Quiet Eye Duration, Expertise, and Task

Complexity in Near and Far Aiming Tasks

A. Mark Williams
Research Institute for Sport
and Exercise Sciences
Liverpool John Moores University
Liverpool, England
ABSTRACT. Skilled (n = 12) and less skilled (n = 12) billiards
players participated in 2 experiments in which the relationship
between quiet eye duration, expertise, and task complexity was
examined in a near and a far aiming task. Quiet eye was defined as
the final fixation on the target prior to the initiation of movement.
In Experiment 1, skilled performers exhibited longer fixations on
the target (quiet eye) during the preparation phase of the action
than their less skilled counterparts did. Quiet eye duration
increased as a function of shot difficulty and was proportionally
longer on successful than on unsuccessful shots for both groups of
participants. In Experiment 2, participants executed shots under 3
different time-constrained conditions in which quiet eye periods
were experimentally manipulated. Shorter quiet eye periods result-
ed in poorer performance, irrespective of participant skill level.
The authors argue that quiet eye duration represents a critical peri-
od for movement programming in the aiming response.
Key words: action, billiards, perception, visual search
ecause of its highly complex and often unpredictable
nature, the sporting arena provides an excellent context
in which to test theoretical assumptions related to movement
control (Williams, Davids, & Williams, 1999). In sports such
as billiards, darts, golf, and pistol shooting, the ability to pro-
gram precise aiming movements appears crucial (Vickers,
1996). In those sports, the processing of critical visual infor-
mation and the ability to self-regulate cognitive and emotion-
al activity are keys to successful execution of self-paced
movement skills. More specifically, many sports involve an
aiming component to some degree. The majority of those
require far aiming skills in which an object is directed toward
a distant target located in extrapersonal space (e.g., archery,
basketball free throw, volleyball serve), but several sports
combine both near and far aiming requirements (e.g., bil-
liards, putting in golf). In motor behavior investigations,
researchers have focused primarily on near aiming tasks, that
is, tasks in which the hand or an object controlled by the hand
is moved to a target located within intrapersonal space (e.g.,
Abrams, 1994; Gauthier, Semmlow, Vercher, Pedrono, &
Obrecht, 1991; Prablanc & Pellison, 1990; Zelaznik,
Hawkins, & Kisselburgh, 1983). Only limited research has
been conducted on how vision and action interact during far
aiming tasks, and even less on the requirements of tasks that
combine near and far aiming skills.
In the present research, the visual gaze behaviors of
skilled and less skilled players were recorded as they per-
formed successful and unsuccessful billiards shots of vary-
ing complexity levels. The billiards shot is a complex tar-
geting skill that involves both near and far aiming
components. To execute shots successfully, players must
accurately align the billiards cue with the cue ball (near
aiming task) and then propel that ball toward a target ball
and ultimately the pocket (far aiming tasks). To execute the
shot, players must successfully integrate visual information
from the cue, cue ball, target ball, and pocket.
Few researchers have examined visual search and motor
behavior during aiming tasks performed in a sport setting.
However, Vickers (1996) examined the gaze behavior of
national-level basketball players during a far aiming task, the
basketball free throw. The less expert free-throw shooters
shifted their gaze early in the movement and fixated on the
target during execution. Experts took significantly longer to
prepare the free throw, made fewer fixations than near-
experts during the preparation and impulse phases of the
shot, and generated a greater frequency of fixations during
the execution phase. More important, the duration of final
fixation before initiation of the movement was significantly
Correspondence address: A. Mark Williams, Research Institute
for Sport and Exercise Sciences, Liverpool John Moores Universi-
ty, The Henry Cotton Campus, 15-21 Webster Street, Liverpool, L3
2ET, UK. E-mail address: m.williams@livjm.ac.uk
197
Journal of Motor Behavior, 2002, Vol. 34, No. 2, 197207
B
Robert N. Singer
Department of Exercise
and Sport Sciences
University of Florida
Gainesville
Shane G. Frehlich
Department of
Kinesiology
California State University
Northridge
longer in expert shooters. Finally, experts suppressed vision
to a greater extent during the execution phase than near-
experts. Experts either blinked or diverted their gaze to areas
other than the hoop, the ball, or their hands during the shoot-
ing action.
As a consequence of those results, Vickers (1996) pro-
posed a location-suppression hypothesis for aimed limb
movements to distant targets. In the location aspect of her
hypothesis, she suggested that fixations of relatively long
duration must be made to specific target locations during
the preparatory phase of the movement. That period of time
is considered to be essential for programming parameters of
the movement. The direction, force, and velocity of the
movement are programmed, as are the timing and coordina-
tion of limbs necessary to produce optimal movement
(Newell, Hoshizaki, Carlton, & Halbert, 1979; Spijkers,
1989; Spijkers & Steyvers, 1984). As the movement is ini-
tiated (the impulse phase), players need relatively slow
movements to maintain fixation on the target and complete
the final structuring of the aiming commands.
During the execution phase, Vickers (1996) suggested,
players use a suppression mechanism to block out interfer-
ing visual information arising as a consequence of their
movement (e.g., hands and ball appearing in front of the eye
during a basketball free throw, thus occluding the target).
Expert performers, she contended, have developed the abil-
ity to divert their visual attention from the target during exe-
cution by blinking or orienting their gaze to other elements
of the visual field. Because the parameters of movement
have already been planned in the preparation phase, and
modified in the impulse phase, she deemed that visual
attention is unnecessary in the execution phase for success-
ful completion of the task. Some degree of support for Vick-
erss suppression hypothesis can be inferred from the results
of experiments demonstrating that intermittent occlusion of
vision does not disrupt performance during walking (Assai-
ante, Marchand, & Amblard, 1989) and ball-catching tasks
(Elliott, Zuberec, & Milgram, 1994).
