Representation of illusory shapes within the topographic areas of the posterior parietal cortex
The human visual system consists of multiple topographic maps that extend from the early visual cortex along the dorsal and ventral processing streams. Responses to illusory shapes within these maps have been demonstrated in the ventral stream areas, in particular the lateral occipital complex. Recently, the intraparietal sulcus of the dorsal stream has been linked to the processing of illusory shapes defined by motion. It therefore remains unclear whether the topographically organized parietal areas also respond to static illusory shapes, which would suggest their generic role in representing illusory content. Here we measured brain responses using fMRI while human participants observed flickering inducers around the fixation task. The inducers either formed an illusory diamond in the center, a triangle in the left or in the right hemifield, or were inverted such that no illusory figure was formed. We compared responses of parietal regions IPS0-IPS5 and SPL1 to each illusory figure with the non-illusory condition. To determine the role of attention in illusory shape responses we manipulated the difficulty of the fixation task. Our results show that all IPS areas responded to illusory shapes. The more posterior areas IPS0-IPS3 additionally displayed a preference towards the contralateral shapes, while the more anterior areas IPS4 and IPS5 showed response attenuation with increased task difficulty. We suggest that the IPS can represent illusory content irrespective of the perceptual mechanism that generated it. These responses may serve as a potential feedback signal that drives illusory shape responses in early and ventral visual areas. Significance statementThe traditional view of the ventral visual pathway being solely responsible for representation of objects has recently been challenged by demonstrating illusory shape representation within the dorsal visual pathway with moving bistable stimuli. Our results provide evidence for the dorsal stream contribution to representing not only moving, but also static illusory shapes. Our results also show a functional subdivision along the topographic maps, with spatially specific shape responses in the more posterior, and attention-dependent responses in the more anterior areas. IPS areas of the dorsal stream should thus be considered in the theoretical accounts and neural models of how subjective content is generated in the brain.