The fourth component of the study consisted of evaluating postural sway and reaction
time. Subjects were asked to remove their shoes and stand on a Kistler force platform with
their feet together, placed in an orthostatic position. Maintaining this stance was the primary
task (subjects were asked to allocate their attention to this task and sway as little as possible).
The secondary task involved recording simple reaction times as all subjects were instructed
to respond verbally “top” as soon as the auditory stimulus was presented while keeping their
posture as straight as possible on the Kistler platform. The auditory stimulus was sampled
at a rate of 5000 Hz and lasted approximately 50 ms. A microphone recorded the verbal
cues and relayed the analog information to the computer system. Four 1 min trials were
collected presenting approximately 7–8 auditory stimuli per experimental trial. Postural
sway was monitored by means of an online digital computer program recording the center
of pressure (COP) from the force plate during each trial. This parameter will be assessed
through the interpretation of oscillation frequency modes (with the mode being defined as
the frequency occurring the most frequently as measured by a Fourier analysis) for each
experimental group. Reaction time was measured from the onset of the auditory beep to the
beginning of the “top” response by the study participants. At the end of the testing session,
visual acuity was evaluated by means of a Snellen visual test in order to ensure subjects
were able to participate in the testing sessions with normal or corrected vision.