Anthony & Jesse
Background
What is Retinotopy?
Early visual areas are organized by a principle known as retinotopy. The basic idea behind this principle is that stimulation of adjacent regions of the visual field activate adjacent locations on the cortical surface, and specific locations in these areas respond to stimulation only in particular parts of the visual field. In addition, the preference for particular locations in the visual field across the cortical surface is highly consistent across individuals and can be mapped with fMRI.
Discovery of Visual Field Maps
In the early 1900s, Inouye and Holmes are the first to discover that the location of lesions to primary visual cortex (V1) is related to visual field deficits. For instance, damage to the V1 in the left hemisphere results in impaired vision in the right visual field and damage to the anterior portion of V1 results in impaired vision at peripheral locations in the visual field.
Electrophysiological recordings in the 1940s identified additional field maps adjacent to V1 such as V2 and V3. Each map contains a representation of the entire visual field when combined across hemispheres. However, transitions between maps are smooth in that the areas near the boundaries of field maps both respond to nearby locations in the visual field, and locations in the visual field can be represented by two paramters: Polar angle and eccentricity.
Polar Angle and Eccentricity Bias
Each visual field map in each hemisphere has either an entire hemifield representation of visual space (or a quarter-field representation if separating maps such as V2 and V3 into ventral and dorsal subdivisions). In order to determine which part of the visual field an area is representing, we can use a stimulus that sweeps across the visual field at different polar angles (see figure F, to the right). BOLD activity that correlates with either the upper, horizontal, or lower visual field will be color-coded accordingly, allowing us to identify when there is a mirror-reversal of polar angle mapping. These reversals mark the dividing boundaries between different visual field maps in human cortex. For example, V1 has a hemifield representation, but V2v has a quarter-field representations in which the mapping of polar angle is mirror reversed relative to V1. Similarly, the representation of polar angle in V3v is mirror reversed relative to V2v.
While the cortex is divided according to reversals of polar angle representations, shared across all of the these field maps is an organization of eccentricity biases. As shown in figure E, foveal representations are more posterior, with increasingly peripheral biases radiating anteriorly. V1 through V4 have their foveal representations clustered in the posterior calcarine, known as the confluent fovea. There are other foveal repesentations, such as those on the ventral occipital surface (VO1 & VO2) or the parahippocampal gyrus.