Objective Concurrent Measures of Open-Loop ...

Objective Concurrent Measures of Open-Loop Accommodation and Vergence Under Photopic Conditions Lyle S. Gray,* Barry Winn* Bernard Gilmartin,X and Andrew S. Eadie\

Purpose. To investigate the relationship between photopic open-loop accommodation and vergence and the effect of mental effort on these positions. Methods. Twenty subjects (11 men and 9 women) viewed monocularly a photopic (25 cdm~2), high-contrast (90%) Maltese cross-target for 3 minutes, through a 0.5-mm pinhole drilled into an infrared filter. Accommodation was measured objectively at 1-second intervals using a Canon Autoref R-l infrared optometer, and vergence was recorded continuously and objectively using a differential infrared eye tracking system. Results. Under passive viewing conditions there was a significant correlation between photopic open-loop accommodation and vergence (R = 0.671, P = 0.0012); for the majority of subjects the imposition of mental effort shifted the passive levels of both open-loop accommodation and vergence, but these shifts were unsystematic and uncorrelated (R = 0.259, P = 0.270). The active open-loop positions of accommodation and vergence were also found to be correlated (R = 0.692, P = 0.0007). Conclusions. The influence of proximal stimuli can explain the correlations identified between photopic open-loop positions of accommodation and vergence. The uncorrelated responses of the accommodation and vergence systems to mental effort are likely to reflect interactions between various spatiotopic stimuli including mental effort and perceived proximity. Invest Ophthalmol Vis Sci. 1993;34:2996-3003.

1 he complex task of maintaining a clear and single binocular image of an object is accomplished by the accommodation and vergence systems, which operate within negative feedback loops to minimize blur and disparity of the retinal image.1 Under normal viewing conditions, synkinetic interactions between the two systems exist in the form of accommodative vergence, wherein blur-driven accommodation produces a vergence response (defined as the AC/A ratio) and vergence accommodation, wherein disparity-driven vergence produces an accommodation response (defined as the CA/C ratio).1"3 In the absence of visual stimuli, the accommodation and vergence systems are found to adopt stable, intermediate resting positions that range between 0.5 and 1.5

From the * Departments of Vision Sciences and fPhysical Sciences, Glasgow Caledonian University, Glasgow, and the %Department of Vision Sciences, Aston University, Birmingham, United Kingdom. The,work was supported in part by a Visual Research Trust grant. Lyle S. Gray is supported by a Science and Engineering Council research scholarship (No. 90800177) which is gratefully acknowledged. Submitted for publication August 7, 1992; accepted February 24, 1993. Proprietary interest category: N. Reprint requests: Lyle S. Gray, Department of Vision Sciences, Glasgow Caledonian University, Cowcaddens Road, Glasgow, G4 OHA, UK.

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diopters for accommodation and between 0.5 and 0.9 metre-angles (MA) for vergence.4"7 The terms tonic accommodation and tonic vergence were proposed because it was believed that the positions adopted by the two systems while under open-loop conditions reflected a "baseline" level of resting activity or tonus.8 There are various methods that can be used to place the accommodation and vergence systems under open-loop conditions, the most frequently used method being darkness. In the current study the accommodation and vergence loops are opened using monocular occlusion and a pinhole pupil under photopic conditions. To distinguish between photopic open-loop conditions and dark-room conditions the following abbreviations will be used; DA and DV for dark accommodation and dark vergence, respectively, as suggested previously,9 and POA and POV for photopic open-loop accommodation and photopic openloop vergence, respectively.

DARK ACCOMMODATION AND DARK VERGENCE Various laboratories have reported nonsignificant correlations between DA and DV using a variety of

Investigative Ophthalmology & Visual Science, September 1993, Vol. 34, No. 10 Copyright © Association for Research in Vision and Ophthalmology

