maintain an alert state. Eye-body coordination is similar to eye-hand coordination, except that the athlete must make synchronized motor responses to visual stimuli by shifting balance of the whole body. Very limited information is available concerning this type of visual-motor reaction skill in athletes.
The ability to maintain balance while processing complex, fast-action visual information is a task demand fundamental to many sports. The athlete is frequently required to preserve balance while the oculomotor system is engaged in pursuit, saccadic, and/or vergence eye movements. Some normative data have been provided for athletes on the basis of subjective assessment protocols, and a series of investigations found that vision played an important role in dynamic balance skill acquisition in gymnastics.
Balance maintenance may also be affected by the athlete’s relative field dependence or independence, which refers to the cognitive style of processing information to discern relevant stimuli from an irrelevant stimulus background. The theory implies that field-dependent persons rely more on external cues during information processing and field-independent persons use internal cues more. Kane postulated that field independence was an advantage for athletes competing in “closed skill” sports (e.g., diving, gymnastics, track and field) because they tend to rely more on internal physical components such as body orientation when executing motor responses. Field dependence was considered an advantage in “open skill” , team sports (e.g., basketball, football, hockey) because athletes must make constant adjustments in performance to external factors (teammates, opponents, etc.). Much is still to be discovered concerning the use of visual, vestibular, and other sensory information to make discrete and accurate adjustments in balance during sports.
Complex Interactions Mediating Effector Mechanism Responses
The ability to predict the arrival of an object or stimulus at a designated place can be measured with a motor response and is referred to as visual coincidence anticipation timing (CAT). Excellent perceptual processing combined with exceptional decision processing in fast-action sports can provide a significant advantage in executing the most appropriate motor responses. A substantial body of research addresses the many factors that influence the impressive human ability to perform the complex visual-motor tasks encountered in sports. The literature contains extensive information—from the basic physiologic and neuronal mechanisms to global models using physics computations—to explain how human beings can catch or hit a ball. Covering all these factors to the degree that would do justice to the collective contributions is beyond the scope of this chapter; however, some basic information distilled from the research is presented in the context of the information processing model.
To hit or catch a ball successfully, the athlete must judge the spatial information of height, rightward or leftward displacement, and distance of the ball. In addition to these three-dimensional space judgments, the temporal aspects of time to contact must be calculated with exacting precision. Several visual cues are available to assist the athlete in making these judgments, including retinal image and disparity information. Some neurons in the visual cortex are tuned to binocular retinal image disparity, providing information about the depth position of an object. The difference in retinal locations for the ball as seen by the right eye and left eye constitutes binocular disparity, supplying the stereoscopic perception of relative distance. Binocular stereopsis judgments can be made at relatively far distances, not just for near distances. In addition, evidence indicates that a system of binocularly driven cortical neurons sensitive to motion in depth is separate from the position in depth system. ,
The perception of motion in depth is also produced by a changing retinal image size information system that operates relatively independent of the changing retinal disparity system. , , Ample evidence shows that human beings possess cortical neurons that are selectively sensitive to changing image size and that these “looming” detectors provide a significant amount of information for judging time to contact even under monocular viewing conditions. Additional monocular visual information about time to contact is also available from the flow pattern of visual information, , and this information also appears to be processed by the changing size system. Regan and Beverly concluded that:
The relative effectiveness of changing-disparity and changing-size as stimuli for motion-in-depth sensation varies as follows: (a) changing disparity grows relatively more effective as velocity increases (according to a power law); (b) changing disparity grows relatively more effective as inspection time increases; (c) changing disparity grows relatively more effective as the linear horizontal width of the target decreases; (d) the relative effectiveness of changing disparity and changing size shows marked intersubject variability (at least 80:1).
The precision of the motion in depth system is hypothesized to be attributable to a relative excitation mechanism in the stereomotion channels for changing size and changing disparity that acts similar to the opponent-color stage of human color discrimination. Separate stereomotion channels also seem to exist for different directions of motion: one for motion that is approaching the head and one for motion of a receding object.