Two critical components related to performance were
outlined in Vickerss (1996) location-suppression hypothe-
sis. The first is associated with the duration of the final fix-
ation on the target during the preparatory phase of the
movement. Vickers termed that duration the quiet eye peri-
od and defined it as the final fixation on the target before the
initiation of movement. Longer quiet periods are assumed
to be associated with better performance in aiming to a far
target. In that time period, the performer presumably sets
the final parameters of the movement to be executed. The
key principle is that quiet eye duration is associated with the
amount of cognitive programming required for successful
aiming to a target. Second, Vickers (1996) argued that bet-
ter performance is characterized by suppression of vision
during the execution phase of the movement. Poorer perfor-
mance is presumed to occur when players maintain fixation
on the target; however, expert shooters tend to terminate
their fixation on the target during the execution phase.
In the sports context, support for Vickerss (1996) loca-
tion hypothesis has been obtained in basketball, pistol
shooting, and golf, with expert athletes displaying longer
fixations on the target than their less expert counterparts
(Ripoll, Bard, & Paillard, 1986; Ripoll, Papin, Guezennec,
Verdy, & Philip, 1985; Vickers, 1992). The visual-suppres-
sion component of Vickerss (1996) hypothesis is more con-
tentious. Results of previous research, particularly from
studies of near aiming tasks, suggest that the target is fixat-
ed throughout the movement (e.g., Abrams, Meyer, & Korn-
blum, 1990; Guitton & Volle, 1987; Prablanc & Pellison,
1990). Moreover, vision appears to be used continuously
during interceptive tasks such as catching or striking balls
(Bootsma, Fayt, Zaal, & Laurent, 1997; Bootsma & van
Wieringen, 1990; Tresilian, 1997).
In the present research, we examined the validity of Vick-
erss (1996) hypotheses by using a billiards task that
involved near and far aiming components. In Experiment 1,
the relationships between the final fixation duration on the
target before initiation of movement (quiet eye period), shot
complexity, and performance in skilled and less skilled bil-
liard players were assessed. According to Vickerss
hypotheses for far aiming tasks, skilled players should gen-
erate longer quiet eye periods during the preparation phase,
whereas the duration of this final fixation should increase as
a function of task complexity for both groups of partici-
pants. Because more complex motor responses require
longer preprogramming times (e.g., Henry, 1980; Henry &
Rogers, 1960; Kerr, 1978; Klapp, 1977, 1980), more com-
plex aiming behaviors should be characterized by longer
quiet eye durations if the final fixation on the target is relat-
ed to cognitive programming. We also expected visual sup-
pression during the execution phase of the movement, as
indicated by an increase in eye blinks and in shifts of gaze
to less pertinent areas of the visual environment.
In Experiment 2, we examined further the validity of
Vickerss (1996) quiet eye period as a measure of cognitive
programming by manipulating temporal aspects of billiards
shots. We expected performance and mean quiet eye dura-
tion to decrease as the time apportioned for initiating the
billiards stroke was reduced. Shapiro (1977) and Summers
(1977) have demonstrated that performance decrements
occur in hand aiming movements when participants are
required to execute the movement under time constraint.
Those researchers and others (e.g., Vickers, 1996) have
argued that reductions in the time allotted for shot prepara-
tion result in a concomitant decrease in the time available
for response programming, thereby negatively affecting
performance.
EXPERIMENT 1
Method
Participants
Twenty-four right-handed men were tested. They were
categorized as either skilled or less skilled billiards players
A. M. Williams, R. N. Singer, & S. G. Frehlich
198 Journal of Motor Behavior
on the basis of their experience level and an initial perfor-
mance test. The skilled group consisted of 12 players (M =
23.17 years of age, SD = 3.01 years) who had an average of
9.1 years of experience (SD = 2.64 years) and played 3.33
days per week (SD = 0.98 days). The group had competed
in an average of 14.25 (SD = 14.97) sanctioned tourna-
ments. Each member of the group successfully completed
the performance test in fewer than 12 shots (M = 10.67
shots, SD = 2.46 shots). The less skilled group also consist-
ed of 12 players (M = 21.83 years of age, SD = 1.85 years),
averaging 2.67 years of playing experience (SD = 0.65
years). Those participants had no competitive experience,
played only 1.25 days per week (SD = 0.45 days), and
required an average of 26.67 shots (SD = 3.17 shots) to
complete the initial skills test.
Apparatus
Eye movement measurement. We collected eye move-
ment data by using a mobile corneal reflection unit
(Applied Science Laboratories; Waltham, MA, Model ASL
4000 SU). That video-based monocular system measures
eye line of gaze with respect to a head-mounted camera by
computing the relative positions of two features of the eye,
the pupil and the corneal reflex (a reflection of a near-
infrared light source from the surface of the cornea), in rela-
tion to the optics. Both the infrared beam and the image of
the participants eye were reflected from a visor mounted on
the helmet, which was coated to be reflective in the near-
infrared region and transparent to visible light. The ASL
system computes the line of gaze by measuring the vertical
and horizontal distances between the center of the pupil and
the corneal reflection after correcting for second-order
effects.
The resulting displacement data were recorded and
processed by an external Gateway 2000 486SX/166 com-
puter connected via a 30-m cable to the participants waist.
To record the field of view as observed by the participant,
we positioned an Elmo MP481 color scene camera near the
eye. That positioning allowed us to obtain a view of the
scene from the same position observed by the participant
while avoiding the problems of parallax error and the
mounting of stationary cameras in the field of view.
To assess the exact location of gaze, the 4000 SU proces-
sor superimposed a white cursor representing 1 of visual
angle on the video image produced by the scene camera.
Those images were recorded by a video recorder (Akai
Electric Co., Tokyo, Japan, Model VS-X9EGN S-VHS) and
used for data analysis. The 4000 SU possesses an accuracy
of 1 in both the vertical and horizontal directions and a
precision of better than 0.5. The system sampled at a rate
of 60 Hz, and point of gaze was updated for each frame of
video (every 33.3 ms).
