Disorders of Attention

Question 1

what evidence is there that neglect can operate in object centred coordinates?

Answer 1

Driver & Mattingley 1998
instructed patients to fixated centrally on a rectangle and discern whether the jagged line running vertically through it matched the isolated outline presented centrally below. One portion of the rectangle on either side of the jagged partition would be filled yellow and the other a striped blue background. Normal viewers perceived the yellow portion as an object while the blue as shapeless.
The position of the jagged line shifted horziontally such that it was sometimes to the left or right of the centre of the rectangle. Crucially whether the yellow ‘shape’ or blue ‘background’ was to the left or right of the line was also changed.
Patients with neglect made more errors when the jagged line fell on the contralesional side of the yellow shape irrespective of whether the line itself sat to the right or left of the centre of the rectangle. This demonstrates intact visual segmentation processes and indicated neglect may affect the left side of objects.

Question 2

what is the evidence for intact semantic priming, and therefore unconscious processing of contralesional stimuli in patients with neglect?

Answer 2

McGlinchey-Berroth et al., 1993)
In a lexical decision task (word/ nonword)
Participants presented with two visual stimuli either side of a fixation cross that were either semantically related to the upcoming word or not
Despite congruent stimuli being presented contralesionally, participants were faster to identify semantically related words correctly than unrelated ones
But when participants were presented with the same images followed by two centrally presented images participants were unable to identify the common picture between the two presentations if it was first presented contralesioanlly

consistent with findings that neglected objects activate the primary visual cortex (Rees et al., 2000)
activity in V1 found in response to presentation of stimuli
but not consciously perceived
activity in V1 does not equal conscious visual awareness

Question 3

what is attention?

Answer 3

a state in which cognitive resources are focused on certain aspects of the environment and CNS is in a state of readiness to respond (APA dict)

Question 4

what is the attentional blink?

Answer 4

Raymond et al., 1992
healthy participants were asked to identify partially specified letter targets whilst detecting the presence of absence of fully specified letter probes
impaired when probes are presented during a 270ms interval starting 180ms post target presentation
attentional blink shows that similar deficits can be induced in controls when multiple stimuli are involved
supports attentional accounts of neglect

Question 5

what is unilateral neglect

Answer 5

common cognitive impairment following stroke characterised by a reduced awareness of stimuli on the contralesional side of space that cannot be explained by a sensory or motor deficit (Parton et al., 2004)

highly heterogeneous - increasingly recognised as a multicomponent disorder rather than a unitary syndrome
e.g different sectors of space, frames of reference, modalities, motor neglect, internal representations,
non-neglect specific deficits in working memory, sustained and selective attention that can exacerbate spatial deficits

Question 6

what is extinction?

Answer 6

patients with extinction are unaware of contralesional stimuli but only when presented simultaneously with a stimulus in the ipsilesional hemifield
reflects a competitve aspect of the spatial bias seen in neglect and can be considered as a milder form of neglect - although others consider it a distinct disorder

Question 7

what is anosognosia?

Answer 7

a disorder where patients are unaware of their condition
commonly comorbid with neglect - patients fail to acknowledge their impairment
limit self-report measures
impacts functional recovery and rehabilitation potential

Question 8

what is the evidence that deficits in shifting covert attention is a cognitive mechanism underlying negect

Answer 8

Kaplan et al., 1991
increasing the number of distractors in cancellation tasks leads to a failure in shifting attention that is particularly costly in neglect
however even the performance of healthy individuals decreases with increasing distractors, just not to the same extent.

