Aging Affects The Direction Selectivity Of MT Cells In Rhesus Monkeys

Visual functions are impaired significantly by aging.The aging-related visual deficits can be observed even after the optical factor has been well controlled.It is suggested that the central nerve system plays an important role in generation of such deficits.In the past two decades,several alterations in response properties of visual cortical cells have been reported in senescent animal models,which were considered as the neural mechanisms underlying the aging-related degradation of vision.In order to a more comprehensive understanding about the aging effects on visual cortices and the relationship between the changes at cellular level and the functional degeneration at the whole system level,in the present study,we used in vivo extracellular single-unit recording techniques to compare the response property of visual cortical cells in young and old monkeys.The study included 2 experiments:1.Aging affects the direction selectivity of MT cells in rhesus monkeys.The ability to accurately perceive the direction and speed of moving objects is critical for survival.Many psychophysical studies have shown that this ability declines in humans during normal aging.This cannot be due to optical changes or changes in the retina alone.Therefore,it has been hypothesized that this decline results from functional degradation in central visual areas.The mechanisms that underlie the visual decline that accompanies aging have been investigated using single-cell recording techniques in different visual areas. Studies have provided evidence that the effects of aging on the dorsal lateral geniculate nucleus(dLGN) are minor,while those in the striate cortex(V1) and extrastriate cortex(V2) are severe.Both the orientation and direction selectivity of V1 and V2 cells degrade during senescence.These declines are accompanied by hyperactivity and decreased signal-to-noise ratios.Such hyperactivity and decreased selectivity are thought to be due to the degradation of GABA-mediated inhibition within the visual cortex.It is known that direction-selective neurons play an important role in motion processing.V1 cells are involved in early stages of motion perception. Motion-sensitive neurons in this area respond to the 'component' motion of moving stimuli.These cells signal motion that is orthogonal to their preferred orientations. Recent studies have shown that aging reduces the direction selectivity of V1 cells.To date,nothing is known about age-related changes in later stages of motion processing.The middle temporal area(MT or V5) of the extrastriate cortex has been linked to higher order motion detection.In this area,more than 90%of the neurons are direction-selective.A proportion of MT direction-selective neurons are pattern cells. These cells can detect the direction of moving objects independently of their particular spatial pattern.Thus,the responses of these cells can be equated with human perception.In fact,a population of pattern cells exists in human MT+,and the activity of these neurons is closely linked to the perception of coherent pattern motion.We studied the effect of aging upon MT cells in macaque monkeys by comparing the proportion of pattern cells and the degree of direction selectivity of MT cells in young and old monkeys.The results show that the proportion of pattern cells in MT decreases in old macaques.A comparison of direction selectivity of areas V1 and MT in old and young monkeys provides evidence that DB in cells in both areas decrease in old monkeys.MT cells showed a larger decrease than V1 cells did.These age-related changes were accompanied by increases in peak response and spontaneous activity,as well as a decrease in signal-to-noise ratio.Taken together,our results indicate that functional degradation occurs in both areas of MT and V1 during normal aging,and that area MT is affected by aging more severely than the striate cortex is. Such functional degradation in area MT may contribute to the decline in perception of moving objects during normal aging.2.Degradation of intra-cortical processing in primary visual cortex of senescent monkey.The primate visual system is broadly organized into two segregated processing pathways,a ventral stream for object vision and a dorsal stream for space vision, termed as P pathway and M pathway.Within both streams,visual information is processed sequentially such that progressively more complex functions are carried out at progressively higher levels in the hierarchy.Over the past decades,the mechanisms underlying visual declines that accompany aging have been investigated using single-cell recording technique in a variety of visual areas and deficits in neuronal response was found,with different extends,along the visual hierarchy.The emerged picture suggested that the aging effect could be inserted into each processing stage and resulted in the progressive functional degradation along the hierarchy of visual information processing.For example,studies have provided evidence that the effects of aging on the dLGN are minor,while those in striate cortex are severe,In P pathway,extrastriate cortex(V2) is found more severe affects by aging than V1.Similarly,in M pathway,MT is also found to be affected more severe than V1.Taken these studies together,it is reasonable to hypothesize that higher hierarchical levels are affected by aging more severe.Besides the visual hierarchy composed of different cortical structures or areas, there are similar hierarchies of processing within individual areas.The most well studied example is the simple cell and complex cell in V1.In 1959,Hubel and Wiesel first reported that in the striate cortex,neurons respond selectively to the orientation of local contours and to the direction of motion of a visual stimulus.Also,they found that those neurons could be classified into simple and complex cells based on their receptive field properties.According to hierarchical processing model proposed by Hubel and Wiesel,simple cells receiving inputs from dorsal lateral geniculate nucleus (dLGN) become selective to the orientation of a local contour in a specific location. Complex cells,which carry out more complex functions,integrating information from simple cells become selective to the orientation of a local contour regardless of its exact position within the receptive field.Therefore,the hypothesis that we want to test in the present study is:the complex cells,located at the higher stage of processing, affected by aging more severe than simple cells.To this end,we investigated the visual response of cortical cells in anesthetized and paralyzed monkeys.Orientation and direction selectivity of simple and complex cells between young and old monkeys were compared.Our results have shown that the orientation and direction selectivities to stimuli of simple cells are relatively normal in old V1 area.However,the selectivities of complex cells decrease in old monkeys.The differential age-related changes of the two types of cells indicate that intra-cortical information processing suffers impairments in different degree during aging.By these two experiments,we studied the different characteristics of aging effects in different brain areas and the degradation of information processing during normal aging.The results indicate that 1) there is more severe functional degradation of higher hierarchy;2) The degradation of MT cells in old monkeys might result in visual motion deficits in old human.3) The hypothesis that higher hierarchical levels are affected by aging more severe was well supported by our experiments and previous studies.