Figure 4.

Dopamine depletion, but not apathy, reduces motor vigour following decision to engage. Motor vigour was indexed by excess force generated above the required effort level, i.e. how much more than was required that a participant squeezed (A). Dopamine (ON state) significantly increased response vigour (indexed by OFF state), while the presence of apathy was not associated with a significant change in the vigour of response (B). Despite greater force production at each effort level in the ON state, there was no change in perceived physical exertion between ON and OFF state, nor between patients and controls (C). Maximal voluntary contraction did not differ between ON and OFF states (D). PD = Parkinson’s disease.