Effect of prism adaptation on left dichotic listening deficit in neglect patients: glasses to hear better?

Unilateral neglect is a disabling syndrome frequently observed following right hemisphere brain damage. Symptoms range from visuo-motor impairments through to deficient visuo-spatial imagery, but impairment can also affect the auditory modality. A short period of adaptation to a rightward prismatic shift of the visual field is known to improve a wide range of hemispatial neglect symptoms, including visuo-manual tasks, mental imagery, postural imbalance, visuo-verbal measures and number bisection. The aim of the present study was to assess whether the beneficial effects of prism adaptation may generalize to auditory manifestations of neglect. Auditory extinction, whose clinical manifestations are independent of the sensory modalities engaged in visuo-manual adaptation, was examined in neglect patients before and after prism adaptation. Two separate groups of neglect patients (all of whom exhibited left auditory extinction) underwent prism adaptation: one group (n = 6) received a classical prism treatment ('Prism' group), the other group (n = 6) was submitted to the same procedure, but wore neutral glasses creating no optical shift (placebo 'Control' group). Auditory extinction was assessed by means of a dichotic listening task performed three times: prior to prism exposure (pre-test), upon prism removal (0 h post-test) and 2 h later (2 h post-test). The total number of correct responses, the lateralization index (detection asymmetry between the two ears) and the number of left-right fusion errors were analysed. Our results demonstrate that prism adaptation can improve left auditory extinction, thus revealing transfer of benefit to a sensory modality that is orthogonal to the visual, proprioceptive and motor modalities directly implicated in the visuo-motor adaptive process. The observed benefit was specific to the detection asymmetry between the two ears and did not affect the total number of responses. This indicates a specific effect of prism adaptation on lateralized processes rather than on general arousal. Our results suggest that the effects of prism adaptation can extend to unexposed sensory systems. The bottom-up approach of visuo-motor adaptation appears to interact with higher order brain functions related to multisensory integration and can have beneficial effects on sensory processing in different modalities. These findings should stimulate the development of therapeutic approaches aimed at bypassing the affected sensory processing modality by adapting other sensory modalities.

Cross-modal processing of auditory-visual stimuli in a no-task paradigm: a topographic event-related potential study.

OBJECTIVE: The aim of this study was to investigate in healthy adults the electrophysiological correlates of auditory-visual interactions involved in perception of bimodal events in a no-task paradigm. METHODS: Event-related potentials were recorded in response to unimodal auditory (A), unimodal visual (V) and bimodal (AV) stimuli. Cross-modal interactions were estimated using the additive [AV-(A+V)] model. RESULTS: The spatio-temporal analysis of ERPs and scalp current densities revealed several interaction patterns occurring at both early and late stages of sensory cortical processing: (1) amplitude decrease of the unimodal auditory N1 wave as early as 55ms, (2) amplitude increase of the unimodal auditory P2 wave from 150 to 195ms concomitant with new neural activity over the right fronto-temporal region, and (3) amplitude increase of the late unimodal visual response within 245-350ms post-stimulus. CONCLUSIONS: Our study provides evidence that several patterns of cross-modal interactions can be generated even if no task is required from subjects. SIGNIFICANCE: The paradigm used here can thus be utilized for studying the maturation of the cross-modal processes in young children and in children with pathological development.

Neurophysiological mechanisms of auditory selective attention in humans.

This chapter reviews the main data on the physiological substrates of auditory selective attention and their contribution to theoretical models of cognitive psychology. While event-related potentials, magnetoencephalography, and more recently neuroimaging techniques have provided fundamental information on the neural correlates of attention in the central cortical system, measurements of the frequency-following responses in the brainstem and evoked otoacoustic emissions at the cochlea strongly suggest attentional phenomena at the auditory periphery. We propose an adaptive filtering mechanism for selective auditory attention that can be flexibly and dynamically tuned depending on the attentional demand.

