The following summary was published in the Fall 2008 issue of the ATSA Forum under the title, “MRI research on pedophilia: What ATSA members should know.”
Understanding MRI Research on Pedophilia
James M. Cantor, PhD
In the past two years, researchers have begun studying pedophilic men by using MRI (Magnetic Resonance Imaging), improving our ability to identify brain mechanisms potentially involved in the development of pedophilia. Unfortunately, MRI research has many technical aspects, and research articles using these techniques are written for neuroscientists rather than for front-line service providers or for the public. This situation makes it difficult for policy-makers and clinicians to integrate these state-of-the-art research findings into their work. This article is an effort to bridge that gap and to provide a concise summary of MRI research on pedophilia, including what it has done, what it has the potential to do, and what its limitations are.
“Structural” and “functional” MRI
Several types and subtypes of MRI have been invented, but our purposes here need to refer only to the two major types: The first type allows us to measure the sizes of many (but certainly not all) of the individual structures that make up the brain and is usually called structural MRI. There are a great many medical applications for structural MRIs, and when someone says “MRI,” they generally mean “structural MRI.” Figure 1 shows an example from a structural MRI scan.
The other type of MRI provides information about the activity of the brain and is called functional MRI or fMRI. This type of brain scan is extremely valuable to researchers for identifying which brain regions respond to specific sensory stimulation (such as speech or sexual images), but it has few medical applications at present. These two types of MRI provide very different kinds of information about the brain; one should not consider fMRI to be better than structural MRI merely because it is the more recent invention.
Structural MRI can be thought of as a high-resolution still-picture of the brain, whereas fMRI can be though of as a movie, that is, a series of still pictures taken over time. Of course, when one pauses a movie, one gets a still picture anyway. One of the key differences between an individual frame of an “fMRI movie” and a structural MRI image is that the structural image will be much clearer. A typical fMRI test records a complete image of the brain every 2 seconds or so, whereas a typical structural MRI takes about 10 minutes. The greater time used by structural MRI scans is for recording more detail (i.e., it has greater resolution). Compare Fig. 1 with the fMRI in Fig. 2.
Figure 2. This is a portion of “one frame of an fMRI movie” of Mike Kuban’s brain, as he observed stimuli from the Kurt Freund Laboratory. Note the decrease in clarity of this fMRI image relative to the structural scan in Fig. 1. During an experiment, however, an fMRI such as this one can reveal changes in brain activity, which is invisible to structural MRIs.
Structural MRI studies of pedophilia
So far, three research groups have published
structural MRI studies of pedophiles: my own group at CAMH in
When comparing pedophilic and nonpedophilic men, one must remain careful not to confuse cause with effect. That is, one must consider carefully whether the brain differences we detected cause pedophilia or whether some aspect of being pedophilic caused the brain differences. Previous research findings suggests that it is more likely for the brain differences to be causing pedophilia than for the other way around: Although it is now known that certain brain structures respond to environmental stimulation, such as the motor cortex, there is no evidence that such stimulation causes any changes in the superior fronto-occipital fasciculus or right arcuate fasciculus (the brain regions in which pedophiles and nonpedophiles differ). Moreover, the brain regions we identified are extremely large, and no previous research has ever found changes in such large regions of the brain. As an analogy, physical exercise will generally stimulate one’s muscle tissue to grow, but one would not grow an extra arm; neurological changes occur only in a very specific manner.
The second body of research suggesting that brain differences are not effects of pedophilia is that pedophilic men have up to three times of the odds of being left-handed (or being ambidextrous) than do non-pedophilic men (Bogaert, 2001; Cantor, Blanchard, Christensen, Dickey, Klassen, Beckstead, Blak, & Kuban, 2004; Cantor, Klassen, Dickey, Christensen, Kuban, Blak, Williams, & Blanchard, 2005). Handedness is determined by brain organization and is determined largely before birth (fetuses show a hand-preference for thumb-sucking that is observable on sonograms; Hepper, Shahidullah, & White, 1991; Hepper, Wells, & Lynch, 2005). This indicates that at least some important differences in brain organization existed in pedophilic individuals long before they engaged in any pedophilic behaviors. Thus, although it remains possible that environment influences interacted with the differences in the brain, the simpler interpretation of these findings is that some factor was present, early in development, that affected brain development in a way that caused the handedness differences, pedophilia, and the other brain-related characteristics that have been found in pedophiles (such as lower than average IQs; Cantor, Blanchard, Robichaud, & Christensen, 2005).
