The stimuli to accommodation in the optometric examination.

The study of the stimuli to accommodation has been a topic of particular interest to vision scientists for the past 40 years. Currently, as revealed by a review of the literature, there are 11 different stimuli to accommodation. While it is difficult enough to hold 10 stimuli constant to measure the effect of a single stimulus, stimulus control becomes an even bigger problem when we consider stimulus interactions. With 11 different stimuli there are 2,047 possible stimulus combinations. In the present paper, the authors discuss at length the relationship between stimulus control problems and clinical optometric routine.     

Accommodation responses to flickering stimuli

It has been suggested that accommodation to visual display unit (VDU) symbols or to stimuli lit by fluorescent tubes or similar temporally modulated light sources may be less accurate due to flicker. Furthermore, if the microfluctuations of accommodation play a part in the control of steady-state accommodation, the effects of stimulus flicker might affect the quality of this control. Experiments are described in which monocular accommodation stimulus-response curves for sinusoidal grating stimuli (1.0, 5.3 and 8.9 c/deg) were measured under the following conditions of temporal modulation: (1) square-wave on-off modulation to give spatial modulation varying between 0 and 37%; (2) sinusoidal modulation to give spatial modulation varying between 0 and 37%; (3) sinusoidal modulation to give spatial modulation varying between 16 and 32%; and (4) on-off modulation with varying mark/space ratio. Temporal modulation frequencies were between 0.5 and 7.8 Hz and above the critical fusion frequency (CFF) i.e. 40 Hz. In general, the results suggest that while lower-frequency flicker may adversely affect the accuracy and stability of the accommodation response, the latter are very little affected by flicker at frequencies ≥ 40 Hz. Thus flicker from fluorescent lamps or VDUs is unlikely to cause systematic accommodation difficulties. The possible relevance of the results to theories which ascribe a role for the higher-frequency (> 0.5 Hz) accommodative microfluctuations in the control of the response is considered.     

Characteristics of dynamic accommodation responses: comparison between the dominant and non-dominant eyes

Accommodation responses of dominant and non-dominant eyes were compared in 18 healthy subjects aged 19–21 years to clarify the characteristics of dynamic accommodation. Internal targets were placed at −0.25D and −4.0D in an infrared optometer of a modified model, and external targets (brightness 30cd/cm², diameter 35 mm) identical in appearance with the internal targets, were placed 4.0 m and 0.25 m in front of the eyes. Three experiments were carried out by monocular viewing of the internal targets and monocular and binocular viewing of the external targets, and the results were compared between the dominant and non-dominant eyes. In viewing the internal targets, near-to-far responses were suppressed. In binocular viewing, the accuracy of accommodative positron was increased, and the function of dynamic responses was improved. Furthermore, myopic shifts were observed in the near position after far-to-near accommodation and in the far position after near-to-far accommodation in the dominant eye compared with the non dominant eye, and shortening of the response time and an increase in the response velocity were noted only in binocular viewing. These findings suggest that the dominant eye is in a tonic state and plays the primary role in far-to-near accommodation in binocular viewing.     

Effects on eyestrain of outward stimuli for accommodation

PURPOSE: We investigated experimentally the effect on eyestrain of outward-shift stimuli for accommodation. METHODS: Six female students fixed optical outward-shift stimuli for accommodation during 2 min immediately after 15 min of a sustained task on a 3-D display. Before and after the trial, their ocular functions were measured and their subjective symptoms were assessed. The outward-shift stimuli were applied by moving the targets of scenery images from far to near and near to far repetitively at around both eyes' far point position. As a control, stationary targets at the position of 0 D were used instead of moving targets. The same experiment was repeated twice for each subject. RESULTS: In the first experiment, there were significant changes in the accommodative contraction time, the accommodation lag, and the myopic shift, and the subjective symptoms of "eyestrain" and "eye dryness" in the control group, but in the stimulus group there were only increases of the complaint of "eyestrain". In the second experiment, the changed items were the contraction time and the complaints of "eyestrain" and "eye dryness" in both groups. The outward-shift stimuli caused a tendency in the far point to shift outward in the first experiment, and there was a statistically significant inward shift in the second experiment. CONCLUSION: From these results, it is suggested that the outward-shift stimuli for accommodation are effective in relieving eyestrain following the deterioration of ocular functions except when the far point is shifted inward.     

