Do more complex stimuli require more processing time?

Abstract

PURPOSE: In humans, Visual Evoked Potentials (VEPs) are obtained to a variety of stimuli, some of which are relatively complex. The purpose of this study was to examine if the processing of more complex stimuli yields a measurable increase in retino-cortical timing. METHODS: Visual evoked potentials (VEPs) and eye-hand reaction times (RT) for luminance- (first-order) and texture-defined (second-order) motions were recorded from normal human subjects. The motion stimuli were constructed by either adding (first-order) or multiplying (second-order) static greyscale noise to a modulating 0.5 cpd sinewave-grating which drifted either to the left or right at 5Hz. VEPs and RTs were measured for each motion class at eight different modulation depths (from 3% to 100%). In order to facilitate data analysis, VEP peak time and RT measurements obtained to first and second order motions were normalized to those obtained using a pattern reversal stimuli. RESULTS: Motion VEPs and RTs are significantly delayed compared to pattern VEPs. Compared to the pattern reversal, VEPs evoked to first and second order motions were significantly (p<.05) delayed by 11.7 ?2.1 msec. and 16.1 ?2.9 msec. respectively. Similarly, RT measurements to first and second order motions were significantly (p<.05) delayed by 22 ?7 msec. and 72.5 ?9 msec. Compared to pattern reversal VEPs. Furthermore, VEP and RT evoked to second order motion are significantly more delayed (p<.05) than those obtained using first order motion stimuli. CONCLUSION: Our results clearly indicate that a gradual increase in the complexity of the stimulus requires more processing time which can be quantified with the VEP or RT measurements. Both methods of measuring the visual processing time yield equivalent results. Funded CIHR, GRENE, Reseau Vision.