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Analysis of LFP Phase Predicts Sensory Response of Barrel Cortex

¹Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts; ²Institute for Psychology of the Hungarian Academy of Sciences, Budapest Hungary; ³McGovern Institute for Brain Research and Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts; ⁴Harvard Massachusetts Institute of Technology/Division of Health Sciences and Technology, Harvard Medical School, Department of Anesthesia and Critical Care, Massachusetts General Hospital, Boston, Massachusetts; and ⁵Department of Neurosciences, University of California at San Diego, La Jolla, California

Submitted 7 December 2005; accepted in final form 5 June 2006

Several previous studies have shown the existence of Up and Down states and have linked their magnitude (e.g., depolarization level) to the size of sensory-evoked responses. Here, we studied how the temporal dynamics of such states influence the sensory-evoked response to vibrissa deflection. Under -chloralose anesthesia, barrel cortex exhibits strong quasi-periodic 1-Hz local field potential (LFP) oscillations generated by the synchronized fluctuation of large populations of neurons between depolarized (Up) and hyperpolarized (Down) states. Using a linear depth electrode array, we recorded the LFP and multiunit activity (MUA) simultaneously across multiple layers of the barrel column and used the LFP to approximate the subthreshold Up–Down fluctuations. Our central finding is that the MUA response is a strong function of the LFP oscillation’s phase. When only ongoing LFP magnitude was considered, the response was largest in the Down state, in agreement with previous studies. However, consideration of the LFP phase revealed that the MUA response varied smoothly as a function of LFP phase in a manner that was not monotonically dependent on LFP magnitude. The LFP phase is therefore a better predictor of the MUA response than the LFP magnitude is. Our results suggest that, in the presence of ongoing oscillations, there can be a continuum of response properties and that each phase may, at times, need to be considered a distinct cortical state.

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