Biomedical subjects

B Gilmartin

Publications and source records attributed to B Gilmartin.

At least 55 records · Page 3Linked to original sources

PubMed1992

Current perspective on microfluctuations of accommodation.

The collaboration of Fergus Campbell, Gerald Westheimer and John Robson in the 1950s produced insight into the nature of accommodation microfluctuations and instigated work which has led to the current view that the nominally steady-state accommodation response exhibits temporal variations which can be characterized by two dominant regions of activity: a low-frequency component (LFC less than 0.6 Hz) and a high-frequency component (HFC greater than or equal to 1.0 less than or equal to 2.3 Hz). A functional role has been attributed to these microfluctuations as they offer a means by which a directional cue could be derived from an even-error stimulus. However, there is no consensus regarding the respective contribution made by each of the dominant components in the accommodation control process. Using a newly-designed measurement and recording system we have conducted a series of experiments to investigate the nature and aetiology of the microfluctuations. The incidence and magnitude of microfluctuations in LFCs and HFCs for central and peripheral lens zones were investigated while five young emmetropic subjects viewed a near target. The form of the power spectra of the fluctuations was found to be similar for central and peripheral zones although an overall reduction in magnitude was observed in the periphery. The HFCs are thus a consistent feature of microfluctuations in central zones and not, as previously suggested, merely a spurious feature of peripheral zones. A significant between-subject variation in the location of HFCs was found and led us to consider the relationship between HFCs and other physiological systems which provide intraocular rhythmic variation.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗

PubMed1991

Adaptation of tonic accommodation to sustained visual tasks in emmetropia and late-onset myopia.

The study complements and extends our previous investigation and reports that late-onset myopes (LOM's, N = 15) exhibit differences in adaptation of tonic accommodation (TA) to sustained visual tasks when compared to emmetropes (EMM's, N = 15). Pre- and post-task TA was measured with a modified Canon R-1 objective infra-red optometer under darkroom conditions. Post-task TA was measured immediately after a 10-min counting task and at 1-s intervals over a period of 90 s; the task was located at distances equivalent to 1, 3, and 5 D. The pattern of regression back to the pre-task TA level was used to assess the degree of adaptation generated by the task. The data indicate substantial differences between regression patterns for EMM's and for LOM's which were enhanced as accommodation stimulus levels increased. Using a two-factor split-plot analysis of variance, statistically significant differences between EMM's and LOM's could be attributed to the rates of change of post-task accommodative levels to pre-task TA levels for both the 3 and 5 D tasks.

Accommodation, Ocular↗

PubMed1991

Effect of a topical beta-adrenergic receptor antagonist on the dynamics of steady-state accommodation.

The ocular accommodation response to a stationary near target exhibits microfluctuations which are characterized by two dominant components of activity: a low-frequency component (LFC: less than 0.5 Hz) and a high-frequency component (HFC: 1.0-2.2 Hz). We have shown recently that there is a significant correlation between the frequency of arterial pulse (AP) and the peak frequency of the HFC. If the mechanisms by which AP induces the HFC are intraocular they are likely to involve both rhythmic variations in choroidal blood flow and intraocular pressure (IOP) pulse: the former inducing changes in ciliary ring diameter from pulsatile blood volume changes in the ciliary body, the latter producing changes in effective lens power following small shifts in lens position or, with each drop in IOP, reduced resistance to inherent lens substance and capsule elasticity. In addition to its well-known ocular hypotensive effect, topical instillation of the beta-adrenergic receptor antagonist timolol maleate has been shown to reduce significantly the magnitude of the IOP pulse, an effect that has been attributed to the vasoconstrictive action of the drug on the choroidal vasculature. Using a modified Canon IR optometer we report on the effect of 0.5% timolol maleate on the accommodative microfluctuations for a group of 10 emmetropic subjects viewing a target monocularly at a distance of 25 cm. Following a double-blind protocol against a saline control, timolol was shown to reduce the rms for both LFCs and HFCs. An experiment to separate local from systemic effects involved four subjects whose treated and untreated eyes were examined to determine the effects of the consensual reduction in IOP: the results on microfluctuations in the untreated eyes were equivocal but the correlation between arterial pulse frequency and location of the peak frequency of HFC was maintained. The results show that factors relating to IOP and vasculature will affect the magnitude of accommodative microfluctuations.