Vision-in-action approach. Participants gaze behaviors
were recorded simultaneously with the action phases of the
billiard stroke. Using a Panasonic WJ-MX10 digital pro-
duction mixer (Matsuchita Electric Corp., Secaucus, NJ),
we obtained simultaneous records by interfacing data from
the 4000 SU system with the image recorded by an external
Panasonic WV-PS03/B S-VHS video camera (Matsuchita
Electric Corp., Secaucus, NJ), which was positioned per-
pendicular to the player. The mixer created a split-screen
effect in which the lower-left portion of the frame showed
the participant performing the billiard stroke while the right
portion of the frame displayed his gaze position as record-
ed by the eye and the scene cameras. Both the gaze loca-
tions and the temporal aspects of each phase of the stroke
were analyzed frame by frame.
Billiards arrangements. A Brunswick 4.5- 9.0-ft (1.37-
2.74-m) billiards table, which had a playing surface area of
100 50 in. (254 127 cm), was used for all the testing.
The corner pockets were 5 in. (12.7 cm) wide, and the side
pockets were slightly wider (5.5 in., 13.97 cm). Standard
modern composition balls measuring 2.25 in. (5.72 cm) in
diameter and weighing 6 oz (170 g) were used. Players
were required to use a standard 57-in. (144.78 cm) billiards
cue, weighing 18 oz (510 g). We controlled the arrangement
of the balls for the initial skill test, as well as for each of the
three tasks of varying complexity levels, by placing small
colored markers on the table surface.
Procedure
Initial performance test for skill classification. After
obtaining informed consent, we gave participants the bil-
liards cue and showed them to the table where we had
placed an arrangement of balls similar to those that might
occur in a typical game of nine-ball. Participants were asked
to pocket the balls (in order from 1 to 9) in as few shots as
possible. On the basis of an initial pilot study, we arranged
the balls in such a fashion that high caliber players were
expected to complete the test in less than 12 shots and the
less skilled participants in 20 or more shots.
Performance trials. Once the initial test was completed,
participants were fitted with the eye-tracking system, and
initial calibration procedures were performed. The calibra-
tion procedure consisted of having the player fixate on a
sequence of nine equidistant points located on a target
board placed on top of the billiards table. Upon completion
of the calibration procedure, a series of shots of three levels
of complexity (easy, intermediate, and hard) were per-
formed. Each player was instructed on the correct type of
speed and english (spin) to place on the cue ball, and three
practice trials for each level of complexity were allowed.
They then performed consecutive shots until 10 successful
and 10 unsuccessful outcomes per complexity condition
were achieved. Shots were considered unsuccessful if the
object ball was not sunk or if the cue ball was inadvertent-
ly pocketed (scratch shot).
After 10 successful and 10 unsuccessful shots were
recorded at a particular complexity level, a 3-min rest peri-
od was given, followed by a recalibration of the eye-track-
ing system. The players then progressed to the next series of
shots of a different complexity level. The sequence of task
Visual Search in Near and Far Aiming Tasks
June 2002, Vol. 34, No. 2 199
complexity was randomly assigned and counterbalanced
across participants. The total time to complete the task for
each participant depended on the number of shots that
enabled the experimenters to record 10 hits and 10 misses.
Typically, participants completed the experiment in 45 min
or less.
Shots of varying complexity. For the performance task, par-
ticipants were required to pocket a series of balls with shots
of three levels of complexity. In the easy condition (EC), the
object ball lay near the corner pocket, and the player could
make the shot by using a cut shot with bottom-left english
(Martin & Reeves, 1993). The intermediate condition (IC)
featured a cushion first approach to sinking the object ball
(Martin & Reeves); that is, the object ball lay near the cush-
ion while the cue ball sat on the opposite side of the table.
The object ball was partially occluded by another ball so that
the player had no way of hitting the object ball directly. To
sink the object ball, the player needed to strike the cue ball
with straight-follow english and hit the cushion before hitting
the object ball. Finally, the hard condition (HC) featured a
carom draw shot, in which the cue ball had to be struck
sharply with straight-bottom english so that it would carom
off the object ball into another ball located near the corner
pocket (Martin & Reeves). The difficulty of the shot lay in the
especially precise angle of the carom and the fact that a num-
ber of other balls were located on the table, limiting the
options available to the player.
Dependent Measures
Performance. The performance measure consisted of the
number of shots required to achieve the criterion level of 10
successful and 10 unsuccessful shots for each of the condi-
tions. Specifically, the ratio of successful shots to unsuc-
cessful shots was calculated as a percentage, with higher
values indicating better performance.
Visual behaviors. Five successful and five unsuccessful
shots were randomly selected for analysis at each complex-
ity level. We performed a frame-by-frame analysis of each
shot by using an Akai VS-X9EGN S-VHS video recorder. A
fixation was defined as three or more consecutive frames
(99.9 ms or more) in which the cursor was located in the
same space in the visual environment (Ripoll, 1991; Vick-
ers, 1992, 1996; Williams & Davids, 1998). The dependent
measures included number of fixations per location, mean
fixation duration per location, mean quiet eye duration, and
number of blinks present for distinct phases. Quiet eye
duration was defined as the final duration of fixation on a
target before the onset of movement time (cf. Vickers,
1996). The values recorded were averaged across all trials.
On trials where there were no fixations on a particular loca-
tion, we included a value of zero when calculating mean
values across trials for each of the dependent measures.
For fixation location, we examined the number of fixa-
tions allocated to four areas of interest: cue ball, object ball,
target cushion (or second object ball in the hard condition
only), and the intended pocket. Any remaining fixations
(e.g., on the cue stick or the felt surface) were defined in a
category labeled other areas. For the dependent measure of
mean fixation duration, the time spent (in milliseconds) fix-
ating each location across trials was calculated. The quiet
eye duration in the preparatory phase of the stroke was
recorded for each participant across the three complexity
conditions. The number of eye blinks, recorded by the eye
camera and the system software, was assessed during the
backswing, foreswing, and flight phases of the stroke.
