The attending brain part II Flashcards
Hemi-spatial neglect
A failure to attend to stimuli on the oppisute side of space to a brain lesion
Hemineglect, also known as unilateral neglect, hemispatial neglect or spatial neglect, is a common and disabling condition following brain damage in which patients fail to be aware of items to one side of space. Neglect is most prominent and long-lasting after damage to the right hemisphere of the human brain, particularly following a stroke. Such individuals with right-sided brain damage often fail to be aware of objects to their left, demonstrating neglect of leftward items.
The deficit may be so profound that patients are unaware of large objects, even people, towards their neglected orcontralesional side - the side of space opposite brain damage. They may eat from only one side of a plate, write on one side of a page, shave or make-up only the non-neglected or ipsilesional side of their face (same side as brain damage). Their drawings may fail to include items towards the neglected side, for example when placing the numbers in a drawing of a clock (Fig.1). Many patients are often also unaware they have a deficit (anosognosia).
Classically, the neglect syndrome has been associated with damage to the right posterior parietal cortex. More recent studies have begun to challenge this view, suggesting instead that a more widespread network of areas may be involved, including those that have been implicated in studies of neuroimaging of attention (discussed further in theNeuroanatomy of neglect below; see also Corbetta & Shulman, 2002 and Husain and Rorden, 2003). Differences in the location and extent of lesions (brain damage) across patients may contribute to the heterogeneity of the condition.
Tests of Hemi-spatial Neglect
copying
line bisection
drawing from memory
cancellation
Neglect is a Disorder of ____ and not _____
Neglect is a Disorder of Attention and Not Low-Level Perception
•Visual stimuli presented to neglect patients __________________ in occipital lobes, even though the patients are not aware of them.
still activate visual regions
Neglect is a Disorder of ?
Neglect is a Disorder of Attention and Not Low-Level Perception
• Visual stimuli presented to neglect patients still activate visual
regions in occipital lobes, even though the patients are not
aware of them.
• It’s not just visual: it affects auditory and tactile judgments as
well (e.g. sounds on left are mislocalized but still heard)
• If cued on the left part of the visual field, patients are often
able to detect objects there, especially if the object is presented
alone
▫ Phenomenon of “visual extinction” suggests different perceptual
representations are competing for attention (and visual
awareness)
VISUAL EXTINCTION
•If cued on the left part of the visual field, patients are often able to detect objects there, especially if the object is presented alone
▫Phenomenon of “visual extinction” suggests different perceptual representations are competing for attention (and visual awareness)
stimuli compete for attention in neglect condition
WHAT HAPPENS TO NEGLECTED INFORMATION?
- Some of it at least may not be completely lost: Ventral stream may continue to process neglected objects up to the stage of object recognition and possibly semantics
- Evidence:
▫Recognition of a previously neglected line drawing is facilitated when it’s subsequently presented on the attended side of space (even if it’s degraded) (Vuilleumier et al., 2002)
▫Burning house experiment (Marshall & Halligan, 1988).
Two houses on the left (neglected side), differed in flames, patients could not tell the difference, but when prompted,
stated they’d rather live in the house without flames!
•
BALINTS SYNDROME
three clinical characteristics
A severe difficulty in spatial processing normally following bilateral lesions of parietal lobe; symptoms include simultanagnosia, optic ataxia, and optic apraxia.
Three clinical characteristics of Balint’s syndrome
(1) Inability to perceive more than one object at a time (simultanagnosia)
(2) Inability to reach in the proper direction for an object under visual guidance (optic ataxia)
(3) Impaired fixation of gaze without a primary deficit of eye movement (optic apraxia)
(4) Basic visual abilities normal (e.g. visual acuity, color perception, contrast sensitivity)
Simultanagnosia
Inability to perceive more than one object at a time. The premotor theory of attention
*Balints Syndrome Simultanagnosic patients can / cannot
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?”)
Balints syndrome : Patient rm has simultanagnosia -
Patient RM has simultanagnosia:
Unable to locate objects verbally or by reaching and pointing
Impaired at locating sounds
BUT: they can state which side of body and which body part is being touched (body space may be different from external space)
Different kinds of neglect
(LIST)
REVEAL DIFFERENT TYPES OF SPATIAL MAPS
• Perceptual vs. representational neglect
• Near vs. far-spaced neglect
• Personal vs. peripersonal neglect
• Object-based vs. space-based neglect
o Theories of attention
Feature Integration Theory
▪ Biased Theory of Attention
▪ Premotor Theory of Attention
Perceptual vs. representational neglect
(seeing and imagining)
A double dissociation has been found between perceptual and representational neglect, suggesting the brain uses different spatial reference frames for representational (mental imagery) and perceptual space.
** THE SPATIAL MAP YOU FORM FROM MEMORIES IS DIFFERENT FROM ACTUALLY BEING THERE
Different spatial reference frames for near and far space
Double dissociation between near and far space
Near space = tested with line bisection using pen and paper
Far space = tested with line bisection using projected image and laser pointer
“Near” defined as within reach; if patient is given a stick rather than a pointer then “near” deficit extends into far space
Different Spatial Reference Frames for object-based and space-based neglect
Some 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
Some 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
FIT and object perception
“illusory conjunctions”
What if you cant pay attention? (eg because objects are presented too quickly)
▫ Report a conjunction of features that you never actually saw
Illusory conjunctions provide evidence that attention is needed to combine an object’s features correctly
FIT and visual search (Feature search)
FIT and visual search:
Case 1
Recall: FIT says attention is needed to bind together multiple features
But only one feature is needed to find the target (So this kind of feature search doesn’t need attention) - Reaction time is independent of number of distractors
Case 2:
• Try to find the blue “T” in the array ((Two features needed to find the target – Color and shape))
Now two features are needed to find the target So this kind of conjunction search DOES need attention — More distractors, longer reaction time
FIT
• Conjunction search:
- Feature search: a single feature is enough to identify the target, attention not needed, RT does not increase with increasing distractors
- Conjunction search: must find a combination of two features to identify the target, attention is needed, RT increases with increasing distractors
Evidence for Semantic Processing of Unattended Stimuli: Negative Priming
Biased Competition Theory
Evidence
Within ventral stream: neurons reduce their specificity of responding when multiple stimuli in their receptive field (whole is less than sum of parts)
• Within parietal lobes: brain damage affects attention when multiple stimuli compete (i.e. extinction)
Premotor Theory of Attention
Evidence (1/2)
Rizzolatti et al. (1987): A centrally presented digit (e.g. 2) indicates where target a likely to appear, but it may sometimes appear elsewhere.
• An unexpected shift of attention is harder when crossing midline (location 3 requires new saccade in the opposite direction) than not (location 1 only requires modifying existing saccade in the same direction).
Notice locations 1 and 3 are equidistant from location 2.
Premotor Theory of Attention
Evidence (2/2)
Moore & Fallah (2001). Monkeys had to press a lever when detecting a change in luminance in a specific region of the visual field.
Detection was faster when Frontal Eye Fields (a prefrontal area involved in planning saccades) were weakly stimulated electrically:
- weakly enough not to produce a saccade.
- importantly, it only facilitated detection if the change in luminance happened in the appropriate receptive field.
