RETINAL IMAGE QUALITY IN THE RODENT EYE

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 com monly assumed that this poor spatial resolving capacity is due to neur al rather than optical limitations, but the quality of the retinal ima ge has not been well characterized in these animals. We have modified a double-pass apparatus, initially designed for the human eye, so it c ould 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 (lambda = 632.8 nm) point source was digitized with a C CD camera. From these double-pass measurements, the single-pass MTF wa s computed under a variety of conditions of focus and with different p upil sizes. Even with the eye in best focus, the image quality in both rats and mice is exceedingly poor. With a l-mm pupil, for example, th e MTF in the rat had an upper limit of about 2.5 cycles/deg, rather th an the 28 cycles/deg one would obtain if the eye were a diffraction-li mited system. These images are about 10 times worse than the comparabl e retinal images in the human eye. Using our measurements of the optic s and the published behavioral and electrophysiological contrast sensi tivity functions (CSFs) of rats, we have calculated the CSF that the r at would have if it had perfect rather than poor optics. We find, inte restingly, that diffraction-limited optics would produce only slight i mprovement overall. That is, in spite of retinal images which are of v ery low quality, the upper limit of visual resolution in rodents is ne urally determined. Rats and mice seem to have eyes in which the optics and retina/brain are well matched.