Abstract
Introduction: Perceptual learning has become a topic of interest as an alternative treatment for amblyopia in many recent studies. This literature review aims to analyse the research currently available and evaluate the effectiveness of perceptual learning as a treatment in situations where traditional occlusion therapy is contraindicated.
Methods: Online databases such as EBSCOhost and Ovid Online were used to find the most appropriate and relevant research pertaining to the topic. The effectiveness of perceptual learning was evaluated using a range of factors for example the duration of treatment needed for an improvement in visual function.
Results: Studies into this topic have found perceptual learning can improve visual function particularly in adults where it is thought there is limited room for improvement due to the end of the ‘critical period.’
Discussion: Research into this area seems promising in using perceptual learning as an alternative treatment. However longitudinal randomised controlled trials need to be carried out to truly evaluate the effectiveness of perceptual learning particularly with larger sample sizes. Studies investigating the use of traditional treatment in adults also need to be carried out in order to fully evaluate the significance of the ‘critical period’ and neural plasticity. In order to implement a treatment effectively into clinical practice the dose-response curve would need to be decided in order to most efficiently prescribe a treatment with the hope of the best possible prognosis.
Introduction
1.1 What is Amblyopia?
Amblyopia is a childhood developmental eye condition characterised by reduced visual acuity where there is no ocular pathology. Amblyopia is a common eye condition estimated to affect ‘approximately 3% of Western populations’ (Astle et al. 2011a). It is a result of disturbed visual development between the eye and brain during childhood. Polat et al. (2004) found the abnormal input to the eye due to uncorrected optical distortions leads to distorted activity in the visual cortex creating abnormal connectivity. It is most commonly manifested unilaterally as the eye receiving a more blurred image develops the amblyopia (Chen et al. 2008). The vision remains reduced despite refractive correction. As well as the primary reduced visual acuity there is also reduced contrast sensitivity (Astle et al. 2011b) and reduced stereopsis due to the monocular effect of amblyopia. Amblyopes also suffer from the effects of crowding which is a result of disruptive surrounding stimuli. Amblyopia is most commonly characterised as anisometropic or strabismic due to its primary cause or a mixture of both. It can less commonly be astigmatic or deprivational due to a congenital cataract or ptosis. The depth of amblyopia is determined by the depth of binocular imbalance (Li et al. 2005; Polat et al. 2009) with the ‘earlier the onset and longer the period of deprivation the worse the visual outcome’ (Astle et al. 2011a).
1.2 What is the ‘Critical Period’?
Clinicians aim to detect and begin treatment as soon as possible as it is believed treatment is most effective during the ‘critical period’ in which amblyopia first develops. The critical period is the time where visual development is occurring therefore any deprivation during this period of sensitivity affects development. Liu et al. (2010) suggest conventional treatment is most effective before the age of seven. Routinely it is only prescribed to children under the age of nine. Thich is believed to be the upper limit of the critical period where research has shown treatment to be most effective and where neural plasticity still remains (Zhou et al. 2005; Liao et al. 2016). Astle et al. (2011b) found amblyopia is not induced beyond the age of nine.
6
1.3 Traditional Treatment
Initial treatment comprises of full correction of refractive error and it is found 25% of amblyopic patients resolve with visual correction alone (PEDIG 2005). The ‘gold-standard’ treatment for amblyopia detected during the critical period is occlusion therapy (Li et al. 2007). The most common form is patching treatment followed by the use of atropine where patching is contraindicated. Occlusion therapy penalises/prevents the use of the good eye or the use of accommodation with atropine to encourage the amblyopic eye to focus and improve the cortical connections by forcing fixation (Polat et al. 2009). It has been found to be an effective treatment and its level of improved visual function correlates to the initial depth of amblyopia (Li et al. 2005). Xi et al. (2013) found where traditional treatment effectively restores monocular visual acuity there is little improvement in binocular function. Loudon et al. (2003) highlighted issues with compliance to traditional patching and Levi et al. (2009) found patching leads to psycho-social problems consisting of loss of self- esteem as well as reduced function regarding binocular vision and stereopsis. Polat et al. (2009) found social implications to patching as well as skin irritation and Chen et al. (2008) found social anxiety particularly in older children resulting in poor compliance. With severe amblyopia, penalisation is contraindicated as it is impractical to degrade the vision sufficiently to induce fixation with the amblyopic eye (Polat et al. 2009). Webber et al. (2008) found lower social acceptance scores for children treated with patching. Al-Yahya et al. (2012) found in a retrospective study average compliance is around 66.68%. Non- compliance is most commonly due to social stigma, irritation, child’s refusal and sweating. This poor compliance combined with the fact that occlusion therapy is less effective after the critical period has elapsed shows there is a need for an alternative treatment in those who were not diagnosed until later or where occlusion was unsuccessful. Stewart et al. (2004) found occlusion therapy fully treats only 30% of amblyopia. Liu et al. (2010) found conventional patching may not always fully restore vision and there may be potential for further improvement in those who are no longer responsive to occlusion.
