1. Selective Visual Attention & Visual Search

2. 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

3. 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.

4. 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

5. Posner Cueing Task
When cues were valid, response times were faster than when cues were invalid.

6. 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)

7. 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).

8. 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.

9. 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

10. 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

11. 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
If we vary the time between the cue and target in a modified Posner paradigm, we find that when the Cue-Target-Onset-Asynchrony (CTOA) gets to around ms, reaction time to the target begins to increase. This is called Inhibition-of-return (Klein, 2000).
To get this effect we actually have to attract attention to the target location and then attract it back to the origin. IOR is one of many examples of an inhibition effect being produced by attention.

12. 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

13. Object Based Inhibition of Return

14. 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

15. Disorders of Visual Attention

16. 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

17. 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  
Chaterjee : Neglect patients sometimes confabulate letters to the left side of short words, reading short words as longer than their objective length
E.g., “pot” is read as ”teapot”

18. Neglect of the Visual Field
Difficulty crossing out items
Difficulty copying items
55 y.o. right handed male R TPJ infarct (Mesulam, 2000)

19. 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 person-centred versus red lines showing actual point where the patient neglected
Marshall and Halligan, 1993

20. 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).

21. Simultanagnosic (Balint Syndrome) patients only attend to one object at a time
Simultanagnosia: inability to identify and attend to more than 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).

22. Balint patients can only attend to one object at a time even if they are overlapping
Balint patient: When he failed to recognize a word, he often reported single letters; sometimes able to piece together a word by serially identifying single letters
Luria, 1959

23. 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)

24. Visual Search

25. Examples of Visual Search
Is there a threat?
Where’s Waldo?

26. Is there a red circle?
MORE DIFFICULT
EASY

27. Search times can be influenced by set size
is there a black circle?

28. 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.

29. RT curves for parallel / serial search

31. Feature-integration theory (Treisman & Gelade, 1980)
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
orientation
size
color
E.g., find the blue circle:

32. 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

33. 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”

34. Problem for Feature Integration TheoryX X O O X X O X O O X O X
Some conjunctions are easy and produce fast search times.
(e.g. Theeuwes and Kooi, 1994)

35. Guided Search X X O O X X O X O 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.

36. Attention is often drawn to local differences
“find the vertical line”
(easy search)
“find the vertical line”
(hard search)

37. Guided Search (Wolfe) Computes saliency maps. Activation determined:
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:
Something silver and shiny
Something small and key shaped