P118 The effect of age and cognitive load on prefrontal neurofunctional lateralization: A tDCS study

Abstract

Question Neuroimaging studies suggest that brain aging can be characterized by a reduction of hemispheric lateralization while performing tasks that require the use of complex cognitive functions. Recently, some authors proposed that even young people show bilateral neural activation as task demand increased. In most studies, however, task’s difficulty had been parametrically increased in a static and pre-specified way, while individual response to task-demand was not systematically controlled. In the present study we investigate changes in neurofunctional lateralization due to increasing subjective cognitive load in young and older participants performing tasks involving different cognitive functions. Methods 24 young participants and 12 older adults performed a digit span backward (complex cognitive function) and a finger tapping task (basic motor function) at 3 levels of subjective task difficulty (“easy” 5/5 correct answers, “medium” 3/5, “difficult” 1/5) which were established before starting the experiment. Participants took part at three sessions in three different days: two real stimulation conditions and a sham condition. In the real conditions anodal tDCS (20 min, 1.5 mA) was delivered through a target electrode (5 × 5 cm) positioned to stimulate both the inferior frontal gyrus and the primary motor area, once over the left hemisphere and once over the right one. The cathode was placed over the contralateral supraorbital area. Results A 3 (left, right, sham) × 3 (easy, medium, difficult) mixed-model with random intercepts for subjects was run within each group. For the young group a significant interaction between difficulty and stimulation was found: right tDCS deteriorates the performance only during the hardest condition. The preliminary results obtained from the 12 elderlies suggest instead that tDCS influence performance only for the “easy” and “medium” conditions. Conclusions The results support the hypothesis that hemispheric lateralization is influenced by the subjective level of difficulty of the task. Our findings are also consistent with the hypothesis of a limited reserve of cognitive resources. Results will be presented and discussed in relation to recent neurocognitive models of aging.