7
Perceptual Learning
2.1 The Role of Perceptual Learning
Research into perceptual learning as a new treatment is of particular interest for those diagnosed after being past the critical period as they do not qualify for traditional treatment. Therefore, it needs to be evaluated whether ‘residual plasticity is present in the adult visual brain and this can be harnessed to improve function in adults with amblyopia’ (Astle et al. 2011a). Huang et al. (2008) found occlusion therapy to be ineffective in older children and adults, but as the adult system retains plasticity, it shows a potential role for perceptual learning as a treatment. Zhou et al. (2005) reported there was visual improvement in adult amblyopes and retention of neural plasticity.
2.2 The Mechanism of Perceptual Learning
Perceptual learning improves visual performance by practicing basic tasks specified for different aspects of visual function (Chen et al. 2008). Perceptual learning is the improvement through practice on different stimuli and tasks. Perceptual learning as a treatment for amblyopia utilises the idea that practice elicits permanent improvement in visual function using stimulus specific tasks (Polat et al. 2009; Hussain et al. 2011; Astle et al. 2011a). The visual system is most sensitive to improvement during the critical period, but research suggests there is potential for improvement later too and learning is thought to use the plasticity of early cortical stages. Levi et al. (1996) first found improved visual acuity after training on a Vernier acuity task with this transfer being used for clinical treatment (Liao et al. 2016). Perceptual learning uses repeated practice on a challenging task to produce lasting improvements in visual function (Astle et al. 2011b) and permanent change in the visual cortex (Levi et al. 2009). Using evidence of perceptual learning improving function in adults with normal vision, the concept has been adapted to see its potential applicability to amblyopes (Astle et al. 2011b). Perceptual learning uses increased familiarity and tasks close to the ‘limits of vision’ to maximise performance. Training on visual discrimination tasks have been shown to have increased activity of V1 neurons through electrophysiological studies (Li et al. 2008) and fMRI studies (Astle et al. 2011a). To produce maximal improvement the patient’s individual threshold is repeatedly measured to ensure
8
targets are close to these limits as this is what challenges the amblyopic vision (Zhou et al. 2006). Large improvements in performance in adult observers were found following training on spatial vision tasks demonstrating adult neural plasticity. It is thought early processing is improved which transfers to higher levels creating a long-lasting visual cortex change.
2.3 Methodology
Perceptual Learning is defined as an active process where participants are actively engaged and responding to visual tasks using stimuli features to make psychophysical decisions. A 2AFC (two alternate forced choice) system is most commonly used where participants make decisions on stimuli features based on two options to progress to the next trial. A threshold staircase is created to ensure targets are near the limits of the individuals vision. Using trial- by-trial feedback the task is progressively made more difficult with these incremental shifts being of vital importance (Astle et al. 2011a). Traditional occlusion therapy is a passive process where improvement is achieved through the brain being forced to improve connections with the amblyopic eye. Patients do not concentrate on specific tasks but are encouraged to carry on with everyday activities to expose the eye to a range of stimuli. Before perceptual learning training begins the participant is refracted (routinely under cycloplegia) and given a period of adaptation to the updated prescription. If a subject had previously undergone occlusion therapy, it was ensured there was a stable period of six months with no further improvement (Liu et al. 2010). The better eye is patched, and the target is viewed monocularly with the head secured in place and eye fixation tracked. It is believed the improvements are due to the active nature of the training (Levi et al. 2009) with stimuli specific to an improvable range, focussing on spatial frequencies with lower sensitivities whereas with patching all spatial frequencies are equally stimulated.
