Functional Degradation Of MT Neurons In Senescent Rehesus Monkeys

Human visual abilities decline during normal aging. Much of the decline cannot be attributed to the optical changes of the eye or changes in the retina, but and most likely reflect aged-related changes in visual cortical areas. To reveal the neural mechanism underlyng the visual function degradation during noral aging is of great practical importance to how to improve life quality for the old population and to slow down the aging process in humans.Using single-unit extracellular recording techniques, we compared the response properties of visual cortical cells in young and old monkeys. Our main findings are as follows:1. Aging affects the neural representation of speed in macaque area MTIn the present study, we used random-dot patterns to study the effects of aging on speed tuning curves in cortical area MT of macaque visual cortex. Our results provide evidence for a significant degradation of speed selectivity in MT. Cells in old animals preferred lower speeds than did those in young animals. Response modulation and discriminative capacity for speed in old monkeys were also significantly weaker than those in young ones. Concurrently, MT cells in old monkeys showed increased baseline responses, peak responses and response variability. These changes were accompanied by very low signal-to-noise ratios. We also found that speed discrimination thresholds in old animals were higher than in young ones. The foregoing neural changes may mediate the declines in visual motion perception that occur during senescence.2. Aging affects contrast response functions of MT neurons in rhesus monkeysIn the present study we studied the effects of aging on the coding of contrast in area V1 (primary visual cortex) and MT (middle temporal visual area) of the macaque monkey using single-neuron in vivo electrophysiology. Our results show that both MT and V1 neurons in old monkeys are less sensitive to contrast than those in young monkeys. Generally, contrast sensitivity is affected by aging more severely in MT cells than in V1 cells. Specifically, MT cells were affected more severely than motion direction selective V1 cells. Our results suggest that there may be an anomalous visual processing in both the magnocellular and parvocellular pathways. The neural changes described here are consistent with an age-related degeneration of intracortical inhibition and could underlie some deficits in visual function during normal aging.3. Aging affects contrast adaptation of MT neurons in rhesus monkeysWe assessed age-related changes of neuronal adaptation to visual motion in MT. We also found age-related changes in neuronal adaptation to visual motion in MT. Compared to young animals, the contrast gain of MT neurons in old monkeys are less affected, but the response gain by adaptation of MT neurons are more affected. This may reflect a 'fatigue' mechanism and excitotoxic hyperactivity in old brains.4. Aging affects response variability of V1 and MT neurons in rhesus monkeysWe examined how aging affects response variability of V1 and MT in rhesus (Macaca mulatta) monkeys. We found that mean response and response variability in both V1 and MT of old monkeys are significantly higher than in young monkeys. And signal-to-noise ratio in old monkeys is significantly lower than in young ones. The results are consistent with an age-related degradation of inhibitory intracortical circuits. The neural changes described here could contribute to declines in visual function during senescence.In summary, age-related changes in macaque MT were observed in the present study. The results indicated 1) the neural representation of speed in macaque area MT is affected by aging; 2) there is more severe functional degradation of higher areas in visual hirerchy; 3) the age-related changes in neuronal adaptation to visual motion in MT occur; 4) spike count variability in V1 and MT of old monkeys is significantly higher than young monkeys. All of the findings provide us a more comprehensive understanding about the mechanisms underlying the age-related degeneration of visual function.