Development of cortical influences on superior colliculus multisensory neurons: effects of dark‐rearing

Rearing cats from birth to adulthood in darkness prevents neurons in the superior colliculus (SC) from developing the capability to integrate visual and non‐visual (e.g. visual‐auditory) inputs. Presumably, this developmental anomaly is due to a lack of experience with the combination of those cues, which is essential to form associative links between them. The visual‐auditory multisensory integration capacity of SC neurons has also been shown to depend on the functional integrity of converging visual and auditory inputs from the ipsilateral association cortex. Disrupting these cortico‐collicular projections at any stage of life results in a pattern of outcomes similar to those found after dark‐rearing; SC neurons respond to stimuli in both sensory modalities, but cannot integrate the information they provide. Thus, it is possible that dark‐rearing compromises the development of these descending tecto‐petal connections and the essential influences they convey. However, the results of the present experiments, using cortical deactivation to assess the presence of cortico‐collicular influences, demonstrate that dark‐rearing does not prevent the association cortex from developing robust influences over SC multisensory responses. In fact, dark‐rearing may increase their potency over that observed in normally‐reared animals. Nevertheless, their influences are still insufficient to support SC multisensory integration. It appears that cross‐modal experience shapes the cortical influence to selectively enhance responses to cross‐modal stimulus combinations that are likely to be derived from the same event. In the absence of this experience, the cortex develops an indiscriminate excitatory influence over its multisensory SC target neurons.     

A temporal/nasal asymmetry for blindsight in a localisation task: evidence for extrageniculate mediation

Some patients with hemianopia due to striate cortex lesions show above chance ability in reporting visual stimuli presented in the blind visual field, a phenomenon commonly known as blindsight. Here we report a patient with a dense right hemianopia whose blindsight shows a temporal/nasal asymmetry. MP was tested in a two-alternative forced-choice localisation task, with either the right eye or the left eye patched in separate blocks. When targets appeared in the contralesional temporal hemifield, MP's localisation performance was extremely accurate, whilst she performed at chance with targets in the contralesional nasal hemifield. This is the first demonstration of a temporal/nasal asymmetry for blindsight in a forced-choice paradigm, and is consistent with blindsight in MP's hemianopic field being mediated by a subcortical, extrageniculate route.     

Two corticotectal areas facilitate multisensory orientation behavior

It had previously been shown that influences from two cortical areas, the anterior ectosylvian sulcus (AES) and the rostral lateral suprasylvian sulcus (rLS), play critical roles in rendering superior colliculus (SC) neurons capable of synthesizing their cross-modal inputs. The present studies examined the consequences of selectively eliminating these cortical influences on SC-mediated orientation responses to cross-modal stimuli. Cats were trained to orient to a low-intensity modality-specific cue (visual) in the presence or absence of a neutral cue from another modality (auditory). The visual target could appear at various locations within 45 degrees of the midline, and the stimulus effectiveness was varied to yield an average of correct orientation responses of approximately 45%. Response enhancement and depression were observed when the auditory cue was coupled with the target stimulus: A substantially enhanced probability in correct responses was evident when the cross-modal stimuli were spatially coincident, and a substantially decreased response probability was obtained when the stimuli were spatially disparate. Cryogenic blockade of either AES or rLS disrupted these behavioral effects, thereby eliminating the enhanced performance in response to spatially coincident cross-modal cues and degrading the depressed performance in response to spatially disparate cross-modal cues. These disruptive effects on targets contralateral to the deactivated cortex were restricted to multisensory interactive processes. Orientation to modality-specific targets was unchanged. Furthermore, the pattern of orientation errors was unaffected by cortical deactivation. These data bear striking similarities to the effects of AES and rLS deactivation on multisensory integration at the level of individual SC neurons. Presumably, eliminating the critical influences from AES or rLS cortex disrupts SC multisensory synthesis that, in turn, disables SC-mediated multisensory orientation behaviors.     

Noise‐rearing disrupts the maturation of multisensory integration

It is commonly believed that the ability to integrate information from different senses develops according to associative learning principles as neurons acquire experience with co‐active cross‐modal inputs. However, previous studies have not distinguished between requirements for co‐activation versus co‐variation. To determine whether cross‐modal co‐activation is sufficient for this purpose in visual–auditory superior colliculus (SC) neurons, animals were reared in constant omnidirectional noise. By masking most spatiotemporally discrete auditory experiences, the noise created a sensory landscape that decoupled stimulus co‐activation and co‐variance. Although a near‐normal complement of visual–auditory SC neurons developed, the vast majority could not engage in multisensory integration, revealing that visual–auditory co‐activation was insufficient for this purpose. That experience with co‐varying stimuli is required for multisensory maturation is consistent with the role of the SC in detecting and locating biologically significant events, but it also seems likely that this is a general requirement for multisensory maturation throughout the brain.     

