Fig 1.
A. Schematic description of a trial sequence with successive screenshots displayed to the monkeys. A trial started with a gray screen (intertrial phase) followed by the presentation of 2 red instruction dots and 2 options, each of them composed of a reward cue and a required force cue (offer phase). When the instruction dots turned green (response phase), the monkeys could choose 1 option by pressing the corresponding handgrip (left or right). A continuous visual feedback was displayed to signal the online force exerted by the monkey (yellow filling of the required force cue). The announced reward (drops of juice) was delivered if and when the exerted force reached the target force (outcome phase). The same trial started again if the monkey responded before the end of the offer phase (premature error) or provided no response at all (omission). B. Experimental design for the combinations of required force and reward size used to define options. Each option, regardless of its side of presentation, was defined by a combination of a given reward level (1, 2, 3, or 4 drops) and a given required force (20%, 40%, 60%, or 80% of the calibration force). The effort and reward components of each option were manipulated systematically and randomly, such that the influence of effort and reward on choices were orthogonalized by design. All possible combinations of pairwise options were sampled during the task.
Fig 2.
Behavior under placebo condition.
Lines and dots represent group-level means (across monkeys), and error-bars represent SEM (across sessions). A. Relationship of exerted force with required force of the chosen option. The force exerted at the peak increased with the required force of the chosen option (β = 0.06, t(147) = 4.45, p = 1.69×10−5). The dotted line represents the optimal relationship between exerted force and required force (the minimum required force to complete the trial, i.e., the position of the force threshold). B. Relationship between exerted force and expected reward of the chosen option. Reward had a positive influence on the exerted force (β = 0.02, t(147) = 3.25, p = 1.42×10−3). C-D. Participation (%) to trials as a function of the sum of required forces (C) or reward (D). C. Participation was negatively modulated by the sum of required forces (β = −0.51, t(147) = −7.93, p = 4.99×10−13) presented in an offer. D. Monkeys’ participation was positively modulated by the sum of rewards (β = 0.49, t(147) = 6.62, p = 7.36×10−5) presented in an offer. E. Proportion of high-reward choices as a function of the difference in offered rewards between the 2 options (1D-reward conditions). There was a significant effect of the difference in required force on choices (β = −1.84, t(147) = −6.95, p = 1.09×10−10). F. Proportion of high-required-force choices across difference in offered forces (1d-required force conditions). There was a significant contribution of the difference in reward in choices (β = 2.39, t(147) = 9.44, p = 7.94×10−17). Underlying data can be found in S1 Data. Fmax: maximal force; HF, high force; HR, high reward.
Fig 3.
Properties of the optimal force selection model.
A. Components of the value function depending on the exerted force. A1: the activation component is a linear invariant benefit, A2: the reward benefit component is a sigmoidal function with a slope scaled by the uncertainty component, A3: the effort cost component is a quadratic function. A4: The net value is the sum of 3 previously described components. B. Impact of main experimental conditions onto the value function: B1: chosen rewards increase the optimal expected value and slightly shift the optimal force. B2: chosen effort decreases the optimal expected value but increases the optimal force (bottom) C. Impact of main parameters onto the value function. C1: increase of Kr (sensitivity to reward) leads to huge increase of optimal value and slight increase of optimal force. C2: increase of Ke (sensitivity to effort) leads to decrease of optimal value and decrease of optimal force with a collapse to zero-force when the optimal value becomes negative. C3: increase of Ko (activation weight) leads to increase of optimal value and increase of optimal force. C4: increase of σE: (action-outcome uncertainty) leads to decrease of optimal value and increase of optimal force, shifting it away from the target force (bottom). Simulations were performed with an initial parameters vector of {= 1; = −1; = 2; = 1; = 0.5; = −1; = 0.1; = 0; = 0; = 0} and an initial input vector of {R = 4; E = 0.6; N = 0}. Red dots indicate the optimal force to exert when the net value function is represented. Color-bars indicate either input or parameter values when there are variations. au., arbitrary units; Fmax: maximal force.
Fig 4.
