Fatigue of Accommodation and Vergence Modifies Their Mutual Interactions

Transcription

1 Fatigue of Accommodation and Vergence Modifies Their Mutual Interactions Clifton M. Schor ond Trocy K. Tsueraki Aftereffects of accommodation and convergence demonstrate the adaptability of these two visual motor systems. These aftereffects were reduced after ramp tracking exercises of either accommodation or vergence, and this reduced aftereffect or fatigue was associated with an increase of accommodative vergence and vergence accommodation. When one motor system (accommodation or vergence) adapted more readily than the other, fatigue caused an increase of the cross-link interaction originating from the more adaptable motor system and a reduction of the cross-link interaction originating from the less adaptable motor system. These results suggest an inverse relationship between adaptation and the cross-link interactions between accommodation and vergence and that adaptation of tonic accommodation and tonic vergence is a process that underlies plasticity of accommodative vergence and vergence accommodation respectively. Invest Ophthalmol Vis Sci 28: ,1987 When our gaze shifts from a far object to a near one, there are simultaneous changes in accommodation and convergence of the eyes that are coordinated by cross-link interactions between these two motor systems. Arguments have been put forward that these cross-link interactions between accommodation and vergence are learned 1 and supporting evidence has come from demonstrations in which the ratio of accomodative vergence per diopter (D) of accomodation (AC/A ratio) has been altered temporarily by various environmental manipulations. For example, previous studies have changed the AC/A ratio with visual fatigue, 2 cycloplegics, 3 miotics, 4 increased interocular separation 5 and orthoptics. 6 The ratio of convergence accomodation per meter angle (MA) of convergence (CA/C ratio) has not been studied as extensively as the AC/A ratio due to the limited ability to measure accommodation while its loop is opened and while vergence is stimulated. However, several investigators have observed changes in vergence accommodation with age. 78 Six prism diopters (PD) equal one meter angle. The CA/C ratio is found to be approximately 0.5 D/MA after the age of 22 yr and gradually becomes reduced as the individual becomes more presbyopia 9 The average AC/A ratio is approximately 4 PD/D, and it remains constant until early presbyopia. 10 While these studies clearly demonstrate that the AC/A ratio can be altered, it is not clear if these are direct-learned responses of accom- From the School of Optometry, University of California, Berkeley, California. Supported by National Eye Institute grant EYO to CS. Submitted for publication: September 26, Reprint requests: Clifton M. Schor, School of Optometry, University of California, Berkeley, CA modative-vergence to environmental demands, or if they are indirect results of other altered adaptable components of accommodation and vergence. Adaptation of tonic accommodation and tonic vergence is demonstrated by the continued responses (aftereffects) of these motor systems when their stimuli have been removed and have not been replaced by a new stimulus (open-loop condition). Aftereffects of accommodation can be stimulated directly by lenses or indirectly by vergence via vergence accommodation." Similarly, aftereffects of vergence can be stimulated directly by disparity or indirectly by accommodation via accommodative vergence.'' The magnitude of these aftereffects has been shown to be inversely related to the amplitude of mutual interactions between accommodation and vergence. 12 Vergence stimulated by accommodation becomes reduced during adaptation of accommodation and accommodation stimulated by vergence becomes reduced during adaptation of vergence." The ratio of the change in vergence due to the accommodative response is termed the AC/A ratio and the ratio of the change in accommodation due to the vergence response is termed the CA/C ratio. We have investigated the possibility of modifying the degree of adaptation of accommodation and vergence as a means of altering the accommodative vergence and vergence accommodation ratios respectively. In this study we performed visual motor tracking tasks to produce changes in the AC/A and CA/C ratios. The effects of these accommodative and vergence tracking exercises upon the amplitude of accommodative and vergence aftereffects were also examined. Tracking tasks which increased the AC/A ratio were found to decrease aftereffects of accom- 1250

