Contents
1 Introduction 1
11 Motivation 2
12 General introduction
6
121 Detection of an object - psychophysical sensitivity 6
122 Local features and object detection 11
123 Attention and the detectability of objects and features 17
124 The detection of Gaborized contours outside of the focus of
attention 18
125 Object, signal and feature numerosity 21
126 Summary 25
13 Experiment overview 26
131 Experiments 26
2 Is the detectability of a 3-D object embedded in multi-scale noise
affected by the presence of neighbouring objects? (Pilot experiment) 29
21 Abstract 30
22 Introduction 30
23 Aims 33
24 Methodology 35
241 Participants 35
242 Apparatus 35
243 Stimuli 35
25 Procedure 43
26 Results 45
27 General Discussion 45
28 Issues related to task procedure 47
29 Issues related to the stimulus 47
210 Conclusion 48
3 Is the detectability of a Gaborized contour modulated by the pres-
ence of nearby flanking contours? 49
31 Abstract 50
32 Introduction 51
321 Local processing and Contour integration 54
322 Experimental Summary 55
33 Experiment 1 56
331 Methodology 56
332 Procedure 63
333 Results 66
334 Discussion 71
34 Experiment 2a 73
341 Methodology 73
342 Procedure 75
343 Results 76
344 Discussion 78
35 Experiment 2b 78
351 Methodology 79
352 Results 81
36 General Discussion 83
37 Conclusion 88
4 Contour integration is facilitated by the presence of adjacent con-
tours that share shape-level features 91
41 Abstract 92
42 Introduction 92
421 The detection of symmetry and its e↵ect on the detectability
of a Gaborised contour 94
422 The e↵ect of familiarity on the perceptual organisation of Ga-
borized contours 95
423 The contextual and lateral enhancement of detection due to
the simultaneous presentation of features and objects 96
424 The e↵ects of flanking contours, features and complexity on
the detection of a target Gaborized contour 97
425 Experimental summary 102
426 Methodology 103
427 Procedure 109
43 Results 112
431 A fit of the increase in complexity due to the addition of ori-
entation noise with respect to the initial compactness of the
target contour 115
44 Discussion 122
45 Conclusion 126
5 The magnitude of the flanker facilitation e↵ect on contour integra-
tion is modulated by changes in spatial location and numerosity of
flanking contours 127
51 Abstract 128
52 Introduction 128
521 The flanker facilitation of a Gaborized closed contour 130
522 Number of flanking contours surrounding the target contour 131
523 Alignment of flanking and target contours 132
524 Experimental summary 134
53 Experiment 1: Number of flankers surrounding target contour 135
531 Methodology 135
54 Procedure 138
541 Results 142
55 Experiment 2 - Alignment of target contour and flankers 144
551 Methodology 144
552 Results 146
553 Summary 149
56 General Discussion 149
561 The sampling of shape information from increasing numbers
of flanking contours 151
562 The enhancement of target contour detectability with di↵er-
ences in alignment 152
57 Conclusion 154
6 Shape similarity modulates the magnitude of the flanker facilitation
effect 155
61 Abstract 156
62 Introduction 157
621 The role of context on object detection and perceptual orga-
nization 157
622 Experimental summary 158
63 Experiment 1 159
631 Methodology 159
632 Results 166
64 Experiment 2 167
641 Methodology 167
642 Results 169
65 General Discussion 170
66 Conclusion 174
7 Conclusion 176
71 Overview of thesis 177
72 Summaries of experimental findings 178
721 Chapter 2 ! Is the detectability of a 3-D object embedded
in multi-scale noise a↵ected by the presence of neighbouring
objects? (Pilot experiment) 178
722 Chapter 3 ! Is the detectability of a Gaborized contour mod-
ulated by the presence of nearby flanking contours? 179
723 Chapter 4 ! Contour integration is facilitated by the presence
of adjacent contours that share shape-level features 180
724 Chapter 5 !The magnitude of the flanker facilitation e↵ect on
contour integration is modulated by changes in spatial location
and numerosity of flanking contours 182
725 Chapter 6 ! Shape similarity modulates the magnitude of the
flanker facilitation e↵ect 183
73 The flanker facilitation e↵ect ! Summary of findings 185
731 How is the detection of a contour e↵ected by the presence of
flanking contours? 185
732 What is the role of the flanking contours and how does the
visual system sample information from the flanking contours? 187
733 Is the flanker facilitation e↵ect a single perceptual mechanism? 189
734 The flanker facilitation e↵ect and low level processing 190
735 The flanker facilitation e↵ect and the redundancy signal e↵ect 191
74 Further directions 192
75 Conclusion 193
8 Appendix 1 194
81 Pov-ray object stimuli 194
811 Purpose 194
812 General description 194
813 Methodology 195
9 Appendix 2 196
91 Multi-scale noise 196
911 Purpose 196
912 General description 196
913 Methodology 197
10 Appendix 3 200
101 Measurement of change in compactness 200
1011 Purpose 200
1012 General description 200
1013 Methodology 201
11 Appendix 4 203
111 Interpolation of intermediate shape contours 203
1111 Purpose 203
1112 General description 203
1113 Methodology 203
12 Appendix 4 205
121 Ethical approval 205
References 206
List of Figures
11 Depictions of Mercury droplets by Arthur Worthington - 6
12 Illusory objects and types of boundaries - - 8
13 A Gabor patch - - - - - 9
14 Gaborized contours - - - - 9
15 Figure-ground segmentation of one region of the visual field
