Dynamic causal modeling of the network underlying the Müller-Lyer illusion

Abstract

The Müller-Lyer illusion is an optical illusion that alters the perceived length of a line via arrows attached to its end. Recent imaging studies revealed that ventral and dorsal stream areas contribute to the processing of the Müller-Lyer illusion (Weidner et al., 2010; Weidner & Fink, 2007). Here we investigated [using dynamic causal modeling (DCM; Friston, Harrison, & Penny, 2003)] the effects of this illusion on the effective connectivity of the brain areas involved in coding illusion strength. Strength of the Müller-Lyer illusion was parametrically modulated while participants performed either a landmark task or a non-spatial luminance task. Relative to the luminance task, the landmark task increased neural activity in lateral occipital cortex and right superior parietal cortex dependent upon illusion strength. Based on these findings DCM was used to investigate the possible interactions between ventral and dorsal visual stream. Bayesian model selection (Stephan et al., 2009) indicated that a model involving bi-directional connections between dorsal and ventral stream areas most accurately accounted for the underlying network dynamics. In this model, illusion strength enhanced bi-directional couplings of bilateral lateral occipital cortex and the right superior parietal cortex. Thereby, both areas seem to fulfill differential roles: Whereas lateral occipital cortex seems to be directly related to size transformation processes, activation in right superior parietal cortex rather reflects subsequent levels of processing, including task-related supervisory functions. Furthermore, our data clearly demonstrate that the effect of illusion strength on these interactions between lateral occipital and superior parietal regions critically depends on an observer’s top-down settings.