In the spirit of numbers in vision research, I ask the question:
What percentage of the retinal cones are in the fovea (in human retina)?
Before giving the answer, let me lay out some of the logic, which
generates some interesting round numbers. I would be delighted to
get feedback on the accuracy of these numbers.
The diameter of the human eye is about 25 mm (one inch, or the
diameter of a American quarter coin). The area of the human retina
is about 1000 sq mm.
The total number of cones in the human retina is about 5 million.
Their density falls with about a -2/3 power of eccentricity out to about
20 deg, then gradually decreases but shows a steep increase by about a
factor of 3 at the ora serrata (where there is a strip of pure cones,
and the rod density falls to zero).
The maximum cone density at the foveola is about 200 thousand
cones/sq mm.
The foveola (avascular center of the foveal pit, containing the cones
with the longest outer segments, at about 50 um) has a diameter
of about 0.5 deg. It contains about 2500 cones.
The bottom of the foveal pit has a diameter of about 1 deg (the
roughly flat and rod-free zone, containing all the cones whose
outer segments are elongated beyond the typical peripheral length
of 20 um). It contains about 6000 cones.
The fovea, with a diameter of about 5 deg (from Polyak), contains about
50 thousand cones.
The proportion of cones in the human fovea, therefore, is about
one percent! (An informal survey at ARVO 1997 produced
answers ranging from 10% to 90% for this proportion, with the
exception of one retinal anatomist who knew the exact answer.)
To continue such thoughts, the region of human retina projecting to the
central half of striate cortex is the central 20 deg, of the total area
of about 120 X 180 deg . The number of cones in this region is about
300 thousand, still only 6% of the total number of cones in the retina.
So the vast majority (94%) of cones lie in the far periphery of the
retinal projection.
Although peripheral cone density falls as low as 4000/sq mm, versus
200 K/sq mm for the maximum rod density, it is important to realize
that the cone diameter at these eccentricities is as large as 5 - 9 um.
The cone inner segments therefore occupy as much as 1/3 of the area
of peripheral retina, and catch as much as 1/3 of the light captured by
the two types of peripheral photoreceptors.
In summary, the round numbers are:
Total number of cones = 5 Million
Number of cones in foveola = 2500 (0.05% of total)
Number of cones in fovea = 50K (1% of total)
Mean foveal density = 30 K per sq mm
Number of cones projecting to central half of striate = 300 K
(6% of total)
Many of these points are quantified in my paper:
Analysis of Human Receptor Density, p 63 - 71
in Basic and Clinical Applications of Vision Science, Ed. V
Lakshminarayanan, Kluwer Academic Publishers: The Netherlands.
Check with me if you would like a copy.
Christopher Tyler (cwt@skivs.ski.org)
Christopher W. Tyler, Ph.D.
Associate Director
Smith-Kettlewell Eye Research Institute
2232 Webster Street, San Francisco, CA 94115 USA
Ph. 415-561-1640 Fax 415-561-1610
Email: cwt@skivs.ski.org Home Page: http://www.ski.org/cwt.html