The change of spherical aberration during accommodation and its effect on the accommodation response

Theoretical and ray-tracing calculations on an accommodative eye model based on published anatomical data, together with wave-front experimental results on 15 eyes, are computed to study the change of spherical aberration during accommodation and its influence on the accommodation response. The three methodologies show that primary spherical aberration should decrease during accommodation, while secondary spherical aberration should increase. The hyperbolic shape of the lens' surfaces is the main factor responsible for the change of those aberrations during accommodation. Assuming that the eye accommodated to optimize image quality by minimizing the RMS of the wave front, it is shown that primary spherical aberration decreases the accommodation response, while secondary spherical aberration slightly increases it. The total effect of the spherical aberration is a reduction of around 1/7 D per diopter of stimulus approximation, although that value depends on the pupil size and its reduction during accommodation. The apparent accommodation error (lead and lag), typically present in the accommodation/response curve, could then be explained as a consequence of the strategy used by the visual system, and the apparatus of measurement, to select the best image plane that can be affected by the change of the spherical aberration during accommodation.     

Visual sensitivity to temporal modulations of temporal noise

The present endeavor is meant (a) to provide a direct comparison between first- and second-order temporal modulation and, by so doing, (b) to eliminate all spatial clues that might have contaminated previous assessments of the second-order temporal modulation transfer function (TMTF). The second aim was achieved by means of the temporal modulation of a purely temporal white noise, a stimulus used frequently in psychoacoustics but not used as yet in visual stimulation. Luminance and contrast temporal modulation thresholds were measured with a 2AFC staircase procedure. In the first case, the mean luminance of a spatially homogeneous, 30 degrees field was modulated sinusoidally over time (first-order modulation). In the second case, the luminance of the same or of a 60 degrees field was randomized over time at a rate of 150 Hz and this temporal white noise (the carrier) was modulated sinusoidally over time (second-order modulation). First-order thresholds reproduce the classical (large field) flicker sensitivity. Second-order thresholds (measured for the first time with purely temporal stimuli) are at least 100 times higher than first-order ones, display a low-pass characteristic (at least up to 0.5 Hz) and yield a critical fusion frequency (measured at 100% modulation) of approximately 10 Hz. The data are in accord with other estimates of the TMTF of the second-order system and thus confirm the effective neutralization of the spatial cues present in these previous studies.     

The effect of accommodation response on the dark focus

The “dark focus” position of the eye is influenced by the level of pre-measurement accommodation. Three effects were found firstly during direct stimulation, accommodation responses away from the dark focus initiated a ‘linear’ shift in dark focus position towards the response. Secondly, the gradient of shift is proportional to the duration of sustained accommodation. And thirdly, with reference to the dark focus level, positive accommodation resulted in a bigger (or more rapid) shift than negative accommodation.     

Effects of temporal frequency on the contrast sensitivity of the human accommodation system

Contrast thresholds were determined for the initiation of accommodative responses to drifting sine-wave stimuli (4.1 c/deg) over a range of temporal frequencies (0-14.0 Hz). Accommodation was monitored with a dynamic infrared optometer. Psychophysical detection thresholds were also determined for the same stimuli. A comparison of the contrast thresholds for detection with those for the initiation of accommodation indicates that these two tasks are mediated by mechanisms of comparable sensitivity. Examination of the effects of temporal frequency on the contrast sensitivity of these two visual tasks reveals modest differences in the tuning properties of the underlying sensory mechanisms. Transient mechanisms tuned to intermediate temporal frequencies appear to have a greater role in the initiation of accommodative responses to grating stimuli than in the detection of such stimuli.     

The increasing sclerosis of the human lens with age and its relevance to accommodation and presbyopia

By means of a fine conical probe and a miniature dynamometer, the resistance to penetration of different lens layers was measured. In clear human lenses the power of resistance of the lens nucleus increases with age, mostly due to the hardening of the nucleus. A distinct hardening of the nucleus as opposed to the cortex has been found to occur in lenses as young as 20 years of age. This firmness of the lens nucleus, occurring between the ages of 20 and 60 years, coincides with the decrease in accommodation range and the onset of presbyopia.Supported by the Ernst-und Berta-Grimmke-Stiftung Offprint requests to: H. Pau     

