Biomedical subjects

K J Ciuffreda

Publications and source records attributed to K J Ciuffreda.

At least 73 records · Page 4Linked to original sources

PubMed1989

Suppression of sensitivity to surround displacement during vergence eye movements.

Suppression was investigated psychophysically in three human observers by measuring their loss of sensitivity to brief (20 ms) simultaneous vertical displacement (up to 0.5 degrees) of horizontal lines during 4 degrees convergence eye movements. A two-alternative forced choice procedure was used in which the stimulus was presented either in the first or second portion of a trial. The amplitude of the displacement pulse, the time of the pulse relative to convergence onset, and the portion of a trial in which the stimulus was presented were randomized. The results showed that suppression began about 200 ms before, and continued until 350 ms after, convergence onset with maximum loss occurring at 25 to 125 ms after convergence onset. The maximum sensitivity loss was about 0.25 to 0.30 log units. Since peripheral factors were minimized by the use of a brief stimulus presentation and an eccentrically placed surround, the suppression found was primarily attributed to central neural mechanisms. Finally, the suppression of sensitivity to pulse displacement during the initial phase of the vergence movement is consistent with a recently developed dual-mode model of the vergence system, in which the initial transient portion of a step response is preprogrammed whereas the final sustained portion is maintained by continuous feedback control.

Choice Behavior↗

PubMed1989

The effect of accommodative hysteresis on apparent distance.

The influence of accommodative hysteresis on monocular distance perception was examined in 18 visually normal young adults. Tonic accommodation and apparent distance were measured immediately before and after a 10 minute period of sustained focus at either the accommodative nearpoint or farpoint. Separate sessions were used for the two sustained focus conditions. On each session, apparent distance was assessed for both a highly effective and a non-effective accommodative target viewed at a constant optical distance of 3D. Sustained focus at the nearpoint resulted in a significant increase in the apparent distance of the highly effective accommodative target, as well as in tonic accommodation. Sustained focus at the farpoint yielded a slight decrease in tonic accommodation, but no significant alteration in the apparent distance of the highly effective accommodative target. No tendency for change in the apparent distance of the non-effective accommodative target was observed under either the nearpoint or farpoint condition. The results suggest that accommodative distance information is linked to the level of monocular blur-driven accommodative innervation necessary for clarity of focus.

Accommodation, Ocular↗

PubMed1989

Wallenberg's syndrome: a first-person account.

As clinicians, we may be faced with the patient who has had a stroke with resultant visual and ocular manifestations. Often it is difficult to appreciate the implications of the symptoms and their effects on the quality of life. In this article the first author describes his experiences as a patient, with the hope that this account will assist others to understand this condition better, thus resulting in both enhanced care and sensitivity.

Eye Diseases↗

PubMed1989

A case of Wallenberg's syndrome: ocular motor abnormalities.

Eye movements were measured objectively in a patient with Wallenberg's syndrome during the course of his 2-year recovery period. The patient exhibited a change in fixational and reading eye movement patterns consistent with concurrent reduction in symptoms. The results demonstrate the importance of clinical eye movement recordings in such patients to understand more fully the relationship between the visually related symptoms and the objective oculomotor correlates.

Eye Movements↗

PubMed1989

Does target orientation influence the accommodative response?

A number of studies have demonstrated that visual responsitivity is optimum when viewing either horizontal or vertical stimuli but is significantly reduced when viewing an obliquely oriented object of regard. Accordingly this study examined the accommodative response in 10 subjects to targets oriented at 45, 90, 135 and 180 degrees to the horizontal. Stimuli were located at distances of 1, 3 or 5 D, and steady-state accommodation was measured subjectively from the right eye using a Hartinger coincidence optometer. The data indicate that, for high contrast, supra-threshold targets, variation in target orientation did not produce any significant change in accommodative response. These results suggest that neural sensitivity for monocular blur-driven accommodation is orientation independent. This finding would imply that accommodation and contrast detection are mediated by dissimilar processes.

Accommodation, Ocular↗

PubMed1988

Accommodation and apparent distance.

The ability to judge egocentric distance was assessed in sixteen normal observers under conditions where monocular blur-driven accommodation provided the only source of information regarding changes in target dioptric distance. Accommodation and apparent distance were measured over an optical range of 2 to 6 D for each of three targets which differed in their effectiveness as accommodative stimuli (excellent, moderate, and poor). For the excellent and moderate accommodative targets, apparent distance decreased as accommodation increased, with both targets sharing a common linear function. Apparent distance, however, tended to exceed accommodative distance and was relatively compressed in scale. As expected, accommodative response and apparent distance diminished in range as target effectiveness was reduced. The poor accommodative target represented the limiting case wherein accommodation failed to deviate from the tonic level, and apparent distance remained constant. There were considerable intersubject differences in the ability to judge distances on the basis of accommodation. The results indicate that accommodation can indeed serve as a source of distance information, particularly for some individuals.

Accommodation, Ocular↗

PubMed1988

Dynamics of human voluntary accommodation.

