TOPOGRAPHY OF GANGLION-CELLS AND PHOTORECEPTORS IN THE RETINA OF A NEW-WORLD MONKEY - THE MARMOSET CALLITHRIX-JACCHUS

We studied the anatomical substrates of spatial vision in a New World monkey, the marmoset Callithrix jacchus. This species has good visual acuity and a foveal specialization which is qualitatively similar to t hat of humans and other Old World primates. We measured the spatial de nsity of retinal ganglion cells and photoreceptors, and calculated the relative numbers of these cell populations. We find that ganglion cel ls outnumber photoreceptors by between 2.4:1 and 4.2:1 in the fovea. T he peak sampling density of ganglion cells is close to 550,000 cells/m m(2). This value falls by almost 1000-fold between the fovea and perip heral retina; a value which approaches recent estimates of the centrop eripheral ganglion cell gradient for human and macaque monkey retina a nd primary visual cortex. The marmoset shows a sex-linked polymorphism of color vision: all male and some female marmosets are dichromats. S ix of the retinas used in the present study came from animals whose ch romatic phenotype was identified in electrophysiological experiments a nd confirmed by polymerase chain reaction (PCR) amplification of cone opsin encoding genes. One animal was a trichromat and the others were dichromats. Antibodies against short wavelength-sensitive (SWS) cones labeled close to 8% of all cones near the fovea of one dichromat anima l, consistent with electrophysiological evidence that the SWS system i s present in all marmosets. The topography and spatial density of cone photoreceptors and ganglion cells was similar to that reported for ma caque retina, and we found no obvious difference between dichromatic a nd trichromatic marmoset retinas. These results reinforce the view tha t the main determinate of primate foveal topography is the requirement for maximal spatial resolution.