Relation of white matter anisotropy to visual memory in 17 healthy subjects

Abstract

We performed a Rey visual design learning test (RVDLT) in 17 subjects and measured intervoxel coherence (IC) by DTI as an indication of connectivity to investigate if visual memory performance would depend on white matter structure in healthy persons. IC considers the orientation of the adjacent voxels and has a better signal-to-noise ratio than the commonly used fractional anisotropy index. Voxel-based t-test analysis of the IC values was used to identify neighboring voxel clusters with significant differences between 7 low and 10 high test performers. We detected 9 circumscribed significant clusters (p < .01) with lower IC values in low performers than in high performers, with centers of gravity located in left and right superior temporal region, corpus callosum, left superior longitudinal fascicle, and left optic radiation. Using non-parametric correlation analysis, IC and memory performance were significantly correlated in each of the 9 clusters (r < .61 to r < .81; df = 15, p < .01 to p < .0001). The findings provide in vivo evidence for the contribution of white matter structure to visual memory in healthy people.

Introduction

The assessment of visual memory is one way to examine memory performance. The Rey visual design learning test (RVDLT) is an elementary motion, color, and word independent visual memory test that consists of 15 abstract, black-and-white lined, straightforward, 2-dimensional figures (Spreen and Strauss, 1991). Subjects are required to memorize the figures in 5 stages. The number of recalled figures counts for the performance. We tested 17 healthy people and found a striking dichotomy in 7 low performing subjects (scoring 5–9) and 10 high performing subjects (scoring 13–15). The previous findings on a relationship between white matter structure detected by diffusion tensor imaging and cognitive functions imply that visual memory performance may be associated with white matter structure (O'Sullivan et al., 2001). As yet, to the best of our knowledge, studies examining visual memory and white matter structure have not been performed in healthy subjects.

Diffusion tensor imaging (DTI) is a new approach to assessing tissue structure and provides information about white matter structure. DTI measures diffusion of water molecules in three dimensions. Diffusion of water perpendicular to the direction of the axons is restricted by the myelin sheath and cell membrane such that diffusion will be greater along the length of the axon than perpendicular to the axon. Thus, DTI measures diffusion-driven displacements of molecules during their random path along axonal fibers, expressed as fractional anisotropy (FA) or intervoxel coherence (IC) ranging from 0 (isotropic medium) to 1 (fully anisotropic medium). FA is a measure that quantifies the degree to which diffusion differs in the three dimensions. IC considers the degree of collinearity between the diffusion tensor of the reference voxel and the adjacent voxels, and, in addition, guarantees a better signal-to-noise ratio than the commonly used FA (Pierpaoli and Basser, 1996). Hence, based on the determination of the similarity of orientation of adjacent voxels, IC reflects a measure of connectivity, expressing fiber coherence at the voxel level with a spatial sampling limited by voxel size. The relation between diffusivity of water molecules in human tissue expressed by anisotropy indices and clinical symptoms was verified post-mortem (Mottershead et al., 2003). After scanning, the myelin content and the axonal density of the specimens were evaluated neuropathologically using quantitative techniques. Myelin content and axonal density strongly correlated with diffusion anisotropy. We aimed to test the hypothesis whether subjects performing low compared to high on the RVDLT (expressed as number of recalled figures) would be different in white matter structure (expressed as IC) (Fig. 1).