Balancing the Self

The vestibular system is composed of otolith organs and semi-circular canals that encode linear and angular accelerations, as well as the position of the head with respect to gravity. Thus, the detection of self-motion, the distinction between self- and object-motion, as well as gaze stabilisation and maintenance of postural stability are the core vestibular functions. Recent research shows that vestibular information interacts with higher-level cognitive processes, such as space perception, attention orienting, body schema and bodily self-consciousness. In order to contribute to these faculties, vestibular information is dynamically combined with visual, somatosensory and proprioceptive signals. In the present thesis we explore such multimodal interactions using a human centrifuge (rotating chair). In Part 1 we show that visual and vestibular cues are integrated in accordance with statistical optimality even when large directional conflicts are introduced between these modalities. Participants were significantly better in discriminating rotation magnitude when simultaneously presented with visual and vestibular cues, as compared to each modality independently, despite the fact that the axes of rotation implied by the two cues were different (Study 1). We also demonstrate that visuo-vestibular integration is present and optimal in patients with unilateral vestibular loss (Study 2). Part 2 of the present thesis examined vestibularsomatosensory interactions. We show that vestibular stimulation in the form of passive whole-body rotations increases tactile sensitivity at the fingertips, as compared to a no-rotation baseline (Study 3). We also demonstrate that the effect of vestibular stimulation on touch is not direct, but mediated by visual information about self-motion: visual and vestibular cues first combine, and only subsequently influence tactile sensitivity (Study 4). In Part 3 of this thesis, we explored how vestibular stimulation affects visual attention and awareness. We show that when acting as an exogenous cue, vestibular stimulation orients attention at short cue-to-target delays. When acting as an endogenous cue, vestibular stimulation strongly orients attention at all cue-to-target delays (Study 5). Vestibular stimulation also affects visual awareness. Using a continuous flash suppression paradigm to suppress an optic flow stimulus during passive whole-body rotations, we show that optic flow that is congruent (i.e. counterdirectional) with the direction of the vestibular rotation breaks suppression faster than incongruent optic flow (Study 6). In sum, our findings refine the existing knowledge on multisensory processing in general and vestibular interactions with other senses in particular. Our results are of relevance for the understanding of how visual, vestibular, proprioceptive and somatosensory information are combined by the brain in order to form a coherent representation of the self in space

Self-monitoring in schizophrenia : weighting exteroceptive visual signals against self-generated vestibular cues

Disturbances in self-monitoring are core symptoms of schizophrenia. Some research suggests an over-reliance on exteroceptive cues and a reduced weighting of self-generated interoceptive signals to guide perception. The vestibular sense provides important self-generated information about the body in space. Alterations of vestibular function are reported in schizophrenia, but it is unknown whether internally generated vestibular information is discounted in favour of exteroceptive input. In this study, we test for evidence of an over-reliance on exteroceptive visual cues and a reduced weighting of vestibular signals in guiding perception. In a group of individuals with schizophrenia and healthy controls, we used a well-studied visual illusion – the Tilt Illusion – to probe the respective weight given to visual and vestibular cues in judging line orientation. The Tilt Illusion reveals that perceived orientation of a vertical grating is biased by the orientation in its surround. This illusion increases when the head is tilted, due to the reduced reliability of vestibular information that would otherwise provide an internally generated reference for vertical. We predicted that an over-reliance on exteroceptive cues in schizophrenia would lead to a reduced susceptibility to the effects of head position on Tilt Illusion strength. We find no difference between patients and controls. Both groups show comparable Tilt Illusion magnitudes that increase when the head is tilted. Thus, our findings suggest that chronic patients with schizophrenia adequately combine self-generated vestibular cues and exteroceptive visual input to judge line verticality. A stronger reliance on exteroceptive information over internally generated signals in guiding perception is not evident in our data. Deficits in self-monitoring might therefore be modality specific or state dependant

Comparing adaptive coding of reward in bipolar I disorder and schizophrenia

Deficits in neural processing of reward have been described in both bipolar disorder (BD) and schizophrenia (SZ), but it remains unclear to what extent these deficits are caused by similar mechanisms. Efficient reward processing relies on adaptive coding which allows representing large input spans by limited neuronal encoding ranges. Deficits in adaptive coding of reward have previously been observed across the SZ spectrum and correlated with total symptom severity. In the present work, we sought to establish whether adaptive coding is similarly affected in patients with BD. Twenty-five patients with BD, 27 patients with SZ and 25 healthy controls performed a variant of the Monetary Incentive Delay task during functional magnetic resonance imaging in two reward range conditions. Adaptive coding was impaired in the posterior part of the right caudate in BD and SZ (trend level). In contrast, BD did not show impaired adaptive coding in the anterior caudate and right precentral gyrus/insula, where SZ showed deficits compared to healthy controls. BD patients show adaptive coding deficits that are similar to those observed in SZ in the right posterior caudate. Adaptive coding in BD appeared more preserved as compared to SZ participants especially in the more anterior part of the right caudate and to a lesser extent also in the right precentral gyrus. Thus, dysfunctional adaptive coding could constitute a fundamental deficit in severe mental illnesses that extends beyond the SZ spectrum

