title: Top-down modulations in the visual form pathway revealed with dynamic causal modeling.
abstract: Perception entails interactions between activated brain visual areas and the records of previous sensations, allowing for processes like figure-ground segregation and object recognition. The aim of this study was to characterize top-down effects that originate in the visual cortex and that are involved in the generation and perception of form. We performed a functional magnetic resonance imaging experiment, where subjects viewed 3 groups of stimuli comprising oriented lines with different levels of recognizable high-order structure (none, collinearity, and meaning). Our results showed that recognizable stimuli cause larger activations in anterior visual and frontal areas. In contrast, when stimuli are random or unrecognizable, activations are greater in posterior visual areas, following a hierarchical organization where areas V1/V2 were less active with "collinearity" and the middle occipital cortex was less active with "meaning." An effective connectivity analysis using dynamic causal modeling showed that high-order visual form engages higher visual areas that generate top-down signals, from multiple levels of the visual hierarchy. These results are consistent with a model in which if a stimulus has recognizable attributes, such as collinearity and meaning, the areas specialized for processing these attributes send top-down messages to the lower levels to facilitate more efficient encoding of visual form.
authors.affiliation: Wellcome Laboratory of Neurobiology, Anatomy Department, University College London, London, WC1E 6BT, UK. velia.cardin@rhul.ac.uk
authors.forename: Velia
authors.initials: V
authors.lastname: Cardin
authors.forename: Karl J
authors.initials: KJ
authors.lastname: Friston
authors.forename: Semir
authors.initials: S
authors.lastname: Zeki
mesh_terms: D000328:Adult; D001931:Brain Mapping; D005260:Female; D005556:Form Perception / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008959:Models, Neurological; D014793:Visual Cortex / Q000502:physiology*; D014795:Visual Pathways / Q000502:physiology*; D055815:Young Adult