EFFECTS OF AGING ON THE PRIMATE VISUAL-SYSTEM - SPATIAL AND TEMPORAL PROCESSING BY LATERAL GENICULATE NEURONS IN YOUNG-ADULT AND OLD RHESUS-MONKEYS

1. Visual abilities decline during normal aging, and many of these dec lines are due to neural changes in the retina or central visual pathwa ys. We have begun studies of the primate visual system to investigate the location and nature of these changes as well as to answer general questions about the effects of aging on neural function. We began with the dorsal lateral geniculate nucleus (LGN) because it is the main st ructure through which visual information passes on the way to cortex a nd because the parallel parvocellular and magnocellular pathways, whic h may be affected differently by aging, are anatomically distinct ther e. 2. Single-cell recordings were made in the LGN of young adult (5-16 yr) and old (25-28 yr) rhesus monkeys. We made quantitative measures of a wide variety of response properties for a large number of parvoce llular (n = 257) and magnocellular (n = 113) neurons in the two groups of animals. As a result, in addition to studying the effects of aging , we were able to make quantitative comparisons between parvocellular and magnocellular neurons using larger samples than have been studied previously and for some properties that have not been studied before. 3. We found that magnocellular neurons have significantly higher maxim al response rates and signal-to-noise ratios than parvocellular neuron s. However, response latencies to visual stimulation were similar for neurons in the two types of layers. In agreement with previous studies , magnocellular neurons had higher maximal contrast sensitivity and hi gher contrast gain than parvocellular neurons. However, the sensitivit y difference occurred because nearly all of the neurons with low sensi tivities (<10) were in the parvocellular layers, not because neurons i n the magnocellular layers had the highest sensitivities. 4. Neurons w ith the smallest receptive-field centers, the highest spatial-frequenc y resolutions, and the highest optimal spatial frequencies were found in the parvocellular layers. However, the overall distributions of eac h of these properties overlapped substantially for neurons in the two types of layers, and the mean values were not significantly different. The mean high temporal frequency cutoff was significantly higher for magnocellular than parvocellular neurons, but the difference was small (only 3 Hz), and it occurred because many parvocellular neurons had l ower cutoffs than any seen in the magnocellular layers, not because ma gnocellular neurons had the highest temporal-frequency cutoffs. Parvoc ellular neurons also had narrower temporal-frequency tuning than magno cellular neurons. However, there was no significant difference in opti mal temporal frequency. 5. Together, these comparisons between the mag nocellular and parvocellular layers suggest that, at least at the leve l of the LGN, the differences between the two pathways are more a matt er of degree than kind. It is quite possible that both pathways partic ipate to different extents in all of the visual functions we have inve stigated.ally significant effects of aging on either parvocellular or magnocellular neurons. The percentages of color-opponent and broadband cells, and the percentages of red-green and blue-yellow types among c olor-opponent cells, did not differ significantly between the two age groups. There also were no differences in latency, amplitude, or signa l-to-noise ratio of responses to visual stimuli, center-surround recep tive-field organization, spatial-frequency resolution, optimal spatial frequency, high temporal-frequency cutoff, temporal-frequency bandwid th, contrast sensitivity, or contrast gain. For parvocellular neurons, mean spontaneous discharge rate (FO) was significantly higher, and op timal temporal frequency was significantly lower in old than in young animals. However, the differences were small and may reflect random va riation (i.e., Type I statistical error given the large number of comp arisons).