Locomotion through the environment and interactions with objects depend on fundamental processes of surface segmentation. This project will study the neural mechanisms that support surface segmentation using a combination of functional Magnetic Resonance (fMRI), electro- electroencephalographic (EEG) and magneto-encephalographic (MEG) imaging studies. Elementary encephalographic aspects of surface segmentation will be studied with reduced cue stimuli that control the availability of temporal frequency, orientation, phase and direction cues as inputs to the figure/ground segmentation process. Cortical areas involved in form and motion-based segmentation will be localized functionally using both fMRI and electromagnetic source estimation techniques. The spatial relationship of this functional activation to visual areas defined by retinotopic mapping will also be determined. These studies will determine how the different segmentation cues are combined in different cortical areas and at different times after stimulus onset. A further goal of the imaging experiments is to determine whether border information is extracted prior to surface information or vice versa. To better understand the neural computations involved in segmentation we will apply a non-linear analysis technique based on binary m-sequences to study the time-evolution of segmentation-related activity recorded from the EEG and MEG. The technique measures time evolution of the strength and sign of interaction (facilitation or suppression) between figure and background as a function of their spatio- spatiotemporal configuration. The technique also yields a functional model of the evoked response that can temporal predict responses to the simpler stimuli that will be used in the imaging experiments. The validity of the model will be tested by comparing model predictions for simple stimuli to those actually measured. Finally, we will examine segmentation performance in strabismus patients with a history of abnormal visual experience during early development that leads to defective or absent stereopsis and nasalward/temporal biases in motion processing. Results from the current project period suggest that these patients will have deficits on figure/ground segmentation that are independent of acuity deficits. This study will also determine whether nasalward/temporal biases previously reported for large field uniform stimuli propagate to the figure/ground segmentation process.
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