Modelling adaptation effects in vergence and accommodation after exposure to a simulated virtual reality stimulus.

Natural viewing conditions place equal demands on the vergence and accommodation systems. The two responses are co-ordinated via the interactive components which couple the two systems: accommodative vergence and vergence accommodation. The crosslink components are usually quantified by the AC/A and CA/C ratios. Whether these ratios are stable entities, genetically determined, or modifiable by experience has been the subject of some debate. A 'Virtual Reality' stimulus was used to place unequal demands on the vergence and accommodation systems. Pre-task and post-task measures of AC/A and CA/C were objectively determined. The changes in the post-task measures are discussed with reference to a dual interactive model of vergence and accommodation. Model simulations suggest that adaptation of the vergence and accommodative controllers (tonic adaptation) may also play a part by altering the open loop bias. The results therefore support the idea that vergence accommodation and accommodative vergence are capable of adaptation.

Evolution of control system models of ocular accommodation, vergence and their interaction.

This paper reviews the evolution of inferential models describing ocular accommodation and vergence, which have been developed using the techniques of control engineering. The models are developed by inferring that the observed behaviour of the ocular systems could be produced by particular types of feedback control systems. The models are subsequently tested and improved by modifications resulting from the comparison of model predictions with physiological experimentation. Current models of accommodation and vergence have had considerable success in describing both the steady-state and dynamic behaviours of the individual systems. However, controversy currently exists among researchers, particularly with respect to the composition of models that describe the interaction of the two systems.

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.

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.

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.

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.

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.

Personality testing in 226 patients with urinary incontinence.

Two hundred and twenty-six female patients undergoing investigation for urinary incontinence completed the Eysenck Personality Inventory questionnaire. The results of the questionnaire were correlated with the symptomatology and urodynamic findings. Incontinence and neuroticism were found to be related. No special relationship was found between neuroticism and detrusor overactivity, although this has been reported in other smaller series.

The Urilos recording nappy system.

A technical and clinical evaluation of the Urilos recording nappy system is presented. The recording pads were calibrated to determine the variation between and within batches of 25 pads supplied by the manufacturer. Fifty patients were assessed by this system for periods of 2 to 7 h. The results show that the system is useful in the verification of patient histories, although it is difficult to obtain a quantitative measure of urine loss.

111Indium-labelled white blood cells in the diagnosis of Felty's syndrome.

The use of isotope scanning of the spleen in conjunction with 51Cr-labelled red blood cells has become an established technique in the evaluation of patients with hypersplenism. As far as we are aware the similar technique using labelled white blood cells to demonstrate splenic sequestration in a neutropenic patient has not been described. We report a case where this technique proved valuable in confirming the diagnosis and in predicting a favourable response to splenectomy.

Assessment of total and divided renal plasma flow by 123I-hippuran renography.

We studied 23 patients with suspected renal hypertension, including 12 with renal artery stenosis, or occlusion. Total effective renal plasma flow (ERPF) was measured in all patients by conventional p-aminohippurate (PAH) clearance and by 123I-hippuran clearance performed on the same day. A close correlation between the two techniques was obtained (r = 0.87, P less than 0.001) with the latter technique underestimating the former by a mean ratio of 0.89:1.00. We describe a technique for deriving ERPF for individual kidneys by 123I-hippuran renography, and the data obtained by this method correlate well with data obtained from bilateral ureteric catheterization studies (r = 0.076, P less than 0.001 for both affected and unaffected sides) in 17 patients. The renographic technique is particularly accurate in quantitating ERPF in the 12 patients with renal artery stenosis, and is recommended as the investigation of choice in the assessment of ERPF in patients with this condition.

Monitoring of airborne contamination during the handling of technetium-99m and radioiodine.

Measurements have been made using an air sampler to measure airborne radioactivity produced during the routine handling of large activities of technetium-99m, iodine-125 and iodine-131. The results indicate that 99Tcm can be safely handled in environments without direct exhaust of the ventilated air but that 125I and 131I should always be handled in a ventilated environment such as a fume cupboard or a down-draught work-station of the total-exhaust type. Monitoring of thyroid uptake proves to be the most reliable means of monitoring airborne contamination by these radionuclides, but burdens and radiation doses for typical procedures are well within the maximum permissible limits of the Code of Practice for the Protection of Persons from Ionizing Radiation arising from Medical and Dental Use.