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

• 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

cue

target

ISI

Central cue peripheral cue

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 ReturnObject 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 object-based 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

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

Neglect of the Visual Field

Difficulty crossing out items

Difficulty copying items

55 y.o. right handed male R TPJ infarct (Mesulam, 2000)

Marshall and Halligan, 1993

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 person-centred versus red lines showing actual point where the patient neglected

Visual neglect syndrome is object-based

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 twolines, but they can tell that a figure made by connecting the endsof 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

• Different visual features are coded in parallel in separate feature maps.

• 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

size

orientation

Feature-integration theory (Treisman & Gelade, 1980)

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

Some conjunctions are easy and produce fast search times.

X

XX

X

O X

O

O OO

O

X

X

(e.g. Theeuwes and Kooi, 1994)

Guided Search

• 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.

X

XX

X

O X

O

O OO

O

X

X

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