Examining motion speed processing in schizophrenia using the flash lag illusion

Research on visual perception in schizophrenia suggests a deficit in motion processing. Specifically, difficulties with discriminating motion speed are commonly reported. However, speed discrimination tasks typically require participants to make judgments about the difference between two stimuli in a two-interval forced choice (2IFC) task. Such tasks not only tap into speed processing mechanisms, but also rely on higher executive functioning including working memory and attention which has been shown to be compromised in schizophrenia. We used the Flash Lag illusion to examine speed processing in patients with schizophrenia. Based on previous research showing a strong dependence between motion speed and the illusion magnitude, we expected a deficit in speed processing to alter this relationship. A motion processing deficit in patients would also predict overall reductions in perceived lag. We found the magnitude and speed dependence of the Flash Lag illusion to be similar in patients and controls. Together, the findings suggest no general abnormality in motion speed processing in schizophrenia

Adaptive coding occurs in object categorization and may not be associated with schizotypal personality traits

Processing more likely inputs with higher sensitivity (adaptive coding) enables the brain to represent the large range of inputs coming in from the world. Healthy individuals high in schizotypy show reduced adaptive coding in the reward domain but it is an open question whether these deficits extend to non-motivational domains, such as object categorization. Here, we develop a novel variant of a classic task to test range adaptation for face/house categorization in healthy participants on the psychosis spectrum. In each trial of this task, participants decide whether a presented image is a face or a house. Images vary on a face-house continuum and appear in both wide and narrow range blocks. The wide range block includes most of the face-house continuum (2.50–97.5% face), while the narrow range blocks limit inputs to a smaller section of the continuum (27.5–72.5% face). Adaptive coding corresponds to better performance for the overlapping smaller section of the continuum in the narrow range than in the wide range block. We find that participants show efficient use of the range in this task, with more accurate responses in the overlapping section for the narrow range blocks relative to the wide range blocks. However, we find little evidence that range adaptation in our object categorization task is reduced in healthy individuals scoring high on schizotypy. Thus, reduced range adaptation may not be a domain-general feature of schizotypy

Belief Revision and Delusions: How Do Patients with Schizophrenia Take Advice?

The dominant cognitive model that accounts for the persistence of delusional beliefs in schizophrenia postulates that patients suffer from a general deficit in belief revision. It is generally assumed that this deficit is a consequence of impaired reasoning skills. However, the possibility that such inflexibility affects the entire system of a patient's beliefs has rarely been empirically tested. Using delusion-neutral material in a well-documented advice-taking task, the present study reports that patients with schizophrenia: 1) revise their beliefs, 2) take into account socially provided information to do so, 3) are not overconfident about their judgments, and 4) show less egocentric advice-discounting than controls. This study thus shows that delusional patients' difficulty in revising beliefs is more selective than had been previously assumed. The specificities of the task and the implications for a theory of delusion formation are discussed

Differential effects of vestibular processing on orienting exogenous and endogenous covert visual attention

Recent research highlights the overwhelming role of vestibular information for higher order cognition. Central to body perception, vestibular cues provide information about self-location in space, self-motion versus object motion, and modulate the perception of space. Surprisingly, however, little research has dealt with how vestibular information combines with other senses to orient one's attention in space. Here we used passive whole body rotations as exogenous (Experiment 1) or endogenous (Experiment 2) attentional cues and studied their effects on orienting visual attention in a classical Posner paradigm. We show that-when employed as an exogenous stimulus-rotation impacts attention orienting only immediately after vestibular stimulation onset. However, when acting as an endogenous stimulus, vestibular stimulation provides a robust benefit to target detection throughout the rotation profile. Our data also demonstrate that vestibular stimulation boosts attentional processing more generally, independent of rotation direction, associated with a general improvement in performance. These data provide evidence for distinct effects of vestibular processing on endogenous and exogenous attention as well as alertness that differ with respect to the temporal dynamics of the motion profile. These data reveal that attentional spatial processing and spatial body perception as manipulated through vestibular stimulation share important brain mechanism

Belief revision in schizophrenia

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Out-of-Body Experiences and Other Complex Dissociation Experiences in a Patient with Unilateral Peripheral Vestibular Damage and Deficient Multisensory Integration

Out-of-body experiences (OBEs) are illusory perceptions of one's body from an elevated disembodied perspective. Recent theories postulate a double disintegration process in the personal (visual, proprioceptive and tactile disintegration) and extrapersonal (visual and vestibular disintegration) space as the basis of OBEs. Here we describe a case which corroborates and extends this hypothesis. The patient suffered from peripheral vestibular damage and presented with OBEs and lucid dreams. Analysis of the patient's behaviour revealed a failure of visuo-vestibular integration and abnormal sensitivity to visuo-tactile conflicts that have previously been shown to experimentally induce out-of-body illusions (in healthy subjects). In light of these experimental findings and the patient's symptomatology we extend an earlier model of the role of vestibular signals in OBEs. Our results advocate the involvement of subcortical bodily mechanisms in the occurrence of OBEs