Visual processing faces two conflicting demands: integration and segmentation. Integration is required by inherently noisy visual signals, while segmentation is needed to extract vital information from spatiotemporal variations in visual input. An understanding of the interplay between these two mechanisms will reveal fundamentals of visual processing and also enable insights into functional roles of segmentation processes. In motion perception, the PI's recent work demonstrated that spatial integration of motion signals is not fixed, but critically depends on basic visual factors such as contrast, with spatial integration giving way to spatial suppression as stimulus visibility increases. The overarching goal of this proposal is to investigate the neural mechanisms involved in this adaptive integration/segregation of motion signals, and to elucidate their role in the segmentation of objects from moving backgrounds. The key property of spatial suppression is impaired motion perception of large, high-contrast stimuli.
In Aim 1, we will determine if and how this suppressive mechanism affects visual and oculomotor processing. Answering this question will constrain possible neural correlates of spatial suppression and, along with Aim 2, provide a test for the hypothesis linking spatial suppression to surround suppression in area MT. Substantiating this link will allow the attribution of links between spatial suppression and motion segregation (Aims 2 &3) to the involvement of MT surround suppression.
In Aim 2, we will seek direct evidence about neural correlates of spatial suppression by impairing processing in MT and early visual areas with TMS. We expect that a disruption of neural mechanisms critically involved in spatial suppression will allow normally suppressed motion signals to reach perception. Concurrently, we will also determine whether the same stimulation that impairs spatial suppression also disrupts motion segregation.
In Aim 3, we test the hypothesis that spatial suppression directly enables rapid segregation of moving objects by suppressing background motion signals. Here, the role of spatial suppression in motion segregation is conceptualized as a coarse, but rapid, region-based segmentation process. This hypothesis predicts that variations in spatial suppression (and, thus, in the visibility of background motion) should predict corresponding changes in motion segregation and vice versa. Exploiting different experimental approaches, we will test this prediction by utilizing stimulus manipulations, individual differences and perceptual learning to produce variations in either spatial suppression or motion segregation. One focus will be on older adults, who are known to exhibit weak spatial suppression. We will determine whether this abnormality predicts motion segregation deficits and whether age-related deficits in spatial suppression can be reversed by the perceptual learning of motion segregation.
The knowledge of mechanisms underlying spatial suppression and motion segregation will contribute to the understanding of age-related changes in these two basic visual processes. Moreover, understanding how these age-related deficits can be alleviated through perceptual learning might lead to the development of viable interventions.
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