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K Shinomori, JS Werner; The Impulse Response of the Aging Visual System . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4733.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To measure the impulse response function (IRF) of an achromatic pathway for 62 observers ranging in age from 16 to 85 years. Methods: A double-pulse method was used; thresholds were measured for two pulses separated by interstimulus intervals from 6.7 to 180 msec. Each pulse was presented in a single frame on a CRT display operating at a 150 Hz frame rate. The pulses had a spatial Gaussian shape (±1 SD = 2.3° diam) with a luminance of 10 cd/m2 background, having the same chromaticity as the pulse. To control for possible changes of criterion with age, a spatial 4-alternative forced-choice method was combined with a staircase procedure. Individual differences in ocular media density were largely compensated by heterochromatic flicker photometry with the red phosphor as the standard. Each IRF was measured four times, in separate sessions, for each observer. We used a model by Burr and Morrone (1993) to calculate IRFs from the threshold data. This model does not assume a minimum phase. Four parameters in the model equation were varied to find the best fit to the threshold data with a least-squares method. Results: For 9 of 25 observers over 55 years of age, IRFs are relatively weak and show that the second (inhibitory) phase in the IRFs is reduced. Consequently the IRF for these observers is quite slow and long. For other observers, the duration of the first (excitatory) phase of the IRFs was about 40-60 msec and the duration is nearly constant with age. The peak value of IRFs is almost constant until about 40 years of age and decreases gradually thereafter. This change appears not only in observers whose IRFs have little inhibitory phase, but on many older observers. Control conditions demonstrated that age-related changes in the IRF under these conditions cannot be ascribed to optical factors. Conclusion: The data suggest that the human visual system basically maintains a stable speed of response to a flash until at least about 80 years of age, even while the response signal level decreases with age. Some elderly observers show, however, a substantial reduction in inhibitory contributions to their impulse response.
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