Expecting the unexpected: Temporal expectation increases the flash-grab effect

Abstract

In the flash-grab effect, when a disk is flashed on a moving background at the moment it reverses direction, the perceived location of the disk is strongly displaced in the direction of the motion that follows the reversal. Here, we ask whether increased expectation of the reversal reduces its effect on the motion-induced shift, as suggested by predictive coding models with first order predictions. Across four experiments we find that when the reversal is expected, the illusion gets stronger, not weaker. We rule out accumulating motion adaptation as a contributing factor. The pattern of results cannot be accounted for by first-order predictions of location. Instead, it appears that secondorder predictions of event timing play a role. Specifically, we conclude that temporal expectation causes a transient increase in temporal attention, boosting the strength of the motion signal and thereby increasing the strength of the illusion.

Predictions drive neural representations of visual events ahead of incoming sensory information

“…In the related flash-grab illusion (20), in which an object flashed on a reversing background is mislocalized, the flashed object is similarly mislocalized in the direction of motion after the flash. It has previously been argued that a postdictive correction of failed extrapolation also contributes to this perceptual illusion (21,22). Motion-position illusions such as the flash-lag effect and related illusions therefore provide a potential conscious correlate to the predictive mechanisms that we reveal here.…”

Concurrent perception of competing predictions: a “Split-Stimulus Effect”

“…Specifically, whether or not participants could anticipate the precise location of the flash did not affect the likelihood of them seeing illusory splitting. This finding is curious, as the Flash-Grab Effect, and other MPIs such as the 'Twinkle Goes' Effect (Nakayama & Holcombe, 2021), have been shown to be significantly affected by attention (e.g., Cavanagh & Anstis, 2013;Coffey et al, 2019). One reason why we may not have observed an effect is because the specific attention manipulation we used was relatively modest.…”

“…One could also interpret the unexpected null effect of post-flash motion velocity as being due to differences in task procedure -the Flash-Grab Effect typically estimated using a continuous measure of the degree of perceived shift (e.g., Blom et al, 2019;Cavanagh & Anstis, 2013;Coffey et al, 2019).…”