Photopic OpemLoop Accommodation and Vergence measurement techniques usually requiring subjective input from the observer, for example, laser optometer, 810 vernier optometer,11 and a dual-haploscope optometer12 to measure DA and nonius alignment811 and a dual-haploscope optometer to measure DV.12 In other investigations nonsignificant correlations have been found between DA and DV using objective techniques, for example, photographic methods to measure both DA and DV,1314 and one study that employed an infrared eyetracker to measure DV while using a laser optometer to measure DA.10 Significant correlations have been reported by other laboratories using a laser optometer to measure DA and nonius alignment to measure DV,15"17 using a dual-haploscope optometer,18 and using a vernier optometer and nonius alignment to measure DA and DV,19 respectively. Results obtained using similar experimental apparatus have been contradictory. For example, no significant correlation between DA and DV was found12 using a dual haploscopic optometer (R = 0.31, P > 0.05, n = 32), which measures accommodative state using the principle of stigmatoscopy and vergence position by subjective alignment of targets presented hap r loscopically in the two arms of the optometer. Another study by Wolf et al18 again using a haploscope-optometer found a significant correlation between DA and DV (R = 0.51, P < 0.01, n = 22). These conflicting results demonstrate the variability in the level of correlation reported by researchers investigating DA and DV810"19 and suggest that the underlying relationship between DA and DV is fairly weak and depends to a large extent on methodological considerations.

NONOPTICAL STIMULI TO ACCOMMODATION AND VERGENCE The method used to measure open-loop accommodation has also been shown to influence the values reported, with laser optometry giving higher mean values and lower standard deviations than objective infrared optometers.2021 Post et al22 showed that modifications of DA induced when subjects performed a mental arithmetic task in a dark field were correlated with those for active judgment of the laser speckle motion, and they concluded that the "effort to see" the laser speckle motion resulted in a higher value of DA than that measured using an infrared method. Bullimore and Gilmartin23 found that significant positive shifts in DA were induced by a concurrent backward counting task. This technique can induce mental activity with a relatively high degree of difficulty, which is independent of the target configuration.24 They deduced that the shifts were mediated principally by the parasympathetic nervous system because the shifts were not affected by a nonselective topical beta receptor antagonist, although there was some evidence of attenuation of positive shifts for sub-

2997 jects with relatively high pretask tonic resting levels, a finding consistent with the proposal that sympathetic innervation is augmented when the level of concurrent parasympathetic activity is relatively high. A recent study25 has found systematic shifts in POA induced by a stimulus-independent task where subjects were instructed to count backward in sevens to themselves while viewing a target monocularly in Maxwellian view. A significant negative correlation was identified between the mentally-induced shift anduhe passive open-loop level of accommodation, which relative to zero shift was found to pivot about 1.61 diopters. The authors suggested that the subjects in attempting to perform this mental arithmetic task were constructing a "mental slate" or specific task distance inferred by mental imagery, which for their study would be placed at 1.61 D. Recent studies have demonstrated the potency of proximal stimuli on the open-loop responses of accommodation and vergence.26"29 A change in target distance from 0.2 to 3.0 D was found to cause a mean change in the open-loop accommodation response of 1.70 D, whichthe authors ascribed to proximally induced accommodation.26 Proximal stimuli have also been shown to influence the open-loop vergence position, changes of 0.37 MA for each MA change in target distance being reported27 although it has also been indicated that under certain experimental conditions proximal stimuli can produce approximately 70% of the required vergence response.28 It has been shown that even prior knowledge of surround propinquity (i.e., nearness) can significantly influence open-loop measures of accommodation.29 The effect of mental effort on the vergence system is less well established although it is known that voluntary mental effort can be used as a stimulus to both accommodation3031 and vergence.30 McLin and Schor30 have examined the open-loop accommodation and vergence response to voluntary mental effort (i.e., "thinking near" or "thinking far") in the light using monocular occlusion and a pinhole pupil. They found that both oculomotor systems responded to voluntary stimuli in a ratio that is very similar to the AC/A ratio, and concluded that voluntary mental effort stimulated accommodation directly with changes in vergence occurring via the accommodative-vergence cross-link. Recent investigations have led to the proposal that negative feedback models of oculomotor control respond to two distinct classes of stimuli: spatiotopic stimuli (e.g., size change), which induce relatively large, coarse but primary responses; and retinotopic stimuli (e.g., retinal blur and image disparity), which refine these initial responses.32 It is of considerable interest to examine the responses of the accommodation and vergence systems to purely spatiotopic stimuli. Previous studies that have attempted to examine the effect of spatiotopic stimuli (e.g., voluntary mental

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Investigative Ophthalmology & Visual Science, September 1993, Vol. 34, No. 10

effort) on POA and POV30 have tended to use small subject numbers because of the complex nature of the measurements.

