Effect of cumulative nearwork on accommodative facility and asthenopia.

PURPOSE: To investigate quantitatively in young adults the relationship between long-term cumulative nearwork, degradation of dynamic accommodative ability and the presence of asthenopic symptoms. METHODS: Subjects consisted of 87 young students and office workers between 18 and 31 years of age with uncorrected visual acuity of 20/30 or better in each eye. The amounts of nearwork, dynamic accommodative facility, and level of asthenopic symptoms were measured for each subject. RESULTS: Total cumulative nearwork time was negatively correlated with accommodative facility and positively correlated with the number of asthenopic symptoms. Furthermore, significant correlations were found between total nearwork time and blurred vision, and blurred vision and reduced accommodative facility. Moreover, the sub-category of "hours spent reading over the years" was found to be significantly correlated with decreased accommodative facility. CONCLUSIONS: The correlations suggest a relationship between cumulative amount of nearwork, decreased accommodative facility and asthenopia.

A unifying theory of refractive error development.

While retinal defocus is believed to be myopigenic in nature, the underlying mechanism has remained elusive. We recently constructed a theory of refractive error development to investigate its fundamental properties. Our Incremental Retinal-Defocus Theory is based on the principle that the change in retinal-defocus magnitude during an increment of genetically-programmed ocular growth provides the requisite sign for the appropriate alteration in subsequent environmentally-induced ocular growth. This theory was tested under five experimental conditions: lenses, diffusers, occlusion, crystalline lens removal, and prolonged nearwork. Predictions of the theory were consistent with previous animal and human experimental findings. In addition, simulations using a MATLAB/SIMULINK model supported our theory by demonstrating quantitatively the appropriate directional changes in ocular growth rate. Thus, our Incremental Retinal-Defocus Theory provides a simple and logical unifying concept underlying the mechanism for the development of refractive error.

Quantitative analysis of the effect of near lens addition on accommodation and myopigenesis.

PURPOSE: To develop a quantitative, objective, and scientific basis for understanding the effects of applying near bifocal additions (ADDs) on oculomotor control and myopia development. This is important because myopia is a major public health problem that affects 25% of the U.S. population and 75% or more in Asian countries. It is also associated with an increased risk for vision-threatening conditions, such as retinal breaks and detachments, as well as glaucoma. METHODS: A comprehensive model of refractive error development was constructed based on a dual-interactive feedback model of accommodation and vergence, which represented the short-term dynamics pathway, with the addition of both genetic and environmental (defocus-induced axial growth) components in a long-term pathway. An alternating near- and far-viewing paradigm was simulated, with varying amounts of ADDs, to obtain a parametric relationship between the root mean square of accommodative error (AE) and the induced refractive error (IRE). The parametric relationship provided the crucial linkage between the long-term growth pathway and the conventional short-term dynamics pathway. ADD is the simulated lens placed before the eyes only during near viewing, whereas IRE is the simulated lens fixed before the eyes that represents the optical effect of slowly progressive refractive development caused by near work. RESULTS: A V-shaped functional relationship was found between AE and IRE. The left half of the curve is associated with hyperopic defocus and myopigenesis, whereas the right half is associated with myopic defocus and hyperogenesis. Introduction of an ADD shifts the V-shaped curve horizontally. Thus, an "optimal" ADD can be used to shift the minimum of the accommodative error curve to the zero IRE point, and thereby reduce or eliminate retinal defocus and its potential towards myopigenesis. On the other hand, sensitivity analysis of model parameters shows that increasing the accommodative convergence crosslink gain (AC) shifts the curve to the right and results in a tendency towards myopigenesis, which is consistent with clinical findings in progressive myopia. CONCLUSIONS: The model can be used to specify the precise ADD needed for an individual to retard or eliminate retinal defocus-induced myopic progression. If future experiments show that using the "optimal" ADD results in the greatest benefit (i.e., least myopia progression), there will be considerable worldwide public health benefit.

Differential retinal-defocus magnitude during eye growth provides the appropriate direction signal.

