Comparison of visual evoked potential and psychophysical contrast sensitivity

We measured contrast processing as a function of spatial and temporal frequency using three measurement techniques: psychophysical thresholds, visual evoked potential (VEP) thresholds and VEP amplitudes. Measurements were made using the same stimulus parameters and on the same group of subjects. The agreement among these functions depended upon the particular spatio-temporal stimulus employed and in general, there were more differences than similarities among these measures. For example, VEP derived functions (both amplitude and threshold) peaked at 4 c/deg, whereas, the psychophysical functions peaked at 1 c/deg. VEP amplitude functions were maximum at > or = 3.5 Hz, whereas both psychophysical and VEP sensitivity were maximum at < or = 2.5 Hz. VEP thresholds were broad and low pass in shape and in contrast, the psychophysical and VEP amplitude functions were band pass. In summary, comparisons among these measures of contrast processing must be made with caution.