| This thesis addresses whether activity and response variability in rat auditory cortex depends on brain state. Previous research has revealed there is a great deal of variability in cortical responses to repeated stimuli. We propose that this variability is not random, but relates to ongoing cortical dynamics. First we identify how spontaneous and evoked dynamics differ between states; then we establish how reliably cortical responses can follow temporally-modulated sensory input in different states. Finally, we test the hypothesis that the internal dynamics of synchronized states can account for sensory response variability.;We first assessed the spontaneous activity of auditory cortex during silence. During "synchronized" slow-wave states spike counts of individual neurons in sequential time bins were irregular, but neurons were coordinated as a population. Spike counts were more regular following a tail pinch-induced shift to higher-frequency EEG, but population-wide coordination disappeared. We also uncovered a set of high-firing neurons with independent, rhythmic activity during desynchronized states, peaking between 8 to 18 Hz.;Next we characterized responses to click stimuli. Many neurons discharged short-latency spikes with similar latency across states. These preserved spike latencies manifested as brief population sequences of activity with similar profiles in different states. In some experiments, we observed a late, long-lasting effect of clicks on firing rates in synchronized states.;In our last study, we show that evoked local field potentials (LFPs) can follow high-frequency amplitude modulations of broadband noise during desynchronized regimes. Spikes also track input more reliably, and can be better predicted from stimuli in desynchronized states than in slow-wave states. Finally, we address whether LFPs reliably predict neural activity, and show that LFPs can explain more spiking variability than our amplitude-modulated white noise stimuli. Thus much 'noise' in neural responses is not cell-specific, but reflects a source shared across cells; such variability is state-dependent, and can be accounted for by LFP dynamics.;Our first studies demonstrate that despite clear changes in spontaneous activity, strong onset responses to discrete broadband stimuli are often preserved across states. The final study suggests the desynchronized state supports improved representation of temporally modulated stimuli in auditory cortex. |
