Anian chatteriee 8 per onse or spects of neglect Experimental Research on Neglect studies and othe not (Binder.Marshall.Lazar.Beniamin.Mohr Neglect has become an important probe in investi 1992.Coslett atrick.Ha sever nce Heilman,1990:McGlinchey-B rroth et al. 1996 ics de scribed ne the One of a large no re depa ught e is divergent. 1998a Intention in Spatial Representation Neurochemistry of Neglect select from among many tions the works with more diffuse effects.Rather than influ of perception serves to guide actions in the world encing specific cognitive domains,these diffuse (Milner Goodale,1995).Some time ago,Watson systems seem to influence the state of brain fund and colleagues advanced the idea that neglect patients may have a premotor intentional deficit,a tions across many domains.Dopaminergic systems are of critical importance in neglect.In rats,lesions disinclination to initiate movements or move toward to ascending dopaminergic pathways produce or into contralateral hemispace (Watson,Valenstein, Heilman.1978).Similarly.Rizzolatti and co behavioral abnormalities that resemble neglect (Marshall Gotthelf,1979).and the dopaminergic workers argued that attention facilitates perception by activating the circuits responsible for motor agonist,apomorphine,ameliorates these deficits preparation (Rizzolatti.Matelli.Pavesi.1983). This improvement can be blocked by pretreatment with spiroperidol,a dopamine receptor blocking In most situations.attention and intention are inextricably linked. since attention is usually agent(Corwin et al.,1986). These observations led to a small open trial directed to objects on which one acts.Several clever of the dopamine agonist,bromocriptine.in two experiments have tried to dissociate attention from intention using cameras pulleys and mir patients with neglect (Fleet Valenstein.Watson Heilman 1987)Both patients'performances rors (Bisiach.Geminiani,Berti.Rusconi.1990: Bisiach et al 1995:Coslett et al 1990:Milner improved during bedside assessments of neglect. One patient's husband reported improvement in her Harvey.Roberts.Forster.1993:Na et al..1998 activities of daily living Recent reports suggest that Tegner&Levander,1991).The general strategy in thes bromocriptine may produce greater impr studies is to dissociate where patients vement than methylphenidate (Hurford.Stringer.Jann looking from where their limb is acting.Wher 1998)and may be more effective in treating the patients perform tasks in which these two are in motor aspects of neglect behaviors than strictly conflict,in some patients neglect is determined by ceptual ones (Geminiani,Bottini.Sterzi wher Some n andn others by where the 1998).The efficacy of pharmacological treatment nts behave as though they mhinatio in the neglect syndr has not bee n of the two fo s of neglect. scale studies nal biase in limb m ciated with fr tal les (Binder r et 199 lett et al.,1990:Tegner&
perceptual aspects of neglect and frontal lesions produce the response or motor aspects of neglect. Some studies report this association and others do not (Binder, Marshall, Lazar, Benjamin, & Mohr, 1992; Coslett, Bowers, Fitzpatrick, Haws, & Heilman, 1990; McGlinchey-Berroth et al., 1996). One study of a large number of patients even reports parietal lesions associated with a bias to respond ipsilesionally (Bisiach, Ricci, Lualdi, & Colombo, 1998a). Neurochemistry of Neglect Distributed neural networks are usually thought of in terms of anatomical connections. However, neurotransmitter systems also form distributed networks with more diffuse effects. Rather than influencing specific cognitive domains, these diffuse systems seem to influence the state of brain functions across many domains. Dopaminergic systems are of critical importance in neglect. In rats, lesions to ascending dopaminergic pathways produce behavioral abnormalities that resemble neglect (Marshall & Gotthelf, 1979), and the dopaminergic agonist, apomorphine, ameliorates these deficits. This improvement can be blocked by pretreatment with spiroperidol, a dopamine receptor blocking agent (Corwin et al., 1986). These observations led to a small open trial of the dopamine agonist, bromocriptine, in two patients with neglect (Fleet, Valenstein, Watson, & Heilman, 1987). Both patients’ performances improved during bedside assessments of neglect. One patient’s husband reported improvement in her activities of daily living. Recent reports suggest that bromocriptine may produce greater improvement than methylphenidate (Hurford, Stringer, & Jann, 1998) and may be more effective in treating the motor aspects of neglect behaviors than strictly perceptual ones (Geminiani, Bottini, & Sterzi, 1998). The efficacy of pharmacological treatment in the neglect syndrome has not been investigated systematically in large-scale studies. Experimental Research on Neglect Neglect has become an important probe in investigating several issues in cognitive neuroscience. The topics described next