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Department of Physiology and Biophysics, Georgetown University, Washington DC, 20057-1421
Submitted 31 December 2002; accepted in final form 18 February 2003
Neocortical "theta" oscillation (512 Hz) has been
observed in animals and human subjects but little is known about how the
oscillation is organized in the cortical intrinsic networks. Here we use
voltage-sensitive dye and optical imaging to study a carbachol/bicuculline
induced theta (
8 Hz) oscillation in rat neocortical slices. The imaging
has large signal-to-noise ratio, allowing us to map the phase distribution
over the neocortical tissue during the oscillation. The oscillation was
organized as spontaneous epochs and each epoch was composed of a "first
spike," a "regular" period (with relatively stable frequency
and amplitude), and an "irregular" period (with variable frequency
and amplitude) of oscillations. During each cycle of the regular oscillation,
one wave of activation propagated horizontally (parallel to the cortical
lamina) across the cortical section at a velocity of
50 mm/s. Vertically
the activity was synchronized through all cortical layers. This pattern of one
propagating wave associated with one oscillation cycle was seen during all the
regular cycles. The oscillation frequency varied noticeably at two neighboring
horizontal locations (330 µm apart), suggesting that the oscillation is
locally organized and each local oscillator is about ≤300 µm wide
horizontally. During irregular oscillations, the spatiotemporal patterns were
complex and sometimes the vertical synchronization decomposed, suggesting a
de-coupling among local oscillators. Our data suggested that neocortical theta
oscillation is sustained by multiple local oscillators. The coupling regime
among the oscillators may determine the spatiotemporal pattern and switching
between propagating waves and irregular patterns.
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