Retinotopy is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'retinally mapped'. Visual field maps are found in many mammalian brains, though the specific size, number, and spatial arrangement of these maps can differ considerably between species. Retinotopic maps in cortical areas other than V1 are typically more complex, in the sense that adjacent points of the visual field are not always represented in adjacent regions of the same area. For example, in the second visual area, the map is divided along an imaginary horizontal line across the visual field, in such a way that the parts of the retina that respond to the upper half of the visual field are represented in cortical tissue that is separated from those parts that respond the lower half of the visual field. Even more complex maps exist in the third and fourth visual areas V3 and V4, and in the dorsomedial area. In general, these complex maps are referred to as second-order representations of the visual field, as opposed to first-order representations such as V1. Additional retinotopic regions include ventral occipital, lateral occipital, dorsal occipital, and posterior parietal cortex.
History
The discovery of visual field maps in humans can be traced to neurological cases, arising from war injuries, described and analyzed independently by Tatsuji Inouye and Gordon Holmes. They observed correlations between the position of the entry wound and visual field loss.
Description
In many locations within the brain, adjacent neurons have receptive fields that include slightly different, but overlapping portions of the visual field. The position of the center of these receptive fields forms an orderly sampling mosaic that covers a portion of the visual field. Because of this orderly arrangement, which emerges from the spatial specificity of connections between neurons in different parts of the visual system, cells in each structure can be seen as contributing to a map of the visual field. Retinotopic maps are a particular case of topographic organization. Many brain structures that are responsive to visual input, including much of the visual cortex and visual nuclei of the brain stem and thalamus, are organized into retinotopic maps, also called visual field maps. Areas of the visual cortex are sometimes defined by their retinotopic boundaries, using a criterion that states that each area should contain a complete map of the visual field. However, in practice the application of this criterion is in many cases difficult. Those visual areas of the brainstem and cortex that perform the first steps of processing the retinal image tend to be organized according to very precise retinotopic maps. The role of retinotopy in other areas, where neurons have large receptive fields, is still being investigated. Retinotopy mapping shapes the folding of the cerebral cortex. In both the V1 and V2 areas of macaques and humans the verticalmeridian of their visual field tends to be represented on the cerebral cortex's convex gyri folds whereas the horizontal meridian tends to be represented in their concave sulci folds.
Methods
Retinotopy mapping in humans is done with functional Magnetic Resonance Imaging. The subject inside the fMRI machine focuses on a point. Then the retina is stimulated with a circular image or angled lines about focus point. The radial map displays the distance from the center of vision. The angular map shows angular location using rays angled about the center of vision. Combining the radial and angular maps, you can see the separate regions of the visual cortex and the smaller maps in each region.
