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Abstract

The phase entrainment of local field potential (LFP) oscillations to the timing of a stimulus may improve attentional selection by increasing neuronal excitability at the time of stimulus onset. Studies in humans and monkeys have demonstrated phase entrainment to rhythmically presented stimuli, whereas other studies have demonstrated phase entrainment to the onset of non-rhythmic, but attended stimuli. These studies support the idea that the brain may intrinsically generate phase entrainment in expectation of a task-relevant stimulus in order to improve attentional selection. While this idea has been investigated using humans and monkeys as subjects, it has not been studied in the rat. Therefore, we recorded LFP in the rat from brain regions that have been implicated in expectancy and preparatory attention: the medial prefrontal cortex (mPFC), the anterior cingulate cortex (ACC) and the ventral tegmental area (VTA). We employed a well-characterized rodent preparatory attention task that requires the rat to anticipate a brief visual stimulus, which would appear at 1 of 3 randomly selected locations on each trial. Phase entrainment to the stimulus onset was measured by performing Rayleigh’s Z test on the distribution of phase angles across all trials, for each subject (n=8 rats for PFC and ACC and n=14 rats for VTA). We assessed phase entrainment between 4 - 50 Hz and only found significant entrainment of ~ 5 Hz oscillations. Across subjects, the excitatory LFP troughs of 5 Hz oscillations were locked to the stimulus onset. Furthermore, this entrainment was only observed in the mPFC, not in the ACC or in the VTA. The 5 Hz entrainment in mPFC was not observed when the rat selected the incorrect stimulus location suggesting a critical linkage between phase entrainment and attention task behavior. At the time of stimulus onset, correct and incorrect trials involved similar motor preparation, motor execution, and reward expectation. Therefore, we suggest that low-frequency oscillations may be a cross-species neurophysiological mechanism for attentional selection of expected stimuli. Selection of an expected stimulus may be implemented by enhancing neuronal excitability at the time of stimulus onset.