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1 Department of Radiology, Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts, United States
2 Department of Radiology, Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts, United States; Institute for Psychology of the Hungarial Academy of Sciences, Budapest, Hungary
3 Department of Radiology, Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts, United States; Brain and Cognative Sciences, McGovern Institute for Brain Research at the Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
4 Department of Radiology, Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts, United States; Department of Anesthesia and Critical Care, Harvard MIT/Division of Health Sciences and Technology, Harvard Medical School, Massachusetts General Hospital, Boston, Massachusetts, United States
5 Department of Radiology, Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Charlestown, Massachusetts, United States; Department of Neurosciences, University of California, San Diego, La Jolla, California, United States
* To whom correspondence should be addressed. E-mail: robhh{at}nmr.mgh.harvard.edu.
Several previous studies have demonstrated 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 investigated how the temporal dynamics of such states influence the sensory evoked response to vibrissa deflection. Under alpha chloralose anesthesia, barrel cortex exhibits strong quasi-periodic ~1Hz 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 local field potential (LFP) and multi-unit 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 upon 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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