Investigative Ophthalmology & Visual Science, Vol. 32, No. 1, January 1991
Copyright © Association for Research in Vision and Ophthalmology
Accommodative Hysteresis
Fundomentol Asymmetry in Decoy Rote After Neor ond For Focusing
Sheldon M. Ebenholrz
The resting level of accommodation, or dark focus (DF), was compared before and after subjects
maintained 8-min of clear focus on targets either at the far point (FP) of accommodation or at an
equivalent dioptric distance on the near side of the DF. Although both conditions showed significant
shifts in tonic accommodation in the direction of their respective fixation targets, only after near
fixation was the shift maintained in darkness for 24-min postfixation. After FP viewing the aftereffect
decayed in darkness with a significant linear trend (P < 0.05). Thus, maintaining focus on a near target
is more likely to induce an enduring adaptive shift in tonic accommodation than is far-target viewing,
for reasons associated with the systems controlling far and near accommodation, respectively. Additional analyses provided further evidence that the degree of separation of the target from the DF is a
highly significant factor controlling individual differences in initial adaptation levels. Invest Ophthalmol Vis Sci 32:148-153, 1991
For many individuals the resting level of accommodation or dark focus (DF) is not fixed but rather
exhibits an adaptive nature, moving inward after sustained focusing of a near target at, eg, the near point
(NP) of accommodation and outward after viewing a
far target at, eg, the far point (FP).1"4 These hysteresis-like effects that follow far- and near-target focusing, respectively, however, do not behave symmetrically. For exposure periods of about 8 min, shifts in
tonic levels in the direction of far targets tend to dissipate rapidly with time in the dark, yielding estimated time constants of from 4.6-14.4 min.'>5 On the
other hand, aftereffects of near-target viewing have
tended to yield slow decay rates, estimated in hr,1 or
actually increasing tonic levels in darkness.56 In these
studies, far targets typically have been placed at the
subject's FP, usually within 2.0 D of the DF. In contrast, the near target typically has been stationed at
the NP of accommodation, frequently at a dioptric
distance from the DF greater than 4.0 D.
Since there is evidence that dioptric distance be-
tween target and DF influences the magnitude of the
after effect,5'7 it is conceivable that decay rates may
likewise be influenced by the same variable. On the
other hand, sheer differences in the autonomic components of the two systems controlling near and far
accommodation, respectively,8 also may account for
differential decay rates. To evaluate these two possibilities, comparisons were made between decay rates
for near and far hysteresis effects, after equating for
dioptric distance. Furthermore, since there are reliable individual differences in susceptibility to hysteresis effects2'7 data were sought for both far and near
after effects from the same individuals.
Materials and Methods
Subjects and Design
Sixteen subjects with Snellen acuities ranging from
6/3.9-6/7.5 took part as paid volunteers who provided informed consent after an explanation was
provided. None of the subjects, 18-22 yr of age, had
any history of visual problems, and none wore correcting lenses.
All subjects took part in four experimental sessions
plus a training session on separate days. During the
former, tonic accommodation was measured before
and after subjects maintained clear focus for 8 min on
a 10.9-cpd horizontal square-wave grating. The target
patternfilleda region 2.3° side"1 and was either at the
FP of accommodation or at an equivalent dioptric
distance on the near side of the resting level, or DF.
The latter was referred to as the near-fixation (NF)
condition. Two sessions were devoted to NF and two
From the Schnurmacher Institute for Vision Research and Department of Vision Sciences, SUNY/College of Optometry, New
York, New York.
Supported in part by NIH Research Grants EY03421 and
EY06699 from the National Eye Institute.
Presented in part at the ARVO meeting, April 29-May 4, Sarasota, FL, 1989.
Submitted for publication: January 3, 1990; accepted July 31,
1990.
Reprint requests: Sheldon M. Ebenholtz, Schnurmacher Institute for Vision Research and Department of Vision Sciences,
SUNY/College of Optometry, 100 East 24 Street, New York, NY
10010.
1./18
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DIFFERENTIAL DECAY AFTER FOCUSING / Ebenholrz
No. 1
to FP fixation in alternating order for one half of the
subjects and in blocked order for the remainder. Target luminance was set at 75 cd m~2, and measurement of tonic levels was made with a laser optometer
to which subjects were exposed initially during the
training session. At that session the subjects also were
interviewed about visual problems, and acuity measurements were made.
Apparatus and Procedure
The resting, or tonic, level of accommodation was
measured with a two-channel optical bench equipped
with a laser optometer, target, and light source, based
on the design of Hennessy et al.9 This device, as previously described3 was modified from the original design to include Badal optics for the target channel and
an electronic stage driven by a precision stepping
motor. The latter permitted steps in dioptric levels of
0.16 D.
The resting level of accommodation was measured
by a procedure in which the plane of stationarity10
was bracketed by noting the changes in the direction
of apparent movement of the laser speckle pattern.