2 No. 8 FATIGUE OF ACCOMMODATION AND VERGENCE / Schor and TsueroW 1251 M, I t AS T 1 IKQXQI IS Fig. 1. A top view schematic illustrates the vergence-accommodation stimulator. Optical channels for the right and left eyes are both Badal stimulus optometers that can present stimuli in maxwellian view (open loop). Each channel has a filament light source that is imaged at a pinhole pupil and then eventually at the entrance pupil of each eye. The pinholes PI and Pr serve as 0.75-mm aperture stops which, when imaged in the 6-mm eye pupil, increase the depth of focus to ±4 D. Targets in the right channel (Tr) are imaged before the Maxwellian and Badal lens (MBr) at T. The image distance from MBr determines the accommodative stimulus (AS) which is controlled by a stepping motor that moves mirrors Mr2 and Mr3. The accommodative loop of the right eye can be closed by inserting a diffusing screen or opal glass (O) behind the target plane (Tr). Asymmetric vergence is stimulated (VS) by lateral motion of the left channel target (T1) which is controlled by an X-Y plotter. Accommodative responses are measured objectively by the optometer (OPT) which receives infrared light reflected off the fundus of the right eye from a beam splitter (BS). Vergence responses are measured objectively by a pair of infrared sensors (IS) that sense light reflected from the front surface of each eye. F = filament; P = pinhole; C = condenser; CL = collimating lens; O = opal glass; T = target plane; MB = Maxwellian and Badal lens; M = mirror; AS = dynamic accommodative stimulator; VS = dynamic vergence stimulator; BS = beam splitter; OPT = optometer (Ophthalmatron); subscripts L and R = left and right eye respectively. modation and tracking tasks which increased the CA/C ratio caused a reduction of vergence aftereffects. These results provide further evidence that the plasticity of accommodative vergence and vergence accommodation depends upon adaptable tonic mechanisms of accommodation and vergence respectively. Apparatus Materials and Methods Two serially-arranged, opto-electronic systems, one for stimulus presentation and the other for response measurements, were used (Fig. 1). The stimulus-presentation system was capable of independent control of the inputs to accommodation and vergence and of rendering those inputs either statically or dynamically and under either closed-loop or open-loop conditions. The response-measurement system was capable of continuous and simultaneous monitoring of the outputs of accommodation and vergence and of producing high-resolution recordings. The stimulus to accommodation was generated by a stepping-motor-operated, Badal optometer (AS, Fig. 1), similar to the design of Crane and Cornsweet, 13 which could vary the vergence (distance) of the target without affecting its spatial frequency or luminance. The motorized Badal optometer could change the target vergence by increments as small as 0.02 diopters (D). The stepping motor was interfaced with a digital computer, which controlled both the static and the time-varying parameters of the stimulus. The stimulus optometer was connected in series with a vergence-measurement optometer, a Bausch and Lomb Ophthalmetron (Bausch and Lomb, Rochester, NY). 14 The point of contact between the stimulus optometer and the response optometer was a beamsplitter, which allowed the subject to view the target while his accommodation was being constantly monitored. The problem of eye-movement artifacts, which is common to dynamic infrared optometers in general, 15 was exacerbated during the measurement of dynamic vergence accommodation, during which lateral eye movements were purposefully evoked in order to study their effects on the accommodative

3 1252 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / August 1987 Vol. 28 response. The subjects were trained to hold the right (measured) eye stationary to within 0.5 degrees. To verify that the eye before the dynamic, infrared optometer did indeed remain stationary, a four-channel strip-chart recorder was used, in which the movements of each eye were monitored separately, while on the other two channels, accommodation and vergence were also recorded. The vergence stimulus was provided by a function generator, which drove an X-Y plotter. The latter device was physically attached to the target slide before the left eye by means of a mechanical linkage, the movements of which adjusted the lateral position of the slide. Thus, an asymmetric vergence demand was generated without affecting the optical distance of the stimulus and without affecting the position of the right eye (which for sake of optometer accuracy was required to remain stationary). Accommodative responses of the right eye were measured with the Ophthalmetron, which was originally designed as a clinical instrument with a rated accuracy of D, but was subsequently modified for laboratory use as a dynamic, infrared optometer. 