Review articleRight brain, left brain in depressive disorders: Clinical and theoretical implications of behavioral, electrophysiological and neuroimaging findings
Introduction
Studies in healthy adults and neurological patients have provided considerable evidence for asymmetries of right and left brain function (Springer and Deutsch, 1998, Hugdahl and Davidson, 2003). For over 30 years, studies have reported abnormalities of right-left asymmetry in depressive disorders. We review the findings of these studies and, in particular, examine how they are relevant to diagnosis and treatment of these disorders. Numerous studies have reported evidence of abnormal frontal and parietotemporal asymmetries in depressive disorders. The bulk of findings for frontal asymmetry have come from electroencephalographic (EEG) studies measuring alpha, which has received considerable attention in prior reviews and meta-analyses (Jesulola et al., 2015, Peltola et al., 2014, Thibodeau et al., 2006), and also in studies measuring glucose metabolism with positron emission tomography (PET) and regional cerebral blood flow (rCBF) with functional magnetic resonance imaging (fMRI). Although parietotemporal asymmetries have received less attention (Stewart et al., 2011), behavioral and event-related potential (ERP) studies using dichotic listening or visual hemifield tasks that tap functions mediated by this region have found considerable evidence of abnormal lateralized cognitive and emotional processing in patients having depressive disorders (see Bruder, 2003 for an earlier review). More recent studies have provided additional evidence of altered laterality in depression, particularly for processing emotional information. The present report gives particular attention to reviewing converging evidence from behavioral, EEG, ERP, and neuroimaging studies, which support hypotheses of reduced left frontal and right parietotemporal function in depression, and provide a new understanding of neural networks that may underlie this altered laterality. The relevance of right-left asymmetries for risk of developing a depressive disorder, diagnostic subtypes of depression, and clinical responsiveness to pharmacological and cognitive-behavioral treatments will also be a focus.
Section snippets
EEG alpha asymmetry
The EEG alpha rhythm (8–12 Hz) is maximal when one is in a relaxed, wakeful state with eyes closed, but is reduced when one becomes alert or opens the eyes. Alpha power has been found to be inversely related to cortical activity as measured by rCBF (Cook et al., 1998) or blood oxygen level in posterior regions where alpha is greatest (Feige et al., 2005). This inverse relation can lead to some confusion as to whether one is referring to hemispheric asymmetry of alpha power or cortical activity.
Dichotic listening asymmetry (Behavioral and ERP studies)
Early behavioral studies using neuropsychological tests of cognitive function in depressed patients reported greater deficits in functions related to the right (non-dominant) than left (dominant) hemisphere (Flor-Henry, 1976, Abrams et al., 1981). The greater right hemisphere deficits in depressed patients were more evident among women than men and clinical improvement during treatment was associated with improved cognitive function (Fromm-Auch, 1983). More recent studies have found deficits in
Visual hemifield asymmetry (Behavioral and ERP findings)
Differences in PA were found between patients having a bipolar (primarily Bipolar II) and unipolar depressive disorders who were tested in a dot enumeration task, which requires subjects to report the number of dots projected to either the right or left of fixation (Bruder et al., 1989). Patients having a unipolar depressive disorder and controls showed the expected right hemisphere advantage seen previously for the dot enumeration test (McClone and Davidson, 1973), whereas bipolar depressed
PET, MRI and NIRS neuroimaging findings
Neuroimaging studies using PET and MRI measures have found evidence of abnormalities involving multiple brain regions in depressed patients, including prefrontal cortex (PFC), superior temporal cortex, anterior cingulate cortex (ACC), amygdala, insula, basal ganglia and cerebellum (Fitzgerald et al., 2008, Pizzagalli, 2011, Rive et al., 2013, Sacher et al., 2012), but the extent to which these are lateralized to the right or left hemisphere has received less attention. Most imaging studies were
Conclusions
Studies using dichotic listening, visual hemifield, electrophysiological, and neuroimaging (PET, fMRI, NIRS) measures have found abnormalities of right-left asymmetries of brain function in depressive disorders. The bulk of evidence across studies using these diverse measures is consistent with reductions of left frontal and right parietotemporal function in depressive disorders. Relatively less left than right frontal activity in depression is in accordance with valence or approach/withdrawal
Acknowledgements
This work was supported by the New York State Office of Mental Health and the National Institute of Mental Health (MH36295).
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