Gender differences in the mu rhythm of the human mirror-neuron system

Abstract

Background: Psychologically, females are usually thought to be superior in interpersonal sensitivity than males. The human mirror-neuron system is considered to provide the basic mechanism for social cognition. However, whether the human mirror-neuron system exhibits gender differences is not yet clear.

Methodology/principal findings: We measured the electroencephalographic mu rhythm, as a reliable indicator of the human mirror-neuron system activity, when female (N = 20) and male (N = 20) participants watched either hand actions or a moving dot. The display of the hand actions included androgynous, male, and female characteristics. The results demonstrate that females displayed significantly stronger mu suppression than males when watching hand actions. Instead, mu suppression was similar across genders when participants observed the moving dot and between the perceived sex differences (same-sex vs. opposite-sex). In addition, the mu suppressions during the observation of hand actions positively correlated with the personal distress subscale of the interpersonal reactivity index and negatively correlated with the systemizing quotient.

Conclusions/significance: The present findings indirectly lend support to the extreme male brain theory put forward by Baron-Cohen (2005), and may cast some light on the mirror-neuron dysfunction in autism spectrum disorders. The mu rhythm in the human mirror-neuron system can be a potential biomarker of empathic mimicry.

Figure 1. Conjecture score of each displayed hand sex between females and males.

The conjecture between female and male participants appears similar (P>0.5). The significant differential scoring across each displayed hand sex (P<0.05) confirms the effective manipulation of the perceived hand sex.

Figure 2

A. The frequency power spectrum induced by the visual stimuli across three conditions. At the Baseline (in blue), only the mu (∼10-Hz) rhythm strongly rebounded after the visual stimuli given. At the viewing conditions, this ∼10 Hz post-stimulus rebound suppresses to a degree. Of note, the female relative to the male participant displayed stronger mu suppressions to watch the hand actions (Hand, in red) whereas both of them showed similarly trivial suppressions to observe the moving dot (Dot, in green). B. Significant interaction between the condition and the gender. The interaction is mainly driven from the differential mu suppression between the female and male participants to watch the Hand stimuli (P = 0.04). Females showed stronger mu suppressions to watch the hand actions than males. Instead, the Dot observation induced similarly trivial mu suppressions (P = 0.85).

Figure 3

A. Topography from the representative female and male subjects during the Hand and Dot. Watching the hand actions suppresses the mu rhythm over sensorimotor areas (C3, Cz, C4) apparently more in the female whereas watching a moving dot tends to suppress more in the male. B. Sensorimotor cortex origin of the mu rhythm.

Figure 4

A. Positive correlation. The personal distress subscale of the interpersonal reactivity index (IRI) positively correlates with the mu suppression during the observation of hand actions. B. Negative correlation. The systemizing quotient (SQ) negatively correlates with the mu suppression during the observation of hand actions.