Aberrations and accommodation

Modern methods of measuring the refractive state of the eye include wavefront sensors which make it possible to monitor both static and dynamic changes of the ocular wavefront while the eye observes a target positioned at different distances away from the eye. In addition to monitoring the ocular aberrations, wavefront refraction methods allow measurement of the accommodative response while viewing with the eye's habitual chromatic and monochromatic aberrations present, with these aberrations removed, and with specific aberrations added or removed. A large number of experiments describing the effects of accommodation on aberrations and vice versa are reviewed, pointing out the implications for fundamental questions related to the mechanism of accommodation.

Effect of phenylephrine on static and dynamic accommodation

PURPOSE

We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics.

METHODS

We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size.

RESULTS

When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p<0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were -0.20±0.51 D, and -0.05±0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p>0.313).

CONCLUSIONS

The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation.

Human eyes do not need monochromatic aberrations for dynamic accommodation

Purpose

To determine if human accommodation uses the eye's own monochromatic aberrations to track dynamic accommodative stimuli.

Methods

Wavefront aberrations were measured while subjects monocularly viewed a monochromatic Maltese cross moving sinusoidally around 2D of accommodative demand with 1D amplitude at 0.2 Hz. The amplitude and phase (delay) of the accommodation response were compared to the actual vergence of the stimulus to obtain gain and temporal phase, calculated from wavefront aberrations recorded over time during experimental trials. The tested conditions were as follows: Correction of all the subject's aberrations except defocus (C); Correction of all the subject's aberrations except defocus and habitual second‐order astigmatism (AS); Correction of all the subject's aberrations except defocus and odd higher‐order aberrations (HOAs); Correction of all the subject's aberrations except defocus and even HOAs (E); Natural aberrations of the subject's eye, i.e., the adaptive‐optics system only corrected the optical system's aberrations (N); Correction of all the subject's aberrations except defocus and fourth‐order spherical aberration (SA). The correction was performed at 20 Hz and each condition was repeated six times in randomised order.

Results

Average gain (±2 standard errors of the mean) varied little across conditions; between 0.55 ± 0.06 (SA), and 0.62 ± 0.06 (AS). Average phase (±2 standard errors of the mean) also varied little; between 0.41 ± 0.02 s (E), and 0.47 ± 0.02 s (O). After Bonferroni correction, no statistically significant differences in gain or phase were found in the presence of specific monochromatic aberrations or in their absence.

Conclusions

These results show that the eye's monochromatic aberrations are not necessary for accommodation to track dynamic accommodative stimuli.

Accommodation to wavefront vergence and chromatic aberration

PURPOSE

Longitudinal chromatic aberration (LCA) provides a cue to accommodation with small pupils. However, large pupils increase monochromatic aberrations, which may obscure chromatic blur. In this study, we examined the effect of pupil size and LCA on accommodation.

METHODS

Accommodation was recorded by infrared optometer while observers (nine normal trichromats) viewed a sinusoidally moving Maltese cross target in a Badal stimulus system. There were two illumination conditions: white (3000 K; 20 cd/m) and monochromatic (550 nm with 10 nm bandwidth; 20 cd/m) and two artificial pupil conditions (3 and 5.7 mm). Separately, static measurements of wavefront aberration were made with the eye accommodating to targets between 0 and 4 D (COAS, Wavefront Sciences).

RESULTS

Large individual differences in accommodation to wavefront vergence and to LCA are a hallmark of accommodation. LCA continues to provide a signal at large pupil sizes despite higher levels of monochromatic aberrations.

CONCLUSIONS

Monochromatic aberrations may defend against chromatic blur at high spatial frequencies, but accommodation responds best to optical vergence and to LCA at 3 c/deg where blur from higher order aberrations is less.

Dynamic accommodation with simulated targets blurred with high order aberrations

High order aberrations have been suggested to play a role in determining the direction of accommodation. We have explored the effect of retinal blur induced by high order aberrations on dynamic accommodation by measuring the accommodative response to sinusoidal variations in accommodative demand (1-3D). The targets were blurred with 0.3 and 1mum (for a 3-mm pupil) of defocus, coma, trefoil and spherical aberration. Accommodative gain decreased significantly when 1-mum of aberration was induced. We found a strong correlation between the relative accommodative gain (and phase lag) and the contrast degradation imposed on the target at relevant spatial frequencies.

