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Locomotion initiation times
when running to intercept virtual fly balls |
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For a stationary person, the sign of
optical acceleration specifies on which side of the point of
observation (front or back) an approaching fly ball will pass at eye
level (e.g., Michaels & Oudejans, 1992). Positive acceleration
specifies a ball going to pass behind the point of observation,
negative acceleration specifies a ball going to pass in front of the
point of observation. We studied the use of this information by
having participants intercept approaching virtual balls (in a CAVE)
with their foreheads. If optical acceleration is used as information
about the future passing side of approaching fly balls, the timing
of the initiation of running should be a function of (the optical
consequences of) the specifics of the ball trajectories. That is to
say, trajectories of different launching distance, passing distance,
and apex height result in different time series of optical speed and
optical acceleration (because the information is in the sign of
optical acceleration, the detection of any change in optical speed
suffices to be informed about the future passing side of the fly
ball). When we assume a threshold for the detection of a change in
optical speed, predictions can be formulated regarding the timing of
the start of the running. For instance, later running initiation
times are predicted for balls that will pass close by than for balls
that will pass farther away (cf. Zaal & Michaels, 2003); it will
also take longer to know the ball’s future passing side if the ball
is launched from a longer distance. One way to test the use of optical acceleration as the information for future passing side is to study running initiation times of catchers of fly balls. These times can be compared with those predicted from the optics related with the specific ball trajectories of those fly balls. For instance, Brouwer, Brenner, & Smeets (2002) performed such a comparison. For this purpose, they determined the sensitivity of the visual system to optical acceleration for stimuli of a short duration, using psychophysical methods. Taking the data from the study of Oudejans, Bakker, & Michaels (1997), they compared the running initiation times of the expert catchers of this study with those predicted from the optical-acceleration thresholds they had established in their psychophysics experiments. The conclusion drawn by Brouwer and colleagues was that the expert fielders of the Oudejans et al. study had started running too early for this running initiation to have been based on optical acceleration. This conclusion, however, probably, was drawn prematurely because some of the assumptions going into the comparison were not realistic (cf. Zaal, Bongers, & Bootsma, 2008). |
Recently, we have started to study the
running initiation times of participants asked to intercept virtual
balls in the CAVE of the Faculty of Sport Sciences in Marseille. We
have our participants intercept the virtual (football) balls with
their foreheads, which is a task football players perform often. The
reason to require an interception with the forehead is the feedback
from optical looming about the success of the interception (cf. Zaal
& Michaels, 2003). A first pilot experiment was designed to
establish the inherent variability in running initiation times (see
also Brouwer et al., 2002). Participants were able to intercept
about 75% of the balls. Interception was not only realized by
running back or forth, but also by moving up or down with the head.
Especially, for balls passing close to the starting location of the
participants, also ducking or jumping turned out to be an adequate
way to make the interception. Therefore, in a second experiment, we
made sure that participants were required to do some actual running
for a successful interception to be possible. In addition, with the
information about the inherent variability in running initiation
times, we varied launching distances, apex heights, and passing
distances such that the predicted initiation times were adequately
spaced relative to the within-condition variability of the actual
initiation times. Unfortunately, at the time of writing, we are not
ready to present the results from this second experiment. References Brouwer, A. M., Brenner, E., & Smeets, J. B. J. (2002). Perception of acceleration with short presentation times: can acceleration be used in interception? Perception & Psychophysics, 64, 1160-1168. Michaels, C. F. & Oudejans, R. R. D. (1992). The optics and actions of catching fly balls: Zeroing out optical acceleration. Ecological Psychology, 4, 199-222. Oudejans, R. R. D., Michaels, C. F., & Bakker, F. C. (1997). The effects of baseball experience on movement initiation in catching fly balls. Journal of Sports Sciences, 15, 587-595. Zaal, F. T. J. M., Bongers, R. M. & Bootsma, R. J. (2008). Fielders might still use image acceleration for the initiation of their running to catch fly balls: A reply to Brouwer, Brenner, and Smeets (2002). Manuscript in preparation. Zaal, F. T. J. M. & Michaels, C. F. (2003). The information for catching fly balls: Judging and intercepting virtual balls in a CAVE. Journal of Experimental Psychology: Human Perception and Performance, 29, 537-555. |
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