Motor imagery: does strategy matter?
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Motor imagery requires individuals to form an internal representation of a specific action within working memory without any overt output. Motor imagery has proven effective in improving motor performance of specific skills. This study explored whether different motor imagery strategies influence corticospinal excitability in young (20-35 years) and older subjects (over 55 years). In addition, the effectiveness of these strategies in targeting modulations in motor cortical output and whether the hand “performing” the task was important were also examined. Motor imagery ability was measured using the Kinesthetic and Visual Imagery Questionnaire (KVIQ) and mental chronometry. Working memory including visuospatial, verbal and kinesthetic domains was measured by immediate serial recall. To determine the effect of imagery on corticospinal excitability transcranial magnetic stimulation (TMS) was applied over the contralateral motor cortex as subjects imagined abducting their index finger. Motor evoked potentials (MEPs) were recorded from first dorsal interosseous (FDI), abductor pollicis brevis and abductor digiti minimi muscles (ADM). As subjects performed motor imagery, they were guided by visual, auditory or a combination of visual and auditory cues. Strategies were introduced in randomized sequence interspersed with a rest and a muscle activation condition. Motor imagery ability and verbal working memory were comparable between young and older subjects (p > 0.05). In both groups, MEP amplitudes in the FDI muscle were significantly increased during motor imagery compared to rest regardless of the strategy used (p < 0.001). Visual cueing was the most effective at isolating facilitation to the target muscle (FDI), whereas with the auditory and combined strategies both FDI and ADM muscles generated MEPs that were comparable in amplitude (p > 0.05). TMS induced MEPs were greater in amplitude when the left hemisphere was stimulated during motor imagery of the right finger while being guided by either auditory or visual cueing. In combination, these findings suggest that motor imagery increases corticospinal excitability and the strategy used may serve to target the facilitation.