Keeping Your Eyes Continuously on the Ball While
Running for Catchable and Uncatchable Fly Balls
Dees B. W. Postma, A. Rob den Otter, Frank T. J. M. Zaal*
Center for Human Movement Sciences, University Medical Center Groningen, Sector F, University of Groningen, Groningen, the Netherlands
Abstract
When faced with a fly ball approaching along the sagittal plane, fielders need information for the control of their running to
the interception location. This information could be available in the initial part of the ball trajectory, such that the
interception location can be predicted from its initial conditions. Alternatively, such predictive information is not available,
and running to the interception location involves continuous visual guidance. The latter type of control would predict that
fielders keep looking at the approaching ball for most of its flight, whereas the former type of control would fit with looking
at the ball during the early part of the ball’s flight; keeping the eyes on the ball during the remainder of its trajectory would
not be necessary when the interception location can be inferred from the first part of the ball trajectory. The present
contribution studied visual tracking of approaching fly balls. Participants were equipped with a mobile eye tracker. They
were confronted with tennis balls approaching from about 20 m, and projected in such a way that some balls were
catchable and others were not. In all situations, participants almost exclusively tracked the ball with their gaze until just
before the catch or until they indicated that a ball was uncatchable. This continuous tracking of the ball, even when running
close to their maximum speeds, suggests that participants employed continuous visual control rather than running to an
interception location known from looking at the early part of the ball flight.
Citation: Postma DBW, den Otter AR, Zaal FTJM (2014) Keeping Your Eyes Continuously on the Ball While Running for Catchable and Uncatchable Fly Balls. PLoS
ONE 9(3): e92392. doi:10.1371/journal.pone.0092392
Editor: Todd W. Troyer, University of Texas at San Antonio, United States of America
Received July 25, 2013; Accepted February 21, 2014; Published March 26, 2014
Copyright:  2014 Postma et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: These authors have no support or funding to report.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: f.t.j.m.zaal@umcg.nl
Introduction
Catching a fly ball not only adds to a good result in a baseball
game but also keeps fascinating spectators and scientists alike. A
particularly famous catch was the one made by Willie Mays in the
1954
World
Series
(e.g.,
see
http://en.wikipedia.org/wiki/
The_Catch_(baseball)).
He
managed
to
catch
a
seemingly
uncatchable ball, after looking at the ball and running to the
interception location about 475 feet (145 m) from the home plate
[1]. Willie Mays’s catch made it to an illustration accompanying
the contribution of Chodosh, Lifson, and Tabin in the 1995
volume of the journal Science [2]. These authors claimed that Willie
Mays, and other adept outfielders, do not need to track the ball
with their gaze because they are able to predict where and when to
intercept the ball from the initial part of the ball trajectory. This
will be the issue that we address in the present contribution: Does
it suffice to view only the initial part of the ball’s flight to predict
the interception location or do fielders continuously track the ball
with their gaze while running to that interception location?
Two types of strategy for the interception of moving targets
have been distinguished in the literature (e.g., see [3–9]). On the
one hand are the predictive strategies. In the context of fly-ball
catching, this type of strategy amounts to looking at the ball’s
trajectory and predicting the interception location from the initial
conditions of the ball’s trajectory (i.e. its initial velocity and initial
angle; cf. [10]). It should be noted that the use of such predictive
strategy depends on a priori knowledge about gravity and air
resistance. Because of drag and spin, fly balls do not follow
parabolic trajectories (cf. [1,8,11]), which implies that a sophisti-
cated internal model of ball-flight dynamics would have to be
postulated.
An alternative to a predictive strategy is to use continuous visual
guidance of locomotion on the basis of prospective information.
