DETECTING A VISUAL OBJECT IN THE PRESENCE OF OTHER OBJECTS: THE FLANKER FACILITATION EFFECT IN CONTOUR INTEGRATION
By
Author
Presented To
Department of Psychology
Abstract
When an observer views a complex visual scene and tries to identify an object, his
or her visual system must decide what regions of the visual field correspond to the
object of interest and which do not. One aspect of this process involves the group-
ing of the local contrast information (e.g., orientation, position and frequency) into
asmoothcontourobject. Thisthesisinvestigatedwhetherthepresenceofother
flanking objects a↵ected this contour integration of a central target contour.
To test this, a set of Gaborized contour shapes were embedded in a randomised
Gabor noise field. The detectability of the contours was altered by adjusting the
alignment of the Gabor patches in the contour (orientation jitter) until a participant
was unable to distinguish between a field with and without a target shape (2-AFC
procedure). By varying the magnitude of this jitter, detection thresholds were deter-
mined for target contours under various experimental conditions. These thresholds
were used to investigate whether contour integration was sensitive to shared shape
information between objects across the visual field.
This thesis determined that the presence of flanking contours of a similar shape
(as the target) facilitated the detection of a noisy target contour. The specific re-
sults suggest that this facilitation does not involve a simple template matching or
shape priming but is associated with integration of shape level information in the
detection of the most likely smooth closed contour. The magnitude of this flanker
facilitation e↵ect was sensitive to a number of factors (e.g., numerosity, relative
position of the flankers, and perimeter complexity/compactness). The implication
of these findings is that the processing of highly localised contrast and orientation
information originating from a single object is subject to modulation from other
sources of shape information across the whole of the visual field.
Contents
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 Detection of an object - psychophysical sensitivity . . . . . . . 6
1.2.2 Local features and object detection . . . . . . . . . . . . . . . 11
1.2.3 Attention and the detectability of objects and features. . . . . 17
1.2.4 The detection of Gaborized contours outside of the focus of
attention . . . . .
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