16 As a clinical instrument, the Ophthalmetron would measure changes in refractive power as a function of the meridian of the eye; since accommodation was assumed to be relaxed (because the subject was fixating a target that was placed at his far point), any change in refractive power with meridian of the eye was attributed to astigmatism. By disabling the axisrotation mechanism, the Ophthalmetron was made capable of measuring changes in refractive power over time at a fixed meridian of the eye (vertical or 90 meridian); since the meridian was held constant, any changes in refractive power must be attributed to a change in accommodation. The eye movements were measured with a Gulf- Western model-200 (Waltham, MA) infrared eyemovement monitor. The sensor assembly was mounted on a pair of spectacle frames which were worn by the subject. The infrared sources and the photodiodes were carefully adjusted to avoid crosstalk with the infrared optometer. The vergence signal was derived from the lateral movements of each eye by means of an independent differential amplifier, which had the effect of canceling out conjugate eye movements. Visual Stimulus The stimulus for accommodation and fusional vergence was a bandpass (1.75 octave) filtered image of a vertical bar whose center spatial frequency was 6.4 cycles/deg. This moderate spatial frequency was used since it is in the optimal spatial frequency range for stimulation accommodation Subjects Four college students were used, ages between 20 and 21 yr. All had normal eye alignment, accommodation, and 20 sec arc stereoacuity. Two subjects had low AC/A ratios and high CA/C ratios (CM. and K.C.), one had a moderate AC/A ratio and moderately low CA/C ratio (M.W.), and one had a high AC/A ratio and low CA/C ratio (J.T.). Each subject was informed of the nature of the procedure, and their informed consent was obtained prior to their participation in the study. Experimental Procedures Pupils were dilated with 2.5% phenylephrine, a weak sympathomimetic agent, to avoid optometer artifacts. The use of a much stronger dosage of phenylephrine (10%) was found to have no effect on the resting focus of accommodation. 19 A headrest and mouthbite were also used for artifact abatement. Subjects were trained to maintain fixation within ±0.5 since movements of this magnitude did not change accommodation responses. They viewed targets monocularly through a 16 circular field. Subjects were instructed to remain alert while fixating targets but were cautioned against exerting any voluntary accommodation. Accommodation and vergence signals were run through a lowpass analog filter, which had a -3 db rolloff at 15 Hz. Data was recorded on a four-channel Hewlett-Packard model 1400 strip-chart recorder (Palo Alto, CA). Before experimentation began, infrared eye-trac monitors were carefully aligned to prevent cross-talk with the optometer. Calibrations for the eye-trac monitors were performed by having subjects make versional movements that subtended eight prism diopters. Accommodative responses were monitored via oscilloscope screen and a voltage meter (which was previously calibrated for dioptric values). Baseline resting states of accommodation and vergence were recorded while subjects viewed the target monocularly, under maxwellian (pinhole) viewing condiconditions. Once subjects were adapted for a pre-determined time to a stimulus of a given amplitude, the accommodative or vergence loop was opened. The accommodative loop was opened using a pinhole maxwellian view system, which projected a pinhole pupil into the plane of the subjects entrance pupil (Fig. 1 ). 20 This small aperture stop (0.75 mm) limited light rays entering the eye to the paraxial bundle and increased the depth of focus. In this study, the depth of focus for perception of blur increased from ±0.35 D in the normal condition to ±4 D in the maxwellian view condition. This maxwellian view system was used in-

4 No. FATIGUE OF ACCOMMODATION AND VERGENCE / Schor and Tsueroki 1253 ACCOMMODATIVE STIMULI Fig. 2. Accommodative and monocular accommodative vergence tracking responses are illustrated for subject CM. in response to a 2 D, 0.75 Hz positive accommodative ramp stimulus presented monocularly for 4 min. Occasionally, as illustrated in the recording, the accommodation response level increases during ramp tracking. Vergence also becomes more esophoric due to accommodative vergence. LEFT EYE RIGHT EYE stead of darkness or Ganzfeld to open the accommodative loop since studies have found that accommodative aftereffects are longest with pinhole maxwellian view. 21 The vergence loop was opened by simply turning off the light source before the left eye channel. The gain of tonic adaptation of accommodation was determined for each subject. The gain of tonic adaptation of accommodation is defined here as the ratio of tonic aftereffects at 30 sec after opening the accommodative loop, to the amplitude of the accommodative response prior to