Temporal components of the billiards stroke. Four dis-
tinct phases were identified from the external camera image
that provided a sagittal view of the performer: the prepara-
tory phase, that is, the time from the start of the task until
the first observable movement toward the cue ball (i.e., the
duration in which the billiards player leaned over the table
to begin the shot until the moment he initiated the final
backswing motion of the cue); the backswing phase, that is,
the time from the final backswing movement until the cue
began to move forward toward the cue ball; the foreswing
phase, that is, the time from foreswing initiation until the tip
of the cue came into contact with the cue ball; and the flight
phase, which was defined as the time the cue ball was in
motion before striking the object ball.
Total time (in milliseconds) spent in each phase of the
stroke was calculated for each participant across the varying
complexity conditions. We accomplished those calculations
by analyzing the total number of video frames (each frame
equated to 33.3 ms) corresponding to each phase of the
stroke.
We analyzed all data by using various factorial analyses of
variance (ANOVAs), with the alpha level set at < .05. Simple
main effects and Scheffs post hoc tests were used as follow-
up analyses, as appropriate. For each repeated measures vari-
able, violations of the assumptions of sphericity were
assessed, and, where appropriate, we calculated Green-
houseGeisser adjustments to the level of significance.
Results
Performance
A 2 (skill) 3 (complexity) ANOVA with repeated mea-
sures on the second variable was undertaken so that we
could assess differences in performance between skilled
and less skilled participants. Significant main effects for
skill, F(1, 22) = 75.99, p < .001, and complexity, F(2, 44) =
116.28, p < .001, were observed. Post hoc analyses revealed
that skilled players made significantly more shots (M =
67.53) than less skilled players (M = 47.88%). There were
significant reductions in performance for both groups as the
level of complexity increased, from M = 74.13% in the EC
to M = 65.01% in the IC and M = 47.88% in the HC. The
skilled players had a higher ratio of successful to unsuc-
cessful shots at all levels of complexity than the less skilled
players. The manipulations of shot complexity affected
members of each group in a proportionate manner. The
interaction was not significant. The results are presented in
Table 1.
A. M. Williams, R. N. Singer, & S. G. Frehlich
200 Journal of Motor Behavior
Temporal Components of the Billiards Stroke
To determine whether manipulations of task complexity
led to specific changes in duration for the four phases of the
billiards stroke, we conducted a 2 (skill) 3 (complexity)
4 (phase) 2 (accuracy) 5 (trial) ANOVA with repeated
measures on the last four variables. Significant main effects
were found for accuracy, F(1, 22) = 4.78, p < .05, and com-
plexity, F(2, 44) = 78.17, p < .001. In both groups, accurate
shots were associated with longer durations (M = 3,093.06
ms) than were missed shots (M = 2,885.69 ms). Overall shot
duration increased significantly as complexity level increased
(EC, M = 2,489.38 ms; IC, M = 2,656.67 ms; HC, M =
3,822.08 ms). In addition, a significant main effect was
found for phase, F(3, 66) = 12.73, p < .01. Both groups of
participants spent significantly more time in preparing the
stroke (M= 1,731.46 ms) than in any other phase. The means
for the other phases included backswing (M = 491.84 ms),
foreswing (M = 125.32 ms), and flight (M = 597.87 ms).
Significant Phase Complexity, F(6, 132) = 4.32, p <
.03, and Skill Complexity, F(2, 44) = 5.33, p < .05, inter-
actions were observed. Follow-up tests indicated that there
were no significant differences in the duration of the back-
swing, foreswing, and flight phases across each level of
complexity. The duration of the preparation phase was sig-
nificantly longer in the HC (M = 2,532.16 ms) than in the
EC or the IC, however (Ms = 1,240.72 and 1,421.52 ms,
respectively). Both groups of participants required signifi-
cantly more time to prepare their shots in the most complex
condition than in the other two levels of complexity. The
total movement duration was significantly longer in the HC
than in the EC or IC for both groups, whereas stroke dura-
tions in the EC and the IC did not differ significantly from
each other. The skilled players took longer than their less
skilled counterparts to complete the stroke under the HC.
The data are presented in Table 2.
Visual Behaviors
To assess whether manipulations of task complexity were
associated with concomitant changes in visual behavior, we
analyzed the number of fixations and mean fixation dura-
tion per location, using separate 2 (skill) 5 (location) 2
(accuracy) 5 (trial) ANOVAs with repeated measures on
the last three variables. A separate ANOVA was undertaken
for each level of complexity.
Easy Complexity Level
Significant main effects of location, F(2, 44) = 579.48, p <
.001, and skill, F(1, 22) = 54.35, p < .01, were observed for
number of fixations per location. Specifically, more fixations
per trial on average were directed to the cue ball (M = 3.00)
and the object ball (M= 2.83) than to the cushion (M= 0.00),
pocket (M= 0.00) or to other areas of the display (M= 0.50).
Less skilled players typically made more fixations (M= 1.48)
to each location than skilled players (M= 1.06). A significant
interaction between skill and location was observed for the
number of fixations per location, F(2, 44) = 20.44, p < .01.
Less skilled participants made significantly more fixations
per trial to the cue ball (M = 3.54) and the object ball (M =
3.33) than the skilled participants (M= 2.46 and M= 2.33 fix-
ations, respectively).
A similar pattern of results was observed for mean fixation
duration per location. A significant main effect for location,
F(2, 44) = 204.87, p < .001, indicated that the longest fixa-
tions were directed to the cue ball (M = 396.02 ms) and the
object ball (M = 418.26 ms) than to any other locations. The
main effect for skill was significant, F(1, 22) = 7.57, p < .05.
Skilled performers made longer mean duration fixations per
location (M = 197.47 ms) than less skilled players (M =
161.24 ms). A significant interaction between skill and loca-
tion was found, F(2, 44) = 6.41, p < .05. Post hoc tests
revealed that skilled performers fixated on the target ball for
a longer time period (M = 498.96 ms) than less skilled play-
ers (M = 337.57 ms). The results are summarized in Table 3.
