SPECIAL ISSUE AUDITORY NEGLECT: WHAT AND WHERE IN AUDITORY SPACE Stephanie Clarke and Anne Bellmann Thiran (Division de Neuropsychologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland) ABSTRACT A sound that we hear in a natural setting allows us to identify the sound source and to localise it in space. Several lines of evidence indicate that the two aspects are processed in anatomically distinct cortical networks. Auditory areas that are part of the What or Where processing streams have been identified recently in man and in non-human primates. Comparison between anatomical and activation studies suggests that processing within either stream can be modulated by specific attentional factors. Attending to auditory events can be affected in neglect. Bisiach et al. (1984) described systematic directional errors to the ipsilesional space, which was considered a manifestation of hemispatial neglect and interpreted as a disruption of the neural network providing the internal representation of egocentric space. The other manifestation of auditory neglect is contralesional extinction in dichotic listening condition (Heilman and Valenstein, 1972). Recently two types of auditory neglect have been described, one corresponding to a primarily attentional deficit associated with basal ganglia lesions and the other to distortions of auditory space representations associated with parieto-prefrontal lesions (Bellmann et al., 2001). Based on studies of sound detection and sound recognition following hemispheric lesions we argue that the two types of neglect correspond to disturbed processing in either the What or the Where stream. Key words: spatial processing, sound recognition, auditory cortex, auditory attention INTRODUCTION Bisiach is one of the few authors who have initiated the investigation of auditory spatial functions in man. He and his colleagues have assessed auditory localisation by means of a stereophonic test simulating spatial lateralisations by interaural intensity differences, and found systematic directional errors to the ipsilesional space (for left-sided as well as right-sided targets) in right-damaged patients (Bisiach et al., 1984). This auditory spatial bias after right-hemispheric damage, also documented in two other studies of the same group (Vallar et al., 1995, Sterzi et al., 1996), was considered a manifestation of hemispatial neglect and interpreted as a disruption of the neural network providing the internal representation of egocentric space. This interpretation is in line with the general concept of neglect as a distortion of represented space, with compression on the ipsilesional side and expansion on the contralesional, defended by Bisiach and collaborators (Bisiach et al., 1996; 1998a; 1998b). In the visual modality, this concept has been classically opposed to the ‘attentional’ theories of neglect (e.g. Mesulam, 1981). The ability to acknowledge the presence of an object must be distinguished from the ability to localise this object with respect to one’s own body. This notion can be related to the now well established dichotomy, within the visual system, between a ventral stream involved in object processing, and a dorsal stream dedicated to spatial localisation and to action in space. Some authors Cortex, (2004) 40, 291-300 have relied on this model to explain different aspects of visual neglect: Humphreys (1998) argues that between-objects and within objects are differently mediated by the dorsal and ventral pathways, while Farah and collaborators develop the idea of an ‘object-based’ attention and a ‘location-based’ attention (Farah et al., 1993; Farah and Buxaum, 1997). The role of multiple representations of space, sustained by the parietal cortex, and their breakdown in visual extinction and neglect are stressed in a recent review by Marshall and Fink (2001). The purpose of this paper is to consider the actual situation of auditory neglect in the light of recent investigations of the anatomical and functional organisation of the human auditory cortex. The evidence favouring the existence of a ventral network dedicated to the recognition of auditory objects, and a dorsal network involved in the processing of the auditory spatial dimension of these objects will be reviewed. We shall argue that auditory neglect phenomena are at least partially related to one of these two auditory networks: a neglect within the dorsal network will lead to spatial bias in auditory localisation, whereas an auditory neglect in the ventral stream will manifest itself by inter-object omissions. WHAT AND WHERE PROCESSING STREAMS IN HUMAN AUDITION A sound that we hear in a natural setting allows us to identify the sound source and to localise it in 292 Stephanie Clarke and Bellmann Thiran space. Several lines of evidence indicate that the two aspects are processed at the cortical level independently in anatomically distinct neural networks. Activation Studies Direct comparisons of networks involved in