opening the loop. That is, a gain of 1 would indicate 100% adaptation to the accommodative response after opening the loop for 30 sec. The subjects adapted for 8 sec, under closed loop conditions to a 2 D accommodative stimulus. Measurement of Accommodative Vergence and Vergence Accommodation The CA/C ratio was measured with the accommodative loop opened, so that any changes in accommodation could be solely attributed to a change in vergence. A binocular pinhole maxwellian view was used to open the accommodative loops of both eyes during measures of convergence accommodation. The vergence stimulus had an amplitude which could be varied, although amplitudes were usually on the order of 6 to 8 prism diopters (PD). The stimuli to disparity vergence were presented in a repetitive stepwise manner (0.065 Hz Hz) from the subject's phoria position. The CA/C ratio was defined as the mean change in accommodation in diopters divided by the change in vergence in meterangles (6 prism diopters) averaged from three cycles of the repetitive step responses of disparity vergence and vergence accommodation (the response CA/C ratio). The AC/A response ratio was always measured with the vergence loop opened, so that any changes in vergence could be attributed to a change in accommodation. The vergence loop was opened by extinguishing the light source to the left eye. The accommodative stimulus had an amplitude which could be varied, but was generally between 1.5 to 4 D from the subjects' far point. The accommodative stimulus was presented in a repetitive step-wise manner at 0.05 Hz. The AC/A ratio was defined as the change in vergence in PD divided by the change in accommodation in diopters averaged from three cycles of the repetitive step-wise responses of accommodation and accommodative vergence (the response AC/A ratio). Results Accommodative and Vergence Tracking Tasks Repetitive ramp stimuli (saw-tooth functions) were produced with a digital to analog converter and a computer look-up table. The positive ramp tracking stimulus for accommodation consisted of a 2 D accommodative stimulus which moved slowly toward the observer and jumped quickly back to zero diopters at a frequency of 0.75 Hz for a period of 4 min. An example of an accommodative tracking response is illustrated in Figure 2 for subject CM. During the first 15 sec of the tracking stimulus, there were some accommodative ramp responses. A.Tter this, the accommodative system was unable to maintain the ramp response, and the mean level of accommodation increased and vergence also became more eso-

5 1254 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / August 1987 Vol. 28 Table 1. AC/A and CA/C ratios before and after accommodative tracking tasks AC/A CA/C Before After Before After CM. <2PD/D 4PD/D 1.35 D/MA 0.9 D/MA K.C. <2PD/D 6PD/D 1.50 D/MA 1.0 D/MA J.T. 8PD/D 4PD/D 0.2 D/MA 0.6 D/MA AC/A and CA/C ratios are compared before and after accommodative ramp tracking tasks for two subjects (CM. and K.C.) who adapted accommodation more than vergence, and for one subject (J.T.) who adapted vergence more than accommodation. The accommodative ramp tracking task increased the AC/A ratios and decreased the CA/C ratios of subjects CM. and K..C, and decreased the AC/A ratio and increased the CA/C ratio of subject J.T. phoric. Similar cumulative effects have been reported by Brodkey and Stark. 22 The AC/A ratio, computed from vergence responses during this fatiguing accommodative task, was 3.6 PD/D, which is nearly double BASE IN VERCENCE RAMP SUM. to. «H,, VERGENCE STIMULI -^^I^^^ ^^NsN^N^N VERGENCE STIMULI I 1 -' Fig. 3. Disparity vergence and vergence accommodation tracking responses to a 5 PD, 0.75 Hz base-out (top) and base-in (bottom) ramp stimuli are shown for subject M.W. As convergence or divergence is stimulated, accommodation makes a concurrent ramp movement due to vergence accommodation. the pre-tracking AC/A ratio listed in Table 1. As will be shown below, the increase in the AC/A ratio is related to reduced adaptability of tonic accommodation caused by tracking fatigue. The gradual increase in the accommodative response level was not seen in subjects who had minimal adaptation of accommodation. Fatigue resulting from tracking the ramp was demonstrated by measuring adaptation of accommodation before and after the 4-min tracking task. Accordingly, fatigue produced by tracking is denned here as a reduction of an aftereffect of accommodation or vergence. Figure 3 illustrates the vergence tracking ramps which were stimulated with a convergence or divergence disparity ramp of 5-6 PD presented for 4 min at 0.65 to 0.75 Hz. During vergence tracking, the left eye was required to make all vergence movements as the right eye viewed a stationary target. Vergence phoria adaptation was measured before and after vergence tracking tasks