Global motion perception (GMP) is an important aspect of visual perception. Numerous studies have found that GMP of cognitively healthy elderly declines with aging and it cannot be explained by age differences in the visual system. The neural mechanism of aging on GMP remains unclear. Resting-state fMRI has been widely used to detect the internal spontaneous activity of the aging brain. Thus, whether the functional activity of brain regions related to the GMP for the elderly in the resting state is an effective indicator of their global motion sensitivity (GMS) still needs further investigation. To reveal the neural basis of GMP decline for the elderly, the relationships between the resting-state functional activities of GMP related brain areas (Regions of Interest, ROI: V1, V2, V3, and MT/V5) and individuals’ motion coherence threshold (MCT) were analyzed by using rs-fMRI technology.
In this study, Random Dot Kinematogram (RDK) paradigm was used to evaluate an individual’s GMS with the MCT as the indicator. A higher threshold of the RDK task meant lower sensitivity. Meanwhile, the rs-fMRI data of 36 younger adults (M = 22.04 years old) and 31 older adults (M = 65.05 years old) were acquired using rapid echo-planar imaging (EPI) sequence from a 3T Siemens Prisma magnetic resonance scanner, with TR = 2s, TE = 30ms, Time points = 240 (young) or 246 (old). Rs-fMRI data were preprocessed and processed using SPM (http://www. fil.ion.ucl.ac.uk/spm) and DPABI (http://rfmri.org/dpabi) toolbox to obtain the functional activities of the ROIs, including ReHo, ALFF, voxel-wise FC, and ROI-wise FC. Then, the regional measures including three nodal centrality metrics (degree K, efficiency Enodal and betweenness b) and the global measures including small-world parameters (clustering coefficient Cp, characteristic path length Lp, normalized clustering coefficient γ, normalized characteristic path length λ, and small-worldness σ) and network efficiency (global efficiency Eglob and local efficiency Eloc) were calculated using GRETNA (http//www.nitrc.org/projects/gretna/) graph toolbox. To determine whether there were significant group differences in these functional properties, two-sample t-tests were performed on each metric. When significant between-group differences in any functional metrics were obtained, the Pearson correlation coefficients among these metrics and individuals’ MCT were further calculated to assess the relationship between changes in brain function and GMP aging.
Results showed that 1) the ReHo values of right V3 and bilateral MT/V5 for older adults were significantly lower than that of younger adults, as well as the ALFF of bilateral MT/V5, and these functional metrics were significantly negatively correlated with individuals’ MCT; 2) The FCs between V2 and left primary motor cortex, V3 and left secondary visual cortex, MT/V5 and left premotor cortex, as well as the FCs between V1, V2, V3 and MT/V5 regions for the older adults were significantly stronger than that of younger adults, and these FCs were correlated with individuals’ MCT; 3) Older adults’ K, Enodal and b of most nodes in temporal lobe were significantly lower than younger adults, and the Enodal of the right temporal cortex were significantly negatively correlated with individuals’ MCT; 4) Older adults’ global network properties including small-world parameters (Cp, γ, Lp, λ, and σ) and network efficiency (Eglob and Eloc) were all significantly lower than younger adults’, with CP and Eloc significantly negatively correlated with their MCT.
These findings suggested that the decline of GMS for the elderly was associated not only with functional changes in the dorsal visual pathway, especially in the MT/V5, but may also with functional changes in broader areas of the whole brain, which supported the theory of “dedifferentiation”.