Placebo effect on gait: a way to reduce the dual-task cost in older adults.

The ability to perform two tasks simultaneously is essential for daily activities. In older adults, this ability is markedly reduced, as evidenced by the dual-task cost on gait. Preliminary evidences indicate that the dual-task cost can be influenced by different types of manipulations. Here, we explored the effectiveness of a new approach to reduce the dual-task cost, based on the placebo effect, a psychobiological phenomenon whereby a positive outcome follows the administration of an inert device thought to be effective. Thirty-five healthy older adults were asked to walk on a sensorized carpet (single-task condition) and to walk while counting backward (dual-task condition) in two sessions (pre-test and post-test). A placebo group, randomly selected, underwent sham transcranial direct current stimulation over the supraorbital areas between sessions, along with information about its positive effects on concentration and attention. A control group did not receive any intervention between sessions. The dual-task cost was significantly reduced in the placebo group at the post-test session compared to the pre-test for several gait parameters (Cohen's d > 1.43). At the post-test session, the dual-task cost was also lower in the placebo group than in the control group (d > 0.73). Cognitive (number of subtractions and number of errors) and subjective (perceived mental fatigability) variables remained stable across sessions. The reduced dual-task cost in the placebo group could indicate the ability to re-establish the allocation of attentional resources between tasks. These findings could contribute to the development of cognitive strategies that leverage positive expectations to boost motor control in older adults.