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K J Ciuffreda

Publications and source records attributed to K J Ciuffreda.

At least 19 recordsLinked to original sources
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

Symptoms related to abnormal tonic state: experimental results and computer simulations.

There have been numerous attempts to relate tonic accommodation and tonic vergence and their adaptive capabilities to a variety of parameters in vision science and clinical optometry. In this paper, investigations attempting to correlate the tonic state with nearwork symptoms will be reviewed. Results suggest that tonic vergence is more closely linked to near symptoms than is tonic accommodation. The results will be discussed in terms of a steady-state model of accommodation and vergence.

Accommodation, Ocular↗
PubMed1992

Contribution of accommodation and disparity-vergence to transient nearwork-induced myopic shifts.

Many investigations have attempted to demonstrate a relationship between sustained near-vision and the aetiology of environmentally-induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity-vergence demand during a near-vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20 cm) for 20 min through base-in, zero power and base-out prisms. The magnitude of the prisms for each individual corresponded to one-third their near vergence range. Pre- and post-task measurements of refractive state were determined objectively using an infrared optometer, with post-task readings being recorded over the initial 50 s following task completion. The results indicated no significant difference between the transient far-point shift for the three disparity-vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10-20 s period immediately following task completion which dissipated within 20-50 s post-task. These findings suggest that the transient post-task myopic shift was not related to the output of disparity-vergence.

Accommodation, Ocular↗
PubMed1992

Accommodative response during distance optometric test procedures.

The control of accommodation is an essential requirement during the clinical optometric assessment of distance vision. Accordingly, the aim of the present study was to examine the steady-state accommodative response during a number of conventional clinical procedures carried out at a viewing distance of 6m (20 feet). Accommodation was measured subjectively to Snellen letters using a Hartinger coincidence optometer during distance viewing, simulated retinoscopy and following the introduction of cylindrical and prismatic lenses in 13 visually-normal, young adults. No significant change in the mean accommodative response was induced by any of the techniques described, with subjects exhibiting a lead of accommodation of approximately 0.25D for all conditions. The results demonstrate the robustness of accommodation to apparently substantial changes in stimulus conditions.

Accommodation, Ocular↗
PubMed1992

Accommodative responses to eccentric and laterally-oscillating targets.

There is little information on the accommodative response to stimuli having naturalistic motion on the retina. In the present experiment, the steady-state accommodative response at various dioptric levels was assessed with a Hartinger coincidence optometer as sinusoidal frequency of a small target was systematically altered across the horizontal foveal region (+/- 2 degrees). The steady-state accommodative response became less accurate as target oscillation frequency increased, approaching the tonic accommodation level when the frequency was between 0.5 and 1 Hz (corresponding to peak velocities of 6.3 and 12.6 deg s-1, respectively). These results suggest that the accommodation system is reasonably robust to naturally-occurring retinal-image motion.

Accommodation, Ocular↗
PubMed1991

Vergence control of central and peripheral disparities.

The results of previous studies using either small or large target configurations suggest that open-loop (OL) vergence response dynamics may depend on the peripheral extent of the target. To investigate systemically the effect of peripheral target extent on dynamic vergence control, OL vergence responses to central, peripheral, and central-plus-peripheral targets were recorded. Open-loop disparity stimuli included steps, ramps, and sinusoids. It was found that the OL step and ramp responses to the central target were more rapid and exhibited significantly more step and multiple-step movements than those for either the peripheral or the central-plus-peripheral targets. During OL ramp stimulation, the maximum disparity for which tracking could be maintained was largest for the peripheral target. Sinusoidal responses showed the greatest gain for the central target. For all three types of stimuli, responses to the central-plus-peripheral target showed characteristics that were somewhat between those for the central and the peripheral targets. These results resolve some previously conflicting findings about open-loop vergence dynamics and suggest that large peripheral disparity stimulation initiates slow fusion of a visual scene, whereas small central disparity stimulation produces more rapid movements for precise binocular tracking of targets moving in depth.

Convergence, Ocular↗
PubMed1991

Proximal vergence and age.

A number of previous investigations have suggested that the relative contribution of proximal vergence to the overall vergence response may increase with age. Accordingly the present study has examined the proximal convergence/test distance (PC/T) ratio in a clinical population of 106 subjects ranging from 6 to 47 years of age. PC/T was assessed by comparing accommodative convergence/accommodative stimulus (AC/As) ratios obtained using the gradient and heterophoria techniques. With the former method, the stimulus to accommodation was produced optically using spherical lenses whereas in the latter technique, the change in accommodative stimulus arose from variation in actual target distance. The difference between these two measurements of AC/As reflected the output of proximal vergence. No significant variation in PC/T was observed with increasing age. It is proposed that increased disparity-vergence, rather than proximal vergence, is more likely to compensate for any age-related decline in AC.

