In reality it is smaller than this, and irregular, because when the observer is looking straight ahead, his or her nose blocks vision of some possible parts of the surface.
In perimetric testing, a section of the imaginary sphere is realized as a hemisphere in the centre of which is a fixation point.
To specify loci in the visual field, a polar coordinate system is used, all expressed from the observer's perspective.
The meridian of the visual field has been found to influence the folding of the cerebral cortex.
In both the V1 and V2 areas of macaques and humans the vertical meridian of their visual field tends to be represented on the cerebral cortex's convex gyri folds whereas the horizontal meridian is tends to be represented in their concave sulci folds.