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Chapter 6

Sensation and Perception Psych 367 Chapter 6.docx

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Douglas Wylie

Chapter 6 Visual attention Attention and perceiving the environment - Divided attention: paying attention to a number of things at once - Selective attention: focusing on specific objects and ignoring others Why is selective attention necessary - We look at things that are interesting - Problem the visual system faces is that there is so much information being sent from the retina to the brain that if the visual system had to deal with all of it, it would overload - To deal with this problem, the visual system is designed to select only a small part of this information to process and analyze How is selective attention achieved - Eye movement is one mechanism - We can pay attention to things that are not directly on our line of vision - We can look directly at something without paying attention to it - There is a mental aspect of attention that occurs in addition to eye movement - James o We focus on some things to the exclusion of others What determines how we scan a scene - Fixations: places where the eye pauses to take in information about specific parts of the scene - Saccades: the lines connecting the dots are eye movements Stimulus salience - Characteristics of the environment that stand out because of physical properties such as colour, brightness, contrast or orientation - Capturing attention by stimulus salience is a bottom up process o It depends on the pattern of stimulation falling on the receptors - Saliency map: taking into account three characteristics of display o Colour o Contrast o Orientation - Parkhurst measured where people fixate when presented with various pictures o Initial fixations were closely associated with the saliency map, with fixations being more likely on high saliency areas Knowledge about scenes - Knowledge we have about the things helps determine where we look Nature of the observer’s task - A person carrying out a task, the demands of the task override factors such as stimulus saliency Learning from past experiences - Shinoda o Measured observers fixations and tested their ability to detect traffic signs as they drove through a environment simulator o Observers were more likely to detect stop signs positioned at intersections than those positioned in the middle of a block - Salient characteristics may capture a person’s initial attention, but cognitive factors become more important as the observers knowledge of the meaning of the scene begins determining where he or she fixates How does attention affect our ability to perceive Perception can occur without focused attention - Reddy o We can take information from a rapidly presented photograph of a face that is located off to the side form where we are attending o Observers looked at the + on the dization screen, and then saw the central stimulus (array of 5 letters) o Letter followed by peripheral stimulus (a picture of a face or a disk half green half red o Three conditions in this experiment  Central task: letters are flashed in the center of screen, task is to indicate whether all of the letters are the same  Peripheral task: letters are flashed in center, task is to indicate whether a face flashed off to the side is male or female, or disk is red green or green red  Dual task: task is to indicate both letter similarity and face stimulus o When observers did the tasks one at a time they performed well o Dual task was almost just as high - Some perception is possible even in the absence of focused attention - Performance of the red green task was around 50% (lower) - All of the factors o Meaningfulness, experience and perceiving as a whole, could make it possible to categorize faces as male or females without focusing attention on the face Perception can e affected by a lack of focused attention - Inattentional blindness: failure to perceive a stimulus that isn’t attended, even if it is in full view Inattentional blindness - Rock o Observers task is to indicate which arm of a briefly flashed cross is longer o A small test object is flashed close to where the observer is looking along with the cross - Simons and Chabris o Created a situation in which one part of a scene is attended and the other is not o Half of the observers failed to report that they say the woman or gorilla - Observers are attending to one sequence of events, they can fail to notice another event, even if right in front of them Change detection - Rensink o Presented one picture, followed by a black field, followed by the same picture but with an item missing o Pictures were alternated in this way until observers were able to determine what was different about the two pictures - Change blindness: difficulty in detecting changes in scenes - When rensink added a cue indicating which part of a scene had been changed, subjects detected the change more quickly - Continuity errors: changes in film, are spotted by viewers who are looking for them - One reason people think they would see the changes may be that they know from past experience that changes that occur in real life are usually easy to see - Changes that occur in real life are often accompanied by motion Does attention enhance perception Effects of attention on information processing - Posner o Does attention to a specific location improve our ability to respond rapidly to a stimulus presented at that location? o Precueing o Observers kept their eyes stationary looking at the + o First saw an arrow cue indicating on which side of the target a stimulus was likely to appear o Observers task is to press a key as fast as possible when a target square is presented off to the side o Valid trial: the square appears on the side indicated by the cue arrow o Observers react more rapidly on valid trials then invalid trials o Information processing is more effective at the place where attention is directed - When attention is directed to one place on an object, the enhancing effect of this attention spreads throughout the object - Egly o Observers first saw two side by side rectangles, and then a pre cue flash at one place o Reaction time was faster when the target appeared where the cue signal had been presented o Observers responded faster
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