Voluntary and reflex accommodation were measured monocularly in five normal subjects using a dynamic infrared optometer. Comparison of the peak velocity/amplitude relation (i.e., "main sequence") showed complete overlapping of the response distributions, suggesting similarity of motoneuronal controller signals for voluntary and reflex accommodation. Voluntary accommodation may represent a preprogrammed maneuver used in a variety of real-life predictable situations to optimize performance. Clinically, it may represent an important component in the training of accommodation dynamics.

Accommodation, Ocular↗

PubMed1988

Noncongruent stimuli and tonic adaptation.

The purpose of this investigation was to determine the influence of noncongruent stimuli on tonic accommodation and tonic vergence. Eight visually normal subjects were tested in a within-subjects design. Tonic states were measured using a haploscope-optometer immediately before and after viewing a reduced Snellen chart binocularly with suppression control in a synoptophore for 8 min. One stimulus was set at either 4 D or 4 MA, with the other set at either the maximally high (e.g., greater than 4 D or 4 MA) or low setting (e.g., less than 4 D or 4 MA) that still permitted clear and single binocular vision. Significant changes in tonic vergence but not tonic accommodation were found. The results suggest that when the oculomotor system is challenged, or stressed, under such noncongruent binocular viewing conditions, it is tonic vergence and not tonic accommodation which adapts rapidly.

Accommodation, Ocular↗

PubMed1988

Relation between distance heterophoria and tonic vergence.

The distance heterophoria was measured using both the von Graefe and Maddox rod clinic procedures under standard reduced illumination conditions and in total darkness. Tonic vergence was measured in total darkness. The phoria measures were more divergent than tonic vergence, with the von Graefe being the more divergent of the two measures. The difference between the phoria and tonic vergence position is believed to be due to accommodative divergence derived from the decrease of accommodation, relative to the tonic level, required to focus the distance phoria target. Equations were derived to predict tonic vergence from the phoria value.

Adult↗

PubMed1988

Accommodative accuracy to harmonically related complex grating patterns and their components.

Monocular steady-state accommodation was measured to harmonically related complex grating patterns and their components to determine the contribution of the various spatial waveforms to accommodative accuracy. Accommodative accuracy was greater for those waveforms containing summed odd-harmonic spatial components than for those waveforms with specific spatial components either removed or isolated. The results suggest that the contrast gradient produced by the summed waveforms, rather than the presence of any particular subset of spatial frequency components per se, was a critical factor for accurate accommodation.

Accommodation, Ocular↗

PubMed1988

The effect of monocular versus binocular fixation on accommodative hysteresis.

Changes in tonic accommodation were compared under monocular (6 or 10 D) and binocular (6 D/6 MA or 10 D/10 MA) inducing conditions in 26 visually normal young adults. Tonic accommodation was measured immediately before and after an 8 min fixation period. Statistically equivalent increases in tonic accommodation were produced under the two modes and levels of fixation. Even those individuals who showed large (greater than or equal to +0.70 D) monocularly induced changes were equally responsive under binocular fixation conditions. The results suggest that monocularly and binocularly induced accommodative hysteresis effects are similar in magnitude, regardless of the potential presence of vergence-driven accommodation under binocular but not under monocular viewing conditions.

Accommodation, Ocular↗

PubMed1988

Stability of tonic vergence.

Tonic vergence was monitored at 30 min intervals over a 10 hr period of a single day and at intervals of approximately 14 days over a period of 19 weeks. Measurements were also made during a continuous 112 min period in total darkness. Tonic vergence showed neither significant systematic variation nor large random fluctuations during the course of either a day or weeks, demonstrating stability under naturally occurring viewing conditions. During the period in total darkness, however, tonic vergence exhibited a significant decreasing trend, ie, eye position became less convergent with time. Stability was therefore poorer in the absence of visual stimuli.

Adult↗

PubMed1988

Auditory biofeedback to control vertical and horizontal eye movements in the dark.

Although there is evidence demonstrating the effectiveness of oculomotor auditory biofeedback on the control of the horizontal component of gaze in the dark, the oculomotor behavior of the horizontal and vertical components under such conditions remained unknown. Horizontal and vertical fixational eye movements were measured monocularly using an infrared limbal eye tracker in three normal subjects under three conditions: in the light, total darkness, and total darkness with two-dimensional auditory biofeedback of the eye movement. With fixation in the light, all subjects showed small drifts and corrective movements of up to about 0.5 degrees horizontally and vertically. With fixation in total darkness, the eye movements generally exhibited drifts and saccades of well over 2 degrees. However, with auditory biofeedback added during fixation in total darkness, the drifts and corrective saccades were reduced to levels more similar to those found with fixation in the light. The percent time on target in the light, dark and dark plus auditory biofeedback conditions was about 100, 50 and 80%, respectively, for both horizontal and vertical eye position. These results demonstrate that incorporation of two-dimensional oculomotor auditory biofeedback alone is sufficient to maintain fixation accuracy of both horizontal and vertical eye movements in total darkness close to that found during normal fixation in the light.

