BOLD signals correlate with ensemble unit activities in rat's somatosensory cortex

Evoked neural activity (ensemble single-unit activity and evoked field potential) and functional magnetic resonance imaging (fMRI) changes of the primary somatosensory cortex in response to electrical stimulation of the hind paw were studied in rats under anesthesia. The effects of stimulation frequency (ranging from 0.3 to 10 Hz) and types of anesthetics (alpha-chloralose and sodium pentobarbital) on blood oxygen level dependent (BOLD) activation and neural activation were compared. Both ensemble single-unit activity and BOLD signal changes achieved maximal activation at 3 Hz of stimulation and responses were significantly stronger under a-chloralose anesthesia. The maximal activation of the integral evoked potential (Sigma EP), in contrast, was the highest at 10 Hz; and the values were similar for alpha-chloralose and pentobarbital. These analyses revealed that fMRI image changes were better correlated with ensemble single-unit activity than with Sigma EP during somatosensory stimulations.