Aging Affects The Spatial And Temporal Frequency Tuning Properties Of Neurons In Visual System Of Rhesus Monkeys And Cats

Aged human show various deficits in visual functions, including but not limited to spatial contrast sensitivity, visual processing speed, and motion perception. Such visual functional deficits severely impact the normal living and working of older adults. For example, they report difficulty in reading and driving.Psychophysical studies indicate that optical degradation of the aged eye could not account for all of the effects, and it has been suggested that there should be age-related neuronal changes in the central visual system occurring in the process of aging. Consistent with this, electrophysiological studies carried out in aged animals revealed that many aspects of neuronal response properties were changed at multiple stages of the visual pathway when compared to young animals, such as delayed response latency (monkeys:V1and V2), elevated response variability to repeated stimuli (monkeys:V1and MT), degraded orientation and direction selectivity (monkeys:V1and MT; cats:V1), and decreased contrast sensitivity (monkeys:V1and MT).To understand the potential neural mechanisms underlying the deficits accompanied by aging in visual perception under various levels of spatial and temporal modulation, we focused on the spatial and temporal frequency tuning properties of neuronal response, which is fundamental in pattern vision and has been extensively studied based on the suggestion that images can be encoded and analyzed by the brain in terms of different spatial and temporal Fourier components. In this study we examined the spatial and temporal frequency tuning properties of neurons in visual areas of both monkeys and cats under paralyzed and anesthetized conditions using electrophysiological extracellular single-unit recording techniques.We quantitatively examined the visual responses of neurons in both young adult and aged macaque monkeys at the stage of the middle temporal area (MT, or V5), which is hierarchically downstream of V1and traditionally deemed a key cortical area for the analysis of visual motion. For both spatial and temporal frequencies, our results show that the neuronal responses of aged monkeys were significantly changed compared to young adult ones. Neurons in area MT of aged monkeys preferred stimuli drifting at significantly lower spatial and temporal frequencies and exhibit wider spatial frequency bandwidths. Spatial frequency resolution and high temporal frequency cutoff were also decreased in aged animals. These changes were accompanied by elevated spontaneous activity and reduced signal-to-noise ratio of MT neurons.We measured the spatial frequency tuning properties of neurons in multiple stages (LGN, A17, A18and PMLS) on the visual processing pathway in both aged and young adult cats. We found that at the stage of the lateral geniculate nucleus of the thalamus, the spatial frequency tuning properties of neurons were not affected by aging. However, in all the cortical visual areas (A17, A18and PMLS), the optimal spatial frequencies of neurons were not changed, but the tuning bandwidths of neurons were significantly broadened in aged cats. These results indicated that the spatial frequency selectivity of neurons were degraded in visual cortical areas during agmg.Our results may help to clarify the neural mechanisms underlying the declines in spatial and temporal contrast sensitivities and impaired motion perception during agmg.