Psych 216: Movement

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Psych 216Movement

• Attention

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What is attention?

• There is too much information available in the
  world to process it all.
• Demonstration change-detection performance
• Thus, attention mechanisms exist to select the
  most relevant subset of the available information
  and focus processing resources on that subset.

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Overt and covert mechanisms

• One way to influence what is processed is via
  overt movements. For example, moving your head or
  eyes toward stimuli leads to faster and more
  accurate processing of those stimuli due to
  having more receptors in your fovea.
• However, selective processing of stimuli can
  occur in the absence of any movements, this is
  known as covert attentional selection.

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What is selected?

• Our goals (top-down factors) and the properties
  of the stimuli themselves (bottom-up factors)
  co-determine what is selected.
• This interaction of factors has been extensively
  studied using visual search tasks.
• Visual search tasks are those in which subjects
  are instructed to look for a certain stimulus
  presented within an array of irrelevant stimuli.

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Attention and eye movements

• One of the primary functions of visual attention
  is to select stimuli that will be the targets of
  future eye movements.
• Movements take a long time to program and execute
  (by the cognitive clock). For example, human eye
  movements take approximately 200-250 ms to
  produce where as the effects of attentional
  selection of stimuli can be observed in as little
  as 20-100 ms.

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Behavioral evidence that covert attention is fast
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Electrophysiological evidence that covert
selection is fast
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Rapid covert selection during visual search
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Neuropsychological studies of attention

• Different types of patients manifest different
  types of attention deficits (or even advantages).
• 1) Split-brain patients
• 2) Visual Neglect
• 3) Balints Syndrome

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Split-brain patients

• In order to alleviate severe epilepsy some
  patients have had their corpus callosum cut.
• This creates a situation in which the two
  hemispheres of the brain are essentially
  independent. For example, only the left
  hemisphere can read.

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Two brains are better than one

• Because covert visual attention mechanisms are
  not lateralized split-brain patients can perform
  visual search at almost twice the rate of normal
  subjects. This suggests that as normals our two
  hemispheres are linked such that we only have one
  focus of attention during visual search.

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Neglect Extinction

• Damage to the right parietal lobe (usually from a
  stroke) leads to unilateral neglect
• Patients do not attend to the contralateral side
  of space (usually the left side following a right
  hemisphere lesion)
• Although they have intact sensory inputs, they
  fail to respond
• Patients typically recover over a period of weeks
  or months, but may be left with long-lasting
  extinction
• They will respond to a single event on the
  contralateral side, but they fail when a stimulus
  is presented simultaneously on the good side

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Line Cancellation Test
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Copying
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A Sensory Deficit?

• Some neurologists believed that neglect is a
  result of diffuse damage to sensory systems
• Several sources of evidence now indicate that
  neglect is not a result of sensory deficits
• Neglect in imagery
• Object-based effects

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Mental Imagery
The Piazza del Duomo in Milan, Italy
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Object-Based Neglect
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Features Conjunctions

• In a visual search experiment, patients with
  neglect showed normal search speeds when looking
  for a simple-feature target

• When they searched for a color-form conjunction
  target, search times were abnormally slowed if
  the target was in the contralateral field

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Balints Syndrome

• Patient R.M. has bilateral parietal damage,
  leading to a complete lack of focused attention
  (Balints Syndrome)

• He can detect simple features, but he cannot
  localize them
• He can perform simple feature search tasks
  accurately, but he is at chance in conjunction
  search tasks
• His performance is improved by an artificial
  attentional prosthesis

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Imaging studies of attention

• Functional imaging studies have been conducted in
  which blood flow or oxygen consumption in the
  brain was measured while visual attention tasks
  were performed by to determine what parts of the
  brain control covert attentional selection.
• These studies show parietal lobe activation
  during visual search and cueing tasks. This
  activity is consistent with several existing
  hypotheses.
• 1) That parietal cortex controls the voluntary
  orienting of attention to a location of interest.
• 2) That parietal cortex controls the reorienting
  of attention to new locations once one object or
  location has been attended.

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Corbetta et al. (2000)
Measured blood flow while subjects performed a
cueing task.