Figure 2. Behavioral task and performance.

(A) Successive frames illustrate the sequence of events in the behavioral task. Depending on the white or blue color of the fixation point, the monkey has to remember either the first or the second of two visual stimuli presented in sequence, respectively. At the end of the trial, the fixation point turns off, and the monkey needs to perform an eye movement toward the remembered location of the visual stimulus in order to receive a liquid reward.

(B) Schematic diagram of a single trial of the task. Blocks represent the successive events in the task: fixation, first visual stimulus presentation, first delay period, second visual stimulus presentation, second delay period, and saccade. Successive trials are separated by inter-trial intervals. NB stimulation, when delivered, always occurs during the inter-trial interval.

(C) Sequence of trials during a stimulation block, in a compressed timescale, relative to (B). Successive trials, labeled 1–4, each lasting approximately 10 s, are followed by 15 s of stimulation. The precise duration of a trial differed depending on how quickly the animal initiated the trial and if the trial was successfully completed or aborted, e.g., due to a break in fixation.

(D) Sequence of trials during a control (no-stimulation) block. The trials are arranged exactly in the same fashion as in the stimulation block, including an extended inter-trial interval every 60 s, during which, however, no stimulation is applied (sham).

(E-H) Percentage of correct trials is shown for each of the two monkeys, for different visual stimulus types, under control conditions (blue bars) and under NB stimulation (orange bars). n = 18 sessions for stimulation and 17 for control for monkey GR; n = 19 for stimulation and 35 for control for monkey HE. (E) Mean performance for trials in which first visual stimulus appears contralateral to the stimulation site, when the monkey is executing the remember-first task and needs to remember the first visual stimulus. (F) Performance in the remember-first task when the first visual stimulus appears ipsilateral to the stimulation site. (G) Performance in the remember-second task, when the second visual stimulus appears contralateral to the stimulation site. (H) Performance in the remember-second task, when the second visual stimulus appears ipsilateral to the stimulation site.

(I) Performance in the remember-first task, for trials grouped by distance between the first and second visual stimulus (180°, 90°, or 45°), under stimulation or control conditions. Data from both monkeys are pooled together.

(J) Performance in the remember-second task, for trials grouped by distance. Error bars represent SEM in all panels.