Figure 4. Patients with PD do not benefit from explicit knowledge of prior probabilities.
(A) Schematic of the dichromatic Glass pattern task showing that participants were informed of the priors explicitly (75:25) by instruction (“explicit task”). This eliminates the need for learning. We first verified the validity of this manipulation by testing a group of 16 naïve yHCs performing the dichromatic task without feedback but with explicit, verbal instructions and we concluded that providing explicit instructions about the priors is a valid way to induce a decision bias in the absence of feedback learning (Figure S1H).
(B) Proportion of positive choices is plotted against the orientation strength from 10 patients with PD (4 females, 6 males, mean age 62 yrs) as in Figure 1C. The grey points and lines show the data and logistic fits from the equal prior conditions whereas the black points and lines show the data and fits for the unequal prior conditions. The prior direction was counterbalanced across participants so different subsets of patients contribute to the two prior direction conditions. All patient data contribute to the equal prior condition. Six of the 10 patients also performed the original version of the dichromatic task ~5 months before performing the explicit task. Only 2 of the patients experienced opposite priors in the two tasks, minimizing the likelihood of learning conflicting priors and maximizing the chances that if patients could use the prior they would. After completing this task, 8 patients reported following the instructions and all 10 reported being aware of the unequal priors even though they failed to use them. This observation rules out an interpretation based on faulty working or short-term memory for the priors.
(C) Starting point of evidence accumulation from the drift-diffusion model during the two experimental epochs (separated by the vertical line) of the explicit task for patients. The solid red line is for the stimuli with unequal priors, the dashed red line for the stimuli with equal priors. Shaded areas indicate 95% confidence intervals.
(D) Parameter estimates for drift rate offset, otherwise like C.