Unilateral occipital lesion causing hemianopia affects the acoustic saccadic programming

Infrared oculographic recordings of saccades evoked by auditory or visual targets in four patients with hemianopia due to an occipital lesion showed that these patients employed a different strategy to find visual and auditory targets in each hemifield. In the seeing hemifield, the patients acquired auditory targets with both monosaccadic and multiple saccadic refixations. The first saccade, the largest, brought the eyes toward the target; the following smaller saccades completed the search as in normal subjects. Saccades to visual targets consisted of one orthometric saccade or two saccades. By contrast, in the blind hemifield the patients acquired auditory targets by a staircase strategy consisting of stepwise saccadic search movements similar to those used for visual targets in the same hemifield. The similarity of strategy to auditory and visual targets suggests a common motor program controlled by the visual input. The latency, accuracy, and velocities of visual and acoustic responses were equal in both hemifields.     

Parallel interhemispheric processing in hemineglect: Relation to visual field defects

Parallel interhemispheric processing is required to explore our visual environment and to integrate visual information from both hemifields simultaneously. Damage to the right temporo-parietal cortex can disrupt such parallel processes and result in neglect and visual extinction of stimuli in the left contralesional visual space. Neglected or extinguished stimuli can still be processed, yet without reaching the patient's awareness. Such unconscious processing has been attributed to structurally intact primary visual areas in neglect. To study whether unconscious parallel processing depends on visual functional integrity, we compared the performance of neglect patients with visual field defects (VFDs) (n = 11) and hemianopic patients with partial or complete blindness of one visual hemifield (n = 11) on redundant targets effects (RTE). The RTE manifests as faster reaction times to redundant paired (two stimuli, one in each hemifield) than single stimulation (in one hemifield). We found RTEs, i.e., unconscious processing, in neglect patients but not in hemianopic patients. Furthermore, neglect patients showed large crossed–uncrossed differences (CUDs), i.e., faster response times to ipsi- than contralesional hemifield stimulation, reflecting a difference in processing speed for single stimuli in the two hemispheres that were correlated with VFDs and visual extinction. The finding that extinction, but not RTE, was correlated with the CUD suggests that under competitive bilateral stimulus conditions the delayed contralesional visual field input may not be detected by the intact left hemisphere, which presumably mediates the task given the impairment of the right hemisphere. By contrast, unconscious parallel processing of contralesional stimuli (RTE) occurred even when contralesional visual field input is lacking (VFD) or delayed (CUD) and is possibly mediated via subcortical visual pathways.     

Eye-movement training-induced changes of visual field representation in patients with post-stroke hemianopia

Changes in neuronal activity have been described in patients with hemianopia following ischemic lesions of the visual cortex. This reorganization may facilitate compensation of lost visual function that is rarely fully restituted. Improving exploratory eye movements with appropriate training has been shown to partially compensate for the visuoperceptive impairment during daily life activities. The changes in cortical processing of visual stimuli that may be induced by these training strategies, however, are less well described. We used fMRI to study the training effects of eye-movement training on cortical representation of visual hemifields. Brain activation during hemifield stimulation was measured in eight patients with an occipital cortical lesion of the striate cortex causing homonymous hemianopia. Starting 8 weeks after the stroke, patients received 4 weeks of eye movement training. fMRI measurements were performed at baseline and after training. In five patients, follow-up fMRI was performed 4 weeks after the end of training. Differences in activation between rest and hemifield stimulation as well as before and after training were assessed with statistical parametric mapping. Twelve healthy subjects were scanned twice at a 4-week interval. During stimulation of the affected hemifield, significant activation at baseline was found bilaterally in extrastriate cortical areas, with the strongest increases in the contralesional hemisphere. This activation pattern was maintained after training. Four weeks after the end of training, there was an additional activation of the extrastriate cortex in the contralesional hemisphere compared to baseline. No changes in the size of visual field defects were found. In this group of patients, eye-movement training induced altered brain activation in the unaffected extrastriate cortex.     

