Suppression of the Rotational Vestibulo-Ocular Reflex During a Baseball Pitch

Suppression of the Rotational Vestibulo-Ocular Reflex during a Baseball Pitch

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Marc A. Burcham

Graduate Program in Vision Science

The Ohio State University

2010

Master's Examination Committee:

Nicklaus Fogt, O.D., Ph.D., Advisor

Gilbert Pierce, O.D., Ph.D.

Andrew Hartwick, O.D., Ph.D.

Marc A. Burcham

2010

Abstract

The purpose of this experiment was to determine to what extent individuals could cancel the rotational vestibulo-ocular reflex (RVOR) in order to track an accelerating, high speed ball. More specifically, this study was designed to determine if subjects could successfully track a baseball pitch while viewing the ball through small apertures. While wearing these aperture goggles, we hypothesized that the batsmen would have to increase the rotational amplitude of their heads while successfully suppressing their rotational vestibulo-ocular reflex in order to accurately track a pitched baseball.

Subjects were tested using a pitching machine called the Flamethrower under normal viewing conditions (no apertures), then while wearing apertures that subtended 3.3 degrees of visual field, and finally under normal viewing conditions again. In the final trial (normal viewing), subjects were encouraged to replicate the eye and head movements adopted while wearing the apertures. Tennis balls were pitched from a distance of 44 feet from the batter at a measured velocity of approximately 80 miles per hour. Eye movements were recorded with the ISCAN infrared eye tracker and horizontal head rotations were recorded with the 3DM-GX1 head tracker. All head and eye recordings were temporally synchronized with each other and with ball position using software.

A total of twelve subjects were enrolled in the study. Each subject viewed 50 pitches under each of the above defined testing conditions. A total of 1796 pitches were successfully

ii recorded with nine subjects identified as able to accurately track a tennis ball under all testing conditions. Thus, 1346 pitches were analyzed.

Mean gaze errors for all three trials indicated that the subjects were able to accurately track the pitched tennis balls for a majority of the ball’s flight path under each testing condition.

Absolute gaze errors were smallest with the apertures compared to the other conditions at 300,

305, and 339ms after the pitch was released. These results were all statistically significant.

The results of the study revealed consistent statistically significant differences in head movement when wearing the apertures at 200, 250, 300, 305, and 339ms. Specifically, this analysis revealed that the introduction of the apertures resulted in a decrease in head rotation.

Finally, examination of the individual data suggested that in most cases once the aperture was removed, individuals generally did not completely adopt the same eye movements and head movements that had been used with the apertures.

Overall these results suggest that wearing the aperture goggles aided in proper tracking.

The improvement in gaze tracking with the apertures indicates that subjects could successfully cancel the RVOR. Further, the apertures generally resulted in a decline in head movement amplitude, in agreement with a previous study showing that individuals tend to show head movement overshoots when aiming the head at moving targets unless provided with a visual cue to head position. Further studies need to be performed to investigate any potential of the apertures to produce behavioral changes post training, and to determine whether these changes in eye and head coordination correlate with batting performance.

iii

Dedication

This thesis is dedicated to my amazing and supportive wife, Stefanie.

Acknowledgments

I would like to thank Dr. Fogt for all of his hard work and long hours put into to computer programming throughout and prior to the study, and for his wealth of insight and knowledge during the entire process. Without his dedication to me and this study, this would not have been possible.

I would also like to thank my wife, Stefanie Burcham, and my parents, Mark and Sharon

Burcham. Individually and as a group, they have always supported and encouraged all of my endeavors.

Vita

May 2002 ...... Mathews High School

May 2006 ...... B.S. Chemistry, Miami University

June 2010 ...... O.D., The Ohio State University

June 2010 ...... M.S., The Ohio State University

Fields of Study

Major Field: Vision Science

Table of Contents

Abstract ...... ii

Dedication ...... iv

Acknowledgments...... v

Vita ...... vi

List of Tables ...... ix

List of Figures ...... xi

CHAPTER 1: INTRODUCTION ...... 1

1.1 Dynamics of a Pitch ...... 1

1.2 Eye Movements for Tracking a Baseball ...... 2

1.3 Studies on Eye Movements in Sports Tracking ...... 6

1.4 Commercially Available Baseball Training Devices ...... 7

1.5 Head Movement and the Rotational Vestibulo-Ocular Reflex ...... 10

1.6 Purpose ...... 11

CHAPTER 2: METHODS ...... 13

2.1 Subject Enrollment and Eligibility ...... 13

2.2 Position of the Ball ...... 13

vii

2.3 Monitoring Eye Movements...... 16

2.4 Monitoring Head Movements ...... 17

2.5 Data Analysis ...... 20

2.6 Study Construct ...... 21

CHAPTER 3: RESULTS ...... 24

3.1 Survey Results ...... 24

3.2 Mean Head Movement Latencies and Maximal Amplitudes ...... 24

3.3 Individual Gaze Error, Head Movements and Eye Movements...... 26

3.4 Analysis of Mean Absolute Gaze Errors, Head Movements and Eye Movements . 33

3.5 Eye Movements within the Apertures ...... 37

CHAPTER 4: DISCUSSION ...... 39

4.1 Head Tracker Calibration ...... 39

4.2 Gaze Error ...... 39

4.3 Limitations ...... 42

4.4 Conclusions ...... 44

References ...... 46

viii

List of Tables

Table 1: Reference points for ball location in time and space. The center of the subject’s

head was about 22 inches from the middle of home plate...... 15

Table 2: Varying angular subtense of tennis ball based on position from the batsman.... 23

Table 3: Head movement latencies for each subject and subsequent runs...... 25

Table 4: Mean maximal amplitudes for each subject and subsequent runs...... 26

Table 5: Mean and standard deviation of absolute gaze errors for subject 1. No baseball experience; Subject in previous baseball study; Age 30...... 27

Table 6: Mean and standard deviation of absolute gaze errors for subject 2. Collegiate baseball experience; Subject in previous baseball study; Age 23...... 28

Table 7: Mean and standard deviation of absolute gaze errors for subject 3. High school baseball experience; Not a subject in previous baseball study; Age 24...... 28

Table 8: Mean and standard deviation of absolute gaze errors for subject 4. High school baseball experience; Subject in previous baseball study; Age 25...... 29

Table 9: Mean and standard deviation of absolute gaze errors for subject 5. Baseball experience before high school; Not a subject in previous baseball study; Age 21...... 29

Table 10: Mean and standard deviation of absolute gaze errors for subject 6. No baseball experience; Not a subject in previous baseball study; Age 26...... 30

Table 11: Mean and standard deviation of absolute gaze errors for subject 7. Collegiate baseball experience; Subject in previous baseball study; Age 25...... 30 ix

Table 12: Mean and standard deviation of absolute gaze errors for subject 8. No baseball experience; Not a subject in previous baseball study; Age 31...... 31

Table 13: Mean and standard deviation of absolute gaze errors for subject 9. High school baseball experience; Subject in previous baseball study; Age 24...... 31

Table 14: Mean and standard deviation of absolute gaze errors for subject 10. Baseball experience before high school; Subject in previous baseball study; Age 30...... 32

Table 15: Mean and standard deviation of absolute gaze errors for subject 11. High school baseball experience; Subject in previous baseball study; Age 42...... 32

Table 16: Mean and standard deviation of absolute gaze errors for subject 12. High school baseball experience; Subject in previous baseball study; Age 26...... 33

Table 17: Mean, standard deviation and repeated measure ANOVA results for the overall absolute gaze errors...... 34

Table 18: Mean, standard deviation and repeated measure ANOVA results for mean overall head movements...... 34

Table 19: Mean, standard deviation and repeated measure ANOVA results for mean overall eye movements...... 35

Table 20: Percentage of eye movements during Run 2 within the tolerance levels of the aperture occluders. Percent within tolerance = (Number of eye movements between +3.3 and -3.3) ÷ (Total number of pitches) x 100%...... 38

List of Figures

Figure 1: Commercially available I-On Eye Trainer...... 9

Figure 2: Angular velocity changing throughout the pitch...... 15

Figure 3: Acceleration error profile of 3DG-XM1 compared to the magnetic search coil for a 30 degree head turn...... 19

Figure 4: Acceleration error profile of 3DG-XM1 compared to the magnetic search coil for a 130 degree head turn...... 19

Figure 5: Gaze error compared to ball flight distance from release point...... 36

Figure 6: Target position-response and resultant head rotation. A head rotation equal to

100% indicates a head rotation exactly equal to the angular change in ball position.

