Friday, April 29, 2011

Mental Imagery and the Right Parietal Lobe in OCD


Obsessive compulsive disorder (OCD) is a mental illness characterized by unwanted and intrusive thoughts, feelings, or ideas (obsessions), and ritualized behaviors (compulsions) the individual feels driven to perform in order to alleviate the disturbing nature of the obsessions. It is a major anxiety disorder classified in Axis I of the DSM-IV, which can be disabling to those who suffer with it.

The specific symptoms of OCD can include fear of contamination (from germs and physical contact with others) and resultant pathological cleaning rituals, fear of causing catastrophic harm to others, disturbing "impure thoughts" often of a sexual nature, and compulsive ordering, organization, and checking.

Currently, major treatments for OCD include cognitive behavior therapy (CBT), considered to be...
...the most effective type of psychotherapy for this disorder. The patient is exposed many times to a situation that triggers the obsessive thoughts, and learns gradually to tolerate the anxiety and resist the urge to perform the compulsion. Medication and CBT together are considered to be better than either treatment alone at reducing symptoms.
The most frequently prescribed drugs are the SSRI (selective serotonin reuptake inhibitor) antidepressants such as:
Neuroanatomical circuit models of the underlying brain dysfunction have implicated fronto-striatal loops that control thoughts and actions. For example, OCD has been associated with overactivity in the orbitofrontal cortex (Menzies et al., 2008), overactive error monitoring processes in the anterior cingulate cortex (Fitzgerald et al., 2005), and reduced activation in dorsal prefrontal-striatal regions during planning (van den Heuvel et al., 2005) and task switching (Gu et al., 2008). In summary, OCD has been conceptualized as an impulse control disorder marked by a breakdown of high-level executive control over behavior.

However, a new study by Koçak et al. (2011) has expanded the range of cognitive processes and brain regions that might be implicated in OCD. The authors proposed that since OCD patients are quite impaired at suppressing complex thoughts and intrusive images, they might also show deficits in suppressing very simple, neutral images and shapes.

Participants in their fMRI experiment were 12 patients with OCD [eight were cleaners, three were checkers (one of the checkers also had contamination obsessions), and one had harming obsessions] and 12 age-matched controls. The tasks performed in the scanner involved forming a visual mental image of a geometric shape and then manipulating this visual image. Three of the tasks involved cognitive control over visual imagery (imagination, suppression, and erasing) and two were baseline tasks (free imagination, resting). Before the scanning session, participants studied the shape until they were able to draw it from memory.



Fig. 1. (Koçak et al., 2011). The shape shown to the participants. The arrow (which did not appear on the paper used in the study) indicates the starting point of the erasing task. The participants were instructed to begin erasing the shape at the point indicated by the arrow and to continue until the shape completely disappeared.

The instructions given to the participants for these tasks were as follows: 1. Imagination task: imagine the shape on the paper continuously until another command is given; 2. Suppression task: imagine the paper with the shape on it immediately after the command is given, and then immediately suppress the image of the shape (try to see the paper as blank) until another command is given; 3. Erasing task: imagine the paper with the shape on it, and then erase the shape by tracing its outline it until another command is given; 4. Free-imagination task; imagine whatever comes to mind and change it with any other intrusive image – a type of free-association task of mental images (this task was used as the baseline condition task); 5. Resting condition: rest while in the scanner.
Since the tasks involved imagery alone and no overt responding, the only measures of performance were the participants' subjective evaluation of how well they were able to continually perform each task during the 25 sec blocks. The OCD group claimed they performed the suppression task better than controls, but we have no external way to validate this finding.

Turning to the neuroimaging results, the major contrasts were the three active imagery tasks, each compared to the free-imagination baseline task. The group comparison across the three active tasks (which did not differ from each other) is shown below.


Fig 3 (Koçak et al., 2011). The whole-brain result depicting significant activations related to the main effect of group (control > than OCD). Threshold at P < 0.05 (corrected for the whole brain). L: left; R: right; A: anterior; P: posterior; SFG: superior frontal gyrus; IPL: inferior parietal lobe; PCC: posterior cingulate cortex.

Individuals with OCD showed less activation than controls in three regions in the right hemisphere: the superior frontal gyrus,1 the inferior parietal lobe, and the posterior cingulate cortex [part of the default mode network]. The right IPL is very important for visuospatial processing (Verden et al., 2010); OCD patients can exhibit visuospatial impairments. Furthermore, activations in both IPL and PCC have been observed in prior studies of visual imagery (Ganis et al., 2004). If the participants with OCD were less adept at imagining, manipulating and suppressing a geometric shape, perhaps this reflects a deficit in imagery that is much more basic than controlling the contents of thought.

