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Chapter 8

Chapter 8 Attention and Scene Perception.docx

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Chapter 8 Attention and Scene Perception Visual acuity is limited in the periphery. Hence you cant read more than 2 books at a time. even if the letters are large. ^ general problem that the retinal array contains more information than that can be processed Why can’t we process everything at once?  We don’t have the brains for it. recall recognizing a single object requires a sizeable chunk of the brain and its processing power.  Processing everything all at once requires a brain that will not fit in the human head. If itisn’t possible to process everything then what should be processesd? --- we pay attention to some stimuli and not others. Attention- is not a single thing and it doesnot have a single locus in the brain.but attention is the name given to family of mechanisms that restrict processing in various ways  Can be overt or covert  Overt attention- refers to directing a sense organ at a stimulus.  covert attention -refers to discreet attention while stimulus organ is pointed somewhere else.  Divided attention – paying attention simultaneously to two things  Sustained attention – paying attention for prolonged period of time  In this chapter, mostly concerned with selective attention.  Selective attention - the form of attention involved when processing is restricted to a subset of possible stimuli. The ability to pick one or few things to focus on.  But these terms are not mutually exclusive.  Operational mechanisms operate in all of the senses not just visual attention  We can also use attentional mechanisms to give one sense priority over another Selection in Space What does it mean to “pay attention”? Michael Posner – created the classic situation. The subject must hit the response key as fast as possible when the probe appears.  Measure of interest is the reaction time (RT)- amount of time that elapses between the point when probe appears and the point when the subject hits the response key.  What if the subject is given a cue? Cue – is a stimulus that might indicate where(OR what) a subsequent stimulus will be.  Valid cue – when the probe appears in the cued location. - RT decreases when valid cue. - RT is faster because the participant is paying attention to the current location.  No cue condition – RT is longe(=slower) compared to valid cue  Invalid cue –occurs when the probe id misleading. Appears in the area opposite of the cued location. - RTs are slower here than in the control condition because the subject has been fooled into attending to the wrong location.  The experimenter doesnot tell the participants whether the trial will be valid or invalid  Peripheral cue vs symbolic cue How long does it take for a cue to redirect our attention? Depends on the nature of the cue. Time 2: when the cue appears Time 3: when the probe appears Stimulus onset asynchrony: the time between the onset of one stimulus and the onset of another. SOA When SOA is o millisec, no time for it to be used to direct attention and there is no diff between the effects of a valid or invalid cue. When SOA is 150 ms, the magnitude of the cueing effect from a valid PERIPHERAL cue increases and plateaus. Symbolic cues take longer to work because we need to do ome work to interpret the arrow. BUT some symbolic cue work like fast, peripheral cues. The spotlight of attention Does attention actually move from one point to another Attention could be deployed from one point to the next in several ways. 1. It might move in the manner analogous to the movement of the eyes.when we shift our gaze, our point of gaze sweeps across the intervening space—attention might sweep across space in similar manner like a spotlight beam. 2. It might expand from the fixation point, growing to fill the region from the fixation spot to the cued location.---zoom lens model 3. When attention is removed from fixation point, it might just melt away etc. Visual Search Visual search experiments- looking for a target in a display containing distracting elements. - Provide a closer approximation of the actions of attention than posner expmnts. - Target item, distractor item - Visual searches are ubiquitous in everyday life but vary in their difficulty and ease. - It is harder to find a target as the number of items increases. - One standard measure ; how much time is added for each item added to the display - As the tasks become harder, the slope relating RT to set size grows steeper. - Saying yes is faster than saying no because even in the hardest task, it is easier to spot the target item than to spot the absence of a target item. - Improving search tasks : efficiency –directing attention to the target as soon as the display appears. Diff types of test tasks differ in in their efficiency. Feature Searches are efficient Feature search – search for a target defined by a single attribute If feature is salient, doesn’t matter how many distracters they are. The target seems to pop out of the display. -we can process the color or orientation of all the items at once parallel - when we measure the RT, it doesn’t change with the set size There are about dozen or so diff basic attributes that support parallel visual search. Many searches are inefficient When the target and basic distracters in a visual search task contain the same basic features, search is inefficient. Serial self-terminating search - A search from item to item ending when a target is found or until all the items are checked. T-among-Ls search interesting is that they are inefficient even though items are easy to see and very familiar. In Real-World Searches, Basic features Guide Visual Search Guided search- search in which attention can be restricted to a subset of possible items on the basis of info about the target items basic features. Vs conjuction searches, no single feature defines the target. Instead, the target is defiend by the conjuction or co-occurrence of two or more features Two-feature conjuction searches lie between the very efficient feature searches and inefficient serial searches.but conjuctions can be made more complex to get closer approximations. The Binding Problem in Visual Search Binding problem : we might be able to analyze a collection of basic features in a preattentive to the object.. the challenge of tying different attributes of visual stimuli which are handled by diff brain circuits to the appropriate object so we percieve a unified object. Feature integration theory - A limited set basic features can be processed in parallel preattentively but that other properties require attention. - Includes guided search - We can use some preattentive feature information to guide the choice of what should be attended and bound. If attention is needed to bind features correctly, what happens if we don’t have time to complete the job? -illusory conjuction Attending in Time : RSVP and Attentional Blink RSVP – rapid serial visual presentation - An experimental procedure in which stimuli appear in a stream at one location. - Rate of presentation - But the task need to be limited to simple characters Attentional blink : - The difficulty in perceiving and responding to the second of the two target stimuli amid a rapid stream of distracting stimuli if the observer has nd responded to the first target stimulus with in 200 to 500msec before the 2 stimulus was presented. - Its as if our ability to visually attend to the characters in RSVP is temporarily knocked out - Fish analogy provided by Chun. - Performance is good if T2 appears immediately after T1 scooping two fishes at once! - Metaphor reveals two processes at work; fast process can identify each object as it appears and disappears. And a slower process is required if the observer will actually respond to a particular item. - Example of two different group who perform differe
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