Photopic Open-Loop Accommodation and Vergence The accommodation loop can be opened under photopic conditions using a variety of techniques such as, bright empty fields,3334 Maxwellian view3435 and Difference of Gaussians,3536 and when measured under these conditions POA is found to be more proximal than DA. Open-loop vergence can also be measured under photopic conditions by opening the vergence loop usinjg monocular occlusion and a pinhole pupil,3437 and POV has been found to be more divergent 3437 than DV. Simultaneous, objective measures of POA and POV have previously been made on a small number of subjects (N = 8 and 5),30>34 although the authors did not indicate if any relationship existed between the two systems. The current study examines objectively and concurrently open-loop accommodation and vergence in 20 subjects viewing a photopic target, using a pinhole pupil to open the accommodative loop and monocular occlusion to open the vergence loop. Measurement of the two open-loop positions using objective techniques avoids any shifts in open-loop accommodation and vergence associated with the mental effort due to subjective judgments. The aim of the experiment was to examine the distribution of and the relationship between open-loop accommodation and vergence under photopic conditions and further to examine the effect of a spatiotopic stimulus (i.e., mental effort) on POA and POV.

MATERIALS AND METHODS Subjects Twenty emmetropic visually normal subjects (11 men and 9 women, mean age of 24.2 years, range 19-29 years) took part in the study. All were students in the Department of Vision Sciences at Glasgow Caledonian University and all gave informed consent to take part in the study after the nature of the procedures had been fully explained to them. All procedures were subject to the principles outlined in the Declaration of Helsinki. A full optometric examination was carried out on all the subjects to ensure normal binocular, refractive, and ocular health status. The subjects had best sphere refractive errors between piano and + 0.50 D with no more than 0.50 D of astigmatism.

Apparatus Accommodation was measured using a Canon Autoref R-l infrared objective optometer (Canon Europa, The Netherlands), in its single-shot mode of recording. The autorefractor operates on the principle of grating

focus and determines the position of focus in three meridians. From this the sphero-cylindrical refractive error can be computed to an accuracy of ± 0.12 D once per second. The reliability and validity of the optometer has been established previously38 and, it has been used successfully for the measurement of static accommodation under a variety of experimental conditions in numerous research laboratories. A differential infrared limbal eyetracker39 mounted on a standard ophthalmic trial frame was used to monitor eye position objectively and continuously. A pair of infrared emitter/detectors are positioned so that when the observer looks at infinity the infrared beam falls centrally on both the nasal and temporal limbal junctions. The outputs from the nasal and temporal detectors are fed into a differential amplifier that produces an output voltage proportional to the amplitude of horizontal eye movements. The infrared sources are chopped at a very high frequency (> 1000 Hz) and the detectors set to recognize only the chopped frequency, which avoids direct current drift and reduces the effect of 1/f noise and any noise from extraneous sources (such as cross-talk between the photodiodes and the infrared optometer). The instrument was found to have a noise level and resolution for horizontal eye movements of 10 minutes of arc under the current experimental conditions. The cross-axis sensitivity was found to be 12 ± 2%, which is very similar to values quoted for other instruments.40 Heterophoria measurement using Maddox rod revealed no vertical heterophoria in any of the subjects, and as movement of the fixating eye would be minimal, the problem of horizontal/vertical cross-talk will arise only if the covered eye showed very large vertical eye movements (> 20°) in response to the mental effort task. The signals from the right and left eyes were fed into a Gould 1604 digital oscilloscope (Gould Electronics, UK) interfaced with a computer (Epson PCeXT [Seiko-Epson Corporation, Japan]). This allowed the traces to be viewed in real time and ensured that data-containing artifacts (e.g., blinks) would be eliminated.