The NP and FP were determined by stepping the
target behind its Badal lens to the point of blur and
then reversing until sharp focus again was restored
and maintained for 5 sec.
For all subjects the order of conditions was such
that initially two DF measures were taken; the average of these constituted the baseline resting level
against which subsequent changes in resting level
were evaluated. An FP measure was then taken, and
depending on the type of session, the subjects maintained focus for 8 min at a target placed either at the
FP or, in NF, at an equivalent dioptric distance on
the near side of the DF. The calculation for this target
setting, ie, DF - (FP - DF), was based on the FP
measure taken immediately preceding the NF fixation period; because NF and FP sessions were scheduled on different days, the setting actually used during the FP session was taken on a separate day. Therefore, the two observation conditions differed slightly
in the absolute degree to which they departed from
the DF. Immediately after the focusing period and at
8-min intervals thereafter, three more DF tests were
made to measure the decay of hysteresis in darkness
through 24 min of postfixation. An NP measure was
then taken to conclude the session.
Results
Fourteen of 16 subjects completed all four experimental sessions. One subject was eliminated because
of inability to maintain focus in the NF condition,
149
and the other excluded subject had a DF that coincided with the NP, thereby precluding participation
in NF. Among the 14 subjects were five who showed
no NF hysteresis effect, with four of the five yielding
FP hysteresis effects on both FP tests; the fifth subject
showed an FP effect on one of two occasions. Only
two subjects did not show any evidence of FP hysteresis effects, but both had NF effects on the two NF
sessions. Four subjects showed hysteresis effects on all
of the NF and FP sessions. Of the remaining three
subjects, one showed an effect on only one of the two
sessions of each type, and two subjects demonstrated
after effects on the two NF sessions but on only one of
the FP sessions. Thus individual differences in susceptibility to NF and FP effects were clearly revealed,
as previously shown.7
Because not all subjects had hysteresis effects under
all conditions, two partially overlapping sets of data
were analyzed. First, a repeated-measures analysis
was done on the data of seven subjects with hysteresis
effects on at least one of the two NF and FP sessions.
Thus each subject contributed to one NF and FP
measure. Where aftereffects were produced during
both sessions of the same type, only the first was
chosen. In a second analysis, data were examined
only from subjects with hysteresis effects during both
sessions of a given type, and the scores were averaged
over sessions. Twelve different subjects contributed
data of this sort, eight under NF and eight under FP.
Since only four subjects contributed to both conditions these data are referred to as "mixed groups."
Data preliminary to those of the main analyses are
shown in Table 1.
For the repeated-measures condition the absolute
distance from target to DF was 0.39 D less under NF
than under FP, but this was not statistically significant (t(6), 1.38, P > 0.05). Over a 1-D difference
occurred in the mixed-group data, also in the same
direction. Since a greater target (T)-DF separation
might favor the FP condition by producing a greater
aftereffect and perhaps less decay, such differences
yield a conservative test of the hypothesis of a greater
decay rate associated with FP than NF conditions.
There were two instances of significant differences
between NF and FP conditions in the repeated-measures group. These occurred in the measures of the
fixation-target distance as expected and with the NP
data. Since this measurement occurred after NF and
FP, the obtained differences were as anticipated from
previous research (Ebenholtz and Zander, 1987). Finally, it may be noted that the correlations between
NF and FP conditions for the various measures as
indicated in Table 1 all were significant at P < 0.05.
Thus the data of the repeated measurements group
were highly reliable.
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150
Vol. 32
INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / January 1991
Table 1. Mean and standard error (am) of prefixation dark focus (DF), fixation-target distance (FTD),
target-dark focus separation (T-DF), near point (NP) and far point (FP) of accommodation,
under near fixation (NF), and FP fixation conditions (units in diopters)
Cond
Repeated measures
(n = 7)
Mixed groups
(n = 8)
NF
FP
r
NF
FP
NP
r-4.98
* L-4.37
0.930
-4.90
-4.86
am
FP
0.45
0.54
0.20
0.25
0.763
0.00
0.57
0.43
0.38
0.27
0.27
0.26
0.20
PreDF
am
am
T-DF
-1.59
-1.94
0.851
-1.63
-2.10
0.33
r-3.39
0.42 * L 0.25
0.59
0.27
0.27
0.32
0.57
0.20
-1.80
2.19
-0.881
-1.64
2.67
FTD
-3.27
0.57
0.32
0.52
0.35
0.34
' Significantly different at P < 0.05.