sound recognition and sound localisation were reported recently and confirmed their anatomical segregation. We have investigated activation associated with performance in sound identification and sound localisation in normal subjects with fMRI using three conditions: i) comparison of spatial stimuli simulated with interaural time differences; ii) identification of environmental sounds; and iii) rest (Maeder et al., 2001; Figure 1A). Conditions i) and ii) required acknowledgement of predefined targets by pressing a button. Sound recognition and sound localisation activated, as compared to rest, inferior colliculus, medial geniculate body, Heschl gyrus and parts of the temporal, parietal and frontal convexity bilaterally. The activation pattern on the frontotemporo-parietal convexity differed in the two conditions. Middle temporal gyrus and precuneus bilaterally and the posterior part of left inferior frontal gyrus were more activated by recognition than by localisation. Lower parts of inferior parietal lobule and posterior parts of middle and inferior frontal gyri were more activated, bilaterally, by localisation than by recognition. Passive listening paradigm revealed segregated pathways on superior temporal gyrus and inferior parietal lobule. Alain et al. (2001) used an S1-S2 match-to-sample task in which S1 differed from S2 in its pitch and/or location. Relative to location, pitch processing generated greater activation in auditory cortex and the inferior frontal gyrus. Conversely, identifying the location of S2 relative to S1 generated greater activation in posterior temporal cortex, parietal cortex, and the superior frontal sulcus. This study demonstrated also a dichotomy between localisation and recognition processing. The What stream was, however, only partially visualised, probably due to a task which did not require the identification of sound objects. A functional segregation of the What and Where processing streams was further confirmed by electrophysiological studies using pitch localisation paradigms (Alain et al., 2001; Anourova et al., 2001). These direct demonstrations of the What and Where processing streams are in agreement with previous activation studies which compared activation to a recognition task vs. rest or to a spatial task vs. rest. Categorisation of environmental sounds, involving recognition, was shown to activate specifically left prefrontal, temporal, parietal and cingulate regions (Engelien et al., 1995). Sound localisation was shown to activate largely distributed cortical networks with an important contribution of the temporal, parietal and prefrontal cortices (Griffiths et al., 1998; 2000, Bushara et al., 1999, Maeder et al., 2001). The activation by auditory spatial stimuli was bilateral in all studies, but some authors suggested a dominance of the right hemisphere (Griffiths and Green, 1999; Weeks et al., 1999; Griffiths et al., 2000), whereas others found no evidence for lateralisation in auditory spatial processing (Bushara et al., 1999, Woldorff et al., 1999). Auditory Short-term Memory Distinct neural populations sustain also shortterm memory for sound content and sound localisation. We have investigated the role of auditory or visual interference tasks in a same/different comparison of two sound stimuli separated by an interval in normal subjects (Clarke et al., 1998). Auditory interference tasks reduced memory for sound content and sound location in a specific way. Memory for sound content was significantly more reduced by auditory recognition than by auditory spatial interference tasks. Visual interference tasks significantly reduced memory for sound location but not for sound content. These results suggest that short-term memory for sound content and that for sound location involve partially distinct processing, and auditory spatial functions are more closely linked to visual functions than auditory recognition. Effects of Focal Lesions The dichotomy of auditory What and Where processing streams, as demonstrated in normal subjects, predicts that focal lesions, centred on one or the other network are associated with the corresponding selective deficits. This has been suggested by published case studies: 3 cases of auditory agnosia without auditory localisation deficits (Spreen et al., 1965; Jerger et al., 1972; Fujii et al., 1990) following right or bilateral lesions; and 1 case of selective impairment of auditory motion perception following a right hemispheric lesion that included the insula and parietal convexity (Griffiths et al., 1996; 1997). A recent study of 15 consecutive patients with right focal hemispheric lesions showed that sound recognition and sound localisation can be disrupted independently (Clarke et al., 2002). In this series, 4 patients were normal in sound recognition but severely impaired in sound localisation, whereas 3 other patients were deficient in recognising sounds but localised them well. The lesions involved the inferior parietal