Chapter 9 Flashcards
Spatial attention processes
Spatial attention is the ability to focus on or orient to spatial locations.
Spatial attention can be facilitated or disrupted by valid respectively invalid spatial clues. The disengagement of attention takes time.
Subtypes of spatial attention
-Covert orienting: without moving the eyes or head.
- Overt orienting: moving the eyes or head along with the focus of attention.
- Exogeneous orienting: externally guided by a stimulus.
- Endogeneous orienting: guided by the goals of the perceiver.
Non-spatial attention processes
Attention can also be directed to objects, to features, or in the temporal domain. Example: attentional blink.
General theory of attention
- Attention affects sensory processing, e.g. in vision.
- The actual attentional processes happen mainly in a frontoparietal network.
Parietal lobes
- Are specialized for spatial processing and have been called the “where” route.
- Bring together different types of spatial representation that are needed for action (e.g. integrating visual space with body space) so also called the “how” route.
Lateral intraparietal areas (LIP)
Single cell electrophysiology shows that LIP:
- has motor properties (generates saccades).
- has sensory properties (both sound and vision), enables sounds to be remapped to eye-centred coordinates.
Important for attention because:
- doesn’t respond to all sensory stimuli (sparseness).
- responds more to stimuli that are unexpected such as sudden flash (i.e. important for exogenous attention).
- responds more to stimuli that are task relevant (i.e. important for endogenous attention).
- can enable covert and overt orienting (with/without eyes).
- codes a spatial ‘salience map’.
Intraparietal sulcus (IPS)
fMRI evidence from humans
- Orienting cues (e.g. arrow) activate IPS and a frontal region, FEF, involved in saccades even if no overt motor response needed.
- Similar network involved in object-based attention.
Fronto-parietal attention mechanisms
- Dorso-dorsal route (including IPS) involved in orienting in a salience map (e.g. from cues).
- Ventro-dorsal route (involving TPJ) acts as a ‘circuit breaker’ (e.g. detecting targets; disengagement).
Hemispheric differences
Spatial attention
Parietal lobes represent full visual field (unlike earlier regions) but in a graded fashion
- BUT: brain damage in humans to right parietal lobe has more profound effects (‘neglect’).
- Possible right IPS contributes more to salience map (natural tendency to be drawn to left).
Non-spatial attention
- Corbetta et al. claim right TPJ has more important role in attention.
- Mevorach et al. suggest right parietal lobe important for attending to salient stimulus; and left parietal lobe for attending to non-salient stimulus (evidence from TMS).
Interference = difference in efficiency for incongruent trials and efficiency for congruent trials.
Biased competition theory
- Competition occurs at multiple stages rather than at some fixed bottleneck.
- Competition occurs in parallel at most stages with serial processing linked to the motor output stage (e.g., you can only fixate one location at a time).
Evidence from parietal lobe damage:
- Focal IPS lesions impair (re)orienting to the contralesional side of space.
Disorders of attention
(1) Bálint’s syndrome, (2) hemispatial neglect, (3) neglect dyslexia
Bálint’s syndrome
Clinical characteristics:
- Inability to perceive more than one object at a time (simultanagnosia).
- Inability to reach in the proper direction for an object under visual guidance (optic ataxia).
- Failure to make proper eye movements without a primary deficit of eye movement (optic/ocular apraxia).
Other characteristics:
- Basic visual abilities normal (e.g. visual acuity, colour perception, contrast sensitivity).
- Unable to locate objects verbally or by reaching and pointing.
- Impaired at locating sounds.
- Can state which side of body and which body part touched (body space may be different from external space).
Neural basis –> single case GK
Spatial localisation –> top down influences
- Case GK could compute face orientation (91%) but not location of ovals (55%).
- Performance improves if other face-like features added (bottom).
Summary:
- Simultanagnosic patients can recognize single objects but cannot compute spatial relationships between objects.
- But when is an object an object? E.g. is a face an object, or several objects (e.g. eyes, nose etc.)?
- Evidence suggests that these patients can group several parts into wholes if they share color, shape, or are connected.
- Top-down factors important too (e.g. “are ovals at top or bottom?” versus “is face the correct way up?”).
Hemispatial neglect
Failure to attend to stimuli at one half of space.
Clinical characteristics: daily life, copying, drawing from memory, cancellation task.
Neural basis: right inferior parietal lobe.
Disorder of attention and not low-level perception:
- Neglect patients still activate visual regions in occipital lobes for stimuli that they claim not to be aware of.
- They are often able to detect objects on the left if cued there.
- Affects auditory and tactile judgments as well as vision (e.g. sounds on left are mislocalized but still heard).
- Phenomenon of visual extinction suggests different perceptual representations are competing for attention (and visual awareness).
Hemispatial neglet versus blindsight
Hemispatial neglect:
- Lack of awareness is not restricted to vision and may be found for other sensory modalities.
- Whole objects may be processed implicitly.
- Lack of awareness can often be overcome by directing attention to neglected region
- Neglect patients often fail to voluntarily move their eyes to neglected region.
- Neglected region is egocentric; relative to the position of the body midline.
Blindsight:
- Lack of awareness is restricted to visual modality.
- Implicit knowledge is restricted to basic visual discriminations (direction of motion).
- Lack of awareness not overcome by directing attention to “blind” region.
- Blindsight patients do move their eyes into “blind” region.
- Blind region is retinocentric.
Allocentric = relative to other stimuli/objects.
Different types of hemispatial neglect
- Perceptual versus representational neglect (dissociation).
- Different spatial reference frames
- Near versus far space
Near space: tested with line bisection using
pen and paper
Far space: tested with line bisection using
projected image and laser pointer - Body versus near space
Body or personal neglect = failure to groom
left of body (fluff test) or notice position of
limbs
Near space or peripersonal neglect = visual
search of array of external objects - Object-based versus space-basedSome neglect patients attend to objects on
the left side of space but omit to attend to
one half of the object itself (object-based
neglect)
Forms a double dissociation with space-
based neglect
More ventral lesions, also
including white matter
- Near versus far space
