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A significant difference in visual acuity scores was noticed whenperformance was compared between the charts having different designpattern. For e.g. the Vision in Preschoolers (VIP) study group (2003) found asignificantly better visual acuity in children (3 to 3.5 years) by 0.25 Log unitswith a flanked HOTV chart (each letter surrounded by 4 equally spacedflanking bars at 0.5 letter width separation) than with a Lea optotype chartthat was similar in design to a Bailey-Lovie LogMAR chart having 1.0 interoptotype separation. Similarly, Flom, Weymouth and Kahneman (1963)found that normal and amblyopic adults showed increased resolutionthresholds when a complex interaction S chart acuity (1.0 inter optotypeseparation and that involves gaze fixations, see Figure 1.7) was comparedto a flanked Landolt C acuity with flanking bars positioned at nearly 0.75optotype width separations. Though the influence of contour interaction wasfound to be less in the Lea LogMAR chart (1.0 optotype separation) andcomplex interaction S chart (1.0 optotype separation) when compared tothe flanked HOTV (0.5 optotype separation) and flanked Landolt C (0.75optotype separation) respectively, poor visual acuity with the Lea LogMARchart and complex interaction S chart may be due to the effect of involvedgaze fixations and attentional components with a whole optotype chart thana flanked optotype chart.In another experiment, McGraw, Winn, Gray and Elliott (2000) comparedvisual acuity scores in adults with normal vision (mean age 21.5 years)obtained using a Glasgow acuity chart (0.5 inter optotype separation) and aBailey-Lovie LogMAR chart (1.0 inter-letter separation). Though poor visualacuity was expected with the Glasgow chart due to the narrower inter-letter15
separation, no significant difference in visual acuity scores (0.07 Log units)was noticed between both charts. This may be because of the cancellationof the increased effect of contour interaction (0.5 letter width separation)and decreased effect of gaze fixations (because of a linear array design) in aGlasgow acuity chart when compared to the decreased effect of contourinteraction (1.0 letter width separation) and increased effect of gazefixations (because of the whole letter chart design) in the LogMAR chart.More recently Norgett and Siderov (2011) determined visual acuity innormal children (4 to 9 years) using crowded Kay pictures, Sonksen LogMARcharts, Glasgow acuity chart, isolated SG letter and isolated Kay picturecharts. The results showed a significant effect of vision chart on acuityscores.In summary, vision charts varied in the design pattern as exemplified by theisolated letter chart, linear letter chart or whole letter chart. The abovementioned studies also explained that the factors such as contourinteraction and gaze fixations contribute to the similarity or discrepanciesbetween the visual acuity scores depending on the design of the visioncharts. Therefore, these factors have to be carefully considered whileattempting to increase the sensitivity of vision charts to screen foramblyopia.Contour interaction and gaze fixations in normal andamblyopic visionThe phenomenon of crowding has long been known (Ehlers, 1936 cited inStuart and Burian, 1962), but the clinical importance of this phenomenon16
Page 1: ANGLIA RUSKIN UNIVERSITYFURTHER INS
Page 4 and 5: ANGLIA RUSKIN UNIVERSITYABSTRACTFAC
Page 6 and 7: TABLE OF CONTENTSACKNOWLEDGEMENTS .
Page 8 and 9: ...................................
Page 10 and 11: Table 4.5(A) and 4.5(B): Depicts th
Page 12 and 13: Tables 5.8(A) and 5.8(B): Depicts t
Page 14 and 15: Chapter 1General IntroductionVision
Page 16 and 17: Visual acuity chartsSnellen chartTh
Page 18 and 19: Bailey-Lovie LogMAR chartThe LogMAR
Page 20 and 21: An accurate and repeatable measurem
Page 22 and 23: elimination of one or more of these
Page 24 and 25: 1985; 1988). Greater crowding in yo
Page 26 and 27: In order to study the reliability o
Page 30 and 31: has gained increased attention foll
Page 32 and 33: Figure 1.3: Depicts the picture of
Page 34 and 35: Landolt C surrounded by flanking ba
Page 36 and 37: Figure 1.4: Depicts the picture of
Page 38 and 39: normal vision and at the fovea and
Page 40 and 41: educed at the abutting condition wh
Page 42 and 43: Figure 1.5: Depicts the data obtain
Page 44 and 45: fixations in normal eyes. Further,
Page 46 and 47: Figure 1.6: Example of a Regan repe
Page 48 and 49: and adults had better isolated lett
Page 50 and 51: (Glasgow acuity poorer than the iso
Page 52 and 53: closer separations based on the fin
Page 54 and 55: identify. On the other hand a 10 mi
Page 56 and 57: Figure 1.7: Example of a PVA test c
Page 58 and 59: the inner letters resulting in miss
Page 60 and 61: gaze instability in amblyopes. A se
Page 62 and 63: each letter in the low contrast Sne
Page 64 and 65: 1999a), the findings are contradict
Page 66 and 67: RationaleTo determine the effect of
Page 68 and 69: In most of the studies (Stuart and
Page 70 and 71: study the effect of contour interac
Page 72 and 73: contour interaction effect and not
Page 74 and 75: contour interaction as the Snellen
Page 76 and 77: In this experiment, the effect of l
Page 78 and 79:
The final set of experiment will in
Page 80 and 81:
Chapter 2The effect of contour inte
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laterality (unlike the Illiterate E
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ProcedureGeneral conditionSix adult
Page 86 and 87:
Figure 2.2: Schematic showing examp
Page 88 and 89:
Figure 2.3: Schematic of the SGRL s
Page 90 and 91:
were corrected by adjusting the deg
Page 92 and 93:
Figure 2.4: The data shown in the f
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thresholds normalised against the i
Page 96 and 97:
Table 2.1: (A) Depicts high contras
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IH was replaced by his slope value
Page 100 and 101:
Table 2.2: Depicts slopes obtained
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Figure 2.7: The data shown are a la
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Table 2.3: (A) Depicts recognition
Page 106 and 107:
Figure 2.9:93
Page 109:
Figure 2.1196
Page 112 and 113:
Figure 2.13:99
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error bars represent ± 1 SD.101
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The effect of the relative letter l
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Figure 2.15: The graph shows mean l