have in common the use of neglect and related disorders as a point of departure, although the issues addressed may be quite divergent. Intention in Spatial Representations Intentional systems select from among many locations those in which to act. This system is yoked to attentional systems, which select stimuli to be processed. There is a growing awareness that much of perception serves to guide actions in the world (Milner & Goodale, 1995). Some time ago, Watson and colleagues advanced the idea that neglect patients may have a premotor intentional deficit, a disinclination to initiate movements or move toward or into contralateral hemispace (Watson, Valenstein, & Heilman, 1978). Similarly, Rizzolatti and coworkers argued that attention facilitates perception by activating the circuits responsible for motor preparation (Rizzolatti, Matelli, & Pavesi, 1983). In most situations, attention and intention are inextricably linked, since attention is usually directed to objects on which one acts. Several clever experiments have tried to dissociate attention from intention using cameras, pulleys, and mirrors (Bisiach, Geminiani, Berti, & Rusconi, 1990; Bisiach et al., 1995; Coslett et al., 1990; Milner, Harvey, Roberts, & Forster, 1993; Na et al., 1998; Tegner & Levander, 1991). The general strategy in these studies is to dissociate where patients are looking from where their limb is acting. When patients perform tasks in which these two are in conflict, in some patients neglect is determined by where they are looking and in others by where they are acting. Some patients behave as though they have a combination of the two forms of neglect. Neglect as ipsilesional biases in limb movements is sometimes associated with frontal lesions (Binder et al., 1992; Coslett et al., 1990; Tegner & Anjan Chatterjee 8
Neglect Levander.1991).However.patients with lesions Mennemeier.Wertman.Heilman,1992:Rapcsak. restricted to the posterior parietal cortex can have Fleet,Verfaellie,Heilman,1988).Bilateral intentional neglect (Mattingley.Husain,Rorden. lesions to temporal-parietal areas may produce Kennard.Driver 1998:Triggs Gold Gerstle neglect for lower and near peripersonal space Adair,Heilman,1994).Mattingley and col- whereas bilateral lesions to the ventral temporal leagues (Mattingley,Bradshaw,&Phillips,1992) r and reported that slowness in the initiation of left far extrapersonal space.Neglect in the vert ward movements is associated with right posterio complex interpl lays between the lesions whereas slowness in the exec ward movements is associated with right anterio (Mennemeier.Chatteriee Heilman.1994) and suboo cal lesions Most nat ients with neglec Left neglect may also vary,depending on whether probably have mixtures of attentional and inten the stimuli are located in close perip onal space onal glect (Adair na Schwa )which may be Heilm or in far rganization ofs ting thar quite comp sp in ace is dis tinct from the nizati n furthe of these (Pr gathe d th e body's nction the e link of gested ini trunk lly by B 941).w and the ect the P construc e reach of one's 1995:Ch terjee,1998).The rele Spatial Reference Frames tinction may a 6 by an tion c Objects in extrapersonal space are anchored to dif other by directi of limb movem ence I ames. interpretation would be conso nant with single-cel Viewe neurophysiological data from we can show that attentional neurons in the postenor pan locate a chair in a room in each of these frames etal cortex are selectively linked to eye or to limb A viewer-centered frame would locate the chair to movements (Colby.1998) the left or right of the viewer.This frame itself is divided into retinal,head-centered.or body- Spatial Attention in Three Dimensions centered frames.An object-centered frame refers to the intrinsic spatial coordinates of the object itselt, Neglect is usually described along the horizontal its top or bottom or right and left.These coordinates (left-right)axis.However,our spatial environment are not altered by changes in the position of the also includes radial (near-far)and vertical (up viewer.The top of the chair remains its top regard- down)axes.Neglect may also be evident in these less of where the viewer is located. coordinate systems.Patients with left neglect An environment-centered reference frame refers frequently have a more subtle neglect for near to the location of the object in relation to its envi- space.On cancellation tasks they are most likely ronment.The chair would he coded with respect to to omit targets in the left lower quadrant in which other obiects in the room and how it is related left and near neglect combine (Chatterjee et al to gravitational coordinates.The vestibular system 1999:Mark Heilman,1997).Patients with bilat through the otolith organs probably plays an impor eral lesions may have dramatic vertical and radial tant role in establishing the orientation of an object neglect (Butter.Evans.Kirsch.Kewman.1989: in relationship to the environmental vertical axis
