Neural mechanisms underlying processing speed in healthy older adults

Abstract

Processing speed corresponds to the rate of information processed in a determined amount of time and is one of the cognitive functions most affected by advancing age (Salthouse & Ferrer-Caja, 2003). For this reason, a better comprehension of the neural mechanisms underlying processing speed in older adults may facilitate the understanding of the aging process and the development of therapeutic strategies aiming at attenuating it or even reversing it (Gao et al., 2020). Therefore, in this study, we evaluated how functional and anatomical connectivity was associated with the processing speed of 57 healthy, right-handed Portuguese older adults (mean age = 68.3; standard deviation = 4.8). All participants underwent magnetic resonance imaging (MRI), consisting of diffusion tensor imaging (DTI) and resting-state functional MRI (fMRI). Additionally, processing speed was evaluated using a Processing Speed index composed of three neuropsychological tests: the Digit Symbol-Coding and the Symbol-Search subtest from WAIS-III (Wechsler, 1997; port. version, Wechsler, 2008), and the Stroop Color-Word Test (port. version, Fernandes, 2013). Our results revealed that the radial diffusivity of several white matter fibers is negatively associated with processing speed, possibly indicating that demyelination is a degeneration process with a strong impact on processing speed performance. Furthermore, functional connectivity between several areas relevant for memory, attention, and motor planning, like left/right entorhinal - right superior parietal, was positively associated with processing speed. In conclusion, our results suggest that processing speed performance is related to the integrity of white matter fibers and to the optimal functioning of memory systems, with particular emphasis on episodic memory, as well as motor planning, attention, and executive functioning.