Patients with functional psychoses show similar visual backward masking deficits
Introduction
Psychiatric diseases are strongly influenced by genetic disposition. For example, genetic analysis has revealed a plethora of candidate genes of schizophrenia of which each gene, however, explains only a small share of the genetic risk of suffering from the disease (Sanders et al., 2008). For this reason, endophenotypes are of particular interest. Endophenotypes are physiological or behavioral traits supervening on certain gene combinations underlying the disease.
We have established a visual masking technique, the shine-through paradigm, as a potential endophenotye of schizophrenia. In visual backward masking, a target stimulus is followed by a mask. The mask impairs performance on target discrimination in both schizophrenic patients and healthy controls. However, masking deficits of patients are much more pronounced than those of controls (Braff, 1981, Braff and Saccuzzo, 1981, Green et al., 1994a, Green et al., 1994b, Slaghuis and Bakker, 1995, Butler et al., 1996, Keri et al., 2000).
In the shine-through effect, a vernier stimulus is presented, i.e. two vertical bars that are slightly offset in the horizontal direction (Fig. 1; Herzog et al., 2004, Chkonia et al., 2010a, Herzog and Brand, 2009, Roinishvili et al., 2008). Observers indicate the offset direction of the bottom line relative to the top line (left vs. right). After the vernier, an inter-stimulus interval (ISI) is presented, followed by a masking grating of 25 or five elements. The vernier shines through the 25-element grating, i.e. the vernier appears as an entity distinct from the grating. For healthy observers, the discrimination of the vernier offset is only mildly impaired. Interestingly, the vernier does not shine through the five-element grating, even though the five-element grating is contained in the 25-element grating and, hence, masking should be stronger for the 25-element grating. In both masking conditions, the vernier offset is fixed and the ISI between vernier and grating is varied. We determined the ISI for which 75% correct responses were reached. With the 25-element grating, patients need almost 10 times longer ISIs compared with healthy controls (Herzog et al., 2004).
Unaffected relatives needed ISIs that were about twice as long as those required by controls (Chkonia et al., 2010a). Performance in the shine-through effects is stable across roughly 1 year (Chkonia et al., 2010a). Hence, the shine-through masking paradigm meets the major criteria for an endophenotype (Gottesman and Gould, 2003). In addition, the shine-through paradigm exhibits a very high sensitivity of 87% (p < 0.0005) and specificity of 89% (p < 0.0005) distinguishing schizophrenic patients form controls (Chkonia et al., 2010a). Here, we tested the specificity of the shine-through paradigm.
Mental diseases strongly overlap in many aspects including psychopathology (Murray et al., 2004), cognition (Hill et al., 2008, Jabben et al., 2010), neurophysiological abnormalities (Thaker, 2008), and genetics (Craddock et al., 2006, Owen et al., 2007). For this reason, the Kraepelinian dichotomy (Kraepelin, 1896, Craddock and Owen, 2005) between schizophrenia and bipolar disorders was questioned recently because of commonalities between diseases which might be based on shared pathophysiological mechanisms (Owen et al., 2007, Craddock and Owen, 2010, Linscott and van Os, 2010). Here, we explored this avenue within the context of visual backward masking. If psychotic diseases belong to one spectrum, we expect strong masking deficits for schizophrenic, bipolar, and schizoaffective patients but not for unipolar depressive patients and abstinent alcoholics.
Section snippets
Methods and materials
We tested schizophrenic, schizoaffective, and bipolar patients in Tbilisi, Georgia (study 1), and schizophrenic, major depressive patients, and abstinent alcoholics in Bremen, Germany (study 2). Control groups were tested at both centers. The equipment in both centers was identical.
Patients were diagnosed using the criteria of the Diagnostic and Statistical Manual of Mental Disorders, fourth edition, (DSM-IV), by means of an interview based on the SCID (The Structured Clinical Interview for
Functional psychosis (study 1)
The patient groups differed with regard to education compared to controls (F[3,90] = 5.8, p = 0.001; Table 1) and in CPT performance (F[3,90] = 5.0, p = 0.003; Table 1). Controls needed shorter SOAs than the three patient groups for both the five- and the 25-element grating (p < 0.0001). There was a significant interaction of group by grating (F[3,90] = 13.0, p ≤ 0.0001; partial eta-square 0.3) (see Fig. 2). For the 25-element grating, SOAs of schizophrenic, schizoaffective, and bipolar patients were about
Discussion
Patients with functional psychoses, i.e. schizophrenia, schizoaffective disorder, and bipolar disorder, show significant masking deficits compared to controls (Fig. 2). Patients with unipolar major depression and abstinent alcohol-dependent patients, however, perform like healthy controls. This is in line with the few previous studies which tested only some pairwise comparisons between disorders such as schizophrenia and mania (Green et al., 1994a, Green et al., 1994b, Keri et al., 2001,
Acknowledgments
This study was supported by the Volkswagen Foundation project ‘Between Europe and the Orient — A Focus on Research and Higher Education in/on Central Asia and the Caucasus’ and the National Centre of Competence in Research (NCCR) “Synapsy” of the Swiss National Science Foundation (SNF). We thank the participants for generously volunteering. We also thank Aaron Clarke for proofreading the manuscript.
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