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Perception
24 September 2014

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21:06

Lecture 1 - perceiving and recognizing objects
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Book : Sensation and perception by Wolfe, 3rd edition
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this means
our brains are doing something pretty complicated after V1
 Extrastriate cortex = Region of cortex bordering the primary visual cortex and containing multiple
areas involved in visual processing
□ Includes areas V2, V3, V4
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part of the "what" pathway

Lecture 3
• Vision is best within the fovea, then rapidly declines
○ Even with big letters, we cannot read the left and the right sides at the same time
• Selective attention = states that we pay attention to some stimuli but not others
○ Processing is restricted to one set of stimuli
• Varieties of attention
○ External = attending to stimuli in the world
 Main focus of lecture
 Ex = looking at the slide during the lecture
Psychology Page 1

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 Ex = looking at the slide during the lecture
○ Internal= attending to one line of thought over the other or selecting one response over another
○ Overt= directing a sense organ toward a stimulus
 Ex = pointing your eyes at something or turning your head
○ Covert = attending without giving outward sign that you are doing so
 Ex = at a party, pretend to listen to a friend but you are really looking at someone else
○ Divided = splitting attention between two different stimuli
○ Sustained = constantly monitoring some stimulus
Reaction time = a measure of the time from the onset of a stimulus to a response
Cue = a stimulus that might indicate where (or what) a subsequent stimulus will be
○ Can be valid, invalid, or neutral
○ Cueing experiments = good tool for examining attention
Stimulus onset asynchrony = the time between the onset of one stimulus and the onset of another
Selection in space = cueing as a tool for examining attention
○ The larger the area attended to, the less time each object gets
The Posner cueing paradigm = simple probe detection experiment
○ Test = hit correct key when probe (x) appears…measures reaction time
○ Gives insight into attention but is rather artificial, no search involved
 When test probe a[[ears in the same space as a valid cue, RT is less, and vice-versa
○ Red dot = symbolic cue (in the middle) …sometimes colour not space determines where x is
 Takes much longer for this symbollic cue to have an effect
Theories of attention
○ "Spotlight" model= attention is restricted in space and moves from one point to the next…areas within
the spotlight receive more processing
○ "Zoom-lens" model = the attended region can grow or shrink depending on the size of the area being
processed
Visual search = looking for a target in a display containing distracting elements
○ Ex = finding your friend in a crowd
○ Target - goal of visual search…distractor - any stimulus other than the target…set size - the number of
items in a visual search display
○ Feature search = target is defined by the presence of a single feature (ex = a different colour)
 Generally very efficient
 Salience = the vividness of an item relative to its neighbors
 Parallel = in visual attention, referring to the processing of multiple stimuli at the same time
○ Conjunction search = target defined by the conjunction (co-occurrence) of two or more features
 Most difficult of the 3 , no colour hints
○ Spatial configuration search = the target and distractors contain the same basic features
○ Generally harder to find the target as the set size increases
 Measured in terms of search slope…the larger the search slope, the less efficient the search was
 Serial self-terminating search =a search from item to item, ending when a target is found
○ When target is only in 50% of trials, can measure how much time is added for every new piece added
 Time is always more when the feature is not present
Real-world search
○ In real world searches, the real world helps to guide your search (ex = scene-based guidance)
○ Guided search = attention is restricted to a subset of possible items based on information about the
item's basic features (eg colour or shape)
○ Conjunction search = search for a target based on the presence of two or more attributes
 No single feature defines the target
○ The binding problem = the challenge of tying different attributes of visual stimuli, which are handled by
different brain circuits, to the appropriate object so we perceive a unified object
 Ex = a vertical red bar moving to the right
Psychology Page 2

 Ex = a vertical red bar moving to the right
□ Color, motion, and orientation are represented by different neurons
□ So how do we combine these features?
○ Feature integration theory = Treisman's theory of visual attention, which holds that a limited set of basic
features can be processed in parallel preattentively, but that other properties, including the correct
binding of features to objects, require attention
 Preattentive stage = the processing of a stimulus that occurs before selective attention is deployed
to the stimulus
○ Illusory conjunction = an erroneous combination of two features in a visual scene
 Ex = seeing a red x in a scene that contains red letters but no red x's
 Provides evidence that some features are represented independantly and must be correctly bound
together with attention
○ Proto-objects = a loose collection of unbound features (size, color, etc
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preference vs

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