Postural Stability Changes in the Elderly with Cataract Simulation and Refractive Blur

Abstract

purpose. To determine the influence of cataractous and refractive blur on postural stability and limb-load asymmetry (LLA) and to establish how postural stability changes with the spatial frequency and contrast of the visual stimulus.

methods. Thirteen elderly subjects (mean age, 70.76 ± 4.14 [SD] years) with no history of falls and normal vision were recruited. Postural stability was determined as the root mean square [RMS] of the center of pressure (COP) signal in the anterior–posterior (A-P) and medial–lateral directions and LLA was determined as the ratio of the average body weight placed on the more-loaded limb to the less-loaded limb, recorded during a 30-second period. Data were collected under normal standing conditions and with somatosensory system input disrupted. Measurements were repeated with four visual targets with high (8 cyc/deg) or low (2 cyc/deg) spatial frequency and high (Weber contrast, ∼95%) or low (Weber contrast, ∼25%) contrast. Postural stability was measured under conditions of binocular refractive blur of 0, 1, 2, 4, and 8 D and with cataract simulation. The data were analyzed in a population-averaged linear model.

results. The cataract simulation caused significant increases in postural instability equivalent to that caused by 8-D blur conditions, and its effect was greater when the input from the somatosensory system was disrupted. High spatial frequency targets increased postural instability. Refractive blur, cataract simulation, or eye closure had no effect on LLA.

conclusions. Findings indicate that cataractous and refractive blur increase postural instability, and show why the elderly, many of whom have poor vision along with musculoskeletal and central nervous system degeneration, are at greater risk of falling. Findings also highlight that changes in contrast sensitivity rather than resolution changes are responsible for increasing postural instability. Providing low spatial frequency information in certain environments may be useful in maintaining postural stability. Correcting visual impairment caused by uncorrected refractive error and cataracts could be a useful intervention strategy to help prevent falls and fall-related injuries in the elderly.

Reports from the United Kingdom and the United States show that a large number of people (∼7,000–10,000) die as a result of falls or fall-related injuries each year, ^{1 2} with the majority of individuals being elderly (84% more than 65 years of age). ¹ Visual impairment has been strongly associated with increased risk of hip fractures ^{3 4 5} and recurrent falls in older adults. ⁴ In addition, visual functioning has been associated with postural instability, ^{4 6 7 8 9 10} and decreases in visual function are strongly associated with fallers compared to nonfallers. ^{6 11} Surveys suggest that approximately half of people aged 65 or more in the United Kingdom could have improved vision with updated spectacles or cataract surgery, ^{12 13} and Jack et al. ¹⁴ showed a particularly high prevalence (76%) of visual impairment in patients admitted to a U.K. geriatric hospital after falling. Seventy-nine percent of this visual impairment was potentially reversible, either by means of correcting refractive errors (40%) or by removal of cataracts (39%). Therefore, it seems highly pertinent to assess the effect of cataract and refractive blur on postural instability. 

Research assessing postural stability in visually impaired individuals has tended to use patients with nonreversible impairment, for example glaucoma, retinitis pigmentosa, and age-related macular disease. ^{15 16 17 18} The studies that have assessed postural stability in individuals with reversible visual impairment have concentrated on the effects of refractive error on young subjects, ^{10 19 20 21 22} and no previous study has assessed the influence of cataractous diffuse blur on postural stability. In the present study, we tested three main hypotheses:

1.  
   That visual impairment due to a cataract simulation would increase postural instability.
2.  
   That the relationship between postural instability and refractive blur ²³ is altered by the spatial frequency and contrast of the visual target used. Given that low levels of refractive blur (>4 D) have been shown to have little or no effect on low spatial frequency contrast sensitivity (CS), ^{24 25} we hypothesized that refractive blur would increase postural instability, particularly when viewing low contrast and/or high spatial frequency targets.
3.  
   That the influence of blur on postural instability is driven by changes in CS rather than resolution. This would support the previously reported association between CS and postural sway ^{6 18 26 27} and CS and falls in the elderly. ^{3 4 8 11 27}

To test these hypotheses, we determined the effect of a binocular cataract simulation and binocular refractive blur on postural stability in healthy, elderly subjects standing upright and viewing targets of different spatial frequency and contrast. The visual contribution to postural stability becomes increasingly important under challenging conditions, ^{9 10 18 23 26 27} and therefore we repeated all measurements while subjects’ somatosensory input was disrupted. 