Posner 1984
neglect patients have a greater cost for incongruent cues due to a deficit in disengaging attention from the ipsilesional field to shift covert attention contrlalesionally
participants had to respond to whether an arrow was up or down by pressing a button
pre-cue with an irrelevant flash of light that was either congruent or incongruent with the target
healthy people are faciliated by congruent cues with an increased cost for incongruent cues in RT and accuracy
patients with right hemisphere parietal damage show an increased cost for incongruent cues in the ipsilesional side of space (right cue for left target)

Question 9

what is the evidence that deficits in the representation of space is a cognitive mechanism underlying neglect

Answer 9

Vuilleumier et al., 1999
bisection deficits may improve with eyes looking to the right but not when the head also turns to the right
suggestive of a head-centred left-sided deficit
visual imagery is not affected by eye/head position nor is auditory extinction as not related to processing visual input from the eyes

Farah et al., 1990
both body and environment centred coordinates can be affected with no clear anatomical distinction
neglect does not rotate with the object

Question 10

what is the evidence for non-spatial deficits in neglect

Answer 10

Sustained attention is a modulatory function that increases arousal localised to the frontal lobe
Posner (1993) proposed that there is a right hemisphere dominant system for endogenously maintaining attention and alertness which modulates the spatial attentional system through noradrenalin

Robertson et al., 1997
unilateral left neglect highly correlated with performance on a spatially non-lateralised auditory sustained attention task
participants had to maintain an internal count of up to 14 tones separated by maximum 3 second intervals (WM component)
auditory sustained attention significantly discriminates between right hemisphere damaged patients with and without neglect
non-spatial attention measure as sensitive as standardised spatial tests in discriminating unilateral neglect among right hemisphere patients

Malhorta et a., 2009
patients with neglect show a vigilance decrement in a central fixation task where they must respond to centrally presented stimuli without competing distractors
this was specific to when neglect patients were required to maintain attention to spatial locations but not letter identity even when task difficulty was equated
highly specifc interaction between deficits in spatial processes and sustained attention in neglect
but the deficit in sustained attention was not correlated with standard measures of neglect
deficits in sustaining attention to spatial location may have different effects on neglect severity depending on interactions with other cognitive components of neglect

improving altertness and sustained attention via self-instructional procedure produces significant and specific improvements in lateralised spatial attention in a group of unilateral subjects

Question 11

How might noradrenaline be involved in neglect?

Answer 11

posner hypothesis that the sustained attention system exerts a modulatory influence over the spatial attention system via the right hemisphere dominant neurotransmitter noradrenaline

spatial orientation is mediated by a posterior orienting system and a right hemisphere dominant alerting/sustained attention system

it is a parietal lobe based spatial orientation systems that is the priviledged recipient of activation from the sustained attention system (alerting)

pardo
PET
sustained attention task caused right hemisphere frontal and parietal activation in normal participants

non-lateralised aspects of right hemisphere attention deficits may be just as important in determining deficits in spatial attention as the spatial bias itself

Question 12

how can the properties of parietal neurons explain some of the deficits seen in neglect

Answer 12

Pouget & Driver 2000
review links the properties of single cells that have been studied in monkey parietal cortex
may reflect partial loss or dysfunction of similar cell populations
would produce a pathological gradient in the number of cells representing particular lateral positions of space for particular functions

spatial frames of reference
whether or not a neglect patient will become aware of a particular stimulus on the retina or hand can depend on current position

previously neglected left visual field stimulus may become detectable with the eye or head deviated to the right or the trunk deviated to the left despite constant retinal input
previously neglect touch on the left hand can enter awareness if the hand is placed to the right of the patient’s trunk

the responses of neurons in the parietal lobe is influenced by posture
the majority of neurons in the parietal area have retinotopic RFs but the amplitude of their retinal response is modulated by postural factors such as eye position

this gain in modualtion by non-retinal factors can explain why neglect arises in a mixture of egocentric coordinates

explains why caloric vestibular stimulation or vibration of left neck-muscles are effective treatments as these produce similar inputs to twisting the trunk leftward

Question 13

what are the neuroanatomical correlates of extinction

Answer 13

controversial localisation due to different criteria for what constitutes extinction producing variability in reporting across samples
Bellas et al., 1998
ERP and fMRI comparison of lesion overlap with activation in healthy controls has implicated parietal regions in both allo and egocentric representations
particularly the right parietal angular gyrus and often within inferior parietal damage

Question 14

How can grouping effects overcome extinction?