and frontal cortices, and the superior temporal gyrus in patients with selective sound localisation deficit; the temporal pole and anterior part of the fusiform, inferior and middle temporal gyri in patients with selective recognition Auditory neglect A A B B Selective deficit in auditory localization (N=4) A B C D E1 E2 E3 F G H 293 Selective deficit in auditory recognition (N=3) A I 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 B C D E1 E2 E3 F G H D E1 E2 E3 F G H I 12 12 Plan A 1 Sagitale 1 2 X B C D E1 E2 E3 F G H I A 1 4 X 2 B C d I 3 4 2 5 5 3 Sagitale 1 3 2 Plan d 3 6 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 6 12 12 Plan Sagitale X c Plan Sagitale 1 1 2 2 3 3 4 4 5 5 X c Fig. 1 – Cortical networks involved in sound recognition and sound localisation. A: Mean activation of 18 normal subjects; green denotes regions activated more by sound recognition than sound localisation and red regions activated more by sound localisation than sound recognition (adapted from Maeder et al., 2001). B: Superimposed right-hemispheric lesions associated with selective deficit in sound localisation (left; four patients) or selective deficit in sound recognition (right; three patients). The lesions are represented in Talairach space, sections c and D. Hatching indicates the number of patients in whom a given Talairach cube was damaged (adapted from Clarke et al., 2002). deficit (Figure 1B). This double dissociation clearly supports conclusions drawn from activation and electrophysiological studies (Maeder et al., 2001; Alain et al., 2001; Anourova et al., 2001). Selective deficits in sound recognition or sound localisation were also found in cases with unilateral left hemispheric lesions (Clarke et al., 2000). In this series, 1 patient was severely deficient in recognition of environmental sounds but normal in auditory localisation and auditory motion perception. The lesion included the left superior, middle and inferior temporal gyri and lateral auditory areas, but spared Heschl’s gyrus, the acoustic radiation and the thalamus. Another patient, with the same profile, had a lesion that comprised the postero-inferior part of the frontal convexity and the anterior third of the temporal lobe. A third patient was severely deficient in 294 Stephanie Clarke and Bellmann Thiran auditory motion perception and partially deficient in auditory localisation, but normal in recognition of environmental sounds; the lesion involved large parts of the parieto-frontal convexity and the supratemporal region. These cases confirm that lesions of the What or the Where processing streams in the left hemisphere cause the corresponding deficits. EVIDENCE FROM A NON-HUMAN PRIMATES Auditory What and Where processing streams have been demonstrated anatomically and functionally in non-human primates. Macaque auditory cortex is subdivided into three sets of areas called core, belt and parabelt; individual areas were defined by electrophysiological and/or architectonic criteria (for review see e.g. Rauschecker et al., 1998; Kaas et al., 1999). The lateral belt areas, situated on the inferior lip of the sylvian fissure and the supero-posterior part of the superior temporal gyrus, tend to respond to complex sounds corresponding to monkey calls and/or to spatial locations. While area ML shows specialisation for either type of information, AL appears more specialised for monkey call-like stimuli and CL for locations (Recanzone et al., 2000; Tian et al., 2001). AL has been proposed to be part of the auditory stream that subserves sound recognition (the What stream) and CL of the stream that subserves sound localisation (the Where stream; Rauschecker and Tian, 2000). HUMAN AUDITORY AREAS, WHAT AND WHERE PROCESSING STREAMS AND ATTENTIONAL MODULATION A similar functional specialisation of nonprimary auditory areas was demonstrated in man. Cytoarchitectonic criteria were used to identify 2 to 6 areas outside the primary auditory cortex, (Brodmann, 1909; von Economo and Koskinas, 1925; Galaburda and Sanides, 1980). More recent histochemical studies visualising cytochrome oxidase and acetylcholinesterase activities revealed several auditory areas: lateral (LA), superior temporal (STA), posterior (PA), and medial (MA) area (Rivier and Clarke, 1997; Wallace et al., 2002: Fig. 2A). This approach, combined with the visualisation of calcium-binding protein reactivity, revealed also a hierarchical organisation (Mesulam and Geula, 1994; Hustler and Gazzaniga, 1996; Rivier and Clarke, 1997; Hackett et al., 2001; Wallace et al., 2002; Chiry et al., 2003), which is further supported by a recent study of intrinsic connectivity of the human auditory cortex (Tardif and Clarke, 2001). In particular, three levels were distinguished: a first level corresponding to the primary auditory area; a second level with four B “Where” “What” Level III Level II Level I Fig. 2 – A. The position of auditory areas on the human supratemporal plane (AI, LA, PA, MA, AA and