There are also important implications of what we did not find. We did not find any group differences in the parts of the brain that are known to relate to self-control or impulsivity. That is, we found no evidence to suggest that pedophilic men have a neurological reason to be less capable of controlling their behaviors than are men who commit nonsexual crimes. Although it is never possible to completely rule out the possibility that pedophiles suffer from an impulsivity problem, our project used very large samples (relative to most MRI research) and powerful statistical techniques. So, if pedophilic men do suffer from a neurological problem in self-control, it is likely to be either small or a type of problem that is invisible to conventional MRI.
Although we found the brain differences to occur in a large proportion of the brain, the magnitudes of the differences were statistically “moderate.” That is, the structural MRI differences were reliable enough for studying groups of pedophiles for research, but they were not reliable enough for the assessment of individuals for clinical purposes. It remains possible, however, that functional MRI could be used clinically to distinguish pedophilic from nonpedophilic men.
Functional MRI studies of pedophilia
In fMRI experiments, researchers typically compare how active that one or more brain regions are under different conditions. (In a structural MRI, the brain looks the same no matter what it is doing.) For example, a study subject might read a brief story, followed by hearing a story over earphones, followed by reading again, and so on. In such an experiment, the language areas of the brain would be active during both conditions (reading stories and listening to stories both involve the processing of language); however, as the subject goes back and forth between reading and listening to stories, the levels of greatest brain activity would switch back and forth between the brain regions involved in visual processing and in auditory processing. By looking at which regions of the brain are active, neuroscientists can distinguish between when the brain is engaged in reading and when it is engaged in listening.
Beginning in the late 1990’s, researchers have started demonstrating that fMRI could be used to detect which brain areas become active when study subjects are seeing stimuli depicting sexual activity versus nonsexual activity (Mouras & Stoléru, 2007; Stoléru & Mouras, 2007). That is, a certain set of brain regions become active when a study subject is perceiving something that is sexually arousing, but remains relatively inactive when the study subject is perceiving something that is not sexually arousing. Gay men show activation in these brain areas when they see depictions of male models (and not female models), whereas heterosexual men show those activations when they see depictions of female models (and not male models) (Safron, Barch, Bailey, Gitelman, Parrish, & Reber, 2007).
This suggests the possibility that fMRI could be used to develop a test for distinguishing pedophilic men from nonpedophilic men. That is, one would reasonably hypothesize that the relevant brain regions of pedophilic men would respond when they are shown stimuli depicting children, but not when they are shown stimuli depicting adults. Some fMRI experiments have already been conducted with pedophilic men, but they were not designed for diagnostic purposes. Nonetheless, these studies demonstrate that the technology already exists for creating a diagnostic test. Although there are a great many technical challenges to conducting such research, I feel confident in predicting that we will see such studies coming out within the next few years.
The use of structural MRI and fMRI holds great potential both for the researchers seeking to understand how pedophilia develops and for clinicians seeking methods of assessing pedophilia more accurately than we can presently. Although many questions remain, this leap in technology has the potential to improve greatly our ability to prevent sexual offenses better than ever before.
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Cantor, J. M., Kabani, N., Christensen, B. K., Zipursky, R. B., Barbaree, H. E., Dickey, R., Klassen, P. E., Mikulis, D. J., Kuban, M. E., Blak, T., Richards, B. A., Hanratty, M. K., & Blanchard, R. (2008). Cerebral white matter deficiencies in pedophilic men. Journal of Psychiatric Research, 42, 167–183.
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