Flicker adaptation improves acuity for briefly presented stimuli by reducing crowding

Abstract

Adaptation to flickering/dynamic noise improves visual acuity for briefly presented stimuli (Arnold et al., 2016). Here, we investigate whether such adaptation operates directly on our ability to see detail or by changing fixational eye movements and pupil size or by reducing visual crowding. Following earlier work, visual acuity was measured in observers who were either unadapted or who had adapted to a 60-Hz flickering noise pattern. Participants reported the orientation of a white tumbling-T target (four-alternative forced choice [4AFC], ⊤⊣⊥⊢). The target was presented for 110 ms either in isolation or flanked by randomly oriented T's (e.g., ⊣⊤⊢) followed by an isolated (+) or flanked (+++) mask, respectively. We measured fixation stability (using an infrared eye tracker) while observers performed the task (with and without adaptation). Visual acuity improved modestly (around 8.4%) for flanked optotypes following adaptation to flicker (mean, -0.038 ± 0.063 logMAR; p = 0.015; BF10 = 3.66) but did not when measured with isolated letters (mean, -0.008 ± 0.055 logMAR; p = 0.5; BF10 = 0.29). The magnitude of acuity improvement was associated with individuals' (unadapted) susceptibility to crowding (the ratio of crowded to uncrowded acuity; r = -0.58, p = 0.008, BF10 = 7.70) but to neither fixation stability nor pupil size. Confirming previous reports, flicker improved acuity for briefly presented stimuli, but we show that this was only the case for crowded letters. These improvements likely arise from attenuation of sensitivity to a transient low spatial frequency (SF) image structure (Arnold et al., 2016; Tagoh et al., 2022), which may, for example, reduce masking of high SFs by low SFs. We also suggest that this attenuation could reduce backward masking and so reduce foveal crowding.

Figure 1.

The experiment protocol for the adapted condition. A 30-second flicker-adaptation phase preceded the first trial, followed by a 50-ms ISI, a 110-ms test, and then a 400-ms post-stimulus mask. Finally, a yellow fixation marker appeared that prompted the observer to respond. The second (and subsequent trials) were similar, except that the adaptation phase lasted only 4 seconds. Unadapted conditions were identical, except that the stimulus appeared immediately on each trial. For the uncrowded letter conditions, the central T-optotype target (and mask) appeared without flankers.

Figure 2.

Impact of flicker adaptation on visual acuity. Gray bars represent mean acuity across all observers in each condition. The error bars denote ±1 SEM, and each pair of colored discs represents data for one participant. Acuity is measured in logMAR: higher values on the ordinate represent poorer acuity, and lower values better acuity. (A) For flanked/crowded T targets, flicker adaptation improved acuity (by around two letters on a Sloan chart; mean gain, −0.038 logMAR; p = 0.015). (B) Removing the flankers produced an overall improvement in unadapted performance (mean gain, 0.16 logMAR; compare the first bars of parts A and B). However, acuity improvement derived from adaptation for unflanked (isolated) letters was not statistically significant (mean gain, −0.008 logMAR; p = 0.5).

Figure 3.

Susceptibility to crowding in our experiment. (A) Unadapted visual acuity for isolated versus flanked letters; plotting conventions are as shown in Figure 2. Mean visual acuity was significantly better when measured with isolated compared to flanked letters (mean change, 0.16 logMAR). (B) Adapted visual acuity for isolated versus flanked letters; mean visual acuity was significantly better when measured with isolated compared to flanked letters (mean change, 0.13 logMAR). (C) The correlation between acuity change following adaptation to flicker and susceptibility to crowding was statistically significant (r = −0.58; p = 0.008). Each solid disc represents a plot of average acuity change versus mean change in susceptibility to crowding for each individual, and the shaded area indicates a worsening of acuity. Positive numbers on the abscissa denote greater susceptibility to crowding. The dotted line shows the trend line, and the error bars represent ±1 SEM.

Figure 4.

Effect of flicker adaptation on fixation and pupil size. (A, B) Plotting conventions are as depicted in Figure 2. Fixation stability is somewhat poorer (quantified by the BCEA measure) following adaptation to flicker, but there was no association between individual differences in BCEA and acuity change. (C) Comparison of pupil size before and following adaptation to flicker; pupil size was significantly smaller following adaptation. (D) However, there was no significant link between individual differences in pupil size and acuity change.