SUPPLEMENTARY ONLINE MATERIAL
Methods
Two male macaque monkeys (Macaca radiata) were prepared for training and physiological
recording using aseptic procedures under isofluorane anaesthesia. The experimental protocol
conformed to United States Public Health Service guidelines and was approved by the
Vanderbilt Animal Care Committee. General procedures have been described previously (1). A
PDP 11/83 presented stimuli and collected eye position, spike and event data. Spikes were
isolated and recorded using a Plexon system. Well-isolated neurons were recorded from the
ACC upon entry into the gray matter, regardless of task relevance. The recordings were
concentrated in the dorsal bank and fundus of the cingulate sulcus.
The countermanding task consists of two kinds of trials (Fig. S1). In no stop signal trials,
monkeys fixated a spot that appeared at the center of a display. After fixation for a period
ranging from 400-600 ms, a peripheral target appeared at one of two locations in opposite
hemifields at the same eccentricity coincident with disappearance of the fixation spot. A
speeded gaze shift to the peripheral target resulted in juice reinforcement after a fixed interval
(400 ms). In stop signal trials, the fixation spot reappeared after presentation of the target. The
fixation spot served as a stop signal. A stop signal trial was classified as an error when monkeys
executed the saccade in spite of the stop signal (non-canceled trials). A stop signal trial was
classified as correct when monkeys canceled the saccade (canceled trials) resulting in juice
reinforcement. The probability of canceling the partially prepared saccade is proportional to the
delay of the stop signal which varied from 150 to 450 ms. The intertrial interval was 1000 ms.
Trials with stop signals were randomly interleaved with no stop signal trials, with 1/3 to 1/2 stop
signal trials.
To test whether activity was related to predictability of reinforcement, on a random 10% of
successful trials reinforcement was not delivered. Also, unexpected juice was delivered
occasionally at random times during the intertrial interval.
The activity of single neurons was compared with respect to different events and outcomes
1
resulting from different conditions by convolving spike trains with a combination of growth and
decay exponential functions that resembled a postsynaptic potential (1). Neural activity was
considered to be significantly different between conditions if it exceeded 6 standard deviations of
the mean difference between trial types during the 600 ms preceding event alignment time and
remained above 2 standard deviations for at least 50 ms. To find the duration of significant
activity, the time at which the activity exceeded the 2 standard deviation criterion was subtracted
from the time when the difference fell below the criterion.
Text
Saccadic eye movements are produced by a network of neurons in the brainstem (2).
Saccades are initiated when strong tonic inhibition from omnipause neurons during gaze fixation
is released on burst neurons that innervate the extraocular motoneurons. A circuit distributed
through the frontal lobe, in particular the frontal eye field (FEF), basal ganglia, cerebellum and
superior colliculus (SC) (3), conveys to the brainstem saccade generator where and when to shift
gaze. This circuit is comprised of two major kinds of neurons – movement neurons and fixation
neurons which exert reciprocal inhibition. Saccades are initiated when fixation neurons decrease
discharge rate and movement neurons increase discharge rate to a particular threshold.
Variability in response time arises from variability in the time taken by movement neurons to
reach a threshold (4). When movements are canceled following the reappearance of the fixation
spot in the countermanding task, movement and fixation neurons in FEF and SC exhibit the
preliminary evolution of discharge rate but this change is reversed when movements are canceled
(1, 5). The time taken to cancel the movement can be derived from the behavioral data through
the application of a race model (6, 7). In the saccade countermanding task the stop signal
reaction time is typically around 100 ms. The movement and fixation neurons in FEF and SC are
modulated within this stop signal reaction time, so they exert an influence early enough to
control the initiation of saccades
The interruption of saccade preparation in response to the stop signal could be mediated by
2
multiple circuits. The stop signal could invoke inhibition from the prefrontal cortex (8) that
cancels the partially prepared saccade. However, the visual latencies of prefrontal neurons are
typically longer than the stop signal reaction time, so under the conditions of this saccade
countermanding task, the prefrontal cortex seems an unlikely source of the inhibition. The final
stage of inhibition controlling saccade production is the omnipause neurons in the brainstem, but
these neurons are not modulated during saccade preparation, only immediately before and during
saccade execution (9). The remaining possibility for the inhibition of the saccade preparation
process is the network of fixation and movement neurons in FEF, SC and related structures. The
most plausible mechanism of interrupting saccade preparation involves the mutual inhibition
between fixation and movement neurons. Fixation neurons have foveal receptive fields, so they
are activated directly by the reappearance of the fixation spot (10). When saccades are canceled
in response to the stop signal, fixation neurons in FEF and SC exhibit a rapid re-activation at the
same time that movement neurons are increasing in discharge rate toward the trigger threshold
(1,5). This brief state of co-activation of opposing gaze-holding and gaze-shifting neurons
satisfies the definition of conflict (11). Critically, on trials in which saccades are produced
erroneously in spite of the stop signal movement and fixation neurons in FEF and SC exhibit
patterns of activation indistinguishable from that observed on trials with no stop signal.
Accordingly, under these conditions any conflict between gaze-holding and gaze-shifting cannot
be present before the saccade on non-canceled error trials.
3
NO STOP SIGNAL Trials
Reaction Time
Correct
Canceled
STOP SIGNAL Trials
Stop Signal Delay
Correct
Non-Canceled
Error
Fig. S1. Countermanding task. Dotted circle indicates the focus of gaze, and arrow indicates
saccade. Trials began upon fixation of the central spot following which a target appeared at one
of two peripheral locations 180Ε from each other with the same eccentricity on either side of the
fixation spot. Simultaneously, the fixation spot disappeared, and monkeys were required to shift
gaze to the target. On a random 1/3 to 1/2 of the trials after a variable delay, the fixation spot
reappeared which instructed monkeys to cancel the planned saccade (i.e. stop signal). On trials
without the stop signal, monkeys were reinforced with juice after a period of fixation of the
peripheral target. On stop signal trials, monkeys received reinforcement after a period of
prolonged fixation of the central spot (canceled trials). If monkeys failed to cancel the saccade
and shifted gaze to the target, no reinforcement was delivered (non-canceled trials).
4
Spikes/Sec
30
20
10
0
-200
2nd sacc < 200 ms (n = 30)
2nd sacc > 200 ms (n = 153)
0
200 400 600 800 1000
Time from erroneous saccade (ms)
Fig. S2. Activity of a representative error-related neuron divided according to the latency of
the saccade following the error. The absence of any significant difference is inconsistent with
the hypothesis that the error-related activity actually corresponds to conflict engendered by
activation for the corrective saccade coinciding with activation for the errant saccade.
5
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