Subjective measurement of the foveal Stiles-Crawford effect with different field sizes

Abstract

Purpose : To measure the Stiles-Crawford effect (SCE) with innovative technology to evaluate how the SCE depends on field size.

Methods : A uniaxial Maxwellian system (spot size in pupil 0.5 mm diameter) was employed incorporating a spatial light modulator (Kopin SLM KCM-SK01-AA CyberDisplay® 1280M) to flicker at 1Hz between two 2.3^o fields corresponding to test (peripheral pupil) and reference (pupil center) positions. The participant’s task was to determine thresholds at 13 positions along the horizontal pupil meridian through indicating if the test field was brighter or dimmer than the reference field. In part 1, three participants were tested after dilating their right eye with 1% tropicamide to compare between two psychophysical methods (Staircase and ZEST methods). Thresholds were determined after four reversals with the Staircase method and from pre-determined limits using a maximum-likelihood adaptive procedure with the ZEST method. In part 2, the field size was varied between 0.46^o and 4.7^o. Six participants (4 low myopes and 2 low hyperopes) were tested with five field sizes with three runs per field size using the preferred method. The data were fit by a Gaussian function with directionality (ρ).

Results : In part 1, ρ ranged between 0.080 to 0.107 mm^-2 and from 0.100 to 0.120 mm^-2 for the staircase and ZEST methods, with similar peak locations in the pupil. The difference between techniques was nearly statistically significant (p = 0.053). The test duration with ZEST method (30-35 mins) was three times that for the Staircase method (8-11 mins). For part 2, the Staircase method was used. All myopes showed increasing ρ with an increase in the field size (average 0.054 to 0.114 mm^-2 for 0.46^o to 4.7^o fields, linear R² = 0.94), but this was not as evident for the two hyperopes (Fig 1).

Conclusions : In the form tried, ZEST is not suitable for measuring the SCE because it is too slow. The SCE directionality increases with field size out to 4.7 degrees with, in a small sample, a different trend for low myopes than for low hyperopes. Results are consistent with previous work finding an SCE directionality, for a fixed field size, increases as eccentricity changes from 0^o to 2.5^o (Carmichael Martins & Vohnsen 2019, BOE, Vol 10, 3760).

This is a 2020 ARVO Annual Meeting abstract.

 

Figure 1. Directionality (ρ) as a function of field size for myopic and hyperopic groups. Error bars are ±1SDs.