Reduced foveal crowding contributes to improvement in acuity following flicker adaptation

Flicker adaptation improves acuity for briefly presented (masked) stimuli (Arnold et al, PNAS, v113 p12556-61, 2016). We investigated the role that fixation-stability, pupil-size and susceptibility to foveal crowding might play in this effect. We measured visual acuity by having observers (n = 20) report the orientation of a white tumbling- “T” target. Observers were either unadapted or had adapted to a 60Hz flickering noise pattern (30s first trial, 4s trial-by-trial top-up). The target was presented for 110ms (immediately followed by a mask) and could appear in isolation or flanked by a pair of randomly oriented “T”s. A second cohort of observers (n = 35) performed the acuity task (flanked target only), while undergoing eye-tracking. This stimulus arrangement led to high levels of foveal crowding with recognition acuity for flanked letters (0.049 LogMAR) being around 0.16 LogMAR worse than for unflanked letters. Adaptation to flicker moderately improved visual acuity for flanked (-0.038 LogMAR) but not for unflanked (-0.008 LogMAR) optotypes. The magnitude of acuity-improvement was correlated with individuals’ (unadapted) susceptibility to crowding; those more susceptible to crowding showed greater improvement (r = -0.55, p = 0.006). In conclusion, we confirm that flicker adaptation improves acuity for briefly presented crowded letters (Arnold et al., 2016). While we agree with suggestions that these improvements arise from a reduction in sensitivity to transient low spatial frequency image structure, our results demonstrate that this may improve acuity at least in part by reducing foveal crowding.