Shifts in tonic level as a result of maintaining clear
focus for 8 min are shown in Figure 1, as a function of
the interval between the end of the fixation period
and the DF test. The best-fit linear functions also are
represented. Analysis of variance showed a significant
main effect for conditions (F(l,6), 23.31; P < 0.01),
but no significant effect of test interval or of the interaction of condition by interval. As shown in Figure
1, the initial aftereffects of both conditions were significant. Three of seven subjects showed decay of the
aftereffect under NF; all subjects exhibited decay
under FP. On average, Condition FP exhibited decay
of the aftereffect somewhat past its baseline value,
while for NF, the aftereffect remained relatively constant over time. These trends were supported statistically by a test of orthogonal polynomials which for
FP showed significant linear components (F(l,6),
22.51; P < .05), but no significant quadratic or cubic
components. None of these components were significant under NF. If similar underlying processes characterized both conditions, one would expect both
functions either to decay, ie, converge to baseline, to
grow and hence diverge from baseline, or to remain
parallel with it. On the contrary, the data indicate
1.0
that after effects resulting from sustained FP focusing
decay completely in darkness, while after NF conditions, the aftereffects tend to be sustained.
Results of the mixed-group analysis, where subjects
were selected who demonstrated hysteresis effects on
both sessions of the same type, are shown in Figure 2.
Trend analysis by polynomial coefficients for linear,
quadratic, and cubic components showed only significant linear components (F(l,7), 7.72; P < 0.05) for
FP and no significant trends for NF. Consistent with
these results were the changes in DF measured immediately after the fixation period relative to those of the
final test at 24 min. Seven of the eight subjects under
FP showed a decline whereas only three of eight declined under NF. All analyses are thus consistent with
the conclusion that FP conditions produced greater
decay rates than NF, even after taking account the
potential differences in the T-DF separation.
A glimpse of the large individual differences typical
of accommodative hysteresis effects7 is provided by
the trends of individual subjects in the repeated-measurements analysis. These are shown in Figure 3, FP,
and for the same seven subjects under NF in Figure 4.
n=7
Q
O
c
a>
c
D
C
<D
C
-0.5-
O
-1.0
o
? cond NF I
|
0
8
16 24
Post—fixation interval (min)
Fig. 1. Repeated measurements: mean change in dark focus (DF)
as a function of time in darkness since termination of sustained
focusing and best fit linear function. The same seven subjects contributed a single datum each to both far point (FP) and near fixation (NF) conditions. Vertical bars represent,! SE.
o
0
8
16 24
Post—fixation interval (min)
Fig. 2. Mixed groups: mean change in dark focus (DF) as a
function of time in darkness since termination of sustained focusing and best fit linear functions. There were eight subjects in each
condition, four of whom were in common. Each subject evidenced
hysteresis effects on two sessions within a given condition. Vertical
bars represent 1 SE.
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151
DIFFERENTIAL DECAY AFTER FOCUSING / Ebenholrz
No. 1
2.0
FP, n=7
1.5-1.0Q
L_
Q
c
<D
c
o
SI
o
0.50.0-
0)
CD
C
D
-C
-0.5-
O
-1.0-1.5-2.0
-2.0
0
8
16 24
Post-fixation interval (min)
0
8
16 24
Post—fixation interval (min)
Fig. 3. Individual functions showing the change in DF as a function of time in darkness after FP focusing for repeated measurements group.
There was considerable variation among subjects
even though they were selected for having exhibited
hysteresis effects. The subject represented by filled
triangles was especially noteworthy. The large FP aftereffect and the rapidly lost NF effect both may reflect an extraordinarily large sympathetic system
component6-8 after both near and far fixation.
Previous studies showed a modulating influence of
the dioptric distance between T and DF on variation
in hysteresis magnitude.5'7 Accordingly, an analysis
of this factor was made, taking advantage of the fortuitous variation among subjects in the relationship
between DF and FP, and hence in the separation
between DF and NF, as well. Figures 5 and 6, representing the repeated-measurements and mixedgroups analyses, respectively, show a strong linear relationship between the change in resting level, after
focusing on NF and FP, and the dioptric separation
between T and DF. Approximately 90% of the intersubject variance in magnitude of tonic shift is accounted for by variance in the separations between T
and DF.
Discussion
Both individual decay functions and averaged
group data support the proposition that shifts in tonic
level after far-target viewing decay more rapidly than
Fig. 4. Individual functions showing the change in DF as a function of time in darkness after near-target focusing for repeated
measurements groups.
n=7
r=0. 94
NF:o
1.5- "
Q
1.0-
FP:»
LJ_
Q
0.50.0-
o
/X)
-0.5-
Oj/
-1.0•
-1.5-
/
o
-2.0- — I — h - -\
1 \
\ 1I
\
\—
-4-3-2-1 0 1 2 3 4 5 6
Target - DF Distance (D)
Fig. 5. Repeated measurements group: bivariate plot of adaptive
shift in tonic accommodation and dioptric separation between target and DF for NF and FP conditions. Solid line represents the best
fit linear function. Separate linear correlations for NF and FP,
respectively, were 0.806 and 0.922.
