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Chapter 7

PSYB57H3 Chapter Notes - Chapter 7: Visual Search, Attentional Blink, Test Probe

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George Cree

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Chapter 7: Attention and Scene Perception
Attention: Any of the very large set of selective processes in the brain. To
deal with the impossibility of handling all inputs at once, the nervous system
has evolved mechanisms that are able to restrict processing to a subset of
things, places, ideas, or moments in time.
Here are some of the distinctions we can make when considering varieties of
- Attention can be internal or external. External attention refers to
attention to stimuli in the world (our primary concern here), but we
should not forget internal attention, our ability to attend to one line of
thought as opposed to another or to select one response over another.
- Attention can be overt or covert. Overt attention usually refers to
directing a sense organ at a stimulus fixating the eyes on a single word,
for example. It you point your eyes at this page while directing attention
to a person of interest off to the left, you are engaging in covert attention.
- Reading this text while continuing to be aware of music playing in the
room is an example of divided attention.
- Watching the pot to note the moment the water begins to boil is sustained
- In this chapter we will be most concerned with selective attention, the
ability to pick one (or a few) out of many stimuli.
Selection in Space:
A good place to start is with a cueing experiment of the sort pioneered by
Michael Posner. Start with the situation shown in Figure 7.3a. The subject in
the experiment fixates on a central point (*). After a variable delay, a test
proble, X, appears in one of the two boxes. All of the subject needs to do is hit
a response key as fast as possible when the probe appears.
Reaction time (RT): A measure of the time from the onset of a stimulus to a
Suppose now that the situation is slightly changed: during the waiting period,
the subject is given a cue, a stimulus that provides a hint about where the
target might appear. In Figure 7.3b, the cue is a change in the outline colour
of one of the two boxes call this a peripheral cue. Because the test probe
appears in the cued location, this peripheral cue is said to be a valid cue.
Cue: A stimulus that might indicate where (or what) a subsequent stimulus
will be. Cues can be valid (giving correct information), invalid (incorrect), or
neutral (uninformative).
In Figure 7.3c, we have a different kind of cue. The red dot is a symbolic
cue, but it can also direct attention. (ere the rule is )f the cue is green, the
probe is likely to be on the left. If the cue is red, the probe is likely to be on
the right. )n Figure 7.c, however, the cue is misleading, or invalid, because
the cued location is on the right but the probe appears on the left.
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Stimulus onset asynchrony (SOA): The time between the onset of one
stimulus and the onset of another.
Visual Search
Visual search experiments provide a closer
Approximation of some of the actions of attention
In the real world. In a typical visual search experiment,
The observer looks for a target item among distractor
Visual search: Search for a target in a display
Containing distracting elements.
Target: the goal of a visual search.
Set size: The number of items in a visual display.
Feature Searchers Are Efficient
Feature search: Search for a target defined by a
Single attribute, such as a salient colour or orientation.
Salience: The vividness of a stimulus relative to
Its neighbours.
Parallel search: A search in which multiple
stimuli are processed at the same time.
Many Searches Are Inefficient
Serial self-terminating search, in which items are examined one after another
(serially) either until the target is found or until all items have been checked.
In Real-World Searchers, Basic Features Guide Visual Search
Guided search: Search in which attention can be restricted to a subset of
possible items on the basis of information about the target item’s basic
features (e.g. its colour)
Conjunction search: Search for a target defined by the presense of two or
more attributes (e.g. a red, vertical target among red horizontal and blue
vertical distractions)
In Real-World Searchers, the Real World Guides Visual Search
Scene-based guidance: Information in our understanding of scenes that helps
us find specific objects in scenes (e.g. objects do not float in air, faucets are
near sinks).
This brief introduction to scenes should raise questions in your mind:
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