Levander, 1991). However, patients with lesions restricted to the posterior parietal cortex can have intentional neglect (Mattingley, Husain, Rorden, Kennard, & Driver, 1998; Triggs, Gold, Gerstle, Adair, & Heilman, 1994). Mattingley and colleagues (Mattingley, Bradshaw, & Phillips, 1992) reported that slowness in the initiation of leftward movements is associated with right posterior lesions, whereas slowness in the execution of leftward movements is associated with right anterior and subcortical lesions. Most patients with neglect probably have mixtures of attentional and intentional neglect (Adair, Na, Schwartz, & Heilman, 1998b), which may be related in quite complex ways. One problem in the interpretation of these studies is that attention versus intention may not be the relevant distinction. Rather, the “attention” experimental conditions may reflect the link of attention to eye movement and the “intention” conditions may reflect the link of attention to limb movements (Bisiach et al., 1995; Chatterjee, 1998). The relevant distinction may actually be between two perceptualmotor systems, one led by direction of gaze and the other by direction of limb movements. Such an interpretation would be consonant with single-cell neurophysiological data from monkeys, which show that attentional neurons in the posterior parietal cortex are selectively linked to eye or to limb movements (Colby, 1998). Spatial Attention in Three Dimensions Neglect is usually described along the horizontal (left-right) axis. However, our spatial environment also includes radial (near-far) and vertical (updown) axes. Neglect may also be evident in these coordinate systems. Patients with left neglect frequently have a more subtle neglect for near space. On cancellation tasks they are most likely to omit targets in the left lower quadrant in which left and near neglect combine (Chatterjee et al., 1999; Mark & Heilman, 1997). Patients with bilateral lesions may have dramatic vertical and radial neglect (Butter, Evans, Kirsch, & Kewman, 1989; Mennemeier, Wertman, & Heilman, 1992; Rapcsak, Fleet, Verfaellie, & Heilman, 1988). Bilateral lesions to temporal-parietal areas may produce neglect for lower and near peripersonal space, whereas bilateral lesions to the ventral temporal structures are associated with neglect for upper and far extrapersonal space. Neglect in the vertical axis probably represents complex interplays between the visual and vestibular influences on spatial attention (Mennemeier, Chatterjee, & Heilman, 1994). Left neglect may also vary, depending on whether the stimuli are located in close peripersonal space or in far extrapersonal space, suggesting that the organization of space in peripersonal space is distinct from the organization in further extrapersonal space (Previc, 1998). This notion of concentric shells of space around the body’s trunk was suggested initially by Brain (1941), who proposed that peripersonal space is a distinct spatial construct defined by the reach of one’s limbs. Spatial Reference Frames Objects in extrapersonal space are anchored to different reference frames. These frames are generally divided into viewer-, object-, and environmentcentered reference frames. For example, we can locate a chair in a room in each of these frames. A viewer-centered frame would locate the chair to the left or right of the viewer. This frame itself is divided into retinal, head-centered, or bodycentered frames. An object-centered frame refers to the intrinsic spatial coordinates of the object itself, its top or bottom or right and left. These coordinates are not altered by changes in the position of the viewer. The top of the chair remains its top regardless of where the viewer is located. An environment-centered reference frame refers to the location of the object in relation to its environment. The chair would be coded with respect to other objects in the room and how it is related to gravitational coordinates. The vestibular system through the otolith organs probably plays an important role in establishing the orientation of an object in relationship to the environmental vertical axis Neglect 9
Anian chatteriee 10 (Mennemeier et al 1994:Pizzamiglio vallar tactile awareness (Vaishnavi et al.,1999).Since Doricchi.1997).Several reports de onstrate that 、with neglect may hay nal neglect ccur in an of these refer ence fra (Bisiach et al..1986)or deficit of their ov vitch Rlack Mozer 1994 schema (Coslett. 