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S. J. Leat, J.-M. Seo; Spatial and Temporal Summation in Low Vision. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4740. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
People with low vision often seem to require longer to make visual judgements than those with normal vision, and they certainly require larger stimuli. This led us to ask whether either spatial or temporal summation is increased.
Twenty adults with low vision due to a variety of causes (mostly retinal) and 20 age-matched controls took part. Binocular contrast sensitivity (CS) was measured with a two-alternative forced choice staircase for 0.7 and 3 cpd sine wave grating stimuli of duration ranging from 26-6617 msecs and circular spatial extent ranging from 1-27 cycles width. Temporal and spatial functions were plotted and the critical point (duration or number of cycles at which CS no longer increased) was determined for each subject.
The shape of the functions for the low vision subjects were similar to those of controls, but shifted downwards because of CS loss. Although the critical points were slightly higher for both temporal and spatial summation for subjects with low vision, this did not reach significance. The critical durations were 778 and 861 msecs for subjects with low vision and 550 and 662 for controls for 0.7 and 3 cpd respectively, while the critical number of cycles was 15 and 13.5 for LV and 12.8 and 10 for controls for 0.7 and 3 cpd respectively. For critical duration, mixed ANOVA (2 groups x 2 spatial frequencies) showed that there was no significant effect of group (p=0.19), no effect of spatial frequency (p=0.31) and no interaction (p=0.95). Similarly for the critical number of cycles, there was no effect of group (p=0.13), spatial frequency (p=0.14) and no interaction (p=0.59). There were no significant associations between age or visual acuity and critical points (p>0.05, Bonferroni adjusted).
Although CS was reduced overall, as expected in this population, there were no significant changes in the critical duration or number of cycles for temporal or spatial summation respectively. However, because of reduced acuity and contrast sensitivity, people with low vision will still benefit from magnification and stimuli of longer duration - since CS is reduced overall, they may still take longer to make judgements in order to benefit from their own optimum CS.
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