Can the Human Eye Accommodate in the Correct Direction Just by Itself Without Relying on Other Visual Cues?

Abstract

The aim of this study is to examine the effectiveness of the accommodative response of the adult human eye when subject to both foveal, parafoveal and perifoveal stimuli. This is a study also to clarify whether foveal and parafoveal vision drives accommodation in the correct direction or not, by removing out-offocus blur before relying on any other cues. In addition, the influence of microfluctuations on accommodation process has been studied as it is considered to play important rule in the direction detection process during accommodation. A near-IR laser diode (850 nm) and a fast Hartmann-Shack wavefront sensor (Thorlabs) were used to capture ocular aberrations up to the 4th Zernike order at 20 Hz framerate. A motorized iris placed in the conjugate pupil plane was used to limit the effective pupil to 2.5, 3.5, or 4.5 mm pupil for foveal accommodation while for parafoveal and perifoveal measurements it was fixed to 3 mm to limit the monocular vergence. In all cases, a green fixation target on a monitor at 1-m viewing distance was used. A random sequence of step defocus changes every 10 sec. was induced by a current-driven tuneable lens within the accommodative range of each subject. A near-IR laser diode was used together with a Hartmann-Shack wavefront sensor to capture Zernike aberrations in real time (20 Hz) during the accommodative response. For foveal and microfluctuations measurements subjects viewed a Maltese cross whereas for parafoveal and perifoveal measurements annular targets from 2° to 16° were used. A total of 20 subjects in the age range of 22 - 49 years were characterized for three different measurements. The results show that the accommodative response acts in the correct direction to compensate defocus but with a reduced amplitude of response at increased eccentricity (largely absent at 14°). Moreover, it was found that the accommodation reaction time is not faster than neural processing time which means the eye cannot complete the accommodation process only by itself. It was also found that in the absence of microfluctuations accommodation would still be in the correct direction. Fig.1. Induced defocus sequence (Tuneable lens: black line) and accommodation response (Eye: red line) measured with a Hartmann-Shack wavefront sensor as a function of time for an emmetropic subject at (a) fovea (2o eccentricity), (b) parafovea (6° eccentricity) and (c) perifovea (16° eccentricity). Blinks have been numerically suppressed in the presented data.