Rather than having to know the interception location and time
from early conditions, prospective strategies (e.g., see [12–14]) involve
continuously available information that can be used to know
whether the current action (e.g., running speed) will lead to a
successful interception. In the context of the interception of fly
balls, one such model states that if a fielder keeps the ball moving
on a linear optical trajectory (LOT), he or she will arrive at the
interception location in time, without knowing when and where
the interception will take place [15–18]. The LOT strategy boils
down to making sure that the horizontal and vertical components
of the gaze angle (the angle between the heading and the gaze
direction) change proportionally. Locomotion patterns of fielders
running to catch fly balls travelling to locations in front or behind,
and to the side of the fielders’ initial positions have been reported
to be in line with a LOT strategy (e.g., [16,19]). However, several
authors have claimed that keeping a linear optical trajectory is not
sufficient to guarantee interception because linear optical trajec-
tories can also occur for unsuccessful interceptions [20–23].
Furthermore, for balls approaching a fielder along the sagittal
plane, a LOT strategy cannot be applied because there is only a
vertical gaze angle; because there is no horizontal component of
the gaze angle, all ball trajectories, whether leading to catches or
not, will result in linear optical trajectories.
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When a fly ball approaches along the sagittal plane, only
running in the forward and backward direction needs to be
controlled. In 1968, the physicist Neville Chapman [24] consid-
ered the mathematics of the situation of a fly ball on a parabolic
trajectory approaching a fielder head on. He showed that the rate
of change of the tangent of the gaze angle (the angle between the
line of gaze and the horizontal, assuming that the gaze is directed
at the ball) would remain constant if the fielder runs to the
interception location at a constant speed. Thus, for fielders to
arrive at the right place in the right time, the Chapman strategy
amounts to keeping this rate of change constant. Because the rate
of change of the tangent of the gaze angle is equivalent to the
speed of the projection of the ball onto an image plane, and
because keeping speed constant is equivalent to keeping acceler-
ation at zero, the Chapman strategy is also known as the Optical
Acceleration Cancellation (OAC) strategy (cf. [25,26]; see also
[6,21,27]).
Empirical studies have shown that participants, running to catch
fly balls, show locomotion patterns that are consistent with the use
of the OAC strategy [6,26,28,29]. Because the OAC strategy is a
strategy based on prospective information, it predicts that
locomotion paths will differ for balls that land in the same spot
but with different trajectories. This has been demonstrated in
catching cricket balls [6], baseballs [23], and in heading virtual
soccer balls [22].
The Chapman strategy specifically applies to fly balls that
approach the fielder head on. As mentioned before, the textbook
(e.g., [30,31]) candidate complementary strategy to deal with the
lateral component of running is the LOT strategy. Recent
research using virtual reality has shown that the LOT strategy
might not be the final answer [22,23], and other strategies to
complement the OAC strategy have been put forward (e.g.,
strategies of keeping constant the bearing angle—the CBA
strategy, see [24], or its first temporal derivative—see [21]). If
fielders control their locomotor trajectories on a moment-to-
moment basis and use prospective information, they need to rely
on a constant informational coupling with the approaching fly ball.
However, according to Chodosh and colleagues [2] (cf. [1,10,11]),
there is no need for such continuous visual coupling because
fielders are capable of predicting the future landing location of the
ball based on early available information of its trajectory: These
authors argued that real fielders, like Willie Mays, simply look at
the ball, predict the interception location, run there at maximal
speed, and wait for the ball to arrive. Quite surprisingly, the issue
of whether or not the catching of fly balls involves a constant visual
coupling has not yet received much scientific attention. A notable
exception is the study by Oudejans, Michaels, Bakker, and Davids
[32], which examined gaze direction of fielders confronted with
approaching fly balls.
Oudejans and colleagues [32] were interested in the potential
contribution of extraretinal systems for picking up the information
to guide running to intercept fly balls (see also [29,33,34]). They
argued that if the ball is tracked with gaze, not only the retinal
system but also vestibular or proprioceptive systems might be used
to pick up optical acceleration. Participants were equipped with a
gaze-tracking system, and were allowed to make a few steps in the
right direction for fly balls projected at them head on. Because the
gaze tracker was connected with a cable to the recording unit,
participants could only move about one or two steps forward or
backward. Interesting in the present context is the finding that
participants in the Oudejans et al. study, indeed, continuously kept
their eyes on the ball, by moving both their heads and eyes.