9
Visual Acuity
3.1 Improvement on Vision
A quantitative method of analysing the effectiveness of perceptual learning as a treatment would be to analyse the physical improvement in VA. The results of various studies are summarised in Table 1 showing the average improvement in the amblyopic eye.
3.2 Duration of Treatment
Li et al. (2007) found the most significant improvement in visual acuity after perceptual learning training occurred with longer periods of training. Improvement eventually reached a maximal plateau. Astle et al. (2011b) continued training after a plateau was reached and found a second period of improvement separated by the stable period. Chen et al. (2008) found an average improvement of 0.25LogMAR which with traditional occlusion therapy would require 385 hours of patching but here was achieved within 29.5 hours. This shows the effectiveness of perceptual learning in producing a rapid improvement in visual acuity. Li et al. (2005) found asymptotic improvement after 14-20 hours of training which is considerably effective in comparison to prolonged occlusion where on average 154 hours are needed for an improvement of 0.1LogMAR. Compared to the dose-response curve for traditional occlusion the improvement in visual acuity achieved was over a shorter time period showing the improvement cannot be solely due to occlusion. Li et al. (2004) summarised ‘3000-6000 trials are generally more than enough to reach stable performance’ which generally equates to five-ten hours. Duration of treatment required affects motivation and therefore the compliance of patients.
10
3.3 Depth of Improvement
Astle et al. (2011b) found the greatest depth of improvement occurred in amblyopes with poorer initial performance and therefore required extended training. Liu et al. (2010) found
11
a similar correlation between depth of amblyopia and improvement. This suggests perceptual learning training would be most effective in those with mild amblyopia. Zhou et al. (2005) and Xi et al. (2013) found similar results although none reached normal limits. Li et al. (2008) found more practice is required to ‘reach asymptote in severe amblyopia.’
Polat et al. (2004) found similar improvements for strabismic and anisometropic amblyopes.
Li et al. (2007) and Astle et al. (2011b) found individual variation to the task as shown in Graph 1, meaning an exact dose-response function is not yet known to predict the highest depth of improvement. Li et al. (2007) and Liu et al. (2010) found improvements in acuity plateaued at a faster rate for uncrowded stimuli. Hussain et al. (2011) suggested training with targets closer to threshold could reduce crowding effects. Further research into the specific effects of different stimuli needs to be undertaken in order to best manipulate the tasks to provide the largest depth of improvement. Liu et al. (2010) found greater improvement in trained eyes showing the role of general learning although this cannot completely account for improvements found. Polat et al. (2004) found patients reached normal visual acuity performance with reduced crowding. Controls had a minimal improvement of 0.02LogMAR indicating the improvement is not solely due to the learning effect. Zhou et al. (2005) found training on spatial frequency tasks transferred well to improve visual acuity. Liao et al. (2016) found improved visual acuity after training on contrast sensitivity functions consistent with other studies where large improvements have been found after training on a non-letter-based task. Chen et al. (2008) and Astle et al. (2011b) found large improvements using a contrast detection task. Astle et al. (2011a) also used a letter contrast task and found improved visual acuity, contrast sensitivity and stereoacuity as it is a crowded broadband stimulus which maximised learning. Li et al. (2007) found improvements on crowded and isolated acuity after training on position- discrimination task particularly on isolated letter acuity. Levi et al. (1997) found significant transfer and improvement to visual acuity with training on Vernier acuity and demonstrated significant adult plasticity. Li et al (2007) found improved visual acuity after training on positional acuity. Polat et al. (2006) had significant improvement which is thought to be due to the stimuli including flankers.
