Kenneth J. Ciuffreda

Dynamic 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.

Accommodation and Apparent Distance

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

Proximal contribution to a linear static model of accommodation and vergence.

To determine the influence of target proximity on accommodation and vergence under both open- (OL) and closed-loop (CL) viewing conditions, a static interactive feedback model, which included proximal accommodation (PA) and proximal vergence (PV) inputs, was developed and analysed quantitatively. It was based on an earlier static dual-interactive feedback model of accommodation and vergence. The proximal inputs were added to both the accommodative and vergence loops at the output of the respective controllers. The values of the PA and PV gains were obtained from experimental dual-OL data. The model equations were analysed over a stimulus range of 1 to 6 D (or MA). It was found that under the dual-OL condition, the contribution of PA to the overall accommodative output ranged from 42.5 to 81.6%, whereas the contribution of PV to the overall vergence output ranged from 56.1 to 88.5%. In contrast, under all other stimulus conditions, with the exception of the PA contribution to the accommodative output under the Aol, Vcl condition (26.5-41.0%), the relative contributions were much smaller, ranging from 0.04 to approximately 7.0%. In particular, under the dual-CL condition, representing the normal binocular visual feedback condition, the relative contributions were only 4.0 and 0.04% for PA and PV, respectively. Thus, although the relative contributions of PA and PV were large under the dual-OL condition, they were generally very small under the various CL conditions that simulated more naturalistic viewing situations. Nevertheless, proximity may still play an important role by providing cues for attaining coordinated and harmonious motor responses under specific viewing conditions.

Convergence and divergence exhibit different response characteristics to symmetric stimuli

The dynamic characteristics of horizontal convergence and divergence eye movement responses to symmetric stimuli were examined. Binocular eye movements were recorded in five, visually normal adult subjects using the infrared reflection technique for symmetric convergent and divergent blur-free, disparity-only, step stimuli of 2, 4, 8, 12, and 16 deg. The main sequence as well as other temporal parameters including latency, time-to-peak velocity, time constant, and total duration were analyzed. A number of fundamental differences in the response characteristics were found between convergence and divergence. First, the slope of the peak velocity vs amplitude curve was approximately twice as high for convergence than divergence. The results are consistent with neurophysiological findings in monkeys and most findings in humans. Second, the initial fast component for convergence exhibited a larger amplitude than for divergence. This may reflect differences in central neural gain for convergence and divergence. And, third, all temporally related components were shorter for convergence than divergence. These findings provide an overall framework for vergence control and suggest fundamental differences in neural processing delays and neural controller pathways for convergence and divergence.

Nearwork-induced transient myopia: a critical review.

The literature on nearwork-induced transient myopia (NITM) is reviewed, with NITM being defined as the short-term myopic far point shift immediately following a sustained near visual task. A majority of these investigations demonstrated the presence of NITM for a variety of test parameters, e.g., visual acuity, contrast sensitivity and far point. Overall, these studies reported relatively small myopic shifts, with a mean of approximately 0.40 D and a range from 0.12 to 1.30 D. The subsequent decay is characterized by an exponential function with a relatively short time course. While the precise etiology and implications of NITM remain unclear, speculations regarding its origin and relevance to clinical myopia are discussed. Studies that did not demonstrate NITM are also reviewed.

Accommodative stimulus/response function in human amblyopia

Three parameters are essential to describe static accommodative behavior in a comprehensive, quantitative manner: the slope of the stimulus/response curve, the depth of focus, and the tonic response. These parameters were obtained in amblyopes, former amblyopes, strabismics without amblyopia, and normals. Results showed that the accommodative response in the amblyopic eye was characterized by a reduction in the slope of the stimulus/response curve and increased depth of focus. Similar abnormalities but of lesser magnitude were found in the non-dominant eye of some former amblyopes and some strabismics without amblyopia. Orthoptic therapy always increased the slope of the stimulus/response curve in the amblyopic eye. We believe that the reduced accommodative responses found in amblyopic eyes reflect a primary sensory loss over the central retinal region that occurs as a result of prolonged, early, abnormal visual experience associated with the presence of strabismus and/or anisometropia.

Eye Movements during Reading: Case Reports*

&NA; Since the time of Javal, it has been well established that normal reading eye movement patterns have 3 principal components: (1) small saccades that move the eyes from word to word, (2) large saccades that return the eyes to the beginning of the next line, and (3) fixation pauses between each saccade for information processing. We discuss the vision analysis results and show the quantitative reading eye movement records, measured with the infrared photoelectric method, of 5 patients examined in the Neuro‐optometry Clinic. The reading records showed a wide variety of behavior: 1 patient performed normal reading movements, 1 “slow reader” manifested an excessive number of fixations as well as extended fixational durations, another “slow reader” only exhibited an excessive number of fixations, a patient with dyslexia performed backward reading movements, and 1 patient exhibited nystagmus superimposed upon the reading pattern.

Contrast and accommodation in amblyopia

Steady-state accommodation as a function of sinusoidal and square-wave grating contrast was measured in amblyopes. The amblyopic eyes exhibited reduced average response levels and required greater contrast to sustain accommodation than the fellow dominant eye or the eyes of visually-normal control subjects. Following therapy, accommodation in the amblyopic eye markedly improved. These results suggest: reduced accommodative controller gain, reduced stimulus effectiveness of target contrast, and considerable residual visual system plasticity with respect to the neurological control pathways of accommodation, in the amblyopic eye.

Tonic accommodation: a review I. Basic aspects

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of ∼ I D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part 1 of this review will consider various aspects of this parameter, including its reference to closed‐loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so‐called ‘stimulus‐free’ conditions, in fact probably represents an aggregate response resulting from multiple, non‐optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus‐free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.

Depth-of-focus of the human eye in the near retinal periphery.

Although the depth-of-focus in the foveal region has been well investigated, knowledge regarding the effect of retinal eccentricity on blur detection and sensitivity is limited. In the present study, the depth-of-focus at the fovea and in the near retinal periphery (0 degrees -8 degrees ) was assessed psychophysically in 7 human subjects using a 5 mm artificial pupil with accommodation paralyzed. The group mean total depth-of-focus progressively increased linearly from 0.89 D at the fovea to 3.51 D at a retinal eccentricity of 8 degrees at the rate of 0.29 D/degree, with response variability (S.E.M.) remaining relatively constant (+/-0.17 D). We speculate that the reduced detection and sensitivity to blur in the near periphery may be attributed to retinal topography, sharpness overconstancy, optical aberrations, and visual attention in peripheral vision.