Hemi-spatial neglect

• Define Hemi-spatial neglect ?

  Or unilateral neglect is an attention disorder that prevents the patient from attending to stimuli on one side
• Most frequent sites of HSN ?

  temporo-parietal junction and inferior frontal region
• Issues encountered by HSN patients can be ...

  • Space representation
  • Porblem with competing stimuli
  • Inability to respond to stimuli coming from one side
• HSN is a deficit of stimulus-driven attention.
• This is consistent with a deficit of stimulus-driven reorienting towards task-relevant targets where patients tend to disengage their attention from the relevant right side and reorient to the irrelevant left side
• Patients show defictis in situations where you have endogenous or voluntary attention, especially in search or cancellation tests.
• Do the patients have baises even in absence of stimuli ?

  Yes, they show spatial biases even in the absence of any (external) stimulus
• When trying to retrieve memory, HSN patients usually have a rightward bias during retrieval from long term memory. Plus research showed they have also an eye-movement bias during active visual search and rest.
• HSN are in fact complex combination of both spatial and non-spatial deficits
• Examples of spatial deficits vs non-spatial deficits ?

  • Spatial ones : shifting attention contralesional side, bias in attentional efficacy and representation of space
  • Non-spatial : anosognosia, alertness and motivation, pb of sustained attention, visual short term memory and executive deficits
• In N+ patients their low-level visual processing was preserved;
• What neuropsychological tests for HSN ?

  • Cancellation/search test = Apples/Ota cancelation tasks that can be for egocentric vs allocentric neglect
  • Line bisection
  • Copying or drawing a clock
  • Non spatial test like CPT = continuous performance tests and TEA = tests of everyday attention
• ALE meta analysis : activation likelihood estimation (here based on lesion coordinates) tried to associate lesion sites with deficits in various tests. They realised there are possible disssociation as a function of the symptom/deficits = so depending on your issue you might have a lesion in that specific part
• Lesions in inferior parietal lobe means there are deficits in line bisection, omission of words during reading.
• Lesions in temporal lobe : deficits for left-side of words and the left-side of targets in Ota search task, linked to representational processing
• Lesions in inferior and middle frontal lobe : omission in bells cancellation and Ota search task.
• Why the dorsal-ventral interaction hypothesis ?

  Because we saw different results depending on the test : In spatial cueing tasks, patients have a deficit of stimulus-driven re-orienting = exogenous attention.
• The dorsal-ventral interaction hypothesis states that right ventral (frontal) damage leads to abnormal activation of structurally intact dorsal (and ventral) parietal regions
• Link between the dorsal and ventral attention networks :

  Possible role of the MFG (medial frontal gyrus) as a communication hub between the dorsal and the ventral attention systems
• Top-down system involves the dorsal part and the bottom up system involves the ventral part
• Deficits of spatial attention would actually stem from damage to the white matter rather than the grey matter.
• Basically there are deficits in the transmission of information especially in the one that connects parietal regions to prefrontal regions (aka ventral attention network and dorsal one). Especially in the SLF II : super longitudinal fasciculus I. So the connectivity between dorsal and ventral attention is fucked up
• Bottom up therapy approaches ?

  • sensory activation : strong stimuli on bad side of patient
  • constraint induced : stimuli on the right is hindered so strengthen the impact of stimulus on the left side = eye patchig for ex
• Top-down therapy approaches ?

  • VST : visual scanning training, like cancellation/search task but you get help to explore the bad side
  • mental imagery training : asking to imagine performing movement on the bad side
• Dorsal-ventral interactions : reduced activation and connectivity of (intact) dorsal areas, in acute phase.
• Deficits in the line bisection task can be interpreted in 3 ways

  • Spatial attention = endogenous pb to orient attention
  • Representation = deficit in representation
  • Graded bias = deficit in how space is encoded and how attntion is allocated within the representation, will impact motor actions in space too
• Graded bias is a decit in both space encoding and attention allocation, within the general representation. This will also affected how motor action will be performed in space.
• A graded bias in the representation of space can be thought to reflect a distortion of the horizontal dimension of space
• What is Kinsbourne hypothesis ?

  • Each hemisphere controls attention of the other side
  • Normally the two compete for attention control (inter-hemi competition/inhibition)
  • DAMAGE causes loss of inhibition so hyper-activity on one side so more attention control on one side
• Why is severe neglect more often associated with lesion of right hemisphere ?

  Because : the right hemisphere regulates attention towards both the left and right hemispace
• What is the Heilman model ?

  Prediction that right hemisphere lesion will cause severe deficit for orienting attention to the left, but also a milder deficit of attention for right space. This means neglect is NOT strictly contralesional.
• How to re-establish a sympetric representation of space ?

  By dirupting hyper activation of left hemisphere and corrct inbalance thanks to transcranial magnetic stimulation (TMS)
• The shape of the bias : it's more of a bell-shaped exploration along the horizontal axis

  
• difference between allocentric and egocentric neglect ?

  • allocentric = neglect regardless the position to person body
  • egocentric = neglect relative to their own body
• In N+ patients you have a shift of egocentric space meaning that there is a new center of distribution of goal-directed spatial attention in neglect.

  
• Example of a test you can do for deficit in egocentric space ?

  The sound localization task

  
• When studying graded bias, researchers realized that primarily associated with frontal damage, and can be dissociated from visual neglect !
• How to check for motor negelct in N+ ?

  Using the task to move the right hand (good or ipsilesional) to press button under visual target.

  
• Studies on motor neglect showed that if you start the test with your left side, knowing all targets require rightward mvt, it reduces the deficit in the parietal patient = suggests that deficit in doing lefwards movement in the left hemispace
• Coding in egocentric space can explain the impact of various (multisensory and motor) signals on space exploration and attention orienting