to determine if adaptation to either convergence or divergence stimuli had been altered from baseline values by the tracking exercises. Pilot observations indicated that 4 min of vergence tracking of disparity ramps at frequencies of 0.65 Hz or greater was sufficient to reduce adaptation of the stimulated system (accommodation or vergence). Effects of Accommodative Tracking of Ramp Stimuli on Adaptation of Accommodation and Vergence Two subjects (CM. and K.C.) who adapted accommodation more than vergence were used in this part of the experiment. Adaptability of tonic accommodation was reduced following 4 min of accommodative tracking in response to the positive ramp stimulus. For example, Figure 4 compares accommodative aftereffects in subject CM., resulting from an 8-sec 2 D stimulus, measured before (top) and after (bottom) performing the accommodative ramp tracking task. Following the tracking task (bottom record), there was a rapid decay of the accommodative response, whereas prior to accommodative tracking (top record) there was a persistent aftereffect of a 2 D accommodative response when the loop was opened by pinhole pupil. Duration of accommodative aftereffects were also reduced for subject K.C. by accommodative tracking from over 3 min to less than 15 sec. Both CM. and K.C. had very modest prism adaptation. Accordingly, neither subject had any noticeable effects of accommodative tracking tasks on the adaptability of tonic vergence. In contrast, subjects who adapted vergence more than accommodation (J.T. and M.W.) had briefer periods of vergence after-

6 No. 8 FATIGUE OF ACCOMMODATION AND VERGENCE / Schor and Tsueraki 1255 effects following the accommodative tracking task. For example, subject J.T. had prolonged aftereffects of disparity vergence (10 min) following adaptation for 15 sec to a 10 PD base out (BO) step disparity. However, following the accommodative tracking task, J.T. had a vergence aftereffect that lasted only 20 sec. This result indicates that accommodative tracking tasks can reduce vergence aftereffects in subjects with robust vergence adaptation and little if any accommodation adaptation. Presumably, the fatigue effect occurs via the accommodative vergence cross-link. Effects of Vergence Tracking of Disparity Ramps on Adaptation of Vergence and Accommodation A vergence tracking task (5 PD BO ramp at 0.5 Hz) (Fig. 3) was performed by two subjects (M.W. and J.T.) who adapted to vergence more than accommodation. Adaptability of tonic vergence was reduced after tracking a positive vergence ramp for 4 min. Figure 5 illustrates the duration of vergence afteref- VERGENCE ADAPTATION VERGENCE ADAPTATION ACCOMMODATIVE ADAPTATION Fig. 5. Tonic aftereffects of vergence are observed in subject J.T. after stimulating disparity vergence with a 10 PD BO stimulus for 15 sec prior to (top) and following (bottom) 4 min of the disparity vergence ramp tracking task. The vergence loop was opened after 15 sec of disparity stimulation by occlusion of the left eye. Vergence decayed slowly prior to the tracking task (45 sec) and it decayed quickly (within several seconds) following the vergence ramp tracking task. J ACCOMMODATIVE ADAPTATION Fig. 4. Tonic accommodative aftereffects are illustrated for subject CM. after stimulating accommodation monocularly by 2 D for 15 sec (A) and then opening the accommodative loop with a projected pinhole pupil (B). The time discontinuity at (C) illustrates the initial decay of the accommodative aftereffect 1 min after the accommodative loop was opened. Aftereffects of accommodation are observed prior to (top) but not following (bottom) performance of the accommodative tracking task. Aftereffects lasted for 10 min before the tracking procedure and they decayed in several seconds following accommodative ramp tracking. fects for subject J.T., measured with one eye occluded, after responding to a 10 PD BO step disparity stimulus for 14 sec before (top) and after (bottom) the convergence tracking task. Before the tracking task, vergence aftereffects lasted 10 min. Following vergence tracking, vergence aftereffects decayed quickly to the resting position in 5 sec while the vergence loop was opened by monocular occlusion. Similar changes in vergence adaptation were obtained with convergence and divergence tracking ramps. M.W. also had similar reductions in the duration of vergence aftereffects of disparity vergence following the vergence tracking task. The unusually low gain of tonic aftereffects of accommodation (0.1) for subjects J.T. and M.W. were unchanged after the vergence tracking task. In contrast, subject CM., who adapted accommodation more than vergence, had a reduction of the tonic accommodative aftereffects following the 4-min vergence tracking task. Prior to the tracking task, CM. had prolonged accommodative aftereffects (10 min) when adapted to a 2 D step stimulus for 15 sec. After the vergence tracking task, accommodative aftereffects were shortened to 10 sec following accom-

