[Visual evoked potential studies on human cortical coding of the speed of movement of a grating pattern]

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.