Improving vision: neural compensation for optical defocus

Abstract

Anecdotal reports abound of vision improving in myopia after a period of time without refractive correction. We explored whether this effect is due to an increased tolerance of blur or whether it reflects a genuine improvement in vision. Our results clearly demonstrated a marked improvement in the ability to detect and recognize letters following prolonged exposure to optical defocus. We ensured that ophthalmic change did not occur, and thus the phenomenon must be due to a neural compensation for the defocus condition. A second set of experiments measured contrast sensitivity and found a decrease in sensitivity to mid-range (5-25 cycles deg-1) spatial frequencies following exposure to optical defocus. The results of the two experiments may be explained by the unmasking of low contrast, high spatial frequency information via a two-stage process: (1) the pattern of relative channel outputs is maintained during optical defocus by the depression of mid-range spatial frequency channels; (2) channel outputs are pooled prior to the production of the final percept. The second set of experiments also provided some evidence of inter-ocular transfer, indicating that the adaptation process is occurring at binocular sites in the cortex.

Effect of Lens-Induced Myopia on Visual Cortex Activity: A Functional MR Imaging Study

“…The lens that was used to induce optical blurring has the same action as a low-pass spatial filter because it attenuates information that is carried at high spatial frequencies and results in degradation of the retinal image. 16 The reduction of the amount of visual cortex activity caused by dioptric defocus probably reflects this loss of visual information transferred from retina to visual cortex.…”

“…There is some evidence that improvement in visual resolution after blur adaptation is ascribed to a compensatory process in visual cortex. For example, Mon-Williams et al 16 found visual acuity improvement after a 30-minute period of ϩ1D induced defocus caused by "neural compensation," which occurred at binocular sites in visual cortex. Georgeson and Sullivan 19 also stated that organization of spatial frequency selective channels in the visual cortex attempts to restore the clarity of the blurred image.…”

Effect of Cycloplegia on Blur Perception Thresholds as Measured by Source Method

“…3). Subjective evaluation of image quality depends on previous blur experience and adaptation (Mon-Williams et al, 1998;Webster et al, 2002;Khan et al, 2013). It was suggested (Mon-Williams et al, 1988) neuronal recalibration occurred, which involved spatial frequency sensitivity changes.…”

Effect of Surrounding Blur on Foveal Visibility

“…Previous studies [1][2][3][4][5] have demonstrated that a certain period of exposure to the defocus blur (blur adaptation) improves visual acuity, suggesting the presence of a neural circuit that compensates for blur. More recently, Artal et al [6] suggested that the mechanism underlying blur compensation functions effectively for more complex blur caused by higher-order aberrations.…”