from another - - - - - 10
16 The segmentation of a Gaborized contour from a background
of Gabor patches - - - - - 16
21 Objects used as a detection target - - - 36
22 Configuration changes for a single object - - 37
23 Example set of multi-scale noise with varying maximum fre-
quency harmonics - - - - - 38
24 Example set of multi-scale noise with increasing numbers of
higher order octaves of a single frequency component (aka,
octave value) added to a single multi-scale noise - 39
25 Example set of multi-scale noise with persistence values - 39
26 The Multi-scale noise used to obscure target - - 40
27 The dependent measurement was a change in the opacity of
a noise image with respect to a target object - - 41
28 The four conditions tested in the pilot experiment - 42
210 The mean detection thresholds for the experimental condi-
tions for objects with identical parts (Red) and Non-identical
parts (Blue) - - - - - 46
31 Shapes used to generate target and flanker Gaborized con-
tours - - - - - - 58
32 Methodology for generating Gaborized contours - 59
33 Orientation and positional adjustments to Gabor patches 60
34 The target region presented to a participant - - 61
35 The experimental measurement of orientation noise - 62
36 Conditions presented to participant in experiment 1 - 63
37 The time course of a single trial - - - 64
38 The mean detection threshold of the target contour as a
function of each experimental condition - - 67
39 The ratio of detectability of the target contour as a function
of each flanking condition for each participant - - 68
310 The ratio of the detection threshold as a function of the
target contour shape - - - - 69
311 The ratio of the mean detection threshold of the target con-
tour as a function of each condition grouped by general pres-
ence of symmetry and familiarity - - - 70
312 Shapes used to generate target and flanker Gaborized con-
tours in experiment 2a - - - - 74
313 Orientation of Gabor patches in periphery - - 75
314 Conditions presented to participants in experiment 2a - 76
315 The mean detection thresholds of the target contour as a
function of the peripheral Gabor field - - 77
316 Shapes used to generate target and flanker Gaborized con-
tours in experiment 2b - - - - 80
317 Examples of the flanking contours embedded in the Gabor
fields of di↵erent types - - - - 81
318 The mean detection threshold of the target contour as a
function of the main experimental interactions (noise and
target-flanker, symmetry and target-flanker conditions) - 89
319 The mean detection thresholds of the target contour of the
main conditions (symmetry and target-flanker) - - 90
41 Change in complexity due to local changes in orientation in a circle - - - - - - 99
42 Shapes used to generate target and flanker Gaborized contours - - - - - - 105
43 The individual complexity values for the generating shapes 106
44 Conditions presented to participant in the experiment - 108
45 The time course of a single trial - - - 110
46 The mean detection thresholds of the target contour as a
function of the target-flanker and symmetry conditions - 114
47 The mean detection thresholds of the target contour as a
function of target familiarity and both the target-flanker and
symmetry conditions - - - - 115
48 The mean detection threshold as a function of the complexity
of the shape of the target contour in the control condition 116
49 The compactness di↵erential for individual contours as a
function of the complexity of the initial contour shape in
the control condition - - - - 117
410 The mean detection threshold as a function of the complexity
of the shape of the target contour in the same condition 118
411 The compactness di↵erential for individual contours as a
function of the complexity of the initial contour shape in
the same condition - - - - 119
412 Comparison of the quadratic fits of the compactness di↵er-
ential as a function of the complexity of the target contour
in the control and same conditions - - - 121
51 The presence of bilateral symmetry in adjacent contours in-
troduces inter-object symmetries - - - 131
52 Shapes used to generate target and flanker Gaborized con-
tours - - - - - - 136
53 The region surrounding the target contour containing flank-
ing contours in comparison to that used in previous experi-
ments - - - - - - 139
54 Conditions presented to examine the role of numerosity of
flanking contours on target detectability - - 139
55 The time course of a single trial - - - 140
56 The mean detection thresholds of the target contour as a
function of the numerosity of flankers - - 143
57 The conditions used to examine the role of relative flanker
position on target detectability - - - 146
58 the mean detection thresholds of the target contour as a
function of flanker alignment in the horizontal arrangement 147
59 The mean detection thresholds of the target contour as a
function of flanker alignment in the vertical arrangement 148
61 Shapes used to generate target and flanker Gaborized con-
tours - - - - - - 161
62 Flanker shapes used to generate target and flanker Gaborized
contours - - - - - - 162
63 The time course of a single trial - - - 165
64 The mean detection thresholds as a function of flanker sim-
ilarity to target contour - - - - 168
65 The mean detection thresholds as a function of matching or
non matching flanker shape - - - - 170
91 Coherent and incoherent noise waves - - - 199
29 The time course for a single trial - - - 44
