Abstract:

During the development, the structure and functions of the visual system can be affected by visual experiences and environments. This is called visual plasticity which is most prominent during the critical period of development after birth. Although the structures and functions of neural circuits tend to be stable in adult visual cortex, mounting evidence has shown that adult visual cortex still retains a certain degree of plasticity, including ocular dominance plasticity.

Ocular dominance in humans refers to a phenomenon that one eye is functionally superior to the other eye. A common method for measuring perceptual ocular dominance is the binocular rivalry task. The typical stimuli in this task are two spatially overlapped but incompatible images, with one presented to each eye. At any moment, observer is usually aware of only one of the images which remains visible for a while before being consciously replaced by the other one. The ratio of dominance duration for each eye in a binocular rivalry task can be used to quantitatively assess observer’s ocular dominance.

One of the most commonly used ways to modulate ocular dominance in adults is monocular deprivation, which shifts ocular dominance to the deprived eye through the temporary occlusion of one eye. In recent decades, researchers have extensively investigated the monocular deprivation effect and its underlying mechanism by constantly changing the way of monocular deprivation, such as depriving the energy information (e.g. contrast) or phase information (e.g. contour) of monocular images. A consistent finding is that the imbalance of visual input between two eyes, whether achieved through complete or partial deprivation of visual information to one eye, leads to a shift of ocular dominance towards the deprived eye. The shift of ocular dominance may reflect neural plasticity in the early stages of visual processing, which is closely related to the reduction of GABA inhibition in the primary visual cortex. Meanwhile, one suggested mechanism for monocular deprivation is homeostatic plasticity, an inherent mechanism that stabilizes neuronal activity and prevents the neuronal system from becoming hyperactive or hypoactive. In the context of short-term monocular deprivation, an imbalance in visual input between the two eyes may trigger a homeostatic upregulation of neural response in the deprived eye to maintain a balance of neural activity within the visual system. This can lead to a shift in ocular dominance towards the deprived eye following the monocular deprivation.

More recently, it has been found that even in the absence of visual input deprivation, directing a greater amount of attention towards one eye can effectively induce an effect of ocular dominance shift. For example, a “dichoptic-backward-movie” adaptation paradigm was invented to study eye-based attention induced ocular dominance shift. The ocular dominance is biased in favor of the eye (unattended eye) that has viewed a backward movie for long during which time the opposite eye (attended eye) is presented with a regular movie. This phenomenon indicates that the neural mechanisms of short-term ocular dominance plasticity not only occur at the lower level of visual processing stage but also receive feedback regulations from higher cortical sites. Notably, the boost of the unattended eye was not observed when testing stimuli were binocularly compatible. Therefore, the attention-induced ocular dominance shift may not be explicable solely by means of the homeostatic plasticity mechanism, because the involvement of homeostasis is not specific to binocular rivalry. Given the crucial role of interocular competition in attention-induced ocular dominance shift, this effect is currently explained by the adaptation of ocular opponency neurons that represents interocular conflict by computing differences between the input signals from the two eyes.

Despite significant advancements in the investigation of short-term ocular dominance plasticity, there are many promising research directions for future studies, especially those that may further our understanding of the complicated mechanisms for short-term ocular dominance plasticity. The article then ends with the outlook in this regard.

Key words: ocular dominance, plasticity, monocular deprivation, attention