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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Arch Gen Psychiatry. Author manuscript; available in PMC Apr 8, 2014.
Published in final edited form as:
PMCID: PMC3979588
NIHMSID: NIHMS565359

Increased fronto-temporal activation during pain observation in sexual sadism: Preliminary findings

Abstract

Context

Sexual sadism is a psychiatric disorder in which sexual pleasure is derived from inflicting pain, suffering, and/or humiliation on others. While the psychological and forensic aspects of sexual sadism have been well-characterized, little is known about the neurocognitive circuitry associated with the disorder. Sexual sadists show increased peripheral sexual arousal when observing other individuals in pain. The neural mechanisms underlying this unusual response are not well understood. We predicted that sexual sadists, relative to non-sadists, would show increased responses in brain regions associated with sexual arousal (amygdala, hypothalamus, ventral striatum) and affective pain processing (anterior cingulate, anterior insula) during pain observation.

Objective

To study the neural correlates of pain observation in sexual sadists and non-sadists.

Design

Case-control, cross-sectional study. Sexual sadists and non-sadists viewed 50 social scenes, 25 which depicted a person in pain (e.g., one person stabbing another person’s hand with scissors) and 25 thematically matched no-pain pictures (e.g., one person stabbing a table with scissors with another person’s hand nearby). Pain severity ratings (0 = none, 4 = severe) were acquired following each picture presentation.

Setting

Sand Ridge Secure Treatment Center, Mauston, Wisconsin.

Participants

15 violent sexual offenders; eight sexual sadists and seven age, IQ, and education-matched non-sadists (defined by the Severe Sexual Sadism Scale).

Main Outcome Measures

Hemodynamic response revealed by functional magnetic resonance imaging (fMRI), and pain severity ratings.

Results

Sexual sadists, relative to non-sadists, showed greater amygdala activation when viewing pain pictures. They also rated pain pictures higher on pain severity than non-sadists. Sexual sadists, but not non-sadists, showed a positive correlation between pain severity ratings and activity in the anterior insula.

Conclusions

These results provide neurobehavioral evidence of unusually heightened sensitivity to the pain of others in sexual sadists.

Keywords: Sexual sadism, pain, fMRI, amygdala, anterior insula

Sexual sadism is a psychiatric disorder in which sexual pleasure is derived from inflicting pain, suffering, and/or humiliation on others. The prevalence of sexual sadism is unknown, and estimates have varied widely, from 5–11% to 45–50% of sexual offenders1,2,3,4. Sexual sadism is highly relevant to the assessment of sexual offenders, and may be underdiagnosed in forensic settings5.

While the psychological and forensic aspects of sexual sadism have been well-characterized6,7, little is known about the neurocognitive circuitry associated with the disorder. Sadists convicted of sexual assault have shown impairment on some neuropsychological tests (e.g., speech perception and trailmaking subtests of the Halstead-Reitan test)8, and computed tomography (CT) scans have shown a significant dilation of the right temporal horn in sadists relative to non-sadists9,10,11. Garnett et al.12 recorded positron emission tomography (PET) in one sadist and two non-sadists during the presentation of erotic and neutral auditory stimuli, and reported right lateralized activity during both stimuli in non-sadists, whereas the sadist showed more bilateral activity. However, this finding has not been replicated in a larger study.

The defining characteristic of sexual sadism is the derivation of sexual pleasure by inflicting pain and suffering upon others. Sadists are also aroused by passively watching others in painful situations. Seto et al.13 used penile plesthymography (PPG) to record penile tumescence in sexual sadists and controls while they viewed stimuli depicting physical injury caused to others. Relative to controls, sadists had increased PPG responses. The increased responses occurred to sexual and non-sexual stimuli, and consenting and non-consenting scenarios. The neural mechanisms underlying this unusual response are not well understood. Studies comparing individuals with sadomasochistic interests to those without have reported greater neural and hemodynamic activity in regions of the frontal and temporal cortex14,15 to pictures with sadomasochistic content. However, the participants in these studies did not meet DSM-IV criteria for sexual sadism because they preferred consensual sadomasochistic activities and did not derive sexual pleasure from victimizing others. Furthermore, the studies included individuals with sadistic and masochistic interests in the same group. Thus the brain mechanisms related to sexual sadism remain relatively unknown.

