Lec3: Spatial attention and spatial neglect Flashcards
Definition of spatial neglect
neglect is a failure to report, respond or orient to novel or meaningful stimuli presented to the side opposite to the brain lesion, when this failure cannot be attributed to either sensory or motor deficits
How do we assess spatial neglect
- standardized tests for apatial attention (e.g., letter and bell cancellation tasks to calculate CoC score, does the score show a lateral bias)
- differential diagnostic (are motor and visual functions adequate?is there a visual field defect? is there extinction?
- neuroanatomical assessment (is there evidence for a brain lesion in a critical area?)
- check patients’ mean to the defined mean. (how much deviation) insignificant, then fit the disease definition
give examples of tests used to assess spatial neglect
line bisection task: typically deviate rightward awat from the true midpoint
cancellation task: typically fail to explore and detect targets on the left side of the display
ota cancellation task: encircling all rings without a gap. missing targets on the left side of the sheet (space-based or egocentric neglect) as well as misses of targets with a left gap (object-based or allocentric neglect).
Copy of line drawing of amulti-itemscene. omission in the left space and omission of the left side of items, compression of space with items clumped together on the right side, underuse of the left side
Perceptual extinction is elicited by comparing the detection of two stimuli presented simultaneously on either side of space, relative to the detection of same stiuli presented alone. patients visualt hemianopia (or sensory loss) often perceive the contralesional (left) stimulus in bilateral but not unilateral stimulation.
what is the critical lesion in spatial neglect
it depends on the assessment method.
a network of interconnected areas that can be anatomically remote but share the same function.
frontal, temporal, parietal lobe
How does brain implements spatial attention?
Intraparietal area (LIP) play independent roles in target selection for saccades and the generation of visual attention
LIP acts as a priority map in which objects are representedby activity proportional to their behavioural priority
Priority map combines bottom-up inputs like a rapid visual response with an array of top-down signals like a saccade plan
The spatial location representing the peak of the map is used by the oculomotor system to target saccades and by the visual system to guide visual attention
what are the two major hypothsis of mechanism of spatial neglect
attention hypothesis: bias in attention in favor of the ipsilesional hemispace
transformation hypothesis:
1) error in transforming sensory input into a supra-modal frame of reference.
2) ipsilsional shift of the subjective egocenter
what are the other hypothsis of mechanism of spatial neglect and what are their limitations
premotor hypothesis: a deficit in the oculo motor pragmatic maps
representation hypothesis: a deficit in memory representations
these hypothesis have limited predictions for rehabilitation and assessment
what are the evidence supporting attentional hypothesis
- symmetric, topographical organization of the fronto-parietal cortex (larger response to left visual field, symmetric activation of attentional system)
- interhemispheric competition in spatial attention
use rTMS to stimulate parietal cortex at parameters know to induce visual lesion , contralateral targets show extinction, iplateral enhance accuracy - exploratory movements are normally directed to salient stimuli (e.g., saliency/ priority map, movemnt follows focus attention thus attention problems lead to postural problems
summary of evidence supporting attentional hypothesis for spatial neglect
fMRI, TMS and eye movement evidence support the hypothsis that a lesion in the frontal-parietal network can cause an attention deficit on the side contralateral to the lesion
the bias in exploration and posture can be a consequence of this postural deficit
problem with attention hypothesis
sensorimotor manipulations alleviate spatial neglect,
this means manipulation postural alleviate attention problems
neck muscle vibration
caloric vestibular stimulation
prism adaptation
somatosensory cortex lesion
what are the evidence supporting transformation hypothsis
ask Ss with right parietal lesion and marked left-sided neglect to direct a laser point to the position which they felt to lie exactly ‘straight ahead’ of their bodies orientation
neglect Ss localized the body’s sagittal midplane 15degrees to the right of the objective orientation
neglect Ss’ horizontal displacement of sagittal midplane to the right could be compensated for either by neck muscle vibration or by caloric vestibular stimulation on the left side
when vestibular stimulation was combined with neck muscle vibration, the horizontal deviation linearly combined by adding or neutralizing the effects observed when both types of stimulation were applied exclusively in the control groups as well as in the neglect patients
neck muscle proprioception and vestibular stimulation directly interact in contributing to the subject’s mental representation of spece
data suggest that the afferent information from these different input channels is used simultaneously for computing egocentric, body-centred coordinaates that allow us to determine our body position in space
what are the problems with transformation hypothesis
it assumes a deviation of the egocenter in neglect patients. This is not always the case
if the deviation of the egocenter is causal in neglect, one would expect a positive correlation between this deviation and neglect signs. this is not the case.
the restoration of the egocenter should improve neglect sgins. this is not always the case
an experimental deviation of the egocenter in healthy should cause neglect. this is not the case
what is shift in priority map hypothesis
spatial neglect is an error in transforming priority maps between visual and body centered coordinates
Neglect shoed improved performance when the target is at the right to the stimulus
what are the evidence for shift in priority map hypothesis
1) one experiment showed a 1~2 degree shift in priority map, Neglect SS showed ipsilesional shift in the locus of attention (location with the largest effect of the cue) for all tested locations the shift in cueing error correlates with neglect severity
2) Neglect showed improved performance when the target is at the right to the stimulus
3) we locate visual objects relative to the body (same location on the retina but different locations in the space around the body
4)localization/ egocenter
corollary discharge of the oculomotor command and eye muscle proprioception provide eye position information to the brain
only the efference coy is used whereas proprioception is a slow recalibrator of the forward model
in normal conditions, eye proprioception is not used for visual localization
eye proprioception is , however, continuously monitored to be incorporated into the eye position estimate when a mismatch with the efference copy of the motor command is detected
what is problem with shift in priority map hypothesis
why is the eye position input distorted in neglect?
can only a 1 degree shift in the priority maps explain severe neglect?
visual information not enough to tell us the location of the objects
we need to integrate sensorimotor information about body posture (e.g., eye position, head position, body midline…)
would it matter if we find out that the problem in spatial neglect is a corrupted eye position input from somatosensory cortex to the brain’s priority maps
