We
have surprisingly low visual acuity (resolution) in parts of the visual field
that are not at the center of gaze — where we are looking. We are not aware of
this because we instinctively direct our center of gaze to where we are looking.
The center of gaze, called the fovea, has a higher density of
cones than anywhere else on the retina. In fact, at the fovea, there are no rods
at all. (In the diagram at right, the cones are shown in green.) The fovea evolved
to have the highest possible visual acuity, and the cones are as small as they
can possibly be and still function. Moreover, in the fovea, the retinal ganglion
cells have smaller receptive fields, and in the periphery, they have much larger
receptive fields.
Curiously, despite the vitality of cones to our vision,
we have 125 million rods and only 6 million cones.
The
density of cones in our center of gaze is shown in the graph above. The peak is
on our fovea. The edges of the graph are our peripheral vision.
The fact
that our vision has the highest acuity in the center of gaze does not mean that
our vision in the rest of the visual field is inferior — it’s simply used for
different things. Foveal vision is used for scrutinizing highly detailed objects,
whereas peripheral vision is used for organizing the broad spatial scene and for
seeing large objects. Our foveal vision is optimized for fine details, and our
peripheral vision is optimized for coarser information.
