1) The study evaluated the effects of high acceleration forces (9G) on visual performance and ocular responses in 14 men using a human centrifuge. 2) Transient visual acuity reduction and temporary corneal thickening were observed immediately after centrifugation. Prolonged increases in anterior chamber depth and pupillary dilation were also observed up to 30 minutes after. 3) Contrast sensitivity was found to decrease significantly at low and medium spatial frequencies and did not return to baseline levels within 30 minutes of centrifugation.

1. Ocular Responses and Visual Performance after
High-Acceleration Force Exposure
Ming-Ling Tsai,1,2,3,4 Chun-Cheng Liu,5,6 Yi-Chang Wu,5,6 Chih-Hung Wang,7
Pochuen Shieh,2 Da-Wen Lu,1,7 Jiann-Torng Chen,1,7 and Chi-Ting Horng2,6,8
PURPOSE. To evaluate ocular responses and visual performance are not clear, because there are no previous reports on this
after high-acceleration force exposure. topic. Further studies are needed. (Invest Ophthalmol Vis Sci.
METHODS. Fourteen men were enrolled in the study. A human 2009;50:4836 – 4839) DOI:10.1167/iovs.09-3500
centrifuge was used to induce nine times the acceleration force
in the head-to-toe (z-axis) direction ( 9 Gz force). Visual per-
formance was evaluated using the ETDRS (Early Treatment of
Diabetic Retinopathy Study) visual chart, and contrast sensitiv-
V ision plays a critical role in humans undergoing high-
acceleration movement. With increases in modern vehi-
cle performance, problems associated with acceleration are
ity (CS) was examined before and after centrifugation. Ocular becoming important.1,2 When an emergency occurs at high
responses were assessed with biomicroscopy and topographic speed, visual performance is important to preventing further
mapping after gravitational stress. catastrophe because the response time is relatively short.1,2
Investigators in prior studies have reported that positive
RESULTS. Transient visual acuity reduction (0.02 0.04 logMar acceleration can cause grayout, blackout, and loss of periph-
vs. 0.19 0.07 logMar VA; P 0.05) and temporary ocular eral vision.1,3,4 Many ocular reactions may also occur when
anterior segment reactions were observed immediately after the head is suddenly forced to stop or start moving or to turn
centrifugation. These reactions included changes in corneal rapidly. As the eye is forced to follow the movements of the
thickening (553.7 21.7 m vs. 591.2 20.6 m; P 0.05), head, this acceleration-deceleration–induced shearing force
increasing anterior chamber depth (ACD; 3.19 0.26 mm vs. may induce ocular structure changes and even injuries.5,6
4.53 0.34 mm; P 0.05), and pupillary enlargement (3.54 Because previous data are usually based on subjective de-
0.73 mm vs. 5.76 0.61 mm; P 0.05). The increase in ACD scriptions and are not quantitative, it is important to clarify
continued for 15 minutes after exposure to acceleration the influence of acceleration force on visual performance
(3.19 0.26 mm vs. 4.39 0.27 mm; P 0.05). Pupillary and ocular reactions. However, this issue has not been
dilation was noted both 15 (3.54 0.73 mm vs. 5.56 0.67 explored thoroughly because of the difficulty in designing
mm; P 0.05) and 30 (5.47 0.59 mm, P 0.05) minutes experiments.
after the gravitational stress. CS decreased significantly at low Recent advances in technology allow the human centrifuge
and medium spatial frequencies (1.5, 3, and 6 cyc/deg) and did to be used to elicit high-acceleration forces.4,7 The centrifuge
not return to the baseline level by 30 minutes. is designed to improve the trainee’s learning experience during
CONCLUSIONS. High-acceleration force may induce transient vi- high G-forces. The desired G-forces recommended for optimal
sual acuity reduction and temporary corneal thickening. Pro- training can be adjusted accurately by the instructor according
longed increase in ACD and pupillary dilation were also ob- to the trainee’s weight, which allows each subject to train
served. The decrease in CS persisted for 30 minutes after under the same G-force influence.2,8
centrifugation. The mechanisms underlying these observations A corneal topography system (Orbscan II; Bausch & Lomb,
Rochester, NY) can be used to evaluate objectively the ocular
reaction after high G-force exposure. The system is a reliable
ocular structure analyzer that uses a calibrated and scanning
From the Departments of 1Ophthalmology and 7Otolaryngology- slit beam to access ocular anterior segment-related structures
Head and Neck Surgery, Tri-Service General Hospital, Taipei, Taiwan; such as the cornea and to measure pupil size and anterior
the 2Department of Pharmacy, Tajen University, Pintung, Taiwan, Tai-
chamber depth (ACD) at the same time. It does not contact the
wan; 3Department of Ophthalmology, Taichung Veterans General Hos-
pital, Taichung, Taiwan; 5Department of Ophthalmology, Kan-Shan ocular surface and thus avoids touching the corneal surface
Armed Force Hospital, Kaohsiung, Taiwan; the 6Institute of Aviation and disturbing the detected reading.9,10
and Space Medicine and the 4Department of Ophthalmology, National The goal of this study was to investigate the influence of
Defense Medical Center, Taipei, Taiwan; and the 8Department of Oph- high-acceleration force on visual performance and ocular
thalmology, Kaohsiung Armed Force General Hospital, Kaohsiung, reactions. In this study, a human centrifuge was used to
Taiwan. elicit an acceleration force set at nine times G-force in the
Supported by grants from the 2007 Research and Development head-to-toe z-axis direction ( 9 Gz force). The influence of
Fund of Kaohsiung Military General Hospital, Grants TSGH-C97-69 and acceleration force on visual performance was evaluated by
TSGH-C98-69 from the Tri-Service General Hospital, the Chen-Han
measuring visual acuity and contrast sensitivity (CS). The
Foundation, and Grants 92-2314-B-016-065 and 93-2314-B-016-016 from
the National Science Council. changes in ocular reactions were assessed by biomicros-
Submitted for publication February 15, 2009; revised April 17, copy, physical examinations, and corneal topography (Orb-
2009; accepted July 23, 2009. scan II; Bausch & Lomb).