INTRODUCTION: The rate of ocular growth can be modified by the imposition of either plus or minus lenses before the eyes, which creates changes in retinal defocus. Several hypotheses with relatively complicated mechanisms have been proposed to explain these changes, but the underlying mechanism has remained elusive. MATERIAL AND METHODS: Our new analysis using schematic models, however, provides a relatively simple and logically-consistent explanation of how retinal defocus magnitude alone during ocular growth gives the requisite sign for an appropriate change in ocular growth rate. RESULTS: During a normal genetically-determined incremental increase in axial length, the presence of either an imposed plus or minus lens results in an increase or decrease, respectively, of the blur circle magnitude. Neuromodulators in the retina are proposed to regulate the sensitivity to retinal-image contrast by means of a local feedback mechanism, and the alteration in retinal-image contrast associated with the change in blur circle causes an increase or decrease, respectively, in the rate of release of neuromodulators as well as the rate of proteoglycan synthesis, the latter being associated with the structural integrity of the sclera. CONCLUSION: This provides the critical sign, as well as amplitude, information needed to modulate appropriately the rate of eye growth, to result in a decrease or increase, respectively, in the rate of ocular growth.

Model of human refractive error development.

PURPOSE: To construct a model of refractive error development that can account for the different interactive mechanisms and time courses of refractive error in the hyperope (HYP), emmetrope (EMM), early-onset myope (EOM), and late-onset myope (LOM) over the first 30 years of life. METHODS: First, a baseline short-term (1 mo.) simulation of a previously developed nearwork-induced transient myopia (NITM) model was performed under both far- and near-viewing paradigms to obtain the critical relationships between AErms and refractive error for the four refractive groups. Then, two control pathways were added to the NITM model. The genetically-controlled pathway was associated with the long-term growth of the cornea, lens, and the eyeball. The environmentally-controlled pathway was associated with retinal-defocus during nearwork, wherein the root mean square (rms) of the accommodative error (AE) above a threshold level resulted in an increase in axial length of the eyeball. The thresholds for defocus-induced axial length change were empirically determined to correspond to the differential susceptibility in the four refractive groups. The combination of effects from the two pathways produced the overall refractive error. The relationship between AErms and refractive error was combined with the two control pathways for the long-term simulations (30 yrs: the initial 15 yrs using a far-viewing paradigm followed by an additional 15 yrs using a near-viewing paradigm) to quantify refractive error development as related to daily nearwork activity in the four refractive groups. RESULTS: All refractive groups began early in life with a genetically-determined hyperopic refractive error. The HYP had the lowest susceptibility or highest threshold to retinal defocus effects, and remained at a hyperopic level. The EMM exhibited a relative myopic shift in the first 2 years to become and remain at emmetropia. In the myopic groups, the EOM exhibited both a genetically-controlled component (starting 2 years of age) and a defocus-induced component (starting at 15 years of age), whereas the LOM manifested only a defocus-induced factor (starting at 15 years of age) in the development of myopia. In addition, simulations indicated that emmetropization occurred only for "induced" refractive error that was less than 0.5 D, which was consistent with the non-monotonic relationship between AErms and refractive error, wherein the minimum AErms occurred at 0.5 D. CONCLUSIONS: The model showed that both genetic and defocus-induced environmental factors play important roles in the development of refractive error in the different refractive groups. The model also provides a framework for further detailed quantitative analysis of the processes of refractive error development and emmetropization.

Adaptation model of nearwork-induced transient myopia.

A nearwork model was developed to determine whether variation in accommodative adaptation gain, KA, can account for the differences in the dynamic decay timecourse following near work in hyperopes (HYP), emmetropes (EMM), early-onset myopes (EOM), and late-onset myopes (LOM). The model incorporated a proximal component into a previously-developed adaptation model of accommodation and vergence. It was used to simulate the nearwork-induced transient myopia (NITM) response following 10 min of congruent binocular near viewing (5 D, 5 MA). The accommodative adaptation gain, KA, value was varied from 1.0 to 6.0 in increments of 0.5. For the hyperopes, an additional constraint was imposed wherein the accommodative response was biased on the under-accommodated side of the deadspace operator (i.e., depth-of-focus). In addition, the effect of prolonged nearwork was simulated by alternating between 1 hr of congruent near viewing (3 D, 3 MA) and 5 min of congruent far viewing (0.25 D, 0.25 MA) over a 160 hr period representing one work-month with 40 hours of nearwork per week. The steady-state rms value of the accommodative error was measured as a function of KA. It was found that the NITM timecourses for HYP, EMM, EOM, and LOM could be simulated accurately using KA values of 2.0, 2.5, 4.0 and 5.5, respectively. The long-term final steady-state rms of the accommodative error was found to increase from 0.182 D to 0.188 D as KA increased from 1 to 6. This indicated a small and progressive increase in residual accommodative error for higher KA values, which was associated with EOM and LOM. Thus, NITM for the different refractive groups could be quantified by the accommodative adaptation gain element, with KA for the HYP, EMM, and EOM and LOM groups having lower, intermediate, and higher values, respectively. The larger rms for higher KA values suggests that a myopic individual may have a predisposition to exhibit a slightly larger long-term accommodative error, which may stimulate axial elongation and in turn promote the progression of axial myopia.