Retinotopic accommodation responses in myopia

PURPOSE: A reduced sensitivity to retinal image blur has been reported in myopes. Diminished blur detection reduces the error signal to the retinotopic (blur-induced) accommodation system and results in impaired accommodation responses under retinotopic conditions. This study was conducted to investigate retinotopic accommodation responses in emmetropia and myopia under dynamic conditions. METHODS: Static accommodation responses to a blur-only target with vergences of 0 to 4.5 D were measured with an optometer. Microfluctuations of accommodation were recorded with the subject viewing the target at a vergence of 4 D, and dynamic step responses were measured for step stimuli from 2.5 to 3.5 D and 2.0 to 4.0 D, with the optometer in dynamic recording mode. Measurements were obtained from a group of 32 visually normal emmetropes (EMMs) and subjects with progressing myopia. RESULTS: Stimulus-response curves were not significantly different between the refractive groups. Subjects with late-onset myopia (LOMs) demonstrated significantly larger accommodation microfluctuations compared with emmetropes and subjects with early-onset myopia (EOMs). Fourier analysis revealed that the increase in the magnitude of the fluctuations was mediated by the low-frequency components. Accommodation step responses revealed longer reaction times in LOMs. Further analysis showed that LOMs responded to accommodation step stimuli only between 43% and 64% of the time. In contrast, the other groups showed a response rate of almost 100%. CONCLUSIONS: The experiments demonstrate a reduction in retinotopic processing in LOMs, which results in an increased variability in their dynamic accommodation response to stationary near targets and reduced performance for dynamic step tasks. The results demonstrate a reduced blur appreciation under dynamic conditions in these refractive groups that may lead to periods of retinal image blur of varying magnitude during near work.     

The development of young infants' ability to detect stimuli in the nasal visual field

We compared infants' ability to detect single lines of varying width in the temporal and nasal visual fields. The smallest lines detected by 1-month-olds at 20 degrees in the nasal visual field were more than eight times wider than those detected at 30 degrees in the temporal visual field. In contrast, 2-month-olds detected smaller lines at 20 degrees in the nasal visual field than at 30 degrees in the temporal visual field. Converging evidence suggests that the observed improvement between 1 and 2 months in detection in the nasal visual field reflects the maturation of a projection from the retina through the visual cortex to the superior colliculus.     

Vergence accommodation and monocular closed loop blur accommodation have similar dynamic characteristics

Retinal blur and disparity are two different sensory signals known to cause a change in accommodative response. These inputs have differing neurological correlates that feed into a final common pathway. The purpose of this study was to investigate the dynamic properties of monocular blur driven accommodation and binocular disparity driven vergence-accommodation (VA) in human subjects. The results show that when response amplitudes are matched, blur accommodation and VA share similar dynamic properties.     

Accommodation responses to stimuli in cone contrast space

The aim was to identify the cone contributions and pathways for reflex accommodation. Twelve illumination conditions were used to test specified locations in cone-contrast space. Accommodation was monitored continuously in a Badal optometer while the grating stimulus (2.2 c/d sine-wave; 0.27 modulation) moved sinusoidally (0.195 Hz) towards and away from the eye from a mean position of 2.00 D (+/-1.00 D). Mean accommodation level and dynamic gain and phase at 0.195 Hz were calculated. Mean accommodation level varied significantly when the long- and middle-wavelength cone contrast ratio was altered in both the luminance and chromatic quadrants of cone-contrast space. This experiment indicates that L- and M-cones contribute to luminance and chromatic signals that produce the accommodation response, most likely through magno-cellular and parvo-cellular pathways, respectively. The L:M cone weighting to the luminance pathway that mediates accommodation is 1.63:1. The amplitude and direction of the response depends on changes in chromatic contrast and luminance contrast signals that result from longitudinal chromatic aberration and defocus of the image.     

C-type horizontal cell responses to annular stimuli.