Traditional patching therapy is only routinely prescribed to children within the ‘critical period.’ From Table 1 it can be seen these studies were carried out on a large range of
12
patients and there were significant improvements regardless of age. Levi et al. (2009) reviewed 15 studies and found ‘no significant relationship between improvement and age, for subjects aged 10 to 40 years.’ Hussain et al. (2011) found significant improvement in adult amblyopes demonstrating remaining neural plasticity and Li et al. (2005) found similar levels of improvements between studies on adult and child amblyopia. However, the adults underwent double the training as it is believed this would be too exhaustive for younger children and children therefore show a greater perceptual learning effect as there was similar improvement for half the training. A dose-response relationship would need to be determined to begin to implement perceptual learning training. Polat et al. (2004) demonstrated that as occlusion is impractical in adults and has been demonstrated to be less effective, a practical treatment for adult amblyopes is needed. They found despite the participants being well beyond the ‘critical period (over the age of 7)’ where it is believed treatment would be most effective there was still a quantifiable improvement in visual acuity. This shows there is still a potential for improvement in vision in adult amblyopes and they concluded ‘plasticity is not limited by age.’ As traditional occlusion is successful in 60- 70% of young children this could be combined with active perceptual learning for the most significant improvement. Polat et al. (2004) found improvements independent of age showing modifications to the adult visual cortex post-training in a RCT comparing amblyopes to a normal control. Liao et al. (2016) and Astle et al. (2011a) discovered that a larger degree of improvement may be expected in children due to research from traditional occlusion, but research into perceptual learning has shown similar levels. However, Li et al. (2007) found a larger plateau for younger patients.
13
Graph 1- taken from Levi et al. (2009) showing variation in improvement of Visual Acuity with task stimulus.
3.4 The Effect of Previous Patching Treatment
Liu et al. (2010) found the greatest improvement in those who had no previous treatment. The improvement in visual acuity was ‘comparable’ to an age-matched control who had undergone extended patching treatment. There was however a greater depth of improvement for those undergoing patching at a younger age. This shows the potential as a
14
treatment for those who were unresponsive to patching or were believed to have reached their maximal gain but actually have further depth for improvement. Polat et al. (2009) found significant improvements in children who had poor compliance to patching using a prospective clinical pilot study. The children played computer games between blocks to maintain attention and improve compliance which was believed to be the reason for the poor compliance to patching due to lack of an interactive task. Li et al. (2005) used participants who had undergone previous occlusion therapy and therefore some had already had a large depth of improvement. Two observers who had not responded to occlusion therapy improved well with perceptual learning.
3.5 Retention of Visual Acuity
A measurement of treatment effectivity also includes how long the initial improved vision is retained. Regression in vision post-treatment could have lifestyle implications e.g. driving/occupational standards. A successful treatment would be long-lasting or permanent with multiple sessions in the future not required. Liu et al. (201) followed up patients one- year post-treatment and found a regression of 0.01LogMAR in those who had undergone perceptual learning training compared to 0.04LogMAR in those who had previously been patched. This shows the potential of perceptual learning, however there was only a small sample size so more extensive longitudinal studies with larger sample sizes would need to be undertaken before clinically implementing this research. Polat et al. (2004) found a slight decrement at a one year follow up and Zhou et al. (2005) found 90% retention. Chen et al. (2008) found 92% retention after eight months. Li et al. (2007) found retained visual acuity of 91% after seven months and when one participant was given an additional training session acuity improved back to the previous level showing it can easily be recovered.
3.6 Perceptual Learning vs Patching
Chen et al. (2008) directly compared perceptual learning and patching with patients being randomised between groups and an even number of patients for each condition. This enables us to effectively compare them as treatments as the same methods are used and
15
the same methods of assessment. Anisometropic amblyopes were used so our results can be generalised to these patients. Treatment continued until stabilisation/resolution in visual acuity. 63% of patching patients achieved resolution compared to 38% perceptual learning and 96% of patching improved by two or more lines in 522.2 hours on average compared to 76% of perceptual learning in 29.5 hours. Four adults withdrew from patching due to intolerance particularly due to driving ability and social stigma and three children grew bored of the active concentrated nature of perceptual learning, so these were excluded from the results. There was no statistical difference in improvement between the two groups however the average age of the perceptual learning group was higher, and patching was more extensive. Patching is negatively correlated with age. It has been suggested the improvements in perceptual learning could be due to the patching/monocular effect of perceptual learning but using Stewarts et al. (2004) dose-response rate this suggests for an improvement of 0.25LogMAR 300 hours of patching is required. The improvements may be due to a combination of both. Perceptual learning is clinically administered which is less cost-effective. These results indicate conventional patching is more effective but there are many contraindications to patching creating a need for an alternative treatment. A larger sample size and longer follow-up is required. It has great potential in those non-compliant to traditional methods and achieved further increased acuity in those who no longer improved with patching. Currently treatment requires participants to come in regularly but ensures monitoring and compliance. A home-based treatment would be more convenient to carry out but requires independent compliance. An online computer game would be cost- effective and also enables monitoring of hours completed. A large scale clinical study is required to evaluate the exact prognosis and dose-response rate.
