Improving vision in adult amblyopia by perceptual learning

Abstract

Practicing certain visual tasks leads, as a result of a process termed ''perceptual learning,'' to a significant improvement in performance. Learning is specific for basic stimulus features such as local orientation, retinal location, and eye of presentation, suggesting modification of neuronal processes at the primary visual cortex in adults. It is not known, however, whether such low-level learning affects higher-level visual tasks such as recognition. By systematic low-level training of an adult visual system malfunctioning as a result of abnormal development (leading to amblyopia) of the primary visual cortex during the ''critical period,'' we show here that induction of low-level changes might yield significant perceptual benefits that transfer to higher visual tasks. The training procedure resulted in a 2-fold improvement in contrast sensitivity and in letter-recognition tasks. These findings demonstrate that perceptual learning can improve basic representations within an adult visual system that did not develop during the critical period.A mblyopia is characterized by several functional abnormalities in spatial vision (for reviews see refs. 1-4), including reductions in visual acuity (VA), contrast-sensitivity function (CSF), and vernier acuity as well as spatial distortion (5), abnormal spatial interactions (6, 7), and impaired contour detection (8,9). In addition, amblyopic individuals suffer from binocular abnormalities such as impaired stereoacuity and abnormal binocular summation. The visual deficiencies are thought to be irreversible after the first decade of life (10-12), by which time the developmental maturation window has been terminated. The loss of vision is thought to result from abnormal operation of the neuronal network within the primary visual cortex, particularly of orientation-selective neurons and their interactions (13). The perceptual learning procedure described in this study was designed to train this network by efficiently stimulating these neuronal populations and effectively promoting their spatial interactions.Spatial interactions in human vision can be probed by contrast detection of a localized target in the presence of flankers (Fig. 1). These experiments show that the contrast threshold of a foveal Gabor signal (GS) is reduced in the presence of cooriented and coaligned (collinear) high-contrast GS flankers (14-18). The excitatory interaction is range-dependent and is maximal for target-flanker separation of approximately three times the GS wavelength. Smaller separations can raise the target threshold, depending on flanker contrast and phase (19). Singleunit recordings suggest that the underlying mechanisms reside within the primary visual cortex (20,21). Neuronal responses in the visual cortex are tuned for location, orientation, and spatial frequency. Recent evidence from studies in the cat and the monkey (20,(22)(23)(24)(25) shows that neuronal responses in the primary visual cortex are modulated by remote image parts, with both excitatory and inhibitory effects observ...

Neural mechanisms of recovery following early visual deprivation

“…Whereas the earliest attempts, such as training on vernier alignment (Levi & Polat 1996), showed only limited generalization beyond the specific trained target orientation, later attempts to ameliorate the deficient spatial interactions in low-level vision in amblyopia by perceptual training led to improvement of other aspects of vision such as Snellen acuity, counting and contrast sensitivity (e.g. Polat et al 2004;Levi 2005;Zhou et al 2006;Li et al 2007; see also Levi & Li 2009). …”

Nurturing brain plasticity: impact of environmental enrichment

“…Strikingly, an increasing number of clinical studies have reported that repetitive visual training based on sensory enrichment procedures may represent a very useful approach for the treatment of amblyopia, providing substantial improvement in a variety of visual tasks [76][77][78][79] (for a review, see Polat 80 and Levi and Li 81 ). One caveat to the therapeutic value of these visual practice procedures, however, is the narrow specificity of achievable improvement, which is typically limited to the selected trained stimulus, condition or task.…”

The application of online transcranial random noise stimulation and perceptual learning in the improvement of visual functions in mild myopia

“…For example, in a study investigating the effects of offline theta burst stimulation on a subsequent visual motion discrimination task, Silvanto and colleagues (Silvanto, Muggleton, Cowy & Walsh, 2007) found that external induction of neuronal plasticity (such as in the case of brain stimulation) also depends on the state of the targeted neurons during stimulation. Despite here induction of neural plasticity was found with above-threshold (simple) stimuli, larger improvements in tasks via perceptual learning (and thus neural plasticity) typically occur with more challenging, near-threshold stimuli (Polat et al, 2004;Sagi, 2011).…”

“…So far, PL has been shown to be effective in improving, among other dysfunctions, visual abilities in amblyopia (Campana, Camilleri, Pavan, Veronese, & Lo Giudice, 2014;Hussain, Webb, Astle, & McGraw, 2012;Levi & Li, 2009;Li, Young, Hoenig, & Levi, 2005;Polat, 2009;Polat, Ma-Naim, Belkin, & Sagi, 2004;Zhou, et al, 2006), mild refractive defects like myopia (Camilleri, Pavan, Ghin, Battaglini, et al, 2014;Tan & Fong, 2008) and presbyopia (Polat, et al, 2012), and central or peripheral vision loss and cortical blindness (Chung, 2011;Das, Tadin, & Huxlin, 2014;Huxlin, et al, 2009;Kasten, et al, 1998;Plank, et al, 2014;Rosengarth, et al, 2013;Sabel, Kenkel, & Kasten, 2005). In myopia, visual input is limited by an optical de-focus, despite normal neuronal connectivity and image processing.…”

“…Despite its proven effectiveness, PL techniques require lengthy protocols in order to yield effective outcomes (usually a minimum of two months training of up to three to four times weekly) (Camilleri, Pavan, Ghin, Battaglini, et al, 2014;Polat, et al, 2004;Tan & Fong, 2008). Random noise stimulation could optimize the effects of a behavioral training with measurable changes in the brain by modulating neuronal excitability that are involved in long-term potentiation (Fritsch, et al, 2010;Stagg, et al, 2009) which may ultimately lead to neuroplasticity.…”