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Glasses Off!

“Training the brain to overcome the effect of aging on the human eye”

Sci Rep. 2012; 2: 278. PMID # 22363834

Presbyopia, from the Greek for aging eye, is, like death and taxes, inevitable. Presbyopia causes near vision to degrade with age, affecting virtually everyone over the age of 50.

First-reported negative effects on reading and near tasks occurring between about 42 and 44 years of age and affecting virtually everyone by age 51.

Presbyopia results from the gradual decrease of accommodative (focusing power with age).

In addition presbyopia results in reduced near visual acuity, reduced contrast sensitivity, and slower processing speed.

Indeed, the most common symptom of presbyopia is difficulty in reading up close, particularly in poor lighting, and early presbyopes often complain that their arms are too short. The most common solution is reading glasses or bifocals.

At near, the visual acuity and contrast sensitivity of uncorrected presbyopes is lower than normal

We reasoned that if the neural signals from the blurred retinal image could be boosted or used more efficiently by the brain, it may be possible to overcome or at least delay the effects of presbyopia.

Perceptual learning with similar, though not identical, tasks has been shown to improve visual functions in normal vision and in patients with both neural (amblyopia) and optical (low myopia and presbyopia) deficits

The mean visual acuity before training is just below the newsprint size. Importantly, for every participant, the post-training acuity letter size was better (smaller) than the pre-training acuity letter size, on average by a factor of ≈ 1.6, from 2.44 arc min at pre-test to 1.56 at post-test

Note that the “control” subjects’ (3 subjects tested 2 months apart without training) data fall along the equality line. In other words, the control subjects did not improve.

Interestingly, the mean visual acuity (VA) was reduced from 2.44 ± 0.24 to 1.56 ± 0.16, equivalent to an effective reduction in age of ≈ 8.6 years

Reading speed of presbyopes with uncorrected near vision was slow prior to training when measured on the smallest letter size that subjects could read, but it improved, on average by about 17 words/minute following training

After training, contrast detection thresholds improved (decreased) at all 3 tested spatial frequencies

These results are consistent with earlier reports of improvement of contrast sensitivity in presbyopic and amblyopic

here we show, for the first time, improvement in suprathreshold contrast discrimination without direct training on a just-noticable-difference (JND) task

there was no improvement of near visual functions following the training resulted from changes in the optical functions of the eye testing accommodative power, pupil size and depth of focus

Therefore we conclude that perceptual changes occur due to changes in the brain.

Our results, consistent with previous studies, show that perceptual learning can improve visual acuity and contrast sensitivity in persons with presbyopia, and in some cases, result in performance levels similar to the young control group. Moreover, here we show that training also improves suprathreshold contrast discrimination and reading speed for small letters. Our study is the first to show conclusively that these improvements are not due to improved optical performance of the eye (accommodation, pupil size or depth of focus).

Our results underscore the remarkable capacity of the brain to increase the efficiency of neural processing in order to perform “de-blurring” of highly blurred images, retrieve the information and deliver it downstream for further processing at sensory and cognitive levels.

we suggest that the improvement is an effect of “de-blurring”. This could be achieved by increasing the sensitivity or gain of neurons by a factor that enables image processing at very low signal-to-noise levels with efficiency similar to processing at normal signal-to-noise levels.

blur detection and discrimination are instances of contrast detection, so it is not unreasonable to assume that when we improve contrast detection through perceptual learning, we also improve the ability to detect and discriminate blurred images.

Increasing sensitivity may, in turn, increase neural processing speed. It is also possible that the training regimen may increase processing speed directly. It was shown earlier that training improved contrast sensitivity and, recently, that training on backward masking decreased the latency by 20 ms in the human brain and shortened the reaction time

Thus, altogether, training may improve both the sensitivity and processing speed of the presbyopic visual system in order to compensate for the optically degraded visual input transmitted by the aging eye.

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