| Discrimination of intensity differences has been studied in the past by both electrophysiological and psychophysical methods. It has been shown that an ERP measure, the P300 (P3) varies as a function of the difference in intensity between two suprathreshold stimuli.;This study investigated: (1) The minimum contrast difference between two grating patterns required to elicit a P3. (2) The change of P3 latency and amplitude as a function of the contrast difference. (3) The psychophysical contrast discrimination threshold for the same stimuli. The major aim was to correlate the electrophysiological and the psychophysical indices of contrast discrimination.;Grating patterns with a sinusoidal luminance profile were presented in a visual "oddball" paradigm. The rare stimulus differed only in contrast from the frequent stimulus. In the electrophysiological study, the latency and amplitude measures were obtained for each value of the ;In the psychophysical study, the two-alternative forced-choice method was used because it is essentially independent of threshold criterion. The observer's task was to indicate in which interval a higher (or lower) contrast was presented. The observer's task in the electrophysiological study was to silently count the rare stimulus. This procedure provided a similar method of stimulus presentation for both studies. The just noticeable difference (JND) of contrast discrimination derived by the two methods was compared.;The P3 data indicated: (1) That P3 latency and amplitude varied with contrast difference. The latency of the N2 and P3 increased while the peak-to-peak amplitude decreased with a decrease in ;The results obtained in the psychophysical study were consistent with the results obtained in the P3 study in that frequent contrast had an effect on the JNDs. The JNDs and the variability around the means decreased with higher frequent contrast. |
