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.

Myopes show increased susceptibility to nearwork aftereffects.

PURPOSE: Some aspects of accommodation may be slightly abnormal (or different) in myopes, compared with accommodation in emmetropes and hyperopes. For example, the initial magnitude of accommodative adaptation in the dark after nearwork is greatest in myopes. However, the critical test is to assess this initial accommodative aftereffect and its subsequent decay in the light under more natural viewing conditions with blur-related visual feedback present, if a possible link between this phenomenon and clinical myopia is to be considered. METHODS: Subjects consisted of adult late- (n = 11) and early-onset (n = 13) myopes, emmetropes (n = 11), and hyperopes (n = 9). The distance-refractive state was assessed objectively using an autorefractor immediately before and after a 10-minute binocular near task at 20 cm (5 diopters [D]). RESULTS: Group results showed that myopes were most susceptible to the nearwork aftereffect. It averaged 0.35 D in initial magnitude, with considerably faster posttask decay to baseline in the early-onset (35 seconds) versus late-onset (63 seconds) myopes. There was no myopic aftereffect in the remaining two refractive groups. CONCLUSIONS: The myopes showed particularly striking accommodatively related nearwork aftereffect susceptibility. As has been speculated and found by many others, transient pseudomyopia may cause or be a precursor to permanent myopia or myopic progression. Time-integrated increased retinal defocus causing axial elongation is proposed as a possible mechanism.

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.

Static accommodation in congenital nystagmus.

PURPOSE: To conduct a comparative study of static accommodative function between individuals with normal vision (n = 10) and patients with congenital nystagmus (n = 12). METHODS: The component contribution to monocular steady-state accommodation (slope of the accommodative stimulus/response function, accommodative controller gain, tonic accommodation, and depth-of-focus) was assessed subjectively using a Hartinger coincidence-optometer, except for depth-of-focus, which was determined psychophysically. RESULTS: The group mean slope for the patients with nystagmus was not significantly different from that found in the normal subjects. However, their variability was markedly increased. Therefore, the patients with nystagmus were divided into three subgroups with regard to the normal accommodative stimulus/response function slope criterion. The majority of patients with nystagmus (n = 10) exhibited slopes that were outside of normal limits, being greater than (n = 4) or less than (n = 6) the normal range. Depth-of-focus was the only parameter found to be significantly different between the normal and the nystagmus groups. When the nystagmus group was divided with respect to etiology--ie, albinotic (n = 4) versus idiopathic (n = 8)--there were no significant differences found for the various accommodative parameters. CONCLUSIONS: We speculate that the primary component contributing to the anomalous accommodative behavior was the increased depth-of-focus, with this perhaps being related to abnormal fixational eye movements and eccentric fixation, and more generally related to overall reduced sensitivity resulting from the early abnormal visual experience.

Increased saccadic latencies in amblyopic eyes.

Increased saccadic latencies were measured in the amblyopic eyes of subjects having amblyopia without strabismus, constant strabismus amblyopia, and intermittent strabismus. The subjects tracked a small, bright spot of light moving with random, horizontal step displacements of 0.25 to 8.5 degrees over the central retina. Normal saccadic latencies were generally found during monocular tracking with the nonamblyopic eye as well as during binocular tracking. Studies of eye-hand reaction time in amblyopic eyes have shown delays to occur over the central retina; our new finding establishes this for saccadic initiation. Normal trajectories found for all tracking saccades, normal saccadic latencies measured when the nonamblyopic eye was utilized for tracking, and synchronous movement of the eyes under all test conditions point to a sensory rather than motor basis underlying these delays. Our results are interpreted in terms of a processing delay in the sensory pathways leading from the central region of the amblyopic eye to those centers involved in saccadic initiation.

Different rates of functional recovery of eye movements during orthoptics treatment in an adult amblyope.

Although it is common clinical knowledge that oculomotor control appears to normalize during the course of successful orthoptics therapy for amblyopia, reports providing a quantitative analysis of eye movements during extended periods of treatment are lacking. We provide for the first time such a report in an adult amblyope. Aspects of eye movement control that tended to normalize with therapy include drift amplitude and velocity, duration and frequency of steady fixation, and pursuit gain. These results suggest that smooth pursuit control can be modified, even in an adult amblyope. Aspects of eye movement control that remained abnormal throughout therapy, in spite of normalization of visual acuity and centralization of fixation, include increased saccadic latencies, use of large saccades during small-amplitude pursuit tracking, and static overshooting. These results suggest that certain aspects of saccadic and pursuit control could either no longer be modified or would require longer periods for this to occur.

Abnormal saccadic substitution during small-amplitude pursuit tracking in amblyopic eyes.