Procedure On entering the laboratory there was no attempt made to disguise the position of either the apparatus, the stimulus, or the relative dimensions of the room. Trie subjects were therefore fully aware of the position of the target and instrumentation, although all subjects were ignorant as to the aim of the experiment. The subjects were first asked to sit in the dark to allow the dissipation of any transient changes in the tonic positions due to previous near work.6 A calibration run was first collected for the eyetracker with the subject viewing a scale of 1 ° steps at 1 m. Saccadic eye movements of varying step sizes were recorded for a range of ± 10°. The output voltage from the eyetracker was

Photopic Open-Loop Accommodation and Vergence

y = - 0.65 + 0.90x

3-

2-

I

1-

-2 -•3

1 2 Passive POA (D) FIGURE l. Relationship between the passive open-loop measures of accommodation and vergence respectively showed a significant regression (P = 0.0012). Error bars have been omitted for clarity, one standard deviation was typically ± 0.25D for accommodation measures and ±0.25 MA for vergence measures. found to be linearly related to the eye rotation in degrees over this range (R = 0.99, P = 0.001). A high contrast (90%) photopic (25 cdm~2) Maltese cross was placed at 0 D vergence in a 5 D Badal stimulus optometer and viewed monocularly through a 0.5-mm pinhole drilled into infrared filter. It has previously been shown that a 0.5-mm pinhole will fully open the accommodative loop. 41 During the experiment the target was viewed through a circular aperture restricting the field of view through the Badal lens to approximately the size of a 35-mm slide, although the angular subtense of the target when viewed through the artificial pupil was 2.39°; the ambient room illumination was constant at approximately 250 lux. Baseline measurements of open-loop accommodation and vergence were then taken for a period of 1 minute while the subjects viewed the Maltese cross target monocularly under photopic conditions through the pinhole pupil. During this period 30 static measures of the accommodation response were obtained, and 5 continuous measures of the right and left eye position each of 10 seconds duration were collected at a sampling frequency of 102.4 Hz. The continuous signals were stored initially in the oscilloscope and

2999 then transferred to an IBM-compatible computer (Epson PCe-XT) for subsequent analysis using Asystant (Asyst Software Technologies) software. The subjects were then required to perform a mental arithmetic task that consisted of counting backward in sevens to themselves from a three-digit number set by the examiner for a period of 3 minutes and further measures of accommodation and eye position signals were recorded. Such a task was previously used to induce a level of mental activity that is relatively difficult but remote from the target viewed by the subject,25 in contrast to tasks (e.g., to identify a particular feature of the target) that initiate mental activity directly related to the stimulus.25 RESULTS Passive Measures of POA and POV A significant correlation (Fig. 1, y = —0.65 + 0.90x, R = 0.671, F = 14.705, d f = 19, P = 0.0012) was found between the passive viewing positions of POA and POV. The mean passive POA position was found to be 1.25 ± 0.72 D (Table 1), a value similar to previous reports of POA measured using the Canon optometer. 2526 - 36 The mean passive POV was found to be 0.48 ± 0.97 MA (Table 1), which is more divergent than previous reports of DV.4>5>7 This is also relatively more divergent than the mean open-loop position of accommodation by approximately 0.65 MA. An examination of the regression shows that for 0 D of POA, subjects will exhibit approximately 0.65 MA of divergence. It can also be seen that several subjects exhibit divergent POV positions under the conditions of the current study. The slope of the regression is very close to unity at 0.90 indicating an almost one-to-one relationship between POA and POV. The Effect of Mental Effort on POA and POV Both positive and negative shifts in accommodation and vergence resulted from the mental arithmetic task (Fig. 2). Linear regression of the mentally induced shift in POA against the mentally induced shift in POV revealed no significant relationship (Fig. 2, y = —0.04 + 0.39x, R = 0.26, F = 1.296, df = 19, P = 0.270). It can be seen from the figure that the direction of the mentally induced shift is often of opposite sign in the

TABLE l.

Mean (± SD) Values (N = 20) of Passive and Within-Task Open-Loop Measures of accomidation and Vergence

POA (D) POV (MA)

Mean Passive

Mean Within-Task

Shift

Significance

1.25(0.72) 0.48 (0.97)

1.41 (0.75) 0.54(1.47)

+0.16 +0.06

t = 1.241, df = 38, P> 0.10 t = 0.341, df = 38, P> 0.10

POA = photopic open-loop accommodation; pov = photopic open-loop vergence; D = diopters; MA = metre-angles.