7 1256 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / August 1987 Vol. 28 ACCOMMODATIVE VERCENCE tive adapter (K.C.) are shown in Table 1. Also shown in Table 1 are results for a subject (J.T.) who adapted vergence but not accommodation. Following the accommodative tracking task, J.T.'s AC/A ratio was reduced from 8 PD/D to 4 PD/D and his CA/C ratio increased from 0.2 D/MA to 0.6 D/MA. Effects of Vergence Tracking of Disparity Ramps on the CA/C and AC/A Ratios CA/C and AC/A ratios, measured before and after vergence tracking tasks, are summarized in Table 2 VERCENCE ACCOMMODATION ACCOMMODATIVE VERCENCE 10 P. d RICH! til Fig. 6. Accommodative vergence responses to a 2 D, 0.05 Hz monocular accommodative stimulus are illustrated prior to (top) and following (bottom) the accommodation ramp tracking task (2 D at 0.75 Hz for 4 min). The AC/A ratio increased from <2 PD/D to 4 PD/D as a result of the ramp tracking task. modation to the same 2 D stimulus. Vergence adaptation for CM. was minimal (lasting only 10 sec) before and after the vergence tracking task. Vergence tracking fatigued the accommodative adaptor, presumably through the vergence accommodation cross-link. VERCENCE ACCOMMODATION I,,.. Effects of Accommodative Tracking on the AC/A and CA/C Ratios Figure 6 (top and bottom) illustrates accommodative vergence for subject CM. recorded before and after the accommodative tracking task respectively. Clearly, the AC/A ratio increased from <2 Pd/D to 4 Pd/D as a result of tracking fatigue. Figure 7 (top and bottom) illustrates vergence accommodation for subject CM., also recorded before and after the accommodative tracking task respectively. Interestingly, the CA/C ratio was reduced from 1.35 D/MA to 0.9 D/MA as a result of the accommodative tracking fatigue. Similar results for another rapid accommoda- Fig. 7. Vergence accommodation responses to a 4 PD BO, 0.07 Hz disparity step stimulus are illustrated for subject CM. prior to (top) and at 5 PD BO, 0.1 Hz following (bottom) the accommodative ramp tracking task (2 D at 0.75 Hz for 4 min). The CA/C response ratio was reduced from 1.35 D/MA to 0.9 D/MA as a result of the ramp tracking task.

8 No. 8 FATIGUE OF ACCOMMODATION AND VERGENCE / Schor and Tsueraki 1257 Table 2. AC/A and CA/C ratios before and after vergence tracking tasks ACCOMMODATIVE VERCENCE.AC/A CA/C Before After Before After J.T. 8PD/D 4PD/D 0.2 D/MA D/MA M.W. 4 PD/D 2 PD/D 0.4 D/MA 0.65 D/MA CM. <2PD/D 4 PD/D 1.35 D/MA 1.25 D/MA AC/A and CA/C ratios are compared before and after disparity vergence ramp tracking tasks for two subjects (M.W. and J.T.) who adapted vergence more than accommodation, and for subject (CM.) who adapted accommodation more than vergence. The vergence ramp tracking task increased the CA/C ratios and decreased the AC/A ratios of subjects M.W. and J.T., and decreased the CA/C ratio and increased the AC/A ratio of subject CM. for three subjects. Since the effects of convergence and divergence ramp tracking were indistinguishable, the results have been lumped together. Figure 8 (top ACCOMMODATIVE VERCENCE VERCENCE ACCOMMODATION 10 P. <! I" Fig. 9. Accommodative vergence responses to a 2 D, 0.05 Hz monocular accommodative stimulus are illustrated for subject J.T. prior to (top) and following (bottom) performance of the vergence ramp tracking task (5 PD BO at 0.75 Hz for 4 min). The AC/A response ratio war. reduced from 8 PD/D to 4 PD/D as a result of the vergence ramp tracking task. VERCENCE ACCOMMODATION Fig. 8. Vergence accommodation responses to an 8 PD BO, 0.05 Hz disparity-step stimulus are illustrated for subject J.T. prior to (top) and following (bottom) performance of the vergence ramp tracking task (5 PD BO at 0.75 Hz for 4 min). The CA/C response ratio increased from 0.2 D/MA to 0.6 D/MA as a result of the vergence ramp tracking task. I" and bottom) illustrates the vergence accommodation response of subject J.T. before and after the convergence tracking task respectively. His CA/C ratio increased three-fold to D/MA from 0.2 D/MA after the vergence tracking fatigue task. Figure 9 (top and bottom) illustrates accommodative vergence for subject J.T., also recorded before and after the convergence tracking task respectively. Interestingly, the response AC/A ratio was reduced from 8 PD/D to 4 PD/D as a result of the vergence tracking fatigue. These results are summarized along with those of a moderate vergence adapter (M.W.) in Table 2. Also shown in Table 2 are results for a subject who adapted accommodation but not vergence (CM.). Following the vergence tracking task, C.M.'s CA/C ratio was reduced from 1.35 D/MA to 1.25 D/MA and his AC/A ratio increased from <2 PD/D to 4 PD/D.