Small-amplitude, low-velocity, predictable triangular tracking was tested in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia by means of a photoelectric eye-movement recording technique. In the majority of amblyopic patients, abnormal saccadic substitution was found; that is, abnormally large saccades rather than small-amplitude smooth movements were used by the amblyopic eye to follow a spot stimulus that moved horizontally with low to high frequencies. Pursuit for the same range of stimuli was normal for binocular tracking and for monocular tracking with the dominant eye, pointing to a sensory rather than motor basis for the defect. This abnormal saccadic substitution response appeared to be related to the presence of amblyopia rather than strabismus. Several possible mechanisms responsible for causing this unusual response are discussed, including impairment of direction sense over small central regions of the amblyopic eye.

Dynamic vergence eye movements in strabismus and amblyopia: symmetric vergence.

Dynamic vergence eye movements in response to step target displacements along the midline were measured by an infrared reflection technique in 11 patients having either intermittent strabismus, constants-strabismus amblyopia, or amblyopia without strabismus. We found the absence of normal disparity (fusional) vergence in all patients having strabismus and in some patients having amblyopia without strabismus. A characteristic response consisting of a binocular accommodative vergence component and an early binocular saccadic component was used to foveate the target of interest with the dominant eye. Vergence responses in our control subjects and patients with the nondominant eye occluded were similar to those recorded in our patients during binocular viewing. These results suggest that disparity information is not utilized by patients, probably as a result of long-term, ongoing suppression in the deviated or amblyopic eye. Accommodative vergence with the aid of an early foveating saccade was the primary mechanism for tracking targets in three-dimensional space.

Prognostic value of laser interferometric visual acuity in amblyopia therapy.

There has been no simple clinical test which accurately predicts post-therapy visual acuity in amblyopic eyes. Since grating test patterns generally yield optimal visual acuity in amblyopic eyes, the authors sought to determine if pre-therapy laser interferometric grating visual acuity would predict conventional post-therapy visual acuity in functional amblyopia. In 90% of the patients who completed therapy, the pre-therapy laser visual acuity was within two lines of the post-therapy Snellen visual acuity. Thus, pre-therapy laser visual acuity is a good prognostic indicator of conventional post-therapy visual acuity in amblyopic eyes.

Quantitative assessment of disparity vergence components.

Earlier evidence suggests qualitatively that at least two control modes may mediate a single vergence response. Thus, in a vergence response to step disparity, the transient component drives the initial fast dynamic portion of the response, while the sustained component maintains the latter slower portion of the response. The authors extended this hypothesis by quantitatively defining the stimulus pattern and dynamics that elicit this dichotomous behavior. The disparity target consisted of vertical lines 2 deg in height presented to each eye. Ramp disparity velocities ranged from 0.7 deg/sec to 36 deg/sec with amplitude of up to 4 deg. Photoelectric recordings of eye movements from both eyes were subtracted to give the vergence response. Fast and slow ramp stimulus velocities were found to elicit transient and sustained component responses respectively. In addition, the finding of staircase-like responses to fast ramp stimuli has strong implications on control mechanisms, indicating a sampling process in the transient component of the disparity vergence system.

Short-term changes in tonic accommodation.

Experiments were performed to determine the diurnal variation of, as well as the influence of total darkness on, tonic accommodation. In general, under both conditions trend analysis showed that variations in tonic accommodation over time were either nonsystematic in nature or could be best described by a simple linear function. Given the relatively small range of mean tonic accommodation values over time (0.5 to 1.1 D), the results demonstrate the robustness and stability of tonic accommodation under naturally occurring viewing conditions during the course of a day. In contrast, during the 2-hr period in total darkness, tonic accommodation exhibited a three-fold increase in mean range (0.6 to 2.5 D) as well as a significant increasing linear trend in some subjects, suggesting less stability of tonic accommodation in the absence of visual stimuli.

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.

Identification of accommodative vergence contribution to the near response using response variance.

An experimental method was developed to isolate accommodative and disparity vergence contributions to coordinated near vergence motor responses. The variability normally associated with the neural control signal was used as an identifying marker or tag. Using this approach, the results showed an increased variability associated with the blur-driven, accommodative vergence component that is particularly noticeable during the latter half of the movement. This indicated that accommodative vergence provides a moderate contribution to the near vergence response primarily in the late and post-transient period when the movement is essentially complete. Presence of this accommodative contribution lends further support to the "dual interactive model" of near triad control.

Accommodation dynamics in divergence excess exotropia.

Objective recording of accommodation dynamics was performed in four patients with divergence excess exotropia (two true and two simulated) and in three visually normal control subjects. The accommodative peak velocity/amplitude relationship was normal in the exotropic patients and in the control subjects. Latency for decreasing accommodation was increased significantly, and overshoot frequency was decreased, in the divergence excess exotropes of the true variety, thus demonstrating slightly slowed accommodation dynamics in this diagnostic group. These subtle deviations in response dynamics reflect central rather than peripheral mechanisms.

