Li, H. Reviewed by Liu, Longqian Reviewer
Amblyopia is a condition of low vision in one or both eyes due to visual deprivation and/or abnormal binocular interactions, without detectable organic lesions. The common treatments for amblyopia are masking therapy and atropine suppression therapy, but they are not effective in all cases of amblyopia. As research on perceptual learning in the treatment of amblyopia has intensified in recent years, more and more attention has been paid to the clinical implications of perceptual learning. This growing body of research has led to the possibility that perceptual learning may be an alternative approach to amblyopia treatment. This article reviews the progress of research on perceptual learning in amblyopia treatment. Li Hua, Ophthalmology, Yongchuan Hospital, Chongqing Medical University
1. Definition and mechanism of perceptual learning
Perceptual learning refers to the ability of a subject to perform a certain perceptual task through training. In visual perception, this task is often referred to as a discrimination task, which involves recognizing small differences in simple visual attributes, such as position, orientation, texture, or shape. The mechanisms of perceptual learning are still not well understood. Recent research on perceptual learning has provided some insight into its mechanisms. Perceptual learning is an important research component in the field of perception, which involves the intersection of cognitive psychology, psychophysics, neurophysiology, and other disciplines. Experiments based on the structural design of perceptual learning models have found that the perceptual learning process involves better removal of external noise . Li RW et al. showed that repetitive training was effective in improving visual behavioral abilities, mainly due to the subjects’ improved efficiency in using relevant stimulus information. Many experimental observational and computational models have also identified functional changes in the nervous system associated with perceptual learning. A unifying theory that links behavioral findings in visual perceptual learning with physiological and anatomical findings is the hierarchical flip theory, which posits that the process of learning begins at high levels of the visual system as a top-down processing of visual information and that this process affects the level of information input, resulting in a better signal-to-noise ratio. Hierarchical flipping theory can explain many of the findings in perceptual learning very well. In perceptual learning, subjects need several trials to improve their behavioral abilities, unlike other episodic learning where behavioral abilities can be improved with a single stimulus; perceptual learning is implicit, and subjects do not need to be consciously involved.
2. The role of perceptual learning in amblyopia
2.1 Common treatments for amblyopia
Among the different treatments for amblyopia, masking therapy is the main and most effective method. A large number of clinical studies have shown that masking therapy is effective for the treatment of amblyopia, but it is not a panacea and does not work well for many children or does not significantly improve visual acuity; atropine suppression therapy, an alternative to masking therapy, is better tolerated and may work better than conventional masking therapy, but there is no conclusive evidence that it is effective for children over 10 years of age. However, there is no conclusive evidence of effectiveness in children with amblyopia above the age of 10 years. There is no definitive treatment for this group of children and adults who do not respond well to treatment. The development of perceptual learning as a treatment method may change this situation. This makes it possible to treat this group of poorly treated children or adults with amblyopia more effectively.
2.2 Mechanisms of perceptual learning in amblyopia
A series of clinical trials on perceptual learning in recent years has led to new insights into the concepts of neuroplasticity and critical periods in amblyopia. There is considerable evidence that adult amblyopia can also be effectively treated, and there are two possible mechanisms by which adult amblyopia improves: first, by exposing inhibited neural connections; and second, by learning to use visual information effectively through the completion of visual training tasks. Perceptual learning in amblyopes may be accomplished by learning to use information from the amblyopic eye (which is usually inhibited) with attention, and may involve the following processes: first, the amblyopic subject’s dominant eye is masked during perceptual learning while performing the training task; second, the subject uses the amblyopic eye for visual discrimination during perceptual learning, which is an “activation” process; and third, the subject uses the amblyopic eye for visual discrimination. “Third, the subject receives the same stimulus repeatedly and is given feedback. The same mechanism may be present in common clinical approaches to amblyopia treatment.
2.3 The role of perceptual learning in the amblyopic eye
Amblyopia is a developmental visual disorder that can manifest itself as a variety of spatial visual impairments, including reduced visual acuity, contrast sensitivity, and vernier visual acuity. In recent years, many studies have shown that perceptual learning can significantly improve visual function, many of which have been conducted in patients with amblyopia. Some of the more researched effects of perceptual learning include the following.
2.2.1 Improving contrast sensitivity
There are now many studies on amblyopia dysfunction, and one of the more common perceptions is that amblyopia has decreased contrast sensitivity in the mid and high spatial frequency ranges. Sowden et al. found that training subjects to perceive a specific, brief threshold contrast with a sinusoidal grating significantly improved contrast sensitivity for this grating, and that this improvement was maintained for at least 195 days, with some subjects also showing improved sensitivity for other stimuli. In a study of 23 adults (mean age 19.3 years) with refractive parallax amblyopia, Zhou et al. found that training at a single spatial frequency improved the contrast sensitivity of the amblyopic eye by approximately 76.5%, and averaged across spatial frequencies, improving visual acuity by approximately 68.4%; training over the entire spatial frequency range measured by the contrast sensitivity function (CSF) improved the mean contrast sensitivity by Training at individual spatial frequencies in the amblyopic eye improved contrast sensitivity by 7.8% and visual acuity by 12.5% in the contralateral eye. In some subjects, the trained visual acuity improvement was retained by at least 90% over a period of 1 year. These studies demonstrate that improvements in contrast sensitivity function can be obtained in both normal and amblyopic subjects after perceptual learning.
