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Table
Tennis Testing
To isolate exactly how ADHD impairs motor skills,
Vickers compared a group of boys with the disorder to a comparable
group of boys without it as they played a modified version
of table tennis.
Overall, the boys with ADHD had significantly lower
accuracy than the boys without the disorder.
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In
any sport, the brain must process both short- and long-duration
visual cues. Motor behavior is controlled by two visuo-motor
systems—one
that processes short-duration
information and a second that processes long-duration information.
These two neural systems work together in normal kids. The
short- and long-duration information passes effortlessly from
one system to the other, resulting in smooth visual-motor
coordination. Vickers' research shows that, in kids with ADHD,
the short-duration system seems to be working normally, while
the long-duration system may be the seat of the problem.
Vickers'
experiment required each boy to return a series of serves
to either the left or right half of the table. Target cues
on the end of the table lit up to indicate the half at which
a boy was to aim during each trial. On some trials, the cue
lit up as late as 300 milliseconds after the serve, giving
the subject "short-duration visual information." For other
trials, the cue lit up two seconds before the serve, giving
the subject "long-duration visual information."
Overall,
the boys with ADHD had significantly lower accuracy than the
boys without the disorder. While the unaffected group hit
the target about half the time, the group with ADHD hit the
mark less than one third of the time.
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Alan
tries out the eye-tracking device used by the kids
in Vickers' study.
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To
find out why, Vickers used an eye tracking system like the
one Alan used in "A Quiet
Eye," to monitor the boys' gaze as they prepared to return
the table tennis serves. Motion detectors measured the timing
and speed of the boys' arm movements. The resulting data revealed
what the boys looked at and for how long, and how these variables
affected the timing and accuracy of their arm movements.
Vickers
found that the boys without ADHD moved their eyes an average
of 2.34 times per trial—a
variable Vickers calls "gaze frequency," or "GF." The boys
with ADHD, however, had much jumpier eyes, averaging 3.83
movements per trial. This unsteady gaze, Vickers hypothesizes,
prevents the ADHD athlete from obtaining all the visual information
he'd need to accurately return the serve.
Moreover,
ADHD athletes have trouble processing the visual information
they do get. Vickers tested the ADHD-sufferers twice—once
while they took their regularly prescribed medication, and
once while they abstained from the drugs for at least 48 hours.
Any differences in performance would shed light on what the
medication actually does in the ADHD brain.
The
drugs significantly reduced the ADHD groups' gaze frequency,
allowing the boys to better track the ball in flight.
"That
means the medication was impacting the visual system in some
way," says Vickers. "But we don't yet know how."
This unsteady gaze, Vickers hypothesizes, prevents
the ADHD athlete from obtaining all the visual information
he'd need to accurately return the serve.
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But
medication did not significantly improve the group's overall
accuracy. More importantly, Vickers' study found that even
though gaze frequency dropped, ADHD boys still could not track
the ball long enough to glean all the information they need
about its trajectory. While the boys without ADHD maintained
tracking for about half the length of the ball's flight, the
boys with ADHD followed the ball for less than 40% of it's
flight, suggesting a barrier was reached beyond which the
ball flight information could not be processed. Medication
did not improve that figure, implying that tracking is governed
by a different part of the brain than eye gaze.
Moreover,
while motion detectors revealed no significant difference
between the arm movements of the boys with and without ADHD
during the short-duration trials, the two groups did differ
significantly during the long-duration trials. These trials,
in which the targets lit up two seconds before the serve,
required the subjects to store information in memory. The
ADHD group performed significantly worse than their counterparts
in these trials than in the short-duration trials, telling
Vickers that ADHD mainly affects the long-duration system.
This finding has important implication for the treatment and
management of ADHD.
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