Higher order, multifeatural object encoding by the oculomotor system

Abstract

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.

Oculomotor target selection is mediated by complex objects

“…Saccade curvature indicated that the magnitude of inhibition increased as distractors became increasingly dissimilar to the target, which we have observed previously using the same stimulus set (17). This is the opposite pattern of results observed in previous behavioral studies of saccade curvature using opponent color singletons (19,20).…”

“…Critically, the secondary excitatory epoch occurred before the inhibitory processing for the low and intermediate-similarity distractors, which suggests that the protracted excitatory epoch encoding highsimilarity distractors does not merely reflect a delayed onset of inhibition. As distractor identity could be decoded from the magnitude and duration of excitatory processing related to the distractor onset, this suggests that the oculomotor system dynamically receives preprocessed object representations from relevant visual modules and encodes these objects as dynamically reweighted oculomotor vectors, as we have argued previously (17,18,22).…”

“…Behavioral experiments have demonstrated that visual features impose additional competition between oculomotor vector representations. Saccade trajectories are more spatially biased by distractors with task-relevant (17)(18)(19)(20)(21) or conspicuous (15,16) features than distractors with task-irrelevant or inconspicuous features. For example, saccades curvature elicited by distractors that are color congruent with targets is greater than saccade curvature elicited by incongruent distractors (19,20).…”

“…Six stimuli used in a previous experiment (17) were constructed offline using MATLAB (MathWorks, Natick, MA) by intersecting six or seven vertical and horizontal line segments (1 Â 0.08 ) together at right angles in a configuration that did not resemble meaningful alphanumerical characters to an English speaker (similar to Ref. 37, see Fig.…”

“…Similarly, saccadic endpoint averaging between stimuli is enhanced for identical stimulus pairs as compared with feature-incongruent stimulus pairs (21). In fact, the magnitude of saccade curvature and endpoint deviation varies continuously as a function of target-distractor similarity for color (18) and even complex, meaningless, novel objects (17).…”