Abstract
Healthy macular function is a necessary prerequisite for the performance of fine detailed visual tasks such as reading and accurate face perception. Impairment of central vision follows the onset of age related macular degeneration (AMD) which has long been identified as the most prevalent ocular pathology affecting the elderly population of developed countries. The inherent coarse sampling ability of peripheral retinal neurones forces the individuals to construct their visual world from blurred low spatial frequency patterns. The magnocellular visual pathway is tuned to such low spatial frequency images and responds vigorously to fast modulations of luminance contrast, exhibiting band-pass temporal frequency characteristics. This suggests that temporal modulations of visual objects could enhance the performance of people who primarily perceive images of low spatial frequency content.Normal fixational eye movements are thought to counteract neural adaptation by jittering stationary images over the receptive fields of visual neurones, producing transient bursts of neural spikes. The retinal image shifts, produced by fixational eye movements, are not large enough to elicit neuronal activity during exposure of low spatial frequency images. Retinal image shifts with larger amplitudes, however, could produce activation of magnocellular neurones during the stimulus exposure and thus enhance contrast sensitivity to low spatial frequency images.
Using a novel method of inducing retinal image motion, we found that for people with AMD contrast sensitivity improved by 34.6 ± 5.3 % and 20.3 ± 2.6 %when viewing luminance gratings and Landolt C optotypes, respectively. Text jittering markedly enhanced reading speed by 101 ± 25 % for people with severe visual loss(VA>1 logMAR), while for those with moderate visual impairment (VA<1 logMAR),this effect was weaker demonstrating a 19 ± 9 % improvement. Ability to discriminate facial emotions of jittering images, produced on a computer screen, improved by a factor of 2 with a similar improvement being found when the facial stimuli were viewed through a prototype of optoelectronic jitter goggle. Finally we determined that retinal image jitter does not interfere with habitual eye movements. This finding suggests that a future low vision tool based upon image jitter may not result in asthenopic symptoms secondary to abnormal or excessive habitual eye movements.
The results suggest that retinal-image jitter with optimal frequency and amplitude is an effective strategy for enhancing visual information processing in the absence of spatial detail. These findings will enable the development of novel tools to improve the quality of life of patients with AMD.
| Date of Award | 2013 |
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| Original language | English |
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| Supervisor | Niall Strang (Supervisor) |