1998).the que of bo Chatteriee 1994:Driver Halli &C Tactil ced 1995:Laday arpent 1990,Hi being ced h patia suggesti ention operates across these different pect of the extrapersona Crss-Modal inte the space (Vaishnavi,Calhoun, Humans have a cohe erent sense of space in which pcpe2001 they perceive objects and act (Driver Spence Guiding mov nents by vision also involve 1998).Neglect studies visual signals nsual-moto r mapping can be altered if a subject of space.Presumably,multiple sensory modalities wears prisms that displace stimuli to the left or right interact in complex ways to give rise to multiple of their field of view.Recent work suggests tha representations of space. patients with neglect who are wearing prisms tha Rubens and colleagues (Rubens,1985)de- displace visual stimuli to their right remap ballisti monstrated that left-sided vestibular stimulation movements leftward,and that this remapping can be mproves extrapersonal neglect.Presumably,ves useful in rehabilitation (Rossetti et al..1998). tibular inputs influence visual and spatial atten- tion in complex ways.Vestibular stimulation can Psvchophvsics.Attention also improve contralesional somatosensory aware and Perception in Neglect ness (Vallar.Bottini.Rusconi.Sterzi.1993) and may transiently improve anosognosia as well What is the relationship between the magnitude of (Cappa.Sterzi.Guiseppe.Bisiach.1987).Spatial stimuli and the magnitude of patients'representa- attention may also be influenced by changes in tions of these stimuli?This question features promi- posture.which are presumably mediated by otolith nently in psychophysical studies dating back to the vestibular inputs (Mennemeier et al..1994) seminal work of Gustav Fechner in the nineteenth Similarly,proprioceptive inputs from neck muscles century (Fechner.1899).How do we understand can influence spatial attention (karnath.sievering the kinds of spatial distortions (Anderson.1996 Fetter,1994:Karath,Schenkel,Fischer,1991) Karnath Ferber.1999:Milner Harvey,1995) and serve to anchor viewer-centered referenc and "anisometries"(Bisiach Ricci Modona frames to an individual's trunk. 1998b)shown in the pe ception of neglect patients? Recent studies of patients with tactile extinc It turns out that pati ents are not always aware of tion have also focused on cross. modal factors in the same n on of space.Nor are they always awareness.Visual input when close to the loca aware of th quantity of stimuli.Rather tion of tactile stimulation may improve contra their awareness is sy stematically related to the lesional tactile awareness (di Pelleg rino Basso antity of stimuli esented (Chatterjee et al. Frassinetti.1998:Ladavas.Di Pelleg 1992b. Zeloni.1998:Vaishnavi iet al 1999)Sin ilarly the The evidence that neglect patients are systemati intention to move may also improve contralesiona cal the magnitude of the stimuli
(Mennemeier et al., 1994; Pizzamiglio, Vallar, & Doricchi, 1997). Several reports demonstrate that neglect may occur in any of these reference frames (Behrmann, Moscovitch, Black, & Mozer, 1994; Chatterjee, 1994; Driver & Halligan, 1991; Farah, Brun, Wong, Wallace, & Carpenter, 1990; Hillis & Caramazza, 1995; Ladavas, 1987), suggesting that spatial attention operates across these different reference frames. Cross-Modal and Sensorimotor Integration of Space Humans have a coherent sense of space in which they perceive objects and act (Driver & Spence, 1998). Neglect studies suggest that multiple spatial representations are embedded within this sense of space. Presumably, multiple sensory modalities interact in complex ways to give rise to multiple representations of space. Rubens and colleagues (Rubens, 1985) demonstrated that left-sided vestibular stimulation improves extrapersonal neglect. Presumably, vestibular inputs influence visual and spatial attention in complex ways. Vestibular stimulation can also improve contralesional somatosensory awareness (Vallar, Bottini, Rusconi, & Sterzi, 1993) and may transiently improve anosognosia as well (Cappa, Sterzi, Guiseppe, & Bisiach, 1987). Spatial attention may also be influenced by changes in posture, which are presumably mediated by otolith vestibular inputs (Mennemeier et al., 1994). Similarly, proprioceptive inputs from neck muscles can influence spatial attention (Karnath, Sievering, & Fetter, 1994; Karnath, Schenkel, & Fischer, 1991) and serve to anchor viewer-centered reference frames to an individual’s trunk. Recent studies of patients with tactile extinction have also focused on cross-modal factors in awareness. Visual input when close to the location of tactile stimulation may improve contralesional tactile awareness (di Pellegrino, Basso, & Frassinetti, 1998; Ladavas, Di Pellegrino, Farne, & Zeloni, 1998; Vaishnavi et al., 1999). Similarly, the intention to move may also improve contralesional tactile awareness (Vaishnavi et al., 1999). Since patients with neglect may have personal neglect (Bisiach et al., 1986) or a deficit of their own body schema (Coslett, 1998), the question of how body space is integrated with extrapersonal space also arises. Tactile sensations are experienced as being produced by an object touching the body, an aspect of peripersonal space. Visual sensations are experienced as being produced by objects at a distance from the body, in extrapersonal space. The integration of tactile and visual stimulation may contribute to the coordination of extrapersonal and peripersonal space (Vaishnavi, Calhoun, & Chatterjee, 2001). Guiding movements by vision also involves integrating visual signals for movement. This visual-motor mapping can be altered if a subject wears prisms that displace stimuli to the left or right of their field of view. Recent work suggests that patients with neglect who are wearing prisms that displace visual stimuli to their right remap ballistic movements leftward, and that this remapping can be useful in rehabilitation (Rossetti et al., 1998). Psychophysics, Attention, and Perception in Neglect What is the relationship between the magnitude of stimuli and the magnitude of patients’ representations of these stimuli? This question features prominently in psychophysical studies dating back to the seminal work of Gustav Fechner in the nineteenth century (Fechner, 1899). How do we understand the kinds of spatial distortions (Anderson, 1996; Karnath & Ferber, 1999; Milner & Harvey, 1995) and “anisometries” (Bisiach, Ricci, & Modona, 1998b) shown in the perception of neglect patients? It turns out that patients are not always aware of the same proportion of space. Nor are they always aware of the same quantity of stimuli. Rather, their awareness is systematically related to the quantity of stimuli presented (Chatterjee et al., 1992b). The evidence that neglect patients are systematically influenced by the magnitude of the stimuli Anjan Chatterjee 10
Neglect with which they are confronted has been studied sensations into mental representations occur within the most in the context of line bisection (Bisiach the central nervous system and not simply at the Bulgarelli Sterzi vallar 1983)Patients make level of sensory receptors,as implied by Stevens larger errors on larger lines.Marshall and Halligan (1972). demonstrated that psychophysical laws could de- Crossover in Neglect Halligan and Marshall (Halligan Marshall,1988; Marshall Halligan.1989)discovered that patients single-word reading tasks,and weight judgm with neglect ter nded to bisect short lines to the an be mathe atically left of the obie ctive midnoint an wd to d 10051008. m e ipsilesional with the et al 1994a.1992b:Chatter This is found in n (Chat jee et al..1994a)with and is sents the objective ma itude of the stim plain sily b In f h e aw refe d to ed pain in th fo neglecthcorist on sing em unction relati nship are ob atie asks. the con ate judgments of m left (S bjective length 1970 nt o hat mental rep behavic a stin o the physi mental repr ents ha n on idea ha plausibl 1n& ccur in the normal judgments of luminance putationa (Monaghan 8 magnitudes suggest that mental representations are compressed in relation to the ange of the physica The crossover in line bisection is also influenced stimulus.Exponents greater thar one.as in judg by the context in which these lines are seen.Thus ments of pain intensity.suggest that tmental repre patients are more likely to cross and bisec sentations are expanded in relation to the range of to the left of the true midpoint if these the physical stimulus are preceded by a longe line (Marshall, Chatterjee and colleagues showed that patients Krakauer.Sharma.1998).Recently.Chatterie with neglect,across a variety of tasks,have power and colleagues(Chatterjee,Ricci,Calhoun,2000 functions with exponents that are lower than those 1998)showed that of normal patients.These observations suggest crossoverlike phenomenon also occurs with weight that while patients remain sensitive to changes in judgments.Patients in general are likely to judg sensory magnitudes.their awareness of the size of right-sided weights as heavier than left-sided these changes is blunted.For example.the exponent weights.However.with lighter weight pairs.this for normal iudgments of linear extension is very bias may reverse to where they iudge the left side close to one.By contrast.neglect patients have to be heavier than the right.These results indicate diminished exponents,suggesting that they,unlike that crossover is a general perceptual phenomenon normal subjects,do not experience horizontal lines that is not restricted to the visual system. of increasing lengths as increasing proportionately It should be noted that these observations also mean that nonlinear transformations of the magnitude of