Intermediate Complexity Level
Significant differences in number of fixations were
observed for location, F(4, 88) = 53.01, p < .001, and skill,
F(1, 22) = 22.56, p < .01. Follow-up tests revealed that less
skilled players used a greater number of fixations per location
(M = 1.49) than their more skilled counterparts (M = 1.18).
Visual Search in Near and Far Aiming Tasks
June 2002, Vol. 34, No. 2 201
TABLE 1
Performance Means and Standard Deviations
(in %) for Skilled and Less Skilled Groups
for Each Level of Shot Complexity
Skilled Less skilled
Complexity M SD M SD
EC 83.30 6.74 64.97 11.15
IC 74.84 6.17 55.19 13.19
HC 44.47 7.52 23.47 10.52
Note. EC = easy condition, IC = intermediate condition, and HC =
hard condition.
TABLE 2
Total Duration (in ms) of the Billiards Stroke
for Skilled and Less Skilled Groups
for Each Level of Shot Complexity
Skilled Less skilled
Complexity M SD M SD
EC 2,322.50 435.60 2,656.25 795.10
IC 2,587.50 511.30 2,725.83 569.90
HC 4,021.25 568.20 3,622.92 632.70
Note. EC = easy condition, IC = intermediate condition, and HC =
hard condition.
Moreover, both groups of participants directed more fixations
to the cue ball (M= 2.65), cushion (M= 2.08), and object ball
(M = 1.33) than to the pocket (M = 0.25) or to other areas of
the display (M = 0.35). In addition to those main effects, a
significant Skill Location interaction was found, F(4, 88) =
5.18, p < .02. The less skilled players directed significantly
more fixations to the cue ball (M = 3.00) than the skilled
players. The number of fixations to other locations was sim-
ilar for both groups.
An analysis of mean fixation duration per location showed
significant differences for location, F(4, 88) = 156.77, p <
.001. Both groups fixated longer on the cushion (M= 445.66
ms), the cue ball (M = 406.32 ms), and the object ball (M =
364.06 ms) than on the pocket (M = 51.56 ms) or on other
areas (M= 53.54 ms). No other main effect or interaction was
significant. The results are presented in Table 4.
Hard Complexity Level
An analysis of the number of fixations and mean fixation
duration per location for HC yielded results similar to those
of the other complexity conditions. The three- and four-way
interactions were not significant for either dependent vari-
able. For number of fixations, significant main effects were
demonstrated for location, F(4, 88) = 377.41, p < .001, and
skill, F(1, 22) = 17.09, p < .01. Less skilled performers
made more fixations per location (M= 1.78) than their more
skilled counterparts (M= 1.50). The greatest number of fix-
ations were directed toward the cue ball (M = 3.60), fol-
lowed by the object ball (M = 2.71), and the second target
ball (M = 1.04). Only a minimal number of fixations were
allocated to the pocket (M = 0.44) or to other areas (M =
0.40). Post hoc analysis indicated that each of those means
was significantly different from each other, except for the
comparison between the pocket and other areas. A signifi-
cant Skill Location interaction, F(4, 88) = 8.26, p < .03,
revealed that the less skilled participants generated more
fixations to the cue ball (M = 4.04) and the object ball (M =
3.00) than the skilled players (Ms = 3.17 and 2.42 fixations,
respectively).
An analysis of mean fixation duration per location
showed significant main effects for location, F(4, 88) =
113.60, p < .001, and skill, F(1, 22) = 14.87, p < .01. The
more advanced players used longer fixations (M = 347.56
ms) than the less skilled participants (M = 274.99 ms), and
the longest fixation for both groups was directed at the cue
ball (M = 582.21 ms), followed by the object ball (M =
447.38 ms), and the second target ball (M = 365.10 ms).
Shorter mean fixation durations were found for the pocket
(M = 89.17 ms) and the other areas (M = 72.50 ms). Post
hoc analyses revealed that each of the means was signifi-
cantly different from one another, save for the pocket and
other areas comparison. Finally, the Location Skill inter-
action was significant, F(4, 88) = 3.57, p < .05. The skilled
players had longer mean fixations to the cue ball (M =
678.47 ms) and the object ball (M = 504.93 ms) than the
less skilled players (Ms = 485.95 and 389.83 ms). Those
results are displayed in Table 5.
Quiet Eye Duration and Number of Eye Blinks
We assessed quiet eye duration (in milliseconds) and sup-
pression of gaze (defined as the number of blinks) by using
separate 2 (skill) 3 (complexity) 2 (accuracy) 5 (trial)
ANOVAs with repeated measures on the last three vari-
ables. For the quiet eye dependent variable, significant main
A. M. Williams, R. N. Singer, & S. G. Frehlich
202 Journal of Motor Behavior
TABLE 3
Mean Number of Fixations and Mean Fixation
Duration (in ms) per Trial for Skilled and Less
Skilled Groups in the Easy Condition
Skilled Less skilled
No. No.
Location fixations Duration fixations Duration
Cue ball 2.46 418.82 3.54 373.23
Object ball 2.33 498.96 3.33 337.57
Cushion 0.00 0.00 0.00 0.00
Pocket 0.00 0.00 0.00 0.00
Other 0.50 69.58 0.50 95.42
TABLE 4
Mean Number of Fixations and Mean Fixation
Duration (in ms) per Trial for Skilled and Less
Skilled Groups in the Intermediate Condition
Skilled Less skilled
No. No.
Location fixations Duration fixations Duration
Cue ball 2.29 428.47 3.00 384.17
Object ball 1.01 397.08 1.67 331.04
Cushion 2.04 470.14 2.13 421.18
Pocket 0.21 38.33 0.29 64.58
Other 0.33 46.67 0.38 60.42
TABLE 5
Mean Number of Fixations and Mean Fixation
Duration (in ms) per Trial for Skilled and Less
Skilled Groups in the Hard Condition
Skilled Less skilled
No. No.
