The British Journal of Psychiatry (2000) 177: 468
© 2000 The Royal College of Psychiatrists
Transcranial magnetic stimulation: asymmetrical excitability and depression
B. J. Moore
The University Department of Psychiatry, Royal Liverpool University
Hospital, Liverpool L69 3GA
EDITED BY MATTHEW HOTOPF
Maeda et al (2000)
have succeeded in demonstrating the interhemispheric asymmetry of motor
cortical excitability in major depression, using transcranial magnetic
stimulation (TMS). This is an important finding that raises questions not only
about the pathophysiology of major depression, but also about the state or
trait nature of the results.
In discussing possible explanations for this functional asymmetry the
authors consider the activity of inhibitory interneurons between cortical
output cells, as proposed by Wasserman et al
(1996), but it is not clear
whether this mechanism is thought to act within the hemisphere being
stimulated. The role of transcallosal inhibitory mechanisms has been
demonstrated in schizophrenia (Davey et
al, 1997; Boroojerdi
et al, 1999) and is likely to be relevant to
understanding asymmetrical motor thresholds in depression. In support of this
view, Menkes et al
(1999) hypothesised that
depression is associated with decreased left hemisphere excitability with
respect to the right hemisphere. They successfully showed that inhibitory
low-frequency repetitive TMS applied to the right frontal lobe produced a
significant anti-depressant effect, in contrast to exciting the left frontal
lobe by means of fast-frequency repetitive TMS, the antidepressant effects of
which have been known for some years.
Furthermore, Maeda et al report mean motor thresholds in the
depression group of 41.13% for the left hemisphere and 37.63% for the right
hemisphere, and in the healthy group of 48.29% for the left hemisphere and
52.7% for the right hemisphere. This gives a mean motor threshold of 39.38%
for the depression group and 50.50% for the controls, which suggests important
differences in both absolute threshold and laterality between the groups. Any
changes to either of these parameters in subjects recovered from depression,
and possibly in their first-degree relatives, not only promises new insights
into the pathophysiology of depression, but also may provide clues about the
most elusive object, a biological marker for depression.
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