Dynamic Accommodation Responses Research Articles

The effect of multifocal contact lenses on the dynamic accommodation step response.

To measure the dynamic accommodation response (AR) to step stimuli with and without multifocal contact lenses (MFCLs), in emmetropes and myopes. Twenty-two adult subjects viewed alternating distance (0.25D) and near (3D) Maltese crosses placed in free space, through two contact lens types: single vision (SVCL) or centre-distance multifocal (MFCL; +2.50D add). The AR level was measured along with near to far (N-F) and far to near (F-N) step response characteristics: percentage of correct responses, magnitude, latency, peak velocity and duration of step response. There was no difference between N-F and F-N responses, or between refractive groups in any aspect of the accommodation step response dynamics. The percentage of correct responses was unaffected by contact lens type. Through MFCLs, subjects demonstrated smaller magnitude, longer latency, shorter duration and slower peak velocity steps than through SVCLs. When viewing the near target, the AR through MFCLs was significantly lower than through SVCLs. When viewing the distance target with the MFCL, the focal points from rays travelling through the distance and near zones were approximately 0.004D behind and 2.50D in front of the retina, respectively. When viewing the near target, the respective values were approximately 1.89D behind and 0.61D in front of the retina. The defocus error required for accommodation control appears not to be solely derived from the distance zone of the MFCL. This results in reduced performance in response to abruptly changing vergence stimuli; however, these errors were small and unlikely to impact everyday visual tasks. There was a decrease in ocular accommodation during near tasks, which has previously been correlated with a reduced myopic treatment response through these lenses. With MFCLs, the estimated dioptric myopic defocus was the largest when viewing a distant stimulus, supporting the hypothesis that the outdoors provides a beneficial visual environment to reduce myopia progression.

Contributed Session II: Bringing color into focus: Dynamic accommodation responses to polychromatic stimuli.

As humans look around the environment, the crystalline lens inside the eye changes optical power to bring retinal images into focus. This visuomotor response is called accommodation. For a given accommodative state, light at only one wavelength can be in focus because the eye contains significant chromatic aberration. We examined how the visual system weights different wavelengths for focusing polychromatic stimuli, especially those with peaks at more than one wavelength. With an autorefractor, we continuously measured human accommodative responses (at 30 Hz) to stimuli comprising various mixtures of short- and long-wavelength content. In a series of trials, seven human observers viewed a three-letter word stimulus spanning 1.5° (24 arcmins per letter) against a black background on an AMOLED display for seven seconds. The optical distance of the screen was varied using a focus-adjustable lens. Halfway through the trial, the stimulus underwent a step change in optical distance (±0.75, 1.00, or 1.50 diopters). Simultaneously, the color of the stimulus changed. Accommodative responses for each subject were analyzed with nested descriptive models, including a color-free model, a weighted-averaging model, and a color-switching model. The results show that stimulus color significantly influences the dynamic accommodative response, and that long wavelengths influence the response more than short wavelengths, even when their luminance is the same.

Study of the Immediate Effects of Autostereoscopic 3D Visual Training on the Accommodative Functions of Myopes.