The lack of neuroimaging studies of sexual sadism may reflect the reluctance of sadists to volunteer for community research, due to the risk of drawing attention to their sexual preferences and potential risk for criminal behavior. It has previously been challenging to conduct neuroimaging research in forensic settings, where the prevalence of sadism is 50 times higher than non-forensic settings16, because they do not contain imaging facilities. Here we used a mobile MRI scanner, which was transported to a sex offender treatment facility, to conduct the first (to our knowledge) fMRI study of sexual sadists convicted of a violent sexual offense and committed for treatment. The comparison group of non-sadists were also convicted violent sexual offenders. This ensured groups did not differ in deviant sexual behavior, violence, incarceration-related stress, and psychological characteristics that may be more prevalent in violent criminals such as psychopathic personality.

To investigate the neural mechanisms underlying pain observation in sexual sadists and non-sadists, participants were scanned using fMRI while they viewed picture clips depicting the intentional infliction of pain upon others (e.g. one person slamming a door on another person’s hand), and matched clips depicting non-painful situations (e.g., one person slamming a door shut with another person’s hand nearby), and rated the ‘pain severity’ of each clip. The task was a modified version of a similar task that has been used in healthy controls17, and engages frontal and temporal brain regions including the anterior midcingulate cortex and the anterior insula. These regions are involved in ‘affective pain’ processing, or emotions evoked when observing someone in pain18. Our prediction was that sadists would show increased hemodynamic activity in these regions when viewing pain clips, reflecting heightened sensitivity to others’ pain. We further predicted that sadists would show increased activity in brain regions implicated in sexual arousal, including the amygdala, hypothalamus, and ventral striatum 19,20,21. We also conducted a parametric modulation analysis of pain severity ratings. This identifies brain regions whose activity while viewing pain clips is correlated with the subsequent severity rating. Because the anterior insula is involved in subjective emotion (awareness of own emotional states)22, we predicted that sadists would show a positive correlation with severity ratings in this region. Functional connectivity was also compared across groups using a psychophysiological interaction analysis, which investigates the covariation between activity in a selected brain region and other regions by experimental condition. We predicted that sadists would show greater connectivity between brain regions involved in the perception of others’ pain (anterior cingulate, anterior insula) and those involved in sexual arousal (amygdala, hypothalamus, ventral striatum) during pain observation.

Method

Participants

17 adult male volunteers were recruited from the Sand Ridge Secure Treatment Center, which provides treatment for persons committed under Wisconsin’s sexually violent persons law. Exclusion criteria were: age younger than 18 or older than 65, non-fluency in English, reading level lower than 4th grade, IQ score lower than 75, seizure history, current DSM-IV Axis I diagnosis23, lifetime psychotic disorder in self or first-degree relative, current alcohol or drug use. Information was obtained via self-report and institutional file review1. All participants were right handed except one sexual sadist who was ambidextrous. All had committed at least two violent sexual offenses. Most committed between two and 10, though one sadist and one non-sadist claimed too many to recall. Each offender’s victims were primarily adult females except one non-sadist whose victims were adolescents and children (his file indicated no diagnosis of pedophilia or hebephilia), and one sadist whose victims were male. Except one non-sadist (the participant with child/victims), none were incest offenders.

