Areas involved in figure-ground segregation based on luminance, colour, motion, and stereoscopic depth visualized with fMRI

Abstract

Figure-ground segregation is an important first step of visual object recognition. We compared the strength and distribution of cortical BOLD responses to the presentation of different figure-ground stimuli with that to homogeneous stimuli. The segregation was based on differences between figure and ground in either luminance, colour, motion or stereoscopic depth. In the case of colour- and luminance-based stimuli, figure and ground were either separated by an abrupt luminance or colour border or else defined by the arrangement of dots of different colour/luminance. Eight observers participated. Stimuli were presented by means of a video projector to the foot end of a Siemens Vision 1.5 Tesla scanner and flickered at 4 Hz. Stereoscopic stimuli were presented in red for the right and in green for the left eye and observers used red/green spectacles for this part of the experiment. Data were analysed using Brain Voyager software. Luminance and colour based figure-ground segregation were correlated with activation in both striate and extrastriate cortex. Motion- and stereo-based segregation, on the other hand, mostly activated extrastriate areas. So while figure-ground segregation based on different elementary features may share some common neuronal mechanism(s), a large part of activity for segregation not based on luminance or colour takes place in extrastriate areas, suggesting partly separate mechanisms for figure-ground segregation for different submodalities.

Footnotes

 Supported by German Research Council (SFB 517).