Potential signal to accommodation from the Stiles-Crawford effect and ocular monochromatic aberrations

The purpose of this study is to determine if cues within the blurred retinal image due to the Stiles-Crawford (SC) effect and the eye's monochromatic aberrations can drive accommodation with a small pupil (3 mm) that is typical of bright photopic conditions.The foveal, psychophysical SC function (17 min arc) and ocular monochromatic aberrations were measured in 21 visually normal adults. The retinal image of a 10.2 min arc disc was simulated for spherical defocus levels of -1 D, 0 D and +1 D in each of four conditions consisting of combinations of the presence or absence of the individual SC function and monochromatic aberrations with a 3 mm pupil. Accommodation was recorded in eleven participants as each viewed the simulations through a 0.75-mm pinhole.The SC effect alone did not provide a significant cue to accommodation. Monochromatic aberrations provided a statistically significant but rather small cue to monocular accommodation.

Effect of third-order aberrations on dynamic accommodation

We investigate the potential for the third-order aberrations coma and trefoil to provide a signed cue to accommodation. It is first demonstrated theoretically (with some assumptions) that the point spread function is insensitive to the sign of spherical defocus in the presence of odd-order aberrations. In an experimental investigation, the accommodation response to a sinusoidal change in vergence (1-3D, 0.2Hz) of a monochromatic stimulus was obtained with a dynamic infrared optometer. Measurements were obtained in 10 young visually normal individuals with and without custom contact lenses that induced low and high values of r.m.s. trefoil (0.25, 1.03 microm) and coma (0.34, 0.94 microm). Despite variation between subjects, we did not find any statistically significant increase or decrease in the accommodative gain for low levels of trefoil and coma, although effects approached or reached significance for the high levels of trefoil and coma. Theoretical and experimental results indicate that the presence of Zernike third-order aberrations on the eye does not seem to play a crucial role in the dynamics of the accommodation response.

Cone contributions to signals for accommodation and the relationship to refractive error

The accommodation response is sensitive to the chromatic properties of the stimulus, a sensitivity presumed to be related to making use of the longitudinal chromatic aberration of the eye to decode the sign of the defocus. Thus, the relative sensitivity to the long- (L) and middle-wavelength (M) cones may influence accommodation and may also be related to an individual's refractive error. Accommodation was measured continuously while subjects viewed a sine wave grating (2.2c/d) that had different cone contrast ratios. Seven conditions tested loci that form a circle with equal vector length (0.27) at 0, 22.5, 45, 67.5, 90, 120, 145 deg. An eighth condition produced an empty field stimulus (CIE (x,y) co-ordinates (0.4554, 0.3835)). Each of the gratings moved at 0.2 Hz sinusoidally between 1.00 D and 3.00 D for 40s, while the effects of longitudinal chromatic aberration were neutralized with an achromatizing lens. Both the mean level of accommodation and the gain of the accommodative response, to sinusoidal movements of the stimulus, depended on the relative L and M cone sensitivity: Individuals more sensitive to L-cone stimulation showed a higher level of accommodation (p=0.01; F=12.05; ANOVA) and dynamic gain was higher for gratings with relatively more L-cone contrast. Refractive error showed a similar correlation: More myopic individuals showed a higher mean level of accommodation (p<0.01; F=11.42; ANOVA) and showed higher gain for gratings with relatively more L-cone than M-cone contrast (p=0.01; F=10.83 ANOVA). If luminance contrast is maximized by accommodation, long wavelengths will be imaged behind the photoreceptors. Individuals in whom luminance is dominated by L-cones may maximize luminance contrast both by accommodating more, as shown here, and by increased ocular elongation, resulting in myopia, possibly explaining the correlations reported here among relative L/M-cone sensitivity, refractive error and accommodation.

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.

Accommodation with and without short-wavelength-sensitive cones and chromatic aberration

Accommodation was monitored while observers (23) viewed a square-wave grating (2.2 cycles/deg; 0.53 contrast) in a Badal optometer. The grating moved sinusoidally (0.2 Hz) to provide a stimulus between -1.00 D and -3.00 D during trials lasting 40.96 s. There were three illumination conditions: 1. Monochromatic 550 nm light to stimulate long-wavelength-sensitive cones (L-cones) and medium-wavelength-sensitive cones (M-cones) without chromatic aberration; 2. Monochromatic 550 nm light+420 nm light to stimulate long-, medium- and short-wavelength-sensitive cones (S-cones) with longitudinal chromatic aberration (LCA); 3. Monochromatic 550 nm light+420 nm light to stimulate L-, M- and S-cones viewed through an achromatizing lens. In the presence of LCA mean dynamic gain decreased (p=0.0003; ANOVA) and mean accommodation level was reduced (p=0.001; ANOVA). The reduction in gain and increased lag of accommodation in the presence of LCA could result from a blue-yellow chromatic signal or from a larger depth-of-focus.