Assignment to sadist and non-sadist groups was based on scores from the Severe Sexual Sadism scale24, an 11-item scale rated according to history of sexual behavior. Items are rated yes (1 point) or no (0 points) and include: 1) Engages in gratuitous violence toward or wounding of victim, 2) Exercises power/control/domination over victim, 3) Humiliates/or degrades victim, 4) Is sexually aroused by the act, 5) Tortures victim or engages in acts of cruelty to victim, 6) Evidence of ritualism in offense, 7) Victim is abducted/or confined, 8) Inserts object(s) into victim’s bodily orifice(s), 9) Mutilates sexual parts of victim’s body, 10) Mutilates nonsexual parts of victim’s body, 11) Keeps trophies (e.g., underwear, ID) of victim or keeps records of the offense. To meet sexual sadist criteria an offender’s behavior must include at least four items, three which must be 2–5 or 925. These criteria distinguished between 100 sexual sadists and non-sadistic sex offenders (diagnosed using the ICD-10 and/or the DSM-IV) with 100% accuracy24, and the SSS has shown high reliability (inter-rater reliability across all items: κ = .86, range .65 – 1) and structural validity (coefficient of scalability: H = .83). In contrast, DSM and PPG-based diagnoses have shown lower reliability26,6,7. Information was obtained via institutional file review (family, education, social, and criminal history), and supplemented by interviews (see below). Each participant was assigned to the sadist or non-sadist group, except one participant who met criteria for five SSS items but less than three were items 2–5 or 9. There were also conflicting sadism evaluations in his file. To ensure a clear separation of sadists and non-sadists, this participant was excluded. The sadists’ SSS scores (N = 8) ranged from 5–7, slightly lower than the range of a large sample of sadists residing in forensic institutions (6–8)24. The non-sadists’ scores (N = 8) ranged from 1–3 (Table 1). All sadists except one had a DSM-IV sadism diagnosis listed in their institutional files.

Table 1
Participant Characteristics*

One sexual sadist had comorbid paraphilias (e.g., voyeurism, fetishism). One non-sadist had exhibitionism; however, this participant was excluded due to motion in the MRI. Four sadists and two non-sadists were diagnosed with paraphilia not otherwise specified (non-consent). No other comorbid paraphilias were present. The sadists did not significantly differ from the non-sadists on age, ethnicity, IQ27, education, or alcohol/drug use28 (Table 1). All participants except two non-sadists were undergoing a 4-phase treatment program and in Phase 1 or 2. Phase level did not differ between groups (p = 0.94), nor did duration (years) of treatment (p = 0.83) or commitment to the facility (p = 0.88).

Several of the core characteristics of sexual sadism, including callousness and low empathy, are also characteristics of psychopathy29. To evaluate psychopathic traits all participants completed the Psychopathy Checklist-Revised (PCL-R)30, which uses a detailed interview and file review to rate 20 psychopathic traits (e.g. grandiosity, remorselessness, impulsivity) on a 0–2 scale. A score of 30 or more indicates high psychopathic traits. PCL-R scores were used to ensure sadists and non-sadists did not significantly differ on psychopathic traits (Table 1).

Participants were paid $7.00/hr., a rate commensurate to pay for work assignments at the facility. Participants provided written informed consent and the study was conducted in accordance with institutional ethical standards.

Stimuli and Task

The fMRI task is shown in Figure 1. Participants viewed 50 dynamic stimuli, which depicted action in a three-picture format timed to mimic natural motion. Each picture series, which lasted for 2.2 seconds, belonged to one of two categories: 1) Two people were shown and one caused pain to the other (e.g., one person stabbing another’s hand with scissors); 2) Two people were shown and one caused no pain to the other (e.g., one person stabbing a table with scissors with another person’s hand nearby). The condition of interest was #1 (pain), with condition #2 (no-pain) matched in thematic content. A third condition was included in which one person caused damage to an object; however this was not of interest in the present study. Overall, 25 non-repeating stimuli were presented in each category. The three-picture trials from each condition were randomly presented, and followed by a rating scale where participants rated the preceding trial on the severity of pain caused to a person. The scale displayed a red bar that began at ‘0’ (none) and progressed to ‘4’ (severe) for 4 seconds (Figure 1). The participant pressed a button to stop the bar when it reached their chosen rating. This format was used for simplicity (pressing one button rather than several)2. A 4-second delay then preceded the next picture. Picture trials were randomly interspersed with fixation trials of the same duration, which were analyzed as a no-picture baseline. This created variable rest periods (10.2, 20.4, or 30.6 seconds when a picture trial was followed by 1, 2, or 3 fixation trials) which induced jitter. The 100 total trials (25 pain, 25 no-pain, 25 damage, 25 fixation) were presented across two separate runs lasting ~ 9 minutes each (50 trials per run). Images were projected into the scanner using an LCD projector, controlled by a PC computer. Tasks were presented and responses recorded using Presentation (http://nbs.neuro-bs.com).

Figure 1
Example pain and no-pain picture set, and pain rating.

