Mental motor representations across the adult life-span: behavioural and fMRI evidence in explicit and implicit motor imagery tasks

Motor imagery (MI) is a cognitive state in which movements are mentally evoked. There is behavioural evidence that MI declines with aging, but limited information on the functional anatomical correlates of these changes. In my thesis, I present a systematic behavioural/fMRI investigation of this issue and demonstrate that aging is associated with modified brain responses ranging from successful compensation to failed compensation depending on the complexity of the motor imagery task. After a theoretical introduction in Chapter 1, I describe the effects of early aging on explicit MI, through a study of 24 young and 24 elderly subjects during two fMRI tasks requiring movement execution (ME) or MI of finger movements; temporal correlations between MI and ME were also measured (Chapter 2). I found significant differences between the two groups: elderly subjects lost the behavioural temporal correlation between MI and ME; moreover, they over-recruited occipito-temporal areas. The temporal discrepancy between MI and ME in the elderly subjects correlated with brain regions that showed increased activations. These observations suggest that elderly subjects have qualitatively different explicit MI abilities. MI can be elicited also by using implicit tasks, in which subjects are involved in motorically driven decisions. The hand laterality task is one such example: subjects are asked to decide whether a depicted hand is a left or a right one. Chapter 3 illustrates the fMRI correlates of this task in 30 young subjects. I found stronger signals in left premotor and parietal cortices for palm-viewed stimuli, whereas back-view stimuli were associated with stronger occipital activations to suggest the existence of brain-encoded, view-dependent representations of body segments. Chapter 4 reports the extension of the 2nd study to the assessment of the effects of early aging on implicit motor processes: I compared those data with the ones of 29 elderly subjects. While there was no specific aging related behavioural effect, I found significant additional activations in the elderly group in occipito-temporal regions, which were negatively correlated with RTs. These results reveal a pattern of graceful aging in the domain of implicit motor representations whereby cognitive performance remains at juvenile level thanks to some compensatory brain processes. It remained to be seen whether the effect of early aging could be detected by using more complex implicit MI tasks, something addressed in Chapter 5, through the study of 22 young and 23 elderly subjects performing a Grip Selection Task in which they were asked to report whether they would grip a portrayed tool with an over- or an under-hand grip. I found a behavioural decline in the elderly group, with hyperactivations in the occipital cortices and hypoactivations in the superior parietal lobule, an area previously associated with object grasping. The greater complexity of the imagined movement may determine a pattern interpretable in terms of a failed attempt of compensation. I conclude with Chapter 6, discussing my data in the light of neurocognitive models of healthy aging.