Illusory touch and tactile perception in somatoform dissociators

Abstract

Objective

The psychological mechanisms of somatoform dissociation (i.e., pseudoneurological symptoms) are poorly understood. This study evaluated recent theoretical predictions regarding the role of tactile perception in the development of somatoform dissociative symptoms.

Methods

Eighty nonclinical participants scoring either high or low on the Somatoform Dissociation Questionnaire (SDQ-20) completed the Somatic Signal Detection Task (SSDT), a novel perceptual paradigm designed to simulate the occurrence of somatoform symptoms in the laboratory. Prior to the SSDT, participants completed a memory task designed to produce either minimal or maximal activation of tactile representations in memory.

Results

The high SDQ-20 group exhibited a more liberal response criterion (c) on the SSDT than the low SDQ-20 group after controlling for negative affectivity, somatosensory amplification and depression. This effect was mainly attributable to an increased number of false alarms (i.e., illusory experiences of touch) in the high SDQ-20 group rather than an increased hit rate. General perceptual ability (i.e., tactile sensitivity) was comparable between the two groups. The memory manipulation had no effect on SSDT performance.

Conclusions

Somatoform dissociators appear more likely to experience illusory perceptual events under conditions of sensory ambiguity than nondissociators, despite comparable perceptual abilities more generally. These findings support theories that identify distorted perceptual processing as a feature of somatoform dissociation. The SSDT has potential as a tool for further research in this area.

Introduction

The World Health Organization defines dissociation as “a partial or complete loss of the normal integration between memories of the past, awareness of identity and immediate sensations, and control of bodily movements” [1]. Although most dissociation research focuses on disturbances in memory and identity, several recent theorists [2], [3], [4] have emphasized the role of dissociation in symptoms that suggest neurological disease, but which cannot be explained by organic pathology. Such “pseudoneurological” symptoms (e.g., paralysis, seizures, sensory loss) are part of a wider group of complaints characterized by medically unexplained physical symptoms, which are common in healthcare settings and often associated with high levels of distress, disability, and resource utilization [5], [6], [7].

Current taxonomies disagree in their classification of pseudoneurological illness. DSM-IV places pseudoneurological symptoms with other medically unexplained symptoms (MUS) in the somatoform disorders category, separate from the dissociative disorders [8]. Consistent with this, patients with pseudoneurological symptoms often report many other unexplained symptoms, and the presence of one or more pseudoneurological complaints is required for a DSM-IV diagnosis of somatization disorder [8]. Patients with multiple pseudoneurological symptoms tend to experience more distress, psychopathology and resource utilization than other somatoform patients [9], [10], [11], even after controlling for the total number of unexplained symptoms [11]. ICD-10, in contrast, categorizes pseudoneurological symptoms as dissociative (conversion) disorders alongside amnesia, fugue, and multiple personality, but separate from other unexplained symptoms [1]. One influential model spans both taxonomies by describing pseudoneurological symptoms as examples of “somatoform dissociation,” linking them with “psychoform” dissociative phenomena such as amnesia and identity disturbance [3], [12]. Consistent with this, pseudoneurological symptoms and other unexplained complaints are commonly reported by patients with psychoform dissociative disorders [12], [13], [14]. The latter also tend to score highly on the somatoform dissociation questionnaire (SDQ-20[15], [16]), which encompasses a range of pseudoneurological phenomena.

Discrepancies in the classification of these phenomena are likely to remain until the psychological basis of these conditions is better understood [17]. Janet's original dissociation model [18] contends that dissociative phenomena reflect the activation of “systems of ideas and functions” that have been separated from awareness by a deficit in attention and integrative capacity due to trauma. Most recent models of pseudoneurological illness [2], [3], [4] have been influenced by this account, although the emphasis placed on particular concepts, such as the role of trauma, varies from one model to another. One such model [2] suggests that pseudoneurological symptoms are disturbances in consciousness and cognitive control, reflecting the over-activation of mental representations of illness in memory. In this account, repeatedly recollecting or otherwise attending to these “rogue representations” increases their activation to the point where they dictate bodily awareness and/or control, in a manner akin to Janet's “systems.”

Although there is a considerable body of theory concerning the psychological basis of pseudoneurological symptoms, there are relatively few empirical studies in this area. With that in mind, this article describes an experimental study designed to test hypotheses concerning the role of tactile perception and memory in somatoform dissociation [2]. We adopted a recent paradigm, the somatic signal detection task (SSDT; [19]), designed to mimic the experience of somatoform phenomena. During the SSDT, participants indicate whether they feel vibrations delivered to their fingertip at perceptual threshold over a series of trials where the presence of the vibration is varied. A light is located next to the vibrating device and is illuminated on half of the trials, creating four trial types: vibration alone, vibration plus light, light alone, no stimulation. Using signal detection analysis, the paradigm can be used to calculate an individual's general perceptual ability (ie, tactile sensitivity or d′) and their overall tendency to respond positively regardless of what stimulus is presented (i.e., their response criterion or c). Previous research indicates that healthy participants report the presence of a vibration on a significant minority of trials where no tactile stimulus has been presented (“false alarms”), particularly when the light is present [19]. It has been argued that these illusory touch experiences are akin to the kinds of misperceptions reported by patients with somatoform symptoms and that similar processes are likely to be operating in each case [19]. If so, the paradigm has potential as a laboratory analogue of somatoform phenomena, facilitating research into the mechanisms of these conditions.

As an initial step towards establishing the validity of this idea, the primary aim of this study was to determine whether there is an association between somatoform dissociation and illusory touch experiences on the SSDT. As the tendency to experience unexplained symptoms is a trait-like phenomenon distributed across both normal and clinical populations [20], we sampled nonclinical participants with and without a history of pseudoneurological symptoms, as indicated by relatively high and low scores on the SDQ-20, and compared them on the SSDT. We predicted that the high SDQ-20 group would have a significantly lower response criterion (c) than the low SDQ-20 group, due to more false alarms (i.e., illusory touch experiences) for the former. No predictions were made regarding the hit rates or tactile sensitivity of the two groups. We also manipulated the presence of the light, with a view to maximizing the occurrence of false alarms and testing whether the SDQ-20 groups are differentially influenced by the addition of this stimulus.

A secondary aim of the study was to test the hypothesis that the frequency of illusory touch on the SSDT varies according to the activation of tactile representations in memory [2], [19]. Prior to the SSDT, all participants were presented with a training phase comprising a series of picture stimuli, half of which were accompanied by a vibration. They then completed a recall phase where they were presented with a subset of the training pictures and asked to identify those that had previously been presented with a vibration during the training. At recall, we varied the proportion of picture stimuli that had been paired with a vibration during training, so that one group (maximal recall) was presented with three times as many such pictures as the other group (minimal recall). In this way, we attempted to manipulate the frequency with which participants recalled vibrations, and thereby the activation of vibration-related memories. We predicted that participants in the maximal recall group would experience more illusory touch (i.e., false alarms) on the SSDT than those in the minimal recall group, following Refs. [2], [19]. We predicted that this effect would be more pronounced for the high SDQ group.