EP 105. The relation of prestimulus alpha power, poststimulus gamma power and perception in human somatosensory cortex

Abstract

Neuronal oscillations are the synchronous firing of many aligned neurons. Different frequency bands have been ascribed specific functions. Two prominent frequency bands are alpha (8–12 Hz) and gamma band (40–150 Hz). Gamma-band activity is commonly related to active processing in a brain area. Gamma power correlates with attention and stimulus detection in sensory tasks ( Hoogenboom et al., 2010 ). Alpha power correlates inversely with attention and stimulus detection. A previous study found that in perception of supra-threshold tactile stimuli, veridical perception is related to low prestimulus alpha power in contralateral somatosensory and posterior brain areas ( Baumgarten et al., 2014 ). Here, we studied the relation of prestimulus alpha power, poststimulus gamma power, and tactile perception in a supra-threshold stimulation task. If gamma power enhances stimuli detection, we hypothesized that veridical perception relates to high gamma power. Due to a relation between veridical perception and low alpha power, poststimulus gamma power should negatively correlate with prestimulus alpha power. We used data recorded by Baumgarten et al. (2014) . 16 subjects received two electrical stimuli with different stimulus onset asynchronies (SOAs) to their left index finger. Subjects were asked to report whether they perceived one or two stimuli. For each subject, we determined the SOA for which 50% of the trials were veridically perceived as two. While performing the task, we measured brain activity with magnetoencephalography. We localized the primary somatosensory cortex (S1). In S1 sensors, we determined individual gamma power peaks (3 subjects excluded for showing no gamma power peaks). We divided all trials in 5 bins with respect to prestimulus alpha power, or with respect to poststimulus gamma power. For each bin, we determined average gamma power and subjects’ responses. Finally, we performed linear or quadratic regression analyses. Regression analysis demonstrated a quadratic relationship between prestimulus alpha and poststimulus gamma power (r = 0.99, p 0.05 ). That is, trials with high and low prestimulus alpha power showed high poststimulus gamma power. With mid prestimulus alpha power, poststimulus gamma power was lowest. While alpha power significantly negatively correlated with perception ( Baumgarten et al., 2014 ), we did not find a direct significant correlation between gamma power and perception. We found, however, a relation between gamma power and perception when taking prestimulus alpha power into account: If prestimulus alpha power was low, poststimulus gamma power correlated positively with perception. In contrast, if prestimulus alpha power was high, poststimulus gamma power correlated negatively with perception. In other words, whether gamma power was associated with more veridical or erroneous perception depended on prestimulus alpha power. These results are contrary to our hypothesized linear relationship between alpha and gamma power. Gamma power was high for either veridical or erroneous responses. High gamma power might account for subjects’ having come to a definite decision. Low gamma power might account for more ambiguous perception.