Position–Response = (Acutual Head Rotation) ÷ (Change in Ball Position) x 100%...... 37

CHAPTER 1: INTRODUCTION

1.1 Dynamics of a Pitch

Imagine Nolan Ryan, Roger Clemens or Randy Johnson standing aloft a ten inch tall mound of dirt armed with a 3 inch in diameter, 5 ounce ball 1. A man equipped with nothing but

a small piece of wood awaits 60 feet 6 inches away confined to a 4 foot by 6 foot box, ready and

willing to defend an imaginary space 17 inches wide and stretching from his armpits to his

knees 2. The most intimidating pitchers in Major League Baseball (MLB) reach speeds upwards of

100 miles an hour, which will cover the above distance in less than 400 milliseconds (ms) 3.

It takes the average batsmen 194ms to initiate a swing 4, so in roughly 200ms, the batter must not only decide if he will swing, but where and when he should swing in order to make contact with the ball. Additionally, once the decision to swing is made, the time from the initial bat movement until the bat crosses home plate takes approximately 160ms 5. By combining the time it takes to initiate a swing and to actually perform a swing, even a well trained batsman with low latency values is left with no time for errors in tracking or judgment. "If a person from another planet was told what's involved ... they would say it's impossible," says Porter Johnson, a physics professor at Illinois Institute of Technology in Chicago 3.

An even closer look at the physics of a 95 mph fastball serves to only further exemplify

Dr. Johnson’s perspective. Even though the linear velocity of a pitch is relatively constant over

the short distance it travels, the angular velocity in which the batter perceives the pitch

increases exponentially as the ball approaches home plate.

Assuming the average pitcher releases the ball 5 feet in front of the pitcher’s mound after striding, the ball will cross home plate in 434ms. Covering the first 47.5 feet or 12 degrees-

—referenced from the batter’s point of view—in only 341ms, the angular velocity is a reasonable 35.19 degrees per second. The next 4 feet of ball flight takes only 0.0287 seconds and represents an angular change of 13 degrees. The angular velocity over this last 4 feet is now

452.96 degrees per second, and will continue to increase to multiple thousands of degrees per second while approaching the batsman 6. Bahill states “that the best imaginable athlete could not track the ball closer than 5 feet from the plate, at which point it is moving three times faster than the fastest human could track.” 7

1.2 Eye Movements for Tracking a Baseball

Dodge was the first to classify human eye movements into categories. While studying eye movements, Dodge was able to classify all eye movements into one of five categories. The five eye movements consisted of vergence eye movements, two compensatory or reflexive eye movements—the vestibulo-ocular reflex and the optokinetic response, one to follow a target in motion—pursuits—and one to rapidly foveate a target located in a different position of gaze — saccades 8. In order to track a high velocity target such as a baseball, one might assume a batsman must possess all five eye movements and be able to accurately perform them in order to be successful; however, for the typical batsmen, not all of these eye movement systems must be utilized.

Vergence eye movements are stimulated by objects falling on disparate retinal locations, causing the eyes to move in opposite directions 8. Convergence is the act of both eyes

turning nasally to binocularly fixate an object the falls on disparate temporal stimuli. Divergence

is temporal rotation of both eyes in order to binocularly fixate disparate nasal stimuli. With the 2 above understanding of the vergence system, it seems reasonable that a subject must diverge in order to fixate distance objects and converge in order to fixate near pitches.

One would then assume since a batter must accurately see the pitcher release the ball from roughly 55 feet 6 inches away and track the ball to a few feet away, that vergences would play a large role in tracking a baseball; however, vergences are not used by batsmen when tracking a pitch. The vergence system is negligible for a majority of distance the ball must travel as there is no demand to converge. Further the latency of convergence makes performing an accurate convergence eye movement impossible.7 While average latencies for convergence are

approximately 182ms for the untrained eye and possibly 65ms with intensive training 9, neither of these reported values leave adequate time to converge once the demand has been met to do so.

The two compensatory eye movements described by Dodge both play a role in actively tracking a pitch. The rotational vestibulo-ocular reflex (RVOR) is a compensatory eye movement intended to offset head motion and is triggered by the semicircular canals of the inner ear being stimulated 10 . Once stimulated by head rotation in one direction, neural impulses are fired that

move the eyes in opposite and ideally equal direction. A previous tracking study indicated that

the gain of RVOR suppression was 0.56.6 Perfect RVOR suppression would result in a gain of

1.00, so evidently RVOR suppression is far from perfect for rapid head velocities.

The optokinetic response is a visually mediated eye movement similar to the RVOR. This reflexive eye movement does not rely on the movement of the head, but rather by the movement of the entire visual scene across the retina. This movement across the retina stimulates eye movements in the opposite direction in an attempt to stabilize fixation.11 Both of the above eye movements actually hinder a batsman’s ability to accurately track a pitch. Any 3 head motion from the pitcher towards home plate while following the ball should stimulate the semicircular canals and drive the eyes away from the balls flight path towards home plate and back towards the pitcher. Background objects beyond the plane of the ball, will appear to move opposite in direction of the ball as it travels towards the batsman, thus stimulating the optokinetic response and again driving the batsman’s eyes away from the true flight of the ball and back towards its point of origin.

The eye movements with the highest possible velocities are the saccades. These are eye movements designated for rapid foveation of peripheral objects. Saccades have been shown to have peak velocities nearing 500 degrees per second12 , and would appear to be most advantageous for tracking high angular velocity pitches. Additionally, saccadic latency has repeatedly been reported in the range of 150-200ms 13 and 180-200ms.14 The latency and velocity of ocular saccades more than allow the saccadic system enough time to initiate and enough velocity to accurately locate a pitch to within 4 feet of home plate.

However, further investigation into saccadic eye movements indicates that unavoidable characteristics of saccades inhibit them from providing accurate and continuous foveation.

Saccadic suppression is a mechanism that allows human perception to remain oblivious to the smeared retinal images during rapid eye movement. The duration of saccadic suppression depends on the testing conditions, but the ranges for duration of saccadic suppression are between 100-200 ms 15 and this suppression extends beyond the completion of the saccade by roughly 20 ms.7 While latency and velocity both appear to be ideal in saccades tracking a

baseball pitch, the complete loss of fixation of the ball for one quarter to one half of the pitch is

a clear disadvantage.

Ocular pursuits are a heavily studied eye movement in sports vision and tracking. The stimulus for ocular pursuits is motion of an object’s image across the retina 16,17 , identically matching the velocity of a moving object such as a baseball pitch. Pursuit latency has been previously reported from 100 ms 18 to 120-250ms 13 , closely mirroring the latencies of saccadic eye movements and conscious physical movement. While pursuits ideally offer the possibility of constant foveation during the pitch, pursuit peak velocity falls short of the necessary speeds required for complete tracking to home plate.

Early studies have reported maximal pursuit velocities of 25-30 degrees/second 13 , while

more recent studies have reported peak pursuit velocities of 75 degrees/second 19 , 90-100 degrees/second 20 or even up to 120 degrees/second 7 in rare individuals. While the ball approaches the batter, the angular velocity easily exceeds 450 degrees per second, leaving no hope for ocular pursuits to maintain tracking accurately eight feet and in.

