Figure 1.

Experimental procedures. A, Saccade-countermanding task. Monkeys initiated trials by fixating on a central point. After a variable time, the center of the fixation point was extinguished. A peripheral target was presented simultaneously at one of two possible locations. On no-stop-signal trials monkeys were required to shift gaze to the target, whereupon after 600 ± 0 ms a high-pitched auditory feedback tone was delivered, and 600-ms later fluid reward was provided. On stop-signal trials (∼40% of trials), after the target appeared the center of the fixation point was re-illuminated after a variable SSD, which instructed the monkey to cancel the saccade in which case the same high-pitched tone was presented after a 1500 ± 0 ms hold time followed, after 600 ± 0 ms by fluid reward. SSD was adjusted such that monkeys successfully canceled the saccade in ∼50% of trials. In the remaining trials, monkeys made non-canceled errors which were followed after 600 ± 0 ms by a low-pitched tone, and no reward was delivered. Monkeys could not initiate trials earlier after errors. B, Countermanding behavior. Top left, Cumulative distribution function of response latencies on no-stop (green) and non-canceled (yellow) trials. Response latencies on non-canceled trials were faster than those on no-stop trials. Top right, Inhibition function plotting the probability of responding across SSDs. Weibull functions were fitted to data from each session. The mean of these Weibull functions across sessions and the corresponding 95% CI is plotted for each monkey (monkey Eu: purple; X: blue). Bottom left, Distribution of mean SSRTs across sessions. Bottom right, Distribution of the proportion of trigger failures across sessions. C, LFP processing. EEG was recorded with leads placed on the cranial surface over the medial frontal cortex at the location analogous to FCz in humans. The EEG lead was located over the supplementary eye field (SEF) and pre-supplementary motor area (pre-SMA). For each session, raw data were extracted. After bandpass filtering between 15 and 29 Hz, this signal was epoched from −1000 to 2500 ms relative to target presentation, saccade initiation, and stop-signal presentation. D, β-Burst processing. The epoched signal for each trial was convolved with a complex Morlet wavelet. Time-frequency power estimates were extracted by calculating the squared magnitude of the complex wavelet-convolved data. Individual β-bursts were defined as local maxima in the trial-by-trial band time-frequency power matrix, for which the power exceeded a threshold of six times the median power of the entire time-frequency power matrix for the electrode. An example burst is shown in the time-frequency plot at the bottom. E, Examples of β band time frequency in 12 randomly selected trials, aligned on SSD (solid red), with the corresponding SSRT (dashed red). These plots are indistinguishable from counterparts derived from human data.