PurposeStereoscopic viewing has an impact on ocular dynamics, but its effects on accommodative functions are not fully understood, especially for autostereoscopic viewing. This study aimed to investigate the changes in dynamic accommodative response, accommodative amplitude, and accommodative facility of myopes after autostereoscopic visual training.MethodsWe enrolled 46 adults (men = 22 and women = 24; age = 21.5 ± 2.5 [range = 18–25] years, spherical equivalent: −4.52 ± 1.89 [−8.88 to −1.75] diopters [D]) who visited the Eye & ENT Hospital of Fudan University. The study population was randomly divided into three-dimensional (3D) and two-dimensional (2D) viewing groups to watch an 11-minute training video displayed in 3D or 2D mode. Dynamic accommodative response, accommodative facility, and accommodative amplitude were measured before, during, and immediately after the training. Accommodative lag and the variability of accommodation were also analyzed. Visual fatigue was evaluated subjectively using a questionnaire.ResultsAccommodative lag decreased from 0.54 ± 0.29 D to 0.42 ± 0.32 D (P = 0.004), whereas accommodative facility increased from 10.83 ± 4.55 cycles per minute (cpm) to 13.15 ± 5.25 cpm (P < 0.001) in the 3D group. In the 2D group, there was no significant change in the accommodative lag (P = 0.163) or facility (P = 0.975), but a decrease in accommodative amplitude was observed (from 13.88 ± 3.17 D to 12.71 ± 2.23 D, P = 0.013). In the 3D group, the accommodative response changed with the simulated target distance. Visual fatigue was relatively mild in both groups.ConclusionsThe immediate impact of autostereoscopic training included a decrease in the accommodative lag and an increase in the accommodative facility. However, the long-term effects of autostereoscopic training require further exploration.

Accommodative Relaxation by Extending the Viewing Distance through the Simple Optical Design of a Double-Mirror System

Purpose: This paper discusses the accommodative relaxation of myopic adults using a simple double-mirror design. This optical design can extend the viewing distance to 2.285 m and can magnify the image up to 3.386 times, and it results in the accommodative relaxation of a single human eye. By using this optical design, accommodative relaxation may improve eye fatigue and, furthermore, it may delay the progression of myopia. Method: This study recruited 32 subjects with an average age of 20.8 ± 0.95 years old. After an examination of their refractive status, disposable contact lenses with a corresponding refractive error were fitted, and the dynamic accommodative response and change in pupil size were measured by using an open-field autorefractor. The subjects gazed at two different viewing distances. First, they gazed at a real object, which was placed 0.4 m in front of them. Second, the subjects gazed at a virtual image through a double-mirror system at a distance of 2.285 m. The dynamic accommodative responses and pupil size data were collected under these two viewing distance conditions. Results: When the subjects gazed at a real object that was 0.4 m away, or a virtual image that was 2.285 m away, the mean value of the accommodative response was 1.69 ± 0.31 D or 0.11 ± 0.05 D, and the pupil size was 3.79 ± 0.49 mm or 4.09 ± 0.72 mm, respectively. The accommodative response decreased, and the pupil size increased when using the double-mirror system, and therefore, accommodative relaxation can be achieved by using this new optical design. Conclusions: In the present study, we first successfully proposed a simple optical design to relax the accommodation, and the fluctuations of the accommodation response were stable, with an extended viewing distance of 2.285 m. This design may be applied for the improvement of visual function in applications such as the reduction of asthenopia and the control of myopia.

The effects of colour and temporal frequency of flickering light on variability of the accommodation response in emmetropes and myopes

BackgroundMyopia is hypothesized to be influenced by environmental light conditions. For example, it has been shown that colour and temporal frequency of flickering light affect emmetropisation in animals. Considering the omnipresence of flickering light in our daily life, we decided to analyze the effect of colour flickers on variability of the accommodation response (VAR) in emmetropes and myopes.MethodsWe measured the dynamic accommodative responses of 19 emmetropic and 22 myopic adults using a Grand Seiko WAM-5500 open-field autorefractor. The subjects focused for more than 20 s on a black Snellen E target against three different backgrounds made up of three colour flicker combinations (red/green, red/blue and blue/green) and under five frequency conditions (0.20 Hz, 0.50 Hz, 1.00 Hz, 1.67 Hz, and 5.00 Hz).ResultsFlicker frequency and colour both had a significant effect on VAR. Lower frequencies were associated with larger variability. Colour had an effect only at low frequencies, and red/blue colour flicker resulted in the largest variability. The variability in myopes were larger than those in emmetropes.ConclusionsThese findings support the hypothesis that further studies on the colour and temporal frequency of flickering light can lead to a better understanding of the development and progression of myopia.

