A surgical planning method for functional MRI assessment of language dominance: Influences from threshold, region-of-interest, and stimulus mode
Ralph Suarez, Stephen Whalen, James P. O’Shea, Alexandra J. Golby
Presurgical determination of language laterality is
an important step for assessing potential risk of dysfunction
resulting from brain resection within or near suspected
language areas. Image-based functional MRI (fMRI) methods
seek to address limitations to the clinical gold-standard
technique by offering a safer, less costly, and non-invasive
alternative. In this study we outline a set of protocols for
objective determination of langue-specific asymmetry from
fMRI activation maps. We studied 13 healthy, right-handed
volunteers using a vocalized antonym-generation task.
Initially, using the standard threshold-dependent laterality
index (LI) procedure, we demonstrated an undesirably high
degree of intra-subject variability and indeterminacy in LI
value. We addressed this issue by implementing a novel
threshold-independent method, resulting in a single, unambiguous
LI for each subject. These LIs were then averaged
across the group and used to compare functional laterality
within the whole hemispheric volumes and six intrahemispheric
regions-of-interest (ROIs). We noted that as a
result of increased bilateral activation from vocalizations,
laterality assessment calculated from the whole hemisphere
resulted in insignificant asymmetry. However, by focusing
the LI exclusively on the inferior frontal (IFG) and supramarginal
gyri (SMG), robust leftward asymmetries were
observed. We also examined the influence of stimulus mode
on the group mean ROI LI, and observed an increase in IFG
asymmetry using visual mode, and in SMG using the
auditory mode. Based on these findings, we make recommendations
for optimized presurgical protocols.
Group-level fMRI activation maps from all subjects (n=13)
performing vocalized antonym-generation in visual mode (left panel)
and in auditory mode (right panel). Consistent activation, independent
of stimulus mode, was observed bilaterally in the precentral gyrus
(PCG), and favoring the left hemisphere in the inferior frontal gyrus
(IFG) and the posterior portion of the superior temporal gyrus (TPG).
Visual mode activation was observed bilaterally in the middle
occipital gyrus (MOG); auditory mode was associated with bilateral
activation in the middle portion of the superior temporal gyrus (TTG).
Image threshold at P<0.000001, uncorrected.
A surgical planning method for functional MRI assessment of language dominance: Influences from threshold, region-of-interest, and stimulus mode
Ralph Suarez, Stephen Whalen, James P. O’Shea, Alexandra J. Golby
Brain Imaging and Behavior
June 2008, vol 2, no 2. 59-73.