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B Gilmartin

Publications and source records attributed to B Gilmartin.

At least 19 recordsLinked to original sources
PubMed1998

The magnitude and distribution of open-loop accommodation using three different methods of opening the loop.

PURPOSE: The present study examines the magnitude, distribution, and relationship of open-loop accommodation obtained using the three most common methods of opening the accommodation loop. METHODS: Open-loop accommodation was measured in 93 young, emmetropic subjects using a Canon R1 objective infrared optometer, and the accommodation loop was opened using the following methods: (1) dark empty field (DA), (2) bright empty field (BA), and (3) viewing a target through a small artificial pupil (PA). RESULTS: PA was found to elicit significantly (p = 0.0001) higher values of open-loop accommodation than either DA or BA and demonstrated a much wider distribution of values than DA or BA. A further experiment demonstrated that the higher PA was attributable to the proximal effect of placing a small artificial pupil close to the eye. CONCLUSIONS: Our results suggest that using a small artificial pupil to open the accommodation loop may not produce a veridical measure of open-loop accommodation.

Accommodation, Ocular↗
PubMed1995

The effect of topical beta-adrenoceptor antagonists on accommodation in emmetropia and myopia.

Using topical instillation of non-selective and selective beta-adrenoceptor antagonists we examine the proposal that a deficit in inhibitory sympathetic innervation of ciliary smooth muscle may be a specific precursor to the onset and development of late-onset myopia (LOM). Post-task accommodative hysteresis, a possible consequence of such a deficit, was assessed by measuring the time-course of regression of accommodation when open-loop (darkness) conditions were immediately imposed following far and near tasks. For the proposal to be feasible only LOMs should exhibit post-task responses which fail to differentiate the type of beta-antagonist employed. As the overall profile of responses to beta-adrenoceptor antagonism was equivalent for each of the three different refractive groups (emmetropes N = 6; early-onset myopes N = 5; LOMs N = 5) we conclude that a propensity to LOM is not associated with a deficit in sympathetic inhibition.

Accommodation, Ocular↗
PubMed1995

Computation of retinal contour in anisomyopia.

It is well documented that myopia is associated with an increase in axial length of the posterior vitreous chamber. Whether equatorial or transverse dimensions are likewise affected in myopia is relevant to further understanding of the development of ametropia. We have utilised a computing method to determine retinal contour from real eye measurements of keratometry, A-scan ultrasonography and peripheral refraction as a means of assessing the transverse dimensions of the vitreous chamber. This technique has been applied to a 21-year-old female Caucasian anisomyope with a refractive error of R -1.50/-0.50 x 130 and L -4.00/-0.50 x 160. Anisomyopia offers a special opportunity for inter-eye comparison of different degrees of myopia. The repeatability of the technique was assessed by taking 10 separate sets of the aforementioned measurements and thus generating 10 retinal contours for each eye. We conclude that this method is repeatable and is capable of demonstrating differences between anisomyopic eyes although validation against in vivo measurements is required.

Adult↗
PubMed1995

The aetiology of presbyopia: a summary of the role of lenticular and extralenticular structures.

Presbyopia is a condition of age rather than ageing and, as such, is devolved from the lamentable situation where the normal age-related reduction in amplitude of accommodation reaches a point when the clarity of vision at near cannot be sustained for long enough to satisfy an individual's requirements. Most of our facility to accommodate has been lost by 55 years-of-age and subsequent deterioration in visual performance at near is attributable to characteristics of senescent vision familiar to the optometrist. Our understanding of the cause of presbyopia has then to be derived principally from our understanding of the mechanism of accommodation in young eyes. Hermann von Helmholtz did much to clarify these mechanisms, but despite much research in the 100 years since his death, there is still no consensus on their precise nature. This paper presents a summary of issues, past and present, which have figured in the literature on the physiology of accommodation and presbyopia, and confirms that the pathophysiology of presbyopia is likely to result from deterioration in structure and function of a number of inter-related tissues. Changes in crystalline lens dimensions with age, the associated change in geometry of zonular attachments, and changes in viscoelastic properties of the lens capsule and lens matrix would, however, appear to be the principal correlates for the onset of presbyopia. Recent models of the biomechanics of accommodation have drawn attention to the feasibility of extralenticular contributions to presbyopia and have examined properties of the elasticity and leverage provided by posterior, anterior and tensile fibre systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1995

Reversal of tropicamide mydriasis with single instillations of pilocarpine can induce substantial pseudo-myopia in young adults.