Predictions of the complete computational model for choice and force production.
The graphs display major predicted dependencies between experimental conditions and measurements under changing parameter values. The 6 columns display the relationship between 1. the participation rate and sum of offered rewards. 2. The participation rate and sum of offered efforts. 3. The proportion of high-reward choices and difference in offered rewards. 4. The proportion of high-effort choices and difference in offered required forces. 5. The exerted force and the chosen reward. 6. The chosen effort. The rows display the impact of the 4 key parameters (top to bottom: Kr, sensitivity to reward; Ke, sensitivity to effort; Ko, activation weight; σE, action-outcome uncertainty.). Color bars indicate parameter values. Simulations were performed with an initial parameters vector of {= 1; = −1; = 2; = 1; = 0.5; = −1; = 0.1; = 0; = 0; = 0} and an initial input vector of {R = 4; E = 0.6; N = 0}. Fmax: maximal force.
Fig 5.
Clonidine affects choice and force production.
Behavior of the monkeys under the placebo (gray) and clonidine (brown) conditions. Lines and dots represent group-level means (across monkeys) and error-bars represent SEM (across sessions). A. Relationship between the percentage of high-required force choices and the difference in offered forces. The weight of the difference in required force increased under clonidine (clonidine = −2.55 ± 0.23, placebo = −1.65 ± 0.28, F(1,145) = 14.81, p = 1.78×10−4). B. Relationship between the percentage of high-reward choices and the difference in offered rewards. Under clonidine, the weight of the difference in reward sizes did not change (clonidine = 2.29 ± 0.21, placebo = 2.45 ± 0.30, F(1,145) = 0.47, p = 0.49). C-D. Relationship between exerted force relation and chosen force level (C) and chosen reward size (D). The dotted line represents the optimal relation (identity) between exerted force and force difficulty (the minimum required force to complete the trial). Clonidine had a main effect on force production (clonidine = 0.55 ± 0.05, placebo = 0.63 ± 0.03, F(1,145) = 20.70, p = 1.12×10−5), without affecting the effects task factors (reward weights: clonidine = 0.02 ± 8.3×10−3, placebo = 0.02 ± 5×10−3, F(1,145) = 0.93, p = 0.33; required force weight: clonidine = 0.06 ± 0.01, placebo = 0.05 ± 0.01, F(1,145) = 0.92, p = 0.33). E-F. Relationship between participation and the sum of offered forces (E) and sum of offered rewards (F). There was a tendency for an increase in the weight of the sum of required forces that did not reach significance (clonidine = −0.75 ± 0.18, placebo = −0.46 ± 0.04, F(1,145) = 2.02, p = 0.16) (E) but no effect of clonidine on the modulation of participation by the sum of rewards (F). Underlying data can be found in S1 Data. au.,arbitrary units; Fmax, maximal force; HF, high force; HR, high reward.
Fig 6.
Specificity of the clonidine effect.
Behavior of the monkeys in the placebo (gray) and clonidine (brown) conditions. Lines and dots represent group-level means (across monkeys) and error-bars represent SEM (across sessions). A. Relation between peak velocity and peak of the exerted force (commonly known as Fitts law). Clonidine did not affect this proxy for muscular contractility (clonidine = 4.32 ± 0.43, placebo = 4.14 ± 0.40, F(1,145) = 1.19, p = 0.27). B. Influence of session progression (trial number) on the percentage of high-required-force choices. Monkeys made less high-required-force choices overall, but the effect was constant over the session (β = −9.73×10−5 ± 9.13×10−5, t(145) = −1.06, p = 0.28). C. Influence of session progression (trial number) on exerted force. Exerted force was decreased under clonidine, but the effect of the drug was constant over the session (p > 0.05). D. Proportion of high-required force choices as a function of success (proportion of correctly executed actions in the session, across sessions). There was no significant relationship between success rate and the proportion of high-force choices (β = 0.04 ± 0.06, t(144) = 0.70, p = 0.17). Underlying data can be found in S1 Data. Fmax Maximal force,; HF, High force.