Varikuti PhD thesis final.pdf - Anglia Ruskin Research Online

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ANGLIA RUSKIN UNIVERSITYABSTRACTFACULTY OF SCIENCE AND TECHNOLOGYDOCTOR OF PHILOSOPHYFURTHER INSIGHTS INTO LETTER CROWDING: THE ROLE OF CONTOURINTERACTION, CONTRAST AND GAZE FIXATIONSBy VINEELA V.N. VARIKUTIFEBRUARY 2012Visual acuity is reduced when optotypes are viewed in the presence ofsurrounding contours. This reduction in acuity is known as the crowdingeffect and is thought to be caused by a varying combination of contourinteraction, gaze instability and attention. Traditional studies have usedsingle optotypes surrounded by flanking bars to investigate crowding. Suchtargets may not realistically replicate the crowding effect inherent in clinicalvision charts. The aim of this <strong>thesis</strong> was to systematically investigate theeffect of crowding on visual thresholds in subjects with normal vision and insubjects with amblyopia, using specially designed charts.In the 1 st and 2 nd experiment, contour interaction was assessed using a high(80 %) and low contrast (5.8%) Sheridan Gardiner repeat letter (SGRL) chartin subjects with normal vision. The effect of contour interaction wasinvestigated by varying the inter-letter separation in the SGRL chart.Significant contour interaction was obtained at the abutting condition forboth the contrast conditions. In the 3 rd experiment the same protocol wasrepeated but in amblyopes. Significant contour interaction was obtained atii
0.2 letter separation and the abutting condition for both the contrastconditions. The effect of contour interaction appears to be less for lowcontrast than for high contrast letters in normal, non-amblyopic andamblyopic eyes. Finally, in the 4 th experiment a Sheridan Gardiner ComplexInteraction (SGCI) chart that requires imposed gaze fixations wasconstructed to measure visual acuity in normal’s and amblyopes. The effectof any gaze instability on crowding was investigated by comparing SGRLthresholds to SGCI thresholds. The SGCI thresholds were higher than theSGRL thresholds at all the separations measured, suggesting an importanteffect of gaze instability on crowding.In conclusion, this research has shown that gaze instability is an importantcomponent of the crowding effect for letter chart acuity measurements.Visual acuity especially when screening for amblyopia should be measuredusing a whole optotype chart that requires optotype to optotype fixation.Key words: visual crowding, amblyopia, repeat letter chart, complexinteraction chart, contour interaction, contrast and gaze fixationsiii
Page 1: ANGLIA RUSKIN UNIVERSITYFURTHER INS
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 28 and 29: A significant difference in visual
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
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gaze instability in amblyopes. A se
Page 62 and 63:
each letter in the low contrast Sne
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1999a), the findings are contradict
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RationaleTo determine the effect of
Page 68 and 69:
In most of the studies (Stuart and
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study the effect of contour interac
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contour interaction effect and not
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contour interaction as the Snellen
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In this experiment, the effect of l
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The final set of experiment will in
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Chapter 2The effect of contour inte
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laterality (unlike the Illiterate E
Page 84 and 85:
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
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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
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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
Page 102 and 103:
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
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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
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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
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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
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Page 186 and 187:
Figure 4.7: Depicts normalised thre
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Further, the estimates of slopes ob
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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
Page 208 and 209:
Further, the estimates of slopes fo
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197
Page 212 and 213:
High and low contrast conditionThe
Page 214 and 215:
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
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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
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Table 5.4(A) and (B): Depicts the s
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and mean complex interaction thresh
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235
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A (non-amblyopic eyes) B (amblyopic
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Figure 5.8: The difference between
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Std error 0.05 0.06 0.06 0.07 0.07
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For the amblyopic subjects, a three
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Page 260 and 261:
Control conditionError analysisData
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249
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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
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REFERENCESABRAHAMSSON, M. and SJOST
Page 280 and 281:
BERNARD, J.B. and CHUNG, S.T.L., 20
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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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