Auditory cortex activation in deaf subjects during cochlear electrical stimulation. Evaluation by functional magnetic resonance imaging.

RATIONALE AND OBJECTIVES: The authors detect activation in the auditory cortex during cochlear electrical stimulation in deaf patients using functional magnetic resonance (MR) imaging. METHODS: Stimulating electrode was inserted gently under local anesthesia close to the round window membrane of the cochlea in seven cochlear implant candidates. These patients suffered from postlingual-acquired deafness. Four patients were stimulated above the electrical perception threshold and three below the electrical discomfort threshold. Functional scans (fast low-angle shot 91 mseconds/60 mseconds) were acquired in an oblique axial plane running parallel to the sylvian fissure. Four consecutive series of six images were obtained in 6 minutes. The acquisition time of each image was 15 seconds. RESULTS: During electrical cochlear stimulation below the discomfort threshold, the three patients described "auditory" sensations with activation of the superior temporal regions. In two patients with electrical stimulation of the left ear, the maximum signal intensity increased by 8.42% in the right auditory cortex and 5.69% in the left. In one patient with a right electrical stimulation only the left cortex was activated. Electrical cochlear stimulation above the perception threshold induces no significant activation in the auditory cortex. CONCLUSION: Functioning MR imaging can detect activation in the auditory cortex during cochlear electrical stimulation in deaf patients using a conventional 1.5-tesla system in a routine hospital environment. Further studies are needed to investigate its usefulness in clinical practice.

Electrophysiological evidence for a shared representational medium for visual images and visual percepts.

Does mental imagery involve the activation of representations in the visual system? Systematic effects of imagery on visual signal detection performance have been used to argue that imagery and the perceptual processing of stimuli interact at some common locus of activity (Farah, 1985). However, such a result is neutral with respect to the question of whether the interaction occurs during modality-specific visual processing of the stimulus. If imagery affects stimulus processing at early, modality-specific stages of stimulus representation, this implies that the shared stimulus representations are visual, whereas if imagery affects stimulus processing only at later, amodal stages of stimulus representation, this implies that imagery involves more abstract, postvisual stimulus representations. To distinguish between these two possibilities, we repeated the earlier imagery-perception interaction experiment while recording event-related potentials (ERPs) to stimuli from 16 scalp electrodes. By observing the time course and scalp distribution of the effect of imagery on the ERP to stimuli, we can put constraints on the locus of the shared representations for imagery and perception. An effect of imagery was seen within 200 ms following stimulus presentation, at the latency of the first negative component of the visual ERP, localized at the occipital and posterior temporal regions of the scalp, that is, directly over visual cortex. This finding provides support for the claim that mental images interact with percepts in the visual system proper and hence that mental images are themselves visual representations.

Cerebral mechanisms underlying orienting of attention towards auditory frequency changes.

Brain mechanisms underlying detection of auditory frequency changes were studied with event-related potentials (ERPs) in 14 human subjects discriminating visual stimuli. Scalp-current density mapping revealed bilateral components of mismatch negativity (MMN) in frontal and auditory cortices. Deviance-related activations in frontal and temporal cortex began to be significant at 94 ms and 154 ms in the right hemisphere, and at 128 ms and 132 ms in the left hemisphere. The magnitude of MMN-neuroelectric currents from the left temporal cortex correlated significantly (r = -0.56, p < 0.05) with distraction caused by MMN-eliciting deviant tones. These results suggest a complex cerebral circuitry involved in frequency change detection and strongly support the role of this circuitry in driving attention involuntarily towards potentially relevant frequency changes in the acoustic environment.

Maturation of frontal and temporal components of mismatch negativity (MMN) in children.

