Biomedical Engineering Reference
In-Depth Information
linking frontal EEG asymmetry to the activity of underlying neural systems involved
in the experience, expression, and regulation of emotion is considerably lacking.”
Although more research in this area is desirable, a number of approaches have
already yielded important results.
One measurable manifestation of lateralized processing is the “perceptual pref-
erence” for one hemisphere over the other during dichotic listening tasks. In
dichotic listening paradigms, different stimuli—a pair of words or of tones—are
simultaneously presented to the left and right ears; these stimuli compete with one
another for identification, and the advantage for hearing items in the right or left ear
is referred to as perceptual asymmetry (PA), an index of which (contralateral) hemi-
sphere is favored for processing this verbal or tonal data. Studies using dichotic lis-
tening tests [116] have indicated that pretreatment measures of functional
asymmetry of the brain are related to subsequent responsiveness to treatment with
the selective SRI (SSRI) fluoxetine [117-119]. Individuals with MDD who
responded well to fluoxetine exhibited greater left-hemispheric PA for perceiving
dichotic words and less right-hemisphere PA for complex tone stimuli [117].
In a two-sample replication/extension study of PA before and after treatment
with fluoxetine, PA did not change with fluoxetine treatment, so this measure may
be considered as a stable, enduring, “trait” characteristic [118]. Replication and
extension studies found the relationship of PA to treatment outcome to be depend-
ent on gender [118]: Women but not men exhibited a heightened left-hemisphere
perceptual advantage for words, while men but not women showed a reduced
right-hemisphere advantage for tones among fluoxetine responders.
Perceptual asymmetries may also be related to resting-state asymmetries in the
EEG. Bruder and colleagues [120] built on their dichotic listening work and studied
patients entering treatment with fluoxetine with EEG as well, and found that
responders and nonresponders differed not only in their pretreatment PA measure
during dichotic listening, but also in their resting-state EEG alpha asymmetry.
Nonresponders showed an alpha asymmetry indicative of overall greater activation
of the right hemisphere than the left, whereas responders did not (eyes-open, resting
state). This relationship between hemispheric asymmetry and treatment response
interacted with gender, being present for female but not male subjects. In a recent
project extending that work [121], it was reported that fluoxetine responders were
characterized by greater alpha power compared with nonresponders and with
healthy control subjects, with the largest differences being detected at occipital sites.
They also reported differences in alpha asymmetry between responders and
nonresponders at occipital sites, with responders showing greater alpha (less
activity) over right than left hemisphere.
As to putative mechanism(s) relating this phenomenon to treatment response,
perceptual asymmetry has been found to be significantly associated with plasma
cortisol levels in MDD subjects [122], a neuroendocrine abnormality found in many
MDD patients. Given that serotonergic activity may be related to arousal [121], it
was hypothesized that the increased alpha power found in depressed patients who
respond to an SSRI might reflect low arousal associated with low serotonergic activ-
ity. Researchers noted that the right temporoparietal and subcortical regions were
particularly important in mediating arousal, which might account for their alpha
asymmetry observations, and suggest that low serotonergic activity, tied to activity
Search WWH ::
Custom Search