Differential aging effects on visuomotor control : evidence for an adaptive aging brain





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Aging in psychology has been investigated for decades and general aging declines in behavioral cognitive tasks have been well established. With advances in neuroimaging techniques, some changes have been found in the brain activity patterns of older adults. Their associations with better performance indicate their functions as compensations rather than impairments. The nature of these compensations is unknown: Are they passive responses to structure impairments or active adaptations to the altered environments. Sensorimotor control is supposed to be a pretty good way to explore it because of its natural connections with the external environments and its strong interactions with cognition. Visuomotor control has been extensively explored with visuomotor adaptation tasks and distinct mechanisms and subcomponents have been identified, such as model based learning and model free learning. In model-base learning, multiple states have been found: fast state, responding strongly to error whereas retaining information poorly, and slow state, responding weakly to error whereas retaining information well. Corroborating these multiple states, fast adaptation and slow adaptation have been identified as well as learning processing and retention processing. For model free learning, recently visuomotor adaptation has been found to be impacted by reward and punishment. How aging impacts these processes has been relative less explored. Saccadic eye movement is so brief that its online trajectory is resistant to any sensory feedback, making it an especially good way to investigate model-based learning. Saccade adaptation could be induced by different factors, ranging from low-level intrasaccadic step to high-level perceptual task. The first study investigated how aging impacts these two different processing with two different paradigms. A double-step paradigm was used in the first experiment which was designed to trigger primarily low-level, gradual motor adaptation and a perceptual task paradigm was used in the second experiment in which adaptation was induced by a perceptual task that emphasizes high-level, fast processes. Equivalent adaptation of saccadic gain was found in the two age groups in the first experiment whereas the fast, strategic adaptation response was significantly more pronounced in the young adult group in the second experiment. Reaching movement is with longer duration and less precision so that it is a pretty good complimentary way to investigate model free learning. The second study investigated age-related effects on learning processing and retention processing in reaching adaptation and motivational modulations of these effects. In the study, a reaching task was used in which participants were asked to make fast shooting movements towards visual targets with their right hand through a robotic manipulandum (vBOT system) allowing to measure reaching trajectories. Adaptation was induced by a 30° screen-cursor visuomotor rotation. Participants were assigned to one of motivational feedback conditions, i.e. neutral, reward, or punishment. Reward and punishment feedback was based on reaching endpoint error. Equivalent retention rate was found in the two age groups whereas learning rate was smaller in older adults. However, the learning rate was equivalently enhanced by reward in the two groups suggesting the benefits from motivational feedback during reaching adaptation so that age-related differences in visuomotor plasticity, though persisting, can be attenuated.Saccade landing position and saccade latency has been found to be impacted by low-level factor salience and high-level factor value, making saccadic eye movement a good way to explore decision making. Recently the integration of salience and value was found to be dynamic, i.e. their influence on saccade landing position is dependent on saccade latency. The third study investigated how aging impacts these dynamics. In the study, salience and expected value was traded off by asking participants to make saccades to target patches with subregions differing in salience and associated reward. No differences were found between two groups on saccade landing position when only salience was administered. A significant interaction was found between value manipulation and age group on saccade landing position with equivalent reward effects in two groups but weaker penalty effects in senior group. When respecting with saccade latency, reward triggers an average latency decrease in senior adults but not in young adults. These differential reward effects on saccade landing position and saccade latency were also reflected by the less dependence of saccade direction on latency although the dependence exists in both groups. Taken together, this thesis consistently found reserved low-level processing in older adults, such as equivalent gradual adaptation in the first study, equivalent retention rate in the second study, equivalent influence of salience in the third study. However, the high-level processing was impaired in older adults, such as impaired faster adaptation in the first study, reduced learning rate in the second study, reduced reward effects on saccade latency in the third study. The impairments could be result from impairments in the brain area the high-level processing involved in or from strategy changes.




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