In addition, a secondary purpose of the present study was to determine whether binocular refractive blur and cataract simulation are associated with increases in limb-load asymmetry (LLA). LLA is thought to be a precautionary balance strategy, whereby more weight is transferred to one limb to shorten the reaction time when moving the other limb in the event of having to step forward or backward to recover balance. ²⁸ Thus, we anticipated that LLA would increase with a cataract simulation and at some level of refractive blur. 

Methods

Thirteen elderly subjects, eight male and five female (mean age, 70.76 ± 4.14 [SD] years) were recruited from a group of volunteer patients who attend the University of Bradford Eye Clinic for teaching purposes. The tenets of the Declaration of Helsinki were observed, and the study gained approval from the University ethics committee. Informed consent was obtained from the participants after the nature of the study had been fully explained. An assessment was made to ensure that all subjects had no history of falls. For this study, a fall was defined as falling all the way to the floor or ground, falling and hitting an object such as a chair or stair, or falling from one level to another, for example from bed to the ground. ²⁹ Subjects were also screened using a self-report health questionnaire. Those with cardiac arrhythmias, vestibular disturbances, diabetes, or severe arthritic conditions and medications affecting balance were excluded. Scores on the Lawton Activities of Daily Living (ADL) questionnaire ³⁰ were high, with all subjects scoring a maximum 16 points. This indicated that the subjects were all independently mobile. 

Measurements of visual acuity (VA) and ocular screening using slit-lamp biomicroscopy, tonometry, indirect ophthalmoscopy, and central visual field were undertaken. To ensure that vision loss was entirely due to refractive blur or the cataract simulation, subjects with a history of amblyopia, strabismus, eye disease, or ocular surgery; binocular VA less than 0.0 logarithm of the minimum angle of resolution (logMAR; Snellen equivalent ∼20/20); and/or any visible ocular disease were excluded. A subjective refraction was performed to obtain the subject’s optimal refractive correction at 4 m. Binocular visual function was subsequently assessed by VA and CS measurements. Binocular VA was measured (mean VA −0.07 ± 0.03 logMAR; Snellen equivalent ∼20/15) with the Early Treatment Diabetic Retinopathy Study (ETDRS) logMAR chart, with by-letter scoring, chart luminance of 160 cd/m² and a 4-m working distance. Binocular CS was measured (mean 1.68 ± 0.08 log CS) with the Pelli-Robson chart at 1 m, with by-letter scoring and a chart luminance of 200 cd/m². Binocular VA and CS were subsequently remeasured with additional binocular blur lenses of +1, +2, +4, and +8 DS and a cataract simulation ³¹ (light-scattering goggles; Vistech Consultants Inc., Dayton, OH) in a randomized order. The cataract simulation used in this study has been shown to mimic the wide angle (between 5° and 20°) light-scattering properties of cataract, in that it scatters light proportional to the inverse of the glare angle. ³¹ In addition, this cataract simulation was chosen because it has been shown to produce greater effects on Pelli-Robson CS than VA, ³¹ which is the opposite of refractive blur, which has a greater effect on VA. ²⁵ By comparing postural stability changes with refractive blur and the cataract simulation, we intended to determine whether increases in postural instability are driven by reduction in CS or VA or in both. 

Postural stability measurements were determined while subjects stood stationary on two adjacent force platforms (OR6-7; Advanced Medical Technology Inc., Boston, MA) mounted flush with the floor. Outputs from each of the force plates were combined to derive displacements of a global center of pressure (COP) in the anterior–posterior (A-P) and medial–lateral (M-L) directions. 

Fluctuations in the displacement of the CP signal were quantified using the root mean square (RMS) of the amplitude. These fluctuations reflect the response of the central nervous system (CNS) to displacements of the center of mass. ^{32 33} The subjects were asked to stand still on the force plate for 30-second periods with their arms by their sides and one foot on each of the adjacent force platforms placed at a distance one tenth of the subject’s height apart, and the long axis of each foot was externally rotated by 15°. ³⁴ To ensure that this stance position was maintained throughout the test procedure, a template was made for each subject according to height and the length of the foot, and placed over the force platform during each trial. Having the subject’s feet placed on two separate platforms allowed the vertical forces exerted by each limb and A-P and M-L force moments to be obtained to assess LLA, ²⁸ which was determined as the ratio of the average (over the 30-second period) body weight placed on the more loaded limb to that on the less loaded limb  

\[\mathrm{LLA}\ {=}\ \frac{\mathrm{average\ weight\ on\ the\ more\ loaded\ limb}}{\mathrm{average\ weight\ on\ the\ less\ loaded\ limb}}\]

According to this definition, an LLA of 1.0 would denote perfect symmetry. 

