How visual eccentricity shapes conflict via target and distractor processing in the Simon task

The visual Simon task is widely used to study action control in the presence of conflicting target and distractor information. However, it is unclear how one of the core parameters of this task, visual target eccentricity, affects conflict processing. Building on a quantitative dual-route model Diffusion Model for Conflict Tasks (DMC), we hypothesized that increased eccentricity alters the relative strength of target- and distractor-based activation at the stage where conflict emerges, through two mechanisms: (a) weakened target processing due to reduced perceptual quality, and/or (b) enhanced distractor processing via continuous spatial coding of location. Using DMC simulations, we demonstrate that relying solely on mean reaction time (RT) to assess eccentricity effects may obscure underlying processing differences: depending on the timing of distractor-based activation, the impact of eccentricity on mean Simon effects may vary - even when the strength of target and/or distractor processing remains unchanged. In the present two experiments, we therefore first conducted distributional (delta plot) analyses to account for the temporal dynamics of distractor processing, revealing generally larger Simon effects for far compared to near targets. Analyses of DMC best-fitting parameters showed that, in both experiments, the increased conflict with greater eccentricity was due to decreased target-based accumulation rates. Moreover, distractor-based activation increased with eccentricity in Experiment 1 but not Experiment 2, suggesting that spatial locations are coded continuously rather than categorically when they can serve as mutual reference points (e.g., within blocks). We discuss implications for the mechanisms underlying the Simon effect by elaborating on how the timing and strength of target and distractor processes jointly shape conflict dynamics.