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Our intention was to obtain a visual evoked potential (VEP) consisting only of a movement-related component for the purpose of further investigations of movement detection. This was attempted by selection of appropriate stimulus conditions. Evoked by initial movement a VEP with five typical waves was observed at the human occipital scalp. The N2-wave with a peak latency of 180-200 ms was most prominent. Following results were yielded in the experiments carried out: 1. Adaptation to a pattern movement: The amplitude of N2 and P1 is significantly reduced (Fig. 4). 2. Relation between amplitude and velocity: The experimental data could be approximated by a power function with an exponent of m = 0.3 for N2 and lower m for later waves (Fig. 5). 3. Pattern variation (grating, checkerboard, zig-zag) had no influence on N2 but on P1 and possibly also on later waves (Fig. 6). These results suggest that the wave N2 is movement-related under our experimental conditions. A pattern-related component may additionally be assumed in wave P2. Components, evoked by further reasons, may be included in the waves following N2. Their specification demands supplementary experiments.

Knowledge referring to the kind of velocity coding in human cortex can be obtained either by psychophysical methods or by recording of visually evoked cortical potentials (VEPs). With the help of adaptation experiments it is possible to distinguish between the additive and substitutive coding principle. VEPs were evoked by the onset of movement of a grating pattern (test stimulus). The most prominent waves N2 and P2 increased with accelerating test stimulus velocity within a range of 0.2-4 deg/s (see strong curve in Fig. 6a and 7a). Moving gratings (adaptation stimuli) were additionally presented in corresponding runs during the intervals between the test stimuli to attain a stable level of movement adaptation. The additional movement stimulation reduced the VEP amplitudes in general (see weak lines in Figs. 6a, 7a and relative amplitudes in Figs. 6b, 7b). When the adaptation stimulus velocity was raised within a range of 0.1-4 deg/s (see values of the abscissas in Figs. 6 and 7 indicated by symbols with arrow) the relative VEP amplitude became smaller (decrease from curve to x). This result points to the validity of the additive coding principle. The different behavior of the delta-curve supports the hypothesis of a second channel for velocities faster than 1 deg/s.

The amplitudes of the motion VEP waves P100, N200 and P300 were investigated on 12 subjects at six electrode positions (2.5, 5, 7.5 and 10 cm above the inion and 5 cm bilaterally from the 2.5 cm midline point). The evaluation was performed on the basis of in general 45 (for P100 32) averaged potentials per subject. Each averaged potential was obtained from 40 single potentials. P100: The amplitudes were nearly equal at all electrodes. N200: The greatest amplitudes could be found at the lateral electrodes. The amplitudes decreased in the midline in occipito-frontal direction reaching the lowest value at the electrode most frontally situated. P300: This wave had its greatest amplitude at the most frontal lead position. The amplitude decreased systematically in occipital direction. The differences in the position of the amplitude maxima and in the topographical distribution of the remaining amplitude values suggest different generator structures of the waves N200 and P300.