Can perceptual learning be used to treat amblyopia beyond the critical period of visual development?

Abstract

Background: Amblyopia presents early in childhood and affects approximately 3% of western populations. The monocular visual acuity loss is conventionally treated during the 'critical periods' of visual development by occluding or penalising the fellow eye to encourage use of the amblyopic eye. Despite the measurable success of this approach in many children, substantial numbers of people still suffer with amblyopia later in life because either they were never diagnosed in childhood, did not respond to the original treatment, the amblyopia was only partially remediated, or their acuity loss returned after cessation of treatment.

Purpose: In this review, we consider whether the visual deficits of this largely overlooked amblyopic group are amenable to conventional and innovative therapeutic interventions later in life, well beyond the age at which treatment is thought to be effective.

Recent findings: There is a considerable body of evidence that residual plasticity is present in the adult visual brain and this can be harnessed to improve function in adults with amblyopia. Perceptual training protocols have been developed to optimise visual gains in this clinical population. Results thus far are extremely encouraging; marked visual improvements have been demonstrated, the perceptual benefits transfer to new visual tasks and appear to be relatively enduring. The essential ingredients of perceptual training protocols are being incorporated into video game formats, facilitating home-based interventions.

Summary: Many studies support perceptual training as a tool for improving vision in amblyopes beyond the critical period. Should this novel form of treatment stand up to the scrutiny of a randomised controlled trial, clinicians may need to re-evaluate their therapeutic approach to adults with amblyopia.

Figure 1

Data from one adult amblyopic observer who practiced a letter based contrast sensitivity task. The subject trained for 25 sessions in total. Data for sessions 1 to 12 are fitted with an exponential function shown by the solid line. Data for sessions 13 to 25 are fitted with an exponential function shown by the dashed line.

Figure 2

Change in performance following training on a variety of acuity and contrast based tasks, expressed as a ratio of start performance (PPR) relative to subject age. A lower PPR corresponds to a greater improvement in performance. The horizontal dashed line PPR=1 corresponds to no improvement. Lines lying below this line denote individuals who improved in performance. There is no significant relationship between improvement and age of subjects (r(27)=0.26, p=0.17).