What do cases of visual neglect tell us? Flashcards
Describe visual neglect
There is a long history demonstrating that unilateral brain lesions in humans often cause an impairment in spatially directing attention to the contralateral hemifield, a syndrome known as visuospatial neglect.
(Rafal, 1994)
In severe cases, PS completely disregard the visual hemifield contralateral to the side of the lesion
PS fail to report, respond or orient towards stimuli
In less severe cases of neglect…
the deficit is more subtle and becomes apparent only if the PS is confronted with competing stimuli, as in the case of visual extinction…
In visual extinction, PS are able to orient attention to a single visual object presented to their impaired visual hemifield; however, if two stimuli are presented simultaneously, one in the impaired and the other in the intact hemifield, the patients will only detect the one presented to the intact side, denying that any other object had been presented.
This essay will…
outline the characteristics of visual neglect patients and discuss the implications of these characteristics for our understanding of visual attention and awareness.
Outline what visual neglect tells us?
Tells us that just because something hasn’t entered conscious awareness, it may have still entered attention.
Tells us that attention is a multi-step process.
Tells us the neural substrates of attention; distributed network
Tells us that there is a specialised role for the right hemisphere in directed attention
Visual attention does not equal visual attention
i.e just because PS aren’t aware of a stimuli, it may have been attended to at some level…
Marshal & Halligan (1988)
Tested patient P.S who had LVF neglect
Presented pictures of houses with flames on left or right side
Results
P.S could not explicitly see flames when on left side of house
- Said there was no difference between the houses on most trails
P.S implicitly preferred to live in the non-burning house
- Therefore neglected stimulus still has influence on cognition at a preconscious level
- These findings replicated by Young et al (1992)
Priming
Lavadas et al (1993)
Patient B.P - right fronto-temporo parietal damage
B.P had to determine whether a letter string was a word or non word
- Priming words presented in blind field before target that sometimes semantically related
Results…
B.P responded faster to target if it was preceded by a related word (even in neglected field)
- B.P could not make explicit semantic judgments about words in blind field
- Suggests neglected stimuli filter through to cognition despite escaping awareness
Driver et al (1999) replicated these findings except using objects as the neglected prime
What do Marshal and Halligan’s (1988) study and Lavadas’ (1993) study tell us?
These studies show how despite no conscious/explicit awareness of neglected stimuli, they are attended to at a lower-level of processing and their associated semantics affect cognition at an implicit level.
- This would suggest attention is a multi-step process and that neglect PS are not deficient at all stages of this attentional pipeline…
What stage does the neglect happen?
Driver et al (1992) – Study 1
Aimed to investigate at what stage of attention does neglect occur…
Patient C.C – lesion including FEF and superior parietal lobule = Severe left side hemispatial neglect
Experiment 1 - C.C. had to remember the shape of a dividing contour between red background and green shape
Results
- C.C performed worse when green figure (and contour) on right hand side
- Why? Because after figure-ground segmentation, contour was on left of green figure
- The fact that FGS has occurred suggests that early attention processes are intact, as for FGS to occur, the object needs to be attended to in some manner
Experiment 2 – C.C. had to tell experimenter what colour the ‘shapes’ were. Stimuli involved either symmetrical or asymmetrical red or green shapes. A symmetrical shape will appear to be an object against an asymmetrical background.
Results
- C.C. could report colour of shapes with no bias to report rightmost colour
- However, C.C. did not overtly report any symmetry – based judgments on ‘closeness
and brightness’
- THEREFORE symmetry, which relies on a bilateral perception of shapes, was a
object-promoting factor in C.Cs visual system meaning both sides of figures are
represented at a preattentive stage
What does Driver et al’s (1992) findings tell us?
These results suggests that neglect is applying at a late stage of attention
- This would explain how neglect patients are able to use semantic features of neglected stimuli despite no concious
Therefore, these cases of neglect support the claim that attention is a process involving multiple steps, not all of which cause awareness.
The notion of multi-stage processing of attention is further demonstrated in neuroimaging data…
Rees et al (2000)
Patient G.K – right inferior parietal damage = left sided extinction
Task: observe brief peripheral presentation of houses and faces
- Either bilateral or unilateral presentation
- PS indicated which side he saw stimuli
Results
97% of bilateral trials, PS indicated ‘right, unilateral’
Despite no awareness, fMRI shows right V1 and extrastriate activations
- Some category specific responses found for extinguished stimuli at lower statistical threshold
THEREFORE
Striate and extrastriate activity not sufficient for visual awareness
The location of lesions…
Visuospatial neglect may follow unilateral lesions at different sites, including the:
parietal lobe, especially its inferior part and the temporo-parietal junction (Vallar & Perani, 1987)
regions of the frontal lobe (Heilman & Valenstein, 1972) ACC (Janer & Pardo, 1991)
basal ganglia (Damasio et al, 1980)
thalamus (Watson & Heilman, 1979).
Therefore, the finding that lesions of many different areas may cause visuospatial neglect has led to the notion that these areas form a distributed network for directed attention (Mesulam, 1981).
Distributed model of attention and neglect is supported by which MEG study?
Rastelli et al (2013)
Found left-sided target omissions are preceded by a specific pattern of activity (less beta-synchronizations) in the ipsilateral, healthy hemisphere
This supports network-based models of neglect, which postulate that this condition does not directly result from focal brain damage, but depends on dysfunction of large-scale brain networks within and across the two hemispheres (Bartolomeo et al., 2012, 2007)
Right hemisphere specialisation
(Vallar, 1993).
The fact that neglect occurs more often with right-sided parietal lesions than left-sided suggests that there is a specialised role for the right hemisphere in directed attention
Based on this hemispheric asymmetry, it has been proposed that the RH mediates directed attention to both sides of visual space
Whereas, the LH mediates direction only to the right side of visual space (Mesulam, 1981)
According to this view, in the case if a LH lesion, the intact RH would take over the attentional function of the LH
Whereas, a RH lesion would result in a left sided hemispatial neglect because the intact LH would have a bias for the RH
This dominance has only been demonstrated in cases of severe neglect; visual extinction appears to result as frequently from left- as from right sided lesions (Rafal, 1994).
Conclusion
In summary, cases of visual neglect are able to demonstrate several characteristics of visual attention and awareness.
Firstly, studies investigating semantic priming reveal that attention is a multi-stage process.
Studies by Driver et al () have identified that visual neglect is the result of dysfunction at the later stages of attention.
Neuroimaging studies have revealed that V1 and other extrastriate areas are activated even when PS report no awareness of certain stimuli.
This suggests that while stimuli are being attended to at some earlier stage of processing, this is insufficient to lead to conscious awareness of stimuli.
Furthermore, the location of lesions in neglect PS suggests the directed attention relies on a distributed network of areas including the parietal and frontal lobes.
And finally, the fact that neglect occurs more often with right sided parietal lesions suggests that the right hemisphere may have a specialised role in directed attention.