The mismatch negativity (MMN) response of auditory ERPs in adults appears to result from several overlapping components involving both frontal and temporal brain areas. Our aim was to test whether a similar configuration could be observed in children, and to examine the maturation rates of the different components. MMN (standard tones: 1000 Hz, deviants: 1100 Hz) was recorded from 28 scalp electrodes in 24 healthy children aged from 5 to 10 and in eight adults for comparison. Scalp current density analysis revealed both temporal and frontal components in children of all ages as well as in adults. Moreover the amplitudes of the temporal components were significantly greater in children than in adults, whereas the frontal components were similar at all ages. The results strongly suggest that MMN is mediated by at least two separate neural systems, and that the frontal system matures earlier than the sensory-specific system.

Early signs of visual categorization for biological and non-biological stimuli in humans.

In a previous experiment aimed at studying gender processing from faces, we had found unexpected early ERP differences (45-85 ms) in task-irrelevant stimuli between a condition in which the stimuli of each gender were delivered in separate runs, and a condition in which the stimuli of both genders were mixed. Similar effects were observed with hand stimuli. These early ERP differences were tentatively related to incidental categorization processes between male and female stimuli. The present study was designed to test the robustness of these early effects for faces, and to examine whether similar effects can also be generated between two classes of non-biological stimuli. We replicated the previous findings for faces, and found similar early differential effects (50-65 ms) for non-biological stimuli (grey and hatched geometrical shapes) only, however, when the two shape categories were separated by conspicuous visual characteristics. While these results can partly be explained by phenomena related to neuronal habituation in the visual cortex, they may also suggest the existence of coarse and automatic categorization processes for rapid distinction between two wide classes of stimuli with strong psychosocial significance for humans.

Auditory selective attention in the human cochlea.

According to current theories, auditory selective attention alters the sensory analysis of acoustic inputs only in the central auditory system. Despite numerous attempts, no evidence of attentional selection has been found in the auditory periphery. Measurements of evoked otoacoustic emissions (EOAEs) during a selective dichotic listening task showed that the EOAEs to tones in one ear had larger amplitude when attention was directed to this ear than when attention was directed to the opposite ear. The results indicate that genuine effects of auditory selective attention can be observed at the cochlear receptor.

Auditory distraction: event-related potential and behavioral indices.

OBJECTIVE: The aim of this study was to illuminate behavioral and event-related potential (ERP) effects of attentional orienting and reorienting obtained in a newly developed auditory distraction paradigm, to provide more precise indicators about the neural generators of the ERP effects using scalp current density (SCD) analysis, and to evaluate the stability of the distraction effects. METHODS: In two sessions separated by 25 days, 10 subjects were presented with tones being of short (200 ms) and long (400 ms) duration equiprobably; tones were of high-probability standard or of low-probability deviant frequency. In Distraction condition, subjects had to behaviorally discriminate short from long tones. In Ignore condition, subjects were reading a book. Behavioral performance and multi-channel EEG were recorded. RESULTS: Task-irrelevant frequency deviations prolonged reaction times in the duration discrimination task by more than 35 ms and elicited the MMN and P3a components of the event-related potential. The P3a was followed by a negative deflection called RON (reorienting negativity). P3a and RON were absent in Ignore condition. All effects were found to be highly stable between sessions (product-moment correlations between 0.76 and 0.90). SCD analysis suggested frontal generators for P3a and for RON. CONCLUSIONS: It is demonstrated that small frequency deviations may yield distinct distraction effects in a tone duration discrimination task on a behavioral and on an electrophysiological level. Results support the hypothesis that frontal areas are involved in the exogenous orienting of attention (P3a) and in the reorienting of attention (RON). Due to the high stability of the deviance-related behavioral and ERP effects, this distraction paradigm may be utilized for clinical research.

Precautions in topographic mapping and in evoked potential map reading.

First, we consider the main points that must be addressed when constructing topographic maps: types of projection, methods of interpolation, number and locations of recording electrodes, and color scales. Data integrity and precautions in map interpretation are then examined for the case of evoked potential data.

Sequential colour mapping system of brain potentials.