Accommodation, Ocular↗
PubMed1991

The Glenn A. Fry invited lecture. Accommodation to gratings and more naturalistic stimuli.

An array of stimulus characteristics, such as spatial frequency content, contrast, color, and size, as well as higher-level control aspects, such as cognitive demand, instruction set, and volitional effort, may influence the steady-state accommodative response. Several experiments are reviewed which demonstrate clearly the remarkable robustness of accommodation to marked variations in these parameters. The retinal luminance gradient appears to be the primary determinant of overall response accuracy. These findings are of theoretical interest in understanding basic aspects of accommodative control, as well as being of practical importance. They suggest that the accommodative response remains relatively stable and accurate despite dynamic changes in retinal-image quality which occur under experimental and clinical, as well as more naturalistic, viewing conditions.

Accommodation, Ocular↗
PubMed1991

Model of tonic accommodation after sustained near focus.

The effect of short-term near focus on tonic accommodation (TA) was previously investigated in 48 visually normal subjects. After sustained focus upon a near target for 10 min, all illumination was extinguished and the decay of accommodation was monitored for 20 min. We calculated from these results that accommodation decayed rapidly with a time constant of about 0.4 min, followed by a slower decay with a time constant of about 32 min. However, the asymptotic level remained slightly above the preadaptation level after 20 min in the dark. A feedback model with two parallel dynamic components (transient and decay), which summed with a "bias" element in the forward loop, was developed to account for the results. The gains of the dynamic elements were varied until the simulation curves matched the experimental adaptation and decay data. During the closed-loop adaptation phase, the outputs of the dynamic elements moved in opposite directions but combined to yield the overall accommodation step response. Moreover, when the model was subsequently open-looped in the dark, the transient component declined rapidly, whereas the decay component remained above the preadaptation level. When the 48 subjects were grouped equally into 4 refractive categories, somewhat different decay time courses could be simulated with changes in model parameter values.

Accommodation, Ocular↗
PubMed1991

Accommodative responses to conflicting stimuli.

In this study we examined the monocular accommodative response when subjects (N = 12) simultaneously viewed two overlapping, horizontally and vertically oriented targets located at disparate dioptric distances from the observer. The proximal and distal stimuli were located at dioptric levels of 5 and 3, 3 and 1, or 5 and 1 D, respectively. A broad spectrum of individual responses was observed for all target conditions. Additionally, a significant correlation was observed between the accommodative response when the ambiguous targets were viewed and the response found in the absence of a blur stimulus to accommodation. This finding suggests that when conflicting information is provided to a particular component of accommodation, the relative importance of that component in the determination of the aggregate accommodative response is attenuated.

Accommodation, Ocular↗
PubMed1991

The linearity of proximally induced accommodation and vergence.

This study sought to determine the linearity and amplitude of both proximally induced accommodation (PIA) and vergence (PIV) by comparing the open-loop accommodation and vergence responses, respectively, to individual stimuli located at viewing distances ranging from 0.20 m to 1,500 m. For the measurement of PIA, the vergence and accommodation loops were opened by subjects (N = 10) monocularly viewing the target through a 0.5-mm pinhole, while accommodation was assessed concurrently using an infrared optometer. In the assessment of PIV, the accommodation loop was opened either by subjects (N = 10) viewing a low-frequency difference of Gaussian (DOG) grating or by viewing the targets binocularly through 0.5-mm pinholes. For both PIV methods, the vergence loop was opened by vertical dissociating prisms while the heterophoria was assessed for the various target distances using the von Graefe technique. The results demonstrated significant changes in both PIA and PIV for stimuli located closer than 3 m. However, both proximal components attained a minimum level and remained constant for targets at or beyond 3 m. Furthermore, the magnitudes of PIA and PIV were linearly related to the target distance measured in diopters and meter angles, respectively. These findings clearly demonstrate the influence of target proximity on the oculomotor system.

Accommodation, Ocular↗
PubMed1991

Effect of surround propinquity on the open-loop accommodative response.

The effect of knowledge of surround propinquity, ie, awareness of proximity of the adjacent surroundings, on the open-loop accommodative response (AR) was determined by comparing measurements of accommodation obtained in total darkness in two different-sized rooms. The AR was measured in two laboratories, one 2.5-m square and the other 6.75 x 2.75 m. Steady-state accommodation was assessed on two occasions in each room using an infrared optometer. On the first occasion, the subjects (n = 10) were fully aware of the laboratory dimensions and topography. For the second trial, they were blind-folded before entering the laboratory and hence were unaware of the experimental location. When subjects were unaware of the laboratory dimensions, no significant difference existed between the recorded values of AR. However, when subjects were initially able to observe the size of the room, the AR in darkness was significantly higher in the smaller laboratory. This suggests that proximally induced accommodation, initiated by prior knowledge of the dimensions of the laboratory, was responsible for this increase in AR. Furthermore, it is proposed that complex interactions exist between proximal, tonic, and other nonoptical accommodative stimuli such that it may be impossible to isolate an individual, nonoptical accommodative component.

