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J.D. Mollon, B.C. Regan, R. Foo, B.J. Morris; Sildenafil Increases Persistence of Vision . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4097.
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Purpose: Subjective reports from patients taking sildenafil citrate (Viagra®) suggest that this drug may increase the persistence of visual sensations. We asked whether such an effect could be demonstrated when visual persistence was estimated by means of a performance measure, that is, when we do not ask the subject to describe his phenomenological sensations but rather measure his accuracy in an objective test. Methods: In psychopharmacological experiments an impairmentof performance can often be attributed to non-specific factors. So we designed a task in which enhanced visual persistence would lead to better performance. Stimuli were presented on a computer-controlled monitor running at a refresh rate of 160 Hz. On each trial, two successive arrays of white dots were presented on a dark background. When the interval between the two arrays was brief, the subject could combine these two sub-arrays and identify a ring of dots randomly placed within the total array. His task was to report the position of a gap in the ring. Of the 11 dots making up the ring, 5 occurred in one sub-array and 6 in the other. In total, the two arrays contained 50 dots. The independent variable in the experiment was the number of milliseconds between the presentations of the two sub-arrays. The computer program adaptively adjusted this temporal separation to find the inter-stimulus interval (ISIc) at which the subject failed to identify the position of the gap. We used a Latin-square, double-blind, cross-over design with five conditions, corresponding to oral doses of 25, 50, 100 and 200 mg of sildenafil, plus a placebo condition. There were 16 young healthy adult male volunteer subjects, who gave informed consent. One subject did not complete all conditions and is excluded from the analysis. Results: A related measures ANOVA showed a highly significant dependence of ISIc on dose (F=8.43, p<.0001). After ingestion of 100 mg of sildenafil citrate (the maximal clinical dose), the value of ISIc was modestly greater [by approximately 20%] than that found in placebo or pre-test conditions. The effect was fully reversible. Conclusions: Sildenafil citrate increases persistence of vision in that it enhances performance in a task where two stimuli must be integrated across a time interval.
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