Dynamic asymmetries in disparity convergence eye movements.

Vergence eye movements have traditionally been considered the product of a single neural control center and are usually studied by combining the movements of each eye into a single 'vergence' response. In the present experiment, disparity-driven eye movements were produced by symmetrical step stimuli, and the dynamic properties of each eye movement were analyzed separately. Although the final positions of the two eyes were symmetrical, large dynamic asymmetries often occurred. The timing between the two eyes showed fair synchrony as they attained maximum velocity at approximately the same time. Since the final static positions were symmetrical, asymmetries occurring during the initial dynamic component must necessarily be compensated by offsetting asymmetries in the latter portion of the response.

Saccade-vergence trajectories under free- and instrument-space environments.

PURPOSE: The purpose of this study was to examine in detail the binocular fixation top-view trajectories of saccade-vergence responses to asymmetrical targets, and to compare latency difference between saccade and vergence, under the free- and instrument-space viewing environments. METHODS: Binocular eye movements were recorded using the infrared reflection technique in five visually-normal subjects. Responses were obtained for various asymmetrical target positions under both free- and instrument-space environments. RESULTS: Four types of top-view trajectories that represented normal variations in saccade and vergence control were found: straight, overshoot, undershoot, and saccade-vergence. Also, it was found that under the instrument-space environment, there was a predominance of saccade-vergence trajectories and a scarcity of overshoot trajectories, whereas under the free-space environment, there was a predominance of overshoot trajectories, and a scarcity of saccade-vergence trajectories. Further, under the instrument-space environment, latency was significantly longer for saccade than vergence (35.9 +/- 15.7 msec; t = 5.1, degrees of freedom (df) = 4, P < 0.01), whereas under the free-space environment, there was no latency difference (-10.5 +/- 14.8 msec; = -1.6, df = 4, P > 0.05). CONCLUSIONS: The differences in response profiles under the two viewing environments could be accounted for by differences in timing of saccade and vergence onset. Moreover, in contrast to some recent investigations that suggest higher center control of individual trajectories, which was dependent on the naturalistic scene, these trajectories could be accounted for by known neural and oculomotor mechanisms, with the higher centers using a priori information about spatial location of the target, to assist in the synchrony of saccade and vergence onset under the free-space environment.

Dynamic model of the vergence eye movement system: simulations using MATLAB/SIMULINK.

A dynamic model of the vergence eye movement system was developed and simulated using MATLAB/SIMULINK. The model was based on a dual-mode dynamic model previously written in FORTRAN. It consisted of a fast open-loop component and a slow closed-loop component. The new model contained several important modifications. For example, in the fast component, a zero-order hold element replaced the sampler and the target trajectory estimator in the earlier model to provide more stable and accurate responses. Also, a periodicity detector was added to automatically detect periodicity in the stimulus waveform. The stored periodic stimulus, with a reduction in latency, was used to drive the fast component output. Moreover, a connection representing the efference copy signal was added from the fast component output to the disparity input to provide an accurate estimate of the stimulus waveform. Further, Robinson's model of the extraocular muscles replaced the earlier 2nd-order plant to provide more realistic muscle dynamics. The entire model, containing the fast and slow components, was simulated using a variety of stimuli such as pulses, positive and negative ramps, square-wave, and sine-wave. The responses showed dynamic characteristics similar to experimental results. Thus, this new MATLAB/SIMULINK program provides a relatively easy-to-use, versatile, and powerful simulation environment for investigating the basic as well as clinical aspects of vergence dynamics. Moreover, the simulation program has general characteristics that can be modified to represent other oculomotor systems such as the versional and accommodation systems. This provides a framework for future investigation of dynamic interactions between oculomotor systems.