The retinal information concerning changing disparity and changing size is sufficiently accurate to judge time to contact with a ball; however, it does not provide exact information concerning the actual distance of the ball or its speed. , Stereoscopic depth perception from calibrations within the vergence system and from motion in depth information provides precise information about relative depth but not about the exact distance location. Ocular vergence information is notoriously unstable when the vestibular signals must contend with a freely moving head, and changes in vergence angle or changes in absolute disparity do not affect binocular fusion or the perception of motion in depth for images beyond a few meters when independently altered. Binocular viewing offers advantages in catching or hitting a ball ; however, monocular catching ability can be trained to similar skill levels as binocular viewing. Comparison of binocular and monocular performance of a table tennis hitting task revealed that only stroke consistency was affected under monocular viewing, not accuracy or movement time. A frame of reference to assist the judgment of relative depth has been shown to improve catching and hitting performance, , and the vergence system demonstrates a rapid ability to recalibrate. It also appears that coupling the CAT skills with the typical physical movements used in the activity, such as batting in cricket, significantly improves anticipation accuracy and is less susceptible to the effects of blur. ,
Because the time to contact can be determined by the changing size and changing disparity information available to the visual system from an approaching ball, Lee derived an optical quantity signified by the Greek letter tau (τ) and the τ-margin to describe the calculation of time to contact on the basis of the relative rate of dilation of the ball’s optical contours during approach. Many studies have confirmed the use of τ in guiding motor responses during acts requiring spatial and temporal judgments. , , , Further studies elaborating the τ-margin found that the rate of constriction of the optical gap separating the moving object from the interception point also provides sensitive information concerning time to contact. , ,
The actual performance of catching tasks has demonstrated that extensive feedback from the visual and kinesthetic systems constantly updates the time to contact judgments , and that observer movement improves the judgments. , , It appears that freedom to intercept a moving target at any location significantly improves the accuracy of the timing of contact and is better in faster moving targets. Visual and kinesthetic feedback during motor task acquisition and performance has been manipulated in many studies, and removal of vision feedback typically produces decrements in performance. , Elaborate models have been constructed (and debated) to explain how baseball fielders select a path and movement speed to intercept a ball in flight , or hit a ball in flight. , , Misjudgment of ball velocity can cause significant perceptual misjudgments concerning the trajectory and appearance of speed changes. , , Similarly, studies of gaze behaviors for hitting a bouncing ball, such as in tennis or cricket, show anticipatory saccades are made prior to the bounce in order to expedite ball tracking after the bounce.
The quality of visual input is a critical factor to sports task performance, and extensive feedback from the sensory receptors provides continuous information for performance adjustment. The value of skill repetition and performance experience cannot be underestimated in its role of providing the effector mechanism with the framework for effectively and efficiently using the information from the perceptual mechanism. The gifted athlete’s ability to make the correct responses routinely and consistently generates the appearance of effortless “natural” ability, a venerated description in sports society, thereby neglecting to duly recognize the contribution of years of demanding practice and experience.
Collectively, the wealth of research information provides extensive insight into the mechanisms responsible for successful sports performance. The information processing model for skilled performance provides a useful structure for applying this information to specific tasks. An application of the model is presented for batting in baseball, often called “the single hardest act in all of sports.” ,
Application of the Information Processing Model to Batting in Baseball
The time frame from when the pitcher releases the ball to when it reaches the average point of contact with the bat must first be considered. Simplified calculations have suggested that a 90-mph fastball pitch reaches the bat approximately 400 ms after release and a 75-mph curveball arrives in approximately 480 ms. The type of pitch thrown will have significant consequences on the flight trajectory of the ball because of the properties of aerodynamics; this is discussed in the decision mechanism portion of the information processing model. The batter will have access to visual and cognitive cues to help anticipate the most likely type of pitch to expect, but time must be allocated for completing the mechanics of the swing. Although batters typically complete the swing in 150 ms, some can perform this feat faster.
For the purposes of this application, a 400-ms time frame is presented for the time course between the pitcher’s release and the contact with the bat, and 150 ms is used for completion of the swing mechanics. The pitcher is at the moment of release and the clock is ticking.
