Abstract
Young people exhibit a negative BOLD response in ipsilateral primary motor cortex (M1) when making unilateral movements, such as button presses. This negative BOLD response becomes more positive as people age. In this study, we investigated why this occurs, in terms of the underlying effective connectivity and haemodynamics. We applied dynamic causal modeling (DCM) to task fMRI data from 635 participants aged 18–88 from the Cam-CAN dataset, who performed a cued button pressing task with their right hand. We found that connectivity from contralateral supplementary motor area (SMA) and dorsal premotor cortex (PMd) to ipsilateral M1 became more positive with age, explaining 44% of the variability across people in ipsilateral M1 responses. In contrast, connectivity from contralateral M1 to ipsilateral M1 was weaker and did not correlate with individual differences in rM1 BOLD. Neurovascular and haemodynamic parameters in the model were not able to explain the age-related shift to positive BOLD. Our results add to a body of evidence implicating neural, rather than vascular factors as the predominant cause of negative BOLD—while emphasising the importance of inter-hemispheric connectivity. This study provides a foundation for investigating the clinical and lifestyle factors that determine the sign and amplitude of the M1 BOLD response in ageing, which could serve as a proxy for neural and vascular health, via the underlying neurovascular mechanisms.
Highlights
Making a unilateral movement is typically associated with activation of sensorimotor cortex in the contralateral hemisphere of the brain
Such movements engage the ipsilateral sensorimotor cortex, according to recent reviews, [1,2]. This can result in a negative BOLD response (NBR) in the ipsilateral hemisphere, in young people, as measured with fMRI
The background to this question can be tackled into three parts: why is there a neural response to unilateral movements in the ipsilateral sensorimotor cortex; why is the BOLD response negative for some people; why does it become more positive with age?
Summary
Making a unilateral movement is typically associated with activation of sensorimotor cortex in the contralateral hemisphere of the brain Such movements engage the ipsilateral sensorimotor cortex, according to recent reviews, [1,2]. This can result in a negative BOLD response (NBR) in the ipsilateral hemisphere, in young people, as measured with fMRI. This study investigates why ageing has this effect on the ipsilateral BOLD response. The background to this question can be tackled into three parts: why is there a neural response to unilateral movements in the ipsilateral sensorimotor cortex; why is the BOLD response negative for some people; why does it become more positive with age?
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