PSYC20007 Lecture Notes - Lecture 8: Visual Search, Simple Features, Mental Chronometry
14 Jun 2018
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Lecture 8 - Tuesday 12 September 2017
PSYC20007 - COGNITIVE PSYCHOLOGY
LECTURE 8
ATTENTION IN SPACE & TIME
THE PSYCHOLOGICAL FUNCTION OF SPATIAL ATTENTION
•(1) We have spatial attention to assign limited-capacity processing resources to relevant stimuli in
environment.
•In order to do so, we must locate stimuli relevant to behavioural goals among the distractors and
process (identify) them.
VISUAL SEARCH
•(2) The best way of making this task a lab
task is known as a visual search.
•Using letters as stimuli you have to identify if
letters are present or not.
•(3) Reaction time is what’s usually
measured; we measure mean RT as a
function of display size.
•(4) Some
tasks are
really easy.
•(5) Some are
very difficult.
POP OUT
EFFECTS
•(6) Some search targets seem to ‘pop out’ from the
background easily (4); others require more attention to be
identified (5)
POP OUT EFFECTS WITH SIMPLE FEATURES
•(7) Unique colours and unique orientations both pop out,
we don’t have to search for the red target or the vertical blue
block.
PARALLEL SEARCH FOR FEATURE TARGETS
•(8) Mean RT doesn’t increase with display size; it stays constant, for both target
present and target absent trials. This is
interesting. It could mean that what we have to
do is make a decision that every one of the
stimuli in the display is or isn’t a target.
•Compare contents of each display location with
mental representation of target at the same
time; parallel search
CONJUNCTION TARGETS DO NOT ‘POP
OUT’
•(9) The target is now defined by a combination
of 2 features which increases the difficulty.
•This loses the pop out effect. RT increases linearly with display size as difficulty increases.
•The slope is twice as steep for target absent and target present trials.
Lecture 8 - Tuesday 12 September 2017
PSYC20007 - COGNITIVE PSYCHOLOGY
EVIDENCE FOR SERIAL SEARCH
•(10) The above results are consistent with a serial search model.
•Seem to need to focus attention on target to detect it – focus attention on each item in turn.
•Constant scanning rate predicts linear RT/display size function.
SELF-TERMINATING SERIAL SEARCH
•(11) Stop when target is found.
•On average, we search half the display on target-present trials and all of the display on target-
absent trials.
•Constant scanning rate predicts 2:1 slope ratio
POP OUT EFFECTS WITH LETTER STIMULI
•(12) Pop out when targets can be identified by a single features (straight lines among curves or
vice versa).
•No pop out when targets can’t be identified by a single feature (straight lines among straight lines
or curves among curves).
FEATURE INTEGRATION THEORY
•(13) Treisman & Gelade, 1980
•Role of attention is to bind features into perceptual compounds.
•Each feature (lines, colours etc.) registered in its own feature map.
•Without attention, features are free-floating and may lead to illusory conjunctions.
•The role of visual attention is to bind the component features that make up a perceptual object
into coherent perceptual wholes.
•Each stimuli are registered in their own map, so the features making up a perceptual map aren’t
conjoined; it is the job of visual attention to make them together.
•(14) Conjunction targets require feature binding, so need
focused attention – leads to serial search.
•Feature targets don’t require feature binding, don’t need focused
attention – leads to parallel search.
•Lots of experiments show this.
PROBLEMS WITH FEATURE INTEGRATION
THEORY
•(15) Pop out sometimes depends on complex object
properties, not just simple features (Enns & Rensink,
1990)
•High-level, not low-level properties predict pop out.
•Inconsistent with idea that pop out only occurs at level
of simple features.
EFFICIENT vs INEFFICIENT SEARCH
•(16) Many tasks show intermediate pattern, don’t
provide clear evidence of either serial or parallel search
•Wolfe: better described as inefficient or efficient search
•No evidence of dichotomous population of search
slopes; parallel and serial functions look like ends of
continuum
GUIDED SEARCH THEORY
•(17) Wolfe, 1989
•Two-stage theory
•Initial parallel stage provides a candidate list of possible targets
Document Summary
Effects: (6) some search targets seem to pop out" from the background easily (4); others require more attention to be identified (5) Pop out effects with simple features: (7) unique colours and unique orientations both pop out, we don"t have to search for the red target or the vertical blue block. Parallel search for feature targets: (8) mean rt doesn"t increase with display size; it stays constant, for both target present and target absent trials. Out(cid:282: (9) the target is now defined by a combination of 2 features which increases the difficulty, this loses the pop out effect. Rt increases linearly with display size as difficulty increases: the slope is twice as steep for target absent and target present trials. Self-terminating serial search: (11) stop when target is found, on average, we search half the display on target-present trials and all of the display on target- absent trials, constant scanning rate predicts 2:1 slope ratio.
