Oculomotor target selection is mediated by complex objects

Abstract

Oculomotor target selection often requires discriminating visual features, but it remains unclear how oculomotor substrates encoding saccade vectors functionally contribute to this process. One possibility is that oculomotor vector representations (observed directly as physiological activation or inferred from behavioral interference) of potential targets are continuously reweighted by task relevance computed elsewhere in specialized visual modules, whereas an alternative possibility is that oculomotor modules use local featural analyses to actively discriminate potential targets. Strengthening the former account, oculomotor vector representations have longer onset latencies for ventral- (i.e., color) than dorsal-stream features (i.e., luminance), suggesting that oculomotor vector representations originate from featurally relevant specialized visual modules. Here, we extended this reasoning by behaviorally examining whether the onset latency of saccadic interference elicited by visually complex stimuli is greater than is commonly observed for simple stimuli. We measured human saccade metrics (saccade curvature, endpoint deviations, saccade frequency, and error proportion) as a function of time after abrupt distractor onset. Distractors were novel, visually complex, and had to be discriminated from targets to guide saccades. The earliest saccadic interference latency was ∼110 ms, considerably longer than previous experiments, suggesting that sensory representations projected into the oculomotor system are gated to allow for sufficient featural processing to satisfy task demands. Surprisingly, initial oculomotor vector representations encoded features, as we manipulated the visual similarity between targets and distractors and observed increased vector modulation response magnitude and duration when the distractor was highly similar to the target. Oculomotor vector modulation was gradually extinguished over the time course of the experiment.NEW & NOTEWORTHY We challenge the role of the oculomotor system in discriminating features during saccadic target selection. Our data suggest that the onset latency of oculomotor vector representations is scaled by task difficulty and featural complexity, suggesting that featural computations are performed outside of the oculomotor system, which receives the output of these computations only after sufficient visual and cognitive processing. We also challenge the convention that initial oculomotor vector representations are feature invariant, as they encoded task relevance.