Disclosure: M.-L. Tsai, None; C.-C. Liu, None; Y.-C. Wu, None;
C.-H. Wang, None; P. Shieh, None; D.-W. Lu, None; J.-T. Chen,
None; C.-T. Horng, None MATERIALS AND METHODS
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked “advertise- Fifteen men (mean age, 21.1 years) were enrolled in the study.
ment” in accordance with 18 U.S.C. §1734 solely to indicate this fact. Informed consent was obtained from each subject before participa-
Corresponding author: Chi-Ting Horng, Department of Ophthal- tion. All experimental protocols were conducted in accordance
mology, Kaohsiung Armed Force General Hospital, No. 2, Jhongjheng with the Declaration of Helsinki. Ethical approval for this study was
1st Road, Lingya District, Kaohsiung, Taiwan; yalesu@yahoo.com.tw. obtained from the institution review board. Subjects with any his-
Investigative Ophthalmology & Visual Science, October 2009, Vol. 50, No. 10
4836 Copyright © Association for Research in Vision and Ophthalmology

2. IOVS, October 2009, Vol. 50, No. 10 Visual and Ocular Changes after High G-Force Exposure 4837
TABLE 1. The Cornea-Related Parameters before and after Gravitational Stress
Immediately After After
Before After 15 min 30 min
Sim K
Maximum 44.52 0.43 45.08 0.63 45.21 1.17 44.89 1.31
Minimum 43.91 0.67 44.57 0.81 44.32 0.61 44.21 0.87
Anterior BFS (D) 43.23 1.51 42.94 1.32 42.98 0.94 43.11 1.34
Posterior BFS (D) 52.37 2.03 52.02 2.21 52.11 0.97 52.29 1.47
CCT ( m) 553.7 21.7 591.2 20.6* 567.4 21.6 556.2 23.6
n 14 eyes.
* Statistically significant difference (P 0.05).
tory of ocular or systemic disease, such as hypertension, diabetes, itational stress. All data were collected and analyzed. Fundus
glaucoma, cataract, or uveitis, were excluded from the investiga- examination was performed 2 hours after centrifugation. Throughout
tion. the experiment, blood pressure (BP) and pulse rate were monitored by
All examinations were performed at the Air Force Health Exam- a digital BP monitor (HDM 704 cm; Omron, Tokyo, Japan). All results
ination and Physiological Training Center, Kan-Shan Armed Forces are expressed as the mean SD. A paired t-test was used to compare
Hospital, Taiwan. All subjects had remained at sea level for the the physiological parameters before and after centrifugation. P 0.05
previous month. Before centrifugation, each subject underwent a was accepted as significant.
complete ocular examination that included tests of visual acuity
(Early Treatment of Diabetic Retinopathy [ETDRS] logMAR chart)
and refraction (AR310; Nidek, Tokyo, Japan) and examination with
RESULTS
the biomicroscope. The letter stimuli of the ETDRS logMAR chart in One subject withdrew from the study because of nausea after
this study are printed on an illuminated cabinet (background lumi- the centrifugation. All data were collected from 14 eyes. Most
nance, 350 cd/ m2). The chart has five letters per row ranging in size of the ocular-related parameters, such as anterior BFS, poste-
from 1.0 to 0.30 logMAR. Visual acuity was tested at 4 m rior BFS, and Sim K, did not change significantly from before to
distance. The corneal topography system (Orbscan II; Bausch & after centrifugation. However, the CCT increased significantly
Lomb) was used to assess the various ocular responses including immediately after centrifugation compared with the value be-
Sim K (maximum K and minimum K values), anterior best-fit sphere fore centrifugation (553.7 21.7 vs. 591.2 20.6, P 0.05),
(BFS), posterior BSF, ACD, pupillary diameter (PD), and central but this was not maintained beyond 15 minutes after high
corneal thickness (CCT). Macular responses such as metamorphop- gravitational stress (Table 1).
