FUSION TIME (SECONDS)

OCCLUDER SHUTTER

CYLINDRICAL SHUTTER
HOUSING

CROSSED ROTARY
PRISMS

MOTOR AND GEAR

ASSEMBLY

Fig.1

12.04
11.04
10.04
9.04
8.04
7.074
6.04
5.04
4.04
3.04
2.04

1.04

Young} Old |Young| Old jYoung| Old

Young] Old [Young] Old [Young] Old

| SQUARE |2 SQUARES/|3 SQUARES

| SQUARE |2 SQUARES|3 SQUARES

IMAGE DISPLACEMENT TO LEFT

IMAGE DISPLACEMENT TO RIGHT

Fig.2





TABLE IT

RATE OF FUSION (SQUARES/SEC)

SUBJECT AGE

1 2
RW 25 1.32 1.85
LR 26 1.85 2.98
JF 27 1.35 2.11
TP 27 1.09 1.64
DW 28 0.78 0.80
JA 29 1.43 2.00
RC 29 1.19 147
WM 29 1.54 2.13
AVERAGE 132 1.87
EA 46 0.69 0.62
OF 47 1.37 2.25
RM 47 1.45 211
NY 49 1.01 0.73
GI 50 0.34 0.21
FA 55 1.02 0.93
EW 56 0.68 0.97
EF 59 0.87 0.40
AVERAGE 0.93 1.03

image was displaced to the right than when it
was displaced to the left.

On the average, the fusion times at all image
displacements for the older group were longer
than those of the younger group for equal dis-
placements. Table I shows that there is overlap
of the individual values in the two groups,
however. The differences in fusion times be-
tween the two age groups became more pro-
nounced as the extent of image displacement
was increased (Figs. 2 and 4, Tables I and II).
All of the young group accomplished fusion of
images at least one time at all displacements.
In the older group, however, only three could
accomplish fusion when the displaced image
was three squares to the right (Fig. 3). One
of the older subjects ceased fusing when image

IMAGE DISPLACEMENT TO LEFT

IMAGE DISPLACEMENT TO RIGHT

3 1 2 3
2.17 0.87 0.77 0.61
2.86 1.32 1.87 1.68
2.29 1.14 145 0.23
178 0.60 0.75 0.50
0.38 0.43 0.28 0.25
2.56 0.92 0.65 0.48
1.35 0.49 0.43 0.32
2.21 1.60 2.09 1.89
1.95 0.92 1.04 0.75
0.29 0.09 No Fusion No Fusion
2.19 0.22 0.20 0.15
1.89 0.60 0.56 No Fusion.
0.35 0.17 0.12 No Fusion
0.17 0.37 0.20 0.26
0.48 0.18 0.53 No Fusion
1.12 0.67 1.43 1.55
0.19 0.23 0.14 No Fusion
0.84 0.32 0.45 0.65

displacement was two squares to the right. A
comparison of mean squares revealed the larg-
est source of variation to be between the two
age groups (759.40). The second largest source
of variance was between subjects within the
two groups (244.70). The data further reveal
that experimental design can be improved in
future experiments by reducing both the num-
ber of runs, the number of readings, and the
number of repeat visits on different days.

DISCUSSION

One of the primary characteristics of the
aging process is a loss of skeletal musculature.
Where these muscles are normally concerned
with movement of bony levers, some of this
loss of muscle might be related to the “rigors

—-5-—



TABLE I

EYE FUSION TIMES (SEC)
HETERO. DOMI- ACUITY (WITH IMAGE DISPLACEMENT IMAGE DISPLACEMENT
SUBJECT AGE PHORIA NANCE CORRECTION ) TO LEFT TO RIGHT
1 2 3 1 2 3
14D R 20/20
RW 22 EXO R UL 20/20 075 108 138 115 260 4.89
R 20/20
LR 26 0 R  L 20/20 054 067 105 076 107 179
R 20/20
JF 27 0 R  L 20/20 074 095 131 088 138 12.98
R 20/20
TP 27 0 R L 20/20 092 122 169 168 265 597
1AD R 20/20
DW ESO R L 20/20 128 249 798 235 712 1178
1AD R 20/20
JA EXO L  L 20/20 070 100 LIT 109 306 625
R 20/20
RC 0 R L 20/20 084 136 223 203 467 945
R 20/20
WM 29 0 R  L 20/20 065 094 136 063 096 150
AVERAGE 080 191 227 132 294 684
R 20/20 No No
EA 48 0 LL 20/20 144 325 1027 1055 Fusion Fusion
14D R 20/25
OF 47 EXO LL 20/20 075 089 137 448 990 19.45
1aD R 20/20 No
RM 47 EXO R  L 20/2 0.69 095 159 168 355 Fusion
R 20/30 No
NY 49 0 RL 20/20 099 273 869 5.72 1636 Fusion
R 20/20
GI 50 0 RL 20/20 294 975 1740 267 1017 1175
R 20/20 No
FA 55 0 R  L 20/20 098 216 630 549 380 Fusion
3AD R 20/20
EW 56 EXO R  L 20/20 148 206 268 150 140 194
1aD R 20/20 No
EF 59 =EXO R  L 20/9 115 5.05 1603 440 1417 Fusion
AVERAGE 130 335 804 456 848 1175