Subjects were asked to keep looking at the middle of one of four visual targets, which consisted of a horizontal and vertical square-wave pattern. ^{10 23} Two patterns had a fundamental spatial frequency of 2.5 cyc/deg and two had a fundamental spatial frequency of 8 cyc/deg. The targets either had a Weber contrast of approximately 25%, which we assumed to be representative of contrast levels typically found in a home environment, or a Weber contrast of approximately 95%, which is representative of high-contrast black-on-white targets. Each of the targets covered an area of 1.1 m² and had a viewing distance of 1 m. The targets were adjusted for height for each subject so that its center was at eye level. Viewing was binocular, and vision in each subject was corrected with the optimal 4-m refractive correction and a 0.75-DS working-distance lens with full-aperture lenses in a trial frame at a distance of 1 m. 

Standing postural stability and LLA were measured under two conditions: first, normal (bare platform) standing, and, second, standing on a 1.8-cm-thick dual-density polyurethane surface (1 cm at 270 kg/m³ and 0.8 cm at 430 kg/m³). The high-density polyurethane layer prevents localized compression under the typical areas of contact (metatarsal, malleolus, and hallucis) during upright standing, and hence maintains the compliant nature of the surface throughout the experimental procedure. The compliant nature of the foam makes it difficult for the kinesthetic system to provide accurate body orientation information in relation to the ground, and this disrupts somatosensory system inputs. 

Subjects attended a familiarization session that involved their standing on the foam surface so that they could become familiar with standing with somatosensory input disrupted. Subjects were also exposed to the various visual conditions. Under each of the surface test conditions, standing balance and LLA were measured with the optimal refractive correction for the 1-m working distance and under six blur conditions for each of the four visual targets. The blur conditions included binocular dioptric blur levels of 0, +1, +2, +4 and +8 D and with diffusive blur using the cataract-simulating goggles. ^{31 35} In addition, standing balance and LLA were measured during normal standing and during standing on the foam surface with eyes closed. The order of the standing, visual, and target conditions were completed in a randomized order, and subjects were given a rest period of 1 minute (in which they could be seated) between each 30-second trial period. 

Changes between conditions in the COP RMS were analyzed with a generalized estimating equation (GEE) population-averaged model that accounted for the correlation of readings within subjects (Stata, ver.7.0 statistical program; Stata Corp., College Station, TX). An exchangeable correlation structure was judged to be appropriate, given the experimental design. The terms in the model are:

1.  
   A-P/M-L, a fixed factor with two levels: A-P and M-L directions of stability
2.  
   Sensory disruption, a fixed factor with the two levels described earlier
3.  
   Blur, a fixed factor with six levels: eyes open with no blur and 1-, 2-, 4-, and 8-D blur and cataract simulation
4.  
   Spatial frequency, a fixed factor with two levels: high (8 cyc/deg) and low (2 cyc/deg)
5.  
   Contrast, a fixed factor with two levels: high (Weber contrast 95%) and low (Weber contrast 25%)

The interactions of blur and sensory disruption and of blur and A-P/M-L were also included in the model.

For each of the standing and target conditions, differences between LLA measures in the eyes-open condition and each level of refractive blur, the cataract simulation and the eyes closed condition, were assessed by means of analysis of variance (ANOVA). 

Results

Blur and Visual Function

Group mean VA (logMAR) and CS for increasing levels of refractive blur and with the diffuse blur of the cataract simulation are shown in Figure 1 . The cataract simulation reduced logMAR VA to 0.13 (Snellen equivalent, ∼20/25), which was comparable to the reduction in logMAR VA caused by 1-D blur (0.19 logMAR; Snellen equivalent ∼20/30). However, the cataract simulation reduced Pelli-Robson CS to 0.95 log units, which was comparable to the reduction in CS of 1.00 log unit caused by 8-D blur. The reduction of CS to 0.95 log unit is similar to the level induced by a dense cataract. For example, Pelli-Robson CS before first-eye cataract surgery has been measured as 1.36 ± 0.22, ³⁶ providing a lower 95% confidence limit of 0.93 log unit. 

