visual attention

Attention

mental processes for seeking out and focusing on relevant (visual) stimuli, which are consequently processed to a greater extent (at the expense of other stimuli)

The amount of information processed is limited

• sensory systems

• motor systems

Sensory systems

e.g., visual fixations, size of foveal gaze

Motor systems

e.g., movements planned/words spoken sequentially

Saccades

small, rapid eye movements

• saccadic suppression

• neither motion of the eye/motion blur nor gap in perception is detected

• begins before eye movement (thus, is not an optical effect)

Saccades suppression

attenuation in some spatial frequencies during saccade

Fixations

pauses that typically indicate where a person is attending

• about 3 fixations/second

fixation locations may be determined by:

• visual salience

• Picture meaning/observer knowledge

• task demands: what are you being asked to do?

Visual salience

colour, contrast, orientation (bottom-up process)

picture meaning/observer knowledge

what do you expect to see? (Top-down process)

Visual scenes are scanned to take in different parts; measured with camera-based eye-trackers

Saccades and fixations

Overt attention

directing foveal gaze (and attention) to a stimulus

Covert attention

directing attention to a stimulus in peripheral vision

Covert attention: Helmholtz (1896)

• put matrix of letters on wall, with fixation cross in the centre

• fixated on cross and turned lights off

• directed his attention (not his eyes) to a certain location, then briefly illuminated room with a spark

• could perceive letters in attended region, but not in other regions

• attention can be used to select stimuli independent of

Selective attention task

requires responding to a stimulus while ignoring others

• e.g., reading a textbook while trying to ignore interruptions

Divided attention task

requires attending to two or more simultaneous stimuli

• e.g., driving, drinking a coffee, and talking on cell phone at the same time

Michael Posner, Nissen, & Ogden (1978)

• attention viewed as a spotlight

• precueing (or Posner spatial cueing) task

Michael Posner, Nissen, & Ogden (1978): Components of attention

• disengaging attention from current location

• Moving attention to new location

• engaging attention in new location

Spotlight

can focus on certain locations

precueing (or Posner spatial cueing) task: Conditions

• neutral: no cue presented

• valid: arrow appeared, pointing to direction of subsequent stimulus

• invalid: arrow pointed in the wrong direction

Michael Posner, Nissen, & Ogden (1978): conclusions

• reaction time affected by type of cue

• attention can be directed to a particular location, which enhances perceptual processing

• if cue is invalid, attention must be disengaged, moved, and re-engaged

• attention is not only exogenous or “stimulus-driven” (bottom-up), but also endogenous or

  “goal-directed” (top-down)

Balint’s syndrome

• patients experience more illusory conjunctions; deficit in locating targets in conjunction

• Simultanagnosia: difficulty in attending to more than object at a time--even when overlapping:

  • supports the idea that attention can be object-based

  • syndrome may be due to inability to shift attention among object

Unilateral neglect (a.k.a. hemispatial neglect)

• deficit in awareness of items on one side of space, due to brain damage

• damage results in neglect of objects (or parts of objects) in the contralesional visual field

  • e.g., may eat food from just one side of a plate, shave/make-up one side of face, fail to Dress one half of their body, or copy only half of a drawing

• patients often unaware of this deficit

• typically due to damage in right posterior parietal cortex, critical to attention and selection

Unilateral neglect: explanations

• problem in disengaging attention from ipsilateral (same) side; cueing (directing attention) helped reduce spatial biases

• disruption of balance between hemispheres in directing attention; more stimuli on ipsilateral side hampers attention to contralateral (opposite side) items

Object based

left side of object neglected, rather than the left side of space

• e.g., control participant, copying drawing upright and rotated Neglect patient

  • patient with object neglect cannot detect differences on the left side of an object--even when it falls into the right visual field

Humphreys & colleagues (2010)

• participants: 41 brain damaged people with neglect

• stimuli: sheet with 150 drawings of apples:

  • 50 apples have a complete outline

  • 50 apples have a gap on their left side

  • 50 apples have a gap on their right side

• task: cross out all complete apples in 5 min

Humphreys & colleagues (2010): Two distinct patterns of results

• egocentric neglect

• Allocentric neglect

Egocentric neglect

unaware of one side of space:

• 11 people overlooked complete apples on the left side of the page; 2 people missed complete apples on the right side

• brain damage was clustered in right anterior cortical regions, and subcortical structures

Allocentric neglect

unaware of one side of objects:

• 8 people crossed out apples with gaps on their left side; 2 people crossed out apples with gaps on their right side

• brain damage was in right posterior cortical regions

Humphreys & colleagues (2010): Implications

• these different forms of neglect may reveal the nature of attentional processing

• attention may be ______ location-based or object-based

• different parts of the brain process different kinds of attention

• location-based processing is better suited to static scenes, whereas object-based processing works well for following a moving object in dynamic scenes

Damage to location

Based system produces egocentric neglect

Damage to object

Based system results in Allocentric neglect

Inattentional blindness

stimulus that is presented, but is not attended, is not perceived (a.k.a. “looked-but-didn’t-see” problem)

Mack & Rock (1998)

• participants briefly shown a cross

• task: which is the longer arm, horizontal or vertical?

• critical trial: small geometric shape also presented in display

• recognition task: identify the shape

• results: 25% correct (chance level) on critical trial

• conclusion: there is no conscious perception of the visual world without attention to it

Simons & Chabris (1999)