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Abstract:
The human visual system is hypothesized to code motion in
discrete time quanta, given that within certain thresholds, moving
objects such as wagon wheels are perceived as rotating in the
reverse direction. To investigate the neurobiological correlates of
motion perception, we designed a paradigm in which two bars of
light (1 apart) were flashed with a stimulus onset asynchrony (SOA)
of 0,3,Ö, to 27ms. After each trial set, subjects reported
whether they perceived apparent motion (AM) and its direction.
Neuromagnetic responses were recorded with a 148-channel
neuromagnetometer system (4D-Neuroimaging). Data were selectively
averaged, filtered 3-70Hz, and analyzed by the time-dependent FFT
and by Independent Component Analysis. Current sources were
localized using a new anatomically constrained algorithm (weighted
iterative truncated-SVD). A 3D cortical surface was reconstructed
from the subject's MRI. The lead field matrix was computed for the
segmented sourcespace. Psychophysics indicate that subjects
perceived AM for SOAs longer than 15-18ms. For shorter SOAs,
subjects perceived two simultaneous bars of light rather than a
single moving bar. The spectogram showed gamma-band activity in the
posterior channels. Magnetic waveforms showed peaks near
75,115,150,200,250, and 300 ms. Current distribution estimates of
these peaks showed recurrent activity in areas corresponding to
V1,V2,VP,V3a,MT,V4 and other areas. In conclusion, AM is processed
in quanta of 15-18ms, and correlates with gamma-band activity in
the visual system.
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