The effect of hemidecortication on the inhibitory interactions in the superior colliculus of the cat.

Single neurons were recorded extracellularly from the superficial layers of the superior colliculus (SC) in 21 curarized cats. Four animals were normal unoperated cats, 17 were animals in which all cortical visual areas were ablated on one side from 7 to 69 days before the electrophysiological experiments. After cortical ablation all animals were blind in the visual field contralateral to the ablated side. In both normal and hemianopsic cats the effect of a visual stimulus located very far from the excitatory part of the unit receptive field, on the neuron responses to visual stimuli was studied. The remote stimulus (extra-field stimulus) was a hand moved black spot 10 degrees in diameter. In normal animals the introduction of the extra-field stimulus in the hemifield contralateral or ipsilateral to the recorded SC produced a marked reduction of unit responses to visual stimuli presented in their receptive field. This effect was particularly strong when the extra-field stimuli were introduced in the hemifield contralateral to the recorded side. In the hemianopsic animals the neurons of the SC ipsilateral to the lesion (receptive fields in the behaviorally blind hemifield) responded well to visual stimuli, but were only weakly inhibited by the extra-field stimuli presented in the blind hemifield. The neurons of this colliculus with the exception of those in the upper part of stratum griseum superficiale were normally inhibited by stimuli presented in the normal hemifield. The neurons of the SC contralateral to the lesion responded well to visual stimuli and were normally inhibited by stimuli presented in the normal hemifield; they were virtually not affected by stimuli presented in the blind hemifield. Mechanisms responsible for the abnormal inhibitory interactions between and within colliculi after cortical lesions and the possible behavioral implications of the findings are discussed.     

Audiovisual integration in patients with visual deficit

In the present study, we investigated the possibility that bimodal audiovisual stimulation of the affected hemifield can improve perception of the visual events in the blind hemifield of hemianopic patients, as it was previously demonstrated in neglect patients. Moreover, it has been shown that "hetero-modal" and "sensory-specific" cortices are involved in cross-modal integration. Thus, the second aim of the present study was to examine whether audiovisual integration influences visual detection in patients with different cortical lesions responsible of different kinds of visual disorders. More specifically, we investigated cross-modal, audiovisual integration in patients with visual impairment due to a visual field deficit (e.g., hemianopia) or visuospatial attentional deficit (e.g., neglect) and patients with both hemianopia and neglect. Patients were asked to detect visual stimuli presented alone or in combination with auditory stimuli that could be spatially aligned or not with the visual ones. The results showed an enhancement of visual detection in cross-modal condition (spatially aligned condition) comparing to unimodal visual condition only in patients with hemianopia or neglect; by contrast, the multisensory integration did not occur when patients presented both deficits. These data suggest that patients with visual disorders can enormously benefit the multisensory integration. Moreover, they showed a different influence of cortical lesion on multisensory integration. Thus, the present results show the important adaptive meaning of multisensory integration and are very promising with respect to the possibility of recovery from visual and spatial impairments.     

Multisensory perception of action in posterior temporal and parietal cortices

Environmental events produce many sensory cues for identifying the action that evoked the event, the agent that performed the action, and the object targeted by the action. The cues for identifying environmental events are usually distributed across multiple sensory systems. Thus, to understand how environmental events are recognized requires an understanding of the fundamental cognitive and neural processes involved in multisensory object and action recognition. Here, we investigated the neural substrates involved in auditory and visual recognition of object-directed actions. Consistent with previous work on visual recognition of isolated objects, visual recognition of actions, and recognition of environmental sounds, we found evidence for multisensory audiovisual event-selective activation bilaterally at the junction of the posterior middle temporal gyrus and the lateral occipital cortex, the left superior temporal sulcus, and bilaterally in the intraparietal sulcus. The results suggest that recognition of events through convergence of visual and auditory cues is accomplished through a network of brain regions that was previously implicated only in visual recognition of action.     

