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Abstract:
The human visual system is hypothesized to code motion in
discrete time quanta, given that within certain thresholds,
rotating objects such as wagon wheels are perceived as stationary
or as moving in the reverse direction. To investigate the
neurobiological correlates of motion perception, we designed a
paradigm in which two bars of light (1degree apart) were flashed
with a stimulus onset asynchony (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 MEG system (4D-Neuroimaging). Data were
selectively averaged, filtered 3-70Hz, and analyzed by the
time-dependent FFT. 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 surface
normals. Psychophysics indicate that subjects perceived AM for SOAs
longer than 15-18ms. For shorter SOAs, subects 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 75,115,150,200,255, and 297ms.
Current distribution estimates of these peaks showed recurrent
activity in areas corresponding to V1,V2,VP,V3a,MT,V4 and other
areas within the temporal and parietal lobes. In conclusion, AM is
processed in quanta of 15-18ms, and correlates with gamma-band
activity in the visual system.
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