Presentation on theme: "Neglect By the end of this lecture you will have learned:"— Presentation transcript:
1 Neglect By the end of this lecture you will have learned: More neglect phenomenaOutlines of some theoretical accounts of neglectThat no theory currently accounts for all neglect phenomenaThat neglect phenomena are informing models of how the brain encodes space.Essential reading:Parkin, Ch 5Additional 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, 87-97Given the time constraints you will be getting an overview of neglect theories - can’t possibly go into all of them in detail, and even to go into one in detail would mean ignoring others.E.g. what is the neural substrate for orienting attention, in what reference frames does attention operate, how does the brain encode spatial information?I will not be answering the essay question in this lecture - I am giving you a general overview, highlighting the problems faced by all theoretical accounts, by illustrating a few of them. You could just as easily write an essay on two accounts which I will not mention at all. E.g (Humphrey’s & Riddoch, 1993 and Rizzolati et al, 1993).
2 Neglect - early theories Focused on the sensory / motoric features of neglectE.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 processingIs the deficit primarily motoric? Evidence againstWalker 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 offsetInterestingly - some of the earliest mentions of neglect phenomena - Pick, 1898; Zingerle, concentrate on concepts such as consciousness and mental representation as their level of analysis.Talk of “body schemes” and “scheme for external space”.Also can’t explain some of the associated phenomena such as anosognosia
3 (Fixation point remains during target presentation) Overlap condition85(Fixation point remains during target presentation)
4 (Fixation disappears simultaneously with target presentation) 0 ms gap condition85(Fixation disappears simultaneously with target presentation)
5 (Fixation disappears 100ms before target presentation) 100 ms gap condition85(Fixation disappears 100ms before target presentation)
6 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)
7 Theoretical Approaches A distinction is commonly made between attentional and representational models of neglectAttentional 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)
8 Neglect - attentional theories The very term ‘neglect’ implies an attentional involvementEffects 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 sideBUT - attention is not a unitary constructOften difficult to establish exactly what is meant by attentional involvementStrongest theories based on reasonably well articulated models of normal attentional functioningLine bisection performance can be improved simply by drawing subjects attention to the left end of the line - most commonly done by asking them to name a number placed there.
9 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 hemispheresEach processor directs attention (controls an orienting response) to contralateral spaceIn neglect the “disinhibited” left processor is in control, biasing attention to the right side of spaceIn normal people these two processors are in equilibrium via reciprocal inhibitionOther attentional models predict that right hemisphere attention is normal.Predictions:There is an attentional gradientAttention is exaggerated towards the right e.g. an ipsilesional bias, not just a contralesional impairment
10 Neglect - Kinsbourne’s model There is some evidence for attentional gradientsSmania et al (1998)RTs to detect targets presented at different eccentricities:Triangles = contralesional Circles = IpsilesionalSlower the further leftFaster the further rightKinsbourne (1993) gives other examples.Kinsbourne -assumes that in healthy righthanders the LH processor prevails over the RH. EG in normal controls there should be a slight rightwards bias in attention - this has been foundBut note that it is not exactly linear.
11 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 hemispaceProblems -not all unilateral lesions produce neglectNot all patients have attentional gradientsCan’t explain object centred neglectKinsbourne - one explanation for lack of right neglect following left hemisphere damage:K assumes that in healthy righthanders the LH processor prevails over the RH. EG in normal controls there should be a slight rightwards bias in attention - this has been found.When the RH is damaged, the strong LH processor results in an attentional bias to the right. When the strong LH is damaged, the weak RH processes do not produce much of a leftwards attentional bias.Interesting prediction - neglect should be cured if a second lesion occurs in other hemisphere - this has been reported.Heilman - Right hemisphere directs spatial attention to left and right, left hemisphere only directs attention to the right.Problem with Heilmans & Mesulams models are that they do not really address the object centred problems.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.
12 Neglect - Posner’s model Shifting attention involves:DisengagingShiftingRe-engagingEvidence from patients with neglect, PSP and pulvinar lesions provide some support for this modelCore deficit in neglect is difficulty in disengaging attention from an ipsilesional stimulusPulvinar is a nucleus in the hypothalamus.