Small foveal targets for studies of accommodation and the Stiles–Crawford effect

The properties of small monochromatic targets as accommodative stimuli are not well understood. We used a dynamic optometer to record accommodation responses to monochromatic disc targets (1.0-27.3 min arc) and to a Maltese cross. Accommodation responded adequately to points as small as 13.6 min arc. The response to these small targets is relevant to the question of whether the Stiles-Crawford (SC) effect could provide a stimulus to accommodation. Previous studies have used pupil apodizing filters to neutralise the natural SC function and so determine how visual performance or accommodation is influenced by the SC effect. However, these filters cannot correct for known inhomogeneities in the SC function across the retina for extended targets. Therefore, we calculated the SC function inhomogeneities across the retinal image of a smaller 13.6-min arc target. Unfortunately, even this small target is too large to permit a homogenous SC function across its extent. Alternatives to the apodizing filter approach are discussed.

Accommodation responses to stimuli in cone contrast space

The aim was to identify the cone contributions and pathways for reflex accommodation. Twelve illumination conditions were used to test specified locations in cone-contrast space. Accommodation was monitored continuously in a Badal optometer while the grating stimulus (2.2 c/d sine-wave; 0.27 modulation) moved sinusoidally (0.195 Hz) towards and away from the eye from a mean position of 2.00 D (+/-1.00 D). Mean accommodation level and dynamic gain and phase at 0.195 Hz were calculated. Mean accommodation level varied significantly when the long- and middle-wavelength cone contrast ratio was altered in both the luminance and chromatic quadrants of cone-contrast space. This experiment indicates that L- and M-cones contribute to luminance and chromatic signals that produce the accommodation response, most likely through magno-cellular and parvo-cellular pathways, respectively. The L:M cone weighting to the luminance pathway that mediates accommodation is 1.63:1. The amplitude and direction of the response depends on changes in chromatic contrast and luminance contrast signals that result from longitudinal chromatic aberration and defocus of the image.

The role of short-wavelength sensitive cones and chromatic aberration in the response to stationary and step accommodation stimuli

The aim of the experiment was to test for a contribution from short-wavelength sensitive cones to the static and step accommodation response, to compare responses from short and long- plus middle-wavelength sensitive cone types, and to examine the contribution of a signal from longitudinal chromatic aberration to the accommodation response. Accommodation was monitored continuously (eight subjects) to a square-wave grating (2.2 c/d; 0.57 contrast) in a Badal optometer. The grating stepped (1.00 D) randomly towards or away from the eye from a starting position of 2.00 D. Five illumination conditions were used to isolate cone responses, and combine them with or without longitudinal chromatic aberration. Accuracy of the response before the step, step amplitude, latencies and time-constants, were compared between conditions using single factor ANOVA and t-test comparisons. Both S-cones and LM-cones mediated static and step accommodation responses. S-cone contrast drives "static" accommodation for near, but the S-cone response is too slow to influence step dynamics when LM-cones participate.

Accommodation to simulations of defocus and chromatic aberration in the presence of chromatic misalignment

Previous studies have demonstrated that accommodation will respond to sine gratings in which the relative modulations of red, green and blue image components have been altered to simulate the effects of defocus and longitudinal chromatic aberration. The present study aimed to determine the tolerance of the accommodative system to relative phase shifts in those components induced by chromatic misalignment. It was found that accommodation can tolerate moderate amounts of chromatic misalignment (6'), but responds adversely when misalignments are large. Applications to visual display terminals and spectacle lens and instrument design are discussed.

Accommodation and the Stiles-Crawford effect: theory and a case study

PURPOSE

Fincham (The accommodation reflex and its stimulus. Br. J. Ophthalmol. 35, 381-393) was the first to suggest that the Stiles-Crawford effect (Type I) might provide a stimulus for accommodation, but the possibility has not been investigated experimentally. The present paper outlines a theoretical basis for such a mechanism, and includes a case study on a subject with a nasally decentred Stiles-Crawford (S-C) function.

METHODS

Accommodation to a monochromatic sine grating was monitored continuously with the natural S-C function intact, or with apodising filters imaged in the subject's pupil to neutralise, reverse or double the natural S-C function.

RESULTS

Mean accommodative gain was not reduced significantly when the normal S-C function was either neutralised or reversed.

CONCLUSIONS

For the present subject, the average S-C effect does not mediate the accommodation response to defocus, but more subjects should be examined. Other methods by which directionally sensitive cone receptors could detect light vergence are discussed.