Intracellular recordings of C-type horizontal cells were made in the isolated carp (Cyprinus carpio) retina. The C-type cells were subdivided into two types in accordance with their spectral responses: biphasic (RG) and triphasic (YRB). The responses of the two types of cell were investigated to large fields of centered and annular stimuli with a given wavelength and increasing intensities combination. The threshold and the properties to both centered and annular flashes were different from one another for each hyperpolarizing and depolarizing component of the two types of cells. The hyperpolarizing component at peaks of spectral response curves of the RG- and YRB-cell evoked to the centered flashes always surpassed that evoked to the annular flashes at any intensities. On the other hand, the depolarizing component of the RG- and YRB-cell and the second hyperpolarizing component of the YRB-cell evoked to the centered flashes were reversed in polarity as the intensity was increased. The depolarizing and the second hyperpolarizing component of the two types of cells evoked to the annular flashes were progressively developed at increasing intensities, and surpassed that evoked to the centered flashes at a given intensity. The results obtained show that there is a not-well defined color dependent center-surround organization in the receptive field of C-type horizontal cells.     

Effect of third-order aberrations on dynamic accommodation

We investigate the potential for the third-order aberrations coma and trefoil to provide a signed cue to accommodation. It is first demonstrated theoretically (with some assumptions) that the point spread function is insensitive to the sign of spherical defocus in the presence of odd-order aberrations. In an experimental investigation, the accommodation response to a sinusoidal change in vergence (1-3D, 0.2Hz) of a monochromatic stimulus was obtained with a dynamic infrared optometer. Measurements were obtained in 10 young visually normal individuals with and without custom contact lenses that induced low and high values of r.m.s. trefoil (0.25, 1.03 microm) and coma (0.34, 0.94 microm). Despite variation between subjects, we did not find any statistically significant increase or decrease in the accommodative gain for low levels of trefoil and coma, although effects approached or reached significance for the high levels of trefoil and coma. Theoretical and experimental results indicate that the presence of Zernike third-order aberrations on the eye does not seem to play a crucial role in the dynamics of the accommodation response.     

MfERG responses to long-duration white stimuli in glaucoma patients

The intent of our study was to evaluate whether the response to a long-duration white stimulus in the multifocal electroretinogram (mfERG) is sufficiently sensitive to detect early retinal dysfunction in glaucoma. On–off mfERGs were recorded from 15 NTG and 15 HTG patients and compared with 14 control subjects. Recording parameters were the following: LED stimulus screen (RETIscan™), 100-ms stimulus duration, 200-ms stimulus interval, 11-min total recording time, stimulus matrix of 61 elements, frame rate: 70 Hz, Lmax: 180 cd/m², Lmin: 0 cd/m², and filter setting: 1–200 Hz. The second negative response following stimulus onset (N2-on), as well as following stimulus offset (N2-off), was analyzed as an overall response and in quadrants, as well as in 4 small central and four adjoining peripheral areas per quadrant. The latency of the N2-on was significantly delayed in HTG in all response averages tested, while in NTG this was only seen in the overall response and in the small central response averages (P < 0.05). The most sensitive measure in HTG was the latency of the N2-on of the small peripheral response average of the superior temporal quadrant with an area under the ROC curve of 0.881. For NTG, the most representative measure was the latency of the N2-on of the small central response average of the inferior nasal quadrant with an area under the ROC curve of 0.793. Our results showed that in stimulation with long-duration flashes, the second negative response following the on response, representative of the early PhNR, is affected in glaucoma where N2-on showed a latency delay in POAG patients. The latency delay of the N2-on was more prominent for HTG than for NTG.     

Wavefront aberration changes caused by a gradient of increasing accommodation stimuli

Purpose

The aim of this study was to investigate the wavefront aberration changes in human eyes caused by a gradient of increasing accommodation stimuli.

Design

This is a prospective, single-site study.

Methods

Healthy volunteers (n=22) aged 18–28 years whose refraction states were emmetropia or mild myopia, with astigmatism <1 diopter (D), were included in this study. After dilating the right pupil with 0.5% phenylephrine drops, the wavefront aberration of the right eye was measured continuously either without or with 1, 2, 3, 4, 5, or 6D accommodation stimuli (WFA1000B psychophysical aberrometer). The root mean square (RMS) values of the total wavefront aberrations, higher-order aberrations, and 35 individual Zernike aberrations under different accommodation stimuli were calculated and compared.