Perceptual Mechanism
The perceptual mechanism is responsible for organizing, processing, and interpreting the sensory information to facilitate the optimal response to the pitch. The successful batter uses efficient and effective visual search patterns during the pitching motion to analyze any advance cues to the pitch type, , hopefully narrowing the trajectory probabilities that need to be considered by the decision mechanism. The batter also has access to information about the inclinations of the pitcher, the pitch count, the current situation in the game, and the presence of any base runners to guide attentional focus. At the moment of release, the batter initiates a saccadic eye movement to direct foveal fixation to the release point or pitcher’s elbow, while processing spatial information from the arm angle and height and locations of the release. Basic physiology indicates that retinal cell information must be encoded and assembled, a process that takes approximately 25 ms. This retinal information must be conducted to the visual cortex, requiring another 20 ms. The visual cortex must process the retinal information to construct the image; because a substantial amount of memory is available, this process takes approximately 30 ms. Therefore the visual information contained in the pitch release takes approximately 75 ms to process, and the ball is now one-fifth of the distance to the plate. The visual images can be continuously processed with only the 25-ms conduction delay requiring that the batter maintain pursuit eye movements. Electroencephalographic studies demonstrate that fastballs are discriminated at the earliest points in their trajectory, relative to the curveballs or sliders. This is advantageous because less time is afforded on a fastball pitch to make a decision whether to swing or not swing, compared to off-speed pitches.
How good is the visual information being received? Previous studies have demonstrated superior visual resolution skills, contrast sensitivity, and DVA in baseball players. , , , , , , Although enhancing the seams of the baseball has been shown to improve curveball hitting, the sport demands that the visual system use the subtle cues of a traditional ball. Some measures of stereopsis have been found to be superior in baseball players, , , as has the visual field size of female softball players. The interaction effect of the dominant eye and dominant hand on batting in baseball has received considerable scrutiny. Although the findings have been somewhat contradictory, the preponderance of evidence indicates no relation between eye dominance patterns and batting performance.
Because the use of pursuit eye movements to track a pitch all the way to contact appears to be impossible, the batter must use a complex combination of pursuit and saccadic eye movements, along with rotational head movements, to track the approaching ball. Similar eye and head movements have been found in cricket batting. , One study of pursuit and saccadic eye movement quality found subjectively better performance in those athletes with better batting averages. Anticipatory saccades are common in tracking a pitch, in which the pursuit eye movements fall far enough behind the ball that a saccade is initiated to a location where the trajectory of the ball can be intercepted. , The mechanism of saccadic suppression prevents the batter from seeing during, and for approximately 20 ms after, the saccadic eye movement. , Therefore a faster pursuit eye movement system, coupled with rotational head movement, offers an advantage to track a pitch. The vestibuloocular system is used to a small degree to stabilize eye posture during head movements, and vergence eye movements do not appear to be used to track the pitch.
How good is the visual system in providing the information for predicting when the ball will arrive? The visual acuity, CSF, and DVA of the batter must be sufficient to detect the seam rotation of the ball in order to judge the pitch type and ultimately the anticipated speed and trajectory the ball will travel. The retinal image information provided by changing size and changing disparity detectors for motion in depth supplies the capability of judging time to contact (τ) within 2–10 ms. , For objects subtending less than 1.5° (a baseball further than 10 feet away), cortical neurons are sensitive to changing size characteristics as low as 0.02°. This capability is well above the visual threshold at the moment of release, providing the batter the opportunity to estimate time to contact with an accuracy of better than ±9 ms. A batter must estimate the time to contact to within 7–9 ms , , to hit a 90-mph ball close to the center of percussion of the bat; other estimates, however, and estimates from cricket suggest that accuracy may need to be even better than ±9 ms.
How good is the visual system in providing the information for predicting what location the ball will arrive at? The batter needs to judge the vertical height of the ball to within 0.75 inches and the inside-outside location of the pitch with respect to the batter’s body to within 3 inches. The batter can estimate the ball’s vertical speed from the retinal velocity information combined with the distance to the ball. The range of speeds encountered in pitched balls is within the human capability for accurately judging time to contact within 5%. , Because the batter can only estimate pitch speed, and the visual system is incapable of providing more than just a relative estimate of distance, judgments of ball location are susceptible to significant uncertainty. This may explain why batters are typically better at timing the arrival of the pitch than at judging the location of the ball.
The perceptual mechanism is under significant time constraints to process the critical visual information through the dorsal (“where”) and ventral (“when”) neural streams to the PFC and PPC. Top-down processing directed by the PFC and PPC based on prior experience and attentional focus direct perceptual binding to selectively process critically relevant visual information regarding the pitch, thereby shortening the processing time needed to make a decision about the batting response.
