A window into visual cortex development and recovery of vision: Introduction to the Vision Research special issue on Amblyopia

Normal visual development requires unimpeded and coordinated input from each eye to the visual cortex during an early critical period of cortical maturation. Disrupted binocular vision during this critical period, due to visual deprivation (e.g. congenital cataract), misalignment of the eyes (strabismus) or unequal refrac-tive error (anisometropia), can lead to amblyopia, a neurodevelop-mental disorder of vision (Daw, 2014; Holmes & Clarke, 2006). Amblyopia is characterized by a loss of visual acuity in the affected eye and impaired or absent binocular visual function in the absence of any ocular disease or abnormality. Amblyopia has been the focus of two parallel and complementary lines of research for many decades: one clinical and one neu-roscientific. Clinically, amblyopia represents the most common cause of visual impairment in childhood (Wong, 2012) and has a significant impact on quality of life (Carlton & Kaltenthaler, 2011). The current evidence-based treatment for amblyopia involves refractive correction (Cotter et al., 2006; Writing Committee for the Pediatric Eye Disease Investigator et al., 2012) followed by occlusion or penalization of the non-amblyopic eye to encourage use of the amblyopic eye (Pediatric Eye Disease Investigator Group, 2002). This treatment can induce recovery of amblyopic eye visual acuity if implemented at an early age (Wallace et al., 2006); however improvements in binocular vision are often limited (Birch, 2013). In humans, early monocular deprivation has catastrophic consequences for the final level of vision the deprived eye can obtain, while the same type of deprivation introduced later in childhood has a diminishing impact and little or no effect at all beyond around 9 years of age (Vaegan & Taylor, 1979). This transition from severe early impairments to virtually no effect on acuity later in development delimits the duration of visual system susceptibility to abnormal visual experience – the so-called 'critical period'. This window is thought to reflect a changing balance of influence between visual mechanisms of plasticity and stability. Currently, there are no treatments for adults with amblyopia that are in general clinical use. This reflects the rather entrenched view that amblyopia is intractable in older patients. Once the period of vulnerability had passed, it was assumed that plasticity was limited in scope and, as a result, treatment would be ineffective. From a neuroscientific perspective, amblyopia is a preeminent model for exploring cortical development and plasticity in both animals and humans. Nobel prize winners Hubel and Wiesel famously used experimentally induced amblyopia in monkeys to …