The three groups did not differ in sleep architecture (one-way ANOVA: F≤2.92, df=(2,110), p>0.05) (), except for sleep latency (one-way ANOVA: F=4.8, df=(2,110), p=0.01). Specifically, post-hoc t-tests showed increased sleep latency in schizophrenics relative to healthy controls (p<0.01). No difference was found between schizophrenics and non-schizophrenic patients taking antipsychotics, or between them and healthy subjects (p≥0.1).
Sleep architecture variables of subject groups.
EEG power analysis
Whole-night NREM EEG power was calculated between 0.5 and 40 Hz for the three groups (). One-way ANOVA revealed group differences in the spindle range (12–16 Hz, F≥3.5, df=2,110, p≤0.03). Post-hoc t-tests showed power reductions in schizophrenics compared to healthy subjects (12–16 Hz, p≤0.03) and non-schizophrenic patients taking antipsychotics (12–16 Hz, p≤0.02), but no difference between non-schizophrenic patients and healthy subjects (p≥0.3, ). Power deficits were present in each sleep cycle, and were restricted to spindle range.
Schizophrenics had deficits in spindle range (12–16 Hz) power and other spindle parameters
An initial analysis was performed for spindle amplitude, duration, number, and ISA in 12–16 Hz, the frequency range decreased in schizophrenics. The topography of each parameter was similar across groups, with peaks in prefrontal and centroparietal areas. Spindle parameters in these regions were markedly reduced in schizophrenics (). Spindle amplitude was decreased in schizophrenics vs. healthy and medicated psychiatric comparison subjects (p<0.0004, SnPM) in centroparietal areas (), while spindle duration was decreased in schizophrenics vs. healthy and medicated psychiatric comparison subjects (p<0.0002, SnPM) in prefrontal regions (). Schizophrenics also showed deficits in spindle number and ISA vs. healthy (p<0.0001, SnPM) and medicated psychiatric comparison subjects (p<0.0001, SnPM) in prefrontal and centroparietal regions. No difference was found between healthy and medicated psychiatric comparison subjects for any spindle parameter ().
Additional analyses were performed for slow (12–14 Hz) and fast (14–16 Hz) spindles. Schizophrenics showed deficits in slow spindle duration, number and ISA compared to both healthy and medicated psychiatric comparison subjects in prefrontal regions (SnPM, p≤0.001, , left panel). We found no difference in slow spindle amplitude across the three groups, or between healthy and medicated psychiatric controls in other slow spindle parameters. Deficits in fast spindle duration (prefrontal), amplitude (centroparietal), number and ISA (prefrontal and centroparietal) were found in schizophrenics vs. both healthy and medicated psychiatric comparison subjects (SnPM, p≤0.001, , right panel). Additionally, schizophrenics had reduced fast spindle number and ISA in the left temporal cortex (SnPM, p≤0.0004). No difference was observed in fast spindle parameters between healthy and medicated psychiatric comparison subjects.
Topography of slow (12–14 Hz) and fast (14–16 Hz) spindle deficits in schizophrenia
To determine the magnitude of spindle deficits in schizophrenics Cohen’s d, which measures effect size (ES), was calculated for spindle parameters. Cohen’s d analysis showed that prefrontal (for spindle duration), centroparietal (for spindle amplitude, number and ISA), and left temporal (for spindle number and ISA) areas were the most affected in schizophrenics. Moreover, Cohen’s d values ranged from ES=1.4, for spindle amplitude to ES≥2.21 for spindle number and ISA.
Slow wave analysis
In addition to slow wave activity (SWA, 1–4.5 Hz) we computed slow wave incidence, negative peak amplitude, down-slope, and up-slope. In previous work we demonstrated that each of these parameters represent different aspects of cortical synchronization during slow waves (11
). The topography of these parameters showed similar antero-posterior gradients across groups, with clear peaks in prefrontal areas, where SWA is usually maximal, and lowest values in posterior parietal and occipital areas (). No significant difference for any slow wave parameter was found across groups (SnPM, p≥0.2).
Slow wave parameters are not defective in schizophrenia
Cognitive ability and spindles
To establish possible correlations between general cognitive ability and spindles schizophrenics (N=20), healthy subjects (N=19) and non-schizophrenic patients taking antipsychotics (N=12) were administered a computerized version of the Ravens progressive matrices (RPM) test (14
), which is widely used in research/clinical settings to measure intelligence (15
). We found that schizophrenics (mean=36, SD=9) and medicated comparison subjects (mean=37.5, SD=13) had similar RPM scores (p=0.94), but both had lower scores relative to healthy subjects (mean=44, SD=9). Moreover, RPM scores of schizophrenics did not correlate with ISA (r=0.18, p=0.42) or spindle number (r=0.17, p=0.48).
Clinical and spindle parameters in schizophrenics
We also performed multiple regression/correlation analyses between clinical characteristics and spindle parameters in schizophrenics (). Spindle number was inversely related to age (r=-.33; p=.021) and stereotyped thinking of PANSS negative symptoms (n7, r=− .32; p=.028). Moreover, both ISA and spindle number were inversely related to positive symptoms of PANSS (ISA: r=−.40; p=.005; spindle number: r =−.37; p=.01). Specifically, spindle number was correlated only with conceptual disorganization (p2, r=−0.34, p=0.03), and hallucinations (p3, r=−0.4, p=0.01), while ISA was correlated exclusively with hallucinations (r=− 0.48, p=0.002).
Correlations between clinical and spindle parameters in schizophrenia patients.