Pattern reversal stimuli: motion or contrast?

Pattern reversal stimuli are widely used for electrophysiological examination of the visual system. To investigate the mechanisms contributing to the pattern reversal EP (PREP) we performed experiments with a trained alert macaque monkey, using both contrast and motion stimuli. It was found that, as in man, for parafoveal and peripheral stimulation the PREP can be constructed from the EPs to the onset and offset of pattern motion. On the other hand foveal stimulation activates the contrast mechanism only. Our conclusion is that pattern reversal data should be considered with caution since pattern reversal can activate motion as well as contrast mechanisms. This ambiguity is not present in the pattern appearance-disappearance stimulus.