Retinotopic Mapping
The brain is organized retinotopically. This means that neighboring points in the brain represent neighboring points of the visual world. There is a “map” inside your brain that represents the basic spatial structure of the visual scene. Two spatial properties are represented: eccentricity (distance from the center of gaze) and polar angle. Functional magnetic resonance imaging (fMRI) provides a powerful tool for measuring these maps. To the left are two example maps from a single individual’s brain (visualized on a flattened representation of the cortex, right hemisphere) (from Gardner, Merriam, Movshon, and Heeger, 2008). The small circles color-code regions of the visual field. The maps show regions of the brain that respond to the corresponding areas of the visual field. The eccentricity maps (below) shows that there is a continuum of nearby neural regions devoted to different eccentricities (distances from the center of gaze). The polar angle map (above) shows that the left half of the visual field is represented in the right hemisphere. Note also in the polar angle that the same regions of the visual field are re-represented in multiple areas (drawn in white). These areas each perform different but related functions.
Retinotopic mapping allows us to clearly pre-define regions of the brain, so that we can then test hypotheses about their function in visual processing. However, there exist a multitude of methods for doing such mapping that differ in both the stimuli and the analysis procedures, and these methods have never been comprehensively compared. We also know little about the nature of these maps at the near-fovea (center of gaze). We are looking at both of these important methodological points.
This work is in collaboration with Elisha Merriam.