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. 2018 Oct 10;7:e36018. doi: 10.7554/eLife.36018

Figure 4. Actual versus optimal adjustments of momentary-evidence (me) and decision-rule (z) biases.

(A) Schematic of the comparison procedure. Choice and RT data from the two reward contexts in a given session were fitted separately using the HDDM. These context- and session-specific best-fitting me and z values are plotted as the monkey’s data (black circles in B and C). Optimal values were determined by fixing parameters a, k, and non-decision times at best-fitting values from the HDDM and searching in the me/z grid space for combinations of me and z that produced maximal reward function values. For each me and z combination, the predicted probability of left/right choice and RTs were used with the actual task information (inter-trial interval, error timeout, and reward sizes) to calculate the expected reward rate (RR) and average reward per trial (RTrial). Optimal me/z adjustments were then identified to maximize RR (purple) or RTrial (orange). (B) Scatterplots of the monkeys’ me/z adjustments (black), predicted optimal adjustments for maximal RR (purple), and predicted optimal adjustments for maximal RTrial (orange), for the two reward contexts in all sessions (each data point was from a single session). Values of me > 0 or z > 0.5 produce biases favoring rightward choices. (C) Scatterplots of the differences in me (abscissa) and z (ordinate) between the two reward contexts for monkeys (black), for maximizing RR (purple), and for maximizing RTrial (orange). Positive Δme and Δz values produce biases favoring large-reward choices.

Figure 4—source data 1. RTrial and RR function for each session and reward context.
The same data are used for Figure 5, Figure 5—figure supplement 1, Figure 6, Figure 6—figure supplement 1, Figure 7, Figure 7—figure supplement 1, Figure 8 and Figure 8—figure supplement 1
elife-36018-fig4-data1.zip (49.9MB, zip)
DOI: 10.7554/eLife.36018.019

Figure 4.

Figure 4—figure supplement 1. Estimates of momentary-evidence (me) and decision-rule (z) biases using the collapsing-bound DDM fits.

Figure 4—figure supplement 1.

Same format as Figure 4B and C, except here only showing fits to the monkeys’ data. As with the model without collapsing bounds, the adjustments in me tended to favor the large reward but the adjustments in z tended to favor the small reward.
Figure 4—figure supplement 2. Hypothetical neural activity encoding a reward-biased perceptual decision variable.

Figure 4—figure supplement 2.

The blue and red curves depict rise-to-threshold dynamics in favor of a particular (say, rightward) choice under the two reward contexts, as indicated. Note that when the rightward choice is paired with larger reward: 1) the slope of the ramping process, which corresponds to an adjustment in momentary evidence (me), is steeper; and 2) the baseline activity, which corresponds to the decision-rule (z) adjustment, is lower.