Neglect

By the end of this lecture you will have learned:• More neglect phenomena• Outlines of some theoretical accounts of neglect• That no theory currently accounts for all neglect phenomena• That neglect phenomena are informing models of how the

brain encodes space.

Essential reading:Parkin, Ch 5

Additional reading:Bradshaw JL & Mattingley JB (1995) Clinical neuropsychology. Behavioral and brain science. Ch 6Vallar, G. (1998) Spatial hemineglect in humans. Trends in Cognitive Sciences, 2, 87-97

Neglect - early theoriesFocused on the sensory / motoric features of neglect

E.g. neglect is consequence of visual field deficit (in addition to a generalised loss of info processing ability)

Clearly not the case in representational or tactile neglectAlso can’t explain implicit processing

Is the deficit primarily motoric? Evidence against

Walker et al (1991):Simple target detection paradigm with 3 conditions:

Overlap: Target and fixation point overlap0 Gap: Fixation offset with target onset100 Gap: Target onset 100 ms after fixation offset

Overlap condition

8 5

(Fixation point remains during target presentation)

0 ms gap condition

58

(Fixation disappears simultaneously with target presentation)

100 ms gap condition

58

(Fixation disappears 100ms before target presentation)

Walker et al

PREDICTIONS:

If neglect is due to a sensory / motor deficit, target detection in neglected field should be equally poor in each condition.

E.g. no reason why a “central” manipulation should effect target detection in neglected field.

In fact, performance was far better in the 100ms gap condition (see Posner’s attentional hypothesis for explanation)

Theoretical ApproachesA distinction is commonly made between attentional and representational models of neglect

Attentional models:Deficits in mechanisms involved in maintaining arousal or vigilance and in controlling orientation toward, and selection of, stimuli as targets to be elaborated for further processing.(Kinsbourne, 1987; Humphreys & Riddoch, 1993; Posner, 1988; Heilman)

Representational models:Deficits in mechanisms involved in reconstructing an internal map of space, based either on sensory or “internally derived” information(Bisiach, 1993; Rizzolatti & Berti, 1990)

Neglect - attentional theories

The very term ‘neglect’ implies an attentional involvement

Effects of cueing in line bisection provides supportive evidence.

There are many other examples of neglect being ameliorated by cueing patients to attend to the neglected side

BUT - attention is not a unitary construct

Often difficult to establish exactly what is meant by attentional involvement

Strongest theories based on reasonably well articulated models of normal attentional functioning

Neglect - Kinsbourne’s model

•Orientational bias model - hemispheric rivalry model -“vectorial model”. One of the oldest models.

•Attention controlled by “paired opponent processesors” controlled by R + L hemispheres

•Each processor directs attention (controls an orienting response) to contralateral space

•In neglect the “disinhibited” left processor is in control, biasing attention to the right side of space

Predictions:• There is an attentional gradient• Attention is exaggerated towards the right e.g. an

ipsilesional bias, not just a contralesional impairment

Neglect - Kinsbourne’s model

•There is some evidence for attentional gradients

•Smania et al (1998)RTs to detect targets presented at different eccentricities:

Triangles = contralesional Circles = Ipsilesional

Slower the further leftFaster the further right

Kinsbourne (1993) gives other examples.

Neglect - Kinsbourne’s model

•Can explain representational neglect because processors assumed to be active in exploration of external and internal space.

•Can also explain why left side of objects, or left of two objects is neglected, even if in right hemispace

Problems - not all unilateral lesions produce neglectNot all patients have attentional gradientsCan’t explain object centred neglect

Not clear why right neglect should be so rare.Heilman (1995) and Mesulam (1998) assume RH controls attention to L&R space, LH only controls attention to R space.

Neglect - Posner’s model•Shifting attention involves:

• Disengaging• Shifting• Re-engaging

•Evidence from patients with neglect, PSP and pulvinar lesions provide some support for this model

•Core deficit in neglect is difficulty in disengaging attention from an ipsilesional stimulus

Posner’s paradigm

Posner’s paradigm

Posner’s paradigm

•

Posner’s paradigm

Cue is a valid predictor on 80% of trials

Delay between cue and target can be varied

Neglect - Posner’s model

Neglect patients miss catch trials, or are delayed in responding because cannot disengage attention from cue

100

1000

10000

0 50 500 1000

Gap Msec

RT

right valid

left valid

left invalid

right invalid

Neglect - Posner’s modelWalker et al results also explained by this model -

performance is best when attention does not have to disengage (e.g. in gap condition)

Mark et al (1988): Neglect patients perform better in a cancellation task if they were allowed to erase the lines rather than cross them.

E.g. attention no longer had to be disengaged from stimuli on right in erase vs cross condition.

Problems - in dark, neglect patients search mainly on their non-neglected side - not predicted if no stimuli are present to capture attention.

Neglect - Limiting theories of attentionIf it assumed that there is no attention (or severely impaired attention) on the neglected side, then data from neglect patients can be used to inform theories of attention

1. Does object segmentation occur pre-attentively?

Triesman - Feature Integration Theory:Preattentive: Simple features registered in parallelAttentive: Objects formed by combining features

Prediction: Objects are not segmented in neglected field

BUT - Demonstrations of object based neglect argue against this.

Neglect - Limiting theories of attention1. Does object segmentation occur pre-attentively?

Driver et al (1997): Extinction

Patients report seeing only circle on right

Patients report seeing a “barbell”

E.g. attentional system capable of object segmentation

Neglect - Limiting theories of attention2. Early vs Late selection:Can unattended information be processed semantically?

