Transcript Syllabus P140C (68530) Cognitive Science

Selective Visual Attention
&
Visual Search
What is attended?
• Location-based theories: we attend to
regions of visual field
– Spotlight theory (Posner)
– Zoom lens (Erikson and St. James)
– Multiple “split” beams
• Object-based theories: we attend to
objects
• Location and object-based
Location-based Attention
• Posner (1980)
– Attentional spotlight: a theory which holds that we can
move our attention around to focus on various parts
of our visual field
• Erikson & St. James (1986): zoom-lens model
• Spotlight is not necessarily at location of the fovea:
spotlight can move in the absence of eye movements.
Posner Cueing Task
Central cue
peripheral cue
cue
ISI
target
• Posner presented either a central cue or a peripheral
cue before a target appeared.
• Cues valid on 80% of the trials, invalid on 20% of the
trials or vice-versa
Posner Cueing Task
• When cues were valid,
response times were
faster than when cues
were invalid.
The Zoom Lens analogy
• Zoom lens is flexible in size
• Experimental subjects can control whether they focus on
a specific target (the middle letter of a five letter word) or
spread their attention (across all the letters) (La Berge,
1983)
Evidence in Favour of the Zoom-lens Model
•
Mean reaction time to the probe as a function of probe position. The probe was
presented at the time that a letter string would have been presented. Data from
LaBerge (1983).
Experiments Demonstrating Split Attention
• Awh and Pashler
(2000).
• (a) Shaded areas
indicate the cued
locations and the near
and far locations are
not cued; (b) probability
of target detection at
valid (left or right) and
invalid (near or far)
locations.
Alternative theory: attention is object based
• Experiment: are number
of bumps on the ends of
the objects the same?
• Faster judgments when
bumps are on the same
object (in spite of slightly
larger distance)
• compatible with an
object-based attention
theory
We can select a shape even when it is intertwined
among other similar shapes
Are the green items the same? On a surprise test at the
end, subjects were not able to recall shapes that had
been present but had not been attended in the task
 Evidence for object-based attention
Inhibition of return
• If we have looked at a particular region of space, it is
less likely we return to that region  bias favoring
novelty. It is better to inspect novel regions of a scene
Object based inhibition of return
• In last study, inhibition could have been applied to a
location or to a given object
• Tipper, Driver & Weaver (1991) showed that inhibition
moves with the inhibited object
• Evidence for an object-based theory of attention
Object Based Inhibition of
Return
Results
• If visual attention is object based, then it tracks the
object, not the region
• Object-based inhibition of return predicts that the
flickered object (rotating 90 deg counter clockwise) will
be attended less
• Results showed cueing effect was less at the objectbased location, in line with object-based theory of
attention
Disorders of Visual Attention
Disorders of Visual Attention
• Neglect
– Failure to acknowledge objects in the field contralateral
to the lesion
– Often no perceptual deficit
• Extinction
– Neglect patients sometimes detect a single stimulus
presented to their left visual field, but fail to detect the
same stimulus when another stimulus is presented to
the right of it
Neglect of the Visual Field
Patients may:
•
•
•
•
fail to dress the left side of their body
disclaim “ownership” of left limbs
not recognize familiar people presented on
the left side
deny the illness
Neglect of the Visual Field
Difficulty crossing out items
Difficulty copying items
55 y.o. right handed male R
TPJ infarct (Mesulam, 2000)
Visual neglect syndrome is object-based
Patients can neglect the left side of the object, rather than the left side of
space. This is because the neglect is object-centred and follows the object
rather than person-centred. Black lines show expected left-sided personcentred versus red lines showing actual point where the patient neglected
Marshall and Halligan, 1993
Visual neglect syndrome is object-based
When a right neglect patient is shown a dumbbell that rotates,
the patient continues to neglect the object that had been on the
right, even though It is now on the left (Behrmann & Tipper, 1999).
Simultanagnosic (Balint Syndrome) patients only attend
to one object at a time
Simultanagnosic patients cannot judge the relative length of two
lines, but they can tell that a figure made by connecting the ends
of the lines is not a rectangle but a trapezoid (Holmes & Horax, 1919).
Balint patients can only attend to one object at a time
even if they are overlapping
Luria, 1959
What Happens to Unattended Visual Stimuli?
•
Explicit processing
requiring conscious
readout is impaired
measure neglect
patients are at chance
matching pictures in
contralesional field
•
However, neglect
patient still show implicit
processing: a lexical
decision priming
experiment shows there
is some semantic
processing at
contralesional field
McGlinchey-Berroth et al. (1993)
Visual Search
Examples of Visual Search
Is there a threat?
Where’s Waldo?
Is there a red circle?
EASY
MORE DIFFICULT
Search times can be influenced by set size
is there a black circle?
Serial Search
• Assumes that items are examined one at a time.
• Search terminates when the target is found or all items
have been examined.
• If the target is present, on average, how many items
need to be examined?
– roughly half
– (n+1)/2
– where n = size of the search set
• If absent, all items will be examined.
RT curves for parallel / serial search
Feature-integration theory
(Treisman & Gelade, 1980)
• Different visual features
are coded in parallel in
separate feature maps.
orientation
size
• Visual search is easy
(“pop out”) when it
involves only a single
feature that can be
computed by a feature
map – no attention is
required
 parallel search
color
E.g., find the blue circle:
Conjunction search
• Visual search becomes more
difficult when conjunctions of
features are involved:
• e.g., is there is red circle?
• Theory states that attention is
needed on a particular location
to synthesize its features into
an object
• Attentional spotlight can only
be deployed locally  viewer
must apply serial search
Illusory conjunctions
(Snyder, 1972; Treisman & Schmidt, 1982)
• Prediction of theory: if attention can conjoin features
correctly, the lack of attention can lead to incorrect
(illusory) conjunctions?
Read the vertical line of digits in the following display
For unattended locations, subjects might report illusory
conjunctions of features, e.g. blue “O”
Problem for Feature Integration Theory
X
O
X
X
O
O
O
X
X
O
X
O
X
Some conjunctions are easy and produce fast search times.
(e.g. Theeuwes and Kooi, 1994)
Guided Search
X
O
X
X
O
O
O
X
X
O
X
O
X
• Guided search model (Wolfe) is a modification of feature
integration theory
• Separate processes search for Xs and for white things
(because they are the target features), and there is a
consequent area of double activation that draws attention
to the target.
Attention is often drawn to local differences
“find the vertical line”
(easy search)
“find the vertical line”
(hard search)
Guided Search (Wolfe)
Computes saliency maps. Activation determined:
1. Bottom up:
Attention is attracted to items that are highly dissimilar from their
neighbors (local differences).

Top-down attentional set.
If you are looking for your car keys, you are looking for:
1. Something silver and shiny
2. Something small and key shaped