Deterioration, Compensation and Motor Control Processes in Healthy Aging, Mild Cognitive Impairment and Alzheimer's Disease

Abstract

Aging is associated with modifications of several brain structures and functions. These modifications then manifest as modified behaviors. It has been proposed that some brain function modifications may compensate for some other deteriorated ones, thus maintaining behavioral performance. Through the concept of compensation versus deterioration, this article reviews the literature on motor function in healthy and pathological aging. We first highlight mechanistic studies that used paradigms, allowing us to identify precise compensation mechanisms in healthy aging. Subsequently, we review studies investigating motor function in two often-associated neurological conditions, i.e., mild cognitive impairment and Alzheimer's disease. We point out the need to expand the knowledge gained from descriptive studies with studies targeting specific motor control processes. Teasing apart deteriorated versus compensating processes represents precious knowledge that could significantly improve the prevention and rehabilitation of age-related loss of mobility.

Keywords: Alzheimer’s; aging; cognition; compensation; deterioration; motor control.

Figure 1

A model of deterioration and compensation of the motor function with aging. Adapted from Papegaaij et al. (2014). Aging alters the structure of the sensorimotor system, causing functional deteriorations (i.e., deterioration in how these structures act during motor tasks) and decreased behavioral motor performances. Decreased performance triggers the need for functional compensations (modifications in how brain structures act to attain motor goals) to maintain behavioral performance. Dashed boxes provide examples of deteriorated and compensating modifications. Examples of age-related structure alterations: degradation of sensory receptors [48,49,50], cortical atrophy [51,52,53], and loss of muscular mass [54]. Examples of functional deterioration: noisier sensory integration [48,50] and nonselective brain activation [17,23]. Examples of behavioral modifications: slower and more variable movements [46,47] and loss of muscular strength and power [45,54]. Examples of functional compensations: increased reliance on sensory predictions [55,56,57], increased internal model recalibration [58], increased minimization of muscle effort [59], and overactivation of cortical areas during motor tasks [26].