Abstract
Healthy aging is accompanied by a continuous decline in cognitive functions. For example, the ability to learn languages decreases with age, while the neurobiological underpinnings for the decline in learning abilities are not known exactly. Transcranial direct current stimulation (tDCS), in combination with appropriate experimental paradigms, is a well-established technique to investigate the mechanisms of learning. Based on previous results in young adults, we tested the suitability of an associative learning paradigm for the acquisition of action- and object-related words in a cohort of older participants. We applied tDCS to the motor cortex (MC) and hypothesized an involvement of the MC in learning action-related words. To test this, a cohort of 18 healthy, older participants (mean age 71) engaged in a computer-assisted associative word-learning paradigm, while tDCS stimulation (anodal, cathodal, sham) was applied to the left MC. Participants’ task performance was quantified in a randomized, cross-over experimental design. Participants successfully learned novel words, correctly translating 39.22% of the words after 1 h of training under sham stimulation. Task performance correlated with scores for declarative verbal learning and logical reasoning. Overall, tDCS did not influence associative word learning, but a specific influence was observed of cathodal tDCS on learning of action-related words during the NMDA-dependent stimulation period. Successful learning of a novel lexicon with associative learning in older participants can only be achieved when the learning procedure is changed in several aspects, relative to young subjects. Learning success showed large inter-individual variance which was dependent on non-linguistic as well as linguistic cognitive functions. Intriguingly, cathodal tDCS influenced the acquisition of action-related words in the NMDA-dependent stimulation period. However, the effect was not specific for the associative learning principle, suggesting more neurobiological fragility of learning in healthy aging compared with young persons.
Highlights
With increasing life expectancy, quality of life and social participation in older people is more and more dependent on fluid cognitive functioning
We evaluated participants’ appraisal of attention, unpleasant sensations and fatigue with questionnaires using visual analog scales (VAS) as control parameters
Percentage of Correct Responses First, we report the effect of Transcranial direct current stimulation (tDCS) on the performance of associative learning: The number of correct responses increased significantly over the five blocks (blocks1–5, F(4,68) = 30.44, p = 0.000)
Summary
Quality of life and social participation in older people is more and more dependent on fluid cognitive functioning. Associative learning of a new lexicon has been successfully implemented in experimental settings with precise control over stimulus frequency and exposure time (Breitenstein and Knecht, 2002; Dobel et al, 2009; Liuzzi et al, 2010). While these paradigms yielded robust results for different word classes in young adults, testing the suitability and determining learning success in an older population was the scope of the present work
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