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Abstract:
Oscillatory population activity is ubiquitous in the mammalian neocortex. Oscillations are present across
many frequency bands, and are postulated to reflect the integration of information over neural populations
[1]. Such oscillations can be evoked by sensory stimuli, and neocortical populations can exhibit distinct
forms of resonance to the stimuli. For instance, when subjects are exposed to flickering light at certain
frequencies, their EEG-waves are prone to entrain, i.e. phase lock, to the stimulation frequency [2].
First, we address the state-dependence of such entrainment using a rat-model of sleep [3]). Under urethane
anesthesia, the electrocorticogram (ECoG) alternates spontaneously between a low-voltage desynchronized
state, which resembles the ECoG during rapid eye movement (REM) sleep, and a synchronized state,
which resembles the ECoG during slow-wave sleep. This biphasic state represents an ideal model system to
investigate state-dependent changes in the network responses to sensory stimuli. As expected, frequency
coupling varied with both stimulus frequency (2.5 Hz to 15 Hz) and cortical state. Overall, the
desynchronized state, indicative of a more activated cortex, is more susceptible to entrainment than the
synchronized state, which is in agreement with previous reports (e.g. [4]). Furthermore, in some trials, the
entrainment outlasts the stimulus train, giving way to poststimulus illusory responses resembling a
dampened oscillation. Next, we use voltage-sensitive dye imaging (VSDI) with high signal-to-noise ratio
[5] to study the spatiotemporal characteristics of single trials of this phenomenon, and describe the spatial
coupling by cortical area and cortical state.