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White matter (WM) functional activity has been reliably detected through functional magnetic resonance imaging (fMRI). Previous studies have primarily examined WM bundles as unified entities, thereby obscuring the functional heterogeneity inherent within these bundles. Here, for the first time, we investigate the function of sub‐bundles of a prototypical visual WM tract—the optic radiation (OR). We use the 7T retinotopy dataset from the Human Connectome Project (HCP) to reconstruct OR and further subdivide the OR into sub‐bundles based on the fiber's termination in the primary visual cortex (V1). The population receptive field (pRF) model is then applied to evaluate the retinotopic properties of these sub‐bundles, and the consistency of the pRF properties of sub‐bundles with those of V1 subfields is evaluated. Furthermore, we utilize the HCP working memory dataset to evaluate the activations of the foveal and peripheral OR sub‐bundles, along with LGN and V1 subfields, during 0‐back and 2‐back tasks. We then evaluate differences in 2bk‐0bk contrast between foveal and peripheral sub‐bundles (or subfields), and further examine potential relationships between 2bk‐0bk contrast and 2‐back task d‐prime. The results show that the pRF properties of OR sub‐bundles exhibit standard retinotopic properties and are typically similar to the properties of V1 subfields. Notably, activations during the 2‐back task consistently surpass those under the 0‐back task across foveal and peripheral OR sub‐bundles, as well as LGN and V1 subfields. The foveal V1 displays significantly higher 2bk‐0bk contrast than peripheral V1. The 2‐back task d‐prime shows strong correlations with 2bk‐0bk contrast for foveal and peripheral OR fibers. These findings demonstrate that the blood oxygen level‐dependent (BOLD) signals of OR sub‐bundles encode high‐fidelity visual information, underscoring the feasibility of assessing WM functional activity at the sub‐bundle level. Additionally, the study highlights the role of OR in the top‐down processes of visual working memory beyond the bottom‐up processes for visual information transmission. Conclusively, this study innovatively proposes a novel paradigm for analyzing WM fiber tracts at the individual sub‐bundle level and expands understanding of OR function. [ABSTRACT FROM AUTHOR]

Objective: To use SIV-mac239-infected Chinese rhesus monkeys to study white matter changes with and without regular combined antiretroviral therapy (cART) and the relationships between the changes and clinical results. Methods: Diffusion tensor imaging (DTI) data were collected at baseline and 10 days, 4 weeks, 12 weeks, 24 weeks, and 36 weeks after viral inoculation. Plasma CD4 T cell counts, CD4/CD8 ratio, plasma viral load, and cerebrospinal fluid (CSF) viral load were collected at baseline and 1 week, 5 weeks, 12 weeks, 24 weeks, and 36 weeks after viral inoculation. Microstructural characteristics were examined within 76 white matter areas defined by the DTI-white matter (WM) atlas for rhesus macaques. Corrections for multiple comparisons were performed using a false discovery rate (p < 0.05, FDR). Correlation analyzes between imaging markers and clinical markers (plasma CD4 T cell counts, CD4/CD8 ratio, plasma viral load, and cerebral spinal fluid viral load) were performed using Pearson correlations. Results: White matter changes in SIV-infected macaques were detected in different brain regions as early as 4 weeks after inoculation. As time progressed, cART reversed, ameliorated, or even enhanced the effects. The CD4 T cell count was mainly associated with DTI metrics before cART, while the CD4/CD8 ratio was associated with white matter changes with and without cART. Viral load was positively associated with mean diffusivity in HIV patients without cART, and the opposite results were seen in HIV patients with cART. Conclusion: SIV-mac239 infection may be an ideal tool for studying HIVinduced changes in the brain. The first white matter changes appeared in a structure adjacent to the periventricular area as early as 4 weeks after inoculation. As time progressed, cART had different effects on different regions, reversing, attenuating, or even progressing the pathology. Moreover, these changes were closely related to the CD4/CD8 ratio and viral load, even after cART. [ABSTRACT FROM AUTHOR]