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C. Hou, A. M. Norcia; Electrophysiological Evidence for Multiple Spatio-Temporal Channels Responsive to Coherent Motion in Random Dot Displays. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5881.
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Previous psychophysical studies have argued for the existence of independent fast and slow speed-tuned systems (Edwards et al., 1998; Khuu and Badcock, 2002; Watamaniuk and Duchon, 1992). Here we asked whether there is evidence for multiple speed mechanisms at supra-threshold levels by measuring Visual Evoked Potential (VEP) responses over a large range of speeds.
We used random-dot kinematograms (12.4 arcmin white dots on a black background, 10% density, lifetime 3 sec, field size 24 deg). A full 0.83Hz cycle of the stimulus (F1) consisted of 0.6s of circular coherent motion (clockwise or anticlockwise rotation around the fixation axis) and 0.6s of incoherent motion at the same dot speed. VEP responses were measured over a wide range of dot displacements at three dot-update frequencies (F2: 30Hz, 20Hz and 15Hz). The combination of spatial displacements and temporal frequencies resulted in range of speeds from 0.8 deg/s to 18.6 deg/s. VEPs were recorded from 14 adults between 17-53 years of age.
The VEP waveform at the slower speeds showed a sustained positivity after motion onset followed by a negative peak at motion offset. At the higher speeds, the onset response was initially negative and less sustained and the offset response had a large initial positive peak. Within a given speed, the responses to the different dot-update rates were similar, indicating that the response waveform is more controlled by speed than spatial displacement.
Slow and fast speeds elicit different response waveforms, suggesting multiple speed-tuned mechanisms, rather than a single one underlie the human response to coherent motion.
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