| With the aggravation of the aging population problem,the visual perception deficit of the elderly has attracted more and more researchers’ attention.The impairment of visual perception,especially the impairment of visual motion perception,seriously affects the older adults’ life quality and even the safety.Global Motion Perception(GMP)is the most important part of visual motion perception.The damage of global motion perception in the elderly may reflect the decline of their cognitive ability,and even predict the severity of the clinical evaluation of Alzheimer’s Disease.However,there are many limitations in the current research on the GMP aging.This study addressed this topic by using psychophysical methods,questionnaire methods and resting functional magnetic resonance imaging technique,and recruited young adults aged18~30 years and older adults aged over 60 years without cognitive impairment as participants.Here,using random dot kinematograms(RDKs),we took the global motion coherence threshold(MCT)as the indicator to evaluate the individuals’ perceptual sensitivity to global motion.In experiment 1a~d,using horizontal,vertical,rotational and radial motion patterns,we compared the MCT between the younger adults and the older adults,and found that younger adults’ MCT was significantly higher than that of younger adults,especially in right,down,clockwise and expansion directions.These results indicated that the GMP had a significant aging effect in the elderly,and this aging effect was more serious in the sensitivity of right,down,clockwise and expansion motion perception.In experiment 1e,the results of experiments 1a~d were integrated with the mini-meta analysis technique to estimate the total effect of GMP aging.It was found that the effect size of the difference of GMP between the older adults and the younger adults was significantly greater than 0,indicating that the GMP sensitivity of the elderly was generally decreased.Experiment 2,using the optimal scale regression model,investigated the possible subjective factors influencing the GMP aging based on data from questionnaire and psychophysical test.It was found,for the elderly,individuals with habits of certain physical exercise and reading exhibited higher sensitivity to GMP that individuals without those habits.For the younger,individuals with habit of game playing exhibited higher sensitivity to GMP that individuals without the habit.The results suggested that the factors affecting the GMP of the elderly were different from those affecting the GMP of the younger ones.Additionally,for the gender factor,the female elderly showed lower sensitivity to GMP only in the expansion motion.In experiment 3,the cognitive mechanism of the GMP aging and its influencing factors were discussed by using the perceptual template model(PTM)and operating the speeds of the dots as the external noise.It was found that,within 1°/s to 9.5°/s’ dots speed,the older adults had significantly higher MCT than the younger adults.Then PTM fitting for the two groups’ threshold showed that compared to the younger adults,both of the Aa and Af of the PTM for the older adults were much more than 1(the default value of the younger adults)and the value of Aa was far greater than that of the Af.These results demonstrated that there was a significant aging of GMP in the older adults,and the decline in their speed sensitivity in GMP might be due to the enhanced additive internal noise combined with the weakened ability to exclude external noise,while the enhanced additive internal noise played a major role.In addition,it was found that: 1)the female elderly showed the lower sensitivity to GMP when the dots moving at a medium speed than the males,and the stronger additive internal noise;2)the old individuals with a habit of certain physical exercise exhibited higher sensitivity to GMP and weaker additive internal noise than individuals without such habit;3)the old individuals with a habit of reading exhibited higher sensitivity to GMP and weaker additive internal noise than individuals without such habit;4)the old individuals with abnormal brain structure exhibited lower sensitivity to GMP and stronger additive internal noise than individuals with normal brain structure aging.These results suggested that physical exercise and reading habits might help to slow the progression of the GMP aging,and an explicit reduction of the GMP might indicate a trend of pathological changes of brain structure in the elderly.Experiment 4,using resting state functional magnetic resonance imaging(rsf MRI),analyzed the functional alterations of brain regions related to GMP processing in the elderly,and the relationship between these functional alterations and their MCT.Here,younger adults and the elderly with normal aging brain structure were recruited.The results showed that the older adults’ Re Ho values at bilateral Ceneus,Middle and Inferior occipital gyrus in right hemisphere were lower than younger adults,and these Re Ho values were significant negatively correlated to their MCT.There was also a negative correlation between MCT and the ALFF of MT/V5 in right hemisphere and the Middle occipital gyrus in left hemisphere.And older adults’ ALFF values of MT/V5 in both hemisphere were significantly lower than younger adults,while the ALFF of left middile occipital gyrus was higher than younger adults.For voxel-wise FC,the following FC were all stronger for older adults than for the younger adults-the FC between V1 and right Rectus,the FC between V2 and left paracentral lobule,right cerebellum,right orbitofrontal inferior frontal gyrus and right Super frontal gyrus,the FC between V3 and left superior temporal gyrus,right Inferior parietal gyrus and right posterior central gyrus,and the FC between MT/V5 and lingualis gyrus,left superior gyrus,right MT/V5 and left middle occipital gyrus.And these voxel-wise FC values were all positively correlated to their MCT.For ROI-wise FC,the FC strengths between V1 and V3,MT/V5,V2 and V3,MT/V5,and V3 and MT/V5 in the older adults were significantly higher than those in the younger adults,and were significantly positively correlated with their MCT.For the brain function network,the node degree,node efficiency and betweenness of the superior temporal gyrus and the inferior temporal gyrus for the older adults were significantly lower than those in the younger.The "small world" attribute of the older adults was significantly lower than that of the younger adults,and there was a significant negative correlation between this attribute and their MCT.These results suggested that the reduction of GMP sensitivity in the elderly is associated with local functional activity,functional connection strength in V1,V2,V3 and MT/V5 regions,and whole brain functional activities.In summary,the results of this study supported the view that there is an aging effect on the GMP.And our study first found that this aging effect has directional specificity.The decrease of GMP sensitivity for the elderly was due to the enhancing of internal additive noise and the weakening of the ability to exclude external noise.The habits of physical exercise and reading and abnormal alteration of brain structure might have impacts on individuals’ GMP sensitivity and parameters of the PTM.With regard to the resting brain function,the reduction of the GMP sensitivity for the elderly was related to alterations of the regional functional activity and the strength of functional connectivity in V1,V2,V3 and MT/V5,as well as the functional activities of the whole brain.These results provide a scientific reference for theory construction on the GMP aging and strategy making for slowing the decline of GMP sensitivity in the elderly. |
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