From wikipedia:
"In 2012, Case and Ramachandran reported the results of a survey of bigendered individuals who experience involuntary alternation between male and female states. Case and Ramachandran hypothesized that gender alternation may reflect an unusual degree (or depth) of hemispheric switching, and the corresponding suppression of sex appropriate body maps in the parietal cortex. They stated that "we hypothesize that tracking the nasal cycle, rate of binocular rivalry, and other markers of hemispheric switching will reveal a physiological basis for AGI individuals' subjective reports of gender switches...We base our hypotheses on ancient and modern associations between the left and right hemispheres and the male and female genders.""