Neural correlates of illusory line motion

Abstract

Illusory line motion (ILM) refers to a motion illusion in which a flash at one end of a bar prior to the bar's instantaneous presentation or removal results in the percept of motion. While some theories attribute the origin of ILM to attention or early perceptual mechanisms, others have proposed that ILM results from impletion mechanisms that reinterpret the static bar as one in motion. The current functional magnetic resonance imaging study examined participants while they made decisions about the direction of motion in which a bar appeared to be removed. Preceding the instantaneous removal of the bar with a flash at one end resulted in a motion percept away from the flash. If this flash and the bar's removal overlapped in time, it appeared that the bar was removed towards the flash (reverse ILM). Independent of the motion type, brain responses indicated activations in areas associated with motion (MT+), endogenous and exogenous attention (intraparietal sulcus, frontal eye fields, and ventral frontal cortex), and response selection (ACC). ILM was associated with lower percept scores and higher activations in ACC relative to real motion, but no differences in shape-selective areas emerged. This pattern of brain activation is consistent with the attentional gradient model or bottom-up accounts of ILM in preference to impletion.

Figure 1. Diagram depicting a trial sequence for the real motion, ILM, and reverse ILM conditions.

Figure 2. Behavioural data as a function of flash location and experimental condition.

Panel A) shows Percept scores and B) Decision Times. Error bars indicate +/− 1 SEM. ILM: illusory line motion.

Figure 3. Activations for each of the three motion conditions superimposed onto the population-average landmark- and surface-based (PALS) standard brain (voxel threshold p _corr<0.05 FWE, k>20).

ACC: anterior cingulate cortex, AIC: anterior insular cortex, DLPFC: dorsolateral prefrontal cortex, FEF: frontal eye field, ILM: illusory line motion, IPS: intraparietal sulcus, PCC: posterior cingulate cortex, SMA: supplementary motor area.

Figure 4. Differential brain activations (initial voxel threshold p _uncorr<0.001, cluster threshold p _corr<0.05 FWE) overlaid on the study-specific average of the normalised anatomical scans for the contrast “Illusory line motion > Real motion” (A), the contrast “Illusory line motion > Reverse illusory line motion” (B) and associated percentage signal change values (C).

Error bars indicate +/− 1 SEM. ACC: anterior cingulate cortex, ILM: illusory line motion, L: left, R: right.