Pupillary dilation for diagnostic purpose has become an increasingly common procedure in UK optometry in recent years and the consensus seems currently to militate against the routine use of miotics; an eye that is judged safe to dilate is thought to be at minimal risk during the natural recovery phase to normal pupil size. Nevertheless, the optometrist does, on occasion, need to consider whether there might be some advantage in minimising the sometimes debilitating effects of cycloplegia and mydriasis produced in young adults by tropicamide. In this context we compare the effects of single instillations of two miotic agents: the alpha-adrenoceptor antagonist thymoxamine HCl (0.5%), and the parasympathomimetic pilocarpine HCl (1 and 2%). Tropicamide was used to induce mydriasis in a group of 12 volunteer student subjects aged 20-26 years (7 males, 5 female; mean 21.67 years) selected to provide low (L; n = 4), medium (M; n = 4) and high (H; n = 4) iris pigment levels. Measurements of pupil diameter (Brocca pupillometer), Snellen visual acuity and accommodative amplitude (near point rule) were made every 3 min over a 90 min recording period for 4 trials: (1) a control condition whereby a miotic was not employed; (2) thymoxamine HCl 0.5% was instilled after 30 min; (3) and (4) pilocarpine 1% and 2% was instilled after 30 min, respectively. Tropicamide induced a mean increase in pupil area from 25 to 50 mm2 after 22 min which was generally sustained over the 90 min period and was enhanced for the lower pigment groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1994

Vergence adaptation and senescence.

PURPOSE: The characteristics of prism adaptation have been evaluated by many previous studies but most have been performed on young subjects (typically 18 to 35 years of age). Because little information is available regarding the effect of age on vergence adaptation, we assessed prism adaptation on 25 visually normal subjects across a wide age range (19 to 85 years) for both convergent and divergent induced disparities. METHODS: A "flashed" (125 ms) Maddox rod procedure was used for distance fixation (5 m) over a 3.5 min adaptation period and a 2.5 min recovery period. Stimulus presentation and occlusion time were controlled with an Apple lle microcomputer through a ClL Microsystems interface. RESULTS: Prism adaptation was shown to be present in all age groups, with convergence showing a greater amplitude than divergence. However, the magnitude of adaptation declined significantly as a function of increasing age at a rate of approximately 0.6% per year. CONCLUSION: An adaptation mechanism is present in all subjects but it operates with reduced gain in older subjects. The finding may explain the clinical observation that older patients readily accept prismatic correction to control oculomotor imbalance.

Adaptation, Ocular↗
PubMed1994

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects.

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Accommodation, Ocular↗
PubMed1994

Assessment of the ocular response to topical beta-adrenoceptor antagonists using accommodative microfluctuations.

Power spectrum analysis of accommodative microfluctuations has identified two dominant frequency components: a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.3 Hz < HFC < 2.5 Hz). Computer-driven models of accommodation and experimental manipulation of accommodative feedback loops indicate that LFCs are likely to have a functional role in monitoring retinal image contrast during sustained accommodation. In contrast HFCs have been shown to be correlated with arterial pulse frequency and consequently their characteristics can be modified by the extra- and intra-ocular vascular (and possibly CNS) effects. For example, topical instillation of the non-selective beta-antagonist timolol maleate has shown previously the ability to modify the HFC. In an attempt to clarify proposed differences between beta-adrenoceptor antagonist agents with regard to their effects on systemic and ocular vasculature, we extend the potential offered by HFCs as a non-invasive method of assessing the ocular response to beta-antagonists to the cardioselective beta-antagonist betaxolol HCl. Accommodative microfluctuations were measured using a continuously recording infrared optometer on 10 emmetropic subjects (mean age 23.9 +/- 2.3 years) who viewed a high contrast target located at a vergence of -4 D. A double-blind protocol was employed between saline and betaxolol (0.5%, 2 x 30 microliters) following corneal anaesthesia. Local and systemic effects were separated by examining the treated and untreated eyes of three subjects. Power spectrum analysis indicated that the root mean square (r.m.s.) value and power of LFCs and HFCs were equivalent for the saline and betaxolol trials.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1994

Repeatability of post-task regression of accommodation in emmetropia and late-onset myopia.