Impact of Fixation Direction on Dynamic Accommodative Response

Purpose : The purpose of this study was to measure accommodative response when looking at different direction such as downgaze and sidegaze either right or left. Methods : This study was performed with 13 adult males and females(22.40±0.41 years old) no subject had ophthalmic surgery or disease and placed the targets at the primary, 10° and 20° sidegaze, 10° and 20° downgaze, The accommodative responses was measured by a open-field autorefractometer under the distances of 3 m, 1 m and 40 cm that enable consecutive measurement. Results : In all gaze directions as distance became shorter the accommodative response increased. At each viewing distance 1m and 40 cm, the accommodative responses showed the peaks 1.09± 0.10 D in 10° and 2.04±0.11 D in 20° downgaze. respectively, while the lowest accommodative responses were 0.44±0.05 D and 1.54±0.06 D in 20° and 10° sidegaze, which showed statistical differences(p＜0.050). There were significant differences in each viewing direction except for in 10° and 20° sidegaze(p＜0.050). The gaze direction and the accommodative responses were significantly different(p＜0.050). Conclusion : This study demonstrates that the accommodative responses vary with the gaze direction and gaze angle for the same modulating stimulus. Therefore, it is assumed that 20° downgaze makes the object sharper and minimizes the eye fatigue.

Can Tinted Lenses Be Used to Manipulate Pupil Size and Visual Performance When Wearing Multifocal Contact Lenses?

BackgroundThe purpose of this study is to investigate the effect of wearing neutral density (ND) filters with different transmittance levels over multifocal contact lenses (MTF CLs) on pupil size, visual functions and the dynamic accommodative response under daylight conditions in early presbyopes.MethodsSeventeen individuals aged between 40 and 48 years (mean age 42.35±2.62 years) were recruited to participate in this study. This study involved a repeated-measures design with two within-subject factors: optical condition (which had five levels), and fixation distance (which had three levels). The five optical conditions were no correction (baseline), MTF CL wear, MTF CL wear + 0.3 ND filter (transmission [Tv]=48.38%), MTF CL wear + 0.6 ND filter (Tv=27.12%) and MTF CL wear + 0.9 ND filter (Tv=14.58%). The three fixation distances were 3 m, 40 cm and 20 cm. Outcome measures were pupil size, the measured accommodative response (optical aid + physiological accommodation), distance visual acuity (VA), near VA and contrast sensitivity (CS).ResultsThere was no effect of optical condition on the accommodative response; however, there was a significant effect of fixation distance. (Accommodative response was 0.44±0.29, 1.83±0.37 and 4.03±0.46 D for fixation distances of 3 m, 40 cm and 20 cm, respectively; p<0.05.) There was a significant effect of optical condition (p<0.05) and of fixation distance on pupil size variation (5.13±0.90 mm for 3 m, 4.71±0.92 mm for 40 cm and 4.11±1.04 mm for 20 cm; p<0.05). Optical condition had a significant effect on distance VA, near VA and CS (p<0.05). Compared to the reduction in distance VA, near VA was relatively small (two-letter reduction with 0.9 ND) and unlikely to be clinically significant.ConclusionThis study demonstrated that in a group of adults with early presbyopia, the use of ND filters enlarged pupil size. However, the combination of enlarged pupils and MTF CL did not improve distance vision, and if transmittance was reduced by 50% it resulted in decrements in vision and CS. MTF CL designs which incorporate tints, or tints that vary in darkness in response to ultraviolet exposure, should consider these findings.