To summarize eye movements used in tracking a baseball, no single eye movement is sufficient enough for a batter to track a MLB fast ball from the pitchers hand all the way to home plate. Vergence eye movements are not used to track the ball, saccades cause loss of visualization of the ball for an average of one third of its total flight time, pursuits lack the velocity required to track the ball beyond 8 feet and the compensatory eye movements only add to drive the eyes away from the ball and back towards the pitcher. In order to legitimately watch the bat make contact with the ball, a batter must incorporate a variety of eye movements and acknowledge that compromises must be made during the flight of the ball, and all of this must occur within 400ms.

1.3 Studies on Eye Movements in Sports Tracking

Multiple studies have been conducted in order to further clarify the differences between novice batsmen and experienced batsmen. In a study involving cricket players, experienced batsmen picked up vital pitch information within the first 0-150ms of the pitch, and they would then make an anticipatory saccade to the location on the ground where the ball was expected to make contact. The study concluded that this accurate anticipatory saccade gave a great advantage to the experienced batsmen that novice batsmen could not account for in other ways. 21 By making an accurate anticipatory saccade, batsmen are able to negate the need to track the ball the complete distance of the pitch.

An early baseball study involving professional batsmen during live batting and monitored with both photographs and recorded video reported that these professional batsmen infrequently made head movements toward the plate during pitches that they swung at. Eye and head movements were not carefully quantified. However, when batsmen decided to take the pitch, or not swing at the pitch, extensive head movements were made in order to watch the ball cross the plate. It was theorized that once the swinging motion was initiated, further tracking information to determine ball position was no longer necessary, meaning the athletes did not need to follow the ball all the way to the plate. During pitches where tracking could actually be confirmed, pursuits were found to be the predominant eye movement used to track the pitches 22 .

In another more quantitative study involving eye tracking during a simulated pitch, novice batsmen, collegiate batsmen and a single professional batsman were monitored with head and eye trackers to determine patterns of tracking within groups. While only 6 complete and 15 partial recordings were reliably obtained, a few important conclusions were drawn from 6 the study. Significant findings from the study included the facts that the lone professional batsmen was able to track the simulated pitches to closer distances than the other batsmen and that he was also able to achieve higher peak pursuit velocities than ever previously recorded.

Additional findings in the study revealed that the professional batsman also displayed a much better ability to suppress his RVOR during tracking7. Either the professional batsman had an

innate ability to pursue and track better than amateurs, allowing him to become a MLB player,

or the professional batsmen was able to progress and develop the ability to pursue at velocities

faster than normals could, through years of batting practice. A unique conclusion drawn from

this study indicates that since a batsman is physically incapable of tracking a ball entirely to

home plate, if one wanted to see the bat make contact with the ball, one would have to make

an anticipatory saccade to the point at which the batsman thought the point of contact would

occur 7.

1.4 Commercially Available Baseball Training Devices

Perhaps now more than ever, developing players with high aspirations and professional baseball players are exploring any and all options to improve their on the field performance. As training regimens are continually developed to get the most out of these world class athletes, training of the visual system has garnered a lot of attention.

Research into some sports vision training devices has begun. Some researchers have used dynamic visual acuity (DVA) and kinetic visual acuity (KVA) to define visual function. DVA is predominantly defined as the ability to discern a moving object and is typically tested by determining a subject’s ability to discern a target moving horizontally in front of the eye. KVA is not well agreed upon in the literature. One study defined KVA as the ability to discern an object as the object of regard moves towards the subject 23 . 7

A training program—SPEESION—which has been used at a Japanese professional baseball team’s training camp, is one such commercial device is used to improve the DVA and

KVA of participants. This Japanese based computer training software was studied to evaluate if this program resulted in improved DVA and KVA. After training for three days a week for eight consecutive weeks on the SPEESION, neither DVA nor KVA improved significantly when measured on a third party measurement device, the Vision Tester. However, the study concluded that since measured values on the SPEESION significantly improved, the use of the

SPEESION with regular baseball skills will improve visual function 24 .

Many training devices currently utilized by major league baseball teams involve colored and or numbered tennis balls shot from pitching machines at high velocities. Starting velocities can average 90 miles per hour (mph) that then increase all the way up to 150 mph before returning back to 90 mph. Batsmen are instructed to intently focus on the balls and read off colors and or numbers written on the balls.

Some professional players engage in further training by performing live batting practice with the machine, selectively hitting either the red or black numbered balls. Others attempt to swing at all the pitches but alter the direction or field of play they chose to hit the tennis balls to—for example they will hit the red numbered balls to right field and hit the black numbered balls to left field. A recent study demonstrates that the absolute gaze errors in tracking a baseball were significantly improved when subjects were required to call out colors and numbers on pitched tennis balls compared to when subjects “tracked” the ball as if they are batting.25

Inexpensive training devices have also surfaced. The I-On Eye Trainer was a commercially available device that utilizes adjustable aperture occluders. The I-On Eye trainer is 8 advertised as a training aid to be worn during batting practice to provide enhanced performance during live situations when not wearing the goggles. This device consists of oval apertures positioned in occlusion lenses of a sporty spectacle frame, as seen in Figure 1. The apertures were approximated to be 4-5 millimeters (mm) in diameter, based upon their size in relationship to the bridge of the frame—the bridge was approximated to be 16 mm, a common distance between lenses. The apertures are on a variable portion of the lens, in order to provide movement to properly align a number of pupillary distances. 26

Figure 1: Commercially available I-On Eye Trainer.

The I-On Eye trainer is based on the age old theory of needing to, “keep your eye on the ball,” in order to be successful while batting. In order to accurately track the ball, a batsman would need to make greater head rotations and subsequently need to suppress their RVOR while wearing these aperture goggles. Two questions arise: First, can an individual track high velocity objects with the head—or a combination of increased head movements and eye

9 movements? Second, should individuals track high velocity objects with the head or a combination of increased head and eye movements?

1.5 Head Movement and the Rotational Vestibulo-Ocular Reflex

We submit that theoretically, head movements should be beneficial in tracking high velocity objects such as baseballs. This is because the head can be moved at very high velocities, but saccadic suppression is not invoked. Saccades with a peak velocity nearing 500 degrees/second are by far the highest velocity eye movement, where as maximal voluntary head velocity has been recorded upwards of 780 degrees/second.12 Steady target fixation and accurate head

movements theoretically could lead to better tracking performance than ocular saccades alone

at these high speeds. Further, combining ocular pursuit with head movements could lead to

even higher gaze tracking velocities.

However, there are two issues that could limit the velocity advantage provided by head

movement during tracking. First, the RVOR would need to be suppressed or cancelled. Studies

have shown that RVOR suppression is possible for voluntary head rotations at rotation velocities

under 170 degrees/second 27 while additional studies have showed successful VOR suppression

up to 350 degrees/second. 28 Since the RVOR is a reflexive eye movement, the latency of the

RVOR is very short at 16 ms 29 and is typically a highly efficient compensatory eye movement.

Though the RVOR is a highly efficient, counterproductive eye movement for gaze tracking, it has

been shown that increased efficacy in VOR cancellation can be obtained with targeted VOR

suppression training.30 So it seems as though RVOR suppression could be useful through at least a portion of a baseball pitch.

Second, it may not be possible for individuals to turn the head at the velocity of the target. While ocular saccades are similar to head saccades in that they are programmed by 10 estimates of positional error, ocular pursuit may not be analogous to head pursuit. Ocular pursuit is driven by the velocity of the stimulus, while head pursuit may only be an adjunct to eye pursuit (adding to eye pursuit to maintain accurate gaze pursuit). Head pursuit may not exist in the sense that the head perhaps cannot be rotated at the velocity of a moving stimulus.

The ability to turn the head at the same velocity as a moving target has only been evaluated in one study. In this study, the investigators used a step-ramp model to investigate a subject’s ability to accurately pursue a target with head motion alone. The study was constructed such that fixated targets would rapidly change position (the step) to a horizontal peripheral location and then begin smoothly moving across the field (the ramp) in a direction opposite to the initial target step.

