Varikuti PhD thesis final.pdf - Anglia Ruskin Research Online

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and adults had better isolated letter acuity than the corresponding repeatletter acuity and vice-versa. The results also showed that the poor visualacuity in amblyopic children was either due to the effect of contourinteraction or gaze instability. However, poor visual acuity in amblyopicadults was due to the effect of gaze instability and not due to a contourinteraction effect. This could be because of more fixational instability orimmaturity in children (Aring, Gronlund, Hellstorm and Ygge, 2007). Theseresults suggest that visual acuity is influenced by contour interaction at 1.0optotype separation in children but not in adults as visual acuity in adults isnot influenced by contour interaction when letters are separated by 1.0letter width in a Regan repeat letter chart. This confirms the findings ofother studies which showed a greater extent of contour interaction inchildren than adults (Atkinson, et al., 1985; 1988; Semenov, Chernova andBondarko, 2000; Bondarko and Semenov, 2005; Jeon, Hamid, Maurer andLewis, 2010). Also, each category of the visual deficit i.e.the repeat letteracuity being better than, equal to or poorer than the Snellen acuity includedany kind of amblyopia (strabismic, anisometropic, strabismic+anisometropic). Regan, Giaschi, Kraft and Kothe (1992) concluded that thecomparison between the repeat letter acuity and Snellen acuity provides anon-invasive method of differentiating the amblyopic vision based on thetype of the visual deficit and confirms the findings of Kothe and Regan(1990b) that a repeat letter chart differentiates subjects whose visual acuityis reduced due to gaze instability.In another study, Simmers, Gray and Winn (1999) determined the effect ofabnormal gaze fixations on visual acuity scores in subjects with congenitalnystagmus and normal adults. Visual acuity was compared between theRegan repeat letter chart (1.0 letter width separation) and Glasgow acuity35
chart (0.5 letter width separation). Though the inter-letter separation seemsdifferent in both the charts, it is evident from the results of Flom, Weymouthand Kahneman (1963) that in normal adults, the intensity of contourinteraction is minimal for separations greater than 0.4 letter widths.Therefore, the effect of contour interaction may possibly be controlled inboth the charts at these optotype separations. Similar to the hypo<strong>thesis</strong> ofKothe and Regan (1990b), Simmers, Gray and Winn (1999) stated that anysignificant difference between the repeat letter acuity and Glasgow acuity isattributed to the effect of gaze instability on visual thresholds. Simmers,Gray and Winn (1999) found that their normal subjects showed nosignificant difference (0.02 Log units) between the repeat letter acuity andGlasgow acuity. This implies that the effect of contour interaction and gazeinstability did not limit visual acuity scores in their normal subjects.Contrary, subjects with congenital nystagmus showed significantly betterrepeat letter acuity by nearly 0.25 Log units than the Glasgow acuity. Thisimplies that in these subjects, abnormal gaze fixations limit visual acuityusing the Glasgow acuity chart. These results support the notion of Kotheand Regan (1990b) and Regan, Giaschi, Kraft and Kothe (1992) that therepeat letter acuity is less sensitive to the effect of gaze fixations or gazeinstability.Subsequently, research continued to monitor the efficiency of occlusiontherapy in amblyopic adult (30 years old) (Simmers and Gray, 1999) andamblyopic children (mean age 5.6 ± 1.3 years) (Simmers, Gray, McGrawand Winn, 1999b). Visual acuity was measured using an isolated letter, highand low contrast Glasgow acuity chart and Regan repeat letter chart. Theclassification of visual deficit or improvement in visual acuity during thecourse of occlusion therapy was assessed in terms of contour interaction36
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 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 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
Page 82 and 83: 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
Page 90 and 91: were corrected by adjusting the deg
Page 92 and 93: Figure 2.4: The data shown in the f
Page 94 and 95: thresholds normalised against the i
Page 96 and 97: Table 2.1: (A) Depicts high contras
Page 98 and 99:
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
Page 104 and 105:
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
Page 114 and 115:
error bars represent ± 1 SD.101
Page 116 and 117:
The effect of the relative letter l
Page 118 and 119:
Figure 2.15: The graph shows mean l
Page 120 and 121:
Normalised recognition thresholds (
Page 122 and 123:
DiscussionThe main results of this
Page 124 and 125:
Since the experiment of Flom, Weymo
Page 126 and 127:
showed that the target flanker simi
Page 128 and 129:
gaze instability and attention comp
Page 130 and 131:
Chapter 3The effect of contour inte
Page 132 and 133:
MethodsMethod of generating low con
Page 134 and 135:
the low contrast letters were more
Page 136 and 137:
was the same as that described in C
Page 138 and 139:
isolated letter in each slide was l
Page 140 and 141:
the identification of the isolated
Page 142 and 143:
Threshold of psychometric functions
Page 144 and 145:
The threshold values obtained from
Page 146 and 147:
AB133
Page 148 and 149:
Figure 3.4: Depicts normalised thre
Page 150 and 151:
Slopes of psychometric functionsThe
Page 152 and 153:
Figure 3.5: Average slope of the ps
Page 154 and 155:
esulting in improved threshold visu
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employed the same size target for t
Page 158 and 159:
Chapter 4The effect of contour inte
Page 160 and 161:
Freeman, 1981). As a result of the
Page 162 and 163:
Table 4.1: Depicts the visual featu
Page 164 and 165:
same as that of the main experiment
Page 166 and 167:
Figure 4.2: The recognition thresho
Page 168 and 169:
Table 4.2: Depicts the test distanc
Page 170 and 171:
Figure 4.3: The percentage correct
Page 172 and 173:
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
Page 178 and 179:
Figure 4.5: Recognition thresholds
Page 180 and 181:
Figure 4.6(A) shows mean recognitio
Page 182 and 183:
Figure 4.6: (A) Recognition thresho
Page 184 and 185:
Page 186 and 187:
Figure 4.7: Depicts normalised thre
Page 188 and 189:
Further, the estimates of slopes ob
Page 190 and 191:
Page 192 and 193:
Low contrast conditionFor all the f
Page 194 and 195:
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
Page 202 and 203:
A189
Page 204 and 205:
The threshold values of all the sep
Page 206 and 207:
Figure 4.12: Depicts normalised thr
Page 208 and 209:
Further, the estimates of slopes fo
Page 210 and 211:
197
Page 212 and 213:
High and low contrast conditionThe
Page 214 and 215:
DiscussionThe current experiment in
Page 216 and 217:
ecognition thresholds were nearly t
Page 218 and 219:
on the target size and the extent o
Page 220 and 221:
the contrast sensitivity loss was d
Page 222 and 223:
Simmers, Gray, McGraw and Winn, 199
Page 224 and 225:
separations are also immune to the
Page 226 and 227:
SGCI chart was surrounded by an add
Page 228 and 229:
ProcedureGeneral conditionThree adu
Page 230 and 231:
Chapters 2 and 4 where the effect o
Page 232 and 233:
ResultsGeneral conditionFor the 3 n
Page 234 and 235:
The effects of crowding on visual a
Page 236 and 237:
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
Page 248 and 249:
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
Page 262 and 263:
249
Page 264 and 265:
DiscussionThe current experiment in
Page 266 and 267:
Based on these findings, we attribu
Page 268 and 269:
and Spowart, 1997), multiple E opto
Page 270 and 271:
Chapter 6Summary and conclusionsThe
Page 272 and 273:
The effect of contour interaction o
Page 274 and 275:
demonstrable at the abutting condit
Page 276 and 277:
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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