STA) as defined by patterns of cytochrome oxidase and acetylcholinesterase activity (Rivier and Clarke, 1997). Scales indicate Talairach coordinates (Talairach and Tournoux 1988). B. Schematic representation of What and Where auditory processing streams based on anatomical (Rivier and Clarke, 1997; Tardif and Clarke, 2001; Wallace et al., 2002; Chiry et al., 2002) and activation studies. Horizontal hatching denotes areas shown to be selectively activated by sound recognition-type processing (LA, STA; Hall et al., 2002) and crossed hatching areas located in the posterior part of the planum temporale, shown to be involved in spatial processing (Weeks et al., 1999; Warren et al., 2002). Asterisks mark areas shown to have increased activation by attention-related factors, STA more than LA and PA (Hashimoto et al., 2000). areas (LA, MA, PA, AA; Figure 2B) and a third level with at least one area (STA). The heterogeneity of parvalbumin and calbindin expression within the four early stage areas suggested that they may belong to different processing streams, areas LA and STA to the What stream, and areas MA and PA to the Where stream (Figure 2B; Chiry et al., 2003). Such an organisation is supported by activation studies, which demonstrated a specialisation for sound recognition-type processing in non-primary areas situated laterally and anterolaterally to the primary cortex, and specialisation for auditory spatial processing in postero-medial areas. Frequency modulation was shown to increase activation within areas LA, STA and a region anterior to AA, but not in AA, MA and PA (Figure 2B; Hall et al., 2002). More generally, stronger activation by complex stimuli was found in belt Auditory neglect areas than in the primary auditory cortex (Wessinger et al., 2001). Anterior part of the superior temporal gyrus (on the left side) was activated in speech processing (Binder et al., 2000). Posterior part of the superior temporal gyrus and the planum temporale were found to be involved in spatial processing (Figure 2B; Weeks et al., 1999; Warren et al., 2002). Non-primary auditory areas have been shown to be specialised either in sound recognition or sound localisation and constitute thus the early stages of the What and Where processing streams. It is interesting to note that some of these areas may be specifically modulated by attentional load. This has been demonstrated by an activation study using dichotically or diotically presented speech stimuli (Hashimoto et al., 2000). Diotic was defined by these authors as the presentation of the same stimulus (without interaural time difference) in both ears, and dichotic as the simultaneous presentation of two different stimuli, one in each ear. The latter task was the more difficult one and required greater attentional resources. Areas PA, LA and STA showed greater responses under dichotic than diotic condition, which was interpreted as reflecting the increased attentional load. Furthermore, stronger effects of attentional modulation were observed in area STA than in areas LA or PA (Hashimoto et al., 2000), suggesting a hierarchical organisation, which is very similar to that revealed by anatomical studies (Figure 2B; Rivier and Clarke, 1997). Discrete attentional modulation within the What and Where processing streams may be the basis of different types of auditory neglect which were observed following right hemispheric lesions. IN THE AUDITORY NEGLECT WHAT AND WHERE SYSTEMS Two different types of deficits have been attributed to auditory neglect. One manifestation of auditory neglect is the presence of systematic directional errors in sound localisation, including alloacusis from the contralesional to the ipsilesional hemispace, in tasks of overt single sound localisation or of determination of auditory subjective straight ahead (Altman et al., 1979; Bisiach et al., 1984; Vallar et al., 1995; Sterzi et al., 1996; Haeske-Dewick et al., 1996; Soroker et al., 1997; Kerkhoff et al., 1999; Bellmann et al., 2001). There is general agreement to consider this type of errors as manifestation of neglect, and to attribute them to a distortion of represented space (Bisiach et al., 1984; Bellmann et al., 2001). Most of these studies have raised the question of the relationship between systematic directional errors in the auditory modality and visual neglect and found significant correlation, though a few cases of dissociations have been reported (Bisiach et al., 295 1984; Soroker et al., 1997, Kerkhoff et al., 1999). A recent study reports also auditory disturbances along the vertical dimension, suggesting a complex disorganisation of the auditory space in visual neglect (Pavani et al., 2002). The other manifestation of auditory neglect commonly described is contralesional extinction (Heilman and Valenstein, 1972; Hugdahl and Wester, 1994). It is generally observed when two stimuli are simultaneously lateralised in the auditory space. Dichotic listening is the procedure most often used