Combined fMRI and DTI of the human low level visual cortex

I. AbstractWe study the anatomical connectivity network in the human low-level visual cortex using in vivo Magnetic Resonance Imaging techniques.

8 different visual areas were functionally mapped in each hemisphere of 4 subjects, and the white matter

fibers network among them was then investigated using DTI. Various intra and inter-hemispheric

connections were found, globally supporting the ventral/dorsal streams segregation.

Keywords: fMRI, DTI, visual areas, retinotopy, functional mapping, Riemannian geometry, fiber tracking

N. Wotawa†, C. Lenglet†, M. Roth*, B. Nazarian*, J.-L. Anton*, R. Majhoub*, R. Deriche†, O. Faugeras†

(†) I.N.R.I.A., 2004 route des Lucioles, 06902 Sophia-Antipolis, France(*) Centre IRMf de Marseille, CHU Timone, 13385 Marseille, France Odyssée Lab

V. Results and Conclusion

III. ROIs definition from fMRI• Two different criteria to define various low-level visual areas: retinotopy and functional selectivity

• Visual stimuli: 300x300 pixels video, 20.9°x20.9° visual angle, 72Hz refresh rate

• Attention and fixation controlled through a discrimination task at the fixation cross (normal: 0.5°, test: 0.77°)

• Preprocessings:

Segmented areas: V1, V2v-V2d, V3v-V3d, V3A, V4.

Volumetric ROIs computed through a 3mm “retro-projection” along the GM/WM interface normals

80°, 8 rotations

4 rotations

2/ Functional mapping of hMT+ (10mns)

• hMT+ complex: visual motion sensitive area, presumably human homologue of MT/V5 + MST (+ FST)

• Classical block design: RDP in coherent radial motion (7.53 deg.s-1, 2Hz alternating) VS flicker or static

• 2 x 5mns run, 3x8 blocks of ~17secs each

• hMT+ volumetric ROIs extracted with SPM99 - Marsbar Toolbox

II. Data acquisition• 3T MEDSPEC 30/80 AVANCE (Bruker) at the Centre IRMf de Marseille, quadrature bird-cage head coil

• Anatomical image: T1-weighted MPRAGE, TR=25msec, TE=5ms, FOV 256x230x180mm 3,

voxel size = 1x0.75x1.22mm3, 15mns

• fMRI: 30 pseudo-coronal slices, TR=2.111sec, TE=35ms, voxel size=2x2x2mm 3

• DTI: 12 directions, repeated 8 times, b=1000 s.mm -2, =38.5ms, =21.6ms, TR=10000ms, TE=83ms.

voxel size = 2x2x2mm3, 20mns

•

Investigation of the low level visual cortex areas considering 3 different characterization criteria within

<1.5h of scans for the whole experiment

Intra and Inter-hemispheric connexions were found in all subjects

Good inter-subjects reproducibility (4 subjects)

Our data support the dorsal (V1, V3A, hMT+) and v entral (V1, V2, V3, V4) streams

1462 W-PM Human Brain Mapping, Toronto, Ontario, Canada, June 12-16, 2005

1/ Retinotopic mapping of occipital areas (25mns) Standard procedure by phase-encoded stimulation of the visual field

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Results of the fiber tracking with every white matter voxels taken as a seed from a single left hMT+

voxel. Dark values correspond to low mean along the fibers, indicating a strong connectivity index.

White matter segmentation (Brainvisa and Absolut).Each white matter voxel is considered as a seed for the

fiber tracking algorithm.

IV. DTI Fiber Tracking & Connections Likelihood

Basic idea: DTI fiber tracking and estimation of an index of connectivity for each fiber are conducted as follows. Diffusion tensor are used as a metric to first compute geodesic distances and then the shortest paths between the ROIs extracted from fMRI. A measure of likelihood for each computed fiber is then derived by looking at the minima of the distance function Euclidean gradient norm.

Estimation of diffusion tensors by a robust gradient descent algorithm (see poster 736 T-

PM)

ROI

Distance function computation:

Statistics of along two distinct fibers toward the same target. Fibers with high

variance (red) are discarded

3D rendering of geodesics with best statistical properties of with left hMT+ (red) as a target.Fiber tracts strongly connect contralateral hMT+ (green) and contralateral as well as ipsilateral (blue) V1 area.

Synthetic example of distance function

Real dataset distance function from a corpus callosum voxel over an axial slice

motion correction (INRIAlign Sofware) cortical surface-based anisotropic smoothing, FWHM=3mm (Odyssee Lab. software) MNI template normalization (SPM2)

Fiber with low confidenceMean = 1815Standard deviation = 1160

Fiber with high confidenceMean = 1020Standard deviation = 180