Saccades curved toward a distractor are accompanied by a burst of neuronal activation at the distractor locus in the intermediate layers of the superior colliculus (SCi) ~30 ms before the initiation of a saccade. Although saccades curve away from inactivated SCi loci, whether inhibition is restricted to a similar critical epoch for saccades curved away from a distractor remains unclear. We examined this possibility by modeling human saccade curvature as a function of the time between onset of a task irrelevant luminance- or color-modulated distractor and initiation of an impending saccade, referred to as saccade distractor onset asynchrony (SDOA). Our results demonstrated that 70 ms of luminance-modulated distractor processing or 90 ms of color-modulated distractor processing was required to modulate saccade trajectories. As these behavioral, feature-based differences were temporally consistent with the cortically mediated neurophysiological differences in visual onset latencies between luminance and color stimuli observed in the oculomotor and visual system, this method provides a noninvasive means to estimate the timing of peak activation in the oculomotor system. As such, we modeled SDOA functions separately for saccades curved toward and away from distractors and observed that a similar temporal process determined the magnitude of saccade curvatures in both contexts, suggesting that saccades deviate away from a distractor due to a rapid accumulation of inhibition in the critical epoch before saccade initiation. In this research article, we propose a novel, noninvasive approach to behaviorally model the time course of competitive oculomotor processing. Our results highly resembled those from previously published neurophysiological experiments utilizing similar oculomotor processing contexts, thus validating our approach. Furthermore, this methodology provided new insights into the underlying neural mechanism subserving oculomotor processing given that we applied it to a context with which the neural mechanism is more contentious, and the results clearly favored one view.

The oculomotor system utilizes color extensively for planning saccades. Therefore, we examined how the oculomotor system actually encodes color and several factors that modulate these representations: attention-based surround suppression and inherent biases in selecting and encoding color categories. We measured saccade trajectories while human participants performed a memory-guided saccade task with color targets and distractors and examined whether oculomotor target selection processing was functionally related to the CIE (x,y) color space distances between color stimuli and whether there were hierarchical differences between color categories in the strength and speed of encoding potential saccade goals. We observed that saccade planning was modulated by the CIE (x,y) distances between stimuli thus demonstrating that color is encoded in perceptual color space by the oculomotor system. Furthermore, these representations were modulated by (1) cueing attention to a particular color thereby eliciting surround suppression in oculomotor color space and (2) inherent selection and encoding biases based on color category independent of cueing and perceptual discriminability. Since surround suppression emerges from recurrent feedback attenuation of sensory projections, observing oculomotor surround suppression suggested that oculomotor encoding of behavioral relevance results from integrating sensory and cognitive signals that are pre-attenuated based on task demands and that the oculomotor system therefore does not functionally contribute to this process. Second, although perceptual discriminability did partially account for oculomotor processing differences between color categories, we also observed preferential processing of the red color category across various behavioral metrics. This is consistent with numerous previous studies and could not be simply explained by perceptual discriminability. Since we utilized a memory-guided saccade task, this indicates that the biased processing of the red color category does not rely on sustained sensory input and must therefore involve cortical areas associated with the highest levels of visual processing involved in visual working memory.

Previous behavioral and physiological research has demonstrated that as the behavioral relevance of potential saccade goals increases, they elicit more competition during target selection processing as evidenced by increased saccade curvature and neural activity. However, these effects have only been demonstrated for lower order feature singletons, and it remains unclear whether more complicated featural differences between higher order objects also elicit vector modulation. Therefore, we measured human saccades curvature elicited by distractors bilaterally flanking a target during a visual search saccade task and systematically varied subsets of features shared between the two distractors and the target, referred to as objective similarity (OS). Our results demonstrate that saccades deviated away from the distractor highest in OS to the target and that there was a linear relationship between the magnitude of saccade deviation and the number of feature differences between the most similar distractor and the target. Furthermore, an analysis of curvature over the time course of the saccade demonstrated that curvature only occurred in the first 20–30 ms of the movement. Given the multifeatural complexity of the novel stimuli, these results suggest that saccadic target selection processing involves dynamically reweighting vector representations for movement planning to several possible targets based on their behavioral relevance. NEW & NOTEWORTHY We demonstrate that small featural differences between unfamiliar, higher order object representations modulate vector weights during saccadic target selection processing. Such effects have previously only been demonstrated for familiar, simple feature singletons (e.g., color) in which features characterize entire objects. The complexity and novelty of our stimuli suggest that the oculomotor system dynamically receives visual/cognitive information processed in the higher order representational networks of the cortical visual processing hierarchy and integrates this information for saccadic movement planning.