Postural Stability

The GEE population-averaged model approach was checked by plotting the predicted values of postural stability (COP RMS data) against the actual values and against the studentized residuals. There was generally close agreement between the actual and predicted values of postural stability, and departures from model assumptions were not severe, which suggests that the model was a good approximation of the data. 

The A-P/M-L COP RMS term in the model was highly significant (χ² = 6.91, P < 0.001), which means that the COP RMS displacement in the A-P direction was always greater than the associated M-L measure. The A-P/M-L COP RMS–blur interaction term was also significant (χ² = 35.72, P < 0.0001), suggesting that blur had a greater effect on postural stability in the A-P than in the M-L direction. Because postural stability was affected more in the A-P than in the M-L direction, the effects of visual blur on postural stability under the various standing and target conditions, are presented using the COP RMS data for the A-P direction only. ^{10 23}  

The effect of somatosensory disruption was significant (χ² =2.48, P < 0.01) and indicates postural instability was greater when the subject stood on the foam surface than when standing on the bare platform (compare Figs. 2 3 ). The sensory disruption–blur interaction term was also highly significant (χ² = 22.74, P < 0.005), which indicates that postural instability increased with blur to a larger extent during standing with somatosensory disruption than in normal standing (Fig. 3) . 

Blur and Postural Stability

As a representative example, mean A-P COP RMS at refractive blur levels of 0 and 4 D and with the cataract simulation are shown in Table 1 . Refractive blur and the diffuse blur from the cataract simulation produced substantial increases in postural stability across all conditions (χ²= 22.74, P < 0.0005), particularly when standing with somatosensory system input disrupted (Fig. 3) . The effects of the cataract simulation were similar to that determined for 8-D refractive blur (Figs. 2 3) . Analysis of the A-P COP RMS showed a significant difference between the level of postural instability with 4-D blur and with the cataract simulation (χ²= 8.22, P < 0.05); however, there was no difference between A-P COP RMS with 8-D refractive blur and that with the cataract simulation (χ²= 0.21, P = 0.97). 

Influence of the Visual Target

Representative means for A-P COP RMS with the four visual targets at refractive blur levels of 0 and 4 D and with the cataract simulation are shown in Table 1 . Target spatial frequency showed a significant effect on postural stability (χ² = 4.88, P < 0.001). For example, under conditions of somatosensory input disruption during the eyes-open (0-D) condition, postural instability was 9% to 10% greater with the 8-cyc/deg targets than with the 2.5-cyc/deg targets (P < 0.05). This difference was increased under 4-D blur (22%) and with the cataractous diffuse blur (23%). However, although target contrast tended to increase postural instability, the effect was relatively small and only approached significance (P = 0.09). The relationship between postural instability and visual blur (refractive and diffuse) for the four visual targets is shown for the normal standing and standing with somatosensory disruption conditions in Figures 2 and 3 , respectively. Preliminary investigations had indicated that the low-frequency targets were always visible, even with 8 D of refractive blur, similar to our earlier findings. ²³ Therefore, simple linear regression analyses were used to define the relationship between postural instability and refractive blur with subjects viewing the 2.5-cyc/deg target. However, the high spatial frequency targets were made invisible by the higher levels of refractive blur. Consequently, we assumed that postural instability would increase as a function of refractive blur ²³ until the visual target became invisible, when instability would remain at a fixed level. For this reason, least-squares bilinear curve fitting (KaleidaGraph, ver. 3.08; Synergy Software, Reading, PA) was used to define the relationship between postural instability and refractive blur with subjects viewing the 8-cyc/deg target. 

Limb-Load Asymmetry

For all standing and target conditions, group mean LLA measures with the eyes open and closed and with each level of refractive blur and the cataract simulation were more or less the same (range, 1.16–1.24), and ANOVA indicated that refractive blur, cataract simulation, or closing the eyes had no effect on LLA (P > 0.05). 

Discussion

Blur and Visual Function

Refractive blur had a significantly greater effect on logMAR VA than on Pelli-Robson CS at low levels of blur (Fig. 1) , which agrees with previous findings. ²⁵ The Pelli-Robson chart measures CS at or slightly below the peak of the CS function at approximately 1.5 cyc/deg, and these spatial frequencies are relatively unaffected by small amounts of refractive blur. ²⁵ The diffuse blur of the cataract simulation had a much greater effect on Pelli-Robson CS than on logMAR VA, which is in agreement with previous findings. ^{31 35} The cataract simulation reduced logMAR VA to 0.13 (Snellen equivalent, ∼20/25), which was comparable to the reduction in logMAR VA caused by 1-D blur, 0.19 logMAR (Snellen equivalent, ∼20/30). However, the cataract simulation reduced Pelli-Robson CS to 0.95 log unit, which was comparable to the reduction in CS of 1.00 log unit caused by 8-D blur (Fig. 1) . 

