Original ArticlesHemispheric asymmetry in schizophrenia: a “dual deficits” model
Introduction
In a previous study, we found that schizophrenic patients were impaired on a dichotic listening (DL) test with verbal stimuli (Green et al 1994; see also Green 1998). Specifically, patients who were experiencing auditory hallucinations failed to show an expected right ear advantage on the DL test. Thus, schizophrenic patients seemed to lack aspects of the normal left hemisphere language lateralization, particularly in a hallucinatory state (cf. Bruder et al 1995, Crow 1997, Wexler and Heninger 1979, Woodruff et al 1997).
The DL test of verbal stimuli is a measure of hemispheric asymmetry, especially left temporal lobe functional integrity Bryden 1988, Hugdahl 1995. In the dichotic listening task of the present study, two different verbal stimuli (consonant–vowel syllables) were presented simultaneously, one in each ear. The typical finding in normal healthy subjects is a right ear advantage (REA) to verbal stimuli. The REA results from a predominance of contralateral auditory pathways from the right ear to the left, speech dominant, hemisphere, as well as an inhibition of the ipsilateral pathways by input from the contralateral ear (Kimura 1961).
Most individuals can, however, increase or decrease the ear advantage through shifting of attention to either the right or left ear Bryden et al 1983, Hugdahl 1995, Hugdahl and Andersson 1984. This probably reflects the influence of attentional and perceptual factors on brain laterality dominance patterns, resulting in a dynamic interaction between structural and cognitive laterality (see Hugdahl 1995 for further discussion). Thus, DL is a relatively uncomplicated behavioral test for studying both automatic (stimulus-driven) and controlled (instruction-driven) performance.
In the previous study (Green et al 1994), we noted that patients were unable to modify their DL performance when specifically instructed to focus their attention to either the right or left ear, whether they hallucinated or not. Although this aspect of performance was not a primary focus of that study, it was a potentially important observation. In essence, schizophrenic patients seemed to be impaired both with regard to a left hemisphere superiority for processing of dichotically presented stimuli, and with regard to the ability to modify the ear advantage through shifting of attention. In other words, schizophrenia may involve “dual deficits” with regard to both automatic and controlled processing (cf. O’Leary et al 1996).
Our previous study had two methodological limitations. First, the schizophrenic patients were on a wide variety of medications at a large range of doses. This variability in medications may have obscured some performance patterns. Second, there was no control or comparison group. Thus the performance of the schizophrenic patients could not directly be compared to healthy comparison subjects.
The main purpose of the present study was to more carefully test the possibility of “dual deficits” in schizophrenia. For this study, we assessed treatment-resistant patients who were participating in a clinical trial protocol and who were receiving a relatively narrow dose of the same antipsychotic medication. As part of the treatment-resistant criteria, all of the patients were experiencing active psychotic symptoms. Hence, they were relatively homogenous on this aspect of illness as well. Furthermore, a clear majority (26 out of 33) of the patients in the present sample scored in the clinical Brief Psychiatric Rating Scale (BPRS) range for hallucinations. We compared the performance of these patients to nonpsychotic healthy individuals, drawn from the dichotic listening database gathered by Hugdahl (1995).
In a more general sense, the purpose of the present study was to: 1) investigate left hemisphere functional integrity for auditory language processing in schizophrenic patients; and 2) investigate the interaction between brain laterality and attentional processing by having subjects shift attention to the left or right auditory space. We hypothesize that schizophrenic patients will show abnormalities in both these performance areas.
Section snippets
Subjects
The subjects included 33 patients with treatment-resistant schizophrenia (23 men and 10 women) drawn from the UCLA Clinical Research Unit at the Camarillo State Hospital, California, USA. Subjects were participating in a larger clinical trials study and were assessed in the baseline phase prior to assignment to study medication (Green et al 1997). Patients were excluded if they had: 1) clinically significant neurological disease (including seizure disorder); 2) a history of head injury; 3)
Results
Table 2shows the percentage of correctly reported syllables from each ear for the schizophrenic patients and comparison subjects in the three attention conditions.
To test for interaction effects of handedness and attentional instructions, a four-way ANOVA was carried out on the design Ear × Group × Handedness × Attentional instruction. This analysis yielded a significant three-way interaction of Ear × Group × Attentional instruction, F(2,124) = 36.36, p < .0001. The interaction was followed up
Discussion
The schizophrenic group did not show a REA in the non–forced attention condition as compared to the normal comparison subjects who showed the typical REA. In fact, the schizophrenic subjects had a tendency toward reversed laterality (see Figure 1), with more patient subjects showing a LEA in the NF attention condition compared with the comparison subjects. Also, the schizophrenic group was not able to shift attention in the FR and FL conditions, again as opposed to the normal comparison
Acknowledgements
The project received support from the NIMH UCLA Clinical Research Center for the study of Schizophrenia (R. P. Liberman, P.I.), an investigator-initiated grant from the Janssen Research Foundation (M.F. Green and B.D. Marshall, Co-P.I.s), and from the school of Psychology, University of Bergen, Norway (to K. Hugdahl).
The data were collected at the Camarillo State Hospital, Camarillo, California, while Else-Marie Løberg was a visiting student. The authors thank B.D. Marshall, Jr, MD, Robert
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