Isolated short-wavelength sensitive cones can mediate a reflex accommodation response

Both long- and middle-wavelength sensitive cones mediate the reflex accommodation signal but the contribution from the short-wavelength sensitive cones is unknown. A short-wavelength sensitive cone contribution could extend the range of the signed defocus signal from chromatic aberration. The aim was to determine whether isolated short-wavelength sensitive cones mediate reflex accommodation independently of long- and middle-wavelength sensitive cones. Accommodation was monitored continuously (eight subjects) to a sine-wave grating (3 cpd; 0.53 contrast) moving with a sum of sines motion in a Badal optometer. Two illumination conditions were used: a 'blue' condition that isolated short-wavelength sensitive cones, and a 'white' control condition that stimulated all three cone types. Of the eight subjects, two responded equally in the 'white' and 'blue' condition, four gave reduced responses in the 'blue' condition and two failed to respond in both conditions. The mean response in the 'blue' condition was reduced by 50% compared to the 'white' condition. Further analysis indicated that four of the eight subjects gave responses that were considerably greater than noise (S.D.>1.82) when short-wavelength sensitive cones were isolated. Some subjects can accommodate using only S-cones.

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.

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 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.

Inhibitory effect of di-catechol rooperol on VCAM-1 and iNOS expression in cytokine-stimulated endothelium

Induced expression of vascular cell adhesion molecule-1 (VCAM-1) and of nitric oxide synthase (iNOS) is believed to play a role in the pathogenesis of atherosclerosis, asthma, as well as other inflammatory disorders. In the current study we examined the effect of the di-catechol rooperol [(E)-1,5-bis (3',4'-dihydroxyphenyl) pent-4-en-1-yne] on the process of microvascular endothelial cell (MME) activation by TNF-alpha and IFN-gamma. We show that rooperol decreases VCAM-1 and iNOS mRNA levels in cytokine-activated MME with subsequent inhibition of VCAM-1 membrane expression as measured by adhesion of P815 cells to MME monolayers, and NO production, as reflected in the nitrite concentration in culture medium. The properties of rooperol now described suggest that rooperol may be an anti-inflammatory agent useful in the treatment of several inflammatory disorders.

Cytokine production in human and rat macrophages and dicatechol rooperol and esters

The ability of dicatechol rooperol and esters to inhibit the production of cytokines in endotoxin-stimulated human alveolar macrophages, human blood monocyte/macrophages, histiocytic cell line U937, and rat alveolar macrophages was examined in vitro. Rooperol derivatives inhibited the production of tumour necrosis factor-alpha, interleukin-1 beta and interleukin-6. Of the esters tested on human cells, rooperol diacetate and tetraacetate were more potent inhibitors of cytokine production (IC50 in the range of 10-20 microM) than rooperol disulphate (IC50 in the range of 25-75 microM). The acetate esters also inhibited cytokine production in rat alveolar macrophages, whereas the sulphate had little effect. Rooperol and acetate esters, in the same concentration range, decreased the production of nitric oxide by rat alveolar macrophages stimulated by endotoxin. These concentrations of rooperol had no effect on cell viability, as indicated by incorporation of 14C-labelled leucine into macrophage proteins and their content of lactate dehydrogenase. The results obtained suggest that rooperol esters are potentially useful antiinflammatory agents.

Pharmacokinetic behaviour and cardiovascular effects of intravenously administered hypoxoside and rooperol

This study concerns the pharmacokinetic behaviour and cardiovascular effects of rapid infusions of hypoxoside (CAS 83643-94-1) and rooperol (CAS 83644-00-2) in anaesthetised Chacma baboons. Institutional approval was obtained and animal care conformed to international guidelines. Hypoxoside (500 mg) and rooperol (240 mg) dissolved in isotonic saline were infused during 15 min. Concentration-time data from high performance liquid chromatography of arterial blood samples were subjected to non-linear curve-fitting to obtain two-compartment mammillary pharmacokinetic models. Mean values were: [Table: see text] Hypoxoside was eliminated without significant metabolite formation and it revealed no cardiovascular effects. Rooperol was metabolized rapidly with formation of nine metabolites of which the major three were the diglucuronide, disulphate and mixed glucuronide sulphate. Rooperol caused moderate, transient increased cardiac output, stroke volume and vascular pressures without increased heart rate or filling pressures, suggestive of increased myocardial contractility probably allied to its catechol structure.

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 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.

