Functional MRI of memory in the hippocampus: Laterality indices may be more meaningful if calculated from whole voxel distributions
Daniel M. Branco, Ralph O. Suarez, Stephen Whalen, James P. O~@~YShea, Aaron P. Nelson, Jaderson C. da Costa, and Alexandra J. Golby
Lateralization of memory by functional MRI (fMRI) may be helpful
for surgical planning related to the medial temporal lobe (MTL). Most
fMRI memory studies have calculated lateralization indices (LI) in the
MTL from suprathreshold voxels only, but the selection of threshold
remains highly arbitrary. We hypothesized that LIs could be reliably
extracted from the distribution of voxels encompassing all positive T
statistical values, each weighted by their own statistical significance.
We also hypothesized that patient LIs that are two or more standard
deviations (SD) away from the control group mean LI may be more
clinically relevant than LIs that are not compared to control group.
Thirteen healthy subjects had memory fMRI, and five epilepsy patients
had both fMRI and the intracarotid amobarbital procedure (IAP). The
fMRI task consisted of encoding patterns, scenes, and words. We found
that normal subject LIs extracted from whole weighted statistical
distributions tended to lateralize to the left for words, to the right for
patterns, and intermediately for scenes, consistent with previous
research. Weighted LIs were less variable than those calculated from
suprathreshold voxels only. Using this approach, all patients had fMRI
memory lateralizations consistent with IAP results. The weighted LIs
provided a more clear-cut distinction of patients from the normal
group (in terms of SDs from the group mean) than the suprathreshold
voxel count approach. Our results suggest that using weighted
distributions can be a useful strategy for assessing memory lateralization
by fMRI in the MTL.
Activations in the anterior hippocampus. Left panel: activations of a healthy subject encoding scenes overlaid on subject’s own structural T1 image.
Crosshairs indicate stronger activations on the right hippocampus. Right panel: random-effects activations for the combined effect of patterns, scenes, and
words encoding overlaid on average T1 image obtained from all subjects. Crosshairs indicate stronger activations on the left hippocampus.
Functional MRI of memory in the hippocampus: Laterality indices may be more meaningful if calculated from whole voxel distributions
Daniel M. Branco, Ralph O. Suarez, Stephen Whalen, James P. O~@~YShea, Aaron P. Nelson, Jaderson C. da Costa, and Alexandra J. Golby
Neuroimage
2006; 32, 592-602.