Laterality in Parkinson's disease may predict motor and visual imagery abilities

Experimental evidence suggests that motor imagery (MI) engages the same neural substrates supporting actual motor activities and is likely impaired when such substrates are damaged, as in Parkinson's disease (PD). MI intuitively relies on visual imagery (VI), because mental simulations of physical movements depend on the visual retrieval of these movements. Although VI is generally considered a right hemispheric function, the hemispheric dominance of MI is still in dispute. Disparities in sidedness of motor disturbances are a distinctive feature of PD, and recent findings indicate that such disparities may similarly characterize cognition. Specifically, the deficits observed may depend upon which hemisphere is principally involved. Essentially, MI and VI are cognitive tasks subject to differential impairment and reflecting the prevalence of hemispheric impairment in PD. Motor imagery (assessed by the Vividness of Motor Imagery Questionnaire [VMIQ]) and VI (assessed by the Vividness of Visual Imagery Questionnaire [VVIQ] and Test of Visual Imagery Control [TVIC]) were examined in patients with asymmetric PD and in healthy elderly control subjects (HC group). VMIQ scores were similar in PD laterality subsets and the HC group, but VVIQ scores were significantly lower in both PD groups compared with the HC group. TVIC scores were significantly lower in the presence of left motor (right hemispheric) impairment and were predictive of left motor (right hemispheric) impairment. We suspect that MI is strongly reliant on VI and that language may mediate these two functions, to the extent that both are evoked through verbal stimuli. Working memory, both visual and verbal, is also involved in MI and VI tasks. Without due attention to laterality of symptoms, any training incorporating MI and VI may not deliver expected outcomes in the setting of asymmetric PD symptomatology.