sia were detected by using the Amsler grid test, and visual perfor- ACD increased considerably immediately after (3.19 0.26
mance was assessed further by testing CS with the CS was tested mm vs. 4.53 0.34 mm, P 0.05) and 15 minutes after
monocularly before and after 9-Gz exposure by a CS test chart (4.39 0.27, P 0.05) 9-Gz force stress. ACD returned to
(VCTS 6500; Vistech Consultants, Inc., Dayton, OH). The chart the pretest value by 30 minutes after exposure to acceleration
consists of 45 circular targets with gratings of different size and (3.24 0.29, P 0.05; Table 2).
contrast. The target size in the distance test is 7.5 cm. The gratings PD enlarged significantly immediately after (3.54 0.73
in the targets are vertical or 15° left or right from the vertical mm vs. 5.76 0.61 mm; P 0.05) and 15 minutes (5.56
direction. In the test chart, the contrast decreases horizontally from 0.67 mm; P 0.05) after centrifugation. PD remained enlarged
left to right, and gratings become smaller vertically from top to 30 minutes after exposure to acceleration (5.47 0.59 mm;
bottom (from 1.5 to 18 cyc/deg; from target A to target E). The P 0.05; Table 3).
subject began at the top horizontal line A and stated whether the In the tests of visual performance, transient visual acuity
gratings in targets 1 to 9 are vertical, left, or right. The last target decreased immediately after the gravitational stress (0.02
that the subject reported correctly was marked down on a result 0.04 vs. 0.19 0.07, P 0.05; Table 4). CS reduction was also
paper provided by the test. The same procedure was repeated in all observed at 30 minutes after exposure to acceleration, and
the other horizontal test lines from B to E. At measurement time, the significant depression of CS was found at low and medium
room luminance was approximately 200 cd/m2 and the test dis- frequencies. CS in the right eye decreased at 1.5 (P 0.05), 3.0
tance, 3 m. (P 0.05), and 6.0 cyc/deg (P 0.05; Fig. 1). Refraction
After these examinations, the subject entered the human centri- remained stable at 30 minutes after gravitational stress (Table
fuge and donned an aviation helmet with a microphone attached so 4). The Amsler grid examination revealed no particular finding
that he could communicate with the aerospace physician outside. A such as metamorphopsia in any subject.
TV monitor was mounted outside the human centrifuge to monitor Throughout the experiment, ocular posterior segment re-
the status of each subject. When the setting was complete, a vealed no specific observations. In addition, no particular oc-
gravitational stress of 9-Gz force was established with a human ular finding was noted such as hyphema, lens dislocation, or
centrifuge which also applied G-forces parallel to the subject’s retinal hemorrhage.
body. The technician outside the centrifuge accelerated the speed
until it reached 9 Gz and held this constant for 15 seconds. The
operating protocol to induced 9 Gz in this experiment was as TABLE 2. Anterior Chamber Depth before and after
follows. The centrifuge was accelerated to 2 G/s from 1 to 1.4 G for Gravitational Stress
15 seconds, then accelerated to 2 G/s from 1.4 to 4 G and held
Immediately After After
constant for 20 seconds, and then reduced to 1.4 G for 30 seconds. Before After 15 min 30 min
The centrifuge was then accelerated rapidly to 6 G/s and then to 9
G for 15 seconds before slowing to 2 G/s and finally to 1 G.4,11 ACD (mm) 3.19 0.26 4.53 0.34* 4.39 0.27* 3.24 0.29
Visual acuity was checked and the corneal topographer was ap-
plied again immediately, 15 minutes, and 30 minutes after the centrif- n 14 eyes.
ugation. Refraction and CS were measured 30 minutes after the grav- * Statistically significant difference (P 0.05).

3. 4838 Tsai et al. IOVS, October 2009, Vol. 50, No. 10
TABLE 3. Pupillary Diameter before and after Gravitational Stress
Immediately After After
Before After 15 min 30 min
PD (mm) 3.54 0.73 5.76 0.61* 5.56 0.67* 5.47 0.59*
n 14 eyes.
* Statistically significant difference (P 0.05).
