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Edelman JA, Xu KZ. Inhibition of voluntary saccadic eye movement commands by abrupt visual onsets. J Neurophysiol 101: 1222-1234, 2009. First published November 19, 2008 doi:10.1152/jn.90708.2008. Saccadic eye movements are made both to explore the visual world and to react to sudden sensory events. We studied the ability for humans to execute a voluntary (i.e., nonstimulus-driven) saccade command in the face of a suddenly appearing visual stimulus. Subjects were required to make a saccade to a memorized location when a central fixation point disappeared. At varying times relative to fixation point disappearance a visual distractor appeared at a random location. When the distractor appeared at locations distant from the target virtually no saccades were initiated in a 30-to 40-ms interval beginning 70 -80 ms after appearance of the distractor. If the distractor was presented slightly earlier relative to saccade initiation then saccades tended to have smaller amplitudes, with velocity profiles suggesting that the distractor terminated them prematurely. In contrast, distractors appearing close to the saccade target elicited express saccade-like movements 70 -100 ms after their appearance, although the saccade endpoint was generally scarcely affected by the distractor. An additional experiment showed that these effects were weaker when the saccade was made to a visible target in a delayed task and still weaker when the saccade itself was made in response to the abrupt appearance of a visual stimulus. A final experiment revealed that the effect is smaller, but quite evident, for very small stimuli. These results suggest that the transient component of a visual response can briefly but almost completely suppress a voluntary saccade command, but only when the stimulus evoking that response is distant from the saccade goal.

Executive control involves the ability to flexibly inhibit or change an action when it is contextually inappropriate. Using the complimentary techniques of human fMRI and monkey electrophysiology in a context-dependent stop signal task, we found a functional double dissociation between the right ventrolateral prefrontal cortex (rVLPFC) and the bi-lateral frontal eye field (FEF). Different regions of rVLPFC were associated with context-based signal meaning versus intention to inhibit a response, while FEF activity corresponded to success or failure of the response inhibition regardless of the stimulus response mapping or the context. These results were validated by electrophysiological recordings in rVLPFC and FEF from one monkey. Inhibition of a planned behavior is therefore likely not governed by a single brain system as had been previously proposed, but instead depends on two distinct neural processes involving different sub-regions of the rVLPFC and their interactions with other motor-related brain regions.

The ability to inhibit actions inappropriate for the context is essential for meeting the shifting demands of complex environments. The stop signal task (SST) has been used in many previous studies to examine the interactions between go and stop responses in a cognitively demanding task involving attention, conflict resolution, and motor plan selection. The current study uses a variant of the SST, in which the continue signal instructs participants to proceed with the go response they were preparing. Reaction times (RTs) on continue trials were bimodally distributed, suggesting that an aspect of inhibition was involved in at least some of the trials. We investigated whether the cognitive processes delaying the generation of a behavioural response on continue trials are the same as for stop trials. We found improvement of stop signal reaction times (SSRTs) following stop trials, but the decrease in continue signal reaction times (CSRTs) was not significant. No improvement in either SSRT or CSRT was found following continue trials, suggesting that activation of the processes delaying the response on continue trials is insufficient to drive subsequent adjustments in SSRT or CSRT. In addition, go RTs only slowed following stop trials. These effects may suggest the presence of a selective learning process, which requires that the initial inhibition captured by SSRT and CSRT be combined with recognition of the stop signal specifically to affect subsequent performance.

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