The results of the study indicated that when intentionally instructed to track a target with only the head, subjects were not able to move the head at the same velocity as the target.

Instead, subjects made a large saccade-like movement with the head. The velocity of this movement was not at all related to the target. Next, a laser was mounted on a head band worn by the subjects. Subjects were instructed to point the laser at the moving target. Subjects were able to move the head with the target when giving this ocular cue (the laser) to head position.

These results indicate that without an ocular clue of the heads position, subjects are unable to efficiently and accurately track with their head.31

1.6 Purpose

The purpose of this study was to evaluate the ability of individuals to suppress the RVOR during a simulated baseball pitch. To make this assessment, subjects were required to view a pitched ball through small apertures placed before the eyes. If subjects are able to suppress their RVOR and track a baseball with head rotation—thereby controlling the tendency of the 11 head to overshoot a moving stimulus—then it stands to reason that this form of tracking could be highly advantageous if this tracking can be learned and implemented in live batting situations.

CHAPTER 2: METHODS

2.1 Subject Enrollment and Eligibility

This study was approved by The Ohio State University Biomedical Institutional Board of

Review. Informed consent was obtained from all subjects, and a Health Insurance Portability and

Accountability Act form was signed by all subjects as part of their enrollment into the study. The study consisted of only one visit by each subject.

Eligible subjects were healthy males between the ages of 18 to 50. A total of twelve subjects were enrolled in the study. The subjects’ average age was 26.25 years. Subjects were required to have a best corrected visual acuity of 20/30 and were screened for global stereopsis in order to confirm binocularity. A brief survey was orally given to each subject regarding their previous baseball playing experience and previous participation in baseball tracking studies.

Nine subjects were unaware of the aims of the study, while 3 subjects were familiar with the goals of the study.

2.2 Position of the Ball

Tennis balls were “thrown” toward the subjects using a highly accurate mechanical pitching device, The Flamethrower (Accelerated Baseball Technologies, Barrington, IL). The pitching machine was placed at a distance of about 44 feet from the subject. The Flamethrower is air compressed pitching machine that uses a rubber bladder to repeatedly and accurately pitch tennis balls.32 The pitching machine sits on top of a 5 foot platform ladder. An

approximately 5 foot long polyvinyl chloride (PVC) tube supported by a tripod emerges from The

Flamethrower and is used to accurately aim the pitched balls. The PVC tube was modified to attach a laser and photocell were aligned with each other and mounted in the end of the tube closest to the subject. As tennis balls emerged from the tube and passed through the laser and

photocell, a drop in voltage from the photocell occurred, as measured through an amplifier.

A 2.5 foot wide by 2.5 foot deep by 5 foot high wooden bin was used to collect and

contain the pitched tennis balls. Towels were hung in the back of the wooden bin in order to

absorb the balls and to prevent possible errant ricochet of previous pitches. This wooden bin

was placed in an area where a typical catcher would be positioned, a few feet behind home

plate.

Previous pilot studies to determine the accuracy of The Flamethrower showed that 56

consecutive pitched balls all landed in a 16.5 inch high by 15 inch wide area 60 feet 6 inches

away. This compares extremely well to the strike zone of an average sized batsman. As

previously mentioned, the typical strike zone is 17 inches wide—the width of home plate—and

extends from a batters knees to the batters armpits, approximately 27 inches high for a 6 foot

tall batsman.

The time that elapsed between when the ball was released and when the ball reached a

particular location was determined using a series of photocells placed along the path of the ball.

The elapsed times and linear distances of interest are shown in Table 1. The final value in this

table appears when the ball reaches the batter. The linear velocity of the ball varies along the

path of the ball, but is between 75 and 80miles per hour over the entire trajectory. Figure 2

shows the angular velocity of the ball along its trajectory. As the ball approaches the batter, the

angular velocity dramatically increases.

Table 1: Reference points for ball location in time and space. The center of the subject’s head was about 22 inches from the middle of home plate.

Distance from Pitching Machine (ft) Time (ms) Angular Change (degree) 17.52 150 1.62 23.36 200 2.77 29.20 250 4.85 35.04 300 9.69 35.62 305 10.54 39.60 339 22.28 43.5664 373 87.60

Figure 2: Angular velocity changing throughout the pitch.

2.3 Monitoring Eye Movements

Eye movements were continuously monitored from the lead or left eye (all subjects stood in the batter’s box with the left leg closer to the pitching machine as if batting right handed) throughout the study using an eye-tracker from ISCAN Incorporated (Burlington,

Massachusetts), only horizontal eye movements were measured for this study. The ISCAN tracks the location of the center of the pupil. The ISCAN consists of a tight-fitting goggle mounted in which beam-splitting mirrors and infrared video-cameras are mounted. The video cameras are mounted above the eyes, and images of the eyes enter these cameras after reflection from the beam-splitters placed in front of and just below the eyes. The estimated spatial resolution of this eye tracker is about 15 minutes of arc. The recording rate of the system is 120Hz. The ISCAN output was fed through a digital to analog converter supplied with the system, and then into an analog to digital converter (USB 1208FS, Measurement Computing, Norton, MA) for subsequent synchronization with the head tracker and the laser/photocell ball tracking combination.

In order to determine any temporal latency, the sclera search coil method was used to directly compare to the ISCAN. The search coil is a more invasive technique to measure eye movements. The search coil consists of silicone annulus containing a small coil of wire inside of it and is worn by the subject directly on the eye. Utilizing a constructed cage that produces a magnetic field, changes in magnetic field are monitored as the annulus of wire changes is rotated within the field. This system of tracking eye movements is so sensitive that the temporal latency is considered negligible. In order to determine the ISCAN’s temporal latency, simultaneous eye-tracking was performed while wearing the search coil and the ISCAN. Direct comparison showed a relatively constant and repeatable 35 ms delay in the onset of the ISCAN recording. 16

To determine the gain or amplification factor of the ISCAN, subjects were instructed to maintain a fixed head and to view a target directly below the pitching machine about 44 feet away and to then view another target much nearer to them and attached to an adjacent wall. At each of these distances, the digital output from the analog to digital converter was recorded.

The procedure required the subject to turn the eyes through an angle of 51.3deg. This procedure was repeated prior to trials 1 and 3 in the experiment described below.

2.4 Monitoring Head Movements

Head movements were continuously monitored during the study using the 3DM-GX1

(Microstrain, Williston, VA) gyro enhanced head tracker, capable of sensing 3-axis orientation.

The 3DM-GX1 combines three magnetometers with three accelerometers and three angular rate gyros to measure each of its 3 axes of interest. For the purposes of this study, only horizontal head rotations were measured. Microstrain reports the 3DM-GX1’s accuracy to be 0.5 degrees under static conditions and 2 degrees under dynamic conditions while recording at 100 hertz. 34

The static, as opposed to dynamic, gain or amplification factor of the head tracker used in this experiment was assessed by asking an individual to point the head at several known marked points on a wall. The output from the head tracker used in this experiment was always smoothed using a 40 point averaging filter.

A laser was also mounted on the head to facilitate accurate head pointing. Measures of angular head position were made with the Flock of Birds (Ascension Technology). This device makes use of a DC magnetic field to record head position. Those data from the Flock of Birds stream out in degrees. Measures of head position were also made simultaneously with the head

17 tracker used in the current experiment. These simultaneous head position measurements could then be used to obtain the static amplification factor.

The true accuracy of the head tracker was assessed by comparing calibrated traces of head movements made simultaneously with the magnetic field coil method—considered the gold standard—and the head tracker used in the current experiment. Measurements were made at 500Hz. For the magnetic field coil method, a coil of wire was attached to the helmet worn by the subject. The subject attempted to track tennis balls with the head as these balls were thrown from the pitching machine, mimicking the task used in the current experiment.

Overall, data from 15 pitches were used for this analysis. The maximum amplitude of these head movements—as assessed with the head tracker used in the current experiment— ranged from 13.4 to 149.8 degrees.