The characteristics of post-task regression of accommodation to pre-task tonic accommodation (TA) levels have been examined in a number of studies to clarify the nature of the within-task facility for accommodative adaptation. Of special interest is the recent observation that significant attenuation of post-task regression occurs in late-onset myopes (LOMs) when compared with emmetropes (EMMs). These findings have led to speculation that such attenuation may reflect a deficit in inhibitory sympathetic innervation to ciliary smooth muscle in late-onset myopia and hence a predisposition to sustained accommodative adaptation which then acts as a precursor to the induced myopia. A consequence of this study was that post-task regression may have some value in predicting those individuals who may be susceptible to post-task accommodative hysteresis. A pre-requisite for such a predictive value is that for a given individual the variation in inter-trial regression patterns is not significant. The aim of this study is principally to investigate the inter-trial variability of post-task regression for individual subjects following a sustained near vision task, and to confirm further differences that have been reported between EMMs and LOMs with respect to the time course of post-task regressions. A modified Canon R1 infrared optometer was used to measure accommodation objectively throughout a near task and for 2 min post-task. Accommodative level was measured following 3 min fixation of a high contrast photopic Maltese cross target placed 3 D above the subject's baseline TA. Repeatability of post-task regression in 10 EMMs and 10 LOMs was assessed by taking measurements on three separate occasions.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1993

Accommodative microfluctuations and pupil diameter.

The nominally steady-state accommodation response exhibits temporal variations which can be characterized by two dominant regions of activity; a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.0 < or = HFC < or = 2.1 Hz). There is no consensus as to the relative contribution made by each of the frequency components of the microfluctuations to the control of steady-state accommodation. We investigate the effect of variations in artificial pupil diameter (0.5, 1, 2, 3, 4 and 5 mm pupils) on the microfluctuations of accommodation, while three young emmetropic subjects view, monocularly, a photopic high contrast Maltese cross target placed at a dioptric distance equal to their open-loop accommodation level. Average power spectra were calculated for five accommodation signals, each of 10 sec duration, collected for each viewing condition at a sampling rate of 102.4 Hz using a continuously recording infrared objective optometer. For artificial pupil diameters < or = 2 mm the power of the LFC was found to increase as a function of reducing pupil diameter, while for artificial pupil diameters > 2 mm the LFC was found to be relatively constant. No systematic change in the HFC with varying artificial pupil diameter was observed. Changes in the root-mean-square (r.m.s.) value of the fluctuations with varying pupil diameter were significant (one-way ANOVA, F = 8.507, P = 0.0001, d.f. = 89) and showed a similar form to the changes in the LFC.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1993

Effect of target luminance on microfluctuations of accommodation.

It is feasible that small temporal variations in steady-state accommodation may provide feedback to the accommodation control system through changes in retinal image contrast and that this feedback may be used to maintain an optimal accommodation response. The complex waveform of microfluctuations is dominated by two distinct regions of activity; a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.0 < or = HFC < or = 2.3 Hz). Whereas the HFCs appear to be correlated with some intraocular manifestation of arterial pulse the contribution of the LFCs to the control of steady-state accommodation is unclear. The present study investigates the effect of target luminance on the waveform of accommodative microfluctuations. Three young emmetropic observers viewed monocularly a high contrast (90%) Maltese cross target placed at a vergence equal to their dark-focus level of accommodation in a Badal stimulus system. The luminance of the target was varied from 0.002 to 11.63 cd m-2 in nine equal logarithmic steps. Five continuous accommodation signals were collected for each viewing condition at a sampling rate of 102.4 Hz, and average power spectra subsequently calculated with a frequency resolution of 0.1 Hz. One-way ANOVA revealed a significant variation in the root-mean-square (r.m.s.) value of the microfluctuations (F = 19.795, d.f. 124, P = 0.0001) which could be attributed mainly to increases in the r.m.s. value for the two lowest luminances (0.002 and 0.004 cd m-2). Power spectrum analysis revealed that these changes in the microfluctuations could be attributed to increases of power in the LFC.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1993

Tonic accommodation: a review. I. Basic aspects.

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of approximately 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, including its reference to closed-loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resulting from multiple, non-optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.

Accommodation, Ocular↗
PubMed1993

Objective concurrent measures of open-loop accommodation and vergence under photopic conditions.