Accommodation, Ocular↗

PubMed1991

The amelioration of myopia using biofeedback of accommodation: a review.

The purpose of this review is to give an account of the issues raised by reports that myopia can be treated by operant conditioning or biological feedback. Devices are available that provide auditory feedback of accommodative state such that variation in tone frequency provides a means by which the patient can monitor his or her ability to induce changes in accommodation response, the task thus being to change the pitch of the tone in a direction corresponding to a more distant far point. The aim of a training programme is to reinforce and establish control over the accommodative response by a process akin to operant conditioning and subsequently to transfer the skill to normal visual environments such that a manifest reduction in myopia becomes apparent. Although there are reports that the technique can induce a reduction in myopia by up to 3 D the findings are of limited value owing to the lack of objective data, for example pre- and post-training cycloplegic refraction. The pre- and post-training measures of subjective letter acuity used by many studies are, with repeated trials, particularly vulnerable to individual differences in the ability to learn how to discriminate and interpret blurred images. The review concludes that more comprehensive clinical trials are needed before accommodation biofeedback can qualify as an established method of clinical treatment of myopia. The trials should be designed to encompass the following issues: the characteristics of a feasible physiological model linking accommodation and myopia development; the rationale with regard to patient selection; the technical performance of the optometer employed; the characteristics of the control group used; the criteria for assessment of myopic change; the transfer of training to performance in normal visual environments; the economic viability of the programme of training and equipment; and the skill, training and knowledge of the clinician implementing the training programme.

Accommodation, Ocular↗

PubMed1991

The effect of mental effort on open- and closed-loop accommodation.

The accommodative response to stimuli in normal visual environments is determined by a complex and subtle integration of optical and non-optical factors. Mental effort associated with the visual task can modify significantly the steady-state accommodative level, but, owing to the diversity of experimental designs, there is no clear consensus on the mechanisms involved. Changes in the accommodation response of ten emmetropic subjects (mean (+/- SD) age = 20.4 +/- 4.5 years) under open- and closed-loop conditions were investigated for three levels of mental activity. (1) A passive task whereby subjects simply read letters to themselves. (2) A stimulus-dependent task (SDT) whereby subjects are instructed to respond only when the letter 'e' appears in one of a series of presentations. (3) A stimulus-independent task (SIT) whereby subjects count backwards in sevens to themselves while viewing the target. An objective infra-red (IR) optometer was used in its static mode of operation to make monocular measurements of accommodation under monocular viewing conditions. Open-loop conditions were achieved by placing a pinhole (0.5 mm diameter), drilled into an IR filter, 12 mm in front of the eye. Under closed-loop conditions the mean accommodation response for passive viewing of the near target was +3.08 D. A significant (F = 5.45 d.f. 9,18 P less than 0.005) accommodative shift induced by mental effort in the mean response of +0.17 D occurred for the SDT. The SIT induced a mean shift of -0.05 D which was not significantly different to the passive viewing response.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗

PubMed1990

The frequency characteristics of accommodative microfluctuations for central and peripheral zones of the human crystalline lens.

