Takemura, H. | Search bioRxiv

bioRxiv2018

Microstructural properties of the vertical occipital fasciculus explain the variability in human stereoacuity

Stereopsis is a fundamental visual function that has been studied extensively. However, it is not clear why depth discrimination (stereoacuity) varies more significantly among people than other modalities. Previous studies reported the involvement of both dorsal and ventral visual areas in stereopsis, implying that not only neural computations in cortical areas but also the anatomical properties of white matter tracts connecting those areas can impact stereopsis and stereoacuity. Here, we studied how human stereoacuity relates to white matter properties by combining psychophysics, diffusion MRI (dMRI), and quantitative MRI (qMRI). We performed a psychophysical experiment to measure stereoacuity, and in the same participants we analyzed the microstructural properties of visual white matter tracts based on two independent measurements, dMRI (fractional anisotropy, FA) and qMRI (macromolecular tissue volume; MTV). Microstructural properties along the right vertical occipital fasciculus (VOF), a major tract connecting dorsal and ventral visual areas, were highly correlated with measures of stereoacuity. This result was consistent for both FA and MTV, suggesting that the structural-behavioral relationship reflects differences in neural tissue density, rather than differences in the morphological configuration of fibers. fMRI confirmed that binocular disparity stimuli activated the dorsal and ventral visual regions near VOF endpoints. No other occipital tracts explained the variance in stereoacuity. In addition, the VOF properties were not associated with differences in performance on a different psychophysical task (contrast detection). These series of experiments suggest that stereoscopic depth discrimination performance is, at least in part, constrained by dorso-ventral communication through the VOF.

neuroscience↗

bioRxiv2017

Computational neuroanatomy of human stratum proprium of interparietal sulcus

Recent advances in diffusion-weighted MRI (dMRI) and tractography have enabled identification of several long-range white matter tracts in the human brain. Yet, our understanding of shorter tracts, such as those within the parietal lobe, remains limited. Over a century ago, a tract connecting the superior and inferior parts of the parietal cortex was identified in a post-mortem study: Stratum proprium of interparietal sulcus (SPIS; Sachs, 1892). The tract has since been replicated in another fibre dissection study (Vergani et al. 2014), however, it has never been identified in the living human brain and its anatomical properties are yet to be described. We used dMRI and tractography to identify and characterise SPIS in vivo, and explored its spatial proximity to the cortical areas associated with optic-flow processing using fMRI. SPIS was identified bilaterally in all subjects, and its anatomical position and trajectory are consistent with previous post-mortem studies. Subsequent evaluation of the tractography results using linear fascicle evaluation and virtual lesion analysis yielded strong statistical evidence for SPIS. We also found that SPIS endpoints are adjacent to the optic-flow selective areas. In sum, we show that SPIS is a short-range tract connecting the superior and inferior parts of the parietal cortex, wrapping around the intraparietal sulcus, and that it may be a crucial anatomy underlying optic-flow processing. In vivo identification and characterisation of SPIS will facilitate further research on SPIS in relation to cortical functions, their development, and diseases that affect them.

neuroscience↗