Effect of Surround Propinquity on the Open-Loop Accommodative Response

Transcription

1 Investigative Ophthalmology & Visual Science, Vol. 32, No. 1, January 1991 Copyright Association for Research in Vision and Ophthalmology Effect of Surround Propinquity on the Open-Loop Accommodative Response Mark Rosenfield and Kenneth J. Ciuffreda The effect of knowledge of surround propinquity, ie, awareness of proximity of the adjacent surroundings, on the open-loop accommodative response (AR) was determined by comparing measurements of accommodation obtained in total darkness in two different-sized rooms. The AR was measured in two laboratories, one 2.5-m square and the other 6.75 X 2.75 m. Steady-state accommodation was assessed on two occasions in each room using an infrared optometer. On the first occasion, the subjects (n = 10) were fully aware of the laboratory dimensions and topography. For the second trial, they were blindfolded before entering the laboratory and hence were unaware of the experimental location. When subjects were unaware of the laboratory dimensions, no significant difference existed between the recorded values of AR. However, when subjects were initially able to observe the size of the room, the AR in darkness was significantly higher in the smaller laboratory. This suggests that proximally induced accommodation, initiated by prior knowledge of the dimensions of the laboratory, was responsible for this increase in AR. Furthermore, it is proposed that complex interactions exist between proximal, tonic, and other nonoptical accommodative stimuli such that it may be impossible to isolate an individual, nonoptical accommodative component. Invest Ophthalmol Vis Sci 32: ,1991 It has been well documented that in the absence of an adequate visual stimulus, the accommodative response (AR) adopts a relatively stable value of around D;'~ 3 the term tonic accommodation 2-4 (TA) being used to describe this intermediate level of accommodation. To obtain a veridical assessment of the tonic level, all exogenous stimuli to accommodation must be eliminated. This is most readily achieved by placing the subject in total darkness. 2 ' 3 ' 5 " 9 However, other studies report that even under darkroom conditions, nonoptical cues may influence the AR. For example, both Westheimer 10 and Malmstrom et al." showed that visual imagery, ie, imagining either near or far targets under open-loop conditions, could significantly influence the AR. These findings suggest an interaction between proximally induced accommodation (PIA), ie, accommodation resulting from knowledge of nearness of the object of regard 13 " 16 and TA. (The term "proximally induced accommodation" was adopted since a recent investigation 12 indicated that accommodation induced by From the Schnurmacher Institute for Vision Research, Department of Vision Sciences, SUNY/State College of Optometry, New York, New York. Presented at the Annual Meeting of the Association for Research in Vision and Ophthalmology, Sarasota, FL, May Submitted for publication: January 3, 1990; accepted July 23, Reprint requests: Mark Rosenfield, PhD, SUNY/State College of Optometry, 100 East 24th Street, New York, NY knowledge of nearness was an aggregate response resulting from a combination of proximal accommodation and proximal vergence driving convergent accommodation.) Recent studies 1516 suggest that the potential contribution of PIA to the aggregate AR may be more substantial than previously considered. For example, Rosenfield et al. 15 observed that with both accommodation and vergence under open-loop conditions, a change in target distance from D produced an initial mean change in AR in ten emmetropic subjects of +1.7 D. This would represent a PIA/accommodative stimulus ratio of 0.60 D/D. Further evidence for an interaction between PIA and TA may be drawn from investigations 17 " 22 showing differences in the magnitude of TA depending on the method used to open the accommodation loop. Schor et al. 17 noted that accommodative adaptation was relatively sustained when subjects viewed an illuminated target consisting of a low spatial-frequency difference of Gaussian (DOG) grating through a 0.5- mm pinhole compared with that response when the accommodative loop was opened by placing the subject in total darkness. Furthermore, Wolfe et al. 19 reported that 19 of 21 tested subjects showed a more myopic value of pretask TA when measured in an empty light field compared with the dark TA measurement (mean difference, 0.50 D). Under illuminated conditions the presence of a fixation target, eg, the DOG target used by Schor et al., 17 and the subject's increased awareness of the surrounding instrumentation may have acted as stimuli to PIA. Thus