When using a predictive strategy, fielders obviously need to look
at the ball during the initial part of its flight. Certainly, there is no
need to keep the eyes on the ball during its entire flight. Although
the use of a prospective strategy does not necessitate such
continuous tracking of the ball (intermittent looking at the ball
would suffice), the finding that participants do continuously track
the ball would fit the use of a prospective strategy better than it
would the use of a predictive strategy. The present paper reports
an experiment in which we tracked the gaze of participants in a
setting in which approaching fly balls either were within their
locomotor reach (i.e., balls were catchable) or were not within their
locomotor reach (i.e., balls were uncatchable). Importantly, the
gaze tracker that we used was mobile, and allowed the participants
to use their natural range of motion. That is to say, whereas
Oudejans et al. [32] have shown that their participants continu-
ously tracked the balls with their gaze for balls falling at or near the
initial position of the participant or when simply watching balls
that landed farther away than two steps, the present study allows
to establish this behavior while participants are free to run much
greater distances, even reaching their top speeds. Furthermore, we
studied two situations. In line with the majority of previous studies
on catching fly balls, we considered balls that would fly close
enough to the participants that they would be able to arrive at the
interception location in time. In addition, we also studied the
situation in which balls were projected so far away from the
participants’ starting position that they would not be able to reach
the ball before it would hit the floor. We instructed participants to
indicate when they knew that a ball would be uncatchable, and
when this occurred we inspected the direction of gaze up until this
point. In short, the present study considered running to catch fly
balls under the demanding circumstances as seen in regular ball
games. Tracking the ball might be regarded much more difficult
when running close to full speed. When even under these
strenuous conditions we would find pursuit tracking of the ball,
we argue, this gaze behavior must have a functional origin, which
most probably would be related with continuous visual control.
Methods
Participants
Ten female volunteers (mean age 21.762.2 years) participated
in the experiment. To be included, they needed to have at least
two years of experience in ball sports. All participants reported
normal, or corrected to normal (lenses) vision. Prior to the
experiment, participants were informed about the procedure of the
study and gave their written informed consent. The study was
approved by the Ethics Board of the Center for Human
Movement Sciences (University Medical Center Groningen, the
Netherlands), and the protocol was in accordance with the
Declaration of Helsinki.
Apparatus
To determine the point of gaze (PoG), participants were
equipped with a monocular, mobile eye tracker (Mobile Eye,
Applied Science Laboratories, Bedford, MA). The tracking system
consists of a scene camera (recording the field of view of the
participant), and an optics module that consists of a near infrared
light source and an eye camera. All components are mounted on a
pair of lightweight spectacles. Calculation of the point of gaze is
based on ‘dark pupil tracking’ and involves detection of the center
of the pupil and the reflection of a cluster of three infrared LEDs
on the cornea. Eye rotations are calculated from the angle and
length of the vector connecting the pupil center and the corneal
reflection. After calibration (see below), eye rotations are mapped
onto the scene view, establishing the PoG in the scene. Interleaved
images of the eye camera and the scene camera were recorded on
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tape using a portable video recorder (Sony GV-D1000E DVCR),
at 30 Hz. The PoG was represented in the scene view by a
crosshair with an approximate accuracy of 1u visual angle. The
visual range of the eye tracker is 50 degrees horizontally, and 40
degrees vertically. The weight of the system, excluding the video
recorder, is 76 grams. During testing, the video recorder was worn
in a pouch around the waist, and allowed near-normal mobility for
the participant.
The eye tracker was calibrated using a 3.3 m high by 4.4 m
wide grid with 20 equally spaced points (4 rows of 5 dots each),
representing a visual angle of 16.9u in the horizontal and 12.6u in
the vertical direction. During calibration, the participants were
positioned 15 meters from the grid, while their head rested on a
chinrest that fixated the head. The gaze tracker was calibrated
prior to the start of the experiment, after each set of 18 trials, and
also when the participant indicated that the tracker had changed
its position on the head during testing.