Acute Effects of Caffeine on Dynamic Accommodative Response and Pupil Size: A Placebo-controlled, Double-blind, Balanced Crossover Study

ABSTRACT Objectives To evaluate the acute effect of caffeine consumption on the accuracy and variability of accommodation, as well as its impact on pupil size and perceived levels of activation. Methods 22 university students (21.68 ± 3.67 years old) ingested a capsule of caffeine (4 mg/kg) or placebo (300 mg of corn-starch) in two different days and counterbalanced order. After 30 min of capsule ingestion, we objectively measured the accuracy and variability of accommodation, and pupil size using the WAM-5500 binocular open-field autorefractometer for 2 min at each of the six viewing distances (5 m, 50 cm, 40 cm, 33 cm, 25 cm, and 20 cm). Subjective levels of activation to check the effectiveness of caffeine/placebo manipulation were also reported. Results We found that after 30 min of caffeine/placebo ingestion, participant perceived higher levels of activation in the caffeine condition (p = .047, Cohen´s d = 0.48). Caffeine consumption induced a statistically significant dilator effect on pupil size (p = .011, η2 = 0.271), and reduced variability of accommodative response (p = .027, η2 = 0.211). However, no differences were obtained for the accuracy of accommodation (p = .321). Conclusions Our data suggest that caffeine consumption reduced the variability of accommodative response and induced pupil dilation. Nevertheless, the accuracy of accommodation was insensitive to caffeine intake. These findings may be explained by the bidirectional relationship between ocular functioning and the nervous system´s state of activation.

Ocular Accommodative Response is Modulated as a Function of Physical Exercise Intensity

ABSTRACTPurpose: The present study aimed (1) to examine the impact of two high-intensity interval training (HIIT) protocols differing in exercise intensity (low intensity and high intensity) on the dynamic accommodative response (AR), and (2) to elucidate whether the ocular accommodation changes are associated with the function of the autonomic nervous system during stimulus processing, as measured by the heart rate variability (HRV).Methods: Twenty physically active university students (9 women; age = 23.9 ± 3.3 years) were tested on three separate sessions. The lag of accommodation and the root mean square of successive R-R interval differences (RMSSD) parameter of HRV were obtained at the beginning and at the end of each testing session, while one of the following protocols was applied in a randomized order between the measurements, low-intensity HIIT (8 sprints with 60 s of rest), high-intensity HIIT (8 sprints with 30 s of rest), and Control (walking for 8 min).Results: Our data demonstrated a higher lag of accommodation after the high-intensity HIIT compared to the low-intensity HIIT (p = 0.006, d = 0.798) and control (p = 0.007, d = 0.741), but no significant differences were observed between the low-intensity HIIT protocol and control condition (p = 0.598, d = 0.12). As expected, lower HRV values were observed with higher exercise intensity, but the changes of AR and HRV were not significantly correlated (p > 0.05 in all cases).Conclusions: The present findings indicate that the acute effects of exercise on ocular accommodation depend on exercise intensity, showing that highly demanding physical effort induces a greater lag of accommodation, which may be of relevance when performing near activities after physical efforts.

Effect of phenylephrine on static and dynamic accommodation

PurposeWe tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics. MethodsWe performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size. ResultsWhen mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p<0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were −0.20±0.51 D, and −0.05±0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p>0.313). ConclusionsThe mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation.

Is 25 Hz enough to accurately measure a dynamic change in the ocular accommodation?

BackgroundAccommodation is often recorded at a low sampling rate using devices such as autorefractors that are designed to measure the static refractive error. It is therefore important to determine if that resolution is sufficient to accurately measure the dynamic properties of accommodation. The current study provides both theoretical and empirical evidence on the ideal sampling rate necessary to measure a dynamic response. MethodsAccommodative and disaccommodative step stimuli ranging from 1–3D (1D steps) were presented using a Badal optical system. Responses from 12 children (8–13 years) and 6 adults (20–35 years) were recorded using a dynamic photorefractor (DPR). Fast Fourier transformation was applied to the unsmoothed dynamic responses including position, velocity and acceleration. Also, velocity and acceleration main sequence (MS) characteristics were compared between three photorefractor conditions on 3 subjects. ResultsThe Nyquist sampling limit necessary to accurately estimate position, velocity and acceleration was at least 5, 10 and 70Hz, respectively. Peak velocity and acceleration were significantly underestimated at a lower rate (p<0.5). However, the slope of MS remained invariant with sampling rate (p>0.5). ConclusionContrary to the previous findings, a dynamic accommodative response exhibited frequencies larger than 10Hz. Stimulus direction and amplitude had no influence on the frequencies present in the dynamic response. Peak velocity and acceleration can be significantly underestimated when sampled at a lower rate. Taken as a whole, low sampling rate instruments can accurately estimate static accommodation, however, caution needs to be exercised when using them for dynamic accommodation.

