Philip B. Kruger

Stimuli for accommodation: blur, chromatic aberration and size.

We investigated the frequency response of the accommodative system (0.05-1 Hz) using three stimuli: defocus blur, the effects of the chromatic aberration of the eye, and changing target size. A high-speed infrared optometer monitored accommodation while the subject viewed a target in a Badal optometer. Blur was provided by moving the target sinusoidally towards and away from the subject (1-3 D) and the size of the target was varied at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white light (3300 K). Gain and phase plots changed systematically as we varied the number of stimuli presented together. This suggests that besides defocus blur both chromatic aberration and changing size are involved in accommodative control.

Accommodation with higher-order monochromatic aberrations corrected with adaptive optics

Higher-order monochromatic aberrations in the human eye cause a difference in the appearance of stimuli at distances nearer and farther from best focus that could serve as a signed error signal for accommodation. We explored whether higher-order monochromatic aberrations affect the accommodative response to 0.5 D step changes in vergence in experiments in which these aberrations were either present as they normally are or removed with adaptive optics. Of six subjects, one could not accommodate at all for steps in either condition. One subject clearly required higher-order aberrations to accommodate at all. The remaining four subjects could accommodate in the correct direction even when higher-order aberrations were removed. No subjects improved their accommodation when higher-order aberrations were corrected, indicating that the corresponding decrease in the depth of field of the eye did not improve the accommodative response. These results are consistent with previous findings of large individual differences in the ability to accommodate in impoverished conditions. These results suggest that at least some subjects can use monochromatic higher-order aberrations to guide accommodation. They also show that some subjects can accommodate correctly when higher-order monochromatic aberrations as well as established cues to accommodation are greatly reduced.

Changing size (looming) as a stimulus to accommodation and vergence

Changing size (looming) produces changes in accommodation and vergence. Dynamic responses of vergence and accommodation to sinusoidal looming of a Maltese cross were recorded with an SRI dual-Purkinje-image eyetracker and optometer. The ratio of these two motor responses was compared with the response accommodative convergence/accommodation (AC/A) ratio and convergence accommodation/convergence (CA/A) ratio determined from sinusoidal variations of blur and disparity respectively. The response to changing size was found to be more similar to the AC/A ratio than the CA/C ratio. In addition, when a changing disparity or changing blur stimulus was combined with a changing size stimulus, the response phase lags of accommodation and vergence were decreased. In addition, the CA/C ratio was increased when changing size was added to changing disparity, but the AC/A ratio was unaltered when changing size was added to changing blur. These results indicate that changing size is stimulating accommodation directly and vergence secondarily through an AC/A crosslink.

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.

The effect of cognitive demand on accommodation.

Abstract An optometer (infrared recording retinoscope) was used to monitor the accommodation of subjects (N = 40, ages 20 to 30 years) to determine whether an increase in cognitive demand would result in an increase in the level of their accommodation. When the subjects in the experimental group (N = 20) changed from reading 2‐digit numbers at 40 cm to adding the numbers, with no change in the visual stimulus, the average level of accommodation increased in 75% of the subjects—the mean level of accommodation increased 0.28 D (P < 0.01). Subjects in the control group (N = 20) showed no significant change in accommodation. The increase in accommodation is attributed to an increase in cognitive demand.

Infrared recording retinoscope for monitoring accomodation.

An optometer (infrared recording retinoscope) that provides high resolution measurements of accommodative changes of the eye has been developed. The instrument employs the principle of retinoscopy to monitor changes in the refractive state of the eye. Infrared radiation is scanned across the eye to provide a moving retinal source as in retinoscopy. An electro-optical system replaces the observer in determining the direction and amount of apparent motion in the pupil of the patient. The optometer is sensitive to accommodative changes of 0.12 D over a range of 12 D. The response of the instrument is faster than the most rapid changes of accommodation, and measurements are unaffected by pupil changes or by small eye movements.

Dioptric and non-dioptric stimuli for accommodation: Target size alone and with blur and chromatic aberration

The frequency response of the accommodative system (0.05-1 Hz) was determined for various combinations of stimuli: changing target size was presented alone, together with defocus blur, and with both defocus blur and chromatic aberration. A high-speed infrared optometer monitored accommodation while the subject viewed the target in a Badal optometer. Target size was varied sinusoidally and blur was provided by moving the target towards and away from the subject at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white (3300 K) light. The target was presented under open-loop conditions when size was the only stimulus. We find that besides the conventional dioptric stimuli, changes in target size that result in changes in apparent distance can have substantial effects on accommodation.

Changing target size is a stimulus for accommodation

Accommodation was monitored by using a high-speed infrared optometer while subjects viewed a target that appeared to approach and recede in a sinusoidal manner. The target was presented under open-loop conditions to prevent blurring because of accommodation. The experiments suggest that changing target size can be an effective stimulus on its own. This supports the view that accommodation responds to both dioptric and nondioptric stimuli.

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.

Accommodation to static chromatic simulations of blurred retinal images

The eye’s longitudinal chromatic aberration (LCA) is known to drive ‘reflex’ accommodation to moving objects, but the evidence is not as clear for stationary objects. The present study examined whether accommodation can be driven by static simulations of the effects of defocus and LCA. Accommodation was recorded continuously while each of 12 subjects viewed images (through a 0.75 mm pinhole) that simulated the appearances of blurred sine wave gratings (3.9 c.p.d.). In two experimental conditions, an eye with normal LCA was assumed and defocus of +1 D or −1 D was simulated. In a control condition, an eye with neutralised LCA was assumed and target defocus of 1 D was simulated. Subjects’ accommodation responses were consistent with the hypothesis that LCA provides a stimulus to accommodation. Chromatic aberration drives accommodation to both moving and stationary objects, and thus is an important stimulus for accommodation in everyday situations. The study findings are discussed in relation to colour vision, visual display terminals and emmetropization.