The seemingly paradoxical findings of enhanced word recognition speed with image jitter found here and the reduced reading speed and visual acuity with image jitter found in the above-mentioned studies (Parish DH, Legge GE, unpublished observations, 1989)
10–11 can be explained by differences in the temporal characteristics of the image jitter. Our control experiment 1 showed that in simulated low vision conditions, jittering text with interjitter intervals of 500, 250, and 100 ms (1–5 Hz fundamental temporal frequencies) enhanced the speed of word recognition, while shorter interjitter intervals of 50 and 25 ms (10–20 Hz) impaired performance in comparison with that for stationary text (
Fig. 3). These results are closely related to the band-pass characteristic of contrast sensitivity to stimuli of low spatial frequencies as a function of stimulus temporal frequency. Robson
4 found that the sensitivity for detecting a central flickering grating of 0.5 cyc/deg, compared with that at a temporal frequency of 0.5 Hz, increased by a factor of 6 at a 6-Hz flicker rate and reduced by a factor of 10 at a 30-Hz flicker rate. Virsu et al.
27 showed that when gratings of 0.75 cyc/deg were presented at different eccentricities, contrast sensitivity decreased as a function of eccentricity, but the temporal contrast sensitivity functions had a similar band-pass form with a peak at approximately 6 Hz. Georgeson
28 showed that the peak of the contrast sensitivity function at 8 Hz for 1.5 cyc/deg flickering gratings was also found in the above-threshold temporal modulation function using a contrast-matching method. Our control experiment 2 also provides direct evidence that contrast modulations of 5 to 10 Hz enhanced contrast sensitivity for discriminating orientation of 0.5 cyc/deg gratings in people with central visual loss (
Fig. 6).Thus, the impaired reading rate
10 (Parish DH, Legge GE, unpublished observations, 1989) and visual acuity
11 could be due to reduced sensitivity of the visual system to high-frequency modulations (interjitter intervals of 24–40 ms) of retinal images.