Visual persistence of stereoscopic stimuli: electric brain activity without perceptual correlate

Dynamic random-dot stereograms (RDS) were used to study cortical neuronal mechanisms related to visual persistence in nine subjects. Electric brain potential components evoked by stereoscopic stimuli were compared to those evoked by conventional checkerboard stimuli with contrast borders. In a pattern-onset/offset presentation visual persistence thresholds were significantly lower for stereoscopic than for contrast stimuli: with temporally modulated patterns stereoscopic stimuli appeared to visually persist at much shorter interstimulus intervals than contrast stimuli. With stereoscopic stimuli all subjects reported not seeing changes of the RDS pattern, while the corresponding evoked potentials showed components related to the stimulus changes indicating a consistent discrepancy between psychophysical and electrophysiological data. The electrical brain activity was not caused by vergence eye movements elicited by the stereoscopic stimulus. In addition, for dynamic RDS stimuli a significant inverse linear relationship between temporal modulation frequency and evoked potential amplitude was found which was not observed with comparable contrast stimuli.