2 No. 1 SURROUND PROPINQUITY AND ACCOMMODATION / Rosenfield ond Ciuffredo 143 PIA could contribute to some of the differences in AR under the two open-loop conditions. The influence of apparent target proximity on the AR was also demonstrated by Hennessy et al. 23 In their investigation, subjects were required to view a defocused spot of light through either a 1 or 4 aperture. The fixation spot was located 6 m from the observer, while the viewing distance of the "wall" containing the aperture varied between m. These authors suggested that subjects tended to localize the spot of light in the aperture plane. They demonstrated that the AR to the spot of light altered with changes in the dioptric distance of the aperture plane. Furthermore, they also observed variations in AR depending on the order of presentation of the various aperture locations. In view of this latter finding, they suggested that the initial viewing condition could influence the state of accommodation. For instance, subjects who first saw the aperture in the nearest position and were unaware of the actual dimensions of the experimental laboratory may have incorrectly assumed that the additional targets used were closer than their actual physical location; they therefore exhibited an increased perceptually driven AR. If the AR can be influenced by knowledge of apparent target position, then it is possible that measurements of accommodation in total darkness may be biased by prior observation of the experimental apparatus and the dimensions and overall topography of the laboratory before the room illumination is extinguished. Accordingly our aim was to measure the AR in total darkness in two different rooms. The length of one room was approximately 2.7 times that of the other. Additionally accommodation was assessed twice in each room. On the first trial, the subjects were fully aware of the laboratory topography. However, for the second assessment, they were blindfolded before entering the laboratory and hence were unaware of the experimental location. Materials and Methods The AR was examined in ten visually normal subjects (two men and eight women), all of whom were students at the SUNY/State College of Optometry. The mean age of the subjects was 27.3 yr (standard error of the mean [SEM], ± 1.4 yr; range, yr). Five of the subjects were emmetropic, ie, had unaided distance visual acuity of at least 6/6 and a mean spherical refractive error not greater than 0.50 D of myopia or hypermetropia (with astigmatism not exceeding 0.50 D). The remaining five subjects were all myopic and wore their habitual spectacle refractive correction on entering the laboratory. However, these subjects were required to remove their spectacles immediately before measurement of the AR because reflections from the anterior surface of the spectacle lens would interfere with the operation of the infrared optometer. The mean spherical equivalent refractive error of the myopic subjects was 3.12 D (SEM, ±0.35 D), and no subject had astigmatism exceeding 1.00 D (mean, D cyl; SEM, ± 0.09). All myopic subjects were able to achieve a visual acuity of at least 6/6 with their refractive correction; no subject was rejected for failing to meet these criteria. The AR was measured objectively using the Canon Autoref R-l infrared optometer (Canon Inc., Kanagawa, Japan). This instrument provides a wide, open field of view with a semisilvered mirror, and the absence of an internal fixation target avoids any extraneous stimuli to PIA. The optometer measures three meridians in 0.2 sec to provide sphere and cylinder powers to an accuracy of ±0.12 D and has been fully described and evaluated by McBrien et al. 24 The linearity of this instrument over a refractive range of ± 8.00 D has been demonstrated previously. 25 All measurements of accommodation were taken from the left eye, and the readings were calculated as mean spheres (ie, sphere + half cylinder power). No calibration factor was added to the optometer reading. To ensure that stray light from the video monitor would not illuminate the laboratory, the monitor was covered with opaque material, and the experimental operator viewed the screen under this material. Before measurement of TA, the subject sat in total darkness for 5 min to allow the dissipation of transient changes in accommodation before the start of the experimental session After this the subject was positioned in a chin and forehead rest, and 50 readings of the refractive ei or were recorded at approximately 2-sec intervals over a 100-sec period. These were then averaged to give the mean value of TA, and in the case of the ametropic subjects, appropriate compensation was made for their uncorrected refractive error. TA was measured in two rooms having dimensions of 6.75 X 2.75 m and 2.5-m square, respectively. Both rooms had an internal height of 2.6 m. In the larger room, the subject was positioned at one end such that they were midway between the two side walls (ie, approximately 1.35 m away from each) and 5.75 m away from the facing wall. In the smaller room, the subject was placed in one corner so that they were approximately 0.90 m from both the facing and right-hand walls, respectively. In both rooms, the ceiling and all four walls were covered with matte black paint. The assessment of TA was conducted on four occasions for each subject, two measurements in each room. Each experimental session was separated by at