Temporal variations in the nominally steady-state accommodation response can be characterized approximately by two principal regions of dominant activity: a low frequency region (LFR less than 0.6 Hz) and a high frequency region (HFR greater than 1.0 Hz). The observed changes of activity in the LFR induced by varying stimulus conditions has led to the proposal that this region may be utilized as an error detector in the accommodation feed-back loop. The role of fluctuations in the HFR is, however, equivocal particularly as several reports have shown their magnitude to be positively correlated with pupil size. The implication is that HFRs are a consequence of the mechanical and elastic properties of peripheral rather than central zones of the crystalline lens and hence do not contribute to the accommodation control process. The aim of the study was to compare the incidence and magnitude of microfluctuations in LFRs and HFRs for central and peripheral lens zones when viewing a near target. Accommodation was measured continuously with a modified Canon Autoref R1 infra-red optometer on five young emmetropic subjects. We show that although the total power of the microfluctuations is reduced in the periphery, the form of the power spectra is similar for central and peripheral zones. Thus rather than being spurious effects caused by the periphery of the lens HFRs are present for both central and peripheral lens zones.

Accommodation, Ocular↗

PubMed1990

Effect of target proximity on the open-loop accommodative response.

Proximal stimuli to accommodation were assessed by comparing the open-loop accommodative responses (AR's) to stimuli placed at viewing distances of 5 m (0.2 D) and 0.33 m (3D) in 10 late-onset myopes (LOM's) and 10 emmetropes. The accommodation and vergence loops were opened by subjects viewing targets monocularly through 0.5-mm pinholes. Pre- and post-task levels of tonic accommodation (TA) were used to assess the accommodative adaptation induced by the task. For the 0.2 D condition there was no significant difference between pre-task TA, within-task AR, or post-task TA, indicating that accommodation was open-loop throughout this distal condition. When viewing the 3 D target both refractive groups exhibited significant increases in AR, emmetropes showing higher levels of proximally induced accommodation (PIA) when compared with LOM's. The data indicate that target proximity will influence AR even when both blur and vergence cues have been stabilized.

Accommodation, Ocular↗

PubMed1990

The influence of alpha-adrenergic agents on tonic accommodation.

This study has examined the effect of an alpha-1 adrenergic agonist (phenylephrine HCl, 2.5%) and a non-selective alpha antagonist (thymoxamine HCl, 0.5%) on tonic accommodation (TA). TA was measured at 1 min intervals throughout a 45 min period using an objective infra-red optometer (Canon Autoref R-1). Additionally pupil diameter and the near-point of accommodation (NPA) were monitored in order to assess the efficacy of the instilled drug. A double-blind protocol was adopted between thymoxamine, phenylephrine and a saline control for 7 emmetropic subjects. There was no significant difference between the values of TA obtained under the three pharmacological conditions. However, both adrenergic agents induced changes in pupil diameter and NPA. Furthermore in all conditions TA increased by approximately 0.25D while the subject sat passively in darkness. The data indicates that alpha-adrenergic innervation alters the closed-loop accommodative response independently of TA.

Accommodation, Ocular↗

PubMed1990

Arterial pulse modulates steady-state ocular accommodation.

Rapid and continuous fluctuations in ocular focus are known to occur when the eye views a stationary target. The advent of high-speed infra-red optometers has established that these microfluctuations of ocular accommodation have two dominant components: low frequency of less than 0.6 Hz and high frequency between 1.0-2.3Hz. Although the retinal image blur associated with microfluctuations has the potential to guide and maintain optimum accommodation levels, there is no consensus with regard to the respective contribution of each of the dominant frequency components. Using a newly-designed measurement and recording system we show that, when viewing a stationary target located at 25cm, individuals exhibit little variation in the frequency of low frequency components but significant variation in high frequency components. Simultaneous measurements of ocular accommodation and systemic arterial pulse demonstrate that the variation in high frequency component is significantly correlated with arterial pulse frequency. Since control experiments indicate that the microfluctuations are derived from activity of the crystalline lens our observations could provide the basis for a non-invasive method of assessing the effects of arterial pulse on ciliary body/choroidal vasculature, the vitreous/lens interface and intraocular pressure. Further, it is feasible that under certain conditions an abnormal increase in the magnitude of arterial pulse may affect the aggregate nature of accommodative microfluctuations to an extent that disrupts the normal control processes maintaining optimum retinal contrast during sustained near vision.