MRI Data Acquisition and Analysis

MR images were collected with a mobile Siemens 1.5T Avanto with advanced SQ gradients (max slew rate 200T/m/s; 346 T/m/s vector summation, rise time 200us) equipped with a 12-element head coil. The EPI gradient-echo pulse sequence (TR/TE 2000/39 ms, flip angle 90°, FOV 24×24 cm, 64×64 matrix, 3.4×3.4 mm in plane resolution, 5mm slice thickness, 30 slices) covers the entire brain (150 mm) in 2.0 seconds. Head motion was limited using padding and restraint. Any participant with motion greater than 6mm was not analyzed, resulting in the exclusion of one non-sadist.

Functional images were analyzed using Statistical Parametric Mapping (SPM5). Images were realigned using a motion correction algorithm unbiased by local signal changes31,32. Motion parameters (3 translation; 3 rotations) were entered as covariates of no interest in the model to regress motion-related variance. Functional images were spatially normalized to the MNI template33 and smoothed (8mm FWHM). High frequency noise was removed using a low pass filter (cutoff 128s). All trial types (pain, no-pain, damage, fixation) were modeled as separate events with the canonical hemodynamic response function (2.2-second duration). The rating period was modeled as one regressor for all ratings (4-second duration).

Functional images were computed for each participant that represented hemodynamic responses associated with viewing pain pictures, no-pain pictures, or no pictures (fixation). Group differences were analyzed using a 2 (Sexual sadist/Non-sadist) × 3 (Pain/No-Pain/Fixation) ANOVA in SPM5, which included between and within-participant effects.

Hemodynamic responses associated with pain severity ratings were also analyzed using a parametric modulation analysis in SPM5, in which the participant’s ratings of each picture were entered as covariates in the first-level analysis. A functional image was computed for each participant that represented the correlation between brain activity and severity ratings across all pain pictures. No-pain pictures were not included, since nearly all were rated ‘1’ on severity by participants. This analysis determined whether increased activity in any brain regions during picture viewing was associated with higher (positive modulation) or lower (negative modulation) severity ratings. Including each participant’s ratings in the first-level analysis also controlled for potential associations between different ratings across participants and brain activity in main effect analyses (e.g. pain vs. no-pain). One-sample t-tests were conducted in each group to assess whether positive or negative modulatory effects were present in any brain regions. To determine whether modulatory effects differed across sadists and non-sadists, a two sample t-test was conducted on the parametrically modulated images.

A psychophysiological interaction (PPI) analysis was also conducted in SPM5. This identifies changes in the coupling between activity in brain regions across different conditions. The maximally activated cluster in the left amygdala was entered as the seed region. The analysis produced statistical maps displaying brain regions whose activity changed significantly with the time course in the amygdala during the pain, no-pain, and fixation conditions. These maps were compared across groups using two sample t-tests.

For all analyses we defined regions of interest related to affective pain processing, including bilateral anterior midcingulate cortex and anterior insula, using central coordinates of these regions previously identified by a meta-analysis of fMRI studies examining pain observation 18. We also defined regions of interest related to sexual arousal, including bilateral amygdala, hypothalamus, and ventral striatum, by averaging coordinates from fMRI studies of sexual arousal19,20,21. Eight-mm radius spheres were defined around the center coordinate of each region, and corrected with a family-wise error (FWE) threshold of p < .05 using small volume correction in SPM5. Whole-brain analyses were also conducted to examine activation in other regions during pain observation (thresholded at p < .05, corrected, 10 contiguous voxels), and differential effects in sadists and non-sadists (thresholded at p < .001, uncorrected, 10 contiguous voxels). Activations were overlaid on a high-resolution structural T1-weighted image from the SPM5 canonical image set, coregistered to Montreal Neurological Institute (MNI) space. All coordinates are reported in MNI space.

Results

Pain Severity Ratings

A Group (Sadist/Non-sadist) × Condition (Pain/No-pain) ANOVA was used to assess group differences in pain severity ratings. A Group × Condition interaction was found (F(1,6) = 8.24, p < .03), indicating higher ratings of pain pictures given by sexual sadists relative to non-sadists (F(1,14) = 7.79, p < .02; Figure 2). No group differences were found for no-pain pictures (F(1,14) = .035, p = .86). Both groups rated pain pictures higher than no-pain pictures (Sadists: F(1,7) = 916.62, p < .0001; Non-sadists: F(1,6) = 47.95, p < .0001).