Morphological characterisation of the cell-growth inhibitory activity of rooperol and pharmacokinetic aspects of hypoxoside as an oral prodrug for cancer therapy

Hypoxoside is the major diglucoside isolated from the corms of the plant family Hypoxidaceae. It contains an unusual E-pent-1-en-4-yne 5-carbon bridging unit with two distal catechol groups to which the glucose moieties are attached. It is non-toxic for BL6 mouse melanoma cells in tissue culture on condition that the fetal calf serum in the medium is heat-inactivated for 1 hour at 56 degrees C in order to destroy endogenous beta-glucosidase activity. The latter catalyses hypoxoside conversion to its cytotoxic aglucone, rooperol, which, when tested as a pure chemical, caused 50% inhibition of BL6 melanoma cell growth at 10 micrograms/ml. Light and electron microscopy revealed that the cytotoxic effect of rooperol manifested as vacuolisation of the cytoplasm and formation of pores in the plasma membrane. Indications of apoptosis were also found. Pharmacokinetic studies on mice dosed intragastrically with hypoxoside showed that it was deconjugated by bacterial beta-glucosidase to form rooperol in the colon. Surprisingly, no hypoxoside or rooperol was detectable in the serum. Only phase II biotransformation products (sulphates and glucuronides) were present in the portal blood and bile. In contrast, however, in human serum after oral ingestion of hypoxoside, the metabolites can reach relatively high concentrations. Rooperol metabolites isolated from human urine were non-toxic for BL6 melanoma cells in culture up to a concentration of 200 micrograms/ml. In the presence of beta-glucuronidase, which released rooperol from the metabolites, 50% growth inhibition was achieved at a 75 micrograms/ml metabolite concentration. The supernatant of a human melanoma homogenate could also cause deconjugation of the metabolites to form rooperol.(ABSTRACT TRUNCATED AT 250 WORDS)

A phase I trial of hypoxoside as an oral prodrug for cancer therapy--absence of toxicity

OBJECTIVE

To assess the toxicity of hypoxoside taken orally by 24 patients with lung cancer.

DESIGN

Open study with patients taking 1,200-3,200 mg standardised Hypoxis plant extract (200 mg capsules) per day divided in 3 doses in order to maintain metabolite blood levels near 100 micrograms/ml.

PARTICIPANTS AND SETTING

Patients with histologically proven squamous, large-cell or adenocarcinoma were hospitalised initially at the radiation oncology ward, Karl Bremer Hospital, Bellville, W. Cape. Thereafter they returned every 2 weeks for full clinical examinations.

METHODS

Routine biochemical and haematological measurements were done. Patients underwent regular full clinical examinations including radiographs and computed tomography scanning according to the discretion of the principal investigator.

RESULTS

Nineteen patients on hypoxoside therapy survived for an average of 4 months with progression of their primary tumours and metastases, while 5 survived for more than a year. One of them survived for 5 years and histological examination of the primary lesion showed absence of cancer. No toxic effects, in clinical examinations or biochemical or haematological measurements, were found that could be ascribed to the ingestion of hypoxoside. Only one occasion of possible drug intolerance, with anxiety, nausea, vomiting and diarrhoea, was noted.

CONCLUSION

The absence of toxicity warrants further investigation of hypoxoside as an oral prodrug, especially in patients with slow-growing necrotising tumours that are inoperable and have high concentrations of beta-glucuronidase and sulphatase as well as a high sensitivity for rooperol.

The pharmacokinetic behaviour of hypoxoside taken orally by patients with lung cancer in a phase I trial

OBJECTIVE

To study the pharmacokinetic behaviour of hypoxoside taken orally by 24 patients with lung cancer.

DESIGN

Randomised open study with three single doses of 1,600, 2,400 and 3,200 mg standardised Hypoxis plant extract (200 mg capsules) and a multiple-dose study on the first 6 patients taking 4 capsules 3 times daily for 11 days.

PARTICIPANTS AND SETTING

Patients with histologically proven squamous, large-cell or adenocarcinoma were hospitalised at the Radiation Oncology Ward, Karl Bremer Hospital, Bellville, W. Cape.

METHODS

Blood was drawn at regular intervals up to 75 hours after single doses and the concentrations of metabolites of the aglucone of hypoxoside, rooperol, were measured with a high-performance liquid chromatography method. For the multiple-dose study blood was drawn before the first dose each day. Concentration-time relationships were analysed according to a conventional single open-compartment model and also by using the NONMEM digital computer programme.