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Normalised recognition thresholds (
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DiscussionThe main results of this
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Since the experiment of Flom, Weymo
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showed that the target flanker simi
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gaze instability and attention comp
Page 130 and 131:
Page 132 and 133:
MethodsMethod of generating low con
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the low contrast letters were more
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was the same as that described in C
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isolated letter in each slide was l
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the identification of the isolated
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Threshold of psychometric functions
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The threshold values obtained from
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AB133
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Figure 3.4: Depicts normalised thre
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Slopes of psychometric functionsThe
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Figure 3.5: Average slope of the ps
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esulting in improved threshold visu
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employed the same size target for t
Page 158 and 159:
Page 160 and 161:
Freeman, 1981). As a result of the
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Table 4.1: Depicts the visual featu
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same as that of the main experiment
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Figure 4.2: The recognition thresho
Page 168 and 169:
Table 4.2: Depicts the test distanc
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Figure 4.3: The percentage correct
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2 X contrastSubject Eye Contrast th
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A (amblyopic eyes) B (non-amblyopic
Page 176 and 177:
Table 4.4(A) and 4.4(B): Depicts th
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Figure 4.5: Recognition thresholds
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Figure 4.6(A) shows mean recognitio
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Figure 4.6: (A) Recognition thresho
Page 184 and 185:
Page 186 and 187:
Figure 4.7: Depicts normalised thre
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Further, the estimates of slopes ob
Page 190 and 191:
Page 192 and 193:
Low contrast conditionFor all the f
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A (non-amblyopic eyes) B (amblyopic
Page 196 and 197:
Page 198 and 199:
Figure 4.10: Depicts the layout of
Page 200 and 201:
Figure 4.11(A) shows mean absolute
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A189
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The threshold values of all the sep
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Figure 4.12: Depicts normalised thr
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Further, the estimates of slopes fo
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197
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High and low contrast conditionThe
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DiscussionThe current experiment in
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ecognition thresholds were nearly t
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on the target size and the extent o
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the contrast sensitivity loss was d
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Simmers, Gray, McGraw and Winn, 199
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separations are also immune to the
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SGCI chart was surrounded by an add
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ProcedureGeneral conditionThree adu
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Chapters 2 and 4 where the effect o
Page 232 and 233:
ResultsGeneral conditionFor the 3 n
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The effects of crowding on visual a
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Tables 5.2(A) and 5.2(B): Depicts t
Page 238 and 239:
Page 240 and 241:
Figure 5.4(A): The top panel depict
Page 242 and 243:
Further, the estimates of slopes ob
Page 244 and 245:
Table 5.4(A) and (B): Depicts the s
Page 246 and 247:
and mean complex interaction thresh
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235
Page 250 and 251:
A (non-amblyopic eyes) B (amblyopic
Page 252 and 253:
Figure 5.8: The difference between
Page 254 and 255:
Std error 0.05 0.06 0.06 0.07 0.07
Page 256 and 257:
For the amblyopic subjects, a three
Page 258 and 259:
Page 260 and 261:
Control conditionError analysisData
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249
Page 264 and 265:
DiscussionThe current experiment in
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Based on these findings, we attribu
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and Spowart, 1997), multiple E opto
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Chapter 6Summary and conclusionsThe
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The effect of contour interaction o
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demonstrable at the abutting condit
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SGCI thresholds when measured for s
Page 278 and 279:
REFERENCESABRAHAMSSON, M. and SJOST
Page 280 and 281:
BERNARD, J.B. and CHUNG, S.T.L., 20
Page 282 and 283:
ELLIOTT, D., BULLIMORE, M., PATLA.
Page 284 and 285:
HARIHARAN, S., LEVI, D.M. and KLEIN
Page 286 and 287:
JACOBS, R.J., 1979. Visual resoluti
Page 288 and 289:
LEVI, D.M., KLEIN, S.A. and HARIHAR
Page 290 and 291:
OWSLEY, C., 1994. Vision and drivin
Page 292 and 293:
ROBINSON, D.A., 1964. The mechanics
Page 294 and 295:
SIMMERS, A.J., GRAY, L.S. and WINN,
Page 296 and 297:
AppendicesAppendix 1: Depicts the p
Page 298 and 299:
three and two amblyopic subjects re
Page 300 and 301:
Appendix 3:287
Page 302 and 303:
Appendix 4: The percentage of subst
Page 304 and 305:
Appendix 5: Depicts the letter conf
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