9 1258 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / Augusr 1987 Vol. 28 Discussion The tonic adaptors of accommodation and convergence have been shown to be independent of one another by studies of accommodation and vergence in darkness and by their dissociated aftereffects. 23 Furthermore, the AC/A and CA/C ratios have been shown to be inversely related to the gain of tonic adaptation of accommodation and vergence respectively." These observations have led to a model of the interactions between accommodation and convergence which predicts that reduced adaptation of accommodation or vergence would lead to an increase in the AC/A and CA/C ratios respectively. It is also known that aftereffects of vergence and accommodation can be stimulated by accommodative vergence and vergence accommodation respectively Accordingly, reduced adaptability of accommodation and vergence would also be expected to lead to a reduction of the CA/C and AC/A ratios respectively. In the current investigation, the adaptable states of accommodation and vergence were reduced with ramp tracking tasks. Fatigue was produced with 4 min of ramp tracking exercises. This duration was found to be the minimal time to cause a reduction of adaptation. Subjects (CM. and K.C.) who adapted accommodation more than vergence showed marked decreases in adaptable accommodation but little change in vergence adaptation after performing both the 4-min accommodative and vergence tracking tasks. In contrast, subjects M.W. and J.T., who adapted vergence more than accommodation, had a marked decrease in vergence adaptation but little change in their minimal adaptation of accommodation after both the vergence and accommodative tracking tasks. Both tracking tasks caused an increase in the AC/A ratio for CM. and K.C. and an increase in the CA/C ratio for subjects M.W. and J.T. Interestingly, the AC/A ratio was lower for J.T. and M.W. after the vergence tracking task, and the CA/C ratio was slightly lower for subject CM. and K.C. after performing the accommodative tracking exercise. This consistent reduction in the AC/A and CA/C ratios may have resulted from the reduction of adaptation by vergence and accommodation respectively, since these aftereffects are normally stimulated by accommodative vergence and vergence accommodation respectively The results demonstrate that individuals who have strong adaptation of one system (accommodation or vergence) with moderate to low adaptation of the other system (vergence or accommodation), have fatigue of their more adaptable system following either a vergence tracking or accommodative tracking task. Subsequent changes in crosslink interactions are predictable following fatigue. Unusually low AC/A ratios are increased by reduced tonic adaptation of accommodation and unusually low CA/C ratios are increased by reduced tonic adaptation of vergence. Similarly, unusually high AC/A ratios are decreased by reduced tonic adaptation of vergence and unusually high CA/C ratios are decreased by reduced tonic adaptation of accommodation. A recent model of interactions between accommodation and vergence"' 12 predicts that these changes will be most pronounced when accommodation and vergence adapt unequally (ie, accommodation more than vergence or vice versa). In these cases, a given observer will have an unusually high AC/A ratio and low CA/C ratio if vergence is more adaptable than accommodation. The reverse case occurs if accommodation is more adaptable than vergence. If both accommodation and vergence are equally adaptable (either low or high), then the cross-link interactions would remain unchanged by tracking tasks as long as adaptation of accommodation and vergence are fatigued equally. The site of fatigue caused by ramp-tracking exercises is not likely to be at the accommodative plant or the extraocular muscles since the plants for accommodation and vergence do not fatigue easily. 24 Rather, fatigue is likely to occur centrally, at sites where adaptable tonic accommodation and tonic vergence are controlled and perhaps also at sites where phasic responses of optical reflex accommodation and disparity vergence are controlled. This fatigue of adaptable tonic mechanisms is produced by both direct stimulation as well as by cross-link interactions between accommodation and vergence. For example, we have found that it is possible to reduce adaptability of accommodation with ramp-tracking exercises of either monocular accommodation or disparity vergence. Similarly, vergence adaptation can be reduced by ramp-tracking exercises of either disparity vergence or monocular accommodation. Consequently, it is possible to elevate a low AC/A ratio with either accommodative or vergence ramp-tracking exercises. There may be other adaptable processes that regulate the cross-link interactions between accommodation and vergence in response to environmental demands, that are independent of adaptable tonic elements of accommodation and vergence. For these processes to be independent of the adaptable tonic elements, they would probably involve mechanisms described by the cross-links themselves. They would also be responsive to environmental demands placed upon accommodation and vergence, such as uncorrected refractive state and altered interpupillary distance. Indeed, uncorrected myopes have higher response AC/A ratios than uncorrected hyperopes or

