Differential Encoding of Factors Influencing Predicted Reward Value in Monkey Rostral Anterior Cingulate Cortex

Background: The value of a predicted reward can be estimated based on the conjunction of both the intrinsic reward value and the length of time to obtain it. The question we addressed is how the two aspects, reward size and proximity to reward, influence the responses of neurons in rostral anterior cingulate cortex (rACC), a brain region thought to play an important role in reward processing. Methods and Findings: We recorded from single neurons while two monkeys performed a multi-trial reward schedule task. The monkeys performed 1–4 sequential color discrimination trials to obtain a reward of 1–3 liquid drops. There were two task conditions, a valid cue condition, where the number of trials and reward amount were associated with visual cues, and a random cue condition, where the cue was picked from the cue set at random. In the valid cue condition, the neuronal firing is strongly modulated by the predicted reward proximity during the trials. Information about the predicted reward amount is almost absent at those times. In substantial subpopulations, the neuronal responses decreased or increased gradually through schedule progress to the predicted outcome. These two gradually modulating signals could be used to calculate the effect of time on the perception of reward value. In the random cue condition, little information about the reward proximity or reward amount is encoded during the course of the trial before reward delivery, but when the reward is actually delivered the responses reflect both the reward proximity and reward amount

Average firing rate of rACC neurons in <b>Fig. 4</b>.

<p>Colors mean reward amount as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030190#pone-0030190-g004" target="_blank">Figure 4</a>. A, Decreasing type activity. B, Increasing type I activity. C, Increasing type II activity.</p

Behavioral task.

<p>A, Time sequence of task events in an individual color discrimination trial in the rewarded trial with 3 drops of reward in the valid cue condition. B, Example of the sequence in 3-trial schedule with 3 drops in the valid cue condition. The visual cue was presented on the top of the screen. Length of the visual cue indicates schedule state (remaining trials to earn the reward; 1, 2, or 3 trials). C, Brightness of the visual cue indicates reward amount (1, 2, or 3 drops of water).</p

Percentage of neurons encoding information about specific schedule levels or reward amount.

<p>Percentage of neurons whose responses in the first trials were larger than those in intermediate and rewarded (purple), whose responses in the intermediate trials were larger than either in the first and rewarded (green), or whose responses in the rewarded trials were larger than in the first and intermediate (orange), is shown in A and C (N = 308). Percentage of neurons whose responses in the small reward amount trials were larger than in the medium and large (purple), whose responses in the medium reward amount trials were larger than either in the small and large (green), or whose responses in the large trials were larger than in the small and medium (orange), is shown in B and D (N = 308). A & B, the valid cue condition. C & D, the random cue condition.</p

Percentage of errors in the task.

<p>Solid lines show the percentage of errors in the valid cue condition. Broken lines show the percentage of errors in the random cue condition. Percentage of errors (%) is shown on the ordinate and schedule state on the abscissa. Reward amount is shown by line color (1 drop, blue; 2 drops, green; and 3 drops, red). Percentage of errors was calculated as the total number of errors divided by the total number of trials (×100) in each schedule state and reward amount across all recording sessions (Monkey T: 233 sessions in the valid cue condition and 75 sessions in the random cue condition, Monkey I: 75 sessions in the valid cue condition and 10 sessions in the random cue condition.</p

Percentage of neurons showing significant task-related activity.

<p>Percentage of neurons showing significant task-related activity.</p

Recording site.

<p>MR image (coronal section at anterior 33 to the interaural line ; monkey T) is shown. An example of electrode position is shown. MRIs were obtained on a 3-T General Electric Sigma unit.</p

Population results of reward proximity and amount effect.

<p>A, Percentages of neurons that showed significant main effect of schedule level and reward amount, and interaction by ANOVA are shown. Black lines show the percentages of neurons that showed schedule level effect, red lines show reward amount effect, gray lines show interaction. Solid lines show the valid cue condition; broken lines for the random cue condition. B, Dynamics of encoding of the schedule state and the reward amount as revealed by the percentage of variance explained for each single neuron. Each line represents the percentage of variance explained in a color heat scale for both valid (upper) and random (lower) cue conditions. The data for each of the 85 (numbered 1 to 85) neurons are the data that were recorded both in the valid and random cue conditions. Neurons are sorted from top to bottom according to the number of events related to a significant response and total value of variance explained of the schedule state, therefore the order of the individual neurons is different in the left and right panels. C, Mean value of valiance explained is shown (summed from data in panel B). Same convention on color as in panel A.</p