We studied patients with conversion tremor using a within-subject comparison of involuntary conversion tremor and voluntary reproduction of their conversion tremor to assess for the correlates of loss of self-agency. We demonstrated that C-R vs V-R was associated with right TPJ hypoactivity, a region involved in multisensory integration. During C as compared with V, the TPJ had lower functional connectivity with sensorimotor regions and limbic regions.
From 156 patients in the database seen over a 5-year period, only 8 patients were included in the study because of technical demands of our study that would permit comparative analysis of voluntary vs involuntary movement. Recruiting for functional imaging studies on conversion disorder has been difficult, with reported sample sizes in the literature ranging from 1 to 8.12–18
The present study was limited by the lack of a healthy control group, which we did not include given the lack of involuntary tremors. We did not include a neurologic control group such as PD or essential tremor because the symptoms would not cease at rest and would be difficult to mimic without triggering their symptoms. Thus, having a patient with conversion disorder performing voluntary movement as a within-subject control was the optimal control condition to answer our question of interest. We controlled for movement differences with video recording and did not observe differences in the cortical motor areas in the contrast of C-V or V-C, confirming that possible differences in motor output were slight and involuntary and voluntary movement involves similar motor pathways. We also note that the inclusion of only patients with positionally triggered tremor symptoms may limit generalizability. Furthermore, the inclusion of patients with different lateralizing symptoms may present a limitation; however, we suggest that our findings may represent more general mechanisms that have been attributed to the right hemisphere.
Decety and Lamm6
have proposed that the fundamental role of the right TPJ is a low-level computational process involving the prediction of external events by functioning as a general comparator of internal predictions with actual external events. This process is suggested to explain the various low- and high-level cognitive processes attributed to the right TPJ, including self-agency,2–4,6,19
theory of mind,20
and spatial reorienting of visual attention.21
Certainly, studies on theory of mind suggesting that the attribution of mental states to self and to others involves the right TPJ may be relevant in conversion disorder. Spatial attention may also be relevant, emphasizing that the semiautonomous generation of C is associated with a different awareness of spatial movement as compared with the voluntary generation of V. However, we suggest that our findings reflect a lack of self-agency, which is not only a symptom fundamental to the experience of conversion disorder but a feature core to the definition of conversion disorder. The mechanisms underlying self-agency, by definition, fit in well with the role of the right TPJ as a comparator of internal sensory prediction and the actual sensory state.
Stimulation of the inferior parietal cortex has been recently demonstrated to be associated with the illusion of controlling movement (i.e., the experience of controlling movement when no actual movement occurred), which the authors termed the sense of “conscious intention,” and has been suggested to be related to activation of the network involved in movement monitoring through forward modeling.3
In healthy volunteers, agency has been studied using self-generated action and visual feedback manipulation implicating the right inferior parietal cortex and TPJ.2,4–7
In this context, loss of agency is associated with right inferior parietal cortex hyperactivity, which is the opposite of what we observed. In visual feedback manipulation experiments of voluntary movement, the mismatch involves an intact higher-level motor intention. However, in our study, motor intention is almost certainly abnormal. The movement arises without conscious intention, and there may not be a feed-forward signal. The lack of feed-forward signal is a possible interpretation of the decreased connectivity of the TPJ and the sensorimotor cortices and cerebellar vermis. Thus, despite proprioceptive feedback from the movement, there is no mismatch detection, and activation is decreased. C vs V was also associated with lower connectivity between the TPJ and limbic regions (ventral anterior cingulate and ventral striatum), suggesting less limbic involvement in conversion movement evaluation.
The right TPJ has also been implicated in the pathologic states of vestibular illusions (of elevation, rotation), multisensory illusions (or visual shortening and movement of limbs), autoscopy (or the experience of seeing one's body in extrapersonal space), and out-of-body experiences (or the experience of seeing one's body and environment from a location outside of the physical body).22,23
For example, TPJ lesions22
and high-frequency stimulation23
targeting the superior temporal gyrus with increased functional activity of the right TPJ have been associated with out-of-body experiences. Similarly, mental imagery of an out-of-body experience in healthy volunteers is associated with TPJ activity, and transcranial magnetic stimulation impairs this specific mental imagery.24
The phenomenon of disembodiment has been suggested to be a failure to integrate proprioceptive, visual, and tactile information regarding one's body (disintegration in personal space) along with an additional disintegration between personal (vestibular) and extrapersonal (visual) space that occurs during impaired consciousness.22
There may indeed be similarities between these phenomena and conversion tremor on the general level of multisensory integration, hence implicating similar regions. However, we have confined our interpretation to the feed-forward model comparing sensory feedback and prediction to explain the clinical phenomena of the experience of subjective involuntary movement. Furthermore, conversion tremor does not involve integration within personal or extrapersonal space, and our findings suggest right TPJ hypoactivity, whereas out-of-body experiences are associated with the opposite. We cannot comment on whether the TPJ is intrinsically impaired and suggest rather that the process of generating the sensory prediction in conversion tremor may be abnormal. It is possible that a range of symptoms, from that of nonconscious “nervous” foot tapping/hand drumming (which presumably also uses voluntary pathways) to l- dopa–induced dyskinesias25
and other involuntary movement disorders (which are less likely to use voluntary pathways), may be perceived as involuntary in part because of reduced feed-forward signaling. Whether this mechanism holds for conversion paralysis or other conversion symptoms is not clear. Further studies will be able to clarify one of these hypotheses.
Conversion movements use voluntary motor pathways and yet are paradoxically experienced as involuntary. Our study highlights a potential abnormality of integration of the internal sensory prediction with the actual sensory state in conversion tremor. We note that this mechanism does not address the question of how or why the conversion tremor is initiated, but may give insight into why it is experienced as involuntary. This theory is further compatible with other theories put forward in conversion motor disorders, including that of abnormal motor conceptualization,13
with motor function, and hyperactive monitoring of internal states.27
This mechanism may reflect a more general process of comparison of internal predictions with actual events attributed to the right TPJ.6
The absence of a feed-forward signal in conversion tremor would lead to a lack of a match in the TPJ, thus leading to the crux of conversion movements, the feeling that one is not the cause of one's actions.