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J R Pugh

Publications and source records attributed to J R Pugh.

15 recordsLinked to original sources
PubMed1995

The use of coherence functions in the study of ocular mechanisms.

The study of ocular mechanisms is often concerned with the possible interaction of two systems. Spectral analysis has been used previously by many investigators as a tool to investigate the interrelationships in the behaviour of two or more systems. This note demonstrates the value of computing coherence functions following spectral analysis to obtain a more rigorous tool for the investigation of synergy between two systems.

Accommodation, Ocular↗
PubMed1993

Peripheral nerve blocks.

This article addresses the use of peripheral nerve blocks as a primary or adjunctive technique of anesthesia for selective operative procedures. The use of these blocks can reduce the amount of inhalational and narcotic agents used during an anesthetic and facilitate a pain-free postoperative period.

Extremities↗
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↗
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

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

Twin channel infrared optometer for recording binocular accommodation.

A twin channel infrared (IR) optometer for recording accommodation from each eye is described. Some modifications to a conventional single channel optometer design are necessary to facilitate binocular recording. The optometer incorporates an on-line microcomputer for data acquisition and processing. Processing includes power spectrum and correlation analysis, and a facility for performing cursor controlled measurement of time delays. Accommodation responses recorded are shown to be independent of horizontal eye movements, and little affected by the drug used for mydriasis.

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↗
PubMed1989

Perceptual detectability of ocular accommodation microfluctuations.

When the eye fixates a stationary stimulus, the power of the lens is known to change rapidly and continuously. Although the basic characteristics of this fluctuating activity are known and the influencing factors have been identified, their exact role in the control of accommodation is uncertain. It is thought that, for the fluctuations to be useful, a detectable change in the retinal image is necessary, yet an accommodation response can be provoked by a stimulus change of only 0.1 D, well within the ocular depth of focus as conventionally measured. In this study we examine whether the fluctuations of accommodation are able to provide perceptual sign information or whether the presence of a sensorimotor mechanism utilizing subthreshold blur is required. Normal accommodation fluctuations for steady-state viewing were recorded using an infrared optometer and subsequently were used to drive a Badal stimulus optometer. The perceptual threshold of the fluctuation signal for an eye under cycloplegia was then determined. The threshold for the detection of the fluctuations at optimal focus was not significantly different from the root-mean-square value of the actual response. Hence a portion of the fluctuations more than spans the dead space (depth of focus) and is therefore capable of providing information to control accommodation without the need for a subthreshold mechanism.

Accommodation, Ocular↗
PubMed1989

A procedural guide to the modification of a Canon AutoRef R-1 for use as a continuously recording optometer.

We have recently reported on the modification of the Canon AutoRef R-1 for use as a continuously recording optometer, while retaining its capacity to measure in 'single-shot' mode. Although the principles of the modification and performance of the dual mode of operation have been described in previous publications, this technical not aims to provide sufficient detail for research laboratories wishing to implement the modification.

Electronics↗
PubMed1988

Modification of the Canon Auto Ref R1 for use as a continuously recording infra-red optometer.

The Canon Auto Ref R1 has been modified to allow continuous recording of accommodation while retaining its capacity to record in 'single-shot' static mode. Possible alternative principles of modification are discussed with emphasis placed on the method used in this paper. Preliminary results are presented for both a novel dynamic model eye and an emmetropic real eye. The system described provides a useful research tool in the study of ocular accommodation dynamics.

Accommodation, Ocular↗
PubMed1987

Power spectrum analysis in the study of ocular mechanisms.

The use of power-spectrum analysis in the study of ocular dynamics is becoming increasingly widespread. This paper summarizes the key points of signal acquisition and power-spectrum analysis, emphasizing aspects which we have found to be of relevance when the technique is applied to experiments in binocular accommodation. The considerations and conclusions are relevant to other areas of investigation into ocular dynamics and similar biological systems.

Accommodation, Ocular↗
PubMed1986

A new adaptable computerized system for the measurement of specific airways conductance.

We have developed an automated system for the measurement of specific airways conductance (SGaw) which differs in several aspects from other reported automated systems. In seven subjects this system was used to measure baseline SGaw and the provocation concentration (PC) of histamine producing a 35% fall in SGaw. The results produced were compared with those obtained by manual analysis using both one limb and two limbs of the inspiratory part of the flow/box pressure curve. Using the automated system, baseline SGaw and PC35SGaw results were similar to those obtained by the two limbs method of manual analysis, but the analysis time was considerably reduced. A single measurement of SGaw was obtained within 2-3 seconds. Both the automated and two limbs manual methods of analysis were more reproducible than the one limb method of manual analysis.

Adult↗