Human Neural Responses to Overlaid Real and Apparent Motion: Implications for Mechanisms of Illusory Motion Perception

Abstract

Abstract: : Purpose: It has been previously shown that the human motion complex (visual area hMT⁺/ V5) is involved in segmentation and integration of global motion signals. Indeed, the measured activation levels are correlated with the number of perceived moving surfaces. However, there is little information on the role of hMT⁺/V5 in integration of illusory motion in particular when it is superimposed on real motion. In this study, we searched for evidence of interactions between real and illusory motion using overlaid fixed– and mixed–direction moving plaid and apparent motion (AM) stimuli. Methods: Functional magnetic resonance imaging (fMRI) was performed at 1.5T in 4 participants. Stimuli were fixed– and mixed–direction moving plaids and static plaid test patterns with and without overlaid AM. We have analyzed responses in visual area hMT⁺/V5 for all experimental conditions. Data analysis included preprocessing, coregistration and Talairach transformation of scans, volume rendering, and multiple regression analysis by fitting a general linear model (GLM) with predictors for each experimental condition, and event related averaging using BrainVoyager QX. Results:We found much weaker activation for apparent motion than for plaid motion in hMT⁺/V5. The superposition of AM signals does not result in a significantly different activation pattern that of plaid stimuli alone. We found higher activation for fixed–direction adaptation than for mixed–direction adaptation during the subsequent static test condition. Fixed–direction adaptation conditions elicited illusory motion aftereffects (MAE) regardless of the presence or absence of superimposed AM. However, we found no difference between AM, MAE, and AM superimposed on MAE activation levels in the human motion complex. Conclusions: The results suggest that hMT⁺/V5 plays a different role in illusory motion integration as compared to real motion integration. Illusory motion (AM or MAE) leads to substantially smaller modulations of hMT⁺/V5 activity than real motion and these modulations are more susceptible to visual selective attention effects.

Keywords: motion-2D • imaging/image analysis: non-clinical • visual cortex