Karan R. Aggarwala

Accommodation Without Feedback Suggests Directional Signals Specify Ocular Focus

Accommodation was monitored continuously under open-loop conditions while subjects viewed a sinusoidally oscillating sine-wave grating (0.2 Hz; +/- 1 D; 2.7 c/d; 0.56 contrast) in a Badal optometer. The target was illuminated by monochromatic light (590 nm) or white light (3000 K) with longitudinal chromatic aberration (LCA) normal, doubled, neutralized and reversed. Subjects (12) accommodated well in white light with LCA normal and doubled (mean gains = 0.85 and 0.94), gain was reduced in the neutralized condition (0.54), in monochromatic light (0.43), and especially when LCA was reversed (0.30). The results suggest that accommodation responds to changes in the relative contrast of spectral components of the retinal image and perhaps to the vergence of light.

Accommodation responds to changing contrast of long, middle and short spectral-waveband components of the retinal image.

We simulated the effects of longitudinal (axial) chromatic aberration and defocus on contrast of the long-, middle- and short-wavelength components of the retinal image to determine whether the effects of chromatic aberration are sufficient to drive accommodation. Accommodation was monitored continuously while subjects (12) viewed a 3 c/deg white sine-wave grating (0.92 contrast) in a Badal stimulus system. The contrasts (amplitudes) of the red, green and blue components of the white grating changed independently to simulate a grating oscillating from 1 D behind the retina to 1 D in front of the retina at 0.2 Hz. Subjects responded strongly to the chromatic simulation but poorly to a luminance control. The results support the hypothesis that focus is specified by the contrast of spectral-wavebands of the retinal image, and that conventional color mechanisms, monitoring chromatic contrast at luminance borders (1-8 c/deg), mediate the signals that specify dioptric vergence.

Spectral bandwidth and ocular accommodation

Previous studies have suggested that targets illuminated by monochromatic (narrow-band) light are less effective in stimulating the eye to change its focus than are black-white (broadband) targets. The present study investigates the influence of target spectral bandwidth on the dynamic accommodation response in eight subjects. The fixation target was a 3.5-cycle/deg square-wave grating illuminated by midspectral light of various bandwidths [10, 40, and 80 nm and white (CIE Illuminant B)]. The target was moved sinusoidally toward and away from the eye, and accommodation responses were recorded and Fourier analyzed. Accommodative gain increases, and phase lag decreases, with increasing spectral bandwidth. Thus the eye focuses more accurately on targets of wider spectral bandwidth. The visual system appears to have the ability to analyze polychromatic blur to determine the state of focus of the eye for the purpose of guiding the accommodation response.

Small Amounts of Chromatic Aberration Influence Dynamic Accommodation

The prevailing view of accommodation is that the eye changes focus to maximize luminance contrast by trial and error. Negative feedback is considered essential in this view because luminance contrast provides no directional information. Fincham proposed an alternate view in which longitudinal (axial) chromatic aberration (LCA) provides a directional stimulus for accommodation. For spatial frequencies above approximately 0.5 cpd contrast of the retinal image is different for long, middle, and short spectral waveband components of the image. We varied the amount of LCA in small steps (0.25 D) to determine how much LCA is needed to enhance or impair the response. An infrared optometer monitored accommodation continuously while subjects viewed a yellow/ black square-wave grating (3.5 cpd) in a Badal stimulus system. The yellow/black grating was produced by superimposing red (600 nm) and green (520 nm) gratings, and LCA was increased, decreased, neutralized, and reversed by repositioning the red grating component along the axis of the optical system. Target vergence was modulated sinusoidally (0.2 Hz) over a 1 D range (1.5 to 2.5 D) and gain and phase-lag of the accommodation response were determined by Fourier analysis. Subjects accommodated well as long as a normal amount of LCA was present—0.5 D in the correct direction enhanced accommodative gain, and 0.25 D in the reverse direction markedly inhibited the response. We conclude that the contrast of the retinal image in different spectral wavebands specifies focus of the eye, and provides a powerful directional stimulus for reflex accommodation.

Accommodation to stationary and moving targets

Purpose. To test the hypothesis that the contrast of spectral components of the retinal image specifies ocular focus and controls reflex accommodation. Methods. Eight subjects viewed a stationary target at 0, 2.5, and 5 D in a Badal optometer, with longitudinal chromatic aberration (LCA) normal and reversed and in monochromatic (550 nm) light. Accommodation was monitored continuously during 40-s trials. Subjects also viewed the grating target as it moved sinusoidally (1.5 to 2.5 D) at 0.2 Hz under the same three conditions. Results. Subjects accommodated relatively accurately at all distances in the normal condition; three subjects had difficulty accommodating in monochromatic light at 5 or 0 D, and seven subjects could not maintain focus with LCA reversed. The accommodative response differed significantly in the three chromatic conditions both for stationary and moving targets. Conclusions. Relative contrast of long-, middle-, and short-wavelength components of the retinal image specifies ocular focus and drives reflex accommodation.

On the short-term variability of measurements of intraocular pressure

Measurement of intraocular pressure (IOP) (tonometry) is conducted routinely in the eye clinic for the diagnosis and management of primary open-angle glaucoma. Mean IOP may be influenced by a number of factors including ocular accommodation, spontaneous pulsations of IOP, and the effect of repeated applanation. A review of some of the factors that influence IOP should help us gain a perspective on the reliability of diurnal tonometry in the diagnosis of primary open-angle glaucoma.

Chromatic stimulus for accommodation to stationary and moving targets

Experiments using moving targets (sinusoidal and sum-of-sine motion) suggest that the longitudinal chromatic aberration (LCA) of the eye provides directional information at edges that specifies ocular focus and drives reflex accommodation (Kruger, Mathews, Aggarwala, Sanchez, 1993). Recently two studies used stationary targets to stimulate accommodation and both concluded that chromatic aberration has no effect on accommodation when the target is stationary (Bobier, Campbell and Hindi, 1992; Kotulak, Morse and Billock, 1995). The issue was addressed in three experiments: First, subjects (8) viewed a stationary target (3.5 cycles per degree square-wave grating) through a 3 mm artificial pupil at three dioptric stimulus levels (0 D, 2.5 D, 5 D) in a Badal stimulus system. There were 3 experimental conditions including Normal LCA, Reversed LCA and Monochromatic (550 nm; 10 nm bandwidth) and the resting position of accommodation was monitored while subjects viewed the grating target through a pinhole pupil. The target remained stationary during randomized 40-second trials, and accommodation was monitored continuously. Most subjects accommodated accurately in the normal condition at all three stimulus distances, a few subjects (3 of 8) had difficulty maintaining focus in monochromatic light at the near (5 D) or far (0) stimulus distances, and most subjects (7 of 8) could not maintain focus at both the near and far distances with LCA reversed. When the target is stationary it must be considerably closer or further away than the (tonic) resting level to demonstrate the effect of LCA.