10 No. 8 FATIGUE OF ACCOMMODATION AND VERGENCE / Schor ond Tsueroki 1259 normals 25 and the response AC/A ratio of uncorrected myopes becomes reduced after they receive a minus lens spectacle correction. 26 In addition, optically increasing the interpupillary distance while vigorously shifting vergence and accommodation from one target distance to another, results in a predictable increase in the AC/A ratio. 27 However, optically decreasing the interpupillary distance while performing the same tracking exercises causes unpredictable increases and decreases in the AC/A ratio. 526 Perhaps fatigue of adaptable tonic accommodation and tonic vergence caused by the tracking exercises used in these prior studies may have contributed, in some portion, to the observed changes in AC/A ratio, irrespective of the optimal change required by the altered interpupillary distance. Clearly, there are several possible ways that interactions between accommodation and vergence might be altered, and changing the adaptability of tonic elements of accommodation is one of them. Key words: accommodative vergence, vergence accommodation, tonic accommodation, tonic vergence, adaptation, fatigue References 1. Helmholtz H von: Treatise on Physiological Optics, Vol 3. New York, Dover, 1962, p Manas L: The inconsistency of the AC/A ratio. Am J Optom Arch Am Acad Optom 32:304, Christoferson KW and Ogle KN: The effects of homatropine on the accommodative convergence association. Arch Ophthalmol 55:779, Parks MM: Abnormal accommodative convergence in squint. Arch Ophthalmol (NY) 59:364, Miles FA and Judge SJ: Optically-induced changes in the neural coupling between vergence eye movements and accommodation in human subjects. In Functional Basis of Ocular Motility Disorders, Lennerstrand G, Zee DS, and Keller EL, editors. Oxford, Pergamon Press, 1982, pp Flom MC: On the relationship between accommodation and accommodation-convergence III. Effects of orthoptics. Am J Optom Arch Am Acad Optom 37:619, Fincham EF: The accommodation reflex and its stimulus. Br J Ophthalmol 35:381, Kent PR: Convergence accommodation. Am J Optom Arch Am Acad Optom 35:393, Fincham EF: The proportion of ciliary muscle force required for accommodation. J Physiol (Lond) 128:99, Morgan MW: Relationship between accommodation and convergence. Arch Ophthalmol 47:745, Schor CM and Kotulak JC: Dynamic interactions between accommodation and convergence are velocity sensitive. Vision Res 26:927, Schor CM: Adaptation of accommodative vergence and vergence accommodation. Glenn Fry Lecture. Am Acad Optom Physiol Opt 63:587, Crane HD and Cornsweet TN: Ocular-focus stimulator. J Opt Soc Am 60:577, Knoll HA and Mohrman R: The Ophthalmetron: principles and operations. Am J Optom Physiol Opt 49:122, Krishnan VV, Shriachi D, and Stark L: Dynamic measurement of vergence accommodation. Am J Optom Physiol Opt 54:470, Johnson CA, Post RP, and Tsuetaki TK: Short-term variability of the resting focus of accommodation. Ophthalmic Physiol Opt 4:319, Kotulak JC and Schor CM: The spatial frequency response of accommodation. ARVO Abstracts. Invest Ophthalmol Vis Sci 26(Suppl):270, Owens DA: A comparison of accommodative responsiveness and contrast sensitivity for sinusoidal gratings. Vision Res 20:159, Garner LF, Brown B, Baker R, and Colgan M: The effect of phenylephrine hydrochloride on the resting point of accommodation. Invest Ophthalmol Vis Sci 24:393, Westheimer G: The maxwellian view. Vision Res 6:669, Schor CM, Kotulak JC, and Tsuetaki TK: Adaptation of tonic accommodation reduces accommodative lag and is masked in darkness. Invest Ophthalmol Vis Sci 27:820, Brodkey J and Stark L: Feedback control analysis of accommodative convergence. Am J Surg 114:150, Kotulak JC and Schor CM: The dissociability of accommodation from vergence in darkness. Invest Ophthalmol Vis Sci 27:544, Duke-Elder S and Abrams D: System of Ophthalmology, Volume V: Ophthalmic Optics and Refraction. London, Kimpton, 1970, p Ogle KN and Martens TG: On the accommodative convergence and the proximal convergence. Arch Ophthalmol 57:702, Flom MC and Takahashi E: The AC/A ratio and undercorrected myopia. Am J Optom Arch Am Acad Optom 39:305, Judge SJ and Miles FA: Changes in the coupling between accommodation and vergence eye movements induced in human subjects by altering the effective interocular separation. Perception 14:617, 1985.