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Regression of the within-task measures of POA and POV was significant (Fig. 4, y = -1.37 + 1.36x, R = 0.69, F = 16.514, df = 19, P = 0.0007). It can be seen that the slope of the regression is now greater than unity (1.36) compared to the slope (0.90) of passive POV against passive POA (Fig. 1). The increased slope of the regression can be attributed to both a narrower distribution of within-task POA values (Fig. 3A) and a wider distribution of within-task POV values (Fig. 3B) as a result of the mental effort task.

3l D

-

1 " 0

g

D

m

0 D

•

-1 --

-2

G

•

D

Passive Measures of POA and POV •

-

1

DISCUSSION

D

i

•

0 1 Accommodation shift (D)

2

FIGURE 2. Relationship between the induced shifts in openloop accommodation and vergence showed no significant regression (P = 0.183). Typical standard deviations were ± 0.25 D for the accommodation measures and ± 0.25 MA for the vergence measures.

The values of passive POA and passive POV reported in this study agree with previously reported values of open-loop accommodation252636 and vergence37 mea4.0 3.5" Q

3.0"

y = 0.49 + 0.73x

2.52.0-

two systems; with positive changes in accommodation often being accompanied by negative changes in the vergence system and vice-versa. This finding describes clearly the disparate and uncorrelated responses of the accommodation and vergence systems to the mental effort task. Table 1 shows the mean pretask and within-task positions of POA and POV and the mean shifts found in both systems as a result of the mental effort task. The mean shift in the accommodation system was +0.16 D, which was not significant (paired t test, t = 1.241, 38 df, P> 0.10). Regression of the pretask and within-task measures of accommodation revealed a significant relationship (Fig. 3A, y = 0.49 + 0.73x, R = 0.706, F = 17.874, df = 19, P= 0.0005). The slope of the relationship being flatter than unity (0.73) indicates that the within-task positions of POA were not quite as widely distributed as the original passive positions of POA. The mean mentally-induced shift in the vergence system was +0.06 MA, and this difference between pretask and within-task POV positions was not significant (paired /-test, t = 0.341, 38 df, P> 0.10). Regression of the within-task POV position against the passive POV position revealed a significant relationship (Fig. 3B, y = -0.09 + 1.31, R = 0.87, F = 56.389, df = 19, P = 0.0001), in this case the gradient was greater than unity (1.31) indicating that the task-induced POV positions tended to have a wider distribution of values compared with the passive POV positions.

1.5 " J.O" 0.5" 0.0 -0.5

0.0

0.5 1.0 1.5 2.0 Passive POA (D)

2.5

3.0

y = -0.0'J + 1.3 I x

4"

2"

^

0-

-2 -

-4 0 I Passive POV (MA)

FIGURE 3. (a) Relationship between pretask and within-task measures of photopic open-loop accommodation showed a significant regression (P = 0.0005). Typical standard deviations were ± 0.25 D. (b) Relationship between pretask and within-task measures of photopic open-loop vergence showed a significant regression (P — 0.0001). Typical standard deviations were ± 0.25 MA.

Photopic Open-Loop Accommodation and Vergence

3001

open-loop accommodation and vergence showed a significant regression (P = 0.0007) Typical standard deviations were ± 0.25 D for the accommodation measures and ± 0.25 MA for the vergence measures.

pic measures of open-loop accommodation and vergence collected in the current study are likely to have been significantly influenced by such factors as perceived proximity and surround propinquity, as there was no attempt made to disguise the room dimensions and surroundings or the position of the target relative to the observer. All these influences could have a proximal effect on the open-loop positions measured. Even factors such as the room surroundings have been shown to significantly influence open-loop measures of accommodation.29 Under the current experimental conditions there is likely to have been a strong targetdependent proximal response in both the accommodation and vergence systems due to a subject's awareness of the target location and the influence of the room dimensions. The drive to accommodation and vergence resulting from these proximal stimuli would be expected to be similar and it may be the responses to these proximal stimuli that underly the correlation between passive POA and POV found in the current study.