Stimulating human accommodation without changes in focus

PURPOSE: Inspired by the finding in chickens that preferential stimulation of the ON retinal system suppresses myopia induced by negative spectacle lens wear and that stimulation of the OFF system suppresses the hyperopia induced by positive lens wear, we sought to determine whether stimulation of the ON-OFF retinal systems could drive directional accommodation responses in humans. If emmetropisation and accommodation use similar image processing algorithms, more accommodation would be expected with OFF stimulation. METHODS: Accommodation responses were measured while viewing a computer-generated pattern designed to stimulate the ON-OFF systems. The stimulus comprised a rectangular field (12 x 9.5 cm) on a black background filled with 196 discs (diameters: 0.4-1.0 cm). These were presented on an LCD monitor in a dark room at a viewing distance of 55 cm (1.8 D). Thirteen subjects aged 21-37 years took part. The individual discs had saw-tooth shaped temporal luminance profiles with the same time period but with random phases with respect to each other, so that the mean brightness of the stimulus was constant. To eliminate accommodation responses based on other cues (i.e. proximity) a 0.5 mm artificial pupil was used to open the accommodation loop. Refraction in the vertical pupil meridian was continuously recorded with an infrared photorefractor (the PowerRefractor). To verify that computer-based stimuli presented within our experimental design were effective in driving accommodation, previously studied stimuli were also tested: changes in size (looming) and incremental low pass filtering. RESULTS: Preferential stimulation of the ON or OFF subsystems produced a convincing depth illusion in all subjects (which was psychophysically confirmed in four subjects). Although the stimulus appeared to move in depth it did not produce accommodation responses that were consistent with that, i.e. the accommodation system did not appear to fluctuate in rhythm with the temporal oscillations of the stimulus. As the target appeared to loom it induced a greater accommodation response then when it appeared to recede. The looming target produced changes in the accommodation response in nine of 13 subjects that were consistent with its perceived change in proximity (although the target did not actually move in depth). Incremental low pass filtering produced non-directional drifts of accommodation in all subjects. Combinations of the stimuli (i.e. looming and low pass filtering, ON/OFF and looming) were not more effective stimuli to accommodation. After removal of the artificial pupil (closed loop conditions), accommodation was no longer induced with any of these stimuli. CONCLUSIONS: Although the preferential ON or OFF stimulation produced a pronounced illusion of motion in depth despite constant average brightness, proximal accommodation was induced in only one subject. Therefore, the ON/OFF stimulation appeared to have only minor input into proximal accommodation. Potential inputs into reflex accommodation need to be defined in further studies.     

Arousal-related changes in dark focus accommodation and dark vergence

Dual-innervation theory suggests that dark focus (DF; the accommodation in effect in total darkness) reflects a balance between sympathetic and parasympathetic inputs. Since it is assumed that relaxation of accommodation is sympathetically innervated, and that accommodative increases are parasympathetically innervated, stimulation that leads to sympathetic activation should produce lower DF values than is the case when there is greater parasympathetic influence. In addition, the synkinesis between accommodation and vergence, and the known effects of fatigue, injury and drugs on fusion, promote an interest in the effects of arousal on dark vergence (DV; the vergence in effect in total darkness). Twenty-three young adult males experienced a variety of stimulus conditions. The conditions, selected because of their likely autonomic effects, included deep muscle relaxation, cold pressor and mental activity, among others. During each condition, measurements were made of DF (with a laser optometer) and DV (with a Nonius alignment system). Results for DF were consistent with dual innervation theory; manipulations expected to induce sympathetic activation (eg, cold pressor) produced lower DF values than did conditions expected to induce more parasympathetic (or less sympathetic) activation (eg, relaxation). DV tended to be influenced in the opposite direction, in that relaxation led to smaller DV angles than did the other conditions. Several analyses confirmed a lack of synkinesis between accommodation and vergence during darkness.     

Time-averaged accommodation response to flickering stimuli

Experiments are described in which the steady-state accommodation response versus stimulus curve was measured with an infrared autorefractor for high-contrast stimuli having a 100% square-wave temporal modulation of luminance in the frequency band 1-200 Hz. Slightly more accurate responses were found at frequencies approximately 50-100 Hz, i.e. above flicker fusion. The relevance of the results to practical situations in which flicker may be experienced is discussed.     

Pupil size, mean accommodation response and the fluctuations of accommodation

We wished to determine how pupil size and mean accommodation response level interact to influence the fluctuations of accommodation. A dynamic infra-red optometer was used to record accommodation responses while subjects viewed a steady target at two stimulus levels (1.5 and 3 D) through four pupils (1, 2, 4 and 6 mm). It was found for most subjects that the fluctuations of accommodation increase at higher mean accommodation response levels, and small pupils lead to an increase in the low frequency (but not the high frequency) fluctuations of accommodation. The effects of mean accommodation response are independent of pupil size, and the effects of pupil size are independent of mean response level.     

Modulation dependence of the accommodation response to sinusoidal gratings

The steady-state, monocular, accommodation response to sinusoidal grating targets (spatial frequencies 0.67, 2.0, 6.0 and 18 c/deg) at effective distances between 0.2 and 1.0 m was measured with a laser optometer, as a function of the grating modulation. In general, only a rather weak dependence of the response on modulation was observed. The results are discussed in terms of the theoretical modulation of the retinal image as a function of defocus. It is argued that the accommodative system usually works to produce a contrast in the grating image which is well above the detection threshold. Considerable inter-subject differences in response were observed, suggesting that a variety of strategies may be employed by different subjects when engaged upon the same voluntary accommodation task.     