DISCUSSION
In this study, we observed a 10% increase in CCT after 9-Gz
force exposure. We assume that the transient hydrostatic pres-
sure increase may explain this finding. The ocular anterior
chamber in the eye is full of aqueous humor.12,13 An exposure
to high gravity may increase hydrostatic pressure in the ante-
rior chamber and increase the tendency for fluid to flow across
the corneal endothelium into the stroma, which would in- FIGURE 1. Mean CS before and after 9-Gz force exposure. *Signifi-
crease the corneal thickness.14 The CCT returned to its initial cant differences at P 0.05.
value within 15 minutes, suggesting that the corneal endothe-
lium was not compromised by high gravitational exposure,
even at 9-Gz force. An interesting observation was that a 10% stress.12,13 When subjects are under high-gravity stress, the aque-
increase in CCT occurred without significant concomitant ous hydrostatic pressure in the anterior and posterior chambers
changes in the corneal curvature. Our data are consistent with may increase toward the gravity direction. Because the volume is
those of Rom et al.,15 who found no significant correlation greater in the anterior chamber than in the posterior chamber, the
between corneal thickness and curvature, even when the cor- aqueous humor pressure is also greater in the anterior chamber
neal thickness increases up to 16%. during high G-force exposure.12,13 The greater hydrostatic pres-
PD increased significantly after high G-force exposure in our sure in the anterior chamber may push the iris toward the poste-
study. Before the stress, PD was 3.54 0.73 mm, and the marked rior chamber and cause the ACD to increase. At the same time,
pupillary dilation persisted for 30 minutes after the stress. The size this iris diaphragm may rest on the lens surface and trap fluid in
of the pupil is regulated by the autonomic nervous system, which the anterior chamber. When the hydrostatic pressure is relieved,
is influenced by many environmental, physiological, and psycho- the trapped aqueous humor maintains the ACD, which may ex-
logical factors. Previous studies show that gravitational stress can plain the prolonged deepening of the anterior chamber. The
increase sympathetic tone to prevent body fluid shifting down- trapped aqueous humor drains with time, and the ACD returns to
ward with gravity.16 This transient sympathetic tone elevation baseline.12,13,19 We also observed that the refractive power did
may account for the early pupil dilation immediately after centrif- not reveal a significant finding at 30 minutes after 9-Gz expo-
ugation. However, an increase in sympathetic activity cannot sure. This observation suggests that axial length did not change at
clarify completely why the pupil enlargement persisted for 30 30 minutes after gravitational stress. However, further research is
minutes. Tran et al.17 reported that high G-force exposure causes necessary to confirm this finding.
a persistent reduction in parasympathetic activity. This prolonged In our study, visual acuity showed a transient reduction imme-
reduction in parasympathetic activity may also explain the persis- diately after centrifugation and returned to baseline at 15 minutes.
tent pupil enlargement after gravitational stress. Neurohormonal We assume that temporary corneal edema plays a role, because a
regulation is another possible explanation for this persistent pupil study has shown that corneal edema can disturb visual acuity.20
dilation. Gravitational stress increases the blood somatostatin con- However, the Amsler grid examination revealed no specific find-
centration. Yamaji et al.18 found that somatostatin may induce ings in our study, suggesting that the macular structure was not
mydriasis by attenuating cholinergic neurotransmitter release. compromised even after the high G-force exposure. For clarifica-
Therefore, neurohormonal regulation may also be involved in this tion of our supposition, further study may be helpful, such as
prolonged pupil enlargement. pinhole, potential acuity testing, and laser interferometry. Visual
In our experiment, the ACD increased immediately and 15 acuity is usually evaluated with the ETDRS visual chart, a basic
minutes after 9-Gz force exposure but returned to baseline method of evaluating visual performance by estimating the visual
within 30 minutes after gravitational stress. Pupil enlargement response to black-and-white contrast. However, performance on
may explain the increase in ACD at 15 minutes after gravitational the ETDRS chart represents one extreme of CS, and in reality,
stress, but it cannot explain why the ACD returned to baseline objects and their surroundings are of varying contrasts. The CS
when mydriasis persisted 30 minutes after centrifugation. We test was developed based on the measurement of visual resolution
believe that the increase in hydrostatic pressure caused by the for a wide range of contrasts. The CS test is useful for evaluating
9-Gz force explains this finding. The aqueous humor in the eye visual quality more precisely, by plotting the reciprocal of the
may shift toward the gravity direction under gravitational threshold contrast for sinusoidal gratings as a function of their
TABLE 4. Visual Acuity and Refraction before and after Gravitational Stress
Immediately After After
Before After 15 min 30 min
VA (logMAR) 0.02 0.04 0.19 0.07* 0.05 0.06 0.04 0.07
Refraction (D) 0.37 0.47 0.23 0.51
n 14 eyes.
* Statistically significant difference (P 0.05).

4. IOVS, October 2009, Vol. 50, No. 10 Visual and Ocular Changes after High G-Force Exposure 4839
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