Vision is limited by two main factors: (a) the quality of the image that is transferred from the eye, and (b) the neural processing in the brain, which needs to integrate information between different neurons located at neighboring brain locations (space). Cortical cells (neurons) are highly specialized and optimized as image analyzers. Thus, to characterize an image, visual processing involves the cooperative activity of many neurons—those neuronal interactions contributing to both excitation and inhibition. The integration of image parts should be performed very quickly, since the time-window in which the first percept is formed is very short. Thus, visual information processing may be limited if the first percept representation is inefficient either due to slow neural processing or to the lack of effective interactions between the neurons.
Contrast sensitivity (CS), i.e., the ability to discriminate between shades of gray, is one of the main determinants of how well people see. It is assumed that the contrast sensitivity function (CSF) describes the combined response of the classical receptive fields of simple cells that have been selectively tuned for location, orientation, and spatial frequency and constitute the fundamental units of analysis. (Wilson, 1991; Wilson & Wilkinson, 1997). Thus, CSF describes the output of an early stage that provides the building blocks for the succeeding steps of visual processing.
During the last two decades, it was demonstrated that contrast response is also determined by lateral interactions in the visual cortex of humans (Bonneh & Sagi, 1999; Cass & Alais, 2006; Cass & Spehar, 2005; Ellenbogen, Polat, & Spitzer, 2006; Polat & Norcia, 1996; Polat & Sagi, 1993, 1994a, 1994b, 2006; Shani & Sagi, 2006; Solomon & Morgan, 2000; Tanaka & Sagi, 1998; Woods, Nugent, & Peli, 2002) and of animals (Crook, Engelmann, & Lowel, 2002; Kapadia, Ito, Gilbert, & Westheimer, 1995; Mizobe, Polat,) Visual acuity (VA) is the most common clinical measurement of visual function and is considered as the gold standard measure of visual functions. VA measures the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. A person with standard (normal) VA can recognize a letter that is specified as 6/6 (20/20).
Visual plasticity is the ability of the visual system to change its responses in order to adapt to changes in the visual input. Evidence for plasticity in the adult visual system has been reported in human studies that have demonstrated that training in specific visual tasks leads to improvement in performance or sensitivity (for a review, see (Fahle & Poggio, 2002)). Perceptual learning has a major influence on our understanding of the development and plasticity of the visual system. Improvement after perceptual learning was demonstrated using a variety of visual tasks showing that the adult visual system can change according to behavioral demands (Fahle, 2005; Fiorentini & Berardi, 1980; Polat & Sagi, 1994b; Sagi & Tanne, 1994). (For a review, see Fahle (2002), Fahle and Poggio (2002), Gilbert, Sigman, and Crist (2001), Sagi and Tanne (1994).
Amblyopia is a reduction of visual functions that cannot be directly attributed to the effect of any structural abnormality of the eye or the posterior visual pathway. It is caused by abnormal binocular visual experience early in life, during the ‘critical period’ that prevents normal development of the visual system. A generally practiced principle of treatment is that therapy can only be effective during the critical period, usually considered to end around the age of 8–9 (Greenwald & Parks, 1999; Prieto-Diaz, 2000; von Noorden, 1981), when the visual system is considered sufficiently plastic for cortical modifications to occur. The standard amblyopia therapy is thus traditionally directed toward children and consists of penalizing the preferred eye by using an eye patch or atropine, thus forcing the brain to use the visual input from the amblyopic eye. However, in adults, the visual deficiencies are thought to be irreparable after the first decade of life, once the developmental maturation window has been terminated; thus the standard treatment is usually not offered. However, recovery of visual functions in adults with amblyopia after occlusion therapy (Birnbaum, Koslowe, & Sanet, 1977; Simmers, Gray, McGraw, & Winn, 1999; Wick, Wingard, Cotter, & Scheiman, 1992) or after loss of vision in the good eye (El Mallah, Chakravarthy, & Hart, 2000) was reported. The first step in a series of controlled studies that provided evidence for plasticity, after perceptual learning, in adults with amblyopia used training for the vernier acuity task (Levi & Polat, 1996; Levi, Polat, & Hu, 1997b). Repetitive practice led to a substantial improvement in vernier acuity in the amblyopic eyes of adults with amblyopia. In two observers, the improvement in vernier acuity was accompanied by a commensurate improvement in VA reaching up to normal vision. These studies provided an optimistic possibility for future treatment of amblyopia based on perceptual learning. Recent studies have provided additional evidence for plasticity in adults with amblyopia (Chung, Li, & Levi, 2006; Fronius, Cirina, Cordey, & Ohrloff, 2005; Fronius, Cirina, Kuhli, Cordey, & Ohrloff, 2006; Levi, 2005; Li & Levi, 2004; Polat et al., 2004; Zhou et al., 2006).