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INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / January 1991
152
1.5- "
1.0-
n=8
r=0. 95
NF:o
#
/
/
0.50.0-
/
•
o
-0.5-
/°
-1.0-
o
-1.5-2.0- — \ — h - -\
1 1 \
-4-3-2-1 0 1 2
1
1
\—
3 4 5 6
Target - DF Distance (D)
Fig. 6. Mixed groups: bivariate plot of adaptive shifts in tonic
accommodation and dioptric separation between target and DF for
NF and FP conditions. Solid line represents the best fit linear function. Separate linear correlations for NF and FP, respectively, were
0.649 and 0.902.
do the corresponding aftereffects of near-target focusing. This occurred even though the respective targets were at equivalent dioptric distances from the
accommodative resting level. On the likely assumption that lens power, except possibly at its extremes, is
linearly related to the magnitude of ciliary muscle
innervation, it follows that the results could not be
attributed to differential focusing effort or ciliary
muscle innervation level. On the same grounds the
linear relationship (Figs. 5,6) between shift in tonic
level and dioptric distance between T and DF may be
expected to be isomorphic with innervation level.
The existence of differential decay rates suggest that,
for subjects capable of adapting, there is a greater
likelihood that near-target focusing will trigger an enduring adaptive process than will far-target viewing.6
The latter, on the other hand, may be expected to
produce aftereffects that decay at a rate reflecting the
initial level of stimulation, which in turn reflects the
residual level of sympathetic neurotransmitter at ciliary neuromuscular junctions. On this account we
may expect to find initial postfixation levels of tonic
adaptation that correlate highly with tonic levels present at the end of the decay period. After near-target
viewing, however, it seems likely that the development of an adaptive process would serve to decorre-
Vol. 32
late the two events. In support of this conjecture, data
of the repeated-measurements and mixed-group sets
show significant correlations between initial and final
DF levels of 0.776 and 0.789 (P < 0.05), respectively,
for FP, but insignificant values of-0.570 and 0.294,
respectively, for NF.
The presence of an enduring adaptive shift after
lengthy far-target focusing2 provides an instance of an
individual difference that represents a limitation to
the generality of these results. Tan et al.2 found that
after 1 hr of reading a book placed at 0 D, four subjects exhibited far-target tonic shifts, two of whom
evidenced complete decay of adaptation to prefixation tonic levels within 6 hr in darkness. The remaining two subjects, however, showed either an adaptive
shift that remained stationary (subject 2) or actually
increased over the 6 hr (subject 1) in the direction of
the 0 D reading material. Thus for certain individuals
an enduring adaptive process may not be unique to
NF conditions but may require much greater sustained focusing time on far targets than near ones.
Our results support the conclusion that overall
there is a fundamental asymmetry in the decay characteristics of the adaptive response to sustained focusing at near and far targets, respectively. Furthermore, to the extent to which near targets tend to be
further, dioptrically, from the resting level than are
far targets, this asymmetry may be yet further enhanced. Its significance may lie in the potential relationship between near-target accommodative hysteresis and the development of work-induced and/or
late-onset myopia1 and associated symptoms." In
light of evidence for concurrent sympathetic and
parasympathetic system activity associated with near
vision tasks,12 Gilmartin et al.13 speculated that the
sympathetic nervous system may thereby serve to attenuate near-task accommodative hysteresis. Furthermore, McBrien et al.14 provided support for the
proposition that variation in sympathetic system capability may distinguish between early- and lateonset myopia, the latter, but not the former, having
exhibited. . ."significant myopic shifts in tonic accommodation after near viewing" (p. 467). In addition reduced sympathetic facility either in association
with near tasks or as a result of far-task viewing that
might follow near work615 would serve to retard the
decay, and hence to sustain the development, of
near-task accommodative hysteresis. It follows that
such a propensity may, when considered together
with the fundamental asymmetry in decay rate, increase the risk of myopia development while conversely, enhanced sympathetic facility would be protective. The fundamental asymmetry in decay rate
also explains the rarity of a paresis of relaxed accommodation induced by sustained viewing of far targets,
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DIFFERENTIAL DECAY AFTER FOCUSING / Ebenholrz
No. 1
as against the relatively common occurrence of
spasm of accommodation after near-target viewing.
Key words: tonic accommodation, adaptation, accommodative hysteresis
Acknowledgments
The author thanks Ms. P. A. L. Zander for her aid in
directing the subjects at the laboratories of the Psychology
Department, University of Wisconsin, Madison.
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