Adult↗
PubMed1990

Suppression of sensitivity to change in target disparity during vergence eye movements.

It has been demonstrated recently in human psychophysical experiments that sensitivity to surround displacement is suppressed during convergence eye movements. To determine whether sensitivity to changes in target disparity is also reduced, responses to test disparities that were superimposed on standard 4 degrees step disparities were investigated. The test disparities consisted of brief (20 ms) positive and negative pulses as well as steps (in the range of +/- 0.6 degrees). A two-alternative forced-choice procedure was used in which the test disparity was presented in either the first or the second portion of a trial. The results showed that suppression of both test pulse and step disparities began before the start of the convergence movement and continued during the movement. Maximum suppression was about 0.50 to 0.85 log units and occurred between 150 ms before to 50 ms after convergence onset. The differences in sensitivity curves for pulse and step stimuli suggest the presence of different central and peripheral neural factors during vergence eye movements.

Eye Movements↗
PubMed1990

Adaptation to optically-increased interocular separation under naturalistic viewing conditions.

Mirror spectacles which enhance binocular disparity by optically doubling the normal separation between the eyes were used to create conditions of combined perceptual and oculomotor conflict. Apparent depth and distance, as well as tonic accommodation, tonic vergence, and accommodative-vergence gain (response AC/A ratio), were assessed immediately before and after a 30 min exposure period of naturalistic viewing with the spectacles. Wearing the spectacles produced an increase in tonic vergence, and perceptual aftereffects consisting of increased apparent distance and depth. The results indicate that oculomotor conflict associated with enhanced interocular separation may be resolved through adaptation of tonic vergence, rather than through alteration of accommodative-vergence gain. The results also demonstrate that perceptual conflict between disparity and multiple veridical depth cues does not necessarily produce adaptive modification of the relationship between binocular disparity and apparent depth.

Accommodation, Ocular↗
PubMed1990

The effect of stimulus duration on tonic accommodation and tonic vergence.

Changes in tonic accommodation and tonic vergence were studied as a function of stimulus duration and time after near fixation. Thirteen visually normal young adults each participated in experimental sessions comprising each of 12 combinations of stimulus type (accommodative or vergence) and stimulus duration (0.25, 0.5, 1, 2, 4, or 8 min). During each session, measurements were made of either tonic accommodation or tonic vergence 30 s before stimulus onset and at 0.5, 2, 4, 6, 8, 10, 12, and 14 min after stimulus offset. The stimulus (a high contrast reduced Snellen visual acuity chart) was viewed monocularly at the accommodative nearpoint and binocularly through pinholes at the fusional nearpoint on the accommodative and vergence sessions, respectively. For tonic accommodation, stimulus duration had no significant effect on either the magnitude of the aftereffect or its rate of decay. For tonic vergence however, the aftereffect was larger and had a slower rate of decay as stimulus duration increased. The results suggest that the adaptive mechanisms within the two systems have different time constants.

Accommodation, Ocular↗
PubMed1990

Distance heterophoria and tonic vergence.

It has previously been suggested that the difference between tonic vergence (TV) and distance heterophoria may relate either to proximally induced vergence or accommodative divergence. To identify which of these components was responsible for this difference, the present study compared TV and distance heterophoria in a population of 20 subjects. However, distance heterophoria was now assessed with open-loop accommodation. No significant difference was observed between these two measures. This suggests that the previously observed variations between TV and distance heterophoria (with accommodation closed-loop) were produced by accommodative divergence. However, in a subgroup of 7 subjects, measurements of TV were more convergent (greater than 2 delta) than the distance heterophoria. In these subjects, proximally induced vergence may have accounted for the difference in response.

Accommodation, Ocular↗
PubMed1990

Accommodative responses to naturalistic stimuli.

Steady-state accommodative responses to a wide range of commonly encountered textural and graphic stimuli were investigated in 12 young, visually normal subjects. Details of two experiments are described. In the first experiment, stimuli were varied in terms of size, contrast, spatial frequency, colour and intellectual demand. In the second experiment, only the colour of the targets was systematically altered. In both experiments the stimulus demand was maintained at 3 D, and accommodation was measured subjectively using a Hartinger coincidence optometer. No significant difference in mean accommodative response was observed between the widely differing targets adopted in either experiment. These findings suggest that an overall threshold exists for accommodative stimuli, with supra-threshold targets of low to moderate contrast being capable of initiating and sustaining an appropriate accommodative response.

Accommodation, Ocular↗