sured under photopic conditions. Previous reports of DV have found mostly convergent positions,4-5 whereas several subjects in the current study show divergent (negative) POV positions. Schor and McLin34 measured tonic accommodation and tonic vergence in darkness and under photopic conditions using a method similar to that used in the current study. They found that the POV positions for four of five subjects tended to be more divergent than DV by up to 2 MA, although they could identify no difference between POA and DA. A recent study measured DV and distance heterophoria using the von Graefe (prism dissociation) method while the accommodation system was placed open-loop by the use of binocular 0.5-mm pinholes.37 The distance heterophoria measurement in that study can be considered equivalent to the passive POV position measured in the current study. The authors found that a few subjects exhibited divergent DV positions and that many more subjects exhibited divergent distance heterophoria positions. It would appear that while DV shows mostly convergent positions, divergent open-loop positions such as those found in the current experiment are more likely to occur when the vergence loop is opened under photopic conditions. The correlation between passive POA and POV measurements agrees with similar results reported for DA and DV.15"19 The effect of proximal stimuli on open-loop measures of accommodation and vergence is now well established.26"29 A recent review discussed the difficulty in obtaining veridical measures of openloop accommodation and vergence regardless of the method used to open the feedback loops.9 The photo-

The Effect of Mental Effort on POA and POV The effect of the mental arithmetic task on the accommodation and vergence systems appears to be unpredictable and indeterminate. Indeed it can be seen from Figure 2 that the direction of change was often opposite in the two systems with negative shifts in accommodation accompanying positive vergence shifts and vice-versa. This lack of correspondence between the mentally induced shifts in accommodation and vergence is puzzling because mental effort is likely to represent a common spatiotopic input to both accommodation and vergence yet the two systems respond in an obviously uncoordinated fashion. It should also be noted that although the mental arithmetic task probably has a proximal effect on oculomotor responses, this cannot explain the occurrence of negative shifts within both the accommodation and vergence systems in response to the mental effort task. Despite the uncorrelated nature of the shifts in the two systems as a result of the mental effort task it can be seen that the within-task measures of POA and POV remain correlated (Fig. 4). The relationship between the accommodation and vergence systems is altered as can be deduced from the increased gradient of the regression (1.36). The shifts in both the accommodation and vergence systems appear to contribute to this altered relationship; it can be deduced that both a narrowing of the distribution of POA values (Fig. 3A) and a widening of the spread of POV values (Fig. 3B) will cause an increase in the gradient of the withintask regression of POA against POV. The shifts identified in the accommodation and vergence systems do not represent the effect of the mental effort task alone but rather the effect of various spatiotopic stimuli in-

4-1

y = - 1.37 + 1.36x 2-

S

0-

> -1 -2-3

0

1

2

3

4

Wthin-task POA (D) FIGURE 4. Relationship between the within-task measures of

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eluding both perceived proximity and mental effort. Thus it appears that while the mental effort stimulus is strong enough to modify the passive positions of POA and POV, the proximal stimulus ensures that correlation between the accommodation and vergence systems is maintained. The occurrence of negative oculomotor shifts in response to the mental arithmetic task can also be explained by the combined effect of these spatiotopic stimuli and therefore will depend on the position at which a particular person perceives the mental effort task to be, such visual imagery has previously been noted to influence DA.42 If a particular subject imagines the mental effort task to be more distant than the position resulting from the various proximal stimuli then a negative shift of accommodation or vergence may occur. However, the occurrence of opposing shifts in the two systems is puzzling and deserves further investigation. Key Words accommodation, vergence, open-loop, photopic, mental effort Acknowledgment The authors thank the reviewers for their useful comments in the preparation of the revised manuscript. References 1. Semmlow JL, Hung GK. The near response: theories of control. In: Schor CM, Ciuffreda KJ, eds. Vergence Eye Movements: Basic and Clinical Aspects. Boston: But-

terworths; 1983:175-195. 2. Schor CM, Kotulak JC. Dynamic interactions between accommodation and convergence are velocity sensitive. Vision Res. 1986;26:927-942. 3. Hung GK, Semmlow JL. Static behaviour of accommodation and vergence: computer simulation of an interactive dual feedback system. IEEE Trans Biomed Eng. 1980;27:439-447. 4. Schor C: Effects on the resting states of accommodation and vergence. In: Grosvenor T, Flom MC, eds. Refractive Anomalies: Research and Clinical Applications.

