The reading strategies in individuals with central scotomas and more than one preferred retinal locus (PRL) were investigated using a scanning laser ophthalmoscope in order to understand the visual requirements that lead to the need to use more than one PRL during reading. It was found that: (1) PRL function can be deduced from variation in PRL usage for different size and length of isolated words; (2) each subject used two or more PRL to accomplish the functions of global viewing and discrimination, suggesting that these are two of the minimum requirements for reading; (3) reading strategies can change depending on the position of words in visual space; (4) line-changing strategies can revert to horizontal and vertical component movements. These findings have implications for improving reading performance through training in patients with central scotomas.
Several studies have shown that growth of the primate eye responds in a compensatory direction to both positive and negative spectacle lenses--eyes grow more slowly and become hyperopic in response to positive lenses, and eyes grow more rapidly and become myopic in response to negative lenses. On the other hand, extended wear soft contact lenses, whether positively or negatively powered, induce hyperopia (Hung & Smith, 1996. Extended-wear, soft, contact lenses produce hyperopia in young monkeys. Optometry & Vision Science 73, 579-584.). We investigated whether responses in a compensatory direction occurred to soft contact lenses worn on a daily wear basis (8 h per day on an 8:16 h light:dark cycle). Ten infant marmosets (8-13 weeks of age) wore a soft contact lens, in one eye only, for 5-9 weeks. Lens powers used were zero (n = 2), +2 D (n = 1), +2 D followed after 5 weeks of lens wear by +4 D (n = 1) for 4 weeks, +4 D (n = 2), -2 D followed after 5 weeks of lens wear by -4 D (n = 2) for 4 weeks, -4 D (n = 2). At the end of the lens-wear period the positive lens-wearing eyes were more hyperopic relative to the fellow untreated eyes [mean +2.39 +/- 0.24 D (SE)] and the negative lens-wearing eyes were more myopic than the fellow untreated eyes [mean -2.48 +/- 0.91 D (SE)]. Fellow eyes were unaffected by lens wear [mean final refraction +0.45 +/- 0.09 D (SE)]. Plano lenses did not affect eye growth in either marmoset fitted with plano contact lenses.
Myopes display hyperopic shifts in the center and near peripheral field during near-viewing, while the farther periphery either remains unshifted or demonstrates a myopic shift. These results are due to the combined effect of lag of accommodation and an increased curvature of field during accommodation.
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