3 144 INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / January 1991 Vol. 32 least 24 hr. In the first two sessions, TA (mean of 50 readings) was recorded in the large and small room, respectively. For these trials, the laboratory was fully illuminated when the subject entered, and therefore they were aware of the experimental location. During these conditions, the subjects were allowed to sit in the fully illuminated laboratory, while wearing their spectacle correction if applicable, for approximately 1 min before the room light was extinguished. This period served to provide the observer with a sense of the laboratory dimensions and overall topography. However, on the third and fourth sessions, the subject initially presented at a third location where they were blindfolded. To promote further confusion as to the experimental site being used for that particular trial, the subject walked around the corridors while blindfolded for approximately 5 min, assisted by the experimenter, in such a manner as to create uncertainty regarding their precise position. Accordingly the subject was led about in an erratic pattern, with frequent turns and reversals of direction, before entering the laboratory. The blindfold was retained during the 5-min dark-adaptation period and was only removed immediately before the measurement of accommodation. Since variations in vestibular inputs have been shown to influence the AR, 28 it is possible that walking about the corridors, with frequent changes in direction, could have induced some change in AR. To control for this possibility, the subjects were required to walk about the corridors in a similar fashion to the two blindfold conditions before the initial two sessions, ie, when the subject was initially allowed to view the experimental location. However, the subject was not blindfolded during these periods. The two "eyes-open" sessions, ie, where the subject was fully aware of the experimental location, were always done before the "eyes-closed" trials so that subjects would appreciate that more than one laboratory was being used. If a subject had an initial eyesopen session followed by an eyes-closed session in the other room, they may have incorrectly assumed that the second session was in the same laboratory as the first, and this could have influenced their AR. However the order of the room-size conditions, ie, large room or small room, was counterbalanced. To discourage subjects from trying to guess their location, they were kept naive as to the actual purpose of the experiment. Subjects were informed that the purpose of the study was to investigate the effects of visualvestibular interactions, produced by walking around the corridor with and without the blindfold, on TA. Thus they were informed of the precise method, with the exception of the fact that two experimental locations were being used, before the experiment. Subjects gave their full consent to taking part in the study. Immediately after completion of the experiment, all subjects were informed in writing of the actual nature of the study. Results To identify any variation in the magnitude of the AR during the 100-sec measurement period, the mean AR (average often readings) was calculated for each 20 sec of recording (ie, 0-20 sec, sec, etc). Four-factor (recording period, room size, eyes open or closed [ie, whether the subject was blindfolded when entering the laboratory], and refractive error [ie, emmetropic or myopic]) analysis of variance was done on this data. This indicated that variations in room size (F, 4.60; df, 1160; P < 0.05), eyes open or closed (F, 4.64; df, 1160; P < 0.05), refractive error (F, 47.78; df, 1160; P < 0.01), and the interaction of room size and eyes open or closed (F, 25.75; df, 1160; P < 0.01) were significant. Neither the recording period (F, 0.77; df, 4160) nor any of the other interactions were statistically significant. Since no significant variation in AR was observed during the 100-sec recording period, the data were averaged for the entire period, and subsequent discussion will refer to this averaged data. The mean values of AR for the four experimental conditions are illustrated in Figure 1. Mean values of AR for individual subjects are presented in Table 1. When subjects were allowed initially to observe the size of the laboratory, the mean values of AR in the large and EYE Test condition EYESC Large room Small room Fig. 1. Mean values of accommodative response recorded in total darkness. Eyes open: where the subject was fully aware of the test room dimensions and overall topography. Eyes closed: the subject was blindfolded prior to entering the laboratory and hence was unaware of the test room dimensions and topography. Error bars indicate 1 SEM.

