to specify loci in the visual field, a polar coordinate system is used, all expressed from the observer's perspective. the origin corresponds to the point on which the observer is fixating. the polar angle is considered to be zero degrees when a locus is horizontally to the right of the fixation point and to increase to a maximum of 360 degrees going anticlockwise. distance from the origin is given in degrees of visual angle; it's a measure of eccentricity. each polar axis is a meridian of the visual field. for example, the horizontal meridian runs from the observer's left, through the fixation point, and to the observer's right. the vertical meridian runs from above the observer's line of sight, through the fixation point, and to below the observer's line of sight.