of life.” The extraocular muscles however, are
largely protected from trauma and do about
the same amount of work day in and day out.
In short, these muscles might make a contribu-
tion toward answering the question of whether
the loss of muscle with age is due to wear or
is a symptom of the aging process itself. This
question has been considered from a histo-
chemical point of view by Dogliotti (7), and
Kny (9) on skeletal muscles, Lowry, et al (10)
on heart and skeletal muscle, and Jayne (8)
on heart. These workers showed the presence
of “aging pigments” in muscles taken from se-
nescent individuals. Bucciante and Luria (5),
more directly pertinent to the subject of this
report, showed that the average diameter of
extraocular muscle fibers increased from 164
in men less than 40 years of age to 23 in men
80 or more years of age. The suggestion was
that the increase in muscle fiber diameter rep-
resented hypertrophy in compensation for de-
generation or atrophy of other fibers. The
degeneration appeared to consist of myofibril-
lar intertwining and eventual distintegration.

Cowdry (6) states that, while the amplitude
of convergence decreases and the extent of
exophoria increases with age, there is no evi-
dence for a weakening of extraocular muscles.
The evidence put forward in this report sug-
gests that such a weakening of the extraocular
neuromusculature does in fact occur with age.
This weakening is manifested in a lessened
ability to fuse when the image is displaced and
by the reduced speed of fusion in the older
group.

Both groups exhibited a decreased fusional
ability, as reflected by fusion time and fusion
rate, when the image was displaced
to the subject’s right, though the effect was
less pronounced in the young group. It must
be recalled that the situation in these experi-
ments does not duplicate heterophoria; the
image of the target light was only prismatically
displaced on the retina. The visual axis was
not affected. This means that an individual
would actually have to diverge his visual axes
in order to fuse an image displaced onto a non-
corresponding part of his nasal retina. A neuro-
muscular mechanism for active divergence of
the visual axes beyond the rest position is not
recognized to exist. The demonstration in
these experiments, however, of fusion of images

displaced to the subjects’ right (nasal retina)
may represent a slight ability to diverge the
axes, at least enough to bring the images onto
Panum’s Areas of the two eyes. The deteriora-
tion of the ability to fuse right-displaced im-
ages in the older group may constitute a test
for neuromuscular aging.

SUMMARY

1. Binocular fusion time increases with the
extent of image displacement. The rate of
fusion of any individual, however, is not
markedly affected by the extent of image
displacement.

2. Young men (25-30 years of age) accom-
plish fusion faster than older men (45-60
years of age).

3. Fusion time is greater (velocity less) when
divergence of the visual axes is necessary
to accomplish fusion than when converg-
ence is called for. The 25-30 year old
group could execute such divergent fu-
sional movements faster than the 45-60
year old group. Further, five of the eight
members of the older group failed to fuse
when the apparent image was laterally dis-
placed to the subjects’ right whereas none
of the younger group failed to fuse under
these circumstances.

REFERENCES

1. Bennett, A. G., A. H. Wells, R, T. Binder and G. A.
Brecher. Am. Jour. Optom. and Arch. Am. Acad.
Optom, Monograph 221, 1958.

2, Brecher, G. A. Am. Jour, Ophthal. 38: Pt. I: 134-
141, 1054.

3. Brecher, G. A., A. P. Hartman, and D. D. Leonard.
Am. Jour. Ophthal. 39: Pt. II: 44-52, 1955.

4, Brecher, G. A, E. W. Pumell, and W. J. Hoover.
Fourth ‘Invitational Conference for Visual Research
Specialists, Lamp Division, General Electric Co., Nela
Park, Cleveland, Ohio, April 23-25, 1958.