RESULTS

Neither hypoxoside nor rooperol appear in circulation. This is due to complete phase II biotransformation to diglucuronide, disulphate and mixed glucuronide-sulphate metabolites, of which the latter is the major component. Considerable interpatient variation in concentration-time relationships was found in the single-dose studies. It was due to an active enterohepatic recirculation in some patients and a distinct lag phase in others together with zero-order rate of formation of rooperol in the colon. Computer modelling indicated a single open-compartment model in which the mass of the patient did not influence volume of distribution and clearance because formation of the metabolites is dependent on the metabolising capacity of the patient. However, the elimination of the metabolites follows first-order kinetics with half-lives ranging from 50 hours for the major metabolite to 20 hours for the two minor metabolites. Multiple-dose studies also showed large interpatient variation.

CONCLUSION

In order to reach metabolite levels near 100 micrograms/ml, which have been shown to be tumouricidal after enzymatic deconjugation to rooperol, maintenance doses need to be individualised for each patient. For most patients, however, a daily dose of 2,400 mg was sufficient.

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.

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.

Studies on hypoxoside and rooperol analogues from Hypoxis rooperi and Hypoxis latifolia and their biotransformation in man by using high-performance liquid chromatography with in-line sorption enrichment and diode-array detection

Methanol extracts of the corms of Hypoxis rooperi and H. latifolia were studied for their hypoxoside content by an in-line sorption enrichment HPLC technique [Kruger et al., J. Chromatogr., 612 (1993) 191]. Hypoxoside is the trivial name for (E)-1,5-bis(3'-hydroxy-4'-O-beta-D-glucopyranosyl-phenyl) pent-1-en-4-yne and rooperol the aglucone obtained from beta-glucosidase treatment. Hypoxoside and rooperol analogues containing 4, 3 and 2 hydroxyl groups resolved as separate peaks with the proportion of the latter two markedly higher in H. latifolia than in H. rooperi. After oral ingestion of hypoxoside by humans, no hypoxoside or rooperol appeared in the serum. Only rooperol was present in the faeces. The serum and urine contained at least three phase II metabolite peaks. Selective enzyme hydrolysis showed that they represent the diglucuronide, disulfate and glucuronide-sulfate conjugates of all three rooperol analogues.

Spatiotemporal transfer function of human accommodation

We have investigated the spatiotemporal transfer function of human "reflex" accommodation. An accommodative mechanism that is sensitive to an intermediate temporal rate of retinal image contrast change is proposed as the basis of the fine focus control hypothesis. To test the proposed mechanism accommodative responses were monitored by a dynamic infrared optometer while the subject focused on sinusoidal gratings (0.98-10.5 c/deg) which were moving sinusoidally at temporal frequencies in the range of 0.05-0.80 Hz over a 0.50 or 2.00 D peak-to-peak amplitude. The accommodative responses were best at 3 and 5 c/deg at both amplitudes of target motion. This result does not support the proposed mechanism or the fine focus control hypothesis for "reflex" accommodation. Fitting the data with first-order response functions showed little evidence of prediction. In addition, a second experiment found that the profile of the accommodative gain function is not altered by instruction at spatial frequencies above 5 c/deg in this type of dynamic accommodation experiment. The use of sinusoidally moving accommodative blur targets, particularly with careful instruction, seems to discourage voluntary accommodation in investigations of "reflex" control mechanisms of accommodation.

beta-Glucosidase activity in fetal bovine serum renders the plant glucoside, hypoxoside, cytotoxic toward B16-F10-BL-6 mouse melanoma cells

By using p-nitrophenyl-beta-D-glucopyranoside as substrate, beta-glucosidase activity was observed in fetal bovine serum (FBS). This activity could be inhibited by heat inactivation of the serum. Gel chromatography of FBS indicated the presence of beta-glucosidase activity with an apparent molecular mass of 29 kDa. In McCoy's 5A medium supplemented with non-heat inactivated FBS, the diglucoside hypoxoside ([E]-1,5-bis[4'beta-D-glucopyranosyloxy-3'-hydroxyphenyl]pent-4-en - 1-yne) showed cytotoxicity toward B16-F10-BL-6 mouse melanoma cells. In incubations where the media were supplemented with FBS previously heat inactivated at 56 degrees C for 1 h or more, no cytotoxicity was observed in the presence of hypoxoside. The aglucone of hypoxoside, rooperol ([E]-1,5-bis[3',4'-dihydroxyphenyl]pent-4-en-1-yne), showed cytotoxicity regardless of whether the serum was heat inactivated or not. The kinetics of the heat inactivation of the beta-glucosidase activity in FBS coincided with the loss of apparent cytotoxicity of hypoxoside. High performance liquid chromatography analysis showed that rooperol could be generated by incubation of hypoxoside in non-heat inactivated FBS, but that this ability was lost in serum that was heat inactivated for 1 h or longer. Newborn bovine serum did not contain any beta-glucosidase activity whereas it was found in three different commercial sources of FBS. This observation is of practical importance because conventional heat inactivation of FBS at 56 degrees C for 30 min was not sufficient to inactivate the beta-glucosidase activity completely.