Accommodation, Ocular↗

PubMed1989

Temporal aspects of accommodative adaptation.

Pre- and post-task measures of dark focus (DF) were used to assess accommodative adaptation induced by brief near-vision tasks in 10 late-onset myopes (LOM's) and 10 emmetropes. The degree of adaptation was examined after tasks of 15, 30, and 45 s duration. Accommodation was measured after the task at 1 s intervals over a 90 s period using an objective infrared optometer. Emmetropic subjects showed a significant increase in DF during the first 10 s of post-task recording, which regressed to the pre-task value within a further 20 to 30 s. No significant difference in post-task accommodative regression was observed after the three task durations. Similar findings were observed for LOM's after the 15 and 30 s tasks. However, after the 45 s task, LOM's exhibited a marked negative shift in DF during the 10 to 50 s post-task period. Instillation of a beta adrenergic antagonist attenuated this negative shift, suggesting that it was mediated via beta adrenergic innervation to the ciliary muscle.

Accommodation, Ocular↗

PubMed1989

The measurement of adaptation of tonic accommodation under two open-loop conditions.

Pre- and post-task tonic accommodation (TA) was measured using an objective infra-red optometer under two open-loop conditions: a darkroom and a bright empty-field (60 cd m-2). The task was of 10 minutes' duration and was located at -1, -3 and -5 D. Post-task TA was measured immediately after the task and over a period of 90 s. Mean data for the group of 10 young men subjects showed that the open-loop condition adopted had no significant influence on the post-task regression patterns of TA towards pre-task values. Three subjects did, however, show significantly more sustained increases in post-task TA for the bright-field condition; one markedly so. Conversely, two subjects showed more pronounced post-task shifts for the darkroom condition.

Accommodation, Ocular↗

PubMed1989

The effect of pupil size on static and dynamic measurements of accommodation using an infra-red optometer.

The study examines to what degree the output of the Canon Auto-ref R1 infra-red (IR) optometer is affected by variations in pupil size for both static measurement of refractive error and for continuous recording of accommodation when switched to its modified mode of operation. In the normal static mode of operation the measurement of refractive state simply depends on the position of the peak photodetector output for each of three meridians: it does not depend on signal amplitude and hence pupil size. However, in continuous mode the actual output of a single photodiode is measured and the measurement is susceptible to changes in pupil size that occur within the area of the exit pupil utilized by the instrument. Using static and dynamic recording on model and human eyes the study shows that a 10% reduction in output signal relative to the maximum output level (for a constant stimulus) occurs for pupil sizes of 3.9 mm. The relatively small exit pupil has important advantages with regard to the use of mydriatic drugs and task-induced changes in pupil diameter.

Accommodation, Ocular↗

PubMed1988

The accommodative response, refractive error and mental effort: 1. The sympathetic nervous system.

The accommodative response of 12 emmetropic subjects 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. The imposition of mental effort induced a significant increase in mean response for the -1 D stimulus, a response equivalent to the passive condition at -3 D and a reduction in response at -5 D. Mental effort induced similar responses for the -1 D and -3 D locations when sympathetic innervation to the ciliary muscle was blocked with the beta-receptor antagonist timolol maleate. Responses for the -5 D location were, however, significantly increased. It is proposed that sympathetic inhibition can modify the effect of mental effort on near accommodative responses.

Accommodation, Ocular↗

PubMed1988

Accommodative adaptation induced by sustained disparity-vergence.

Pre- and post-task measures of dark focus (DF) were used to assess the response of the accommodative system to within-task disparity-vergence in 10 late-onset myopes (LOM's) and 10 emmetropes. Disparity was induced by 3 and 6 delta base-out prisms imposed during a 3 D near-vision task undertaken for 3- and 10-min periods. Accommodation was measured after the task at 1-s intervals over a 90-s period using an objective infrared optometer to determine post-task regression of DF toward pretask values. After the 3-min task both groups show a reduction in DF adaptation with increasing disparity-vergence. After the 10-min task, disparity continues to have a significant effect on accommodative adaptation in LOM's but not in emmetropes, suggesting diminished prism adaptation in LOM's. The implications of these findings are discussed with regard to both current models of accommodation-vergence interaction and the etiology of late-onset myopia.

