Visual Attention

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Visual Attention

• Chapter 6

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Attention

• Attention is a very important aspect of
  perception as we cannot process all the inputs to
  our senses
• E.g Priority of visual over auditory attention
  when reading?? Even if there is music on in the
  background.
• Attention refers to a large set of selective
  mechanisms that allow you to focus on some
  stimuli at the expense of others
• Attention is a general term for mechanisms that
  select certain stimuli for further processing.

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Attention

• Divided attention paying attention to a number
  of things at once e.g.
• Selective Attention focusing on specific
  objects and ignoring others
• Why? design of the visual system
• Eye movements to scan a scene

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Scanning a scene

• Study of eye movements and how to measure it
• Saccades eye movements - with pauses
  (fixations) fig.6.4
• What determines the fixations? depends on the
  scene characteristics, its meaning, our knowledge
  and goals.
• Leads to individual variation in saccades and
  fixations

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Is attention necessary for perception?

• Can perception occur without attention?
• Figure 6.7. experiment that shows perception
  without attention
• However, attention is necessary to determine
  specific details of a scene
• Figure 6.8 Inattentional blindness
• Figure 6.9 when attending to a particular
  sequence of events, we may fail to notice another
  that occurs simultaneously in our visual field

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Attention

• Change Detection spot the difference
  demonstration
• The pictures may have to be alternated back and
  forth a number of times before people can detect
  the differences
• Change blindness difficulty in detecting
  changes in scenes illustrates the importance of
  attention for perception

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Effects of attention on information processing

• Does attention to a specific location improve
  processing of a stimulus presented at that
  location?
• Precueing experiment in figure 6.14 shows
  that information processing is more efficient at
  the place where attention is directed

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Effects of attention on perception

• Can attending to an object have an effect on its
  appearance?
• Experiment can attention enhance the perceived
  contrast of grating stimuli (alternating light
  and dark bars)?
• Observers look at mark and indicate the
  orientation of the grating that has the higher
  contrast

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Attention

• Another function of attention
• Binding the process by which features (colour,
  form, motion, location) are combined to create
  our perception of a coherent object the binding
  problem
• Feature integration theory preattentive stage,
  focused attention stage proposes that focused
  attention is necessary for binding
• Illusory conjuctions under some conditions the
  features associated with one object can be
  incorrectly associated with another

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Visual Search

• Looking for an object among other objects
• Typical search experiment observer looks for a
  target item among some number of distractor
  items.
• Real world visual searches face in a crowd, car
  in a parking lot, book on a shelf
• Measure reaction time for an observer to find a
  target to indicate yes if target is present or
  no if not
• Target may be present in 50 and absent in the
  other 50 of the trials

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Visual Search Experiments

• Calculate mean reaction time for each set size,
  for target present and target absent trials
• What makes some search tasks easy and others more
  difficult?
• find the target (a red vertical bar)
• Figure where is this easiest?
• What is the difference between each of these
  searches?

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Visual Search

• Two factors being varied
• number of items in the display general rule
  harder to find a target as the number of
  distractors increases - number of items in the
  display increases from top to bottom.
• From left to right the nature of the task changes
  searches can be easier (efficient) or harder
  (inefficient), depending on the relationship of
  the target to the distractor items

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Feature Searches are Efficient

• Feature search search for a target defined by a
  single attribute, such as a salient colour or
  orientation
• The defining feature may be sufficiently salient
  so as to stand out visually from neighbouring
  features in this case it may not matter how may
  distractors there are
• In this case - process the colour or orientation
  of all items at once (in parallel) indicated by
  the v. short reaction times

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Visual Search

• Visual search is inefficient when the target and
  distractors share the same basic features final
  column in figure all items in each display have
  red, blue, horizontal and vertical features
  inefficiency is indicated by the increase in the
  reaction times
• Conjunction search search for a target defined
  by the presence of two or more attributes e.g. a
  red, vertical target among red horizontal and
  blue vertical distractors

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Attending in time

• Rapid serial visual presentation (RSVP) an
  experimental procedure in which a stream of
  stimuli appear at one location at a rapid rate
  (8 per second)
• Example viewing a stream of letters that all
  appear at the same location in space determine
  if there is a number in the stream of letters
• How fast can the characters be presented and
  still permit you to do the task with high
  accuracy?

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Attending in time

• With fairly large, clearly visible stimuli, you
  can reliably pick a digit out of the letters when
  the characters are appearing at a rate of 8 to 10
  items per second
• Not limited to simple characters - Other examples
  determine if a particular image is in a stream
  of rapidly presented photographs
• Find two digits in the stream of letters?

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Attending in time

• Critical variable the interval between the two
  digits
• If second digit appears 200-300ms after the
  first, and the first is correctly reported -
  likely to miss second digit attentional blink -
  Attending to the first digit can make it
  difficult to attend to the second

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Attentional blink

• 2 separate processes
• The first process monitors the RSVP sequence and
  identifies each letter as it goes by.
• The second process comes into play when you
  actually have to do somethinge.g., make a
  decision about whether there is an A or a B in
  the sequence.
• The second process takes away resources from the
  first process

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Attentional blink

• See an A deploy attention noting this is the
  first target small amount of time in which
  there are not enough attentional resources left
  over to detect and note the second target.
• After 300 ms or so attention can be redeployed
  back to the first process allowing the second
  target to be accurately detected
• Why is performance good at the very initial times
  (see graph)?

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Attentional blink

• two consecutive letters / digits can be responded
  to simultaneously.
• The second process will act on both targets
  together if they occur very close to one another
  (e.g. with no distractors between them)
• But when at least one letter intervenes between
  the first and second targets, the attentional
  blink comes into effect making the second
  target difficult to detect