Results

The average induced accommodations using 1, 2, 3, 4, 5, or 6D accommodation stimuli were 0.848, 1.626, 2.375, 3.249, 4.181, or 5.085 D, respectively. The RMS of total wavefront aberrations, as well as higher-order aberrations, showed no significant effects with 1–3 D accommodation stimuli, but increased significantly under 4, 5, and 6 D accommodation stimuli compared with relaxed accommodation. Zernike coefficients of significantly decreased with increasing levels of accommodation.

Conclusion

Higher-order wavefront aberrations in human eyes changed with increased accommodation. These results are consistent with Schachar''s accommodation theory.     

Preferences and asymmetries in saccadic responses to delayed bihemifield stimuli

Saccaadic response preferences and latencies were measured using dual targets presented at 5, 10 and 15 deg along the horizontal meridian in either one or both visual hemifields. In the unihemifield condition, subjects exhibited a strong preference (about 95%) in favor of the target presented closer to the original fixation point. Further, the presence of the second target did not increase the response latency to the preferred target. In the bihemifield condition using spatially symmetric targets, directional preference varied across subjects from 57 to 100% (mean = 83%). There was also a significant increase in response latency as compared with responses to either single targets or dual targets presented in one hemifield. This latency increase was observed for responses in both the preferred and nonpreferred directions and was taken as evidence of mutual interaction between the two hemispheres. The response preferences could be eliminated by delaying the onset of the preferred target by about 100 msec in the unihemifield condition and by about 50 msec in the bihemifield condition. Further, the interactions between the effects of the targets on response time in the bihemifield condition were found to be asymmetrical in that a target presented in the preferred hemifield has a greater effect than a target in the nonpreferred hemifield on responses to targets presented in the counterhemifield. The results suggest an asymmetry in hemispheric interaction which has not previously been demonstrated in the visualoculomotor system.     

Eye-movement responses to disparity vergence stimuli with artificial monocular scotomas

The effects of artificial monocular scotomas on eye-movement responses to horizontal disparity vergence stimuli were studied in six subjects with normal binocular vision. Subjects viewed stereoscopic 1.5 degrees horizontal step disparity vergence stimuli through liquid crystal shutter glasses. The central portion of the stimulus presented to the right eye was removed to simulate monocular artificial scotomas of variable diameters (2 degrees to 10 degrees ). Eye movements were recorded with a binocular head-mounted eye tracker. Responses included pure vergence, vergence followed by saccades, and pure saccadic eye movements. The rate of responses with saccadic eye movements increased with the diameter of the artificial scotoma (p < 0.0001); there was an increase in the rate of responses starting with saccades (p < 0.0001), as well as an increase in the rate of saccades after initial vergence responses (p < 0.01). The probability of saccades after initial vergence responses was affected by the open-loop gain of the vergence response (p < 0.001). The open-loop gain decreased with increased diameters of the artificial scotomas (p < 0.0001). As the diameter of the artificial scotomas increased, the amplitude of the initial vergence eye-movement responses decreased, and the prevalence of saccadic eye movements and asymmetric vergence increased. The effects of the diameter of artificial monocular scotomas on eye-movement responses in subjects with normal binocular vision are consistent with the effects of diameter of suppression scotomas on eye-movement responses to disparity vergence stimuli in patients with infantile esotropia.     

Macaque ganglion cell responses to probe stimuli on modulated backgrounds

In the natural environment, visual targets have to be detected and identified on changing backgrounds. Here, responses of parasol (magnocellular) ganglion cells to probes on modulated backgrounds are described. At low frequency, the adaptation level of the background influences the probe response, but with increasing frequency there is a strong interaction with the response to the background per se, so that on- and off-center cell responses are modulated in different phases. Interactions with the background response include both thresholding effects (when the cell's firing is suppressed and no pulse response occurs) and saturation effects (when the background response is vigorous the pulse generates few additional spikes). At 30 Hz, the effect of the pulse is largely a suppression or phase shift of the background response. The data are relevant to the probed-sinewave paradigm, in which pulse detection thresholds are modulated with pulse phase relative to a sinusoidal background. The physiological substrates of the psychophysical results with the probed-sinewave paradigm appear complex, with on- and off-center cells likely to contribute to detection at different pulse phases.