Figure 2
Severity of pain ratings by condition in sexual sadists and non-sadists. Bars = standard error.

Brain Activity during Pain Picture Viewing

The Group (Sadist/Non-sadist) × Condition (Pain/No-pain/Fixation) analysis revealed a main effect of group in the ventral striatum (Table 2). Sadists, relative to non-sadists, showed greater hemodynamic responses to pain pictures (t(14) = 3.53, p = .024, x = −9, y = 15, z = 6) and no-pain pictures (t(14) = 3.50, p = .026, x = −6, y = 9, z = 9), but not during fixation. An interaction was present in the left amygdala (Table 2), indicating greater responses to pain vs. no-pain pictures in sadists (t(7) = 3.49, p = .041, x = −18, y = 6, z = −18) but not non-sadists (Figure 3). When the pain condition was examined without reference to the no-pain condition, sadists again showed increased amygdala response relative to non-sadists (t(6) = 3.28, p = .04, x = −18, y = 6, z = −18). An interaction was also present in the right temporo-parietal junction (Table 2, Figure 4). Sadists, relative to non-sadists, showed increased temporo-parietal activity while viewing both pain and no-pain pictures. Non-sadists did not show greater responses relative to sadists in any condition.

Figure 3
Interaction in left amygdala, representing increased response during pain vs. no-pain picture viewing in sexual sadists but not non-sadists. Bars = standard error.
Figure 4
Interaction in right temporo-parietal junction, representing increased response during pain and no-pain picture viewing in sexual sadists but not non-sadists. Bars = standard error.
Table 2
Brain Regions Showing Differential Response to Pain Pictures in Sexual Sadists and Non-sadists

Brain Regions Modulating Pain Ratings

One sample t-tests conducted on the parametric modulation results for each group separately revealed a significant positive modulation in the left anterior insula in sexual sadists, indicating that increased activity in this region during pain picture viewing was associated with higher pain severity ratings. A similar effect was found in the right anterior insula at a reduced statistical threshold (p < .001, uncorrected). This effect was not present in the non-sadists, and the between-group difference was significant (Table 3, Figure 5). Non-sadists showed a greater modulation in the a more dorsal region of the left insula as well as the left parahippocampal gyrus compared to sadists, but these effects were not explained by a significant positive modulation in the non-sadists or a significant negative modulation in the sadists (Table 3).

Figure 5
Positive association between left insula activity and pain severity ratings in sexual sadists (A), and greater when compared to non-sadists (B).
Table 3
Brain Regions Associated With Severity of Pain Ratings

Psychophysiological Interaction

The PPI results showed greater functional connectivity between the left amygdala and the right anterior insula in sadists relative to non-sadists during pain versus fixation (t(13) = 3.96; p = .056, x = 39, y = 21, z = 3). Functional connectivity did not differ across groups during the pain versus no-pain and no-pain versus fixation conditions.

Discussion

This study explored whether sexual sadists differ from non-sadists in neural systems underlying pain observation. Consistent with hypotheses, sadists showed increased activity in the left amygdala in response to pain relative to no-pain pictures, whereas non-sadists did not. Sadists rated pain pictures higher on pain severity than non-sadists, and showed a positive association between pain ratings and activity in the left anterior insula that was not present in non-sadists. Sadists also showed increased activity in the right temporo-parietal junction when viewing both pain and no-pain pictures relative to non-sadists. Sadists, but not non-sadists, showed greater connectivity between the left amygdala and right anterior insula when viewing pain pictures. These results are indicative of heightened sensitivity to others’ pain in sadists, are consistent with prior studies demonstrating increased sexual arousal during pain observation in sadists, and point to underlying neural mechanisms.

