Influence of motion smear on visual acuity in simulated infantile nystagmus

Abstract

Purpose: In persons with infantile nystagmus (IN), visual acuity correlates with the duration of the foveation period of the nystagmus waveform, i.e., when the retinal image is on or near the fovea and moves with low velocity. In this study, we asked how acuity is affected by the non-foveating phases of the nystagmus waveform, when the velocity of retinal image motion is substantially higher.

Methods: Visual acuity was measured in three normal observers for high contrast, four-orientation single T-stimuli, presented during image motion that simulated either the whole jerk-IN waveform (whole-waveform) or only the foveation periods of the IN waveform (foveation-only). Simulated foveation durations ranged from 20 to 120 ms. For both motion waveforms, we displayed the acuity target for different number of cycles to examine whether acuity benefits from multiple presentations of the stimulus.

Results: As expected, visual acuity improves with longer simulated foveation durations in both the whole-waveform and foveation-only conditions. Acuity is consistently better (by ∼0.1 logarithm of the minimum angle of resolution) in the foveation-only than the whole-waveform condition, indicating that the high-velocity image motion during the simulated IN waveform has a detrimental effect. This difference in acuity between the two waveform conditions increases with the number of cycles, apparently because summation occurs across cycles in the foveation-only condition but not in the whole-waveform condition.

Conclusions: In normal observers, visual acuity in the presence of a simulated nystagmus waveform is limited not only by the duration of the foveation periods, but also by the non-foveating phases of the waveform. However, because persons with IN report little or no motion smear in association with their nystagmus, it remains unclear whether the rapid retinal image motion during the non-foveating phases of the nystagmus waveform generates a similar degradation of visual acuity in IN.

Figure 1

Idealized position traces showing the accelerating ramp waveforms used to simulate the retinal image motion in jerk nystagmus for the whole-waveform condition with simulated foveation durations of 20 and 120 ms. Each waveform has a frequency of 4 Hz and an amplitude of 8°. For comparison, traces for the foveation-only condition are shown underneath. Three different numbers of cycles of stimulus presentation are illustrated: one (N=1), two (N=2) and unlimited.

Figure 2

Visual acuity (logMAR) is plotted as a function of simulated foveation durations (ms) for image motion that simulated the whole waveform of IN image motion (filled circles) or only the simulated foveations (unfilled circles). The six panels show data for different number of cycles of stimulus presentation, represented by the value of N. Data shown are averaged across the three observers. Error bars represent ± 1 SE. The smooth curve drawn through each set of data represents the best-fit exponential function fit (see text for details).

Figure 3

Visual acuity (logMAR) is plotted as a function of simulated foveation durations (ms) for (left) the whole-waveform and (right) the foveation-only condition. In each panel, the different curves represent the acuity vs. duration functions fit to the data for different number of cycles of stimulus presentation. These curves are the same as those shown in Fig. 2.

Figure 4

The visual acuities predicted from the weighted-average model that is described in the text (triangular symbols with thick solid line) are compared with the empirically determined acuities for the whole-waveform (thin solid line) condition. For reference, the empirical data for the foveation-only (dotted line) condition is shown also. The acuity vs. duration curves for the empirical data are the same as those shown in Fig. 2. For a single cycle of stimulus presentation (left panel), the predicted and empirical values match reasonably well. However, with an increase in the number of cycles of stimulus presentation (N=2, right panel), the model systematically underestimates the measured acuities at longer simulated foveation durations. Model predictions for larger numbers of cycles of the whole-waveform condition exhibit similar deviations from the empirical whole-waveform data (not shown).