Summerside Erik M, Courter Robert J, Shadmehr Reza, Ahmed Alaa A
bioRxiv. 2023 Aug 29:2023.08.28.555022. doi: 10.1101/2023.08.28.555022.
As people age, they move slower. Is age-related reduction in vigor a reflection of a reduced valuation of reward by the brain, or a consequence of increased effort costs by the muscles? Here, we quantified cost of movements objectively via the metabolic energy that young and old participants consumed during reaching and found that in order reach at a given speed, older adults expended more energy than the young. We next quantified how reward modulated movements in the same populations and found that like the young, older adults responded to increased reward by initiating their movements earlier. Yet, their movements were less sensitive to increased reward, resulting in little or no modulation of reach speed. Lastly, we quantified the effect of increased effort on how reward modulated movements in young adults. Like the effects of aging, when faced with increased effort the young adults responded to reward primarily by reacting faster, with little change in movement speed. Therefore, reaching required greater energetic expenditure in the elderly, suggesting that the slower movements and reactions exhibited in aging are partly driven by an adaptive response to an elevation in the energetic landscape of effort. That is, moving slower appears to be a rational economic consequence of aging.
Healthy aging coincides with a reduction in speed, or vigor, of walking, reaching, and eye movements. Here we focused on disentangling two opposing sources of aging-related movement slowing: reduced reward sensitivity due to loss of dopaminergic tone, or increased energy expenditure movements related to mitochondrial or muscular inefficiencies. Through a series of three experiments and construction of a computational model, here we demonstrate that transient changes in reaction time and movement speed together offer a quantifiable metric to differentiate between reward- and effort-based alterations in movement vigor. Further, we suggest that objective increases in the metabolic cost of moving, not reductions in reward valuation, are driving much of the movement slowing occurring alongside healthy aging.
随着人们年龄的增长，他们的动作会变慢。与年龄相关的活力下降是大脑对奖励的估值降低的反映，还是肌肉努力成本增加的结果？在这里，我们通过年轻和年长参与者在伸手动作过程中消耗的代谢能量客观地量化了运动成本，发现为了以给定速度伸手，老年人比年轻人消耗更多能量。接下来，我们量化了奖励如何调节同一人群的动作，发现与年轻人一样，老年人通过更早开始动作来对增加的奖励做出反应。然而，他们的动作对增加的奖励不太敏感，导致伸手速度几乎没有或没有调节。最后，我们量化了增加努力对年轻人奖励调节动作的影响。与衰老的影响类似，当面对增加的努力时，年轻人对奖励的反应主要是反应更快，动作速度变化很小。因此，伸手动作在老年人中需要更大的能量消耗，这表明衰老中表现出的较慢动作和反应部分是由对努力的能量格局升高的适应性反应驱动的。也就是说，动作变慢似乎是衰老的一种合理经济后果。
健康衰老与步行、伸手和眼球运动的速度或活力下降同时出现。在这里，我们专注于解开与衰老相关的运动减慢的两个相反来源：由于多巴胺能张力丧失导致的奖励敏感性降低，或与线粒体或肌肉效率低下相关的能量消耗增加的运动。通过一系列三个实验和构建一个计算模型，我们在此证明反应时间和运动速度的瞬态变化共同提供了一个可量化的指标，以区分基于奖励和基于努力的运动活力变化。此外，我们认为运动代谢成本的客观增加，而不是奖励估值的降低，是导致健康衰老过程中许多运动减慢的原因。
