Improvement Of Visual Functions In Normal Adults Through The Combination Of Adaptive Optics Correction And Perceptual Learning

Although the visual cortex can be most easily modified early in life (the so-called critical period), it is also now recognized that some plasticity persists well into adulthood. However, the observed plasticity that demonstrated through the process of perceptual learning is highly limited in normal adults, which casts a shadow on the application of perceptual learning as a possible way to improve visual functions. Most researchers believe that the low observed plasticity could be fully accounted for by the limited cortical plasticity. Nonetheless, it is still not clear whether there are other factors that may hinder the adult brain's potential to change. On the other hand, previous studies in visual optics have demonstrated that higher order optical aberrations, if corrected with adaptive optics techniques that initially used in astronomy, can result in visual improvement. However, those experiments failed to display significant medical values since the improvement following the correction of optical aberrations is too weak and can't exist without of the adaptive optics system. In this dissertation, on the basis of recent advances on perceptual learning and visual optics, we provide the first evidence that the quality of the retinal image limits the potential improvement of both contrast sensitivity and visual acuity through perceptual learning, which demonstrating an important inter-dependence between recent advances in the disciplines of optics and biology.In particular, we compared the effectiveness of perceptual learning (PL) that trained with and without correction of the eye's higher order aberrations (HOAs) for contrast sensitivity and visual acuity to explore the limits to the plasticity of adult visual system.Visually normal adults were separated into two training groups. All 13 subjects in Group 1 were trained with HOAs corrected conditions, whereas all 8 subjects in Group 2 were trained under normal viewing conditions (i.e. HOAs uncorrected). All subjects were trained with high spatial frequency sinusoidal gratings every day for a training session which lasts one hour. After a training of 10 consecutive days, Group 1, the HOAs corrected group, showed a 5.39dB (or 86.1%) improvement of contrast sensitivity, which was strongly statistically significant (P＜0.00001). The slope of the average learning curve was 0.41 log units per session, and reached plateau after 7.35 training sessions; Group2, the HOAs uncorrected group, showed a smaller (3.42dB (or 48.2%)) but also significant (P=0.03) improvement. The slope of the average learning curve was 0.35 log units per session, and reached plateau after 4.12 training sessions. Contrast sensitivity functions (CSFs), a measurement of the sensitivity to stimuli of different spatial frequencies, were tested at pre-and post-training stages for both groups. There were significant improvements after training in both Group 1 (post-vs. pre-training:F (1,12)=75.43, P＜0.00001) and Group 2 (post-vs. pre-training:F (1,7)=5.46, P=0.05). The average magnitude of the contrast sensitivity improvements across observers and spatial frequencies were 3.11 dB and 1.31dB in Groupl and Group2, respectively. For selected subjects in Group 1 and Group 2 who showed significant contrast sensitivity improvement (All 13 subjects in Group 1 and only 4 subjects in Group2), the magnitude of improvements at the training spatial frequency were not significantly different (P＞0.5). However, a different spatial frequency dependency was evident:in Group 1, there was a specific learning effect (bandwidth 1.11 octaves) together with a general increase in sensitivity to all spatial frequencies tested. The magnitude of this general increase was half the magnitude of the peak increase; while in Group2, there was only a specific learning effect (bandwidth 1.42 octaves). Another interesting finding was that the training also significantly improved visual acuity in Group 1 (P＜0.000001), but not in Group2 (P=0.199). Average improvement of visual acuity in Groupl was 2.32dB (or 31%), which is larger than that in Group2 (P=0.0008). All subjects in Group 1 had visual acuity improvements after training that were retained for at least 5 months (4 subjects in Group 1 has visual acuity retested 5 months after training). By analyzing the post-and pre-training optical modulation transfer functions (MTF) and the neural transfer function (NTF), we demonstrate that optical quality did not alter significantly after training; the improvement to the CSF reflected neural changes.To show that the beneficial effect we found in Group 1 is not just a consequence of the HOAs corrected conditions per se, we undertook another training experiment under HOAs corrected conditions (Group 3,6 adults). Group 3 went through an otherwise same HOAs-correcting procedure and training process as Group 1 except that the former was trained at the "optimal spatial frequency" on the low spatial frequency end, i.e. the corresponding spatial frequency at the peak of the CSF curve obtained in the process of HOAs correction. Interestingly, we found much less improvement in Group 3 than Group 1. These results indicate that the HOAs-corrected environment is necessary but not sufficient for the improvement; training at a near-cutoff spatial frequency is also required.To identify the mechanism of the improvements in contrast sensitivity after training in Group 1, we assessed the training improvement in contrast thresholds for stimuli with and without added spatial noise. Training improvements were measured at a spatial frequency of 16 cycles per degree for 4 subjects in Group 1 within the HOAs corrected environment. We demonstrate that perceptual training improvements are the result of improved neural efficiency rather than diminished neural internal noise. This finding is consistent with a number of studies that have investigated this distinction from other perceptual learning tasks.We conclude that after the critical period, the visual areas of our brain still have considerable plasticity but the extent of the visual improvement that can be induced by perceptual learning is limited by the optical quality of the eye. If the higher order aberrations are corrected, the full extent of brain plasticity can be uncovered.Our demonstration of enhanced visual plasticity in the adult not only offers hope for the effectiveness of new therapies applied in later life to redress brain dysfunction resulting from anomalous visual development earlier in life, but also shows that "supernormal" vision is achievable through the combination of recent advances in our understanding of optics and brain plasticity.