Peripheral vision

Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the visual field that is included in the notion of peripheral vision. "Far peripheral" vision refers to the area at the edges of the visual field, "mid-peripheral" vision exists in the middle of the visual field, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze.

The inner boundaries of peripheral vision can be defined in any of several ways depending on the context. In everyday language the term "peripheral vision" is often used to refer to what in technical usage would be called "far peripheral vision." This is vision outside of the range of stereoscopic vision. It can be conceived as bounded at the center by a circle 60° in radius or 120° in diameter, centered around the fixation point, i.e., the point at which one's gaze is directed. However, in common usage, peripheral vision may also refer to the area outside a circle 30° in radius or 60° in diameter. In vision-related fields such as physiology, ophthalmology, optometry, or vision science in general, the inner boundaries of peripheral vision are defined more narrowly in terms of one of several anatomical regions of the central retina, generally the fovea.

The fovea is a cone-shaped depression in the central retina measuring 1.5 mm in diameter, corresponding to 5° of the field of vision. The outer boundaries of the fovea are visible under a microscope, or with microscopic imaging technology such as OCT or microscopic MRI. When viewed through the pupil, as in an eye exam (using an ophthalmoscope or retinal photography), only the central portion of the fovea is visible. Anatomists refer to this as the clinical fovea, and say that it corresponds to the anatomical foveola, a structure with a diameter of 0.35 mm corresponding to 1 degree of the field of vision. In clinical usage the central part of the fovea is typically referred to simply as the fovea.

In terms of visual acuity, "foveal vision" may be defined as the part of the retina in which visual acuity is at least 20/20 (6/3 metric). This corresponds to the foveal avascular zone (FAZ) with a diameter of 0.5 mm representing 1.5° of the visual field. Although often idealized as perfect circles, the central structures of the retina tend to be irregular ovals. Thus, foveal vision may also be defined as the central 1.5–2° of the field of vision. Vision within the fovea is generally called central vision, while vision outside of the fovea is called peripheral, or indirect vision.

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