5. Bucciante, Luigi and $. Luria. Arch. Ital. di anat. ¢
di embriol. 33: 110-187, 1934.

Cowdry, E. V. Problems of Agi . 244-245, Wil

liams and Wilkins, Baltimore, te a, 1952,

7. Dogliotti, Giulio C. Zeitschr. f, Anat. u. Endwickl. —
Geschichte 96: 680-722, 1931.
8, Jayne, Edgar P. Jour. Geront. 5: 319-325, 1950.

9. Kny, Walter. Virchow’s Arch. f. Path, Anat. u, Physiol.
299: 468-478, 1937.

10. Lowry, O. H., A. B, Hastings, T. Z. Hull, and A, N.
Brown. Jour. Biol. Chem. 143: 271-280, 1942.

—-T— 67-3063



AM 66-35

PILOT VISION CONSIDERATIONS:

The Effect of Age on Binocular Fusion Time

Carlton E. Melton, Jr.

Marlene Wicks
Approved by Released by
Mp ZL By I ) VA Py? »)
J. Rosert Drie, M.D. P. V. Sureen, M.D.
Curer, Civ Arromepicar InstrruTe Froeran Arr Surcron
October 1966

FEDERAL AVIATION AGENCY
Office of Aviation Medicine



VELOCITY OF FUSION
(SQUARES / SEC.)

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Fig.3
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1.0
° Young| Old |Young} Old |Young] Old Young] Old [Young] Old | Young] Old
| SQUARE |2 SQUARES|3 SQUARES | SQUARE |2 SQUARES|3 SQUARES
IMAGE DISPLACEMENT TO LEFT IMAGE DISPLACEMENT TO RIGHT
Fig.4



The work cited above shows that the time
necessary for a person to accomplish binocular
fusion is affected by fatigue, drugs, and alco-
hol. The purpose of the investigation reported
here is to determine to what extent the binocu-
lar fusion reflex is affected by age.

METHODS AND RESULTS

The method employed was, in principle, the
same as described by Brecher (2). The meas-
urement device consisted essentially of a rotat-
ing cylindrical shutter slotted on two sides.
The cylinder was driven by an electric motor
and geared to rotate at a rate of 2.5 rps, thus
yielding an exposure rate of 5 per sec. The
measured duration of each exposure was 0.09
sec. Crossed Risley rotary prisms were mounted
in a metal tube which was fitted to the front of
the shutter housing so that the target image
could be positioned as desired. An iris dia-
phragm shutter was positioned behind the cy-
lindrical shutter to act as an occluder. The
diaphragm shutter was opened by a wire con-
nected to a push-button and closed by a spring.
A switch controlling a 0.01 second timer was
wired to the same push-button so that the timer
was started at the instant the occluder shutter
was opened (Fig. 1).

The fixation target was a white light posi-
tioned in a hole at the center of a black screen
30 x 40 cm in size. The board was marked off
with white plastic tape into 5 cm squares. All
measurements were made at an observation
distance of 6 meters.

For determinations of their binocular fusion
times, the subjects were seated at a table to
which was clamped the headrest and shutter
assembly. The subject was made comfortable
with his head positioned in the headrest and
with his right eye centered in the open aper-
ture of the occluder shutter. A rubber bite
piece was used to assure reproducibility of
head position. The occluder shutter was then
closed and the cylindrical shutter set into ro-
tation. The subject was allowed to fixate with
his left eye while his right eye was occluded
for 30 seconds. He was then requested to open
the occluder shutter by pressing the switch but-
ton momentarily and to report the position of
the “second light” in terms of squares to the
right or left and above or below the center line.
The rotary prisms were turned appropriately to

displace the “second light” to the desired ex-
tent and direction. The extent of displacement
was tested by repeated trials until variability
was minimal.

Trials were made at one, two, and three
squares of lateral displacement in each direc-
tion. Ten determinations were made at each
setting. The endpoint of each determination
was the subject’s appreciation of singleness of
the light. The eccentric light appeared to
“hop” with each exposure toward the center
steady light. When the “two lights” became
“one light”, the subject released the switch but-
ton, thus stopping the timer and simultaneously
closing the occluding shutter.

