Stranger rape - is it a strategy to avoid female erotic signals? An fMRI and parallel penile plethysmography study (preliminary results)
Abstract
Introduction: Some instances of rape are marked by attacking stranger victims without any preparatory erotic interaction. It points to a possibility, that this type of rape is a strategy to avoid female pre-copulatory erotic signals.
We hypothesized, that showing a female naked body and accessible genitalia in the context of her flirtatious and/or lustful behaviour will fail to elicit strong penile reaction in rapists. On the brain level, we hypothesized that the pattern of reactivity of the key brain areas will differ between the tested groups.
Methods: Subjects: 12 in-patients with a history of hands-on sexual assault against a stranger and 26 control heterosexual men. Stimuli: We used sequences of still pictures, showing Flirtatious, Lustful, Neutral, Distressed faces followed by overt genital display and Control (overt genital display without a facial cue) condition.
Data: fMRI data were acquired using Siemens 3 T Prisma device. Data for hypothalamus, amygdala bilat, nucl.accumbens bilat. and orbital cortex bilat. were extracted using Marsbar toolbox for SPM8.
Penile response was measured with the custom-built MR-compatible volumetric device Heinrich SOM-4. Data were z-transformed for each individual and analysed using IBM SPSS Statistics (ANOVA with within and between subject term, full factorial).
Results: Brain response: We found main effect of category in hypothalamus (F4,126 = 2.89, p = 0.031) and significant stimuli*group interaction in the left orbital cortex (F4,133 = 2.55; p < 0.046). Hypothalamus reacted identically in both groups.
Its activation ranked stimuli as follows: flirtatious, lustful, neutral, control and distressed. Left orbital cortex reacted most to the flirtatious and lustful condition and weakly to the distress and neutral stimuli, in controls.
In rapists, it reacted weakly to the lustful condition and strongly to flirtatious, neutral and distress stimuli. Penile response: We found main effect of category only (F3,85 = 8.36; p = 0.000).
Both groups seemed to favour flirtatious and neutral condition. Conclusions: Rapists and controls showed similar hypothalamic reactivity, strongly favoring flirtatious and lustful contexts.
Yet, in rapists, this pattern of hypothalamic activation was not in accord with responses from the orbital cortex. Orbital cortex, which signalizes motivational value, did not activate in response to the lustful context in rapists.
On the other hand, it reacted to contexts which were not favoured by hypothalamus (distress, neutral). Penile reactivity in both groups seemed to reflect hypothalamic activation, at least partly.
Further enlargement of the sample is necessary to bring definitive conclusions.