Association of Asthenopia with Sustained Attention, Accommodation and Encephalographic Activity

Purpose: Greater accommodative lag and vergence deficits have been linked to attentional deficits similar to those observed in attention deficit hyperactivity disorder (ADHD). The purpose of the present study was to assess the effect of accommodative-vergence stress on sustained attention, lens accommodation and frontal electroencephalographic activity in young adults with high and low symptoms of asthenopia. Methods: Forty-six college students participated in the study. Based on their scores on the Convergence Insufficiency Symptom Survey (CISS), 18 participants were assigned to the high CISS symptom group, while 21 participants comprised the low CISS symptom group. All of the participants completed a test of sustained attention (Conners CPT) twice while wearing -2.00 D lenses and normally in a counterbalanced order one week apart. Their dynamic accommodative responses and electroencephalographic activity (EEG) during attentional testing was also recorded. Results: The results showed that the high CISS symptom group performed a significantly greater number of commissions, perseverations and poorer target detectability than the low CISS symptom group across both testing sessions. Accommodative lag was significantly smaller in the non-stress condition compared to the -2.0 D stress condition for both groups. The low CISS symptom group, however, showed lower high beta power (21- 29Hz) during completion of Conners CPT than the high CISS symptom group, suggesting greater cortical activation in the high CISS symptom group. Conclusions: Overall the results of the study demonstrated that symptoms of asthenopia are also correlated with symptoms of inattention and predict performance deterioration in sustained attention. Additional increase in accommodative-vergence stress, however, was not shown to disrupt performance of the high symptom group to a greater extent than in the low symptom group possibly due to the absence of accommodative deficits in the high symptom group, absence of sustained vergence deficits and/or a relatively short duration of accommodative-vergence stress.

Dynamic accommodative response to different visual stimuli (2D vs 3D) while watching television and while playing Nintendo 3DS Console

The aim of the present study was to compare the accommodative response to the same visual content presented in two dimensions (2D) and stereoscopically in three dimensions (3D) while participants were either watching a television (TV) or Nintendo 3DS console. Twenty-two university students, with a mean age of 20.3 ± 2.0 years (mean ± S.D.), were recruited to participate in the TV experiment and fifteen, with a mean age of 20.1 ± 1.5 years took part in the Nintendo 3DS console study. The accommodative response was measured using a Grand Seiko WAM 5500 autorefractor. In the TV experiment, three conditions were used initially: the film was viewed in 2D mode (TV2D without glasses), the same sequence was watched in 2D whilst shutter-glasses were worn (TV2D with glasses) and the sequence was viewed in 3D mode (TV3D). Measurements were taken for 5 min in each condition, and these sections were sub-divided into ten 30-s segments to examine changes within the film. In addition, the accommodative response to three points of different disparity of one 3D frame was assessed for 30 s. In the Nintendo experiment, two conditions were employed - 2D viewing and stereoscopic 3D viewing. In the TV experiment no statistically significant differences were found between the accommodative response with TV2D without glasses (-0.38 ± 0.32D, mean ± S.D.) and TV3D (-0.37 ± 0.34D). Also, no differences were found between the various segments of the film, or between the accommodative response to different points of one frame (p > 0.05). A significant difference (p = 0.015) was found, however, between the TV2D with (-0.32 ± 0.32D) and without glasses (-0.38 ± 0.32D). In the Nintendo experiment the accommodative responses obtained in modes 2D (-2.57 ± 0.30D) and 3D (-2.49 ± 0.28D) were significantly different (paired t-test p = 0.03). The need to use shutter-glasses may affect the accommodative response during the viewing of displays, and the accommodative response when playing Nintendo 3DS in 3D mode is lower than when it is viewed in 2D.