Setup and procedure
The experiment was performed in a well-lit gymnasium
(50630610 m). A ball-projecting machine (Louisville Slugger,
type UPM45 Blue Flame) with adjustable force and projection
angle was used to deliver tennis balls along the sagittal plane of the
participant, at different projection distances. Because the projec-
tion angle could be manipulated only within a limited range, we
used wooden blocks that were placed underneath the ball
projection machine to generate the desired trajectories. The
projected distance of the fly balls was varied systematically by
adjusting the projection force and angle, and ranged approxi-
mately from 10 to 29 m. The apex of the trajectory was about
8.5 m for every trial, so that all fly balls had an approximate flight
time of 2.5 s. The ball-projecting machine was occluded from sight
to prevent visual anticipation of the ball’s trajectory.
Participants completed 54 trials. The initial position of the
participant was 20 m from ball projection, and was identical in all
trials. At the start of each trial, the experimenter verbally cued the
participant before ball delivery. Participants were instructed not to
make a dive to perform a successful catch. Other than that,
participants were free to move as they felt necessary to catch the
ball. No instructions were given with regard to catching strategies
(e.g. overhand or underhand catching). Not all projected balls
were catchable. Participants were instructed to call ‘no’ at the
instant that they realized that they were unable to catch the ball.
Data analysis
We used EyeVision software (Applied Science Laboratories,
Bedford, MA) to convert the video data that were stored on tape
into AVI files. We analyzed the data from the moment of ball
projection until the moment the ball was either intercepted or until
the moment that the participant indicated that the ball was
uncatchable by calling ‘no’. We used the audio signal from the
internal microphone of the eye tracking system to detect these
moments. More specifically, we marked the first video frame in
which the sound of ball projection was audible as the first video
frame for further analysis. The final frame that was analyzed for
each trial corresponded either with the first frame in which the
sound of the ball hitting the hand of the participant was audible, or
the first frame in which the sound of the participant calling ‘no’
was audible. Audio analysis of the video data was performed in
Adobe Premiere CS6.
To assess whether gaze tracked the ball, we considered the
distance between the point of gaze (PoG) and the ball image, for
each video frame. We used the EyeVision software to establish the
2D position of the PoG in the scene plane. Next, we used ASL
Results Plus GM software (Applied Science Laboratories, Bedford,
MA) to filter invalid values for the PoG. With custom-made
software in MATLAB (Mathworks R2012b), we determined the
2D position of the ball in the scene plane, by hand. Finally, we
computed the absolute distance in pixels between the PoG and the
ball in the scene image.
We assigned points of gaze to one of two categories: either on
the ball (‘tracking’) or not on the ball (‘other’). A PoG was
considered to be on the ball whenever the absolute distance
between the ball and the PoG in the scene plane was smaller than
75
pixels
(corresponding
to
6.25u
visual
angle).
Although
theoretically this criterion allows for changes in distance of 150
pixels between successive frames to be assigned to the ‘tracking’
category, it turned out that these changes were smaller than 25
pixels in 95.8% of the frames identified as ‘tracking’, and that in
only 0.4% of the ‘tracking’ frames the change was greater than 75
pixels.
For each trial, the relative contributions of frames associated
with ‘tracking’ and ‘other’ behavior will be expressed as a
percentage of the total number of frames that had a valid PoG.
Furthermore, we will present median distances between the PoG
and the ball, as well as interquartile ranges.
Results
The relation between the PoG and the ball could be established
in 74.7% of all video frames. In the remaining frames, the relation
between the ball and the PoG could not be assessed, either because
the ball could not be identified in the video frame or because the
PoG was lost. Unsuccessful calibration of the Mobile Eye led us to
exclude the data from two participants from further analysis.
Finally, 20 trials were excluded from further analysis because
participants did not catch the ball but also did not indicate that it
would be uncatchable (16 trials) or because we were unable to
determine whether participants had touched the ball (4 trials).
