Healthy aging is accompanied by changes in cognitive and motor functions that result in impairment of activities of daily living. This process involves a number of modifications in the brain and is associated with metabolic, structural, and physiological changes; some of these serving as adaptive responses to the functional declines. Up to date there are no universally accepted strategies to ameliorate declining functions in this population. An essential basis to develop such strategies is a better understanding of neuroplastic changes during healthy aging. In this context, non-invasive brain stimulation techniques, such as transcranial direct current or transcranial magnetic stimulation, provide an attractive option to modulate cortical neuronal assemblies, even with subsequent changes in neuroplasticity. Thus, in the present review we discuss the use of these techniques as a tool to study underlying cortical mechanisms during healthy aging and as an interventional strategy to enhance declining functions and learning abilities in aged subjects.

During the last century, average life expectancy in developed countries was prolonged approximately 30 years. In addition, a significant decline in fertility generated a shift in the distribution of the population with important socio-economic, political, and public health consequences (UN-Report, 2005). The fact that there are more old people today than ever before, and that this tendency is expected to increase further, underlines the importance of understanding the mechanisms of healthy aging as well as developing novel innovative strategies to adapt for age-relate declines.

Besides age-associated diseases, like cardiovascular diseases or cancer (Balducci and Extermann, 2000), age-related neurobiological changes with consecutive declines in cognitive functions, perceptual, and motor abilities impair activities of daily living, independence, and quality of life (Logsdon et al., 2002; Craik and Bialystok, 2006). In cognition, age-related deficits encompass multiple domains, including attention, memory, reasoning, and executive functions (Celsis, 2000; Hogan et al., 2006). Age-related motor impairments are also ubiquitous, with deficits in the planning, the execution and the control of movement (Krampe et al., 2002; Sawaki et al., 2003). Physiologically, aging consists of a dynamic process in the brain, involving a number of modifications associated with metabolic, structural, and functional changes, part of them hypothesized as adaptive responses to the functional declines (Burke and Barnes, 2006).

In recent years, different forms of non-invasive brain stimulation techniques have been explored in patients and healthy volunteers offering the attractive option to modulate neuronal plasticity and to improve behavior and learning processes (Hummel and Cohen, 2006; Nitsche et al., 2008; Reis et al., 2009). In the present review, we will focus on studies using non-invasive brain stimulation techniques to evaluate cortical mechanisms during healthy aging, especially the one involved in preserving cognitive and motor funcNon-2010.00149tions. Furthermore, the novel field of applying these techniques to increase behavior, neuronal plasticity and learning, e.g., in the sensorimotor domain as a model system, will be presented. Rather than attempting to be comprehensive in terms of reviewed work, this article intends to provide a clearly structured framework of the application of these techniques to understand and support plastic changes in the aging population.