Boston: Butterworths; 1991:310-317. 5. Owens DA, Leibowitz HW. Perceptual and oculomotor consequences of tonic vergence. In: Schor CM, Ciuffreda KJ, eds. Vergence Eye Movements: Basic and

Clinical Aspects. Boston: Butterworths; 1983:25-74. 6. Krumholz DM, Fox RS, Ciuffreda KJ. Short-term changes in tonic accommodation. Invest Ophthalmol VisSci. 1986; 27:552-557. 7. Fisher SK, Ciuffreda KJ, Tannen B, Super P. Stability of

tonic

vergence.

Invest

Ophthalmol

Vis Sci.

1988;29:1577-1581. 8. Gilmartin B, Hogan RE, Thompson SM. The effect of timolol maleate on tonic accommodation, tonic vergence, and pupil diameter. Invest Ophthalmol Vis Sci. 1984;25:763-770.

9. Rosenfield M, Ciuffreda KJ, Gilmartin B. Factors influencing accommodative adaptation. Optom Vis Sci. 1992;69:270-275. 10. Bohman CE, Saladin JJ. The relation between night myopia and accommodative convergence. Amf Optom PhysiolOpt. 1980;57:551-558. 11. Wolf KS, Bedell HE, Pederson SB. Relations between accommodation and vergence in darkness. Optom Vis Sci. 1990; 67:89-93. 12. Fisher SK, Ciuffreda KJ, Levine S, Wolf-Kelly KS. Tonic adaptation in symptomatic and asymptomatic subjects. Amf Optom Physiol Opt. 1987;64:333-343. 13. Fincham EF. Accommodation and convergence in the absence of retinal images. Vision Res. 1962; 1:425440. 14. Aslin RN, Dobson V. Dark vergence and dark accommodation in human infants. Vision Res. 1983; 23: 1671-1678. 15. Owens DA , Leibowitz HW. Accommodation, convergence, and distance perception in low illumination. Am J Optom Physiol Opt. 1980;57:540-550. 16. Hogan RE, Gilmartin B. The relationship between tonic vergence and oculomotor stress induced by ethanol. Ophthal Physiol Opt. 1985; 5:43-51. 17. Hogan RE. Aspects of tonic accommodation and tonic vergence. PhD Thesis, University of Aston, Department of Vision Sciences 1985. 18. Wolf KS, Ciuffreda KJ, Jacobs SE. Time course and decay of effects of near work on tonic accommodation and tonic vergence. Ophthal Physiol Opt. 1987;7:131135. 19. Owens DA, Wolf-Kelly K. Near work, visual fatigue, and variations of oculomotor tonus. Invest Ophthalmol VisSci. 1987;28:743-749. 20. Johnson CA, Post RB, Tseutaki TK. Short-term variability of the resting focus of accommodation. Ophthal PhysiolOpt. 1984;4:319-325. 21. Post RB, Johnson CA, Tseutaki TK. Comparison of laser and infrared techniques for the measurement of the resting focus of accommodation: mean differences and long-term variability. Ophthal Physiol Opt. 1984;4:327-332. 22. Post RB, Johnson CA, Owens DA. Does performance of tasks affect the resting focus of accommodation. Am} Optom Physiol Opt. 1985;62:533-537. 23. Bullimore MA, Gilmartin B. Tonic accommodation, cognitive demand, and ciliary muscle innervation. Am J Optom Physiol Opt. 1987;64:45-50. 24. Lindsay PH, Norman DA. Human Information Processing. An Introduction to Psychology, 2nd ed. New York:

Academic Press; 1977. 25. Winn B, Gilmartin B, Mortimer L, Edwards, NR. The effect of mental effort on open- and closed-loop accommodation. Ophthal Physiol Opt. 1991; 11:335-339. 26. Rosenfield M, Gilmartin B. Effect of target proximity on the open-loop accommodative response. Optom Vis Sci. 1990; 67:74-79. 27. Rosenfield M, Ciuffreda KJ, Hung GK. The linearity of proximally induced accommodation and vergence. Invest Ophthalmol Vis Sci. 1991; 32:2985-2991. 28. Wick B, Bedell HE. Magnitude and velocity of proxi-

Photopic Open-Loop Accommodation and Vergence

29. 30. 31. 32.

33. 34. 35.

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