Many rodents do not see well. For a target to be
resolved by a rat or a mouse, it must subtend a visual
angle of a degree or more. It is commonly assumed that
this poor spatial resolving capacity is due to neural rather
than optical limitations, but the quality of the retinal
image has not been well characterized in these animals.
We have modified a double-pass apparatus, initially designed
for the human eye, so it could be used with rodents to
measure the modulation transfer function (MTF) of the eye's
optics. That is, the double-pass retinal image of a monochromatic
(λ = 632.8 nm) point source was digitized with a CCD
camera. From these double-pass measurements, the single-pass
MTF was computed under a variety of conditions of focus
and with different pupil sizes. Even with the eye in best
focus, the image quality in both rats and mice is exceedingly
poor. With a 1-mm pupil, for example, the MTF in the rat
had an upper limit of about 2.5 cycles/deg, rather than
the 28 cycles/deg one would obtain if the eye were a diffraction-limited
system. These images are about 10 times worse than the
comparable retinal images in the human eye. Using our measurements
of the optics and the published behavioral and electrophysiological
contrast sensitivity functions (CSFs) of rats, we have
calculated the CSF that the rat would have if it had perfect
rather than poor optics. We find, interestingly, that diffraction-limited
optics would produce only slight improvement overall. That
is, in spite of retinal images which are of very low quality,
the upper limit of visual resolution in rodents is neurally
determined. Rats and mice seem to have eyes in which the
optics and retina/brain are well matched.
An Objective Scatter Index Based on Double-Pass Retinal Images of a Point Source to Classify Cataracts