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44 TATE

One of the most frequently cited limitations of the improvements in DTI metrics also correlated
tDCS is its limited spatial precision in terms of the with measures of functional ability (i.e., motor) and
brain areas that are stimulated. tDCS spreads current task acquisition. Though these metrics can be influ-
over a wide area of the brain, and this creates diffi- enced by a number of biological factors, including
culties with relating the effects of tDCS to a specific membrane and myelin integrity, fiber density, and
brain region or network (64). tDCS not only affects even inflammatory processes (39,83), the combina-
the brain regions near the electrodes but may also tion of prospective changes noted after treatment
modulate functional connectivity between remote and important functional correlations with these
but functionally associated brain areas (63) or influ- changes emphasizes the value of utilizing DTI as a
ence within-network connectivity (46). The degree to non-invasive marker of tDCS therapeutic effects.
which the effects of tDCS applied to frequently-in- Early task related functional MRI studies using
vestigated regions such as the primary motor cortex tDCS therapeutic technologies reinforce and extend
(M1) extrapolate to additional areas of the cortex is the findings of the DTI studies. In healthy controls,
also unclear, particularly those involved in complex, anodal tDCS applied to the motor cortex results in
higher-order cognitive processes. Although promis- increased BOLD activation both during and/or for
ing short-term results have been reported in several a short period of time after stimulation (38,68,78).
populations, evidence from larger randomized con- Another interesting observation of these studies
trolled trials (RCTs) remains limited. Finally, optimal includes increased/decreased activation in adjacent
stimulation protocols (in terms of intensity, duration, areas known to be functionally and structurally con-
and repetition of stimulation) that lead to sustained nected to the area being directly stimulated. The signal
improvements in outcome measures relevant to activ- changes are consistent with changes in oxygen usage
ities of daily life have not yet been established and and indicate functional connectivity or co-active net-
should be investigated in future studies (41,64,69). works. Importantly, it appears that neuromodulation
There is a growing MRI literature examining the occurs on a network level rather than simply at the
structural and functional effects of tDCS. However, site of stimulation. This has significant implications
there are a number of caveats to consider when exam- for rehabilitation efforts that have not been examined
ining this literature. Though the motor cortex has thus far. As such, structural and functional MRI rep-
been a primary target for stimulation using tDCS, resents an interesting technology for evaluating the
location or placement of the electrodes does vary therapeutic effects of tDCS and promise to elucidate a
from study to study. In addition, the placement of number of important underlying biological processes
anodal and cathodal electrodes and the strength of the of neuromodulation technologies.
current used may or may not be consistent between
the studies. Regardless, MRI has proven effective in DISCUSSION AND FUTURE DIRECTIONS
elucidating structural and functional changes during Neuromodulation is a rapidly growing industry
and after tDCS stimulation. that promises a number of positive therapeutic and
To date, there do not appear to be any studies educational enhancement results. Given the intense
utilizing standard structural MRI (i.e., volumetric interest, there are a number of neuromodulation tech-
or shape analyses) to examine the effects of tDCS. nologies that have already obtained FDA approval for
More advanced, experimental structural imaging treatment of various neurologic conditions. Studies
methods like DTI have demonstrated a number of demonstrate a good safety record and report very
positive changes in diffusion metrics (i.e., decreased few side effects. Thus, these types of interventions
apparent diffusion coefficient (ADC) and increased may be particularly valuable in patients sensitive or
fractional anisotropy (FA)) in the areas targeted with vulnerable to the side effects of central nervous system
direct current (90). The improvements observed in (CNS) acting medications or where polypharmacy is
these metrics appeared only in the region and tracks already a concern, including pediatric, geriatric, and
ipsilateral to the stimulated hemisphere. Functionally, veteran populations. These technologies might also