Subjective measurement of the foveal Stiles-Crawford effect with accommodation

Abstract

Purpose : To measure the Stiles-Crawford effect (SCE) with innovative technology to evaluate the changes in the directionality and photoreceptor alignment with accommodation.

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®) to flicker at 2 Hz 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 by indicating if the test field was brighter or dimmer than the reference field. Thresholds were determined by a simple staircase procedure after four reversals at each pupil location. After dilating right eyes with 2.5% phenylephrine, seven emmetropes were tested at 0 D to 6 D accommodation stimulus levels in 2 D intervals. The data were fit by the Gaussian function, both if the fits were un-forced or forced to pass through the threshold expected for the reference point. The directionality (ρ) and peak location values (x_max) were compared for unforced and forced fits.

Results : As there were only small differences between the two fitting approaches, only unforced fits are mentioned in the text. The regression slopes on accommodation stimulus were not significant (−0.0001 mm⁻²/D, R²=0.0002, p = 0.94) (Fig 1). There was a tendency for (x_max) to shift temporally with increasing accommodation across the 6 D stimulus range. While this was not significant for regression fitting (–0.058 mm/D, R² = 0.06, p = 0.20) (Fig 2), a paired t-test for 0 and 6 D accommodation stimuli showed significant change of 0.62 mm (t(6)= 2.50, p = 0.046).

Conclusions : The directionality did not change with accommodation, but the pupil peak location showed a significant temporal shift of approximately 0.62 mm with 6 D accommodation stimulus. It is possible that substantial changes in the directionality and a shift in the direction of peak location might occur at very high levels of accommodation.

This is a 2021 ARVO Annual Meeting abstract.

 

Fig 1: Directionality as a function of accommodation for unforced and forced fits. The dashed red and blue regression lines are based on individual mean directionalities. Error bars are ±1 SDs.

 

Fig 2: Peak location as a function of accommodation for unforced and forced fits. The dashed red and blue regression lines are based on individual mean pupil peak locations. Error bars are ±1 SDs.