with which they are confronted has been studied the most in the context of line bisection (Bisiach, Bulgarelli, Sterzi, & Vallar, 1983). Patients make larger errors on larger lines. Marshall and Halligan demonstrated that psychophysical laws could describe the systematic nature of these performances (Marshall & Halligan, 1990). Following this line of reasoning, Chatterjee showed that patients’ performances on line bisection, cancellation, single-word reading tasks, and weight judgments can be described mathematically by power functions (Chatterjee, 1995, 1998; Chatterjee et al., 1994a, 1992b; Chatterjee, Mennemeier, & Heilman, 1994b). In these functions, y = Kfb , f represents the objective magnitude of the stimuli, and y represents the subjective awareness of the patient. The constant K and exponent b are derived empirically. Power function relationships are observed widely in normal psychophysical judgments of magnitude estimates across different sensory stimuli (Stevens, 1970). An exponent of one suggests that mental representations within a stimulus range are proportionate to the physical range. Exponents less than one, which occur in the normal judgments of luminance magnitudes, suggest that mental representations are compressed in relation to the range of the physical stimulus. Exponents greater than one, as in judgments of pain intensity, suggest that mental representations are expanded in relation to the range of the physical stimulus. Chatterjee and colleagues showed that patients with neglect, across a variety of tasks, have power functions with exponents that are lower than those of normal patients. These observations suggest that while patients remain sensitive to changes in sensory magnitudes, their awareness of the size of these changes is blunted. For example, the exponent for normal judgments of linear extension is very close to one. By contrast, neglect patients have diminished exponents, suggesting that they, unlike normal subjects, do not experience horizontal lines of increasing lengths as increasing proportionately. It should be noted that these observations also mean that nonlinear transformations of the magnitude of sensations into mental representations occur within the central nervous system and not simply at the level of sensory receptors, as implied by Stevens (1972). Crossover in Neglect Halligan and Marshall (Halligan & Marshall, 1988; Marshall & Halligan, 1989) discovered that patients with neglect tended to bisect short lines to the left of the objective midpoint and seemed to demonstrate ipsilesional neglect with these stimuli. This crossover behavior is found in most patients (Chatterjee et al., 1994a) with neglect, and is not explained easily by most neglect theories. In fact, Bisiach referred to it as “a repressed pain in the neck for neglect theorists.” Using performance on singleword reading tasks, Chatterjee (1995) showed that neglect patients sometimes confabulate letters to the left side of short words, and thus read them as longer than their objective length. He argued that this crossover behavior represents a contralesional release of mental representations. This idea has been shown to be plausible in a formal computational model (Monaghan & Shillcock, 1998). The crossover in line bisection is also influenced by the context in which these lines are seen. Thus, patients are more likely to cross over and bisect to the left of the true midpoint if these bisections are preceded by a longer line (Marshall, Lazar, Krakauer, & Sharma, 1998). Recently, Chatterjee and colleagues (Chatterjee, Ricci, & Calhoun, 2000; Chatterjee & Thompson, 1998) showed that a crossoverlike phenomenon also occurs with weight judgments. Patients in general are likely to judge right-sided weights as heavier than left-sided weights. However, with lighter weight pairs, this bias may reverse to where they judge the left side to be heavier than the right. These results indicate that crossover is a general perceptual phenomenon that is not restricted to the visual system. Neglect 11
Anian chatteriee Implicit Processing in Neglect Hemispheric Asymmetries The ge Heilman and colleagues (Heilman Van Den tial mation is Abell.1980)as vell a Me am1981) tulated sed p eaentively before l sys that the right hemisphe diffusely are engaged neglect is al diso rde rig left sides of spac the lef preattentively.Negl o seem ab ach a h uld pr spheric organ nof spatia es nake ngure activation the emisphere al 2000) much can stimuli be process the right ed and yet no do show greate right hemi Garraniga 1979) and initially re hemispaces porte that patients with exuncuon to pi sphere activations with rightward sh fts (Corbetta tures shown simultaneously were still able to make Miezen, Shulman,Petersor 1993:Gitelmar same-different judgments more accurately thar et al..1999;Kim et al.,1999).Because intentiona would be expected if they were simply guessing. neglect follows right brain damage,one might Since then,others have reported that pictures neg expect similar results for motor movements.Right lected on the left can facilitate processing of word hemisphere activation is seen with exploratory centrally located (and not neglected)if the pictures movements,even when directed into right hemi- and words belong to the same semantic category space (Gitelman et al..1996).Despite these asym- such as animals (McGlinchey-Berroth. Milberg metries,homologous areas in both hemispheres are Verfaellie.Alexander.Kilduff.1993).Similarlv often activated,raising questions about the func- lexical decisions about ipsilesional words are tional signifcance of left hemisphere activation in aided by neglected contralesional words (Ladavas. these