Investigation Of The Effects Of Aging And Amblyopia On Motion Perception

The primate visual motion system performs numerous functions essential for survival in a dynamic visual world. Prominent among these functions is the ability to recover and represent the trajectories of objects in a form that facilitates behavioral responses to those movements. To achieve this goal, the visual motion information undergoes a two-step analysis, which has been suggested by a large amount of evidence from psychophysical and neurobiological studies. In the first step which may occur at an earlier cortical stage, the displacement of local retinal image features is detected; then in the second step which may occur at higher levels of visual cortical regions, such as MT and MST, all local retinal motion signals are selectively integrated according to the object of origin. By this way, the retina image motion gives rise to our perceptual experience of moving objects.However, the function of the visual motion system can be impaired by a lot of factors, which surely reduces the people's life quality. The first step aimed to improving this reduced function is to outline the characterics of the influeced motion perception. Here, we explored the effect of normal aging and a type of development disorder, amblyopia, on the motion perception. In the first study, we put our emphasis on the local motion perception since the effect of aging on global motion perception has been widely investigated while this is not for local motion perception. In the second study, taking account of the fact that little is known about the perception of amblyopes on moving plaids, a kind of global motion, and the perceptual capbility of moving plaids may suggest more about the nature of global motion processing deficits in amblyopia, we investigated the effect of amblyopia on perceptual capbility of moving plaids.1. The effect of normal aging on first-/second-order local motion perception First- and second-order stimuli are two types of patterns which have been used to characterize early visual processing. The former are defined by modulation of luminance, and the latter are defined by changes in features, such as contrast or texture. It has been suggested that these two types of stimuli are ubiquitous in everyday visual scenes, and that natural images are rich in both kinds of information. In a noisy environment, first- and second-order stimuli can be combined to improve perceptual accuracy. Therefore, perception of these two types of stimuli plays an important role in people's daily life.However, little is known about the effect of aging on second-order stimuli, both static and dynamic. The purpose of the present study is to make some progress in understanding this issue.We measured contrast sensitivity for both first- and second-order stimuli, both static and dynamic, in 141 subjects with normal visual acuity and aged from 19 to 79 years old. The results have shown no gender effect but an evident aging effect, i.e., a progressive decline during aging, for perception of both types of stimuli. We have also proposed a piecewise linear model to interpret our data. Based on this model, contrast sensitivity for second-order stimuli begins to decline significantly earlier than for first-order stimuli, but with a slower rate of progression. We suggest the earlier decline for the perception of second-order stimuli may be interpreted as reflecting a greater complexity of second-order processing,consistent with Jocelyn Faubert's theory on visual perception and aging.2. The effect of amblyopia on moving plaid perceptionA moving plaid, composed of two moving gratings with different orientations which are physically superimposed, is a different type of global motion stimuli. It has suggested that a two-stage visual analysis underlying the moving plaid perception, a decomposition into two one-dimensional component motions which can be implemented by component-selective neurons such as those in V1 and a recombination of these component motions which may be implemented in MT.The effect of amblyopia on moving plaids perception is not as well known as that on the random dot kinematograms which are traditionally used in global motion perception research. Additionally, since the physiological basis for processing of moving plaids and random dot kinematograms are both neurons in MT, it can be used to test the hypothesis whether there is a general loss of MT functions in amblyopia. Therefore, we investigated the effect of amblyopia on moving plaid perception here. We measured contrast sensitivities for moving plaids and their corresponding component gratings over a range of stimuli durations, spatial and temporal frequencies in 10 normal subjects and 13 anisometropic amblyopes by using motion direction discrimination task. We found reduced contrast sensitivities for moving plaids and their corresponding component gratings in anisometropic amblyopia. Additionally, the reduction in sensitivity for plaids was statistically identical to that for their component gratings, suggesting the former can be almost completely accounted for by the latter. In other words, there are no residual deficits in plaid perception at the threshold level after excluding the influence of low-level processing deficits, suggesting the global processing for plaid perception, whose physiological bases are supposed to be MT neurons, is largely intact in anisometropic amblyopia. This is against the hypothesis that there is a general loss of MT functions in amblyopia.