Sexual sadists showed a greater pain vs. no-pain picture distinction in the left amygdala relative to non-sadists. The amygdala has been implicated in sexual arousal in healthy controls20, 21 is more active in men relative to women when viewing erotic stimuli, particularly in the left hemisphere21, and may be related to sexual orientation and regulating sexual behavior34,35. Increased amygdala activity is not typically reported in studies of pain observation in healthy controls (though not all pain observation studies depict pain caused by another person)18, and was not found in our non-sadists. This is consistent with our prediction that only sadists would find pain pictures sexually arousing. It should be noted, however, that the amygdala is also implicated in general positive and negative arousal36, particularly the left amygdala37. It is possible the increased amygdala activity in sadists represents a more general positive emotion than sexual arousal, such as excitement. It is unlikely that pain pictures evoked negative emotions in sadists, given their clinical pathology. It is also worth noting that the study by Seto et al.13, which used stimuli similar to ours, found that sadists showed increased sexual arousal (measured by penile plethysmography) during pain observation. Finally, all sadists had voluntarily consented to treatment, meaning they had previously admitted sexual arousal to causing pain to others.

Sexual sadists also rated pain pictures higher on pain severity than non-sadists. This result is consistent with sadists’ heightened sensitivity to others’ pain. To explore the association between brain activity and pain severity ratings, we conducted a parametric modulation analysis of severity ratings and brain activity in response to pain pictures. Consistent with predictions, sadists showed a positive correlation between pain severity ratings and activity in the left anterior insula, meaning that increased activity was associated with higher subsequent pain ratings. The anterior insula is implicated in the subjective experience of emotion (being aware of one’s own emotional states)22. While both sadists and non-sadists showed increased insula activity when viewing pain pictures, the association between insula activity and pain ratings was present only in sadists. Thus, sadists may draw from subjective emotional experience when evaluating other’s pain severity. Non-sadists showed a significant modulation in a more dorsal region of the left anterior insula which is implicated in executive attention and cognitive control38. The parametric modulation analysis cannot determine whether associations between brain activity and pain ratings are causal. The association may occur because subjective emotional experience influenced the subsequent rating, or they may occur because the pain rating influenced the emotional experience.

Our prediction that sexual sadists would show greater activity relative to non-sadists in the anterior cingulate was not confirmed. Both groups showed increased anterior cingulate activity during the pain and no-pain conditions. This may reflect non-specific effects in the anterior cingulate related to picture viewing39,40. Another possibility concerns the anterior cingulate’s demonstrated involvement in pain anticipation41,42. The close thematic matching of the pain and no-pain pictures likely engenders pain anticipation in the both conditions, since each series consisted of three pictures leading up to the pain/no pain event in the third picture. Sadists may have additionally experienced anticipatory arousal, as suggested by their greater response in the ventral striatum during the pain and no-pain conditions. This region has shown selective recruitment during reward anticipation43. In contrast, sadists’ heightened amygdala activity to pain but not no-pain pictures suggests specificity to pain outcome, a critical point given that sadists may be aroused by other sadistic activities such as control or domination. A person using a knife to strike a table with someone’s hand nearby (no-pain) rather than directly striking the person’s hand (pain) would still be considered the ‘dominant’ actor in the scenario. The association between left insula activity and pain ratings in the sadists also appears specific to pain outcome, because no such association was present in the no-pain condition.

Sexual sadists also showed increased right temporo-parietal activity relative to non-sadists in response to both pain and no-pain pictures. The role of the temporo-parietal junction in mentalizing (attribution of mental states such as beliefs and intentions to others), is well established44. However, studies have highlighted other functions of this region. It has been implicated in empathy, agency, self-other discrimination, and redirecting attention45,46. Decety and Lamm45 proposed that the right temporo-parietal junction is associated with multiple lower-level processes that contribute to higher-level functions including mentalizing. An explanation that encompasses all of these proposals may lie in the fact that sadists showed increased activity to both pain and no-pain pictures. As discussed above, the close thematic matching of the pain and no-pain pictures likely engenders pain anticipation during no-pain pictures. In any scenario where pain is imminent, sadists may pay closer attention than non-sadists to the thoughts and feelings of the victim, because this enhances their sexual arousal when pain is inflicted. In other words, whereas sadists lack sympathy for their victims, they may exhibit empathy (simulating their victims’ feelings) when consistent with their goals.

Relative to non-sadists, sexual sadists showed greater functional connectivity between the left amygdala and right anterior insula during pain observation. This result is consistent with our predictions and suggests covariation between the perception of others’ pain and sexual arousal in sadists but not non-sadists. The difference in connectivity was significant between pain pictures and baseline (fixation), but not between pain and no-pain pictures. Thus the group difference in connectivity might include pain perception and pain anticipation. It is important to note that these results do not indicate causation. It is possible that increased insula activity represents the detection of others’ pain and feeds forwards information to the amygdala, resulting in increased sexual arousal, but the opposite (or both) may also occur.