tasks. Paladini,&Cubelli,1993).Whether neglected stimuli may be processed to higher levels of se Frontal-Parietal Networks mantic knowledge and still be obscured from the patient's awareness remains unclear (Bisiach Most functional imaging studies of visual and Rusconi.1990:Marshall Halligan.1988). spatial attention find activation of the intraparietal sulcus (banks of BA 7 and BA 19)and adiacent regions,especially the superior parietal lobule (BA Functional Neuroimaging Studies of Spatial 7)Corbetta and colleagues (Corbetta et al 1993) Attention and Representation used PET and found the greatest increases in blood owin theright parictal lobue (BA7ad Positron emission tomographv (PET)and functional dorsolateral prefro tal cortex(BA 6)when subjects magnetic resonance imaging (fMRI)studies offer were cued endogenously to different locat insights into neurophysiological changes occurring They found bilateral activation.but the activation during specific cognitive tasks.Functional imaging contralateral to the has the advantage of using normal subjects.These ts The right infer methods address several issues relevant to neglect. (BA40).the uperio d
Implicit Processing in Neglect The general view of the hierarchical nature of visual and spatial processing is that visual information is processed preattentively before attentional systems are engaged. If neglect is an attentional disorder, then some information might still be processed preattentively. Neglect patients do seem able to process some contralesional stimuli preattentively, as evidenced by their abilities to make figure-ground distinctions and their susceptibility to visual illusions (Driver et al., 1992; Mattingley et al., 1997; Ricci et al., 2000). How much can stimuli be processed and yet not penetrate consciousness? Volpe and colleagues (Volpe, Ledoux, & Gazzaniga, 1979) initially reported that patients with visual extinction to pictures shown simultaneously were still able to make same-different judgments more accurately than would be expected if they were simply guessing. Since then, others have reported that pictures neglected on the left can facilitate processing of words centrally located (and not neglected) if the pictures and words belong to the same semantic category, such as animals (McGlinchey-Berroth, Milberg, Verfaellie, Alexander, & Kilduff, 1993). Similarly, lexical decisions about ipsilesional words are aided by neglected contralesional words (Ladavas, Paladini, & Cubelli, 1993). Whether neglected stimuli may be processed to higher levels of semantic knowledge and still be obscured from the patient’s awareness remains unclear (Bisiach & Rusconi, 1990; Marshall & Halligan, 1988). Functional Neuroimaging Studies of Spatial Attention and Representation Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies offer insights into neurophysiological changes occurring during specific cognitive tasks. Functional imaging has the advantage of using normal subjects. These methods address several issues relevant to neglect. Hemispheric Asymmetries Heilman and colleagues (Heilman & Van Den Abell, 1980) as well as Mesulam (1981) postulated that the right hemisphere deploys attention diffusely to the right and left sides of space, whereas the left hemisphere directs attention contralesionally. From such a hemispheric organization of spatial attention, one would predict relatively greater right than left hemisphere activation when attention shifts in either direction. By contrast, the left hemisphere should be activated preferentially when attention is directed to the right. Normal subjects do show greater right hemispheric activation with attentional shifts to both the right and left hemispaces, and greater left hemisphere activations with rightward shifts (Corbetta, Miezen, Shulman, & Peterson, 1993; Gitelman et al., 1999; Kim et al., 1999). Because intentional neglect follows right brain damage, one might expect similar results for motor movements. Right hemisphere activation is seen with exploratory movements, even when directed into right hemispace (Gitelman et al., 1996). Despite these asymmetries, homologous areas in both hemispheres are often activated, raising questions about the functional significance of left hemisphere activation in these tasks. Frontal-Parietal Networks Most functional imaging studies of visual and spatial attention find activation of the intraparietal sulcus (banks of BA 7 and BA 19) and adjacent regions, especially the superior parietal lobule (BA 7). Corbetta and colleagues (Corbetta et al., 1993) used PET and found the greatest increases in blood flow in the right superior parietal lobule (BA 7) and dorsolateral prefrontal cortex (BA 6) when subjects were cued endogenously to different locations. They found bilateral activation, but the activation was greater in the hemisphere contralateral to the attended targets. The right inferior parietal cortex (BA 40), the superior temporal sulcus (BA 22), and Anjan Chatterjee 12