16 • Posner’s paradigm Cue is a valid predictor on 80% of trials Delay between cue and target can be varied
17 Neglect - Posner’s model Neglect patients miss catch trials, or are delayed in responding because cannot disengage attention from cue
18 Neglect - Posner’s model Walker 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.
19 Neglect - Limiting theories of attention If 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 attention1. Does object segmentation occur pre-attentively?Triesman - Feature Integration Theory:Preattentive: Simple features registered in parallelAttentive: Objects formed by combining featuresPrediction: Objects are not segmented in neglected fieldBUT - Demonstrations of object based neglect argue against this.
20 Neglect - Limiting theories of attention 1. Does object segmentation occur pre-attentively?Driver et al (1997): ExtinctionPatients report seeing only circle on rightPatients report seeing a “barbell”E.g. attentional system capable of object segmentation
21 Neglect - Limiting theories of attention 2. Early vs Late selection:Can unattended information be processed semantically?Berti & Rizzolatti (1992):Speeded Animal / Vegetable decision task for object in right hemifieldIN other words, categorisation of object in Right Hemifield was effected by the drawing on the left. Semantic information must be involved.Favours a late selection viewRTs for correct categorisation of right object:Identical: 777ms Same Category: 795ms Different Category: 890
22 Neglect - Limiting theories of attention 3. 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 operateAlso, 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 onlyPersonal - space occupied by bodyPeripersonal - space within reaching distanceExtrapersonal - within pointing or throwing distance
23 Neglect - Limiting theories of attention 3. 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 structurePatient NG (see previous lecture) has object centred neglectTipper & Behrmann, 1996:Target detection task -error rateStatic condition:Left: 75%Right 25%Rotate condition:Left: 10%Right 65%
24 Neglect - representational theories E.g Piazza del Duomo effectAssume 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, “representational scotoma”Attention is not allocated to objects, but to representations of objects provided by the perceptual systemAttentional theories disregard this step.
25 Neglect - Bisiach’s model Many other examples of representational neglectProblems - Not clear how well this model accomodates object centred neglect - although objects are clearly “represented” at some levelAlso some patients demonstrate “visual” but not representational neglect - Brain, 1941Bisiach 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 secondaryBrain described a patient who could perfectly well describe the route from ward to toilet, but could not actually nagivate it (kept turning right).
26 Neglect - transformational theories Suggest that processes which translate sensory input into motor output are impairedProblems:- not all patients show no displacement of the subjective straight ahead- Only address ego-centric neglect (not allocentric or object centred)
27 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, 1993The 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.
28 Neglect Attentional 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“A rivalry is often claimed or tacitly understood to exist between explanations based on either of these concepts. I think this is a mistakeHowever, Bisiach et al stronly resist any attempt to reframe their theory in attentional terms - “attention is not a process acting on a representation”
29 Neglect - The neural coding of space The 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?)So eye position modulates the gain (amplitude) of the cells response.Note that the information is falling on exactly the same part of the retina - not an effect of eye position in that sense.Head position also has same effect - maybe attention could too.
30 Neglect - The neural coding of space Animal 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)This model incorporates many of the ideas in the attentional models - eg gradients.Also includes model of a selection process, although not necessary - could just be selected on basis of function fields - e.g. spatial representation and attentional selection are the same thing.
31 Neglect - Computational Models Pouget & 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 simultaneouslyComputational model developed entirely on the basis of computational principles and neurophysiological data.NP data can be used as an independent test of the modelModel “lesioned” by deleting right basis function mapsAppears to account well for most neuropsychological dataAlso includes model of a selection process, although not necessary - could just be selected on basis of function fields - e.g. spatial representation and attentional selection are the same thing.
32 Neglect - Difficult data Halligan & Marshall (1991)Many neglect patients do not have a problem with marking the centre of a sqaure
33 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 neglectTo what extent is the neglected information “recreated”What strategies are most effective for rehabilitation? Robertson - general attentional problems (reduced capacity) in addition to neglect.
34 Neglect - ConclusionsGiven the range of phenomena regarded as manifestations of “neglect” it is unlikely any unitary theory will ever provide an adequate explanationAdvances in cognitive neuropsychology tend to reflect the introduction of a detailed model of normal performanceThere 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