Sensitivity analysis of the stimulus-response function of a static nonlinear accommodation model.

The effect of parameter variation of a nonlinear static feedback control model of the accommodation system was investigated. Simulations of a MATLAB/SIMULINK model showed a nonlinear relationship between stimulus and response in which the response curve was above the 1:1 line in the region to the left of the crossover and below the 1:1 line in the region to the right of the crossover. At the crossover, the response curve exhibited an inflection that was constant and equal to the tonic accommodation value (ABIAS). Sensitivity analysis showed that increasing depth of focus (deadspace range between +/- DSP) increased the separation between the boundary lines of the deadspace region, with a larger separation associated with late-onset myopia and congenital nystagmus. Increasing accommodative controller gain (ACG) increased the slope of the function on either side of the deadspace, with lower ACG values corresponding to lower slopes that indicated an amblyopic deficit. Increasing ABIAS increased the accommodative level at the inflection region. In addition, the saturation level of the accommodative response decreased with increasing age, while the slope remained the same, which was consistent with the Hess-Gullstrand theory of presbyopia. The accuracy and relative simplicity of the model indicated that it could serve as a basis for further comprehensive investigation of the basic and clinical aspects of the accommodation system.

Initial component control in disparity vergence: a model-based study.

The dual-mode theory for the control of disparity-vergence eye movements states that two components control the response to a step change in disparity. The initial component uses a motor preprogram to drive the eyes to an approximate final position. This initial component is followed by activation of a late component operating under visual feedback control that reduces residual disparity to within fusional limits. A quantitative model based on a pulse-step controller, similar to that postulated for saccadic eye movements, has been developed to represent the initial component. This model, an adaptation of one developed by Zee et al. [1], provides accurate simulations of isolated initial component movements and is compatible with the known underlying neurophysiology and existing neurophysiological data. The model has been employed to investigate the difference in dynamics between convergent and divergent movements. Results indicate that the pulse-control component active in convergence is reduced or absent from the control signals of divergence movements. This suggests somewhat different control structures of convergence versus divergence, and is consistent with other directional asymmetries seen in horizontal vergence.

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.

Quantitative analysis of the accommodative convergence to accommodation ratio: linear and nonlinear static models.

Ogle and his colleagues proposed two measures of oculomotor linkage called the accommodative convergence to accommodation (AC/A) ratio. This ratio provided a clinically useful assessment of the drive of accommodation, or focusing system, on vergence, or binocular fixation system. The phoria method measured the relatively large deviation in eye alignment under the monocular condition, whereas the fixation disparity method measured the relatively small misalignment of the eyes under the binocular condition to obtain the AC/A ratio. Ogle et al. indicated that these two measures should be equal. However, experimental results showed a substantial difference between the AC/A ratios obtained by the two methods. To quantitatively assess the difference between the two methods, a linear static model was first evaluated. This model was based on an earlier successful model of the accommodation and vergence system. The linear-model solution showed that these two methods were equivalent and thus could not account for the differences found. Then, a nonlinear static model, containing the deadspace operators depth of field and Panum's fusional area (PFA), was evaluated. Since two solutions were possible for each deadspace operator, there were four basic solutions. However, there were two binocular-viewing paradigms. This resulted in four prism-viewing and four lens-viewing solutions. Finally, a difference was taken between the prism- and lens-viewing measures, giving a total combination of 16 solutions. Only four of these solutions were equal to that using the phoria method. Some of the other solution lines were widely separated, thus providing a range of possible data values across different solution lines. Calculations showed that the variation in AC/A ratio values for data across difference solution lines was comparable to that found experimentally. Thus, the deadspace operators in the nonlinear model were able to account for the discrepancy between the AC/A ratio determined by the phoria and fixation disparity methods.

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.

Context effect of common objects on visual processing.

