Ocular dominance is a dynamically changing phenomenon that exhibits varying levels of plasticity during the development and maturation of the visual system. Recent studies over the past decade have suggested that certain visual training paradigms are capable of regulating adult ocular dominance. However, the neural mechanisms and behavioral training paradigms underlying the reshaping of ocular dominance in adulthood remain unclear and have yet to reach a consensus. This dissertation examines the neural mechanisms underlying adult ocular dominance plasticity using a paradigm based on long-term adaptation to altered-reality, as investigated in two studies.
In Study 1,we conducted a six-day continuous, three-hour per day behavioral training using the altered-reality adaptation paradigm on adult amblyopia patients with extreme interocular imbalance. Visual acuity and electroencephalogram (EEG) testing were performed on the subjects at three time points: before training, 24 hours after training completion, and one month follow up test. In the EEG experiment, we used frequency tagging and steady-state visual evoked potential (SSVEP) to measure the EEG signals of each eye and the intermodulation frequency (IM) neural signal that reflects interocular interaction. By calculating the changes in response amplitude to complementary-patchwork stimuli (i.e., adaptation stimuli during the training phase) between the two eyes before and after training, we examined the neural activity of long-term altered reality adaptation in regulating eye dominance. The results showed that the amplitude of the IM component decreased significantly one month after the training ended. This training effect was only found in the amblyopia group of Experiment 1 and not in the interocular balanced healthy group of Experiment 2. Given that the stimuli in this study were complementary-patchwork natural images to both eyes, we tend to believe that the measured IM component reflects the degree of interocular inhibition in interocular interaction, and the decrease in IM amplitude represents a weakening of inhibition. We explained this de-inhibition mechanism using the ocular opponency model and provided initial direct neural evidence for interocular de-inhibition mechanism of adult ocular plasticity.
In Study 2, we investigated the training effect on the neural signals of the visual processing pathway in adults with normal vision but relative interocular imbalance. We first conducted a screening experiment using black and white grating binocular rivalry task to select participants with interocular imbalance index. After this, in Experiment 2a, using event-related potential (ERP) and motion coherence detection paradigm, we explored the changes in neural responses related to motion processing area of the visual dorsal pathway before and after long-term altered-reality adaptation. The results showed that the N1 component in the occipital lobe had significant changes after training but with eye specificity, as evidenced by a decrease in the signal amplitude of the stronger eye during the follow-up test. In addition, in Experiment 2b, using steady-state visual evoked potential (SSVEP) and red-green grating binocular rivalry paradigm, we examined how the neural signals in the primary area of the visual ventral pathway changed before and after training. The results showed that there was no significant change in the signal strength of the tagged frequency for both eyes, nor in the intermodulation frequency. These preliminary findings suggest that regulating eye dominance through complementary-patchwork adaptation method could affect the neural signals in the motion processing area of the visual dorsal pathway.
In conclusion, Study 1 suggests that long-term altered reality adaptation can decrease the amplitude of the IM component, which reflects the degree of interocular suppression, in individuals with extreme interocular imbalance. Study 2 reveals that the amplitude of the stronger eye to motion stimuli in the visual dorsal pathway is weakened after long-term altered reality adaptation in individuals with relative interocular imbalance. These findings provide empirical evidence for the regulation of adult eye dominance from both the low-level interocular suppression and high-level dorsal pathway motion processing neural responses in the visual system, complementing our understanding of the neural mechanisms underlying the long-term altered reality adaptation paradigms.
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