Figure 5.

Pyramidal and extrapyramidal. These plots show the consequences of four specific synthetic lesions to the performance of the task shown in Fig. 4 and highlight the difference between pyramidal (i.e. corticospinal) and extrapyramidal lesions. These include the same lesions as in Fig. 2, but additionally include two perturbations to the discrete parts of the model. The first plot illustrates the preservation of coordination following overestimation of sensory precision (Π, compare with the trajectory of Fig. 4). The second illustrates a decrease in the precision of the contribution of the expected free energy to beliefs about ‘how I am going to act’ (γ). The lack of confidence in selecting a sequence of fixed points leads to their estimation as being somewhere in the middle. Note that there is no impairment in reaching these fixed points. The initialization of movement via a series of small amplitude movements resembles the ‘festinant’ gait sometimes observed in Parkinson’s disease (although this is typically observed in the lower limbs). Note the series of superimposed red spheres following this, indicating a decrease in movement amplitude following the initial movement. The third plot shows the overestimation of smoothness we saw earlier, with hypermetric overshoots at the end of each segment of the movement. The changes following overestimation of λ are strikingly similar to those plotted in Holmes⁴⁸ for patients with cerebellar injuries. More modern studies also show the same kind of overshoot in limb trajectories.⁶⁷ The final line shows what happens when the strength of the connections between the highest and middle levels of the hierarchy are attenuated (attenuating the precision associated with A). This shows successful completion of the task, but an apparent confusion each time the target changes position (often leading to a move towards the centre). While rapidly compensated for, this implies a discontinuous sequence of movements that fail to be synthesized into a coherent motor narrative.