Answer 14

Vuilleumier & Rafal 2000
4 patients with RHD
partial report paradigm
tirangle or star stimuli appeared on the screen either to the right or left of a central fixation cross
single displays: one shape in one or both hemifields
double displayes: two shapes in one or both hemifields
location task: report where shapes appeared
enumeration task: report number of shapes
discrimination task: report number of stars

changes in instructions produced different pattern of performance across tasks in all patients
patients extinguished many contralesional stimuli in bilateral displays when reporting location but not when counting them regardless of location and shape
however reaction time was slower than unilateral presentation - implicit processing of the extinguished shape
but they often only detected one of two stimuli within the contralesional hemifield - some grouping mechanisms survive extinction
attending to shapes produced extinction and increased reaction time

extinction can impact processing of simultaneously presented and competing items
unconscious processing of extinguished stimuli is continued
ability of task demand to modulate performance

Question 15

what is simultanagnosia

Answer 15

difficulty in integrating and interpreting a global scene due to an inability to attend to more than one object at a time
patients lack the final step in percpetion: realisation of the whole from elements

Question 16

what lesions are associated with simultanagonsia

Answer 16

rare and associated with a wide range of lesion sites
Chechlacz et al., 2012
voxel-based morphometry and fMRI overlap/subtraction analyses have allowed identification of key sites
compared simultanagonsia patients with a group of uniltareal visuospatial deficits or bilateral posterior lesions with no deficits
identified bilateral grey matter damage to the cuneus area BA46, calcarine and parieto-occipital fissure

Farah 1990
distinction between dorsal and ventral simultanagnosia - dorsal occipital-parietal and ventral occipital temporal regions
similar patient profiles but more severe after dorsal damage

Question 17

what do we know about visual processing in simultanagonsia

Answer 17

Holmes & Horrax 1919
WF - bullet wound causing impaired processing of multiple objects even when they shared the same location but normal perception when only one object was presented
normal visual acuity and fully functioning retina and early visual system

but there is evidence that depth perception may be impaired (Farah, 2001)
patients cannot report the distance an object is to them nor if the examiner is moving the object towards or away from them
don’t exhibit a typical reflexive blink response to an object impending on them
cannot interpret depth cues and respond appropriately
conscious of only the object and not its context
difficulty in integrating local features into a global Gestalt
exhibited in many areas impairing performance on clinical tests and resulting in a real-life impact

Question 18

what have individual case studies of simultanagnosia contributed to our understanding of the disorder

Answer 18

Patient SA (Humphreys & Price, 1994)
performance on reading, imagery and scene description
slow and often inaccurate word reading
response time increased with word length
letter-by-letter reading illustrated by need to ‘sound-out’
failure to see cohesive unit of a word, rather sequential identification of letters
(what would happen if letters were degraded)

able to identify individual parts of a scene but misinterpretation of what is going on and items missing
Boston Cookie Theft Picture (Goodglass & Kaplan, 1972)
correctly identified a girl and a woman
incorrectly reported the woman as holding a bat rather than a plate and water as a dishcloth
failed to see the boy, when probed said ‘he’s gone now’

difficulty in imagery and understanding the spatial and semantic relationships between objects in a scene
intact perception of faces/people as whole rather than parts e.g in prosopagnosia - ventral fusiform area

Patient SL (Dalrymple, 2007)
unable to process an entire Navon figure (global letter composed of smaller composite letters) but able to attend to individual letters
Patients are unable to report the global letter for stimuli composed of local letters organised into the shape of a larger congruent or incongruent letter. There is processing of the global letter as there is a cost incurred by incongruence due to interference despite inability to consciously report.
better at reporting the global face of a portrait made out of fruit than reporting the individual types
flexibility may depend on task demands and stimulus salience (Moscovitch et al., 1997)

Question 19

what is the evidence for covert attention as a cognitive mechanism underlying simultanagnosia

Answer 19

Kinesbourne & Warrington 1962
difficulty shifting to a second object
when presented with object pairs patients can only name one
not affected by relative location or size
many errors when items are presented close together
staggering onset times removes cost, longer delays in sequential presentation
more than just shifting attention
how does priming/habituation interact