Therapeutic monitoring of antituberculosis drugs by direct in-line extraction on a high-performance liquid chromatography system

A direct in-line pre-column extraction technique in which guanidinium and ammonium sulfate are used, followed by column switching, was employed to analyze serum, plasma and cerebrospinal fluid samples of patients treated for tuberculous meningitis. Resolution of a wide range of polar to non-polar xenobiotics was obtained on a C8 silica column by using a linear gradient from a binary system consisting of solvent A (0.05 M KH2PO4) and solvent B (acetonitrile-isopropanol, 4:1, v/v). Apart from the antituberculosis drugs (isoniazid, pyrazinamide, ethionamide and rifampicin) the patients received up to sixteen different medicines for prevention of complications and the treatment of symptoms. Qualitative resolution of all the drugs was obtained by the chromatographic system. Quantitation of pyrazinamide and ethionamide was achieved with high precision and low inter-sample variation.

Accommodation and chromatic aberration: effect of spatial frequency

When subjects view an edge in white light, a colour fringe, produced by longitudinal chromatic aberration (LCA) of the eye, is formed at the edge. The colour fringe changes with changes in focus, and serves as a complex colour-coded cue for reflex accommodation. Fincham found that 60% of his subjects failed to accommodate appropriately when the colour fringe was removed with an achromatizing lens or by the use of monochromatic light. Our experiment sought to determine the spatial frequencies at which LCA is most effective. We monitored accommodation in 10 subjects while they viewed sinusoidally moving sine-wave gratings (1-3 D at 0.2 Hz; 1-10.5 c/deg) in a Badal optometer. The targets were 'white' gratings with LCA normal, doubled, neutralized or reversed. Doubling the aberration has minimal effect, removing the aberration reduces gain and increases phase-lag, and reversing the aberration severely disrupts accommodation. Sensitivity to these chromatic cues exists at all spatial frequencies tested, but is most prominent between 3 and 5 c/deg. These results support the view that the system monitors focus by comparing contrast in red-green and perhaps blue-yellow colour-opponent mechanisms.

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.

Use of guanidine hydrochloride and ammonium sulfate in comprehensive in-line sorption enrichment of xenobiotics in biological fluids by high-performance liquid chromatography

A novel approach has been developed for direct injection of physiological fluids on an in-line extraction pre-column followed by column switching in order to introduce the adsorbed xenobiotic onto the analytical column. The physiological fluid is pre-treated with guanidinium solution in water (200 microliters of fluid plus 300 microliters of a reagent containing 8.05 M guanidinium and 1.02 M ammonium sulfate) in order to denature protein binding sites and to serve as a universal solvent for a divergent range of polar to non-polar xenobiotics in a hydrophilic medium. A 0.5 M ammonium sulfate solution (500 microliters) is used as a pre- and post-flush reagent for the extraction pre-column (30 mm x 2.1 mm I.D.). The pre-flush reagent prepares the sorbent environment of the C18 pre-column for the hydrophobic retention of analytes. The post-flush reagent flushes non-retained sample proteins and salts to waste prior to switching the pre-column in-line with the analytical column. Universal chromatographic conditions for the analytical phase allows elution of a range of polar to non-polar xenobiotics within 20 min from an end-capped C8 silica analytical column (250 mm x 4.6 mm I.D.). This is effected by a linear gradient from a binary system consisting of solvent A (0.05 M KH2PO4) and solvent B (acetonitrile-isopropanol, 80:20, v/v).

Accommodation to size and blur changing in counterphase

We explored the influence of changing size and blur on accommodation by presenting the two stimuli sinusoidally in counterphase. The frequency response of the accommodative system (0.05 to 1 Hz) was determined using a high-speed infrared optometer while the subject viewed the target in a Badal optometer. Blur was provided by moving the target dioptrically toward and away from the subject, and size of the target was altered by a variable aperture. Both stimuli were varied sinusoidally at the same frequency, but in counterphase. We find that both size and blur can have an influence on accommodation: blur is particularly powerful at low temporal frequencies, whereas size becomes effective at moderate and high temporal frequencies.

