Quantifying visual pathway overlap to identify training-related microstructural change in hemianopia

BackgroundVisual training can improve residual vision in hemianopia, but there is significant variability in patient outcomes which may reflect differences in neural structures preserved after injury. Diffusion MRI tractography offers a means of assessing white-matter microstructure in vivo, however substantial overlap between neighbouring visual pathways complicates attribution of differences to specific tracts. We quantified tract overlap to identify relatively unique pathway segments to assess training-related microstructural change. MethodsDiffusion MRI data was acquired in six participants with homonymous hemianopia before and after 3-6 months of visual training. Tractography was used to reconstruct three pathways implicated in residual vision: lateral geniculate nucleus (LGN)-V1, LGN-V5 and pulvinar-V5, together with secondary superior colliculus-LGN and superior colliculus-pulvinar pathways. Streamline overlap was quantified along neighbouring tracts, and comparison of pre- to post-training fractional anisotropy was restricted to unique or pair-specific non-overlapping segments. Exploratory relationships with visual outcomes were also examined. ResultsFractional anisotropy increased after training in both early and late pair-specific segments of the LGN-V5 pathway in the lesioned hemisphere. In the late segment, this increase was significantly greater than in the corresponding unique LGN-V1 segment, whereas change in the early LGN-V5 segment did not differ significantly from the unique pulvinar-V5 segment. No significant training-related changes were observed in collicular pathways. Brain-behaviour analyses also preferentially implicated LGN-V5: baseline fractional anisotropy was strongly associated with post-training Gabor detection, and training-related fractional anisotropy change showed a positive association with improvement in Gabor detection, although the latter did not survive multiple comparison correction. ConclusionThese findings support a role for the LGN-V5 pathway in visual plasticity after hemianopia and demonstrate the importance of explicitly quantifying tract overlap when attributing microstructural change to small, neighbouring white-matter pathways.