to display two different simultaneous stimuli. It remains, however, disputed how far dichotic extinction reflects a primary attentional deficit and is thus appropriate for the diagnosis of auditory neglect (Beaton and McCarthy, 1995). An alternative interpretation considers extinction as a consequence of defective transmission or processing of the sensory stimuli delivered from the ear having privileged links to the lesioned hemisphere (Kimura, 1967; Sparks and Geschwind, 1968; De Renzi et al., 1984; Beaton and McCarthy, 1995). In a recent study, we have circumvented the interpretation impasse proper to the dichotic procedure, by lateralising each of the two stimuli by an interaural time difference (ITD) of 1 msec favouring the left ear in one case and the right ear in the other case. An illusion of double lateralisation similar to that encountered with the dichotic procedure was thus created (one stimulus perceived to the left, the other simultaneously to the right), but without any difference in the intensity level and content received by each ear. With this task, we were able to document contralesional omissions in condition of double stimulation, which were clearly attributable to auditory spatial neglect. These results confirmed data reported by other authors who used free-field presentations by means of two lateralised loud-speakers (Tweedy et al., 1980; Soroker et al., 1997; Deouell and Soroker, 2000). These two different profiles of auditory neglect, i.e., systematic directional errors in sound localisation versus contralesional omissions when an ipsilesional stimulus is simultaneously presented, were found to double dissociate in cases of right-hemispheric lesions. We have assessed auditory neglect with a task of sound localisation (by means of ITD simulations) and with an ITD diotic listening task in four right-damaged patients (Bellmann et al., 2001). Two patients (JCN and MB) presented a marked hemispatial asymmetry favouring the ipsilesional hemispace in the ITD diotic test, but did not show any spatial bias in sound localisation. Two other patients (AJ and ES) had the reverse profile: no hemispatial asymmetry in the ITD diotic test, but a severe spatial bias directed to the ipsilesional side in sound localisation (Figure 3). JCN and MB had mainly subcortical lesions affecting basal ganglia. AJ and ES had cortical lesions in the prefrontal, superior 296 Stephanie Clarke and Bellmann Thiran A B Fig. 3 – Two types of auditory neglect. Four patients (JCN, MB, AJ, ES), with right hemispheric lesions and left ear extinction on dichotic listening task, presented different profiles in auditory spatial attention versus auditory localisation tests. A. Asymmetry on the diotic listening test as compared to controls (mean value of the control population is indicated by CTRL, limit of normal performance by the dotted line). JNC and MB, but not AJ and ES, presented extinction of sound stimuli presented within the left hemispace. B. Sound localisation of positions simulated with interaural time differences of 1 ms (LL) or 0.3 ms (L) in favour of the left ear, 1 ms (RR) or 0.3 ms (R) in favour of the right ear or without interaural time difference (C). JCN and MB performed as normal controls (not shown here), while AJ and ES had a rightward bias in their performance. temporal and inferior parietal areas. These results suggest the existence of two functionally and anatomically distinct types of auditory neglect: i) contralesional omissions in condition of double stimulation, following lesions centred on basal ganglia; or ii) ipsilesional spatial bias following fronto-temporo-parietal lesions. The dissociation between these two types of auditory neglect is not rare. It was found in 40% of cases in a series of 15 consecutive patients with right unilateral hemispheric lesion entering our rehabilitation programme (Bellmann et al., 2001; Bellmann, 2001; Bellmann Thiran and Clarke, 2003). Although the role of basal ganglia in neglect is supported by anatomo-clinical correlations (Hier et al., 1977; Damasio et al., 1980; Healton et al., 1982) and by the relative success of dopaminergic treatment (Fleet et al., 1987; Geminiani et al., 1998; Hurford et al., 1998; Mukand et al., 2001), their specific involvement in the attentional type of auditory neglect needs to be further investigated. The manifestations of the two types of auditory neglect described above occurred in very different situations. In the localisation task, subjects had to process one given object at a time, whereas in the ITD diotic task, two simultaneous sounds were provided together. As emphasised already by Efron et al. (1983) and demonstrated in dichotic listening Auditory neglect tasks (Hugdahl and Wester, 1994), contralesional omissions of single sound targets are very rare. Further evidence of dissociation between the processing of one versus multiple objects in the auditory modality has been provided by Cusack and collaborators (2000). They