Gender differences of cortical neuroplasticity in humans, as revealed by transcranial direct current stimulation

Introduction: It is a well-known fact that brain functions differ gradually between males and females, as shown by certain aspects of cognitive performance and the susceptibility to develop certain neurological diseases. However, the neuronal foundations underlying these differences are still not well understood. We aimed here to explore gender differences of neuroplasticity in the human motor cortex, as induced by transcranial direct current stimulation (tDCS). tDCS induces changes of motor cortical excitability both during and after stimulation. The effects depend on stimulation polarity: anodal tDCS enhances cortical excitability and cathodal stimulation diminishes it. The after-effects last for up to one hour after the application of tDCS for about ten minutes. Methods: The data collected from previously conducted motor cortex tDCS studies, where excitability changes were monitored by single-pulse transcranial magnetic stimulation (TMS), were re-analyzed retrospectively. tDCS protocols eliciting excitability modulations during stimulation, but no after-effects, as well as protocols inducing long-lasting after-effects were included. Results: During a short DC stimulation, which elicits no after-effects, the female group showed more inhibition. Similarly, in women the excitability-diminishing after-effects of cathodal tDCS were relevantly prolonged, as compared to the male group. In contrast, no significant differences between male and female subjects were revealed for the results of excitability-enhancing anodal tDCS. Conclusions: These results suggest gender differences, possibly due to the effects of sex hormones, in the modulation of human cortical plasticity, as elicited by tDCS. Prospective studies, in which development of neuroplasticity is systematically related to different phases of the ovarian cycle, are needed to explore the relationship of hormone level and neuroplasticity.