Berti & Rizzolatti (1992):Speeded Animal / Vegetable decision task for object in right hemifield

RTs for correct categorisation of right object:Identical: 777ms Same Category: 795ms Different Category: 890

Neglect - Limiting theories of attention3. What is the object of attention?

Does attention operate on the basis of locations in space or objects?

Neglect phenomena highlight the variety of reference frames in which attention appears to operate

Also, dissociations have been observed for neglect in personal, peripersonal and extrapersonal reference frames:

Halligan & Marshall (1991): Severe neglect in peripersonal space (e.g. conventional line bisection)Little or no neglect in extrapersonal space (pointing a light, throwing a dart)

Cowey et al, 1994: extrapersonal neglect onlyGuariglia & Antonucci, 1992: Personal neglect only

Neglect - Limiting theories of attention3. What is the object of attention?

See previous lecture for evidence that attention can also be object based.

Object centred neglect: In an object centred frame of reference, the objects parts are computed in relation to its structure

Patient NG (see previous lecture) has object centred neglect

Tipper & Behrmann, 1996:Target detection task -error rate

Static condition:

Left: 75%Right 25%

Rotate condition:

Left: 10%Right 65%

Neglect - representational theories

• E.g Piazza del Duomo effect

• Assume a construct such as Baddeley’s VSS - a “screen on which spatial information may be represented”

• In neglect part of screen (e.g. left) is dysfunctional“There are circumscribed brain areas where lesions would result in

a representational loss limited to definite regions of (egocentric) space” Bisiach et al, 1985 - “representational scotoma”

• Attention is not allocated to objects, but to representations of objects provided by the perceptual system

• Attentional theories disregard this step.

Neglect - Bisiach’s model

• Many other examples of representational neglect

• Problems - Not clear how well this model accomodates object centred neglect - although objects are clearly “represented” at some level

• Also some patients demonstrate “visual” but not representational neglect - Brain, 1941

• Bisiach concedes “representational scotoma” analogy may have been overkill… (Bisiach, 1993). Also lists other problems and potential answers.

• Posner data also difficult to accommodate.

• Rizzolatti’s model is also representational - attentional deficits are secondary

Neglect - transformational theories

Problems: - not all patients show no displacement of the subjective

straight ahead- Only address ego-centric neglect (not allocentric or

object centred)

Suggest that processes which translate sensory input into motor output are impaired

Neglect - adequacy of explanations

“...few current ‘explanations’ of ‘neglect’ are anything more than a description of the phenomena phrased in terms that insinuate understanding without actually delivering… to say that neglect is a ‘representational’ disorder is to describe, not explain, the fact that some patients show the Piazza del Duomo effect…”

Marshall et al, 1993

The term “neglect” should be treated by CNs in the same way as “apahsia” is - e.g. it does not exist as an “entity” -It is a useful shorthand for conveying a range of perceptual, attentional, intentional and representational problems in dealing with left space - all of which may have different underlying causes.

NeglectAttentional and representational models of neglect

are not necessarily mutually exclusive

“…such evidence widens the scope of the interpretations that can be given of these disorders in terms of attention to such an extent as to make attentional explanations logically indistinguishable from representational explanations.” Bisiach, 1994

“Attentional networks are important in the creation and scanning of a visual image just as they must work with object recognition in actual perception” Posner, 1993

Neglect - The neural coding of spaceThe beginnings of a resolution? Recently investigators have begun to relate neglect to the neural coding of space (e.g. Pouget & Driver, 2000; Pouget & Sejnowski, 2001)

Neurons in monkey parietal cortex have retinotopic receptive fields which can be modulated by non-retinotopic information (e.g. posture, eye position - possibly attention?)

Neglect - The neural coding of spaceAnimal work suggests that the same parietal neurons involved in encoding representations of objects are also involved in directing attention. (e.g Colby & Goldberg, 1999)

E.g. representational vs attentional debate is not really relevant.

In addition, many neurons in the parietal lobule are involved in multisensory integration (e.g. cells exist with congruent receptive fields for stimuli in different modalities)

So can explain some of the cross modal effects in neglect (e.g cross modal extinction)

Neglect - Computational ModelsPouget & Sejnowski (2001):

• Neurons in parietal cortex represent space using “basis functions”

• Given eye position and retinal position, basis functions allown an objects position can be transformed into other reference frames (e.g. head centered).

• This allows objects to be encoded in multiple frames of reference simultaneously

• Computational model developed entirely on the basis of computational principles and neurophysiological data.

• NP data can be used as an independent test of the model

• Model “lesioned” by deleting right basis function maps

• Appears to account well for most neuropsychological data

Neglect - Difficult data

Halligan & Marshall (1991)

Many neglect patients do not have a problem with marking the centre of a sqaure

Neglect - Outstanding questions

• Given the many different codes in which space is represented neurally, why is the human phenomonological experience of space unitary?

• What is role of ST / LT memory in neglect

• To what extent is the neglected information “recreated”

• What strategies are most effective for rehabilitation? Robertson - general attentional problems (reduced capacity) in addition to neglect.

Neglect - Conclusions

• Given the range of phenomena regarded as manifestations of “neglect” it is unlikely any unitary theory will ever provide an adequate explanation

• Advances in cognitive neuropsychology tend to reflect the introduction of a detailed model of normal performance

• There are no detailed models of “spatial cognition”

• Recent research has concentrated less asking “what is neglect?” and more on asking “what can data from brain damaged individuals tell us about how the brain encodes spatial information?”.

• Data from neuroscience and computational modelling may provide a way forward