reported betweenobjects attention deficits without within-object attention deficit in patients suffering from visual hemineglect1. The authors concluded that withinobject comparisons and between-objects comparisons are separately represented in the auditory modality as well as in the visual modality. In the study of Cusack and collaborators, the sound objects followed each other in the temporal and not the spatial dimension as in our study. This suggests that attentional mechanisms (not necessarily spatial) are involved when multiple objects have to be processed. Interestingly, theories of visual neglect elaborated on the basis of studies on extinction have generally advocated an asymmetry between the attention allocated to the right vs. left hemi-spaces and/or a reduction in processing speed or capacity (Di Pellegrino and De Renzi, 1995; Driver et al., 1997). In line with these propositions, it has been found that patients with left-sided extinction perceived ipsilesional visual events earlier than physically synchronous contralesional stimuli (Rorden et al., 1997). This suggests that these patients have a chronic bias of spatial attention towards the ipsilesional side, and that stimuli occurring at that attended location are processed more rapidly and receive privileged access to awareness. This phenomenon of ‘prior entry’ has recently been found in the auditory modality (Karnath et al., 2002). These attentional theories contrast with conceptions of hemispatial neglect as a distortion of egocentric space representation (Bisiach et al., 1996; 1998a; 1998b; Karnath, 1997), which are particularly convincing to explain directional spatial bias in tasks where only one ‘object’ is processed at the same time (for example: line bisection, Milner’s landmark task, straight ahead pointing). The differential involvement of attention may thus underlie, at least partially, the dissociations between contralesional extinction and other forms of neglect, documented in the visual (Barbieri and De Renzi, 1989; Di Pellegrino and De Renzi, 1995) and auditory modality (Bellmann et al., 2001). The sound localisation and ITD diotic tasks also differ along another dimension. In the first case, the subjects are explicitly required to attribute a spatial co-ordinate to the sound target. In the second case, they are instructed to acknowledge and report the content of the information contained in the auditory field. We have reviewed above the growing evidence that the dichotomy between a 1The respective involvement of the What and Where processing streams has not been determined in these cases and further studies with precise anatomoclinical correlations are needed. 297 dorsal-spatial system and a ventral-object system also exists in the auditory modality. Other authors have tried to relate auditory neglect to this organisation of the auditory system (Soroker et al., 1995; Deouell and Soroker, 2000; Cusack et al., 2000). For instance, Soroker and collaborators (1995) have shown that extinguished verbal stimuli delivered through a loudspeaker on the left side could be reinstated if a fictitious loudspeaker was visible on the right, ipsilesional side. They interpreted the extinction phenomenon as a disconnection between the ‘Where’ system and the ‘What’ system (which could identify the target but was unable to locate it). The presence of a rightsided potential ‘where’ target would allow the processing of the information. Similar ability of right-damaged patients to identify left-targets that were mislocalised to the right side of space has been demonstrated more recently by the same group (Deouell and Soroker, 2000). The directional bias observed in auditory localisation in our study and by others can clearly be related to a distortion error within this dorsal system. The cerebral lesions responsible for this behaviour are also compatible with the dorsal auditory network revealed by functional studies (Maeder et al., 2001; Warren et al., 2002). They are mainly located in the right parietal lobe (Bisiach et al., 1984; Vallar et al., 1995; Sterzi et al., 1996; Haeske-Dewick et al., 1996) or in the right parietofronto-temporal region (Bellmann et al., 2001). The dichotic or diotic tasks have more links with the ventral system: they do not require overt space processing, the instruction being oriented to the analysis of the content of information. There is however a spatial component in the fact that the extinction is spatially lateralised. We shall argue below that a part of spatial processing is dedicated to the What system. Psychophysical studies in normal subjects suggest two different roles for auditory spatial cues. One is auditory localisation, i.e. the ability to attribute precise egocentric spatial co-ordinates to a sound, involving overt perception of sound location. Auditory spatial information can also be used as a cue for sound object segregation, also referred to as ‘auditory streaming’ (Bregman, 1990; Yost, 1991) or ‘cocktail