Longer VEP latencies and slower reaction times to the onset of second-order motion than to the onset of first-order motion
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We compared visual evoked potentials and psychophysical reaction times to the onset of first- and second-order motion. The stimuli consisted of luminance-modulated (first-order) and contrast-modulated (second-order) 1 cpd vertical sine-wave gratings drifting rightward for 140 ms at a velocity of 6 degrees /s. For each condition, we analysed the latencies and peak-to-baseline amplitudes of the P1 and N2 peaks recorded at Oz. For first-order motion, both P1 and N2 peaks were present at low (3%) contrast (i.e., depth modulations) whereas for second-order motion they appeared only at higher (25%) contrasts. When the two types of motion were equated for visibility, responses were slower for second-order motion than for first-order motion: about 44 ms slower for P1 latencies, 53 ms slower for N2 latencies, and 76 ms slower for reaction times. The longer VEP latencies for second-order motion support models that postulate additional processing steps for the extraction of second-order motion. The slower reaction time to the onset of second-order motion suggests that the longer neurophysiological analysis translates into slower detection.
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