Attention

Psy391h - Classes 13-14

(This page last updated 3 April 2006.)
 

 Why is attention necesary?

• Convergence within the WHAT pathway
  • Higher levels of the hierarchy have larger and larger receptive fields.
  • At the top levels , each neuron can receive input from many different locations and many different objects.
  • Potential for interference and crosstalk.
  • The flow of information through the What Pathway has to be regulated to prevent interference.
  • Information from some objects must be blocked so that information from other objects can be fully processed.
  • Gating of information.
  • This selection process is often referred to as attention.
  • The same sort of selection process is needed outside visual processing.
    • Within the auditory system and other perceptual systems.
    • To select between different perceptual modalities.
    • To select internal memories and thoughts for attention.

     

Two Different Types of Visual Attention

• Overt Attention
  • Shifts of eye position
• Covert Attention
  • Selecting certain locations or objects within a fixation, without moving the eyes.
  • First studied by Helmholtz in 1894

Two Different Types of Factor Affect how Attention is Allocated

• Voluntary Attention
  • Governed by the subject's goals
  • Endogenous
  • Top-down
• Reflexive Attention
  • Driven by salient aspects of the stimulus
  • Exogenous
  • Automatic
  • Bottom-up

The neural responses to perceptual stimuli are shaped by attentional selection.

• Effects of attention demonstrated experimentally.
  • Same stimuli used in different conditions.
  • Only the allocation of attention is different across conditions.
  • Differences in neural response show effects of attention.

Measuring Attention with ERP's

• Measuring endogenous visual attention
  • Stimulus on left in both conditions.
  • In one condition, subject expects stimulus on left and attends there.
  • In other, attention is on right.
  • P1 response in occipital lobe contralateral to stimulus is greater when that location is attended.
    • ~100 msec after stimulus onset
  • Also differences between upper and lower visual field targets.
  • Inhibition of return at longer delays.
• Similar P1 effects, although somewhat weaker, with exogenous attention.
• Also attentional effects in N2pc
  • 200-300 msec after stimulus onset
  • pc = posterior contralateral
• Stronger on attended side than on unattended side.
• Can be used to track movement of attention from one side to the other.
  • Visual search task.
  • Target has gap on left.
  • On 75% of trials, target is red.
  • On 25% of trials, target is blue.
  • Attend first to red, then blue.
  • On some trials, red and blue will be on same side of display.
    • Attention should stay on that side.
  • On other trials, red and blue will be on opposite sides.
    • Attention should shift from one side to the other.

ERP studies show the temporal dynamics of attention. To get better information about the spatial layout of the brain images involved, we need studies using PET and fMRI.

Attention modulates visual processing in a number of brain areas.

• When task requires attention to color, activity increases in extrastriate areas devoted to color.
  • Similar pattern for motion and shape.
• Attentional modulation is strongest in higher areas of the ventral (what) pathway.
• Harder to find attentional modulation in V1.
• Possible that there is attentional modulation in LGN.
  • Connections from cortex back down to LGN
  • However, evidence for attentional effects is weak.
• No connections back to retina to allow attentional effects there.

Spatial attention can help to control interference from distractor objects.

• Compare brain activity when target object appears alone, and when it appears with distractors.
  • When distractors are present and target is attended, response is similar to when target appears alone.
• Additional evidence on attentional control of distractor interference comes from single-cell recordings.
  • In one block of trials, the monkey attends to stimuli with one color. In another block, monkey attends to the other color.
  • Two stimuli with different colors in the neuron's receptive field.
    • One (effective) triggers a strong response in this neuron.
    • The other (ineffective) does not.
  • When ineffective stimulus is attended, neural response is weak, similar to response when effective stimulus is not present.

The above experiments demonstrate the measurement of neural changes associated with allocating visual attention to a specific location. Other experiments have measured neural changes when attention is object-based, rather than location-based.

• Two objects overlapping, occupying same region in visual field
  • one face, one house
  • In some experiments, each stimulus is presented to a different eye, producing rivalry.
• Attention to face produces activity in fusiform face area (FFA)
• Attention to house produces activity in parahippocampal place area (PPA)
• Shift attention from one stimulus to the other produces corresponding shift in brain activity from one area to the other.

Some brain areas serve to control attention.

• parietal cortex
  • Task requires shifting attention to different locations.
    • Shifts generate activity in posterior parietal cortex.
  • Similar activity during visual search, which also requires attentional shifts.
  • For patients with parietal lobe damage, the delay for a stimulus at an unexpected location contralateral to lesion is much greater.
    • This difficulty in shifting attention contralateral to lesion comparable to neglect.
  • Shifts from one sensory modality to another can also produce parietal activity.
  • Parietal damage can also cause spatial neglect and extinction.
    • There is some evidence of neglect in imagery as well.
    • Neglect can be object-based rather than location-based.
• pulvinar
  • Inhibitiing pulvinar limits attentional benefit.
  • Activating pulvinar increases attentional benefit.
• superior colliculus
  • Single cell activity can be recorded during saccades (fast eye movements).
  • Inhibiting superior colliculus can increase distractor interference.

     

next: Memory

Psych 391h: Cognitive Neuroscience Kyle Cave Psychology Dept. U. Mass.