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Chapter 8

Chapter 8 - PSYB51

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University of Toronto St. George
Mathias Niemier

Chapter 8 – Attention and Scene Perception  Attention: any of very large set of selective processes in brain. To deal with impossibility of handling all inputs at once, nervous system has evolved mechanisms that are able to restrict processing to subset of things, plates, ideas or moments in time o Overt attention: directing sense organ at a stimulus – fixating eyes on single word o Covert attention: pointing eyes while directing attention to person of interest off to left o Divided attention: reading text while continuing to be aware of music playing o Sustained attention: watching pot to note movement of water begins to boil  Selective attention: form of attention involved when processing is restricted to subset of possible stimuli  Use attentional mechanisms to give priority to one sense over others Selection in Space  Posner – subject in experiment fixates on central point (*) and after variable delay, test probe appears in one of 2 boxes. All subject needs to do is hit response key as fast as possible when probe appears  Reaction time: measure of time from onset of stimulus to response  Cue: stimulus that might indicate where (or what) subsequent stimulus will be. Cues can be valid (correct info), invalid (incorrect), or neutral (uninformative) o Given valid cue, Posner found that reaction time decreases o Compared to no-cue control, subject generally responds faster to probe, because she is “paying attention” to correct location o Could have valid or invalid instances of either peripheral or symbolic cues  In typical experiment, cue might be valid on 80% of trials and invalid on remaining 20%  How long does it take for cue to redirect attention? – depends on nature of cue  Stimulus onset asynchrony (SOA): time between onset of 1 stimulus and onset of another o If SOA is 0 ms, cue and probe appear simultaneously o as SOA increases to about 150 ms, magnitude of cueing effect from valid peripheral cue increases, but after, effect of cue levels declines a bit  symbolic cues take longer to work presumably because need to do work to interpret arrow The “Spotlight” Of Attention  In cueing experiment, attention starts at fixation point and ends up at cued location  Spotlight metaphor makes good sense and has become most common way for cognitive psychologists to talk about attention  Evidence suggests that attention isn’t moving from point to point the way a physical spotlight would move Visual Search  Cueing experiments provide important insight into deployment of attention  Visual search: looking for target in display containing distracting elements  Target: goal of visual search  Distractor: in visual search, any stimulus other than target  Visual searches are ubiquitous in real world – we look for faces in crowds, mugs in cupboard, etc.  Quest to understand what makes some search tasks easy and others hard proved to be one of most productive and interesting lines of cognitive psych  Set size: number of items in visual display. As general rule, harder to find target as number of items increases  One of standard measures of ease of search is to ask how much time is added (on average) for each item added to display  Observers perform same type of search over and over while experimenter measures RT required for subject to say “yes” if target is present or “no” if there isn’t target in display  In typical experiment target is present 50% of trials and absent in other 50% o At end, researcher averages RTs for target present trials, and upper line represents RTs for target absent trials  One way to compare search tasks is to talk about “Efficiency” with which we can work way through display o If we can direct attention to target as soon as display appears, regardless of set seize, we have efficient search Feature Searches Are Efficient  Feature search: search for target defined by single attribute, such as salient color or orientation  Salient: vividness of stimulus relative to its neighbors  Parallel: in visual attention, referring to processing of multiple stimuli at same time  When measuring RT, doesn’t change with set size  Between 1 dozen and 2 dozen basic attributes seem to be able to support parallel visual search Many Searches are Inefficient  When target and distractors in visual search task contain same basic features, search is inefficient  Serial self-terminating search: search from item to item, ending when target is found In Real World Searches, Basic Features Guide Visual Search  In real world, it would be rare to have true feature search for only red item among homogeneous distractors  Guided search: search in which attention can be restricted to subset of possible items on basis of info about target item’s basic features  Conjunction searches: search for target defined by presence of 2 or more attributes. No single feature defines target  In efficiency, 2 feature conjunction searches tend to lie between very efficient feature searches The Binding Problem in Visual Search  Binding problem: challenge of tying different attributes of visual stimuli which are handled by different brain circuits, to appropriate object so that we perceive unified object  Preattentive stage: processing of stimulus that occurs before selective attention is deployed to that stimulus  Idea that there is preattentive stage of basic feature processing followed by second, attention-demanding stage is core of Anne Treisman’s feature integration theory  Feature integration theory: theory of visual attention, which holds that a limited set of basic features can be processed in parallel preattentively, but that other properties, including correct binding to features to objects, require attention  Illusory conjunction: erroneous combination of 2 features in visual scene  We conjoin only those features that are actually in display  When we can’t complete task of binding, we do best we can with info we have Attending in Time: RSVP and Attentional Blink  Rapid serial visual presentation (RSVP): experimental procedure in which stimuli appear in stream at one location (typically point of fixation) at a rapid rate (typically about 8/sec)  Attentional blink: difficulty in perceiving and responding to second of 2 target stimuli amid rapid stream of distracting stimuli if observer has responded to first target stimulus within 200-500 ms before second stimulus is presented  Chun suggests metaphor – imagine you’re fishing with net in less-than-pristine stream, with boots and tires and occasional fish floating or swimming by o You can monitor stream and identify each item as it passes – boot, tire, fish, boot and so on o You can dip net in and catch fish and once catch fish, takes time before you can get back into water to catch more and might miss fish if it swims by too soon o Metaphor illustrates idea that 2 processes are at work  Fast process can identify each object as it appears and disappears  Slower process is required if observer will actually respond to particular item  Green and Bavelier took 2 groups of non-video game players where one group got experience with first person shooter game, and other group played tetris o First person shooter group improved on attentional tasks; and others didn’t o Suggests that attentional abilities can be changed by training and that those first person shooter video games produce change  Repetition blindness: failure to detect second occurrence of letter, word, or picture in rapidly presented stream of stimuli when second occurrence falls within 200-500 ms of first Physiological Basis of Attention  In oft quoted passage, William James declared, “everyone knows what attention is”  Difficult to know what attention does because attention performs variety of tasks Attention Could Enhance Neural Activity  If asked to attend to one location in visual field, as in a Posner cueing task, neurons that respond to stimuli in that part of field will become more active  Used to be thought that response didn’t apply to striate cortex, first part of visual cortex to process visual input  fMRI experiments in humans and electrophysiological studies of monkeys, have shown that even first stages of cortical processing are influenced by attention  Effects seen at early stages in cortex are quite possibly results of feedback from later stages of processing Attention Could Enhance Processing of Specific Type of Stimulus  Mechanisms
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