In previous experiments on visual processing it was shown that correct identification of sequentially and simultaneously presented numerals declined as the presentation interval was decreased from 50.0 to 16.7 ms/numeral. To determine how performance was influenced in context, common everyday objects were used in place of the numerals. These objects were grouped into 36 different categories with 4 objects per category. An example of a category was fruit: apple, banana, pear, pineapple. For all experimental trials, objects (ranging in number from one to four) were presented randomly at four possible positions 1.5 degrees from a central fixation point. In a sequential trial, the objects were displayed in succession at different locations for a stimulus onset asynchrony (SOA) of either 16.7, 33.3, or 50.0 ms. The masking of a previous object coincided with the onset of the object that followed in the sequence. A simultaneous trial consisted of 2, 3, or 4 objects presented concurrently. For these trials, simultaneous masking occurred at SOA's of 16.7, 33.3, or 50.0 ms. For both experimental paradigms, one-half of the trials consisted of objects from the same category, and one-half from mixed categories. Results taken from four observers showed that performance for all three SOA's was consistently better when the objects were related as opposed to when the objects were not for both presentation protocols. This lent support to the theory that there was simultaneous interaction of top-down and bottom-up processes in visual processing. In addition, as was found in the numerals study, performance was better when the objects were presented simultaneously as opposed to sequentially.

Simultaneous better than sequential for brief presentations.

During perceptually intensive tasks such as reading, there is a bottleneck in the information transfer between the large number of alphanumeric characters available and the acquiring of these characters. This is due mainly to the limited number of characters that one can report at a glance (also known as the "magic number 7 +/- 2") [Psychol. Rev. 63, 81 (1956)]. To examine where in the perceptual pathway this bottleneck occurred, several investigators tested and compared performance with simultaneous and with sequential target presentations [J. Exp. Psychol. 79, 1 (1969); 93, 72 (1972); Percept. Psychophys. 14, 231 (1973)]. They found that performance was nearly equal in the two cases and concluded that the bottleneck must be due to the limitation of short-term memory. However, these studies were limited either by a long stimulus-onset asynchrony (SOA), or time interval between onsets of icon presentations, or by a lack of poststimulus masking. We report on experiments designed to overcome these limitations. We used shorter SOA's than did previous investigators, and we removed persistence effects by poststimulus masking. Our stimuli were presented either sequentially or simultaneously. For the sequential presentation a numeral ranging from 0 to 9 was displayed at any one of eight positions 1.5 deg from a central fixation cross. The appearance of the next numeral in another part of the display coincided with the masking of the previous numeral. This was done for trials of one to four numerals and SOA's of 16.7, 33.3, and 50.0 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects.

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Disparity vergence eye movements exhibit preprogrammed motor control.

To determine the control strategy used to mediate disparity vergence eye movements, an optical technique was employed which eliminated the visual feedback normally associated with a vergence movement. The resulting "open-loop" vergence responses from four subjects exhibited multiple step-like movements. This finding augments previous demonstrations of multiple-step tracking strategies in vergence eye movements and supports the theory that the initial portion of the vergence response is under preprogrammed control. Additional evidence for preprogramming is given by the similarity of the initial dynamics between open- and closed-loop responses as evidenced by similar ratios of peak velocity to amplitude (i.e. "main sequence" plots).

Sensitivity analysis of relative accommodation and vergence.

A sensitivity analysis was performed to determine the variation in response to changes in parameter values of a previously developed nonlinear static model of accommodation and vergence. To determine normal behavior, model simulation responses were computed using previously obtained parameter values in 4 subjects under 2 conditions. In the first, relative accommodation was evaluated by maintaining the vergence stimulus constant at 2.5 meter angles (MA) and varying the accommodative stimulus from -2.5 to 2.5 diopters (D) in 0.25-D steps. In the second, relative vergence was evaluated by maintaining the accommodative stimulus constant at 2.5 D and varying the vergence stimulus from 25 prism diopters (PD) base-in to 25 PD base-out in 5-PD steps. Sensitivity of the model parameters, consisting of controller gains for accommodation (ACG) and vergence (VCG), crosslink gains for accommodation-to-vergence (AC) and vergence-to-accommodation (CA), deadspace operators for accommodation (AE +/- AD) and vergence (VE +/- VD), and the tonic levels for accommodation (ABIAS) and vergence (VBIAS) were assessed by varying them at 50% and 150% of their normal values. It was found that the accommodation and vergence systems were most sensitive to variation in crosslink gain, moderately sensitive to variation in controller gain and tonic level, and least sensitive to variation in size of the deadspace. These results may provide a quantitative basis for the occurrence of ocular dysfunctions associated with abnormal crosslink gains, such as strabismus, in clinic patients.