Some insight into the mechanism underlying neural plasticity, which may improve the contrast sensitivity, comes from lateral masking experiments (Polat & Sagi, 1994b, 1995; Polat et al., U. Polat / Vision Research 49 (2009) 2566–2573 2567 2004). These studies suggest that practice on lateral interactions increases the efficacy of the collinear interactions between neighboring neurons, an effect that enables connectivity with remote neurons via a cascade of local interactions. Thus, the results suggest a possible tool for the use of lateral interactions for improving CS in people with normal vision and in people with impaired lateral interactions such as amblyopia. Polat has developed a perceptual learning procedure that was designed to improve the abnormal lateral interactions in amblyopia by stimulating the deficient neuronal populations and effectively promoting their collinear interactions (Polat, 2006, 2008; Polat et al., 2004). Since the amblyopic deficit is not identical among subjects (Bonneh, Sagi, & Polat, 2004; Bonneh et al., 2007; Polat, 2008; Polat et al., 2005), the treatment was tailored and specifically designed for each individual’s deficiencies.
Amblyopes exhibit abnormal lateral interactions (Bonneh et al., 2004, 2007; Ellemberg et al., 2002; Levi et al., 2002; Polat, 2006, 2008; Polat et al., 2004). The lateral interaction function of the amblyopes at the beginning of the treatment showed no facilitation and in fact, increased the amount of suppression. However, after the treatment, the amount of suppression was significantly reduced to a normal level (Polat, 2008; Polat et al., 2004).
In the study of Polat et al. (2004), the amblyopic eyes exhibit the typical lower CS before treatment, as compared with normal sighted eyes, with the low spatial frequencies near the normal values and the high spatial frequencies showing a worse deficit. The treatment produced a significant improvement in sensitivity, by about a factor of two, in all spatial frequencies including the high spatial frequency range, raising the function to within the normal (lower) range. Most interesting is the result that after 12 months, CSF was not only retained, but it also increased toward an average range at the high spatial frequencies. This result suggests that the high spatial frequencies are used after the treatment in daily tasks and thus are naturally practiced.
The procedure of Polat et al. (2004), when applied to people with normal vision or corrected to normal vision, improved their visual acuity to better than 66. It has been recently applied to improve the vision of people with low myopia (Tan & Fong, 2008). The vision of myopic (short sighted) subjects is blurred without optical correction. Therefore, the CSF is reduced, especially at the higher spatial frequencies, when compared with people with corrected vision. This reduction in CS is reminiscent of the CS of amblyopic subjects. This study used a protocol similar to the one used for the amblyopia (Polat et al., 2004); it showed that when subjects practiced with uncorrected moderate myopia it improved their CS. Thus, even in cases when the lateral interactions are normal (low myopia), training improves CS.
The VA was found to improve after training on contrast detection of amblyopes (Polat et al., 2004), anisometropic amblyopes (Huang et al., 2008; Zhou et al., 2006), and after training on verneir acuity (Levi & Polat, 1996; Levi, Polat, & Hu, 1997a). The training of low myopia on lateral interactions also shows improvement of VA (Tan & Fong, 2008). Thus, the training can be generalized to the letter recognition task (VA), an effect that supports the relationships between these perceptual tasks and letter recognition.
In the studies of Polat and colleagues, during the treatment, the fellow eye was covered; thus the treatment was monocular, targeting the abnormal lateral interactions of the amblyopic eye. Very surprisingly, after treatment, the binocular functions improved, indicating that both the binocular fusion and the stereo acuity improved (Polat, 2006, 2008). A significant improvement in stereoacuity was also found in a retrospective study (Lichter, 2007).
In addition to Amblyopia and Myopia, several other conditions that cause reduced VA were studied. Presbyopic patients showed an increase of 1.5 to 2 lines and an increase of close to 100% in CS following treatment (Polat 2009; Tan 2005; Stahl & Durrie 2008; Durrie & McMinn, 2007). Patients who have undergone refractive surgery have shown similar results (Tan 2005, Waring IV et al., unpublished data). Patients who have undergone intra-ocular-lens (IOL) implant surgery following cataract extraction showed high CS improvement and an increase of 1 to 1.5 lines of VA in a variety of monofocal and multifocal or accommodating IOLs (Waring IV et al. 2010). A possible positive treatment outcome in myopia control is suggested by seminal work done on school children (Chua et al. 2007), and although this is yet to be determined by a randomized double blind controlled study, over several years, the current findings suggest optimistic outcomes.
Due to the promising findings in the above studies and in light of the fact that the treatment is safe and non-invasive, several practitioners have used this treatment in managing several types of ocular pathologies. Among others these include: congenital nystagmus (CN) (Morad 2012), age related macular degeneration, retinitis pigmentosa (Lyra, 2009) pathological myopia and congenital stationary night blindness. A retrospective multicenter study which is being carried out during this year has already showing positive results in cases of CN.
Different studies measured the persistence of the results over a retention period. While amblyopia showed a surprising increase in CS function over time (Polat et al 2004), others showed a mild regression of about 15% of the treatment effect over the first six months post treatment. However, in the following eighteen months of retention no further regression was noted (Siow & Tan, 2008). Studies aimed at presbyopia and post refractive correction procedures have shown no regression over twelve months retention (Tan et al. 2005, Ng et al. 2007)
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