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Abstract:
Abstract: Three systems serve human motion perception: the
first-order motion system is sensitive to drifting modulations of
luminance, second-order to modulations of texture contrast,
third-order to modulations of salience (figure-ground). An
Electrical Geodesics system of 128 electrodes was used to measure
brain potentials (VEPs) evoked by visual stimuli directed
specifically to first- and second-motion systems. Previous studies
had found no important differences between VEPs evoked by first-
and second-order stimuli, so four critical improvements were made
in stimulus construction. (1) The carriers for first- and
second-order stimuli were identical, only the moving modulation
varied. (2) The first- and second-order stimuli were equated for
flicker. (3) The stimuli were balanced for black-white asymmetry so
that second-order stimuli did not generate spurious first-order
components. (4a) Motion direction changes within first-order and
within second-order were measured, not merely motion onsets or
terminations. (4b) Changes from first-order to second-order motion
were measured for stimuli moving in the same direction. With the
contributions of flicker and stimulus onsets equated throughout,
the first- and second-order stimuli are perceptually very similar,
but VEPs recorded for both presentation types, 4a and 4b, clearly
differentiate first- and second-order systems in both temporal
pattern and spatial topography. The second-order VEP lags 70-100
msec behind first-order. MEG and fMRI studies to localize these
sources are underway.
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