Location fixations Duration fixations Duration
Cue ball 3.17 678.47 4.04 485.95
Object ball 2.42 504.93 3.00 389.83
Second ball 1.04 376.88 1.04 353.33
Pocket 0.46 102.92 0.42 75.42
Other 0.42 74.58 0.38 70.42
effects were found for skill, F(1, 22) = 68.41, p < .001;
complexity, F(2, 44) = 76.25, p < .001; and accuracy F(1,
22) = 165.19, p < .001. Successful shots were characterized
by longer quiet eye durations (M = 561.94 ms) than were
unsuccessful shots (M = 213.61 ms). Skilled players had
significantly longer quiet eye measures (M = 499.86 ms)
than less skilled players (M = 275.69 ms), and quiet eye
duration increased linearly with increases in task complex-
ity (Ms = 229.79 ms in the EC, 314.17 ms in the IC, and
619.38 ms in the HC). A significant three-way Accuracy
Complexity Skill interaction was observed, F(2, 44) =
5.05, p < .05. The mean values for the respective measures
are displayed in Figure 1.
There were no significant main effects or interactions for
the eye blink variable. Very few eye blinks were observed
throughout the experiment; the skilled players averaged
0.17 and the less skilled group 0.24 blinks per shot. It
appears that blink rate was not related to skill level, perfor-
mance accuracy, or level of shot complexity.
EXPERIMENT 2
Method
Participants
Participants were the same as in Experiment 1.
Apparatus
All apparatus, including the eye-tracking unit, video
recording and mixing devices, and billiards table, cue, and
balls, were identical to those described in Experiment 1.
However, only the arrangements of balls in the intermediate
complexity condition (IC) were included in Experiment 2.
We used the countdown timing mechanism feature of the
ReAction Coach (S.T.A.R.T. Technologies, New York)
movement timing system to direct the participants as to the
time they were allotted for the task. The ReAction Coach pre-
sents auditory and visual cues so that quicker and more
explosive motor abilities can ostensibly be evoked (see
Singer, Cauraugh, Chen, Steinberg, & Frehlich, 1993). Audi-
tory cues are provided as a series of beeps, and those cues can
be adjusted in volume and sensitivity. The countdown mode
of the ReAction Coach was used in Experiment 2: Three pre-
liminary auditory beeps were presented (priming cues indi-
cating that the trial was imminent), followed by a tone of
longer duration (indicating the initiation of the trial). Once
that tone sounded, the internal timer in the device was acti-
vated, and it was terminated when a loud sound was generat-
ed by the participant (i.e., the striking of the cue ball with the
billiard cue). We recorded and used durations (in millisec-
onds) between onset of the final tone and termination of the
timer to provide feedback to participants.
Procedure
Participants were required to perform under two different
conditions presented in a randomly assigned and counterbal-
anced order. In the 25% constrained condition, the shooter
was required to initiate his stroke within 75% of the average
time spent in the initiation of the stroke as recorded in an
unconstrained condition. The unconstrained-condition times
were based on data obtained from Experiment 1, where the
participant was allowed to take as much time as required to
perform the IC task. For example, if a particular participant
took an average of 4 s to execute the entire shot in the uncon-
strained condition, he was required to perform the shot in 3 s
in the 25% constrained condition. In the 50% constrained
condition, the player had to execute the shot within a duration
of 50% of the average time he had used to initiate the shot in
the unconstrained condition. To use the same example, a
player averaging 4 s per shot in the initial testing session had
to initiate the shot in 2 s in the 50% constrained condition.
Three practice trials were given in each condition.
To aid participants in determining when they would be
allowed to initiate a stroke, we used the countdown timing
mechanism feature of the ReAction Coach movement timing
system. Players stood in a ready position over the shot and
closed their eyes, and were informed that an experimental
trial would begin upon their hearing the initial warning tones
produced by the ReAction Coach. At that time, the timer was
activated and the participants were allowed to open their eyes
and begin preparatory motions to execute the shot. The
experimenter, through observation and use of the video
images, ensured that the participants did not open their eyes
before the signal. On the basis of the times recorded by the
ReAction Coach, the experimenter provided feedback to the
participants after each trial as to whether they were initiating
the stroke within the allotted time frame. Less than 2% of tri-
als had to be repeated because participants had failed to do
so. The players performed consecutive shots until they
reached 10 successful and 10 unsuccessful outcomes in each
condition. Trials initiated after the constrained period were
not included in the analyses.
Dependent Measures and Statistical Analyses
The dependent measures and statistical analyses were
identical to those described in Experiment 1.
Visual Search in Near and Far Aiming Tasks
June 2002, Vol. 34, No. 2 203
FIGURE 1. Mean quiet eye duration (in ms) for successful
and unsuccessful shots for skilled and less skilled groups
across shot complexity. EC, IC, and HC = easy, intermedi-
ate, and hard conditions, respectively.
Results
Performance
A 2 (skill) 3 (shot duration) ANOVA with repeated mea-
sures on the last variable indicated significant main effects
for skill, F(1, 22) = 32.32, p < .001, and duration, F(2, 44) =
6.83, p < .003. Post hoc analyses revealed that skilled players
made significantly more successful shots (M= 67.39%) than
less skilled players (M = 52.07%). There were significant
reductions in performance for both groups as the time avail-
able to execute the shot decreased, from M = 65.01% for the
100% (unconstrained) condition, to M= 61.11% for the 25%
constrained condition, and to M = 53.06% in the 50% con-
strained condition. The interaction was not significant. The
skilled players sank more shots under each condition than the
less skilled players. Also, the manipulations in duration
affected members of both groups in a proportionate manner.
Overall performance for each group is presented in Table 6.
Phase Duration
To determine whether constraining the time allowed to
complete the billiard stroke led to specific changes in dura-
tion for each of the four phases of the stroke, we conducted a
2 (skill) 3 (duration) 4 (phase) 2 (accuracy) 5 (trial)
ANOVA with repeated measures on the last four variables.