As shown in Figures 3 and 4, the beginning of each head movement as assessed with the two head tracking methods matched closely; however, differences in the angle output between the two head trackers were apparent. The error in the head tracker was only important for this study during the time it would take for the tennis ball to reach home plate. Total ball flight time was 373ms and typical head movement latency values were 200ms. Therefore, correcting for head movement recording errors within approximately 150ms after initiation of head movement accounted for accurate head recordings during an entire pitch. Over the period of time we were generally interested in, the head tracker used in this experiment lagged behind the magnetic field coil. These differences increased throughout the first portions of the trajectory as shown in the Figures 3 and 4.

Figure 3: Acceleration error profile of 3DG-XM1 compared to the magnetic search coil for a 30 degree head turn.

Figure 4: Acceleration error profile of 3DG-XM1 compared to the magnetic search coil for a 130 degree head turn. 19

The (linear) regression slopes of the initial portion of the difference plots were found to be dependent on the maximum amplitude of the head movement. Thus, a regression equation was created relating the slope of these difference plots—the slope relates the difference to time—to the maximum amplitude of the head movement. Once this equation was derived, it was then possible to calculate the slope of the difference plot for all amplitudes of head movement. Finally, by knowing the maximum amplitude of a head movement, one could calculate the error and the compensation factor necessary to make the output from the head tracker used in the current experiment equal to the output from the search coil.

The analysis program used in this experiment was modified to include a calculation of the maximum amplitude of head movement for a given pitch. Further, the program also included a calculation of the head movement latency. For each subject, the mean maximum head amplitude for a given trial and the mean head movement latency for that trial were then used to derive a single compensation factor for all of the head movements measured in that trial at each time of interest during the pitch.

2.5 Data Analysis

Analog data from the eye tracker, the head tracker, and the photocell at the end of the tube were recorded at 2000Hz using the analog to digital converter. The data recordings were facilitated by a program written in Visual Basic 6.0 (Microsoft Corp., Redmond, WA). The program was written to synchronize those data from the ISCAN eye tracker and the 3DM-GX1 head tracker with the photocell indicating the initiation of a pitch. Only the initiation of the pitch was recorded because in preliminary investigations the pitching machine was found to be very consistent —within about 5ms—in terms of the time elapsed prior to the ball reaching a particular distance. Continuous streams of data were gathered during each pitch from the head 20 and eye tracker. A specific measurement used for this study was gaze error. Gaze error was calculated by subtracting the recorded eye and head movements from the corresponding change in angular position of the tennis balls during its flight at specific times. Following the completion of each run, a single analysis program was performed to calibrate and report the

(smoothed and corrected) head movement, eye movement and the gaze error at 150, 200, 250,

300, 305, 339 and 373ms after the initiation of the pitch.

2.6 Study Construct

The study was designed to accurately monitor and record eye, head and gaze errors of subjects when wearing aperture occluding goggles. It was hypothesized that the subjects would be able to initially suppress their RVOR and limit their gaze error, but as the ball continued to approach, they would eventually be unable to efficiently suppress their RVOR due to an increase demand for head turning, and their gaze error would increase.

The study was divided into 3 distinct trials of tracking for each subject. The first 50 pitches involved the subject tracking the tennis balls from the pitching machine all the way across home plate. The second 50 pitches utilized the aperture goggles to restrict the possibility of eye turning while accurately tracking the ball. The third and final 50 pitches again had the subjects tracking the balls without the aperture goggles on. Before firing pitches, eyetracker calibration was performed on the first and third set of pitches.

The initial 50 balls were shown to the subjects in order for them to become accustomed with the study equipment and to spotting the balls as they exit the pitching machine. The instructions given to the subjects for the first 50 balls were, “Keep your eye on the ball all the way from the pitching machine until the ball crosses the plate.” The pitches were separated in

21 time by approximately 5 to 7 seconds and only 50 tennis balls were fired consecutively from the pitching machine.

The subsequent 50 balls were tracked while wearing aperture goggles over the ISCAN eye tracker. The apertures were approximately placed 87mm in front of each eye, each measuring 5mm horizontal x 3mm vertical in size. Binocular spotting of a small, distant target was used in order to properly align and center the target in front of each eye. At 87mm in front of the eye, the apertures subtended a visual field of 3.3 degrees. The aperture goggles were worn to limit a subject’s ability to perform a saccadic eye movement, as no peripheral visual information would be available if gaze error was larger than this limited 3.3 degrees. Though the apertures limit peripheral visual stimulus, the visualization of the tennis balls through the 3.3 degree apertures would not be limited if the subject was successful at the task of tracking the tennis ball. At the last point of regard, 4 feet from the batsman, the tennis ball subtends its largest angle of 3.23 degrees, as seen in Table 2. The subjects were again told to, “Keep your eyes on the ball all the way from the pitching machine until the ball crosses the plate.” These 50 pitches were also separated in time by approximately 5-7 seconds, giving the subject adequate time to locate and re-foveate the pitching machines delivery tube before each pitch.

The final 50 balls were shown in an unobstructed manner identical to the first 50 pitches. The instructions given to the subjects for the last 50 balls were, “Keep your eye on the ball all the way from the pitching machine until the ball crosses the plate, trying to replicate the techniques you used to track during the previous aperture trial.” The pitches were separated in time by approximately 5 to 7 seconds and only 50 tennis balls were fired consecutively from the pitching machine.

Table 2: Varying angular subtense of tennis ball based on position from the batsman.

Time (ms) Distance From Batsman (feet) Angular Subtense of Tennis Ball (degrees) 150 26.06 0.49 200 20.22 0.64 250 14.38 0.90 300 8.54 1.51 305 7.96 1.62 339 3.99 3.23

CHAPTER 3: RESULTS

3.1 Survey Results

The following results are subjects’ subjective answers to questioning of previous baseball experience and previous participation in baseball eye tracking studies. Three subjects reported to have no prior baseball playing experience, one subject reported to have played baseball competitively prior to high school, five subjects played baseball competitively in high school and two subjects reported playing baseball in college. Eight of the twelve subjects reported to have previously participated in an eye movement study involving the tracking of simulated pitches and performed in the same laboratory as that of the current study.

3.2 Mean Head Movement Latencies and Maximal Amplitudes

The mean head movement latencies were calculated for each subject for each run and are listed in tabular form in Table 3. Mean maximal amplitudes for each subject for each run were also calculated and are as shown in Table 4. The algorithms used to find the head movements and calculate the values in Table 3 and Table 4 did not always report a value for a pitch. The minimum number of pitches used in calculating these means was 39. Mean maximal amplitudes and latencies were not reported for subjects deemed unable to accurately track a baseball and due to these subjects not being included in the statistical analysis.

Table 3: Head movement latencies for each subject and subsequent runs.

Subject Run Mean (ms) Standard Deviation (ms) 1 221.64 16.94 1 2 214.24 18.94 3 217.81 17.905 1 200.72 18.655 2 2 200.31 21.07 3 200.76 21.69 1 272.305 34.475 3 2 235.49 37.92 3 203.57 25.065 1 184.01 41.485 5 2 197.47 35.99 3 204.385 30.145 1 195.81 31.605 6 2 199.3 28.025 3 201.335 25.77 1 195.59 21.36 7 2 204.64 17.025 3 182.61 26.405 1 204.665 46.94 10 2 189.55 43.29 3 160.24 45.42 1 158.64 31.66 11 2 183.39 56.94 3 166.425 42.905 1 153.11 39.355 12 2 207.27 29.13 3 169.35 36.11

Table 4: Mean maximal amplitudes for each subject and subsequent runs.