An event‐related FMRI study of exogenous orienting across vision and audition

The orienting of attention to the spatial location of sensory stimuli in one modality based on sensory stimuli presented in another modality (i.e., cross‐modal orienting) is a common mechanism for controlling attentional shifts. The neuronal mechanisms of top‐down cross‐modal orienting have been studied extensively. However, the neuronal substrates of bottom‐up audio‐visual cross‐modal spatial orienting remain to be elucidated. Therefore, behavioral and event‐related functional magnetic resonance imaging (FMRI) data were collected while healthy volunteers (N = 26) performed a spatial cross‐modal localization task modeled after the Posner cuing paradigm. Behavioral results indicated that although both visual and auditory cues were effective in producing bottom‐up shifts of cross‐modal spatial attention, reorienting effects were greater for the visual cues condition. Statistically significant evidence of inhibition of return was not observed for either condition. Functional results also indicated that visual cues with auditory targets resulted in greater activation within ventral and dorsal frontoparietal attention networks, visual and auditory “where” streams, primary auditory cortex, and thalamus during reorienting across both short and long stimulus onset asynchronys. In contrast, no areas of unique activation were associated with reorienting following auditory cues with visual targets. In summary, current results question whether audio‐visual cross‐modal orienting is supramodal in nature, suggesting rather that the initial modality of cue presentation heavily influences both behavioral and functional results. In the context of localization tasks, reorienting effects accompanied by the activation of the frontoparietal reorienting network are more robust for visual cues with auditory targets than for auditory cues with visual targets. Hum Brain Mapp 35:964–974, 2014. © 2013 Wiley Periodicals, Inc.     

Behavioural benefits of multisensory processing in ferrets

Enhanced detection and discrimination, along with faster reaction times, are the most typical behavioural manifestations of the brain's capacity to integrate multisensory signals arising from the same object. In this study, we examined whether multisensory behavioural gains are observable across different components of the localization response that are potentially under the command of distinct brain regions. We measured the ability of ferrets to localize unisensory (auditory or visual) and spatiotemporally coincident auditory–visual stimuli of different durations that were presented from one of seven locations spanning the frontal hemifield. During the localization task, we recorded the head movements made following stimulus presentation, as a metric for assessing the initial orienting response of the ferrets, as well as the subsequent choice of which target location to approach to receive a reward. Head‐orienting responses to auditory–visual stimuli were more accurate and faster than those made to visual but not auditory targets, suggesting that these movements were guided principally by sound alone. In contrast, approach‐to‐target localization responses were more accurate and faster to spatially congruent auditory–visual stimuli throughout the frontal hemifield than to either visual or auditory stimuli alone. Race model inequality analysis of head‐orienting reaction times and approach‐to‐target response times indicates that different processes, probability summation and neural integration, respectively, are likely to be responsible for the effects of multisensory stimulation on these two measures of localization behaviour.     

Retinal projections and functional architecture of cortical areas 17 and 18 in the tyrosinase-negative albino cat

The visual field representation and functional architecture of cortical areas 17 and 18 in albino cats were studied. In the same animals the distributions of ipsilaterally and contralaterally projecting retinal ganglion cells were determined by injecting horseradish peroxidase into the dorsal lateral geniculate nucleus or optic tract. All cats were tyrosinase-negative albinos (cc), not deaf white cats (W). The proportion of ipsilaterally projecting ganglion cells in the temporal retina of the albino cat was found to be much smaller than in the normal cat or in the Siamese cat. In the albino cat less than 5% of ganglion cells in temporal retina project ipsilaterally. Recordings from areas 17 and 18 provided evidence of a substantial representation of the ipsilateral hemifield in albino visual cortex; cells representing the contralateral and ipsilateral hemifields were often segregated into alternating zones in area 17 and were always segregated in area 18. Cells recorded at the borders of zones representing the ipsilateral and contralateral hemifields often had abnormal properties. Some border cells had two receptive fields separated by as much as 60 degrees of azimuth; one field subserved the contralateral hemifield (contralateral nasal retina) and the other subserved the mirror-symmetric part of ipsilateral hemifield (contralateral temporal retina). Receptive fields of cells subserving the two hemifields did not differ in size. The preferred orientations, preferred velocities, and other characteristics of the two fields were approximately the same; preferred orientation changed gradually and systematically across the borders of zones representing the two hemifields. Our results indicate that afferents representing nasal and temporal regions of retina of the same eye can segregate and form "hemiretina" domains in albino visual cortex. These afferents can also converge upon individual cortical cells in a fashion reminiscent of convergence of afferents from the two eyes upon binocular cells in the normal cortex. The organization of albino visual cortex is therefore different from the organization of Siamese visual cortex. This may be because, in the albino cat but not the Siamese cat, nearly all cells in temporal retina project contralaterally; afferents representing contralateral temporal retina are not at a significant competitive disadvantage in the albino.     

