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D. Wiggins, T.H. Margrain, M. Woodhouse, J.T. Erichsen; The Consequences of 'Effort To See' for Nystagmus . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2503.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the effect of 'effort to see' on the nystagmus waveform. Individuals with congenital nystagmus (CN) commonly report that making an 'effort to see' can intensify their nystagmus and adversely affect vision. However, such an effect has never been confirmed experimentally.
The eye movement behaviours of congenital nystagmats were recorded for different gaze angles while viewing targets which were above and then at resolution threshold. Eye movements were recorded by infra–red oculography (Skalar IRIS) and visual acuity (VA) was measured using Landolt C targets and a 2–AFC staircase procedure. Eye movement data were analysed for changes in: amplitude, frequency, intensity and foveation characteristics. Waveform type was also noted under the two conditions.
Data from 9 subjects reveal a significant reduction in nystagmus intensity with increased 'effort to see'. At their null zone intensity was reduced on average by 40% (CI 20–60% reduction; p = 0.008; paired t–test). Amplitude and frequency were also reduced with increased visual effort. Furthermore, a change in waveform type with increased visual demand was exhibited by two subjects.
This study indicates that ‘effort to see’ produces a reduction in nystagmus intensity. This contradicts previous reports that 'effort to see' intensifies the nystagmus eye movement. However, this discrepancy may be attributed to the lack of psychological stress involved in the visual task reported here and is consistent with the suggestion by Tkalcevic & Abel (2005)1 that the visual importance of the task to the individual rather than visual demand per se exacerbates CN. Our findings may have additional implications for studies that have examined the relationship between visual acuity and nystagmus because most researchers have recorded eye movements only after acuity has been measured, i.e. when the "effort to see" is minimal. This study strongly suggests that quantifying the relationship between nystagmus eye movements and VA is only possible if the two are measured simultaneously.
1 Tkalcevic L A, and Abel L A (2005) The effects of increased visual task demand on foveation in congenital nystagmus. Vision Res 45: 1139–1146.
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