Subject Run Mean (deg) Standard Deviation (deg) 1 59.51 8.46 1 2 56.11 10.57 3 69.56 11.57 1 91.52 10.22 2 2 58.06 9.31 3 32.31 6.18 1 26.76 16.89 3 2 27.6 13.66 3 41.54 11.63 1 98.18 15.69 5 2 48.59 18.43 3 39.15 19.19 1 80.9 10.2 6 2 110.81 9.98 3 89.87 9.82 1 110.24 4.84 7 2 123.58 4.86 3 114.49 4.02 1 19.96 10.07 10 2 19.82 8.37 3 20.76 6.51 1 41.44 9.05 11 2 13.08 7.97 3 12.62 3.81 1 36.31 13.82 12 2 20.05 7.97 3 21.73 9.52

3.3 Individual Gaze Error, Head Movements and Eye Movements

The gaze error, head movements and eye movements for all of the subjects for each pitch were recorded. Out of the 1350 pitches successfully recorded, 4 pitches were initially discarded due to equipment malfunction or the presence of blinks during the pitch. Subjects 4, 8

26 and 9 had significant gaze errors for multiple trials and were considered unable to perform the task of tracking successfully. Thus, those data from these subjects were not analyzed further.

Gaze error was converted into absolute gaze errors in order to appropriately account for leading and lagging gaze errors. It had been noted in previous pilot studies that the mean gaze error could be quite misleading if the standard deviation of the mean was high. Mean values were calculated for all subjects for their absolute gaze error, head movement and eye movements.

The results of the means are as shown in Table 5 through Table 16. It is important to note that if

RVOR suppression occurred in the second/aperture trial, then the eye movement amplitude should be zero. That is, the eye should remain at the same angle in the orbit throughout the pitch.

Table 5: Mean and standard deviation of absolute gaze errors for subject 1. No baseball experience; Subject in previous baseball study; Age 30.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.50 0.36 2 0.46 0.35 3 0.64 0.53

250ms 1 0.45 0.30 2 1.30 0.57 3 2.10 1.07 300ms 1 1.46 1.74 2 0.92 0.58 3 3.86 2.26

305ms 1 1.82 2.06 2 0.78 0.54 3 4.17 2.48

339ms 1 3.79 3.76 2 4.72 2.33 3 4.83 3.65

Table 6: Mean and standard deviation of absolute gaze errors for subject 2. Collegiate baseball experience; Subject in previous baseball study; Age 23.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.36 0.24 2 0.82 0.63 3 0.60 0.38

250ms 1 2.00 0.87 2 2.51 1.04 3 0.66 0.80

300ms 1 10.65 5.36 2 6.47 3.91 3 15.11 5.76

305ms 1 12.6 5.04 2 7.83 4.06 3 16.76 5.78

339ms 1 28.88 14.77 2 11.53 4.15 3 21.44 4.58

Table 7: Mean and standard deviation of absolute gaze errors for subject 3. High school baseball experience; Not a subject in previous baseball study; Age 24.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 1.26 0.94 2 1.27 0.62 3 1.04 0.63

250ms 1 1.79 1.61 2 1.64 0.86 3 1.10 1.51 300ms 1 3.57 3.43 2 3.02 1.49 3 3.53 3.98

305ms 1 3.87 3.56 2 3.35 1.57 3 4.27 4.42

339ms 1 7.08 5.13 2 10.92 3.46 3 6.59 5.02

Table 8: Mean and standard deviation of absolute gaze errors for subject 4. High school baseball experience; Subject in previous baseball study; Age 25.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.47 0.33 2 2.07 1.07 3 0.55 0.67

250ms 1 2.44 2.69 2 5.83 2.94 3 2.05 2.26

300ms 1 18.14 8.68 2 20.00 10.30 3 11.53 8.32

305ms 1 20.74 8.90 2 21.91 10.63 3 13.48 8.67

339ms 1 32.28 8.94 2 29.10 11.09 3 23.47 9.28

Table 9: Mean and standard deviation of absolute gaze errors for subject 5. Baseball experience before high school; Not a subject in previous baseball study; Age 21.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.79 0.82 2 0.60 0.49 3 0.92 1.30 250ms 1 2.50 4.10 2 1.32 1.32 3 1.20 1.49

300ms 1 12.60 10.45 2 2.31 2.98 3 1.93 1.85

305ms 1 14.02 11.45 2 2.68 3.23 3 2.29 1.99 339ms 1 22.61 18.18 2 5.58 4.07 3 5.68 3.87

Table 10: Mean and standard deviation of absolute gaze errors for subject 6. No baseball experience; Not a subject in previous baseball study; Age 26.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.59 0.50 2 1.22 0.90 3 1.63 1.16

250ms 1 1.99 1.85 2 3.69 1.50 3 5.74 2.64

300ms 1 6.47 6.40 2 6.31 5.48 3 14.97 7.75

305ms 1 7.54 7.07 2 6.70 6.20 3 16.41 8.12

339ms 1 16.52 12.32 2 8.48 7.92 3 21.21 8.54

Table 11: Mean and standard deviation of absolute gaze errors for subject 7. Collegiate baseball experience; Subject in previous baseball study; Age 25.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.64 0.47 2 0.92 0.56 3 1.48 0.44 250ms 1 3.07 1.75 2 2.77 1.07 3 3.55 1.05

300ms 1 15.40 12.87 2 4.60 8.35 3 9.47 13.79

305ms 1 17.91 13.47 2 5.93 11.59 3 11.37 15.11 339ms 1 34.19 13.27 2 20.93 27.12 3 41.59 26.56

Table 12: Mean and standard deviation of absolute gaze errors for subject 8. No baseball experience; Not a subject in previous baseball study; Age 31.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 1.02 0.57 2 3.89 1.85 3 1.13 0.47

250ms 1 2.21 1.44 2 12.51 7.01 3 2.10 2.58

300ms 1 9.52 7.35 2 30.71 11.79 3 16.49 8.67

305ms 1 10.90 7.71 2 32.16 11.95 3 18.26 9.15

339ms 1 17.61 9.43 2 36.65 11.08 3 25.75 10.35

Table 13: Mean and standard deviation of absolute gaze errors for subject 9. High school baseball experience; Subject in previous baseball study; Age 24.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 2.94 3.15 2 3.28 4.67 3 4.04 3.23 250ms 1 14.97 13.20 2 15.33 11.79 3 10.33 8.48

300ms 1 36.20 16.12 2 29.90 14.72 3 20.83 11.42

305ms 1 38.22 16.12 2 31.06 14.94 3 21.53 11.49 339ms 1 46.29 15.85 2 33.45 13.85 3 21.49 11.34

Table 14: Mean and standard deviation of absolute gaze errors for subject 10. Baseball experience before high school; Subject in previous baseball study; Age 30.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.66 0.53 2 0.78 0.94 3 0.81 0.56

250ms 1 1.04 2.06 2 2.12 2.06 3 1.08 0.93

300ms 1 3.04 4.86 2 3.52 4.02 3 3.22 3.75

305ms 1 3.50 4.89 2 3.74 4.12 3 3.99 4.18

339ms 1 10.91 6.28 2 9.16 3.98 3 12.94 11.90

Table 15: Mean and standard deviation of absolute gaze errors for subject 11. High school baseball experience; Subject in previous baseball study; Age 42.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.97 0.84 2 1.09 0.59 3 0.73 0.52 250ms 1 4.37 5.09 2 1.38 1.11 3 1.14 0.67

300ms 1 14.31 9.04 2 2.69 2.31 3 1.60 1.98

305ms 1 15.12 9.39 2 3.21 2.36 3 1.91 2.05 339ms 1 16.29 11.81 2 10.68 3.73 3 9.07 3.41

Table 16: Mean and standard deviation of absolute gaze errors for subject 12. High school baseball experience; Subject in previous baseball study; Age 26.

