Biomedical subjects

K R Aggarwala

Publications and source records attributed to K R Aggarwala.

9 recordsLinked to original sources

PubMed1997

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, Ocular↗

PubMed1997

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.

Accommodation, Ocular↗

PubMed1995

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, Ocular↗

PubMed1995

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, Ocular↗

PubMed1995

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.

Accommodation, Ocular↗

PubMed1995

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.

Accommodation, Ocular↗

PubMed1995

Accommodation to monochromatic and white-light targets.

PURPOSE: The objective of the current study was to compare accommodation to targets illuminated with monochromatic light from different regions of the visible spectrum with accommodation to white-light targets. METHODS: One of 10 marrow-band interference filters (430, 450, 470, 500, 530, 550, 570, 590, 630, and 670 nm) was used to produce monochromatic light from a tungsten-halogen source to illuminate a Maltese cross-target in Maxwellian view. Luminance of each monochromatic light was matched by minimum border photometry against a standard white light (3000 K) that was maintained at 200 cd/m2. Chromatic difference of focus of the eye was minimized for all monochromatic targets by the use of an achromatizing lens. A white-light target also was used, and the subject's eye was achromatized or the eye had normal chromatic aberration. The target was moved sinusoidally toward and away from the eye at a temporal frequency of 0.2 Hz over a 1 D amplitude (peak to peak). Accommodation was monitored continuously by an infrared recording optometer, and responses were Fourier analyzed to obtain gain and phase lag at the temporal frequency of stimulation. RESULTS: Accommodative gain was highest and phase lag was smallest when the target was illuminated by white light in the presence of normal chromatic aberration. The achromatized white-light gain of accommodation was statistically similar to the gain for monochromatic targets, indicating that the presence of chromatic aberration facilitates accommodation. Significant intersubject variability was present in the accommodative tracking ability to monochromatic targets. CONCLUSIONS: Accommodation to monochromatic targets is not as accurate as accommodation to a white-light target, and this effect is related to the presence of ocular longitudinal chromatic aberration for the white-light target.

Accommodation, Ocular↗

PubMed1995

Do changes in pupil size and ambient illumination affect the duochrome test?

Vague and conflicting suggestions currently exist within the literature regarding the appropriate level of ambient illumination to use while conducting the clinical duochrome (bichrome) test. One proposal is that reduced illumination should be adopted since this allows the pupils to dilate, thereby increasing the chromatic interval and resultant test sensitivity. The aim of the present study was to determine whether changes in pupil size and ambient illumination do indeed alter the subjective duochrome response. Accordingly, eight young, visually normal subjects were dilated with 2.5 percent phenylephrine and tested while monocularly viewing the duochrome chart through 1, 3, 5 and 7mm artificial pupils. Additionally, the effects of performing the test through a 1.0 log unit neutral density filter and varying the overall room illumination were examined. There were no significant differences between the mean responses under all test conditions. The variability of individual responses decreased with increasing pupil size; however, a 4mm change in pupil diameter was required to alter the response variability significantly. Therefore it is recommended that the test be performed under the minimum practical ambient illumination conditions in order to minimize this variability.

Accommodation, Ocular↗

PubMed1993

Chromatic aberration and ocular focus: Fincham revisited.

Longitudinal chromatic aberration of the eye (LCA) produces "color fringes" at edges that specify focus. Fincham [(1951) British Journal of Ophthalmology, 35, 381-393] concluded that these chromatic effects were important for accommodation, but most investigators disagree. We monitored accommodation in 25 subjects while they viewed a sinusoidally moving target (1.5-2.5 D at 0.2 Hz) in a Badal optometer. The target was monochromatic (590 nm with 10 nm bandwidth), or white (3000 K) with LCA normal, neutralized or reversed. Sensitivity to the effects of LCA is profound and widespread. Gain decreases substantially and phase-lag increases when LCA is eliminated, and reversing the aberration severely disrupts accommodation. The ordered arrangement of spectral foci produced by LCA seems to be a fundamental aspect of the stimulus for "reflex" accommodation.

Accommodation, Ocular↗