Subjects selected to have good vision in both
eyes were recruited by the University of Okla-
homa Research Institute. They were paid sub-
jects with only casual interest in the experi-
ment. Eye dominance, visual acuity with cor-
rection in both eyes (Snellen chart), and heter-
ophoria (Maddox rod) were determined. The
instructions given to each subject were stand-
ardized and kept at the minimum consistent
with their understanding of their duty. A learn-
ing period was necessary, however, in order to
be certain that the subjects were seeing the
phenomenon that was being measured. When
they could describe with alacrity what had
transpired during a trial, readily detect changes
in prism settings, and when the times necessary
for fusion became consistent within limits, the
subjects were considered to be ready for ex-
perimentation.

Subjects were selected to fall into two age
groups, those between the ages of 25 and 30
and those between 45 and 60. Eight subjects
were studied in each group. Table I shows the
distribution of the subjects with regard to age,
eye dominance, visual acuity and heterophoria.

Figure 2 shows that the average fusion time
increases with the extent of image displacement
in both age groups. Table I shows the indivi-
dual averages of fusion times and affirms. that
in all cases fusion time increases with increas-
ing image displacement. The rate of fusion,
expressed as squares per second, is fairly con-
stant, however (Table II and Fig. 4). Fusion
rate is uniformly slower in the older group than
it is in the younger group. Figure 4 also shows
that fusion rate was consistently less when the

-2-


PILOT VISION CONSIDERATIONS:
THE EFFECT OF AGE ON

BINOCULAR FUSION TIME

Carlton FE, Melton, Jr.
and
Marlene Wicks

When a person looks at one object that is
nearer to him than six meters, other objects at
other distances are out of focus and appear
double. When the gaze is directed to objects
that are more than six meters distant, objects
that are nearer than six meters appear double
and are out of focus. Singularity of vision in
people with two functional eyes occurs because
(1) the visual axes of their eyes can be aligned
so that similar images fall on corresponding
parts of their retinas or (2) vision is suppressed
in one eye when the visual axes cannot be prop-
erly aligned. In the latter case images fall on
non - corresponding parts of the retinas and
double vision results unless suppression occurs.

Binocular single vision is attained at a cost
of the time necessary for proper alignment of
the visual axes. The time required for such
alignment is a component of total reaction time
and occupies a certain primacy when an action
is to be initiated. That is, before a person can
initiate an appropriate reaction to a stimulus,
he must sense and understand that stimulus. A
good example is a situation in which a pilot
shifts his gaze from the empty sky to his in-
strument panel. Before any of the instruments
are meaningful to him he must focus and con-
verge his eyes on them.

Clinical determinations of “ocular motility”
in reality are determinations of the extent and
direction of visual axis deviation and the ability
to accomplish fusion. The cover test gives the
examiner some appreciation of rate of fusion
but no objective measure of it.

Brecher (2) has described a method for the
quantitative determination of the time neces-
sary for an individual to accomplish fusion.
The method involved the interrupted presen-

tation of a light to one eye and the steady pre-
sentation of that light to the other eye. When
heterophoria was present, diplopia occurred;
reflex fusion was slowed depending upon the
rate and duration of presentation of the light
to the covered eye. The slowed fusional proc-
ess was easily measured with an electric timer.
It was also possible to produce standard dis-
placement of the image of the light with
crossed Risley rotary prisms. Fatigue had little
effect on binocular fusion time until virtual ex-
haustion occurred, at which time fusion was
broken up and diplopia resulted (4). Caffeine
and dexadrine afforded subjective relief of fa-
tigue symptoms but did not improve fusion
time. These workers also found that barbitu-
rates interfered with binocular fusion by slow-
ing the fusion reflex time and not by increasing
heterophoria (4). At high blood alcohol levels
the eyes tended to assume a rest position with
the visual axes crossing at about 82 cm. These
alcohol - intoxicated subjects were esophoric
when tested with an object six meters away
and exophoric when tested with an object at
33 cm (3).

Bennett, Wells, Binder, and Brecher (1)
compared the subjective method of interrupted
stimulus presentation with an objective motion
picture method. These workers found good
agreement between the two methods and fur-
ther found that fusion time was not necessarily
related to the extent of heterophoria. Some
subjects with large phorias fused more rapidly
than others with small phorias. They also
found that subjective fusion could occur at dif-
ferent vergences, as objectively measured, be-
cause of the extent of Panum’s Area.

-1-



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