Time Run Mean (degrees) Standard Deviation (degrees)

200ms 1 0.80 0.53 2 0.36 0.31 3 0.52 0.32

250ms 1 1.74 0.82 2 0.47 0.43 3 0.88 0.66

300ms 1 3.21 1.62 2 1.22 0.88 3 1.19 0.79

305ms 1 3.38 1.91 2 1.40 0.95 3 1.23 0.86

339ms 1 3.55 2.44 2 9.34 2.14 3 6.79 3.31

3.4 Analysis of Mean Absolute Gaze Errors, Head Movements and Eye Movements

The means of the absolute gaze errors, head movements, and eye movements for the three trials were compared at each of the distances of interest. Repeated measures analysis of variance (ANOVA) was used for this comparison. The factors in the model were trial number and subject. Subject was listed as a random factor. As part of the repeated measures ANOVA, a multiple comparison test (Tukey) was performed ( α = 0.05). The results are as shown in Table 17 through Table 19.

Table 17: Mean, standard deviation and repeated measure ANOVA results for the overall absolute gaze errors.

Time Run Mean (degrees) Standard Deviation (degrees) ANOVA Results 1 0.73 0.67 1=2 200ms 2 0.84 0.69 1 not equal 3 3 0.93 0.80 2=3 1 2.11 2.73 1=2 250ms 2 1.91 1.50 1=3 3 1.93 2.06 2=3 300ms 1 7.86 8.78 1 not equal 2 2 3.45 4.45 1 not equal 3 3 6.11 7.99 2 not equal 3

305ms 1 8.86 9.48 1 not equal 2 2 3.96 5.45 1 not equal 3 3 6.94 8.72 2 not equal 3

339ms 1 15.98 15.03 1 not equal 2 2 10.15 10.76 1=3 3 14.51 15.51 2 not equal 3

Table 18: Mean, standard deviation and repeated measure ANOVA results for mean overall head movements.

Time Run Mean (degrees) Standard Deviation (degrees) ANOVA Results 1 3.37 2.47 1 not equal 2 200ms 2 2.65 1.45 1=3 3 3.27 2.52 2 not equal 3 1 7.80 4.22 1 not equal 2 250ms 2 6.34 2.20 1=3 3 7.37 3.16 2 not equal 3

300ms 1 14.98 6.79 1 not equal 2 2 12.31 3.89 1 not equal 3 3 13.93 4.29 2 not equal 3 305ms 1 15.87 7.10 1 not equal 2 2 13.04 4.16 1 not equal 3 3 14.72 4.54 2 not equal 3

339ms 1 22.66 9.59 1 not equal 2 2 18.44 6.37 1 not equal 3 3 20.74 7.03 2 not equal 3

Table 19: Mean, standard deviation and repeated measure ANOVA results for mean overall eye movements.

Time Run Mean (degrees) Standard Deviation (degrees) ANOVA Results 1 -0.56 2.22 1 not equal 2 200ms 2 0.47 1.42 1 not equal 3 3 -0.2 2.67 2 not equal 3 1 -1.37 3.63 1 not equal 2 250ms 2 -0.12 2.22 1=3 3 -1.01 3.85 2 not equal 3

300ms 1 -0.51 10.12 1=2 2 -1.08 5.01 1=3 3 -0.08 9.68 2=3 305ms 1 -0.2 11.32 1=2 2 -1.02 6.1 1=3 3 0.35 10.59 2 not equal 3

339ms 1 -0.43 22.22 1=2 2 -1.01 13.75 1=3 3 0.04 22.77 2=3

These data suggest that the mean absolute gaze error was smallest for Trial 2 (the aperture trial), and that head movement tended to be smaller in Trial 2. The eye movement data are harder to interpret, as the standard deviations tended to be quite large. However, the standard deviation is smallest for Trial 2, indicating that subjects tended to be more consistent in the amount of eye rotation used to track the ball in this trial compared to trials 1 and 3.

The relationship between change in absolute gaze error and total distance that the tennis balls had traveled from the pitching machine was graphed for each run. The results are as pictured in Figure 5.

20 18 16 14 12 10 Run 1 8 Run 2 6

Gaze ErrorGaze(degrees) Run 3 4 2 0 20 25 30 35 40 45 Distance Tennis Ball Traveled (feet)

Figure 5: Gaze error compared to ball flight distance from release point.

The relationship between the degree of head rotation during each run and the head rotation required to equal the angular change of the target of regard reported as a percentage.

A 100% response to the target of regard would indicate a head movement equal to that of the angular change of the tennis ball. A 50% response would indicate a head movement equal to half the amplitude of angular change of the tennis ball, and a 200% response would indicate a head movement equal to twice the amplitude of angular change of the tennis ball. If subjects had utilized only head movements to track the ball accurately, actual head rotation would equal the angular change of the tennis ball—100%. The results are pictured in Figure 6. Head movement overshoots were more common in Trials 1 and 3.

180% 160% 140% 120% 100% Run 1 80% 60% Run 2 40% Run 3 20% 0% Target Position-ResponseTarget (percentage) 0 5 10 15 20 25 Angular Degree Change in Tennis Ball (feet)

Figure 6: Target position-response and resultant head rotation. A head rotation equal to 100% indicates a head rotation exactly equal to the angular change in ball position. Position–Response = (Acutual Head Rotation) ÷ (Change in Ball Position) x 100%.

3.5 Eye Movements within the Apertures

In order to determine if gaze errors were representative of a subject’s ability to view the tennis ball through the aperture, the number of eye movements between -3.3 to 3.3 degrees— in order to account for all maximal eye movements made potentially within the width of the apertures—were counted for each subject in run 2 and converted to a percentage of the total pitches for run 2. The results are presented in Table 20.

Subject # 150 200 250 300 305 339 1 100% 100% 94% 96% 94% 64% 2 100% 100% 90% 60% 44% 0% 3 100% 98% 86% 78% 76% 70% 5 98% 98% 82% 44% 50% 52% 6 98% 90% 78% 66% 62% 52% 7 100% 98% 94% 64% 64% 26% 10 98% 96% 88% 52% 50% 42% 11 94% 94% 84% 42% 40% 60% 12 100% 100% 96% 84% 82% 78% Average 99% 97% 88% 65% 62% 49%

CHAPTER 4: DISCUSSION

4.1 Head Tracker Calibration

The construct of this study was consistent with the laboratory’s goal to have the ability to collect all data effectively and efficiently on-site or off-site. For this reason, the search coil method was not utilized as a technique to monitor eye or head movements. As the ISCAN had already been thoroughly evaluated and delays in recording appropriately adjusted for, the invasive search coil was deemed to be unpractical for the purpose of this study. For monitoring head movements, the limitation that a cage to conduct a magnetic field would have to be constructed was deemed sufficient enough to investigate other means of head monitoring. The magnetic field producing cage would not only limit the transportability of the equipment, but could have possibly hindered subjects’ ability to swing at pitches in future studies.

The 3DM-GX1 head tracker was successfully calibrated to account for its head rotation acceleration and maximal amplitude dependent errors. This calibration now allows future studies to incorporate a repeatable and accurate means of recording head movements non- invasively without altering or limiting one’s ability to swing. This will also allow future studies to be conducted outside of the laboratory and on a field or other training arena.

4.2 Gaze Error

39 standard deviation is smallest for Trial 2 indicating that subjects tended to be more consistent in the amount the eye was rotated from trial to trial. Thus, it seems that restricting the amplitude of eye rotation results in more consistent foveation of the target. Further, the apertures resulted in head movements that were closer to the velocity of the pitched ball. This seems consistent with the study cited in the introduction, in which a cue to head position can result in head movements that match the velocity of a moving target.

What is unexpected is that the head movements are larger in Trials 1 and 3. This suggests, similar to the findings of previous studies, that head movements in the absence of a visual cue to head position are correlated with larger gaze errors 33 . It may be that individuals

commonly make head movements larger than those required to track a target in the absence of

a visual cue to head position. In turn, these head movements may make it difficult to accurately

foveate a moving target. Further studies are needed to assess whether larger head movements

are simply correlated with larger gaze errors, or whether larger head movements are the cause

of larger gaze errors.

Finally, with the regard to the mean absolute gaze error data, two subjects showed a

“carry over” effect from Trial 2 to Trial 3. For these two subjects, there is an improvement in the

absolute gaze error from Trial 1 to Trial 2, and this improvement seems to carry over to Trial 3.

