A comparison of the information about rich naturalistic stimuli carried by phase and power of LFP recordings

Local field potentials (LFPs) reflect sub-threshold integrative processes that complement spike train measures. Here we investigate how LFPs encode rich naturalistic sensory stimuli. We addressed this question by recording LFPs from the primary visual cortex of anesthetized macaques with an array of electrodes while presenting binocularly a color movie. The electrodes were arranged in a 4 by 4 matrix and interelectrode spacing varied from 1 to 2.5 mm. We decomposed the LFP into narrow frequency bands (1-4Hz, 4-8Hz, 8-12Hz and up to 100Hz in 4-Hz wide non-overlapping frequency intervals by using standard bandpassing techniques. We then computed the amount of information about the movie that is carried by the phase or by the instantaneous power of the bandpassed LFP obtained from each individual electrode. When considering the information in LFP power, we found that the instantaneous power was most informative in the low frequency range (< 8 Hz) and in the high gamma frequency range (60-100 Hz). When considering the LFP phase, we found that only the phase of low frequency LFPs (< 12 Hz) was informative about the movie. We found that phase was more informative about the movie than power was. On average across all recording sites and experimental animals, the maximum across LFP frequencies of information about the movie that could be extracted from LFP phase was approximately 0.4 bits, whereas the maximum across LFP frequencies of information about the movie that could be extracted from LFP power was approximately 0.2 bits. We finally investigated whether the phase or power or LFPs recorded from different electrodes convey similar or different information about the movie. Our preliminary results suggest that the informative parts of the LFP signal, i.e. low frequency phases, low frequency power and high gamma power have stimulus selectivity that remains largely similar (though not identical) across the range of distances covered by the electrode grid. Currently we are investigating how phase and power of LFPs encodes stimuli in auditory cortex