Accommodative response and cortical activity during sustained attention

Greater accommodative lag and vergence deficits have been linked to attentional deficits similar to those observed in Attention Deficit Hyperactivity Disorder (ADHD). The purpose of the present study was to assess the effect of accommodative–vergence stress on a measure of sustained attention (Conners CPT) used in the diagnosis of ADHD. Twenty-seven normal non-ADHD adults completed the Conners CPT twice: wearing −2.00 D lenses and normally (without the −2.00 D lenses) in a counterbalanced order with at least 24h between the sessions. Simultaneous recording of participants’ dynamic accommodative responses was performed from the right eye using the Grand Seiko WAM-5500 auto-refractor and electroencephalographic activity (EEG) in the left prefrontal region using the Neurosky Mindset headset. The results demonstrated a significantly greater accommodative lag in the −2.00 D stress condition and a significantly poorer performance on the Conners CPT as indexed by slower reaction time, greater standard error of hit reaction time, grater response variability, poorer stimulus detectability and a greater number of perseverations. No differences were observed on measures of EEG in the theta (4–7Hz), alpha (8–12Hz), and beta (12–20Hz) bands. Moreover, when directly juxtaposed with each EEG band in multiple linear regression analyses, greater accommodative lag in the stress condition was significantly associated with a greater probability of clinical classification on the Conners CPT, and was also marginally predictive of the number of omissions recorded in the stress condition. The results demonstrated that sustained attention can be influenced by such factors as accommodative–vergence stress and suggest that bottom-up processes can contribute to and potentially exacerbate attentional problems in individuals with ADHD. The study also showed that cortical dysfunction (while sufficient) may not be a necessary condition for attentional deficits.

Clinical importance of spherical and chromatic aberration on the accommodative response in contact lens wear

The aim of this study was to evaluate the accommodation response under both mono- and polychromatic light while varying the amount of spherical aberration. It is thought that chromatic and spherical aberrations are directional cues for the accommodative system and could affect response time, velocity or lag. Spherical aberration is often eliminated in modern contact lenses in order to enhance image quality in the unaccommodated eye. This study was divided into two parts. The first part was done to evaluate the amount of spherical and other Zernike aberrations in the unaccommodated eye when uncorrected and with two types of correction (trial lens and spherical-aberration controlled contact lens) and the second part evaluated the dynamic accommodation responses obtained when wearing each of the corrections under polychromatic and monochromatic conditions. Measurements of accommodation showed no significant differences in time, velocity and lag of accommodation after decreasing the spherical aberration with a contact lens, neither in monochromatic nor polychromatic light. It is unlikely that small to normal changes of spherical aberration in white light or monochromatic mid-spectral light affect directional cues for the accommodative system, not in white light or mid-spectral monochromatic light, since the accommodative response did not show any change.

Effect of temporal location of correction of monochromatic aberrations on the dynamic accommodation response

Dynamic correction of monochromatic aberrations of the eye is known to affect the accommodation response to a step change in stimulus vergence. We used an adaptive optics system to determine how the temporal location of the correction affects the response. The system consists of a Shack-Hartmann sensor sampling at 20 Hz and a 37-actuator piezoelectric deformable mirror. An extra sensing channel allows for an independent measure of the accommodation level of the eye. The accommodation response of four subjects was measured during a +/− 0.5 D step change in stimulus vergence whilst aberrations were corrected at various time locations. We found that continued correction of aberrations after the step change decreased the gain for disaccommodation, but increased the gain for accommodation. These results could be explained based on the initial lag of accommodation to the stimulus and changes in the level of aberrations before and after the stimulus step change. Future considerations for investigations of the effect of monochromatic aberrations on the dynamic accommodation response are discussed.

