Supplemental Data


Abstract
Full Text
Anterior Cingulate: Single Neuronal Signals Related to Degree of Reward Expectancy
Munetaka Shidara and Barry J. Richmond

Supplementary Material

Experimental procedures

Behavioral and single unit data were collected from two young adult (4-6kg) monkeys (Macaca mulatta). Both monkeys were initially trained to fixate a small target spot to obtain a fluid reward (1). After this training, a cylinder for microelectrode recording and a head holder were fixed to the skull during an aseptic surgical procedure carried out under anesthesia. Electrophysiological recording sessions generally began a week after surgery.

The behavioral paradigms and visual stimuli used in the present study are similar to those of Shidara et al. (2) (Fig. 1). Visual stimuli were presented on a computer video monitor subtending 10.5 deg of visual angle in front of the animal. In each trial, a white cue, which will be described later, was present at the top of the computer video screen and after at least 800 ms a small white fixation spot (0.07 deg) appeared in the center. Then, after 400 ms, a red Wait signal (0.2 deg) appeared on the fixation point after the monkey touched the bar in the chair and fixated the fixation point. After a randomly selected Wait time (400 - 1200 ms), the red Wait signal changed to become the green Go signal, indicating that the monkey could release the bar to earn a liquid reward. If the monkeys responded within 1 s, the target turned blue (OK signal), signaling the monkey that the trial had been completed correctly. The target then disappeared. If the monkeys responded in less than 200 ms after the Go signal, we counted this as an early error. The target disappeared and the trial was terminated immediately. If the monkeys did not respond within one second after the onset of the Go signal, we counted this trial as a late error. We call this task "visual color discrimination trial". Initially each correct trial was rewarded by a drop of juice at a randomly chosen time for 250 - 350 ms after the target turned blue. When the monkeys completed more than 80% of the visual color discrimination trials correctly, the cued multi-trial reward schedules were introduced. The monkeys were required to complete randomly interleaved schedules of one, two, three, or four correct trials to obtain a reward. The brightness of the rectangular cue (10.5 x 0.26 deg) at the top of the screen varied from black to white in direct proportion to the schedule fraction. The schedule fraction ([trial number] / [schedule length]) quantified progress toward the rewarded trial, that is, 1/4, 2/4, 3/4, 4/4, 1/3, 2/3, 3/3, 1/2, 2/2, 1/1. We call this "cued condition". The monkeys had to complete each schedule before beginning a new one no matter how many errors they made. There was no explicit punishment for errors; the same cue appeared in the next trial, and the monkey still needed to complete the requisite number of correct trials for that schedule before a reward was given. After a schedule was completed, a new schedule was randomly picked. When the monkey's performance on this cued condition of the multi-trial reward schedule task was stable (2-3 days), the random condition was introduced in which the cue brightness was not related to the trial sequence. The monkey recognized that the cue's sequence was randomized on the day it was introduced and the performance became stable after 1-2 days (the error pattern shown for the random condition in Fig. 2). These behavioral patterns were similar to the previous reports using similar multi-trial reward schedules (2-4). Once the behavioral performance was stable, we switched between blocks of trials in the cued and random conditions on the same testing day while recording from single neurons, yielding data such as those shown in Fig. 3. These conditions were switched without warning. Conventional single unit recording techniques were used (2). We used MRI (5) to confirm that our recordings were taken from the anterior cingulate cortex (area 24c). All of the experimental procedures described here were approved by the Animal Care and Use Committee of the Neuroscience Research Institute / Electrotechnical Laboratory and were in accordance with the Guide for the Care and Use of Laboratory Animals as adopted by the NRI/ETL.


References

1. R.H. Wurtz, J. Neurophysiol32,727 (1969).

2. M. Shidara, T.G. Aigner, B.J. Richmond, J. Neurosci. 18, 2613 (1998).

3. Z. Liu, E.A. Murray, B.J. Richmond, Nature Neurosci. 3, 1307 (2000).

4. Z. Liu, B.J. Richmond, J. Neurophysiol. 83, 1677 (2000).

5. R.C. Saunders, T.G. Aigner, J.A. Frank, Behav. Brain Res. 81,443 (1990).


ANOVA with repeated measures

The individual neurons were placed in classes by inspection. For the neurons showing changes through the course of the schedule, the data from each of the two longer schedules (3 and 4 trial schedules) were each tested against the null hypothesis that the responses in different trials of the schedule were indistinguishable. ANOVA with repeated measures was used. The neuron's class was maintained if the pattern in the three and four trial schedules had pattern of means characterizing the class, e.g. for those with increasing responses through the schedule, the responses had to increase significantly (p < 0.05) through the course of both the 3 and 4 trial schedules.