Visual angle
Visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.
The diagram on the right shows an observer's eye looking at a frontal extent (the vertical arrow) that has a linear size
can represent the eye's nodal points at about the center of the lens, and also represent the center of the eye's entrance pupil that is only a few millimeters in front of the lens.
heading toward the eye indicate the bundle of light rays that pass through the cornea, pupil and lens to form an optical image of endpoint
The central line of the bundle represents the chief ray.
[1] However, for visual angles smaller than about 10 degrees, this simpler formula provides very close approximations: As the above sketch shows, a real image of the object is formed on the retina between points
For small angles, the size of this retinal image
is the distance from the nodal points to the retina, about 17 mm.
If one looks at a one-centimeter object at a distance of one meter and a two-centimeter object at a distance of two meters, both subtend the same visual angle of about 0.01 rad or 0.57°.
That is just a bit larger than the retinal image size for the moon, which is about
Also, for some easy observations, if one holds one's index finger at arm's length, the width of the index fingernail subtends approximately one degree, and the width of the thumb at the first joint subtends approximately two degrees.
[2] Therefore, if one is interested in the performance of the eye or the first processing steps in the visual cortex, it does not make sense to refer to the absolute size of a viewed object (its linear size
In astronomy the term apparent size refers to the physical angle
But in psychophysics and experimental psychology the adjective "apparent" refers to a person's subjective experience.
Additional confusion has occurred because there are two qualitatively different "size" experiences for a viewed object.
(or apparent visual angle) which is the subjective correlate of
, also called the object's perceived or apparent angular size.
(or apparent linear size) which is the subjective correlate of
, the object's physical width or height or diameter.
Widespread use of the ambiguous terms "apparent size" and "perceived size" without specifying the units of measure has caused confusion.
The brain's primary visual cortex (area V1 or Brodmann area 17) contains a spatially isomorphic representation of the retina (see retinotopy).
Loosely speaking, it is a distorted "map" of the retina.
of a given retinal image determines the extent of the neural activity pattern eventually generated in area V1 by the associated retinal activity pattern.
Murray, Boyaci, & Kersten (2006) recently used Functional magnetic resonance imaging (fMRI) to show that an increase in a viewed target's visual angle, which increases
, also increases the extent of the corresponding neural activity pattern in area V1.
The observers in experiment carried out by Murray and colleagues viewed a flat picture with two discs that subtended the same visual angle
of one was about 17% larger than for the other, due to differences in the background patterns for the disks.
It was shown that the areas of the activity in V1 related to the disks were of unequal size, despite the fact that the retinal images were the same size.