Adolescent↗

PubMed1988

Dual-mode behaviour in the human accommodation system.

The human accommodation system was investigated using a dynamic infra-red optometer to determine whether its feedback control process exhibited dual-mode behaviour. Ramp changes of target accommodative demand presented monocularly at various velocities (0.5-0.6 D s-1) with a fixed 2 D amplitude elicited two modes of behaviour. Smooth ramp movements were observed for low-velocity stimuli, whereas step responses predominated for high-velocity stimuli. At intermediate velocities, combined step-ramp responses were more prevalent. The characteristic of these step-ramp responses showed two important properties that suggest a pre-programming mechanism. The amplitude of the step portion of a step-ramp response approximated the value of the ramp stimulus at the time corresponding to the end of the step movement. Also, the initial step amplitude increased with increasing target ramp velocity. It appears that the amplitude of the preprogrammed accommodative movement was based on an estimate of the expected ramp target position at the end of the response. These findings are consistent with a recent unifying theory of oculomotor control in which the various oculomotor subsystems (versional and vergence eye movements and accommodation) each has fast and slow components. The fast component exhibits preprogramming, while the slow component shows continuous feedback control.

Accommodation, Ocular↗

PubMed1987

Tonic adaptation in symptomatic and asymptomatic subjects.

Tonic accommodation and tonic vergence were measured in symptomatic and asymptomatic subjects before, during, and after 45 min of binocular reading at near (5 D). Reading produced significant increases in both tonic levels, with the magnitude of change being similar in the two groups. After reading, both groups exhibited rapid loss of the tonic vergence effect, but showed no evidence for decay of the tonic accommodation effect. Differences between the groups were observed in the rate of adaptation during the course of reading, with both tonic components increasing more slowly for the symptomatic subjects than for the asymptomatic subjects. In addition, baseline tonic accommodation was significantly higher in the symptomatic group than in the asymptomatic group. The results suggest that although the failure to undergo tonic oculomotor adaptation is not a causal factor in asthenopia, the initial steady-state level and/or subsequent temporal changes of tonic accommodation and tonic vergence may be related to nearwork symptomatology.

Accommodation, Ocular↗

PubMed1987

Tonic accommodation, accommodative hysteresis, and refractive error.

Accommodative hysteresis was compared in high myopes, low myopes, emmetropes, and hyperopes. Subjects consisted of 48 visually normal young adults who were fully corrected for their ametropia and equally distributed among the four refractive categories. Baseline measures of tonic accommodation, nearpoint, and farpoint were followed by a 10 min period of monocular fixation on a target which provided a stimulus equal to the subject's accommodative amplitude. The tonic measures were repeated immediately after fixation and were followed by either reassessment of the nearpoint and farpoint, or a 20 min period in darkness with reassessment of the tonic level at 5-min intervals. The period of sustained maximal focus produced significant increases in both tonic accommodation and the accommodative nearpoint in each refractive group. Although the magnitude of change was statistically equivalent across groups, a tendency to greater increases in tonic accommodation was noted for the high myopes. Although the farpoint remained relatively stable in the individual refractive groups, a slight inward shift was found in the combined sample. The tonic change diminished primarily during the first 5 min in darkness, with no significant difference in either the rate or degree of decay in the various refractive groups. The results indicate no clear association between susceptibility to accommodative hysteresis and refractive category.

Accommodation, Ocular↗

PubMed1987

Time course and decay of effects of near work on tonic accommodation and tonic vergence.

Changes in tonic accommodation and tonic vergence were monitored before, during and immediately after a 45 min period of reading adult level text displayed on either a computer monitor or hard copy at 20 cm under binocular viewing conditions. Both tonic accommodation and tonic vergence gradually increased during the reading period, with changes in tonic accommodation occurring more rapidly. The average increases in tonic accommodation and tonic vergence following reading were 0.35 dioptres and 0.38 metre angles, respectively. Decay of both tonic components to the baseline in either light or total darkness occurred within the first 3 min. These results suggest that a moderate period of interrupted reading at a close distance can produce a transient increase in the tonic oculomotor levels.

Accommodation, Ocular↗

PubMed1987

Impairment of contrast discrimination in amblyopic eyes.

A successive two-alternative, forced-choice procedure incorporating a double-interleaved staircase was used to measure monocular contrast discrimination in strabismic and/or anisometropic amblyopes, strabismics without amblyopia, and normals. Standard contrast was 25%, with comparison contrasts starting at 10 and 40%. Spatial frequencies were 0.5, 2.0, and either 4.0 or 8.0 c deg-1. The amblyopic eye consistently required more contrast than the fellow dominant eye for the task. Results in each eye of the normals and strabismics without amblyopia were similar and normal. The findings clearly demonstrate impairment of contrast discrimination in amblyopic eyes, presumably due to early abnormal visual experience. Such impairment could contribute to the increased steady-state accommodative error found in amblyopic eyes.

Accommodation, Ocular↗