Maintenance of visual fixation using a reaction-time task

The VEP to pattern reversal was recorded in eight healthy subjects who were instructed to fixate on a central LED in the stimulating screen. The LED could be extinguished at irregular intervals approximately once every 10 s and the subjects were then required to re-light it as rapidly as possible in a reaction-time task. In the first series of recordings subjects were reminded verbally to maintain visual fixation throughout the averaging sequences. When the reaction-time paradigm was used to maintain visual fixation, the VEP was slightly less variable in latency but otherwise identical to that recorded when the subjects maintained visual fixation on a constantly illuminated LED. In a separate series of 8 consecutive averaging runs, subjects were given the reaction-time task but no verbal reminders. VEP parameters differed from those in the first series when verbal reminders had been given. There was no correlation between VEP parameters and the reaction time or its variability in individual subjects or in the group. The reaction-time task is a satisfactory means for ensuring accurate visual fixation but does not obviate the need to provide subjects with verbal encouragement to maintain full attention.