Effects of age on dynamic accommodation

Visual accommodation plays a critical role in one's visual perception and activities of daily living. Age-related accommodation loss poses an increased risk to older adults' safety and independence. Although extensive effort has been made towards understanding the effect of age on steady-state accommodation, dynamic aspects of accommodation is still unknown. A study was therefore conducted to investigate age-related dynamic accommodative characteristics utilising a modified autorefractor. Ten individuals from each of three age groups (i.e. younger group: 20 to 29 years old; middle-aged group: 40 to 49 years old; older group: 60 to 69 years old) were recruited and their dynamic accommodation responses were examined. The laboratory experiment was designed to assess dynamic accommodation associated with an abrupt change from a constant far target (400 cm, 50 cd/m2) to a near target (70 cm, 100 cd/m2 or 20 cd/m2), which aimed to simulate car dashboard reading behaviour while driving. The results of the study indicated that age and target intensity both had a significant impact on dynamic accommodation. These effects were attributed to both the age-related physiological limitation of the eye as well as to central neural processing delay. A method of measuring dynamic accommodation and the implications of the study are discussed. Statement of Relevance: The results of the study indicate that age and target intensity both have a significant impact on dynamic accommodation. These effects are attributed to age-related physiological limitation of the eye as well as central neural processing delay and to decreased sensitivity of the cone photoreceptors. To enhance the visual performance of the ageing population involving dynamic accommodation, target distance and target light intensity should be carefully evaluated to facilitate effective viewing.

Investigating the mechanisms that may underlie the reduction in contrast sensitivity during dynamic accommodation

Head and eye movements, together with ocular accommodation enable us to explore our visual environment. The stability of this environment is maintained during saccadic and vergence eye movements due to reduced contrast sensitivity to low spatial frequency information. Our recent work has revealed a new type of selective reduction of contrast sensitivity to high spatial frequency patterns during the fast phase of dynamic accommodation responses compared with steady-state accommodation. Here were report data which show a strong correlation between the effects of reduced contrast sensitivity during dynamic accommodation and velocity of accommodation responses, elicited by ramp changes in accommodative demand. The results were accounted for by a contrast gain control model of a cortical mechanism for contrast detection during dynamic ocular accommodation. Sensitivity, however, was not altered during attempted accommodation responses in the absence of crystalline-lens changes due to cycloplegia. These findings suggest that contrast sensitivity reduction during dynamic accommodation may be a consequence of cortical inhibition driven by proprioceptive-like signals originating within the ciliary muscle, rather than by corollary discharge signals elicited simultaneously with the motor command to the ciliary muscle.

The relationship between CA/C ratio and individual differences in dynamic accommodative responses while viewing stereoscopic images

The relationship between CA/C ratio and individual differences in dynamic accommodative responses while viewing stereoscopic images

The relationship between CA/C ratio and individual differences in dynamic accommodative responses while viewing stereoscopic images

The oculomotor synergy as expressed by the CA/C and AC/A ratios was investigated to examine its influence on our previous observation that whereas convergence responses to stereoscopic images are generally stable, some individuals exhibit significant accommodative overshoot. Using a modified video refraction unit while viewing a stereoscopic LCD, accommodative and convergence responses to balanced and unbalanced vergence and focal stimuli (BVFS and UBVFS) were measured. Accommodative overshoot of at least 0.3 D was found in 3 out of 8 subjects for UBVFS. The accommodative response differential (RD) was taken to be the difference between the initial response and the subsequent mean static steady-state response. Without overshoot, RD was quantified by finding the initial response component. A mean RD of 0.11 +/- 0.27 D was found for the 1.0 D step UBVFS condition. The mean RD for the BVFS was 0.00 +/- 0.17 D. There was a significant positive correlation between CA/C ratio and RD (r = +0.75, n = 8, p < 0.05) for only UBVFS. We propose that inter-subject variation in RD is influenced by the CA/C ratio as follows: an initial convergence response, induced by disparity of the image, generates convergence-driven accommodation commensurate with the CA/C ratio; the associated transient defocus subsequently decays to a balanced position between defocus-induced and convergence-induced accommodations.