4 No. 1 SURROUND PROPINQUITY AND ACCOMMODATION / Rosenfield ond Guffreda 145 Table 1. Individual accommodative responses (D) measured in total darkness for the four experimental conditions* Subject Mean SEM Large room Eyes open Small room Large room Eyes closed Small room * Subjects 1-5 were emmetropic while subjects 6-10 were myopic. Eyes open: where the subject was fully aware of the test room dimensions and overall topography. Eyes closed: the subject was blindfolded prior to entering the laboratory and hence was unaware of the test room topography. small room were D and 1.23 D, respectively. A paired t-test indicated that this difference was statistically significant (t, 3.63; df, 9; P < 0.01). Examination of Table 1 indicates that all ten subjects showed an increased response in the small room, with respect to the large room, when they were aware of the laboratory being used. However, when subjects were unaware of the experimental location, the mean values of AR in the large and small room were 0.89 D and 0.69 D, respectively. This difference was not statistically significant (t, 1.49; df, 9; P = 0.17). The mean AR for the myopic and emmetropic subjects for the four experimental conditions is shown in Table 2. Although the small sample size should be noted (onlyfivesubjects in each subgroup), it is interesting to observe that the AR in darkness for the myopic subjects was significantly lower than that of the emmetropic subjects. This finding is consistent with a number of previous investigations 8 ' 29 ' 30 which all reported lower levels of TA in myopic compared with emmetropic subjects. Discussion These results suggest that prior knowledge of differences in test-room dimensions and overall topography produced a significant shift in the steady-state, open-loop AR. It is likely that the change in AR resulted from an increased awareness of surround propinquity. This finding would suggest that in the smaller room, the measured AR reflected not only the level of tonic innervation, but also the output of other nonoptical accommodative stimuli, eg, PIA. It should again be emphasized that all measurements of AR were obtained in total darkness and, therefore, under open-loop conditions. Other investigations showed complex interactions between tonic, proximal, and cognitively induced accommodation. For example, both variations in cognitive demand, 31 " 38 and visual imagery 10 " (ie, "thinking near" or "thinking far") have been shown to alter the magnitude of TA. Recent data from our laboratory 39 indicated that the level of cognitive demand can also influence the output of PIA. Under alternative stimulus conditions, other nonoptical stimuli to accommodation, eg, vestibular 28 and auditory 40 inputs, may also affect the so-called "stimulusfree" AR. 41 In view of these complex interactions, it is likely that many previously described measures of TA reflected not only the level of tonic innervation to the ciliary muscle but also responses resulting from other nonoptical accommodative stimuli. For example, any optometer which either has an internal fixation target, or alternatively presents the observer with an object of regard (eg, the speckle pattern used in the laser optometer 42 " 44 ), is likely to affect the magnitude of PIA. In two recent investigations, Rosenfield 21 ' 22 demonstrated that viewing an open-loop target, ie, one which does not provide a blur stimulus to accommodation, will still alter the AR compared with that response obtained in darkness. Additionally, our results indicate that to minimize the stimulus to PIA, the subject should either be unaware of the laboratory topography, both the dimensions of the room and experimental instrumentation, or alternatively the apparatus and other laboratory structures should be located as far from the subject as possible. Even the optometer we used, which provides the subject with an open field of view measuring 18 vertically by 50 horizontally, may still have some influence on the overall AR due to the subject's knowledge of proximity of the instrument. Table 2. Mean accommodative responses (D) for the myopic and emmetropic subjects, for the four experimental conditions* Emmetropes (N=5) Myopes (N=5) Large room, eyes open 0.95 (0.22) 0.45 (0.15) Small room, eyes open 1.46 (0.27) 1.00 (0.19) Large room, eyes closed 1.18 (0.27) 0.60 (0.15) Small room, eyes closed 0.95 (0.28) 0.42 (0.22) * Eyes open: where the subject was fully aware of the test room dimensions and overall topography. Eyes closed: the subject was blindfolded prior to entering the laboratory and hence was unaware of the test room topography. Figures in parentheses indicate ± I SEM.

5 146 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / January 1991 Vol. 32 It is likely that the AR measured in darkness represented an aggregate response resulting from multiple, nonoptical stimuli. Thus the response may have reflected not only the level of tonic innervation, but also other extrinsic, nonoptical factors, eg, PIA, cognitive demand, mental imagery, and vestibular and auditory influences. 41 This conclusion has important implications with regard to previous reports of accommodative adaptation, 5 " 9 ie, the posttask increase in TA observed after a period of sustained fixation. It is not apparent which nonoptical accommodative stimulus has, in fact, been adapted. For instance, PIA may also exhibit such adaptation. Further work is required to verify whether the posttask shift in "TA" is actually mediated via a change in the level of tonic innervation or results from variation in the output of an alternative, nonoptical accommodative component. 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