Significant main effects were found for phase, F(3, 66) =
9.90, p < .01, and duration, F(2, 44) = 7.29, p < .02. Both
groups spent significantly more time in the preparation phase
of the stroke (M = 742.51 ms) than in any of the other phas-
es. Means for the other phases included backswing, M =
397.48 ms; foreswing, M= 118.53 ms; and flight, M= 537.66
ms. The 100% unconstrained condition led to a longer mean
phase time (M = 664.17 ms) than the 25% (M = 398.02 ms)
or the 50% constrained conditions (M= 284.95 ms). No other
main effects were significant.
A significant Phase Duration interaction was observed,
F(6, 132) = 4.68, p < .05. Follow-up tests indicated that the
backswing, foreswing, and flight phases for each level of
constraint did not differ across the constrained and uncon-
strained conditions. However, the duration of the prepara-
tion phase was significantly greater (M = 1,476.19 ms) in
the 100% unconstrained condition than in the 25% or the
50% constrained conditions (Ms = 548.00 ms and 203.33
ms, respectively). No other interactions were significant. It
appears that the reduction in overall duration of the billiards
stroke came at the expense of only the preparation phase for
both groups. The backswing, foreswing, and flight phases
all remained relatively similar for each condition.
Quiet Eye Duration
We assessed quiet eye duration by using separate 2 (skill)
3 (duration) 2 (accuracy) 5 (trial) ANOVA with repeat-
ed measures on the last three variables. Significant main
effects were found for skill, F(1, 22) = 72.21, p < .001, dura-
tion, F(2, 44) = 61.50, p < .001, and accuracy F(1, 22) =
208.55, p < .001. Successful shots were characterized by
longer quiet eye durations (M= 310.42 ms) than were unsuc-
cessful shots (M = 118.06 ms), skilled players had signifi-
cantly longer quiet eye measures (M = 270.83 ms) than less
skilled players (M = 157.64 ms), and quiet eye duration
decreased linearly with increases in time constraints (Ms =
314.17 ms for the 100% unconstrained, 190.63 ms for the
25% constrained, and 137.92 ms for the 50% constrained
conditions).
A significant three-way Accuracy Duration Skill
interaction was observed, F(2, 44) = 9.91, p < .01. For both
the skilled and the less skilled groups, quiet eye duration
was significantly longer for accurate than for inaccurate
shots at each level of complexity. The longest quiet eye
duration was observed for successful shots in the 100%
unconstrained condition. Moreover, for accurate shots,
skilled players had significantly longer mean quiet eye
durationsnearly double that of their less skilled counter-
parts. For both skill levels, missed shots in the 25% and the
50% constrained conditions were characterized by short
quiet eye durations. Finally, in the skilled group, missed
shots were associated with mean quiet eye durations that
were less than half those associated with successful shots.
The mean values are displayed in Figure 2.
GENERAL DISCUSSION
In this research, we examined the relationships among
quiet eye duration, expertise, and task complexity in a bil-
A. M. Williams, R. N. Singer, & S. G. Frehlich
204 Journal of Motor Behavior
TABLE 6
Performance Means (in %) and Standard
Deviations for Skilled and Less Skilled Groups
for Each Movement Condition
Skilled Less skilled
Condition M SD M SD
Unconstrained 74.84 6.17 55.19 13.18
25% constrained 68.00 7.74 54.23 14.58
50% constrained 59.33 10.42 46.78 13.83
FIGURE 2. Mean quiet eye duration (in ms) for successful
and unsuccessful shots across skilled and less skilled
groups for each task duration level.
liards task involving near and far aiming requirements. Our
primary intention was to examine the explanatory power of
Vickerss (1996) location-suppression hypotheses in far
aiming tasks. According to Vickerss hypotheses, the pre-
sent participants should fixate predominately on the target
during the preparatory phase of the movement and should
exhibit longer quiet eye periods than their less expert coun-
terparts. We expected that during the execution phase of the
movement, participants would suppress visual input from
the target either by an increase in the number of eye blinks
or by shifts in gaze toward other potentially less pertinent
areas. We also examined the potential value of Vickerss
quiet eye period as an index of cognitive programming by
manipulating the complexity of the task and the time avail-
able to execute the stroke. We expected quiet eye duration
to increase with greater task complexity, whereas we
expected task proficiency to decrease as we reduced the
duration of quiet eye by placing various time constraints on
performance. Finally, the potentially mediating effects of
expertise on quiet eye period during manipulations of shot
complexity and time available for preprogramming were
examined.
EyeHand Coordination
Participants focused primarily on the near target (i.e., cue
ball), irrespective of task complexity or shot phase, with
secondary fixations directed toward the object ball. The sec-
ond ball and cushion were also of importance in the IC and
HC, respectively. There were surprisingly few fixations on
the pocket, limbs, or billiards cue (classified as other
areas). The data provided support for the location aspect of
Vickerss (1996) hypothesis. Most interesting, more fixa-
tions of longer duration were directed at the cue ball than at
the object ball or other areas, particularly for the skilled
players in the IC and HC conditions, indicating that the bil-
liards shot may be primarily seen as a near rather than a far
aiming task.
No evidence was obtained for the suppression aspect of
Vickerss hypothesis. Vickers (1996) noted that expert free-
throw shooters suppressed their gaze immediately follow-
ing the initiation of movement toward the target by either
blinking or shifting their visual attention to areas away from
the target. In the sport of billiards, very few eye blinks or
shifts of gaze to locations other than the cue ball, object
ball, or target cushion were observed. Regardless of skill
level, participants appeared to fixate only on the pertinent
aspects of the display and did not make any eye blinks
throughout each phase of the billiards stroke. Once the
backswing phase was initiated, both groups focused on
either the cue or the object ball and then tracked the cue ball
until it came into contact with the object ball.
Vickerss (1996) suppression hypothesis may be specific
to the basketball free-throw task. When performing that
task, the hands, ball, and arms all come into view of the
eyes and occlude the target (the hoop) during the execution
phase. The appearance of potentially distracting stimuli in
the visual field may cause players to blink or shift their gaze
away from the target. To that end, Vickerss suppression
hypothesis may apply only to tasks in which a body part or
a piece of equipment interferes with the participants view
of the target.
