Motion induced positional shift is modulated by attentional load

Abstract

The motion-induced position shift (MIPS) is an illusion where a stationary object in the periphery appears displaced due to local motion. According to a Bayesian model (Kwon, Tadin, & Knill, 2015), positional uncertainty causes the perceptual system to rely on local motion velocity to infer object positions, with attention potentially playing an important role in perceptual accuracy. Recently, Nakayama & Holcombe (2020) found that increasing bottom-up attention can reset illusory object positions and motion trajectories, highlighting the role of attention. This study tested whether increased attentional load reduces the illusory shifts of object position. Noise patches with Gaussian kernels were presented in the periphery (10–12° eccentricity) with local motion (9–11°/s) for 1 second. The number of objects presented simultaneously (1, 2, or 3) varied across blocks to manipulate attentional load. Participants fixated on the screen center and reported the final object position, and the speed and direction of the local motion. We mainly analyzed the first responded object within a trial to isolate effects of attention from working memory. Results showed that MIPS magnitude increased with the number of objects, indicating a trade-off between attentional availability and illusory positional shifts. Divided attention to multiple objects likely impaired the precision of positional judgments, forcing the perceptual system to rely more on motion cues. These findings align with theories suggesting attention causes a reset of illusory shift of object position.