Hemifield effects of spatial attention in early human visual cortex

Early visual areas (V1, V2, V3/VP, V4v) contain representations of the contralateral hemifield within each hemisphere. Little is known about the role of the visual hemifields along the visuo-spatial attention processing hierarchy. It is hypothesized that attentional information processing is more efficient across the hemifields (known as bilateral field advantage) and that the integration of information is greater within one hemifield as compared with across the hemifields. Using functional magnetic resonance imaging we examined the effect of distance and hemifield on parallel attentional processing in the early visual areas (V1-V4v) at individually mapped retinotopic locations aligned adjacently or separately within or across the hemifields. We found that the bilateral field advantage in parallel attentional processing over separated attended locations can be assigned, at least partly, to differences in distractor position integration in early visual areas. These results provide evidence for a greater integration of locations between two attended locations within one hemifield than across both hemifields. This nicely correlates with behavioral findings of a bilateral field advantage in parallel attentional processing (when distractors in between cannot be excluded) and a unilateral field advantage if attention has to be shifted across separated locations (when locations in between were integrated).     

Verbal visual extinction in right/left hemisphere lesion patients and the problem of lexical access

In order to explore perceptual processes preceding visual lexical access, we compared single words reading in both visual hemifields in right or left hemisphere damaged patients. Words were tachistoscopically presented in three conditions: (1) one word in one visual hemifield, (2) two words, one in each visual hemifield aside the fovea, (3) one longer word in both visual hemifields, centred on the fovea. In many patients, suffering from right as well as left hemisphere lesions, reading remains normal for unilateral presentation whereas bilateral presentation induced extinction in the hemifield contralateral to the lesion. Nevertheless, extinction was not frequently observed for the right or the left part of long centred words. Extinction proved to be most frequent in lesions centred on the occipito-temporo-parietal area. In addition, we found visual extinction not to be related to auditory extinction in verbal dichotic listening.     

Disinhibition of the superior colliculus restores orienting to visual stimuli in the hemianopic field of the cat

Following unilateral removal of all known visual cortical areas, a cat is rendered hemianopic in the contralateral visual field. Visual orientation can be restored to the blind hemifield by transection of the commissure of the superior colliculus or by destruction of the superior colliculus (SC) or the substantia nigra pars reticulata (SNpr) contralateral to the cortical lesion. It is hypothesized that a mechanism mediating recovery is disinhibition of the SC ipsilateral to the cortical lesion. The ipsilateral nigrotectal projection exerts a robust inhibitory tone onto cells in the SC. However, ibotenic acid destruction of SNpr neurons, which should decrease inhibition onto the SC, does not result in recovery. The failure of ipsilateral SNpr lesions to produce recovery puts into question the validity of SC disinhibition as a mechanism of recovery. We directly tested the disinhibition hypothesis by reversibly disinhibiting the SC ipsilateral to a visual cortical lesion with a gamma-aminobutyric acid (GABA)_A antagonist, bicuculline methiodide. In accordance with the hypothesis, transient disinhibition of the SC restored visual orienting for several hours in three of eight animals. Recovery was not a volume or pH effect and was distinct from the release of irrepressible motor effects (i.e., approach and avoidance behaviors) seen within the first hour after injection. Thus, in the absence of all visual cortical areas unilaterally, disinhibition of the SC can transiently restore the ability of the cat to orient to visual stimuli in the previously “blind” hemifield. J. Comp. Neurol. 387:568–587, 1997. © 1997 Wiley-Liss, Inc.     

Cortical and subcortical projections to primary visual cortex in anophthalmic,enucleated and sighted mice

The purpose of this study was to identify and compare the afferent projections to the primary visual cortex in intact and enucleated C57BL/6 mice and in ZRDCT/An anophthalmic mice. Early loss of sensory‐driven activity in blind subjects can lead to activations of the primary visual cortex by haptic or auditory stimuli. This intermodal activation following the onset of blindness is believed to arise through either unmasking of already present cortical connections, sprouting of novel cortical connections or enhancement of intermodal cortical connections. Studies in humans have similarly demonstrated heteromodal activation of visual cortex following relatively short periods of blindfolding. This suggests that the primary visual cortex in normal sighted subjects receives afferents, either from multisensory association cortices or from primary sensory cortices dedicated to other modalities. Here cortical afferents to the primary visual cortex were investigated to determine whether the visual cortex receives sensory input from other modalities, and whether differences exist in the quantity and/or the structure of projections found in sighted, enucleated and anophthalmic mice. This study demonstrates extensive direct connections between the primary visual cortex and auditory and somatosensory areas, as well as with motor and association cortices in all three animal groups. This suggests that information from different sensory modalities can be integrated at early cortical stages and that visual cortex activations following visual deprivations can partly be explained by already present intermodal corticocortical connections.     

