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Chapter 6: Visual Attention

Chapter 6: Visual Attention. Overview of Questions. Why do we pay attention to some parts of a scene but not to others? Do we have to pay attention to something to perceive it? Does paying attention to an object make the object “stand out”?. Attention and Perceiving the Environment.

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Chapter 6: Visual Attention

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  1. Chapter 6: Visual Attention

  2. Overview of Questions • Why do we pay attention to some parts of a scene but not to others? • Do we have to pay attention to something to perceive it? • Does paying attention to an object make the object “stand out”?

  3. Attention and Perceiving the Environment • Divided attention - paying attention to more than one thing at one time • This ability is limited, which has an impact on how much we can process at once • Selective attention - focusing on specific objects and filtering out others

  4. Why is Selective Attention Necessary? • Economy: Some aspects of the environment are more important and interesting than others • The visual system has evolved to operate in this fashion • There is too much incoming stimulation at the retina to process everything • Selection is achieved partially through use of the fovea

  5. How is Selective Attention Achieved? • Scanning a scene - eye movements can take in different parts of a scene • Measuring eye movements - camera-based eye trackers show: • Saccades - small, rapid eye movements • Fixations - pauses in eye movements that indicate where a person is attending • Approximately 3 fixations per second

  6. What Determines Location of Fixations? • Characteristics of the scene: • Stimulus salience - areas of stimuli that attract attention due to their properties • Color, contrast, and orientation are relevant properties • Saliency maps show fixations are related to such properties in the initial scanning process • Bottom-up process that is unrelated to meaning (it’s automatic)

  7. A visual scene and a computer-generated salience map based on color, contrast, and edge orientation.

  8. Characteristics of the Scene • Picture meaning and observer knowledge • Scene schema - prior knowledge about what is found in typical scenes • Fixations are influenced by this knowledge • Influence of the observer’s task • Task demands override stimulus saliency • Eye movements are usually preceded by motor movements by fraction of a second

  9. Does Perception Require Attention? (Li et al., 2002) • Observers focused on center of display where 5 letters were flashed. Task: Are letters the same or different? • A photograph of a scene was flashed in the periphery for 27 ms followed by a mask • Position of scene moved for each trial • Duration of scene was too short for eye movements to occur • Observers could tell whether an animal was in the scene 76% of the time

  10. Perception doesn’t require attention!

  11. When Is Attention Necessary for Perception? • Inattentional blindness - a stimulus is not perceived even when the person is looking directly at it. • Simons and Chabris Gorilla Experiment, 1999 • 46% of observers fail to report the gorilla

  12. A simpler experiment (Mack and Rock, 1998)

  13. When Is Attention Necessary for Perception? - continued • Change blindness • Observers were shown a picture with and without a missing element in an alternating fashion with a blank screen • Results showed that the pictures had to alternate a number of times before the change was detected • When a cue is added to show where to attend, observers noticed change more quickly

  14. Simons and Levin, 1998

  15. When Is Attention Necessary for Perception? - continued • Change blindness blindness • People are “blind” to the fact that they experience change blindness • Past experience shows people that they do notice sudden changes • Disrupting attention leads to missing changes

  16. Attention “focuses and sharpens” interesting objects.

  17. Effect of Attention on Information Processing • Experiment by Posner et al. • Observers saw a square with two lights on each side • Precueing was used to indicate on which side the light would turn on • Lights turned on consistent or inconsistent with the cue • Task was to push button when light was seen

  18. Experiment by Posner et al. • Results showed that observers responded fastest when cue was consistent with light • Information processing is most efficient where attention is directed

  19. Posner et al. (1980) • Results showed that observers responded fastest when cue was consistent with light • Information processing is most efficient where attention is directed

  20. Egly et al. (1994). Attention spreads through objects and doesn’t “radiate” evenly.

  21. Effects of Attention on Perception • Experiment by Carrasco et al. • Observers saw two grating stimuli with either similar or different contrast between the bars • Task was to fixate on center point between gratings and indicate orientation of bars with higher contrast • Small dot was flashed very quickly on one side before gratings appeared

  22. Experiment by Carrasco et al. • Results showed that: • When there was a large difference in contrast, the dot had no effect • When the contrast was the same, observers were more likely to report that the grating preceded by the dot had higher contrast • Thus the shift of attention led to an effect on the perception

  23. Attention can enhance brightness! Carrasco et al. (2004).

  24. Attention and Experiencing a Coherent World • Binding - process by which features are combined to create perception of coherent objects • Binding problem - features of objects are processed separately in different areas of the brain • So, how does binding occur?

  25. Feature Integration Theory • Treisman and Gelade • Preattentive stage - features of objects are separated • Focused attention stage - features are bound into a coherent perception • Attention serves as the “glue” between the physiology of the what and where streams

  26. Figure 6.20 (a) A single object. Binding features is simple in this case because all of the features are at one location. (b) When multiple objects with many features are present, binding becomes more complicated.

  27. Illusory Conjunctions • Features that should be associated with an object become incorrectly associated with another • Experiment by Triesman & Schmidt • Stimulus was 4 shapes flanked by 2 numbers • Display flashed briefly followed by a mask • Task was to report numbers first followed by shapes at 4 locations

  28. Treisman and Schmidt’s (1982)

  29. Look for Letters! XTRMJ

  30. Illusory Conjunctions - continued • Results showed that: • Incorrect associations of features with objects occurred 18% of the time • Asking observers to focus on the target objects eliminated this effect • Balint’s syndrome - patients with parietal lobe damage show lack of focused attention results in incorrect combinations of features

  31. Visual Search • Conjunction search - finding target with two or more features • Patients with parietal lobe damage cannot perform conjunction searches well compared to people without such damage • Parietal lobe is the destination for the where stream

  32. Table 6.1

  33. Physiological Approach to Binding(Bringing it all together) • Synchrony hypothesis - neurons firing to same object synchronize with each other • The firing of the neurons shows the same pattern • Synchrony may also occur between neurons firing for different qualities of the same object • Attention may help synchronize neural firing

  34. The Physiology of Attention • Experiment by Colby et al. • Monkey trained to keep eyes fixated on a dot while peripheral light was flashed on the right • Task in “fixation only” condition: • Keep eyes fixated and release lever when dot dimmed • Task in “fixation and attention” condition: • Keep eyes fixated and release lever when peripheral light dimmed

  35. Experiment by Colby et al. • Recordings from a neuron in the parietal lobe that responded to the peripheral light were made while monkey did task • Neuron responded well when monkey was attending to peripheral light • Neuron responded poorly when monkey was not attending to it • Important to note that response was due to attention since the eyes were always fixated on dot • Attention is a spotlight that may or may not be shining on the object of fixation

  36. Do Neurons Notice Stimuli? • Experiment by Sheinberg & Logothetis • Monkey was trained to pull lever different directions for specific objects • Recorded from IT neurons in monkey’s cortex that responded to specific objects • Recordings showed response to both single object and to object placed in a scene • When monkey scanned a scene containing the target objects, the neuron only fired when monkey noticed the object

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