Question 20

what is the argument for simultanagnosia as non-spatial extinction

Answer 20

Humphreys et al., 1994
pictures extinguish words - stronger cues to the visual system
fixation doesn’t help
closed shapes are preferentially perceived - pull attention more than other cues
report shapes when corner cues over edges as these pull attention more than other cues

Question 21

what is the spatial attention account of extinction

Answer 21

Diver & Humphreys 2001a
extinction is a spatial deficit that is apparent in cases of feature/objects competing for representation in visual awareness
bias towards the ipsilesional side of space so that when stimuli are presented to both hemifields those contralesional are ignored

e.g
Siman-Tov
Pouget & Driver

Question 22

what is the object vs spatial attention argument for simultanagnosia

Answer 22

Balint 1909 described suffers as having a restricted attentional window limited to one object
Thiass & De Bleser 1992
argued against this -
provided evidence of a patient who could process complex stimuli as wholes if they covered a sufficiently small visual angle
Dalrymple et al., 2013
argued that simultanagnosia patients have a reduced window of attention that has dynamic flexibility
explains why objects were selected for attention
due to spaced-based deficits, attention would resort to focusing on single objects
made easier by implicit processing of congruent properties including continuation, shape and size
explains difficulties in object localisation and depth perception
the inability to perceive more than one object at a time means that simultanagnosia patients lack reference points to anchor objects in space and gain cues to their relative locations

But fails to account for some symptoms such as the high proportion of illusory conjunctions and binding errors
failure to see even spatially-overlapping objects

Question 23

what is the spatial mapping account of simultanagnosia

Answer 23

FIT (Treisman & gelade)
the features of a scene are registered early in parallel
before attention is focused these are integrated into a master map which specifies locations

Laeng et al., 1999
parietal lesions damage the ability to form this master map impeding the ability to focus attention on different spatial locations
spatial indexing: features are usually extracted then indexed in terms of properties and their spatial relationships to each other
indexed features can then attract attentional focus to salient objects
impairment results in deficits seen in simultanagnosia where attentional focus is unable to shift between features
patients are impaired in switching between objects and integrating the scene

Question 24

what is the competition in spatial attention account of extinction

Answer 24

Duncan & Desimone 1995
limited capacity system where only select information on the retina can be processed at a given moment
unilateral lesions throughout the sensorimotor networks seen in extinction weaken the local representation of contralateral space
because the competitive weight afforded to stimuli in the affected portion of the contralesional field are lost

converging evidence
animal models (Moran & Desimone, 1985)
single-unit recordings of primate V4 and IT indicate that neuronal responses are specific to goal-relevant stimuli in the visual field
in a discrimination task monkeys had to attend to target stimuli in the visual field while ignoring simultaneously presented distractors in a second location depending on the spatial cue at the start of the trial
when targets and distractors were within the RF of the recorded cell the neuronal response was determined by the target and the response to the distractor was attenuated

e.g effective treatment (Rossetti et al., 1998)
success of treatments of extinction and neglect which aim to correct competitive imbalance by introducing local processing biases
displacing prisms which make objects appear further in the ipsilateral hemifield shift global competitive processing to the neglected side thus pulling attention

extinction effects are especially strong when the ipsilateral stimulus is relevant to behaviour but the same benefit is not seen for the contraleisonal hemifield

Question 25

how do deficits in object-centred attention account for neglect

Answer 25

Driver & Mattingley 1998
patients asked to fixate at the centre of a rectangle and compare the single jagged edge inside to isolate the isolated edge presented centrally below
normal viewers see the rectangles as comprising a yellow figural shape against a striped shapeless blue background due to figure-ground segementation processes
patients performed worse when the inital jagged edge fell on the right of the pateint but on the left of the segmented yellow figure
than when the jagged edge fell on the good right side of the segmented figure even though it was further to the patients left

extinction is modulated by grouping effects (Driver et al., 1997)
simultaneous presentation of two separate circles either side of a central fixationi cross
patients report seeing the circle on the right only
presentation of the same circles with an additional horizontal line spanning their width
patients report seeing a barbell
gestalt cues for perceptual grouping abolishes neglect for the left hemifield
removes competition

Question 26

what do representational accounts argue for?