Although these results are very preliminary, the idea that obsessive thoughts could be associated with problems in the right parietal lobe and control over visual mental imagery is intriguing. One might speculate that repeated practice at generating and then suppressing simple geometric figures could potentially benefit many individuals with OCD, who must cope with disturbing and intrusive images on a daily basis.

Footnote

1 One potential confound that is not discussed is possible differences in eye movements between the groups. The participants were instructed to keep their eyes closed, but of course that didn't prevent them from moving their eyes. However, the activated SFG area in this experiment is anterior to the frontal eye fields, and the IPL region is lateral to a saccade-sensitive area in the IPS. Also, it's not clear why the activations would be right lateralized.

References

Fitzgerald KD, Welsh RC, Gehring WJ, Abelson JL, Himle JA, Liberzon I, Taylor SF (2005). Error-related hyperactivity of the anterior cingulate cortex in obsessive-compulsive disorder. Biol Psychiatry 57:287-294.

Ganis, G., Thompson, W. L., & Kosslyn, S. M. (2004). Brain areas underlying visual mental imagery and visual perception: an fMRI study. Cogn Brain Res 20:226-241.

Gu BM, Park JY, Kang DH, Lee SJ, Yoo SY, Jo HJ, Choi CH, Lee JM, Kwon JS. (2008). Neural correlates of cognitive inflexibility during task-switching in obsessive-compulsive disorder. Brain 131:155-64.

Koçak, O., Özpolat, A., Atbaşoğlu, C., & Çiçek, M. (2011). Cognitive control of a simple mental image in patients with obsessive–compulsive disorder Brain and Cognition DOI: 10.1016/j.bandc.2011.03.020

Menzies L, Chamberlain SR, Laird AR, Thelen SM, Sahakian BJ, Bullmore ET. (2008). Integrating evidence from neuroimaging and neuropsychological studies of obsessive-compulsive disorder: the orbitofronto-striatal model revisited. Neurosci Biobehav Rev. 32:525-49.

van den Heuvel OA, Veltman DJ, Groenewegen HJ, Cath DC, van Balkom AJ, van Hartskamp J. (2005). Frontal-striatal dysfunction during planning in obsessive-compulsive disorder. Archives Gen Psychiatry 62:301-309.

Verdon V, Schwartz S, Lovblad KO, Hauert CA, Vuilleumier P. (2010). Neuroanatomy of hemispatial neglect and its functional components: a study using voxel-based lesion-symptom mapping. Brain 133:880-94.



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4 Comments:

At April 29, 2011 12:01 PM, Anonymous Anonymous said...

No one is ever going to really learn about OCD, or Tourette's, or schizophrenia, etc etc by way of IMAGING.

The problems in lots of neurological conditions are a matter of chemistry. Biochemistry.

 
At May 01, 2011 1:17 AM, Blogger Murfomurf said...

I don't think Anonymous realises that those brain images in the blog are showing evidence of chemistry. Tut tut. You may not be able to see a thought, but you probably can see where it is happening- and that's all we've got.

 
At May 04, 2011 6:13 PM, Anonymous Emmy said...

Thanks for this interesting post. I've always wondered about what I call "clean" OCD as opposed to "hoarding" OCD. I mean, the folks arranging their books to be the same height and their blue jackets to be next to other blue jackets on the coat rack are *not* going to tolerate hoards of cats and stacks of newspapers in their living rooms.

So are there several variations, or is it situational? I realize it's about intrusive thoughts, but really, the two seem quite distinct.

Also, this may be way off topic but I wonder if a visuospatial impairment would also translate to something like a deficiency in map reading.

 
At May 13, 2011 3:22 PM, Anonymous Jessica said...

Another notion that Anonymous has failed to understand is that the chemical imbalance hypothesis of psychopathology is merely conjecture. In fact, very little empirical support has been borne from this idea. I shall implement the oft used analogy regarding this issue; just because aspirin has the ability to reduce headaches does not mean that headaches are caused by an aspirin deficiency.

One of the maxims in statistics is that correlation does not entail causation. fMRI has its failings, to be sure; it is a correlational and linear method that attempts to characterize exceedingly complex and, most likely, nonlinear processes. It has its weaknesses, but it is one of the best methodologies we have to understand brain function.

 

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