Postural Stability

The mean COP RMS displacement in the A-P direction was always greater than the associated M-L measure, which is in agreement with the literature. ^{10 23 32 33} The A-P/M-L COP RMS–blur interaction term was also significant (χ² = 35.72, P < 0.0001), suggesting that blur has a greater effect on postural stability in the A-P than the M-L direction, which is again similar to previous findings. ^{10 19 23} Postural instability was greater when standing on the foam surface, because of the disruption of the somatosensory input, and this instability increased with both refractive and diffuse blur to a larger extent for standing with somatosensory disruption than in normal standing (Fig. 3) . This supports findings that the visual contribution to postural stability becomes increasingly important under challenging conditions. ^{9 10 18 23 26 27}  

Blur and Postural Stability

The diffuse blur of the cataract simulation produced significant increases in postural instability, with up to a 30% increase under normal standing conditions and up to 64% with somatosensory disruption (Table 1) . The results confirmed our hypothesis of an increase in postural instability with diffuse blur from a cataract simulation. We are presently determining the extent of this increase in instability in patients with age-related cataract and whether there are improvements after first- and second-eye cataract surgery. 

The effects of the cataract simulation on postural stability were similar to those with 8-D refractive blur (Fig. 3) . Given that the cataract simulation decreased CS to the level of 8-D blur and VA only to the level of 1-D blur, this suggests that postural stability is driven by changes in CS rather than resolution. The results support the reported association between CS and postural sway ^{6 18 26 27} and CS and falls in the elderly. ^{3 4 8 11 27} Given this finding, it was surprising that the study found only a slight and nonsignificant (P = 0.09) change in postural instability due to the difference in contrast of the visual targets. This may be due to two factors. First, for the 8-cyc/deg targets at the higher levels of refractive blur, the targets were invisible so that no difference in postural stability would be expected. Indeed, we cannot explain the differences in the levels of the plateaus of postural instability for the 8-cyc/deg targets in Figure 3 . We had expected them to be at similar levels. Second, we suggest that the effects of contrast would have been greater if the low-contrast targets had been lower than 25%. For example, 4- and 8-D refractive blur reduced Pelli-Robson CS to approximately 1.5 log CS (3.2% contrast threshold) and 1.0 log CS (10% contrast threshold) respectively, so that a visual target of 10% would probably have had a greater effect on postural instability. 

Influence of the Visual Target

The results confirmed our hypothesis that refractive blur would particularly increase postural instability when viewing high spatial frequency targets compared with lower ones (Figs. 2 3 ; Table 1 ). The results indicate that postural instability was greater under optimal conditions (0-D blur) when viewing the higher spatial frequency target and that this difference increased under conditions of refractive blur, until the high spatial frequency target became invisible. This supports and extends the findings of an earlier study by Kunkel et al. ³⁷ who measured postural sway velocity in a group of healthy young subjects by using a visual target of circular sine-wave gratings of various spatial frequencies under reduced somatosensory input. They found that the minimum sway velocity in both the A-P and M-L directions occurred at 1.33 cyc/deg, whereas higher spatial frequencies (5.33 cyc/deg) elicited increased sway velocity. ³⁷  

Visual Blur and LLA

LLA remained more or less constant (at approximately 1.21 ± 0.14) across all conditions (i.e., refractive blur, cataract simulation, or closing the eyes had no effect on LLA). This indicates that subjects either made no postural control adaptation in the visually disrupted conditions, including when the eyes were closed, or that such an adaptation was used in all conditions, even when the eyes were open. Given that LLA measures for all conditions are comparable to those reported by Blaszczyk et al. ²⁸ when their subjects closed their eyes (approximately 1.19), it seems likely that the subjects in the present study used a postural control adaptation in all conditions. Why this was the case is unclear. As subjects were asked to complete 50 repeated trials, familiarity of the task may have been a contributing factor to the use of an adapted balance strategy in all trials. Alternatively, the discrepancy between the findings of the present study and those of Blaszczyk et al. ²⁸ may be due to methodological differences. For example, they collected data for 120 seconds and found LLA to increase initially, before stabilizing after approximately 20 to 30 seconds. In the present study, data were collected for only 30 seconds, and thus subjects may have had insufficient time to stabilize. To check this, LLA was recalculated using the first and the final 10 seconds of data, but scores for each time interval were found to be more or less the same (P > 0.05), suggesting that subjects may have begun each trial in an adapted position. Another difference between the two studies was the type of visual information available to the subjects. In the study by Blaszczyk et al., ²⁸ there was no mention of using a visual target, whereas in the present study subjects were asked to look at a specific target 1 m in front of them. Because the frequency and contrast of the visual target used were found to influence postural stability, it is possible that it also affected the balance strategy used. Future research is needed to clarify exactly which subjects use and/or which conditions initiate a precautionary balance control adaptation. 