Proximally induced accommodation and accommodative adaptation.

To determine the effect of proximally induced accommodation (PIA) on accommodative adaptation, this study has examined the posttask shift in tonic accommodation (TA) following 5-min monocular viewing of equivalent-sized targets located at distances of 0.33 and 5 m. The distal target was viewed through a negative lens to equate the dioptric stimuli (3 D). The steady-state accommodative response was measured subjectively in 10 subjects using a Hartinger coincidence optometer. A significant correlation was observed between the degree of adaptation following the two conditions, with the magnitude of adaptation for the distal target being approximately half that for the nearer target. Furthermore, adaptation magnitude was inversely correlated with pretask TA under both conditions. These results indicate that PIA can produce accommodative adaptation. The implications of this finding are discussed with regard to models of the accommodative mechanism.

Vergence eye movements under natural viewing conditions.

PURPOSE: To determine whether there are any fundamental differences in vergence dynamics under different viewing conditions, both in instrument space and free space. METHODS: Symmetric vergence responses were measured for a variety of conditions under the traditional instrument space as well as the more natural free space viewing environment. Vergence eye movements were recorded objectively in three subjects using the infrared reflection technique. Within each environment, four conditions were tested: aperiodic self-initiated voluntary gaze shifts between two simultaneously viewed targets; periodic (0.33 Hz) voluntary gaze shifts between the same two targets; gaze shifts in total darkness to the near target following initial far target fixation in the light; and gaze shifts in total darkness to the two remembered target positions. In addition, an experiment was performed in instrument space using randomized step changes of target disparity, in which the responses served as the standard for comparison. For all conditions, target disparities ranged from 0.5 degrees to 10 degrees. The peak velocity of each vergence response was calculated and plotted versus its amplitude. RESULTS: It was found that the data for all conditions tested fell within the standard "main sequence" cluster, indicating similarity in dynamics and thus similarity in the motoneuronal controller signal. Also, the data from investigators who claimed differences in dynamics were also typically found to fall within the normal cluster. CONCLUSIONS: This indicates that the vergence motoneuronal controller signal produced the same dynamics for a particular response amplitude, independent of both viewing environment and test condition.

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.

Saccadic intrusions in strabismus.

Fixational eye movements were studied to determine the presence of and to quantify saccadic intrusions under monocular and binocular viewing conditions in subjects with intermittent strabismus, amblyopia without stabismus, or constant strabismus amblyopia. Saccadic intrusions were present under most test conditions in intermittent strabismus, were rarely observed in amblyopia without strabismus, and were prominent during monocular fixation with the amblyopic eye in constant strabismus anblyopia. This suggests that the presence of saccadic intrusions was related to strabismus and not amblyopia. There was no relationship between saccadic intrusion amplitude and visual acuity. Two possible mechanisms for producing intrusions are abnormally rapid regional visual adaptation and strabismus-induced fixation degradation.

Spatial frequency dependence of accommodative responses in amblyopic eyes.

Monocular, steady-state accommodative responses were measured as a function of spatial frequency of simple sinusoidal gratings presented at high contrast and target vergence levels in amblyopes, as well as in strabismics without amblyopia and in visually-normal control subjects. In general, spatial frequency dependence of the accommodative response was the rule. However, the amblyopic eyes exhibited markedly reduced accommodative responses over most of the spatial frequency range tested, and this was attributed to reduced accommodative controller gain in the sensory pathways involved in the control of accommodation in the amblyopic eye. Due to the diversity of accommodative response spatial frequency profiles found across all groups, the results suggest that reflex, voluntary, and higher-level perceptual aspects of accommodation may interplay in a complex manner in the act of accommodation on a simple sinusoidal grating.

Static aspects of accommodation: age and presbyopia.

Although the progressive reduction in accommodative amplitude with increased age is well documented, little is known about several other aspects of static or steady-state accommodation to provide a comprehensive assessment of changes related to age and presbyopia. Static components of accommodation (tonic accommodation, depth-of-focus, slope of the stimulus/response function, and accommodative controller gain) were assessed objectively using an infrared (IR) optometer in 30 human subjects aged 21-50 years; depth-of-focus was also determined psychophysically as was accommodative amplitude. Tonic accommodation and the amplitude of accommodation decreased with increased age, whereas the subjective depth-of-focus increased; the other parameters remained unchanged. The decrease in tonic accommodation and amplitude of accommodation was attributed to biomechanical factors, whereas the increase in subjective depth-of-focus was believed to result from increased tolerance to defocus related to the gradual onset of presbyopia. Constancy of the objective depth-of-focus suggested absence of age effects on the neurologic control of reflex accommodation, whereas the lack of systematic change in slope and controller gain provided support for the Hess-Gullstrand theory of accommodation and presbyopia.