Quiet Eye Period as an Index of Preprogramming
Longer quiet eye periods were recorded on successful
than on unsuccessful shots, regardless of task complexity or
participant skill level. Quiet eye duration averaged 561.94
ms for successful shots, compared with only 213.61 ms for
unsuccessful shots. That finding provides support for the
results of previous research by Vickers and her colleagues
and suggests that quiet eye duration is an important element
of successful performance in various aiming tasks. Our
assumption is that the quiet eye period reflects a critical
period of cognitive processing during which the parameters
of the movement such as force, direction, and velocity are
fine-tuned and programmed.
As expected, the quiet eye period increased proportional-
ly with the difficulty of the shot, particularly for successful
attempts. Quiet eye duration in the EC averaged 230 ms,
with 314 ms in the IC and almost 620 ms in the HC. In pre-
vious research, longer reaction times have been observed
for more complex tasks (e.g., Klapp, 1980; Sternberg, Mon-
sell, Knoll, & Wright, 1978); therefore, the finding of an
increase in quiet eye duration with shot complexity was pre-
dicted. If quiet eye duration reflects a period of cognitive
response programming, then more complex tasks should be
characterized by longer quiet eye periods.
In the second experiment, externally imposed time con-
straints led to reductions in the preparation phase of only
the shot. The reduction in the relative amount of time avail-
able for shot preparation had a concomitant affect on the
quiet eye period for both groups. Mean quiet eye values
dropped from 314 ms in the unconstrained condition to 138
ms in the 50% constrained condition. Quiet eye duration,
therefore, appears to reflect underlying cognitive processes
that are highly influential in the preparation of effective bil-
liard stroke responses. As proposed by Vickers (1996), the
quiet eye period is related to the amount of time spent in the
response-programming stage of the information-processing
model and may serve as evidence that higher order cogni-
tive processes control gaze behavior.
Mediating Effects of Expertise on Performance
Skilled players performed significantly better than their
less skilled counterparts across the three complexity condi-
tions. They had a higher ratio of successful to unsuccessful
shots in each condition. More accurate shots were charac-
terized by longer overall movement duration, with success-
ful shots taking approximately 210 ms longer to complete.
In general, skilled players took more time to execute their
shots than less skilled players, particularly in the HC. The
level of shot complexity also influenced the overall dura-
tion, with shots in the HC having the longest duration. The
Visual Search in Near and Far Aiming Tasks
June 2002, Vol. 34, No. 2 205
duration of the backswing, foreswing, and flight phases of
the stroke did not differ significantly across the three levels
of task complexity, although increases were observed dur-
ing the shot preparation phase. That result provides support
for the finding in other studies that reaction time measures
are related to task complexity (e.g., Klapp, 1980) and sug-
gests that changes in task complexity influence response
programming.
Skilled and less skilled players were differentiated on the
basis of their visual search behaviors. The less skilled play-
ers used more fixations of shorter duration per trial, partic-
ularly to the cue and object balls, than the skilled partici-
pants. In contrast, the skilled players fixated for longer on
the cue and object balls than the less skilled participants.
The less skilled players seemed to allocate their visual
attention equally to the cue ball, the object ball, and the sec-
ond target ball. It appears that the skilled performers visu-
al search behaviors were more economical, with longer fix-
ations to only one or two key areas of the display. That
efficient search strategy is similar to what has been
observed in experts engaged in other motor skills such as
golf putting (Vickers, 1992), baseball batting (Bahill &
LaRitz, 1984), and volleyball service reception (Vickers &
Adolphe, 1997).
Skilled players used longer quiet eye durations than their
less skilled counterparts did, regardless of shot complexity or
the time constraints imposed. The average quiet eye duration
for the skilled players was around 500 ms, compared with
275 ms for the less skilled participants. That finding provides
support for previous results involving the basketball free
throw and volleyball service reception (Vickers, 1996, 1997;
Vickers & Adolphe, 1997). Quiet eye duration appears to be
a key factor in explaining differences in performance
between and within each of the skill level groups.
More investigation is needed so that the exact nature of
the quiet eye duration can be determined. Vickers (1996)
contended that the parameterization of variables associated
with successful far aiming movements is delineated during
the quiet eye period. Those variables may include temporal,
distance, force, and velocity measures. It is still unclear,
though, whether the quiet eye duration is indeed reflective
of underlying cognitive processing. Although the present
results suggest that quiet eye duration is influenced by fac-
tors known to have an impact upon the programming of a
motor response, further research investigating manipula-
tions in each parameter of the aiming movement might pro-
vide more detailed information as to the exact nature of the
quiet eye measure. An alternative perspective needs to be
explored as well. Perhaps a longer target fixation period
serves primarily as a means of self-regulation enabling the
performer to enter and sustain an optimal attention state;
consequently, the quiet eye period may be part of the ath-
letes preperformance routine. The performer presumably
fixates his or her visual attention on the target but does not
initiate the action until a state of mental readiness is
achieved (Singer, 2000).
Finally, research is needed to determine whether training
programs can be designed to enable performers to enhance
the control of visual attention in near and far aiming tasks.
Because the experts perceptual superiority over the novice
is the result of enhanced computational sophistication and
improved strategic processing of information rather than
differences in visual abilities (Williams et al., 1999),
researchers should determine whether those skills could be
developed through instructional training programs (see
Williams & Grant, 1999). For example, Adolphe, Vickers,
and Laplante (1997) developed a 6-week training program
to improve ball-tracking and -passing skills in high skilled
volleyball players. The results indicated that the training
program resulted in faster tracking onset times, a longer ball
tracking duration, and longer quiet eye periods. The
improvements were still observed after a 3-year follow up,
whereas performance statistics demonstrated that those
skills transferred from laboratory to field settings. Further
research is necessary so that we can increase our under-
standing of the mechanisms underlying the training of visu-
al attention control during near and far aiming tasks.
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Submitted November 19, 2000
Revised June 26, 2001
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June 2002, Vol. 34, No. 2 207