A possible change of refractive index with age and its relevance to chromatic aberration

Summary The Purkinje-Sanson images of the eyes were photographed in red and blue light in two groups of subjects, young (19–38 years) and old (52–63 years). Densitometric examination of the negatives showed a significant increase in the amount of red light and a significant decrease in the amount of blue light reflected from the anterior surface of the lens (third image) in the group of old subjects as compared to the group of young persons. There is also a significant decline in the blue light reflected from the posterior surface of the lens (fourth image) but no change in the red light in old versus young eyes. From these results it is suggested that the index of refraction of the lens increases for red (n _C) and decreases for blue light (n _F ) in the aging eye and similarly the index of the vitreous varies differentially in blue and red light. These specific changes in the indices of the lens and vitreous are believed to account for the large decline of axial chromatic aberration of the aging eye. In addition, the examination of the first Purkinje-Sanson image tends to indicate that the index of refraction of the cornea does not vary with age.     

Changes in the dark focus of accommodation in patients with asthenopia]

The authors studied tonic accommodation in cases of asthenopia. The subjects consisted of ten college students, aged 20 to 22 years old as normal control and seven patients with asthenopia, aged 20 to 27. The dark focus of accommodation was measured with the infrared optometer before and after a 15 min sustained visual task. No significant differences were found between the two groups on the pre-task dark focus of accommodation. After the task, significant differences were found between the pre- and post-task dark focus of accommodation in patients with asthenopia. The results suggest that the patients with asthenopia are more susceptible to autonomic nervous system imbalance than healthy persons.     

Contrast sensitivity, spatial and temporal tuning of the larval zebrafish optokinetic response

PURPOSE: To characterize the quantitative properties of the optokinetic response (OKR) in zebrafish larvae as a tool to test visual performance in genetically modified larvae. METHODS: Horizontal OKR was triggered in 5-day-old zebrafish larvae by stimulation with projected computer-generated gratings of varying contrast, angular velocity, temporal and spatial frequency, and brightness. Eye movements were analyzed by a custom-made eye tracker based on image analysis. RESULTS: The gain of the OKR slow phase was dependent on angular velocity, spatial frequency, and contrast of a moving grating, but largely independent on brightness. Eye velocity was a logarithmically linear function of grating contrast with a slope of approximately 0.8 per log unit contrast. CONCLUSIONS: The OKR of the larval zebrafish is not scaled for stimulus contrast and spatial frequency. These properties make the OKR a valuable tool to quantify behavioral visual performance such as visual acuity, contrast sensitivity, and light adaptation. This behavioral paradigm will be useful for analyzing visual performance in mutant and gene-knockdown larval zebrafish.     

Absence of pupil response to blur-driven accommodation.

The drive to the pupil constriction associated with near fixation has generally been attributed to accommodation with convergence and fusional convergence having secondary roles. However, our previous investigations have shown that significant changes in accommodation can take place without concomitant pupil response. To investigate further, the present study recorded pupil and accommodation responses to a blur-only accommodative stimulus using a target moved sinusoidally at a range of temporal frequencies. Care was taken to minimise target size change and apparent lateral or vertical target displacement. Results show that pupil response could be very much reduced or absent irrespective of stimulus temporal frequency and despite maintained accommodation response. The results suggest that blur-driven accommodation alone is not sufficient to drive pupil near response and that the presence of cues such as size change and lateral or vertical displacement of an approaching object may be necessary to elicit a response.     

Visual accommodation and sustained visual resolution in multiple sclerosis.

Many multiple sclerosis (MS) patients frequently experience transient blurring. We investigated the possibility that this symptom is due to the inability of patients to sustain an accommodative response to stimuli viewed at distances nearer than or farther from the individual tonus position of accommodation. In a group of MS patients and age-matched healthy control subjects, we measured (1) accommodative range and tonus position; (2) reaction time (RT) to detect a change in a small optotype (viewed in a Badal lens system) as a function of viewing distance; and (3) contrast sensitivity at a fixed viewing distance. MS patients did not differ significantly from healthy controls on near point, far point, pupil size, accommodative range, or tonus position measures. However, as a group, MS patients showed significantly slower RTs than controls to detect optotype changes for stimuli viewed at distances nearer to or farther from the individual tonus position of accommodation. All subjects showed significantly slower RTs to detect changes in optotypes viewed at extreme near and far optical distances compared to RTs to detect changes in stimuli viewed at the tonus position. This difference was significantly larger for MS patients than for controls. These data also suggest that dynamic dioptric factors contribute to the magnitude of contrast sensitivity deficits in this patient population and indicate that the relationship between the individual tonus accommodation position and viewing distance is an important variable in CS testing.