Auditory cortical projection from the anterior ectosylvian sulcus (Field AES) to the superior colliculus in the cat: an anatomical and electrophysiological study

Sensory activity in the deep layers of the superior colliculus (SC) is strongly influenced by descending cortical inputs. Elimination (permanent or reversible) of specific regions of visual or somatosensory cortex, known to have direct access to the SC, abolishes or dramatically reduces SC responses to stimuli from those modalities. While many SC neurons are also responsive to auditory cues, the origin of auditory corticotectal connections is not clear at present and their affect on activity in the SC is unknown. Therefore, the present study was undertaken to determine the origin, organization, and functional characteristics of auditory corticotectal projections. Of the auditory cortices (AI; AII; Fields A, P, and VP), only the auditory subregion of the banks of the anterior ectosylvian sulcus (Field AES) showed a robust anatomical projection to the SC. These data were confirmed physiologically: auditory neurons in Field AES projected to the SC and auditory SC neurons responded to stimulation of the Field AES. However, neither anatomical nor physiological techniques revealed a clear topographic relationship between the Field AES and the SC but suggested instead a diffuse and extremely divergent/convergent projection. Stimulation and cryoblockade of Field AES demonstrated the excitatory nature of this corticotectal pathway, whose influence was most evident on SC responses to stimuli of reduced intensity. Given the short latency of this ear-cortex-SC circuit and its excitatory influence on unimodal as well as on multisensory auditory neurons, it seems likely that Field AES plays a significant role in facilitating SC responses to auditory stimuli.     

Is the prefrontal cortex organized by supramodal or modality-specific sensory demands during adolescence?

Attention is inherently biased towards the visual modality during most multisensory scenarios in adults, but the developmental trajectory towards visual dominance has not been fully elucidated. More recent evidence in primates and adult humans suggests a modality-specific stratification of the prefrontal cortex. The current study therefore used functional magnetic resonance imaging (fMRI) to investigate the neuronal correlates of proactive (following cues) and reactive (following probes) cognitive control for simultaneous audio-visual stimulation in 67 healthy adolescents (13–18 years old). Behavioral results were only partially supportive of visual dominance in adolescents, with both reduced response times and accuracy during attend-visual relative to attend-auditory trials. Differential activation of medial and lateral prefrontal cortex for processing incongruent relative to congruent stimuli (reactive control) was also only observed during attend-visual trials. There was no evidence of modality-specific prefrontal cortex stratification during the active processing of multisensory stimuli or during separate functional connectivity analyses. Attention-related modulations were also greater within visual relative to auditory cortex, but were less robust than observed in previous adult studies. Collectively, current results suggest a continued transition towards visual dominance in adolescence, as well as limited modality-specific specialization of prefrontal cortex and attentional modulations of unisensory cortex.     

Contrasting spatial hearing deficits in hemianopia and spatial neglect

Spatial hearing deficits have been described in widely differing pathologies, including bilateral temporal or unilateral parietal lesions, hemispherectomy, spatial neglect and right-sided cortical lesions without neglect. However, the topography of spatial hearing deficits after cortical lesions is only poorly understood, unlike that of vision and touch. We investigated the auditory subjective straight ahead (SSA) with a new technique of binaural sound source simulation using broad-band single pulses which were filtered with head-related transfer functions and delivered with a 5 degree resolution over headphones in front space. Normal subjects showed quite accurate judgments of the SSA, with a small but significant shift to the left of centre (-1.7 degrees) in the horizontal plane. Hemineglect without a scotoma, produced a large ipsilesional deviation of the auditory SSA (+22 degrees), while two hemianopic subjects, both without neglect, showed the opposite deviation of their perceived auditory SSA towards their contralesional, blind hemifield (+10 vs -28 degrees). Two control patients with unilateral lesions, both without neglect and without hemianopia, produced normal judgments of their auditory SSA (-3.0 degrees, +3.8 degrees). These results suggest at least two contrasting influences on directional spatial hearing after unilateral cortical lesions: hemianopia vs hemispatial neglect. The results are interpreted in favour of multisensory convergence of visual and auditory information in directional spatial hearing.