Summary

Our study found significant increases in postural instability due to diffuse blur from a cataract simulation and refractive blur in a healthy elderly population. These increases were particularly large when the input from the somatosensory system to postural control was disrupted. We also found that the relationship between postural instability and refractive blur ²³ was altered by the spatial frequency of the visual target used. A lower spatial frequency target provided less postural instability, particularly under conditions of refractive or diffuse blur. Finally, the results suggest that the influence of blur on postural instability is driven by changes in CS rather than resolution. This supports the previously reported association between CS and postural sway ^{6 8 26 27} and CS and falls in the elderly. ^{3 4 8 11 27}  

These findings suggest that correcting common forms of visual impairment in the elderly, such as refractive errors ^{12 13} and cataract, ^{31 36} may be an important intervention strategy in improving postural stability and avoiding falls. The cumulative effect of visual impairment with somatosensory perturbation indicates that correcting refractive and cataractous blur may be particularly important in patients with somatosensory system dysfunction—for example, those with diabetes and/or peripheral neuropathy. Findings also indicate that visual stimuli of low spatial frequencies could aid in the maintenance of postural stability, whereas, in isolation, higher spatial frequency stimuli may be debilitating, especially in the presence of visual impairment. This suggests that the availability of lower spatial frequency high-contrast information in certain environments (on stairways or when reaching into cupboards above head height, which require head tilt and disruption of vestibular input to the postural control system ²³ ) may help in the prevention of falls in the elderly. 

 Supported by the PPP Foundation Grant 3991/882.

 Submitted for publication May 13, 2003; accepted July 10, 2003.

 Disclosure: V. Anand, None; J.G. Buckley, None; A. Scally, None; D.B. Elliott, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Figure 1.

 

LogMAR VA and Pelli-Robson CS scores as a function of refractive blur and cataract simulation. (•) Binocular VA (logMAR); (▪) binocular CS (log units).

Figure 2.

 

Mean COP RMS measurements in the A-P direction (A-P COP RMS) as a function of refractive blur and cataract simulation, across all visual targets. Mean (±1 SE) A-P COP RMS data are shown for normal standing conditions: (•) 8 cyc/deg, high contrast; (▪) 8 cyc/deg, low contrast; (♦) 2.5 cyc/deg, high contrast; (▴) 2.5 cyc/deg, low contrast.

Figure 3.

 

Mean center of pressure RMS measurements in the A-P direction (A-P COP RMS) as a function of refractive blur and cataract simulation, across all visual targets. Mean (±1 SE) A-P COP RMS data are shown for standing conditions with somatosensory disruption. (•) 8 cyc/deg, high contrast; (▪) 8 cyc/deg, low contrast; (♦) 2.5 cyc/deg, high contrast; (▴) 2.5 cyc/deg, low contrast.

Table 1.

 

Mean A-P Center of Pressure RMS across the Various Target, Sensory and Visual Conditions

Target	Normal Standing					Somatosensory Disruption
		COP (0 D)	COP (4 D)	COP Cataract	COP (0 D)	COP (4 D)	COP Cataract
2.5 cyc/deg high contrast	3.5	4.5	4.0	4.5	5.2	5.7
(% inc from 0 D)	(0)	(27)	(15)		(17.5)	(28)
2.5 cyc/deg low contrast	4.0	4.4	4.7	4.7	5.6	6.4
(% inc from 0 D)	(0)	(8.5)	(15)		(20)	(38)
8 cyc/deg high contrast	4.2	5.2	5.1	4.9	6.9	8.0
(% inc from 0 D)	(0)	(23)	(20)		(41)	(64)
8 cyc/deg low contrast	3.7	5.4	4.8	5.1	6.3	7.1
(% inc from 0 D)	(0)	(45)	(30)		(23)	(39)