EEG power, frequency, asymmetry and coherence in male depression
Introduction
The longstanding use of quantitative waking electroencephalography (EEG) in the study of depression is based on the belief that major depressions have a biological basis and that glimpses of the determining causes and/or of the factors mediating its expression and relief can be objectively captured by non-invasive neuroelectric probes. Although studies assessing the sensitivity and specificity of computer-analyzed EEG features have shown these to be somewhat wanting with respect to their implementation as routine clinical markers in the diagnostic work-up of depressive disorders (Shagass et al., 1984, Prichep et al., 1986), their utility as valuable investigational, rather than clinical tools, is evidenced by the plethora of findings describing characteristic EEG patterns of depressed patient groups (Nuwer, 1988).
Of the spectrally analyzed parameters assessed in depressive disorders, activity in the alpha (≈8–12 Hz) frequency band has received the lion's share of attention, a focus promulgated in part by the importance attributed to the highly replicated inverse association, in healthy non-psychiatric individuals, between alpha voltage suppression and behavioural/emotional/cerebral activation (Lindsley, 1952, Shagass, 1972). Despite a number of inconsistencies, resting (eyes-closed) EEGs of unmedicated, actively depressed unipolar patients have shown elevated alpha and beta (≈13–28 Hz) compared with controls (Pollock and Schneider, 1990). Less consistently, delta (≈1–4 Hz) and theta (≈5–7 Hz) band activity has also been found to be increased, and when reported, delta has appeared to be more prominent in the right than in the left hemisphere of depressed patients (Knott and Lapierre, 1987, Kwon et al., 1996).
Hemispheric EEG activation asymmetry in subclinically depressed college students and clinically depressed patients has been frequently observed with findings of relative excess left mid-frontal (F3>F4) and lateral-frontal (F7>F8) alpha-band activity (Schaffer et al., 1983, Henriques and Davidson, 1991, Debener et al., 2000). Although, also evident in seasonal affective disorder (Allen et al., 1993), frontal alpha asymmetry in depression has not always been shown (Reid et al., 1998) and, when found, it has been shown to be specific to anxious depressed patients (Bruder et al., 1997). Posterior alpha asymmetry has either not been demonstrated (Schaffer et al., 1983, Henriques and Davidson, 1991, Reid et al., 1998) or has been evidenced by way of decreased right compared with left relative parietotemporal activation in non-anxious depressed compared with non-depressed individuals (Henriques and Davidson, 1990, Bruder et al., 1997). Just as the range of normal emotional experiences may depend on a complex patterning of hemispheric neural activation (Heilman, 1997), the theoretical/clinical significance of neuroelectric asymmetries in affective disorders has been frequently couched in approach-withdrawal theories of hemispheric specialization (Kinsbourne, 1978), with emphasis on the complementary functioning of the two hemispheres. A relative decrease in left compared with right hemispheric EEG activity in a given individual is believed to be associated with a paucity of approach or an excess of withdrawal behaviors (Fox, 1991, Davidson and Hugdahl, 1995, Davidson, 1998a, Davidson, 1998b, Reid et al., 1998).
The degree to which EEG features of depression reflect state-like processes sensitive to treatment effects and outcome, as opposed to trait-related vulnerability processes, predisposing to depressive disorders, is as yet unclear as one report has found no differences between chronically medicated and unmedicated patients (Shagass et al., 1988) and no changes with pharmacotherapy (Kwon et al., 1996), while other individual studies have observed decreases in delta (Knott and Lapierre, 1987) and beta (Tarn et al., 1993) and increases in theta (Knott et al., 1996) band voltage following chronic treatment with the tricyclic antidepressant, amitriptyline. Although findings of temporal instability of anterior EEG asymmetry in depressed individuals challenge the notion that this neuroelectric alpha index is a trait marker of clinical depression (Debener et al., 2000), evidence that frontal alpha asymmetry in depression may reflect stable state-independent cerebral processes is seen with reports of diminished left anterior activation in asymptomatic individuals previously diagnosed with depression (Henriques and Davidson, 1990), and of stable anterior activation profiles in seasonal affective disorder patients before and after successful phototherapy treatment (Allen et al., 1993).
This investigation, taking advantage of an on-site recruitment for depressed males to be enrolled in a Phase II drug trial, aimed to contribute to the literature relating EEG to depression by attempting to replicate previous power and inter-hemispheric power findings in a sample of clinically depressed males and by expanding the scope of the regional and hemispheric electrophysiologic study of depression in: (a) casting the measurement net to include the assessment of inter-hemispheric and intra-hemispheric relationships; and (b) extracting measures of EEG frequency and coherence as well as the traditional measures of absolute and relative power. It was reasoned that these additional measurement probes, useful in their own right, may also be of use in interpreting findings with the more traditional power and measures, and they may serve as a preliminary database for formulating testable hypotheses that may be examined in future confirmatory investigations enquiring into gender differences in the neuroelectrophysiology of depression.
Section snippets
Subjects
Seventy right-handed males (outpatients) with a DSM-IV (American Psychiatric Association, 1994) diagnosis of unipolar major depressive disorder, scoring 18 or higher on the 17-item Hamilton Depression Scale (HAM-D: Hamilton, 1967), being free of any centrally acting medications, and exhibiting no concomitant psychiatric (including alcohol/drug abuse) disorders, clinically significant physical/neurological (including seizures) conditions or laboratory abnormalities, were entered into the study
Results
One patient evidenced excessive EEG artifact, resulting in 69 patient recordings being compared with 23 control recordings.
Discussion
This is one of a few studies, as per John et al., (1988) and Prichep and John (1992), to examine a relatively wide range of spectrally and statistically derived EEG features in relation to clinical depression. Although significant patient vs. control group differences were seen with a number of the features, the significance of the response profiles must be viewed within the limitations of the study, including the restricted sampling of male patients, the use of unweighted (vs. weighted)
Acknowledgements
This research was supported in part by Boehringer Ingelheim (Canada) Ltd.
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