Abstract
The most robust and clear biological index differentiating persons with schizophrenia from healthy controls is the drastic reduction of the amplitude of their P300b event-related brain potential (ERP). However, the cause of that reduction remains obscure. Nevertheless, the P300b belongs to the family of the late posterior positivities (LPPs) which are closely related to the consciousness of the meaning of the stimulus in the task for the participants themselves (e.g., the: I am seeing the target stimulus for which I have to respond). The fragmentation of the self present in schizophrenia, could thus be the cause. If this were true, then P300bs should be somewhat reduced in healthy participants when their self representations are temporarily and minimally fragmented. We tested this hypothesis by using the innocuous fragmentation of the self that occurs in virtual reality (VR). There, participants can have a fragment of their self in an avatar they feel embodied in, within a VR room, while having another fragment of their self in their real body in the real room where they know they are. Our participants were thus equipped with a head mounted display in which they viewed a virtual room where a female humanoid avatar was facing them. She was lifting her right hand in synchrony with the participants, in order to induce in them a feeling of embodiment. Stimuli were a frequent green- and a rare red-disk, the oddball stimulus, occurring over the right hand of the avatar. Participants had to perform a Go/NoGo task, lifting their right hand to the frequent green disk and repressing this action for the oddball red disk. In the syncMove block of trials the avatar was lifting her right hand synchronously with the participant, disturbing her self representation as confirmed by the debriefing session. In the noMove block, the avatar remained immobile. In the classic block, only the red and the green disk were displayed on a monochrome background, neither the room nor the avatar were shown. As predicted, P300bs were found to be smaller in the syncMove block than in the noMove- and the classic-block in participants who had the classically large P300b oddball effect between ERPs to the frequent and those to the rare stimuli. Reduced P300bs of schizophrenia could thus be partly due to self fragmentation. Results may also open an avenue of research to the functional significance of LPPs and the content of the consciousness indexed by these potentials.
Highlights
7 years after the first description of the P300b event-related brain potential (ERP; Sutton et al, 1965), it has been shown that its voltage is drastically reduced in schizophrenia patients relative to healthy controls (Roth and Cannon, 1972), even more so when their symptoms are severe (Mathalon et al, 2000), with partial normalization when treatment is efficient to curtail these symptoms (Coburn et al, 1998)
Post hoc paired comparisons revealed that P300b amplitudes in the syncMove condition (Mean = 9.33) were lower than in the noMove condition (Mean = 10.58, z = 3.18, P = 0.006) and in the classic condition (Mean = 10.66, z = 3.36, P = 0.004) and that there was no difference between the noMove and the classic condition (z = 0.18, n.s.)
We tested whether the amplitudes of the P300b ERP would be smaller in case the stimulus eliciting this ERP could be bound to only a fragment of the self of participants rather than to their whole self. This restricted binding was made possible by embodying our participants in an avatar who was in a virtual room and by presenting stimuli there
Summary
7 years after the first description of the P300b event-related brain potential (ERP; Sutton et al, 1965), it has been shown that its voltage (or amplitude) is drastically reduced in schizophrenia patients relative to healthy controls (Roth and Cannon, 1972), even more so when their symptoms are severe (Mathalon et al, 2000), with partial normalization when treatment is efficient to curtail these symptoms (Coburn et al, 1998). P300bs are found to be reduced in many other pathologies (for a brief review, see Picton, 1992), but to a much lesser extent. Despite the fact that their P300bs have often less than half of the amplitude of those of average healthy persons, patients do not appear to suffer from a severe lack of consciousness of the meaning of the stimulus. The radical reduction of their P300b appears out of proportion with their behavioral deficits in the task. These deficits mainly consist in delayed responses (Nuechterlein, 1977; Vinogradov et al, 1998) and not in a gross lack of the awareness that a target stimulus just occurred. Together with the much more modest, but sizable, P300b reductions found in pathologies other than schizophrenia (Picton, 1992), this surprising phenomenon points to P300b factors besides the conscious appraisal of the meaning of the stimulus for the task
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