Preliminary video analysis suggested that participants almost
exclusively directed their point of gaze to the ball and rarely
directed their gaze to locations elsewhere in the scene (for
representative examples of a trial in which the ball was caught and
of a trial in which a ball was judged to be uncatchable, see Figure
S1 and Movies S1 and S2). The average median distance between
the PoG and the ball was 24.26 pixels (with an average
interquartile range of 22.67 pixels); medians (interquartile ranges)
were 23.10 (20.28) pixels in the trials in which balls were caught
(n = 230), and 25.85 (25.58) pixels in the trials in which balls were
judged to be uncatchable (n = 168). Participants tracked the ball,
on average, in 95.5% of the trial when they caught the ball, and in
92.9% of the trial when they called a ‘no’.
As detailed in the Methods section, video frames in which the
distance between the ball and the PoG was more than 75 pixels
formed the ‘other’ category. Further investigation of this category
(representing 5.7% of all frames with a valid PoG) showed that
3.1% of all ‘other’ gaze behavior constituted meaningful gaze
behavior and could be classified as ‘fixations’ (operationally
defined as stable gaze for three or more consecutive frames). That
is, fixations on items other than the ball accounted for 0.2% of all
displayed gaze behavior.
Figure 1 gives the percentage of frames that were categorized as
‘tracking’ as a function of time. Data points are represented in bins
of 100 ms, combining sets of three consecutive video frames. In
Figure 1A, which shows the trials in which the balls were caught,
the abscissa represents the time until contact with the ball. It can
be seen in Figure 1A that the contributions of tracking behavior to
total gaze behavior remained relatively constant throughout the
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trial. Only the last 100 ms before the catch deviated substantially
from this trend. Figure 1B represents the trials in which
participants judged balls to be uncatchable. In Figure 1B, the
abscissa represents the time until the moment that a ‘no’ was
called. Also for this type of trials, the contribution of tracking
behavior to total gaze behavior remained relatively constant
throughout the trial. A slight deviation from this trend can be seen
on the left side of Figure 1B (i.e., the two bins spanning from
t = 2.2 to t = 2.0). Because these early bins included only few trials,
the percentages of these bins were sensitive to the presence of
single frames with a PoG that was coded as ‘other’. More
particularly, one trial that had a few consecutive frames that were
classified as ‘other’ early on in the trial (more than 2 s before the
participant called ‘no’) was mostly responsible for the apparent
decrease in tracking behavior.
Discussion
In his famous catch in 1954, Willie Mays looked at the ball,
turned his back to the ball while running, and finally looked back
at the ball again. Is this the usual way for fielders running to catch
a fly ball? Do they simply know where to run from a single glance
on the ball’s trajectory (cf. [1,2,10,11]), or do fielders need
continuous monitoring of the ball’s position? The results of the
present study show that participants running to catch an
approaching fly ball continuously keep their eyes on the ball.
Although the use of a predictive strategy would not preclude
continuous tracking of the ball, and the use of a prospective
strategy would not necessitate 100% tracking, the gaze behaviour
of our participants suggests that their running is under continuous
visual control, characteristic for a prospective strategy.
Earlier work by Oudejans et al. [32] showed that their
participants reliably tracked the ball in both a fly-ball watching
and a catching task. In the majority of trials administered by
Oudejans and colleagues, participants were asked to simply
observe fly balls approaching head on. Watching these balls
resulted in pursuit tracking, with both head and eye movement
contributing to keeping the gaze on the ball. In a catching
condition, participants were allowed to move and actually catch
the approaching balls. Because the gaze tracker used by Oudejans
et al. was wired, it restricted the participants’ mobility, such that
they were only able to make a few steps to intercept a fly ball. As a
consequence, balls in their catching condition had to be projected
within a few meters from the participants’ initial position. Also in
the catching condition, participants tracked the ball with their
gaze, although the contributions of head and eye movement to
directing the gaze were different than in the watching conditions
(see also [29]). The present study allowed participants to move as
they would naturally do when catching a fly ball. With the mobile
gaze tracker that we used, we were able to study gaze in situations
comparable to real catching in the outfield.
