Power modulation of electroencephalogram mu and beta frequency depends on perceived level of observed actions

Abstract

Introduction: The ability to understand actions and intentions of others is of great importance to social relationships and is associated with the mirror neuron system of the human brain. Whether conscious perception of specific actions is necessary to trigger activity in this system, or alternatively whether this response is independent of conscious perception is not known.

Methods: We addressed this issue by rendering videos of right hand movements invisible to conscious perception, and measuring electroencephalogram (EEG) power suppression in the mu (8-13 Hz) and beta (15-25 Hz) range as index corresponding to the magnitude of mirror neuron activity.

Results: In the beta range over bilateral sensorimotor sites, we find that suppression indices follow the reported perceptual level of subjects with stronger suppression for consciously perceived trials. Furthermore, in the nonperceived trials, oscillation power is significantly suppressed relative to baseline. In the low mu range (8-10 Hz), oscillation power over the left sensorimotor site is significantly more suppressed in the consciously perceived versus nonperceived trials.

Conclusions: Our data suggest that the intensity of mirror system responses during action observation decreases with the observers' perception level yet remains significant during observation of invisible actions. Such subliminal activity could help explain phenomena such as covert imitation.

Keywords: Action observation; conscious perception; electroencephalogram; mirror neuron system.

Figure 1

Experimental Design. During each trial two separate video clips were presented in an interleaved fashion such that odd frames (corresponding with clip 1) were presented to one eye, and even frames (corresponding with clip 2) were presented to the other eye. Clip 1 consisted of 3 sec masking video, whereas clip 2 consisted of a black screen for 1 sec followed by 2 sec of a target video. The first second of each trial was used as baseline for the analysis. At the end of each trial, participants reported which action was presented and their confidence level on a scale from 1 to 4.

Figure 2

Behavioral Results. Correct reports corresponded to higher confidence levels, whereas incorrect reports corresponded to lower confidence levels (Error bars represent Standard Deviation).

Figure 3

Mu and Beta suppression in Central sites (N = 17). Mu and Beta oscillation power for the “Perceived” and “Nonperceived” trials over the left (“C3”) and right (“C4”) sensorimotor cortices in the range of beta (15–25 Hz), high mu (11–13 Hz), and low mu (8–10 Hz). (A) Event‐Related Spectral Perturbations representing changes in oscillation power locked to target display (time 0 ms) relative to baseline (−500 to 0 ms). (B) Stronger suppression for perceptually perceived trials was found in bilateral beta and left low mu frequency ranges. Additionally, power in the beta range in both hemispheres was significantly suppressed in the Nonperceived trials relative to baseline (*P < 0.05 corrected, Error bars represent Standard Error).

Figure 4

Alpha and Beta suppression in Occipital sites (N = 17). Alpha and Beta oscillation power for the “Perceived” and “Nonperceived” trials over the left (“O1”) and right (“O2”) visual cortices in the range of beta (15–25 Hz), high alpha (11–13 Hz), and low alpha (8–10 Hz). (A) Event‐Related Spectral Perturbations representing changes in oscillation power locked to target display (time 0 ms) relative to baseline (−500 to 0 ms). (B) Stronger suppression for the perceptually perceived trials was found only in the low alpha frequency range over O1 (and marginally significant in O2, see text). Power in the beta frequency was significantly suppressed in the nonperceived trials relative to baseline (*P < 0.05 corrected, Error bars represent Standard Error).

Figure 5

Power suppression in low Mu\Alpha and Beta (N = 11) and perception level. Low mu (8‐10 Hz) and beta (15‐25 Hz) oscillation power for the “Full”, “Partial”, and “None” perception level trials over the left (in dark) and right (in light) sensorimotor and occipital cortices. The graphs represent the changes in averaged suppression index across subjects at each of the three perception levels. A significant linear trend was found only for the averaged suppression index in the central sites at the beta range (*P < 0.05, Error bars represent Standard Error).