Answer 26

Bisiach et al., 1998
91 RBD neglect and 43 RBD no neglect asked to double the size of horizontal segments either from their left or right endpoints
neglect patients have a tendency to overextend towards the left side
contralesional overextension is also evident in most RBD patients without neglect
in a minority of RBD neglect patients there is right overextension
argue that neglect was more severe in patients showing right overextension as there was a negative correlation with line bisection error
however this was not significant at the group level and line bisection does not correlate well with other measures of neglect

they argue that USN is a disorder of mental space representation consisting in a left-right pathological anisometry of the medium for space representation: the left-side would be ore relaxed and the right-side would be more contracted/compacted
the contralesional relaxation of the medium might still sustain conscious representation of the contents in space but with a horizontal size distortion
beyond a critical point the overrelaxed medium no longer sustains conscious representation

Biasiach’s 1998 theory also forsees that in some individuals the disorder affects response level of stimulus processing

the transformational hypothesis suggests USN is a failure of the transformation of sensory to motor output which is based in different reference frames (Karnath, 1997)
since such coordinate transforms mainly occur in the parietal cortex, lesions here would impair this process
as a result the egocentric representation of the surrounding environment would be deflected towards the ipsilesional side

Bender & Teuber 1946 argue that the contralateral sensory input to the damaged right hemisphere is weakened and so cannot compete with the ipsilateral hemifield
But Smania et al., 1996 demonstrated that modulatiing the physical characteristics of contralesional stimuli such as brightness and size has no effect on detection

Question 27

what are the different theories for why neglect occurs after lesions to the right hemisphere

Answer 27

right hemisphere dominant spatial attention system
Mesulam et al., 1999
competition of spatial coding
left hemifield is represented by the right hemisphere but the right hemifield is represented bilaterally
the intact left hemisphere is not able to take over the spatial representation of the right

Kinsbounre 1970
interhemispheric competition between the left and right hemisphere parieto-frontal circuits where the left hemisphere shifts attention rightward more strongly than the right hemisphere shifts attention to the left
leading to an imblanace that causes the right hemifield to be over-attended
unilateral damage to the right hemisphere in neglect leads to the left hemisphere becoming hyperexcitable

right hemisphere advantage, with representation of both hemifields bilaterally
Siman-Tov 2007
fMRI study investigating lateralisation of facial expression processing and control house stimuli
space-dependent differential activation of a brain network consistent with visuospatial attention
stimuli presented in the left visual field more robustly activated components of dorsal and ventral fronto-parietal attention related systems and subcortical structures (inferior parietal sulcus, frontal eye fields, anterior insula, basal ganglia)
left field activation reflects a right hemisphere advantage
the ability to attend to both hemispheres subserved by both both hemispheres
bilateral left visual field superiority in representation
interhemispheric transfer of information
right hemisphere has a connectivity advantage

Thiebaut de Schotten 2011
parieto-frontal network in the right hemisphere is larger than the left one
this specialisation is associated with an imbalance in processing speed
diffusion imaging tractography demonstrated that the human brain is organised in longitudinal parieto-frontal tracts separated into a dorsal SLF1 middle SLF 2 and ventral SLF3
dorsal is symmetrically distributed, middle trends towards RH, and ventral is significantly right lateralised
healthy individuals exhibit a slight deviation when completing spatial attention tasks such as the line bisection - pseudoneglect (Orr & Nicolls, 2005)
measurement of the relationship between left and right SLF 2/3 volumes to deviation on the line bisection task demonstrated that participants who showed greater deviation to the left had larger right hemisphere SLF 2 volumes
a minority of patients who showed a rightward deviation on the spatial attention task exhibited the opposite pattern of lateralisation with larger left hemisphere SLF 2 volumes
underpinned by greater myelination, diameter and axon number, increasing the speed of conduction
result in an imbalance in processing speed between the two hemispheres in favour of the right
latger SLF volumes correlate with faster direction time for stimuli presented in the left visual field in a modified Posner paradigm