Motor-evoked potentials as biomarkers for sexual arousal?

Background: Motor cortex excitability may represent the neuronal endpoint of motivational processes and was shown to be modulated by both sexual arousal and deceptive behavior.

Aim: This is the first study to investigate the influence of lying and sex in heterosexual women and men based on motor-evoked potentials (MEPs) measured while viewing sexually arousing pictures.

Methods: Sixteen heterosexual couples were shown 360 trials consisting of pictures displaying both almost naked females and males and neutral control images. In a subsequent forced-choice question about wanting to see the respective pictures fully naked, they were instructed to either answer in agreement with or opposite to their sexual preference. Participants went through 2 blocks of answering truthfully and 2 blocks of lying, with these 4 blocks being shown in a randomized alternating order.

Outcomes: To measure cortical excitability, MEPs were used, evoked by single transcranial magnetic stimulation pulses between image presentation and response.

Results: In normalized MEPs, women and men showed higher amplitudes for preferred over non-preferred sexual stimuli, but only on a descriptive level. Planned contrasts showed higher non-normalized MEPs for lying in all picture categories. Direct comparisons to a preliminary study showed overall lower effect sizes.

Clinical implications: Both sexes tend to show higher MEPs in response to their sexually preferred stimuli. MEPs are not stable markers for willful volitionally controlled deception although lying does increase cortical excitability. The present experimental design does not seem valid enough to serve as a diagnostic marker for sexual preference or paraphilia and malingering.

Strengths and limitations: This is the first study investigating whether sexual motivational stimuli modulate MEPs in women, while also examining the influence of lying for both sexes. The sample was too small for some found effects to be significant. Also, the experimental setup may have been less suited for female participants in comparison to male ones.

Conclusion: The operationalization of sexual motivation via MEPs seems to highly depend on different experimental factors including the sex of the participants, induced motivation, and lying.