3.7 Generalisability of Results
Appraisal of these studies demonstrate the effectiveness of perceptual learning and can be appropriately recommended as a treatment for amblyopia with the intention of improving visual acuity. Significant quantifiable improvement has been demonstrated but it is difficult to directly compare the results as different measuring tools are often used.
16
Contrast Sensitivity
4.1 Improvement in Contrast Sensitivity
Polat et al. (2004) found amblyopes had higher thresholds with lower sensitivity and stronger sensitivity loss for higher spatial frequencies.
Polat et al. (2004) found significant improvement for all spatial frequencies with high spatial frequencies reaching a normal range. Zhou et al. (2005) found an average improvement of 5.7dB over a range of spatial frequencies and the cut-off increased by 2.7dB compared to a control group where there was no intervention and no significant improvement. Training/practice effects had the most significant role in improvement with improvement correlating to duration. Polat et al. (2009) found post-training contrast sensitivity reached normal ranges in children. Liao et al. (2016) found an average improvement of 6.35dB across all sensitivities and improved cut-off spatial frequency by 3cpd. The greatest improvement was found for those with worse initial performance with no correlation to age. Chen et al. (2016) found improvement of 9.7dB on average with transfer of training effect. Huang et al. (2008) found a larger depth of improvement of 10.7dB in comparison to a control group of normals.
Liu et al. (2010) used Gabor stimuli and found improved contrast sensitivity at lower spatial frequencies. Zhou et al. (2005) used stimuli close to the expected threshold based on results from pilot testing. Liao et al. (2016) found improved contrast sensitivity function and modulation transfer function which transferred to improved visual acuity. Huang et al. (2008) trained amblyopes at the cut-off spatial frequency and used a grating detection task resulting in improved contrast sensitivity which transferred to improved visual acuity.
4.2 Previous Patching Treatment
Liu et al. (2010) found significant improvement in contrast sensitivity after undergoing perceptual learning training but to a lesser extent in those participants who had previously undergone patching. Astle et al. (2011b) found 30% of patching patients regressed to pre- treatment levels but long-lasting improvement in perceptual learning.
17
4.3 Retention of Contrast Sensitivity
Polat et al. (2004) found retention of contrast sensitivity 12 months post-treatment. Zhou et al. (2005) found retention of 90% one-year post-treatment in adult amblyopes. Astle et al. (2011b) found retention of improvement after 18 months but there is limited research into post-treatment visual performance so longitudinal research needs to be undertaken to validate the idea of permanent improvement with perceptual learning.
4.4 Perceptual Learning vs Patching
Chen et al. (2008) found for amblyopes with lower peak sensitivity there was a significant improvement across all spatial frequencies for patching and perceptual learning. Patching had a mean improvement of 3.18 and perceptual learning 3.82. There was also a larger improvement for higher spatial frequencies with perceptual learning and a larger transfer/correlation to visual acuity.
18
Stereopsis
5.1 Relevance of Stereopsis
Stereopsis is an important aspect of real-world vision ‘critical for human visual perception, such as perceiving the 3D layout of our surroundings, reading, hand-eye coordination’ (Xi et al. 2013). Fielder et al. (1996) suggests normal stereopsis is less than 40’’. Studies have found perceptual learning improves stereopsis independently of visual acuity improvement suggesting in order to create maximal gain in stereopsis and visual acuity different tasks are needed. Under binocular conditions stereopsis is the most significant amblyopic deficit. It can impair sports ability and locomotion and can also present occupational limitations e.g. pilots, surgeons, architects and professional athletes e.g. tennis players. It is specifically reduced for strabismic amblyopes. For anisometropic amblyopes the stereopsis deficit correlates to the reduced visual acuity and they have been shown to have a better prognosis. Levi et al. (2015) found a link between strabismic amblyopes with abnormal stereopsis and high crowding. Stereopsis and binocular viewing play vital parts of everyday visual function e.g. visually guided hand movements which are slower during monocular viewing and require more corrective movements. Slower walking performance can also be found as well as reduced adaptation to surface change e.g. stairs.
5.2 Measurement of Stereopsis
Measurement of stereopsis e.g. Randot circles are contraindicated due to the presence of monocular cues. Some tests are also not sensitive enough at higher level of deficit and have been found to record some patients as stereo blind when they in fact have stereopsis albeit very little. Similarly, some patients have stereopsis above the highest level the test used can record. This is implicated when presenting the apparent improvement in stereopsis.