PURPOSE: To investigate the relationship between photopic open-loop accommodation and vergence and the effect of mental effort on these positions. METHODS: Twenty subjects (11 men and 9 women) viewed monocularly a photopic (25 cdm-2), high-contrast (90%) Maltese cross-target for 3 minutes, through a 0.5-mm pinhole drilled into an infrared filter. Accommodation was measured objectively at 1-second intervals using a Canon Autoref R-1 infrared optometer, and vergence was recorded continuously and objectively using a differential infrared eye tracking system. RESULTS: Under passive viewing conditions there was a significant correlation between photopic open-loop accommodation and vergence (R = 0.671, P = 0.0012); for the majority of subjects the imposition of mental effort shifted the passive levels of both open-loop accommodation and vergence, but these shifts were unsystematic and uncorrelated (R = 0.259, P = 0.270). The active open-loop positions of accommodation and vergence were also found to be correlated (R = 0.692, P = 0.0007). CONCLUSIONS: The influence of proximal stimuli can explain the correlations identified between photopic open-loop positions of accommodation and vergence. The uncorrelated responses of the accommodation and vergence systems to mental effort are likely to reflect interactions between various spatiotopic stimuli including mental effort and perceived proximity.

Accommodation, Ocular↗
PubMed1992

Steady-state accommodation and ocular biometry in late-onset myopia.

The steady-state accommodative responses of emmetropes and late-onset myopes was measured for an array of numbers located at -1, -3 and -5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. For the passive condition, the late-onset myopes showed a significantly lower accommodative response than the emmetropic group. No significant differences were found between the two groups for the active condition. Ocular biometric characteristics were also measured in emmetropes, late-onset myopes and early-onset myopes using keratometry and ultrasonography. No significant differences in corneal curvature, anterior chamber depth and crystalline lens thickness were found between the groups. Late-onset myopes exhibited significantly deeper vitreous chambers than emmetropes, which more than accounted for the difference in refractive error between the two refractive groups. We conclude that, while significant differences exist in the accommodative responses of late-onset myopes and emmetropes, late-onset myopia is due predominantly to elongation of the vitreous chamber.

Accommodation, Ocular↗
PubMed1992

Absence of pupil response to blur-driven accommodation.

The drive to the pupil constriction associated with near fixation has generally been attributed to accommodation with convergence and fusional convergence having secondary roles. However, our previous investigations have shown that significant changes in accommodation can take place without concomitant pupil response. To investigate further, the present study recorded pupil and accommodation responses to a blur-only accommodative stimulus using a target moved sinusoidally at a range of temporal frequencies. Care was taken to minimise target size change and apparent lateral or vertical target displacement. Results show that pupil response could be very much reduced or absent irrespective of stimulus temporal frequency and despite maintained accommodation response. The results suggest that blur-driven accommodation alone is not sufficient to drive pupil near response and that the presence of cues such as size change and lateral or vertical displacement of an approaching object may be necessary to elicit a response.

Accommodation, Ocular↗
PubMed1992

Factors influencing accommodative adaptation.

Previous studies have ascertained that a broad range of parameters have the facility to induce or modify accommodative adaptation, i.e., the post-task shift in dark accommodation (DA) which may be observed after a period of sustained fixation. This paper reviews the optical (i.e., related to a change in the form of the retinal image) and nonoptical (i.e., independent of the composition of the retinal image) stimuli which can influence the degree of adaptation. It is concluded that DA probably represents an aggregate response resulting from multiple inputs whose interactions may be too complex to allow the isolation and measurement of any single nonoptical component. Accordingly, the exact mechanism of accommodative adaptation remains unclear at the present time.

Accommodation, Ocular↗
PubMed1992

Pharmacological effects on accommodative adaptation.

We review the research which has attempted to determine whether the characteristics of autonomic innervation of ciliary smooth muscle are relevant to the process of accommodative adaptation. The effect of various topical autonomic drugs on the three phases of adaptation were analyzed: pre-task tonic accommodation (open-loop); within-task accommodative response (closed-loop); and post-task regression of accommodation to pre-task tonic levels. Although it is clear that parasympathetic innervation predominates, there is evidence that some individuals utilize supplementary inhibitory sympathetic innervation. When sympathetic innervation is augmented by substantial levels of concurrent parasympathetic accommodative activity, it may serve to attenuate the magnitude and duration of post-task shifts in tonic accommodation. It is proposed that individuals with a deficit in sympathetic inhibition may therefore be predisposed to anomalies of accommodative adaptation. However, the mechanism by which the oculomotor system responds to such predisposition is at present obscure.

Accommodation, Ocular↗