2.2.2 Improvement in orientation discrimination
Dosher et al. reported the application of a perceptual learning model to evaluate perceptual learning in the presence of systematically varying environmental noise in order to facilitate a systematic theoretical analysis of the mechanisms that improve behavioral performance through training. They tested this by learning a direction discrimination task over several days and found that perceptual learning improved direction discrimination in the presence of a large range of ambient or external noise.Lu ZL et al. applied external noise and a perceptual learning model to investigate the eye-specific mechanisms of peripheral vision in perceptual learning of Gabor direction discrimination and found that equal improvements in behavioral ability were obtained by monocular learning in the presence of varying levels of external noise (see Table 1). They also found that perceptual learning in the training eye could be fully generalized to the non-training eye.
2.2.3 Improvement in position discrimination
It has also been found that visual perceptual learning can improve position discrimination and that significant improvements are obtained in amblyopic children. li RW et al. 2004 showed in a study of visual position discrimination that repetitive training can significantly improve position discrimination. They attributed this improvement primarily to an increase in efficiency, with repetitive training resulting in a significant increase in the brain’s ability to use relevant stimulus information; the increase in efficiency reflected a retuning of the subject’s perceptual template to more closely match the ideal template needed to perform the perceptual task. Later, Li RW et al. found in a 2005 study of amblyopic children who improved their visual abilities with positional discrimination training that there was also a significant improvement in the positional discrimination ability of amblyopic children after perceptual learning.
3. Application of perceptual learning in amblyopes of different ages
In recent years, research on perceptual learning in amblyopia has been intensifying, and many studies have found that visual ability of amblyopic patients has been significantly improved through visual perceptual learning. In a recent study by Polat U et al. with a larger sample, a total of 77 (9 to 55 years old) amblyopic and 16 normal vision subjects participated. They performed some functionally more basic training (contrast perception) with a different stimulus (Gabor signal) than the visual acuity examination (letters). The results showed a 2-fold improvement in contrast sensitivity and letter recognition, and the improvement in visual acuity in the training group was not significantly dependent on age. The maintenance of these visual function gains also suggests that this learning is not just a temporary adaptation effect but a long-term sustained change in visual cortex. They concluded that their training method is neurophysiologically based like the other commonly used treatment for amblyopia, the visual stimulation method (CAM), but with the following differences: they use low-contrast targets, only one directional and spatial frequency target appears in a training session, and individualized parameter settings are given as the subject progresses. Their findings suggest that an individualized approach can be used to improve visual function in amblyopes by targeting amblyopia-specific functional deficits.
3.1 Perceptual learning in children with amblyopia
In a study of perceptual learning in children with amblyopia, Li RW et al. showed that visual training with a repetitive position discrimination task resulted in a definite improvement in visual ability in amblyopic children, and they suggested that this improvement in visual ability in amblyopic children may be due to a reduction in equivalent input noise and an increase in efficiency. They asked five amblyopic children (aged between 7 and 10 years) to discriminate the position relationship of three pairs of lines during training in a position discrimination task. As a result, four subjects showed a significant improvement in positional discrimination after training; five subjects also showed an increase in Snellen’s visual acuity after training (approximately 26%). They hypothesized that children under 6 years of age may show better results through perceptual learning and suggested that perceptual learning techniques may provide a new and effective approach to amblyopia treatment. However, there is a lack of research on perceptual learning in children younger than 6 years of age. Such research would be beneficial for the clinical management of amblyopia in children.
3.2 Application of perceptual learning in adult amblyopia
In a study of 11 adults with amblyopia whose visual ability was improved by perceptual learning training, Levi et al. found that all subjects had a significant improvement in upstream visual acuity in the direction of training, and that this improvement was transmitted to other directions (n=4). 4). In their study of adult refractive amblyopia, Zhou et al. found that the improvement in contrast sensitivity after training was accompanied by an increase in visual acuity in the contralateral eye. Moreover, in some subjects, the improvement in visual acuity after training was preserved by at least 90% for at least 1 year. They suggest that the visual system of adults with amblyopia may retain some plasticity.
In summary, many recent studies have identified a role for perceptual learning, particularly in the clinical implications of amblyopia treatment. This has led to the beginning of a new understanding of the boundaries of amblyopia sensitivity and the discovery of some of the neural mechanisms of perceptual learning. As the research on perceptual learning continues, it may change the status quo in the clinical treatment of amblyopia, which is dominated by masking therapy, and improve the visual acuity and contrast sensitivity of amblyopic patients through perceptual learning, thus improving their visual function. Perceptual learning may even become another conventional treatment or an important complement to conventional masking therapy, and may allow us to effectively treat children and adults with amblyopia for whom current treatments are not effective.