Participants in the present study tracked the ball with their gaze
nearly exclusively, regardless of the projected distance. Further-
more, they also showed tracking of balls that they realized were
uncatchable. This latter condition is not commonly part of a study
into the control of interception, although it is part and parcel of
the reality of outfielders. The results suggest that the information
for knowing that a ball cannot be caught should not be sought in a
failure to keep tracking a ball. That is to say, our participants
tracked the ball up until the moment that they indicated that the
ball was out of their reach. They had no problems doing so, even
when running at their maximum speed. Clearly, a failure to track
the ball was no indication for the participants that an approaching
fly ball would be uncatchable.
As discussed before, our results demonstrate that participants
tracked the ball throughout its trajectory. We would like to stress
that especially the fact that tracking continued to just before the
actual catch speaks in favour of the use of continuous guidance
rather than early prediction. Both the use of a predictive and of a
prospective strategy would predict gaze pursuit during the early
part of ball flight. However, when using a predictive strategy, in
which the interception location and time are inferred from the first
part of the ball trajectory, there seems to be no advantage of
keeping an eye on the ball for the rest of its flight; continuous
tracking fits more naturally with continuous visual control. Only
during the very final part of the ball’s approach, approximately
during the final 100 ms, did tracking become inconsistent. A
reason for this might be that the participants had actually stopped
tracking the ball with their gaze because it was not needed for
running to the interception location anymore. It has been
suggested that fly ball interception consists of two phases;
locomotion to the interception point and making the actual catch
(e.g., see [23,26]). The last 100 ms might reflect the latter phase.
Alternatively, participants might have started to prepare for a
follow-up action, such as throwing the ball to a teammate.
In conclusion, the present results paint a picture that is
consistent with the use of a prospective strategy in dealing with
the outfielder problem. Gaze data are not able to show
indisputably that fielders do not use a predictive strategy, in
Figure 1. Percentage of tracking as a function of time. The
number of frames in which participants tracked the ball expressed as a
percentage of the total number of frames with valid data in a trial. A)
Average percentages for trials in which the ball was caught; t = 0
represents the time of contact with the ball; B) Average percentages for
trials in which the ball was judged to be uncatchable; t = 0 represents
the time that a ‘no’ was called.
doi:10.1371/journal.pone.0092392.g001
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which they know where to run from looking at the early
part of ball flight. However, the finding that participants
continuously kept their eye on the ball, while running several
meters to catch a ball that might or might not be catchable, fits
naturally with a continuous visual control on the basis of
prospective information.
Supporting Information
Figure S1
Gaze in two representative trials. Distance
between the ball image and the point of gaze as a function of time.
A) Gaze for a participant who successfully caught the projected fly
ball (after 2.64 s); B) Gaze for a participant who indicated that the
projected fly ball was uncatchable for her (after 1.37 s). See also
Movies S1 and S2, which show scene camera recordings of these
trials.
(PDF)
Movie S1
Gaze in a trial in which the ball was caught.
Eye-tracker recording of a trial in which the participant caught the
ball. The point of gaze is indicated by the red crosshair that
overlays the scene-camera images. Figure S1A gives the distance
between the ball and the point of gaze as a function of time for this
trial.
(AVI)
Movie S2
Gaze in a trial in which the ball was judged to
be uncatchable. Eye-tracker recording of a trial in which the
participant judged the ball to be uncatchable. The point of gaze is
indicated by the red crosshair that overlays the scene-camera
images. Figure S1B gives the distance between the ball and the
point of gaze as a function of time for this trial.
(AVI)
Acknowledgments
We thank Anna Kersten and Pawel van der Steen for their assistance in
collecting the data, and Emyl Smid for his help with the ASL Mobile Eye
tracker.
Author Contributions
Conceived and designed the experiments: DP FZ. Performed the
experiments: DP FZ. Analyzed the data: DP RdO FZ. Contributed
reagents/materials/analysis tools: DP RdO FZ. Wrote the paper: DP RdO
FZ.
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Keeping Your Eyes Continuously on the Ball
PLOS ONE | www.plosone.org
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March 2014 | Volume 9 | Issue 3 | e92392

Keeping your eyes continuously on the ball while running for catchable and uncatchable fly balls.
Postma, Dees B W,den Otter, A Rob,Zaal, Frank T J M