5.3 Improvement in Stereopsis
Liu et al. (2010) found restored acuity up to 50’’ in two subjects who had failed initial stereoacuity testing. Polat et al. (2009) found improved ocular alignment and reduction of esophoria in one patient with reduction of suppression measured in Worth’s light and
19
improvement in stereopsis from 400’’ to 25’’. This occurs due to reduced perceptual difference between the two eyes and reduced suppression creating the improved vision. Despite the monocular basis of the treatment there was improvement in binocular function which could be due to ‘cooperative interactions of the normal and amblyopic eyes.’ Xi et al. (2013) found reduced disparity of 36.9% (P<0.01) and stereoacuity improved from 200.3’’ to 81.6’’ (P<0.01). Chen et al. (2016) found improved binocular function and increased stereo sensitivity in a very small sample of anisometropic amblyopes. Li et al. (2007) found 18% of strabismic and anisometropic amblyopic children who began with no stereopsis improved up to 70’’. Levi et al. (2015) found a higher proportion of strabismic amblyopes improved with dichoptic perceptual learning compared to monocular where two images are presented to the two eyes simultaneously to reduce interocular suppression. There was also recovery in ‘stereo blind’ adults and similarly Astle et al. (2011a) found improved stereoacuity after binocular training.
Xi et al. (2013) used visually demanding stereo tasks but accomplishable in order to leave room for improvement. They found no relationship to visual acuity. Chen et al. (2016) used monocular training on a contrast detection task with no correlation to visual acuity. However, learning effects transferred well to improve binocular function as well as visual acuity and contrast sensitivity. Xi et al. (2013) found greater improvement in those who had worse initial. Levi et al. (2015) found those with some degree of stereopsis had a better overall prognosis for recovery of amblyopia. Xi et al. (2013) found improved stereoacuity was retained over five months. Chen et al. (2016) also found retention.
5.4 Perceptual Learning vs Patching
Liu et al. (2010) found greater depth of improvement in those who had previously never been patched.
Levi et al. (2015) found correcting the refractive error can improve stereoacuity by 30’ in 3- 5-year olds. There was also a 22% improvement in strabismic amblyopes and 43% in anisometropic with patching. However, there is limited evidence to show normal values being reached.
20
Discussion
6.1 Perceptual Learning Summarised
Levi et al. (2009) reviewed perceptual learning studies and found large variation in improvement. There was no correlation with improvement to age with adults achieving significant improvement. However, there is limited research into occlusion therapy with adults due to poor compliance. There was significant correlation between depth of amblyopia and length of treatment needed but there is variation in the optimum stopping period between studies. There was limited variation with type of amblyopia. The impact of publication bias has not been assessed. It has been suggested improvement in visual function after participants underwent perceptual learning training is due to observers learning to more accurately fixate but if this was the only factor task used would not have an impact on results.
6.2 Benefits of Perceptual Learning
The research appraised shows the potential of perceptual learning as a future treatment in children not compliant to traditional methods or no longer responsive but also to adults who are not currently considered for treatment or teenagers who were diagnosed too late to qualify for traditional treatment. This could prevent the negative psychosocial consequences associated with patching and improve binocular stimulation. Liu et al. (2010) also found improved acuity in those who had initially responded well to traditional patching. As amblyopia is a common eye condition said to ‘account for the majority of children’s hospital eye appointments in the UK’ (Astle et al. 2011a) an effective treatment is vital. Perceptual learning is thought to provide a ‘permanent and consistent improvement’ (Astle et al. 2011a) showing the effectiveness. The active process of perceptual learning means concentration is required throughout the duration of treatment with ‘intense periods of training under strictly controlled experimental conditions’ (Astle et al. 2011a). During patching the visual experience is not controlled as well as hours of treatment so compliance cannot be ensured. Chen et al. (2008) demonstrated the same improvement with perceptual learning can be achieved in a fraction of the time as with patching showing its effectiveness. It has generally been found the most effective tasks are contrast based as
21
they have the greatest amount of improvement and transfer to improved visual acuity (Astle et al. 2011b). Prolonged patching has also been shown to reduce stereopsis and binocular function and impacts self-esteem. It is difficult to implement occlusion therapy in older children and adults due to school and work making perceptual learning ideal in these cases.