party effect’ (Cherry, 1953). Spatial cues facilitate the grouping of sound components that belong to the same sound object and the distinction from other sound objects. Segregation of sound objects contributes thus decisively to sound recognition in noisy environment. Efron and collaborators have proposed the existence of a ‘temporal lobe enhancement mechanism’ whose function is to facilitate perception of sound sources located in the opposite side of space when other sounds are present throughout the auditory field. They found that this ‘cocktail party’ competence was impaired in the hemi-space contralateral to a temporal 298 Stephanie Clarke and Bellmann Thiran TABLE I Relationship between Processing within Auditory What and Where and Neglect What stream Type of spatial processing Role of spatial processing Manifestation of neglect Process underlying neglect Sound object segregation Facilitation of perception Contralesional omissions Attentional lobectomy (Efron et al., 1983). Carlyon and collaborators have described less stream segregation of tone sequences presented to the left than to the right ear of patients suffering from unilateral neglect (Carlyon et al., 2001). We have investigated sound localisation and spatial segregation of sound objects in a patient (NM) who complained of difficulties in localising sounds in everyday life after a right temporo-parieto-frontal ischemic lesion (Bellmann et al., 2003). Two groups of tasks were used, in which spatial dimension was simulated by interaural time difference (ITD): i) active localisation of stationary or moving sound targets, and ii) sound segregation on the basis of spatial cues. The latter included a spatial-release-from-masking paradigm and two ITD diotic tasks. NM failed to localise stationary and moving sounds: she perceived all the stimuli at the centre of the head, and could not differentiate stationary from moving targets. In contrast, NM was able to use ITD cues to segregate simultaneous sound sources in the spatial-release-from-masking paradigm and in ITD diotic tasks. These results suggest that sound localisation and sound object segregation based on spatial cues do not rely on the same mechanisms. We have thus described so far two different and at least partially independent uses of spatial cues, one implemented in the Where system, aimed to provide a precise spatial description and allow oriented responses to a sound target, the other at the service of the What system, involved in the segregation of simultaneous sound sources to facilitate perception. Neglect can be considered a high-level process which operates upon preliminary spatial processing (either localisation or segregation). We propose that each of the two types of neglect described above, i.e., contralesional omissions and directional errors, is more intimately linked with one form of auditory processing. Neglect of left-sided sound sources presented simultaneously with right-sided sources, can occur only if sound sources have been spatially segregated from each other. Alain and Arnott (2000) have argued that the ability to focus attention selectively on a particular sound source depends on a preliminary analysis that partitions the auditory input into distinct perceptual objects. Previous electrophysiological studies on sound object segregation have shown that these mechanisms occur at a pre-attentive level (Sussman et al., 1999; Yabe et al., 2001). The other type of Where stream Sound localisation Orientation to the sound Directional errors Distorted spatial representation auditory neglect (directional errors) is clearly linked to the Where system. For example, the alloacusis displayed by the patient AJ occurred only for extreme left stimuli (the more central leftsided stimuli were correctly set in the left hemispace). This suggests that the bias operated on a prior analysis of the spatial position. In summary, there is increasing evidence that processing within the auditory What and Where networks can be differentially affected in neglect (Table I). The ventral network is dedicated to the analysis of stimulus content and the dorsal to that of spatial position with respect to the body. In the auditory modality, spatial aspects are computed on the basis of monaural spectral cues or interaural differences in time or intensity level. These spatial cues serve, however, different goals. The most obvious is overt localisation of sounds with respect to the own body, implemented in the Where system. These spatial cues are, however, also used by the What system to help segregate simultaneous sound sources to facilitate perception in noisy environment. We propose that different types of auditory neglect may reflect disturbed attentional processing within the What and Where systems. 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Stephanie Clarke, Divisione de Neuropsychologie, CHUV, 1011 Lausanne. e-mail: [email protected]
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