THE EFFECTS OF LUMINANCE AND CHROMATIC BACKGROUND FLICKER ON THE HUMAN VISUAL-EVOKED POTENTIAL

Previous studies report that background luminance flicker, which is as ynchronous with signal averaging, reduces the amplitude and increases the latency of the pattern-onset visual evoked potential (VEP). This e ffect has been attributed to saturation of the magnocellular (m-) path way by the flicker stimulus. In the current study, we evaluate this hy pothesis and further characterize this effect. We found that flicker h ad similar effects on the pattern-onset and pattern-reversal VEP, sugg esting that the reversal and onset responses have similar generators. Chromatic flicker decreased latency of the chromatic VEP whereas lumin ance flicker increased peak latency to luminance targets. This result indicates that luminance flicker saturates a rapidly conducting m-path way whereas chromatic flicker saturates a more slowly conducting parvo cellular (p-) pathway. Finally, evoked potentials to chromatic and lum inance stimuli were recorded from 34 electrodes over the scalp in the presence of static and asynchronously modulated backgrounds. An equiva lent dipole model was used to assess occipital, parietal, and temporal lobe components of the surface response topography. Results showed th at chromatic flicker reduced activity to a greater extent in the ventr al visual pathway whereas luminance flicker reduced activity to a grea ter extent in the dorsal visual pathway to parietal lobe. We conclude that the VEP to isoluminant color and luminance stimuli contains both m- and p-pathway components. Asynchronous flicker can be used to selec tively reduce the contribution of these pathways to the surface record ed VEP. Our results provide evidence of parallel pathways in the human visual system, with a dorsal luminance channel projecting predominant ly to the posterior parietal lobe and a ventral color channel projecti ng predominantly to inferior temporal lobe.