6.3 Disadvantages of Perceptual Learning
A popular contraindication of perceptual learning is the idea that as perceptual learning is monocular it is suggested the improved visual function is due to the use of occlusion. However, Polat et al. (2004) found 62% improvement in those undergoing perceptual learning training compared to no improvement in a control group receiving occlusion therapy. Improvements were also greater for letter-based contrast tasks than grating acuity tasks despite the same duration of training showing the improvement cannot be solely due to the use of occlusion (Astle et al. 2011a). Li et al. (2007) found greater improvements when training was combined with occlusion than occlusion alone. Patching can successfully be applied to babies and infants whereas as perceptual learning is an active process it would be more difficult to apply this to these subjects. Chen et al. (2008) found young infants grew bored and dropped out.
6.4 Perceptual Learning Combined with Patching
PEDIG (2005) carried out a prospective study combining occlusion with one hour of near visual activities for 7-17-year olds with a 53% improvement in those under 12 and 25% in those under 17. Greatest improvement occurred with a combination of near activity and occlusion. Li et al. (2007) found active perceptual learning when combined with occlusion significantly increased recovery. This suggests perceptual learning can augment the effects of occlusion therapy.
22
6.5 Improvement Following Visual Loss in the Good Eye
Through a prospective observational case report Fronius et al. (2004) found over a period of ten months visual function improved in an adult amblyope following visual loss in the non- amblyopic eye. This shows the presence of neural plasticity although limited as early diagnosis and intervention is still essential. There was increased single and crowded acuities which could be clinically significant as 1.2% of amblyopes are left visually impaired following loss of vision in the non-amblyopic eye. It was reported reading and the environmental surroundings became easier. As this is a single case report we cannot determine whether the improvement found was due to perceptual learning or spontaneous improvement. A similar study by Fronius et al. (2006) found improved contrast sensitivity and acuity. Research in this field is rare and, in these cases, prognosis is often poor, so the potential application of perceptual learning could present as a viable treatment. This suggests connections to the amblyopic eye are suppressed rather than destroyed. Mallah et al. (2000) found improved visual function in the amblyopic eye after the fellow eyes visual function was lost as a result of AMD. The improvement was also retained over eighteen months.
6.6 Adaptation into a Video Game
The success of clinical perceptual learning has encouraged research into video game formats which would improve compliance and ease of treatment making it a home-based intervention as currently participants are required to come in several times a week. Astle et al. (2011b) found improved contrast sensitivity after subjects played computer games. Li et al. (2014) also found improved visual function in children playing an adapted binocular iPad game for a short duration. Vedamurthy et al. (2015) and Li et al. (2008) found similar results after adapting a popular video game. These studies rarely have controls and use small samples. When assessing the effectiveness of a treatment there is always the implication of under-reporting of negative results due to publication bias. However, the research does provide valuable information particularly displaying improvement beyond the critical period is possible. Computer games have the potential to be easily adapted to become a perceptual learning task as there is a range of visual stimuli and tasks with active
23
engagement and levels of increasing difficulty. However more research into specific stimuli and training characteristics first needs to be undertaken and optimised. Pineles et al. (2016) adapted a perceptual learning-based video game used to treat low vision for amblyopia treatment improved visual acuity as shown in table 1 as well as reduced suppression.
6.7 Future Application
Astle et al. (2011a) highlighted the need for extensive randomised control trials into occlusion for adult amblyopes and also directly comparing perceptual learning and occlusion. Research would also need to be undertaken with the effectiveness of perceptual learning prescribed in clinical practice rather than lab-based as research is now. Liu et al. (2010) found perceptual learning would have significant therapeutic impact for mild amblyopes. Although perceptual learning uses task specificity, transfer of visual function has been commonly found and demonstrated. Improvement is also affected by inter-individual differences with improvement in overall responsiveness. The influence of the placebo effect also needs to be fully considered. We would need to consider the ideal stimulus and who is most suitable for the treatment. Prognosis with traditional occlusion depends on age and depth of loss. It is worth investigating traditional occlusion for adults using the idea that adults retain neural plasticity. Research into the exact mechanisms behind perceptual learning also need to be undertaken as this could aide in selecting optimum stimuli.
24