Accommodation, Ocular↗

PubMed1988

Assessment of the CA/C ratio in a myopic population.

The convergent accommodation/convergence (CA/C) ratio was measured in 30 subjects comprising 10 late-onset myopes (LOM's), i.e., myopia onset at 15 years of age or later, 10 early-onset myopes (EOM's), and 10 emmetropes. The accommodation loop was opened by two methods: (1) the subject viewed a 0.1-mm diameter spotlight at distances of 33 and 100 cm and (2) the subject viewed near-type (viewing distance 33 cm) through 0.5-mm pinholes. Convergence was stimulated by the introduction of 3 and 6 delta base-out prisms before the right eye and the accommodative response of the left eye was measured using an objective infrared optometer (Canon Autoref R-1). There was no significant difference in CA/C between the three refractive groups with either method. The implications of this finding are discussed with reference to previous work on accommodative convergence/accommodation (AC/A) and disparity-induced accommodation and in relation to accommodation-vergence synkinesis in late-onset myopia.

Accommodation, Ocular↗

PubMed1988

Accommodative adaptation to monocular and binocular stimuli.

Pre- and post-task measures of dark-focus (DF) were used to assess accommodative adaptation induced by a 45-s near-vision task at 33 cm. Adaptation was measured under monocular and binocular conditions for a group of 10 young emmetropic subjects (mean age 21.6 years). The accommodative response was measured objectively using an infrared optometer (Canon Autoref R-1). Post-task DF was sampled immediately after the task at 1-s intervals over a 90-s period. No significant difference in accommodative adaptation was observed between the monocular and binocular near-vision tasks. The implications of this finding are discussed with regard to the oculomotor constituents of the closed-loop accommodative response.

Accommodation, Ocular↗

PubMed1988

The effect of vergence adaptation on convergent accommodation.

This study investigates whether, during a sustained near-vision task, the convergent accommodation (CA) derived initially from the fast (reflex) vergence response is subsequently maintained by tonic (adaptive) vergence. CA was examined during the course of a 3 min near-vision task (viewing distance 33 cm) in 10 emmetropic subjects. Vergence was stimulated by the introduction of a 6 delta base-out prism before the right eye while the open-loop accommodative response of the left eye was measured at approximately 1 s intervals using an objective infra-red optometer. In a control condition the subject viewed the target through a plano lens (0 delta). The accommodation loop was opened by the subject viewing near-type through 0.5 mm diameter pinholes. Pre- and post-task measurement of heterophoria was made using a Maddox rod and prism bar. Regression of post-task heterophoria to the pre-task value indicated that adaptation to the vergence stimulus had occurred. However, there was no significant reduction in induced CA during the task. As the onset of adaptation does not produce any fall in CA, it is concluded that both reflex and adaptive vergence act as a stimulus to CA. This finding is discussed with regard to models of accommodation-vergence interaction.

Accommodation, Ocular↗

PubMed1988

Factors affecting the normal perimetric profile derived by automated static threshold LED perimetry. I. Pupil size.

The relationship between pupil size and perimetric sensitivity was investigated in a group of clinically normal emmetropic subjects using the Dicon AP3000 computer-assisted perimeter at bowl luminances of 10 asb and 45 asb. Pupil size was modified with the miotic thymoxamine 0.5% and the mydriatic phenylephrine 10%; saline was used as the control. Perimetric sensitivity increased with an increase in pupil size. The effect was greatest for peripheral locations for both bowl luminances. The fluctuations in perimetric sensitivity increased with peripheral angle and with decrease in pupil size for both bowl luminances.

Adult↗