Dynamics of human voluntary accommodation

Voluntary and reflex accommodation were measured monocularly in five normal subjects using a dynamic infrared optometer. Comparison of the peak velocity/amplitude relation (i.e., "main sequence") showed complete overlapping of the response distributions, suggesting similarity of motoneuronal controller signals for voluntary and reflex accommodation. Voluntary accommodation may represent a preprogrammed maneuver used in a variety of real-life predictable situations to optimize performance. Clinically, it may represent an important component in the training of accommodation dynamics.

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.

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.

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 dynamics in divergence excess exotropia

Objective recording of accommodation dynamics was performed in four patients with divergence excess exotropia (two true and two simulated) and in three visually normal control subjects. The accommodative peak velocity/amplitude relationship was normal in the exotropic patients and in the control subjects. Latency for decreasing accommodation was increased significantly, and overshoot frequency was decreased, in the divergence excess exotropes of the true variety, thus demonstrating slightly slowed accommodation dynamics in this diagnostic group. These subtle deviations in response dynamics reflect central rather than peripheral mechanisms.

Stimulus and response AC/A ratios in intermittent exotropia of the divergence-excess type

Objective infrared recording devices were used to measure simultaneously and continuously both accommodation and accommodative vergence to near stimuli in 4 subjects with intermittent exotropia of the divergence-excess type (2 simulated and 2 true). In addition standard clinically determined stimulus accommodative convergence to accommodation (AC/A) ratios were measured. Results showed the mean group response AC/C ratio to be 5.9/1 (range 4.5-8.0/1) with no differences between true and simulated divergence-excess. Similar AC/A ratios were found after 45 minutes of monocular occlusion. Our results clearly demonstrate relatively normal response AC/A ratios in these subjects. Thus, contrary to what is believed by many clinicians, the reduced ocular deviation at near compared with distance vision cannot be attributed primarily to an abnormally high AC/A ratio. We believe than fusional convergence after-effects and/or proximal convergence effects contribute to inflate the clinically determined stimulus AC/A ratios.

The effect of cognitive demand on accommodation

An optometer (infared 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 damand.

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.

The effect of accommodative changes on the brightness of the fundus reflex

This paper demonstrates how accommodation and refractive changes affect the brightness of the reflex observed during retinoscopy. The amount of light entering the peephole of the retinoscope was monitored while accommodation was varied. The light entering the peephole reaches a maximum when the far-point of the eye is co-incident with the retinoscope and drops when the far-point moves behind or in front of the retinoscope. A reduction in the accommodative lag during book retinoscopy would result in brightening of the reflex along with a shift in the "against" direction.

Brightness changes during book retinoscopy

Book retinoscopy involves the observation of fundus reflex changes that are said to be related to the reading levels or interpretive levels at which persons are functioning. Recent investigations have shown that increasing the cognitive demand on a person can result in accomodative changes in his eye that increase the luminance of the fundus reflex. Since the ability of observers to distinguish such changes is not known, this experiment was designed to determine the minimum detectable increase in luminance. The experiment involved a 2-alternative forced-choice procedure in which observers were required to indicate during which of 2 designated intervals the reflex from a schematic eye became brighter. The luminance of the reflex had to increase by at least 11% beforeit could be detected. A reduction in the lag of accommodation and dilation of the pupil probably account for the changes observed during book retinoscopy.

Changes in fundus reflex luminance with increased cognitive processing

A previous study using photoelectric retinoscopy at near showed an increase in fundus reflex luminance when the subject's task was changed from reading simple material to adding numbers. Unfortunately, it was not possible to attribute the luminance change to the change in cognitive task. The present experiment was designed to determine whether an increase in cognitive processing would produce an increase in fundus reflex luminance. When the subjects (N = 19, ages 22 to 27) changed from viewing 2-digit numbers at 40 cm to adding the digits, with no change in the visual stimulus, the increased in 47 of 54 trials--the mean luminance increased approximately 10% (p less than 0.01). This change in fundus reflex luminance is not attributed to change in eye position but rather to a decrease in accommodative lag.

Luminance changes of the fundus reflex

A recording retinoscope was developed to measure the luminance of the fundus reflex during problem-solving tasks. 10 students were used as subjects. The luminance of the reflex was recorded while the subjects read 5 passages of graded reading material and performed an addition task. A reversal design was used to determine the effects of pupil changes and accommodation on changes in the luminance of the reflex. Relatively large changes in the luminance of the reflex were recorded. These were due to accommodation. The subjects' comprehension of the reading material did not appear to affect the luminance of the reflex in the manner predicted by previous studies. However, a change in task from simple reading material to an addition task appeared to increase the luminance of the reflex.