Such a trend was not as apparent in the overall data for absolute gaze error. It may be that more

pitches must be viewed through the aperture in order for individuals to show a definitive change

in behavior after wearing the aperture.

From those data in which the ability of subjects to view through the aperture was

assessed, it is clear that individuals were quite successful in maintaining vision through the

aperture throughout much of the pitch trajectory. In some cases, this was true even when the 40 velocity of the object exceeded 500 degrees/second at the elapsed time of 339ms. Since the gaze error was relatively low in some of these latter cases, the implication is that the RVOR was successfully suppressed in this experiment at very high head velocities.

The two subjects (1 and 12) with the most consistent ability to limit their eye movements in Run 2 to either -3.3 degrees or +3.3 degrees—the theoretical imposed limitations enforced by the apertures—at 300 and 305ms correspondingly had the lowest gaze error. At

300ms, the rate of success for staying within the predicted limitations of the apertures for subject 1 was 96% and for subject 12 it was 84%. The corresponding gaze error at 300ms was

0.92 and 1.22 degrees respectively. At 305ms, the rate of success for staying within the predicted limitations of the apertures for subject 1 was 94% and for subject 12 it was 84%. The corresponding gaze error at 305ms was 0.78 and 1.4 degrees respectively. The success of these subjects in limiting their eye movements to the restrictions imposed by the apertures, and correspondingly demonstrating the best gaze errors among subjects at 8 feet—between 300 and 305ms—indicates that the subjects most capable of suppressing their RVOR have the best chance of tracking a pitch close to the plate.

In conclusion, the repeated measures ANOVA on the group results revealed consistent

statistically significant differences in head movement when wearing the apertures at 200, 250

300 and 305ms, and 339ms. The initial hypothesis was that subjects would alter their actions

when responding to the occlusion apertures by increasing the rotation of their head and

decreasing their eye movements by suppressing their RVOR. However, this did not turn out to

be the case. The introduction of the apertures resulted in a decrease in head rotation.

The data suggest that the decreased head rotation while wearing the apertures results

in more accurate head tracking. This decrease in head rotation not only aids the subject by 41 producing a smaller residual gaze error by head movement alone, but also results in a lower

RVOR reflex and therefore a decreased demand to suppress the RVOR.

Further data are needed to confirm this, but these data suggest that if one’s tracking behavior with the aperture can be carried over to one’s tracking behavior without the aperture

—something that did not occur in this experiment— then large head movement overshoots that negatively impact one’s overall gaze tracking may be avoided.

4.3 Limitations

During this study, a significant limitation in order to make the obtained results applicable to the sport of baseball, and in particular to the act of batting, is the fact that no attempt to hit the ball was made. Though this study was designed to only investigate subjects would be able to suppress their RVOR and successfully track pitches, additional studies would ideally look at change in performance of the actual act of batting under different testing conditions.

This study was performed under the assumption that the use of the head to increase gaze tracking would be beneficial to tracking and therefore to batting. However, the judgment of baseball outfielders predicting the position of fly balls has been shown to be speed up in head fixed conditions. Baseball players wearing a neck brace showed decreased latencies in accurately predicting whether the fly ball was hit beyond or in front of them compared to head free conditions, even though actual ability in catching the ball decreased. The study theorized that even though head movement is important for the act for catching a ball, the decrease in neural input resulting from restraining the movement of the head allowed for less positional data to be processed by the brain and resulted in the lower latencies.35 Non-conclusive results

from additional studies have yet to confirm a possible detrimental effect between head-free and 42 head-fixed baseball pitch tracking, showing a statistical benefit for head-fixed at 8ft but not at

4ft. The baseball pitch tracking study theorized that novices are unable to make appropriate head movements to aid in tracking.6

Additionally, is it adequate to assume that increased tracking abilities will lead to increased batting performance? This assumption of improved tracking correlating to improved batting performance was not based solely on the logical idea that a subject would be able to more accurately hit an object that they were able to track closer to the point of contact; but, it is also based on a previously mentioned study that has documented better tracking abilities of a professional baseball player when compared to novices.7 Obviously, coordination and athleticism will ultimately help to determine one’s ability to accurately and consistently hit a baseball; however, it theoretically is logical to assume that in the presence of equal athletic ability, unequal tracking abilities between subjects could lead to unequal batting performances.

However, previously mentioned studies have indicated that professional batsmen appear to not track pitches in which a swing is initiated. 22 An additional study has indicated that

experienced fast pitch softball batsmen are able to successfully perform the act of batting while

any one third of the balls path is occluded from view.5 A neuropsychological approach to

swinging hints that a batsman must gather enough information in the initial 200ms of a pitch to

allow oneself enough time to make an accurate motor response 36, while 48% of subjects’ runs in

the current study have head movement latencies exceeding 200ms. If the best batsmen do in

fact not track the ball they intend to hit, if the entire balls flight path is not needed to accurately

track a pitch or if a batsman’s ability to accurately hit a ball is based solely on one’s initial motor response, then attempting to track a pitch with either the head or the eye would be unnecessary. 43

Occlusion studies have been performed for multiple sports, including tennis, cricket and squash. In one such study, advanced tennis players were shown to be significantly faster than the less skilled players in anticipating the direction of opponent’s tennis strokes. The study utilized simulated tennis volleys in which visual information was stopped either immediately before the volley or varying times following the opponent’s volley. While the less advanced players showed initial poor abilities at predicting the flight of the tennis ball based on limited visual information, through training, the less advanced tennis players showed statistically significant improvements in ball location anticipation compared to less advanced players receiving placebo training.37 This ability to obtain significant and beneficial visual information earlier in actions than novices may explain experienced athletes decreased Go/Nogo reaction time when deciding to initiate a sport specific task. 38 Since professional athletes have been

shown to have increased reaction time, need less visual information to accurately perform a

task and have increased tracking abilities, further training in all of these aspects may prove

beneficial in increasing batting success rates.

4.4 Conclusions

The results from this study suggest that gaze tracking utilizing head movements and suppression of the RVOR is an adequate and possibly advantageous strategy to track a baseball pitch to relatively close distances. In lieu of the above mentioned limitations, subjects appeared to be able to accurately track a baseball pitch while wearing an ocular rotation limiting device with no prior training or experience.

The results of this study of head gaze tracking correlate well to those of pursuit eye movement studies. While the ocular saccade and rapid head orienting common neural pathway is well established in the literature, the results of this study may suggest the existence of a 44 common neural pathway between head and ocular pursuit movements. This is not the first time that the head pursuit pathway has been theorized. Though not much is known of this pathway, other studies have theorized this pathway may not be as adaptable as the head saccade pathway 31 and others confirming that during head-unrestrained moments, head and eye

movements are at least in part controlled by a common upstream controller.39 The apertures in

this study may be more indicative of an ocular cue of head position leading to improved head

control and efficiency rather than a means to force the suppression of the RVOR. No matter if

the apertures are cueing improved head movement or forcing RVOR suppression, it is evident

that apertures provide a means to efficiently track a high velocity baseball pitch.

Commercial devices, such as the I-On Eye Trainer, may provide the means to enforce the

suppression of the RVOR and develop it as a learned attribute, or they may aid in developing a

subject’s kinesthetic head awareness. By learning to properly control one’s head, eye and head

movements can be efficiently combined. Further studies would be needed to test whether

training with the apertures can improve non-occluded baseball tracking, and subsequently if

they can improve non-occluded batting performance.

Additional studies should also investigate how much visual information is needed to

accurately hit a baseball. Possible studies could incorporate liquid crystal occlusion goggles to

limit the amount of visual information obtained from a pitched ball at varying time intervals, in

order to possibly determine a threshold of visual information needed to successfully hit a ball.

Once the threshold of visual information needed to successfully hit a baseball is determined, the

most appropriate tracking strategies can then be implemented to improve one’s success rate at

batting.

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