It is interesting to consider whether the neural abnormalities that characterize incarcerated sexual sadists generalize to individuals in the community with sadistic sexual preferences. Sadists who offend may differ from those who do not in early environment (e.g. abuse, inadequate social and family environment)7. Individuals involved in consenting sadomasochistic relationships are also different from criminal sadists, because criminal sadists do not generally engage in sadomasochism, nor do those in sadomasochistic relationships generally victimize others47. Nonetheless, all experience sexual arousal to pain infliction. Thus, whether the present results generalize to all sadists is a question for future research.

Limitations of this study should be noted. First, all participants were from an antisocial/incarcerated population. Although the study results were consistent with those that have been demonstrated in healthy populations (e.g., increased anterior cingulate and anterior insula activity in response to pain pictures), it will be important to directly compare these populations in future studies. Second, our exclusion criteria and controls for potentially confounding variables across sadists and non-sadists (e.g. IQ, comorbid psychiatric conditions) resulted in a relatively specialized sample of sadists. We note, however, that only two sadists were excluded from the study (one due to reading level and one due to MRI incompatibility), and the remaining sadists and non-sadists were matched on all relevant variables with no specific matching efforts (e.g. further exclusions) necessary. Nevertheless, the sample size was small and the results should be replicated in additional samples to help generalize the results to the larger population of sexual sadists. Third, we did not collect self-report ratings regarding sexual arousal to the pictures because we wanted to get a spontaneous reaction from them rather than one which was complicated by their conscious considerations of the degree of sexual arousal they were experiencing (especially if they made responses that did not truly reflect what they were feeling). Although all sadists previously admitted sexual arousal to causing pain to others in general, it will be helpful in future studies to assess this during scanning. Finally, the participants were relatively old in age and had resided in a treatment facility for several years. Although there are no published studies similar to ours in younger and/or non-institutionalized populations to compare our results, it is possible we would have observed even stronger group differences in offenders who were temporally closer in proximity to their most recent sexual offense.

It is important to note that the results of the present study may not indicate inherent abnormalities (e.g. structural) within these brain regions in sexual sadists, but may reflect context-dependent engagement of brain regions that differs across groups. Furthermore, the results could reflect atypical brain mechanisms for pain processing that result in sadism, or sadism could result in atypical recruitment of regions involved in pain processing. These questions can be addressed in future studies using anatomical MRI and longitudinal studies of the development of sadism over time.

Acknowledgments

Funding Support: MIND Research Network grant funded by the Department of Energy (C.H.); NIDA grant R01DA026505 (K.K.)

We thank Christopher Lee, Kristin Macias, Daniel Valenti, Whitney Schulte, and Justin Jobelius for assistance with data collection and scoring. We also thank the staff of the Sand Ridge Secure Treatment Center in Mauston, WI, and the residents who volunteered for the study. We also thank Dr. James Cantor for helpful comments on the manuscript.

Footnotes

1One sexual sadist had conflicting reports of psychotic history in his institutional file, including schizoaffective disorder, psychotic disorder NOS, personality disorder NOS, or sexual sadism only, on separate evaluations by different clinicians. The most recent psychiatric assessment in the participant’s file indicated no history of psychotic disorder. His file also indicated that he had malingered on one of the cognitive tests, and in one evaluator’s opinion he was also malingering psychosis to avoid spending time in a maximum security institution and obtain preferential treatment from staff. Confidential research interviews with the participant indicated no past or present psychosis. Thus, the participant was not excluded from the study.

2The continuous presentation format of the rating scale could affect the ratings of individuals who did not fully attend to the stimuli. In other words, a higher rating could be given because the participant was slow to respond rather than intending to give a high severity rating. Thus, responses were not accepted after the bar reached ‘4’. If a participant was not paying attention during the task, they should have many ‘missed’ ratings. Participants who missed more than 7 out of the 75 pictures (10%) would be excluded from analysis. No participant had this many missed ratings.

The authors do not have any financial or personal conflicts of interest relevant to the content of this manuscript.

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