Decision Mechanism
Because the swing will take approximately 150 ms to initiate to the point of contact, the decision of where and when to swing must be made by 250 ms after the release. If the ball has been followed by head rotations and pursuit eye movements, then a nearly continuous stream of information has been available with information critical to selecting the proper response. As mentioned previously, the batter may also have additional information concerning the idiosyncrasies of the pitcher, the current situation in the game, the status of the pitch count, and the wealth of information stored from previous experience. This additional information, combined with the pitching motion information acquired before the moment of release, provides the potential for preselecting the most likely pitch scenarios for quicker recognition.
Many models have been proposed to explain how cognition occurs in situations such as batting in baseball, , but a minimum of 50 ms is needed to select the appropriate response and send it to the effector mechanism to begin the action. This means the batter has approximately 200 ms to process the visual information to make an accurate decision; more time than that is a luxury that is not available for a pitch of this speed. This time is not much longer than measurements of simple RT in human beings (approximately 150 ms) and is much shorter than the RTs to complex choice conditions demonstrated by Hick. The batter has many issues of spatial and temporal uncertainty to resolve because of the aerodynamics and Magnus forces produced by the seam and texture of a baseball and these issues must be resolved rather quickly. A variety of pitch types can be delivered in a manner that can lead a batter to misjudge velocity and trajectory. Occlusion studies have demonstrated that the early portion of the pitch provides sufficient information for experienced batters to set accurate probabilities about different types of pitches. ,
The skill of the pitcher can place the batter at the edge of human physiologic capacity. A fastball thrown at 95 mph will arrive approximately 25 ms faster than a 90-mph fastball and arrive at a height approximately 3 inches higher. Considering the human weakness for estimating velocity, a misjudgment of the pitch speed would affect the time to contact assessment as well as the height location of the ball. The loss of 25 ms also needs to be subtracted from some aspect of the visual information processing model. The 25 ms will most likely be sacrificed from the decision mechanism because the perceptual mechanism is crucial for making the spatiotemporal judgments, and the time needed for the effector mechanism is relatively set.
Effector Mechanism
At 225 ms from the moment of release, the initial muscle response for a swing must occur. The first muscles to respond are the back leg muscles, and although approximately 25 ms are required to initiate the response, the movement does not commit the batter to a swing at the pitch. The rest of the swing mechanics takes approximately 150 ms, but further visual processing can provide feedback for adjustments. After the first 50 ms of the swing, the bat is moving at approximately 30% of its final velocity and the swing can be changed substantially (or checked) on the basis of continued visual information processing. By 100 ms into the swing, the bat is moving at approximately 75% of its final speed and cannot be changed because of the time factors to the muscles. Some can execute the swing more rapidly than the time course described, which may allow more time for visual information processing and decision-making. However, studies have not found correlation between simple motor RT and batting skill or a difference during play in cricket batting.
Optometry/Ophthalmology and Visual Information Processing
Many vision performance evaluations and sports vision training programs attempt to assess and improve overall processing of visual information. Most sports vision training programs attempt to affect the perceptual mechanism by improving the requisite visual skills for successful sports performance. The fundamental goal of sports vision training programs is to focus the athlete to process larger quantities of information in a shorter amount of time while simultaneously priming the perceptual and effector mechanisms for subsequent information. Ultimately, this improves the speed and efficiency of the decision mechanism, which is additionally enhanced by procedures that provide feedback on visual attention and encourage the development and use of mental imagery. Training options that provide natural or simulated sports-related conditions may provide additional benefits to a sports vision training program by assisting in the transfer of improvements to the sport. Although many unanswered questions regarding the role of sports vision training and sports performance still exist, the conclusion that improved visual skills compounded with improved ability to modulate attention and use positive mental imagery should have a salutary effect on overall visual information processing is a logical one.
Chapter 4 discusses the evaluation of visual skills in athletes, and Chapter 8 discusses the development of sports vision training programs that use the framework of the visual information processing model described in this chapter. Many other professionals also affect the visual information processing and motor responses of the athlete. For example, the development of biomechanically advantageous motor performance skills and an optimal mental mindset directly affect the effector and decision mechanisms and also have a salutary effect on the perceptual mechanism. The athlete can potentially reap tremendous benefits from directly addressing all aspects of sports performance.
References
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