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Impressum
Adaptive VEP recordings, an accelerated method for objective estimation of contrast thresholds
Meigen T., Karl S.
Univ.-Augenklinik Würzburg, Josef-Schneider-Str. 11, 97080 Würzburg
Introduction: The visual evoked potential (VEP) can be used for
objective estimation of sensory thresholds (e.g., contrast thresholds).
Unfortunately, such estimations require long sessions, as VEPs to several
stimulus patterns are to be recorded, which differ in contrast. Recently,
we developed an electrophysiological procedure which performs a data analysis
during the recording and which allows to adaptively adjust the stimulus
parameter for the following trials. In this study we tested to which degree
the duration of contrast-dependant VEPs may be restricted to stimulus
conditions near threshold when this adaptive procedure is applied.
Methods: 22 normal subjects participated in this experiment. Checkerboard
patterns with a mean luminance of 50 cd/m^2, with 3 different check sizes
(0.05°, 0.24°, and 2.4°), and with 8 different contrast values
(0.6%, 1.6%, 2.6%, 5.5%, 11.3%, 23.4%, 48.3% and 100%) were used as visual
stimuli. Steady-state VEPs to pattern reversal stimulation (f=8.3Hz) were
recorded binocularly with a Oz-Fpz derivation. The statistical significance
of a VEP response was derived from the signal-to-noise ratio of the Fourier
magnitude at the reversal frequency (16.6Hz) to the magnitude average
at the two neighbouring frequencies. For each of the 3 check sizes a non-adaptive
recording was performed with 40 trials per contrast condition. Additionally,
an adaptive recording was performed, where the number of trials for each
contrast condition was determined by the adaptive strategy.
Results: For the non-adaptive recordings the transition from non-significant
to significant VEP responses occurred between contrast values of 1.6%
and 11.3% for the check sizes 0.24° and 2.4°, and above 11.3%
for the 0.05° check size. For the adaptive recordings the 3 conditions
that yielded the largest number of trials had contrast values within these
threshold regions for each check size and contained 78% (check size 0.05°),
81% (check size 0.24°) and 52% (check size 2.4°) of the total
number of trials.
Discussion: By applying the new adaptive VEP procedure a large
portion of the recording duration could be spent near the contrast threshold
for the corresponding check size. Thus the adaptive procedure may help
to accelerate the objective estimation of contrast thresholds with visual
evoked potentials in the clinical routine.