We present a colour mapping system for the visualization of both the spatial scalp distribution and the temporal evolution of brain potentials. The system is applicable to recordings of auditory, visual and somatosensory potentials. It uses a Tektronix 4113 colour terminal connected to a Solar 16 (SEMS) mini-computer. The brain potentials are recorded on up to 16 scalp electrodes. The gain and the baseline are corrected separately on each channel. At each point of the scalp the potential is reconstructed by a linear interpolation of the measured potentials of the four nearest electrodes. Simultaneously n2 (1 less than n less than 8) colour maps can be presented on the screen. This allows the study of the temporal evolution of full scalp evoked potentials. The user chooses the two extreme latencies defining the time window to be explored and the latencies of the maps are regularly time-spaced within this window. In a typical case, in which four maps are desired, the latencies of the maps can be chosen independently. The 16-colour palette is predetermined but the user has three possibilities to establish the correspondence between the electrical potential and the colours. Examples are shown in the visual and somatosensory stimulation modalities. The advantages and limitations of such a representation are discussed.

Electrophysiological correlates of age and gender perception on human faces.

In a previous experiment using scalp event-related potentials (ERPs), we have described the neuroelectric activities associated with the processing of gender information on human faces (Mouchetant-Rostaing, Giard, Bentin, Aguera, & Pernier, 2000). Here we extend this study by examining the processing of age on faces using a similar experimental paradigm, and we compare age and gender processing. In one session, faces were of the same gender (women) and of one age range (young or old), to reduce gender and age processing. In a second session, faces of young and old women were randomly intermixed but age was irrelevant for the task, hence, age discrimination, if any, was assumed to be incidental. In the third and fourth sessions, faces had to be explicitly categorized according to their age or gender, respectively (intentional discrimination). Neither age nor gender processing affected the occipito-temporal N170 component often associated with the detection of physiognomic features and global structural encoding of faces. Rather, the three age and gender discrimination conditions induced similar fronto-central activities around 145-185 msec. In our previous experiment, this ERP pattern was also found for implicit and explicit categorization of gender from faces but not in a control condition manipulating hand stimuli (Mouchetant-Rostaing, Giard, Bentin, et al., 2000). Whatever their exact nature, these 145-185 msec effects therefore suggest, first, that similar mechanisms could be engaged in age and gender perception, and second, that age and gender may be implicitly processed irrespective of their relevance to the task, through somewhat specialized mechanisms. Additional ERP effects were found at early latencies (45-90 msec) in all three discrimination conditions, and around 200-400 msec during explicit age and gender discrimination. These effects have been previously found in control conditions manipulating nonfacial stimuli and may therefore be related to more general categorization processes.

Auditory-visual integration during multimodal object recognition in humans: a behavioral and electrophysiological study.

The aim of this study was (1) to provide behavioral evidence for multimodal feature integration in an object recognition task in humans and (2) to characterize the processing stages and the neural structures where multisensory interactions take place. Event-related potentials (ERPs) were recorded from 30 scalp electrodes while subjects performed a forced-choice reaction-time categorization task: At each trial, the subjects had to indicate which of two objects was presented by pressing one of two keys. The two objects were defined by auditory features alone, visual features alone, or the combination of auditory and visual features. Subjects were more accurate and rapid at identifying multimodal than unimodal objects. Spatiotemporal analysis of ERPs and scalp current densities revealed several auditory-visual interaction components temporally, spatially, and functionally distinct before 200 msec poststimulus. The effects observed were (1) in visual areas, new neural activities (as early as 40 msec poststimulus) and modulation (amplitude decrease) of the N185 wave to unimodal visual stimulus, (2) in the auditory cortex, modulation (amplitude increase) of subcomponents of the unimodal auditory N1 wave around 90 to 110 msec, and (3) new neural activity over the right fronto-temporal area (140 to 165 msec). Furthermore, when the subjects were separated into two groups according to their dominant modality to perform the task in unimodal conditions (shortest reaction time criteria), the integration effects were found to be similar for the two groups over the nonspecific fronto-temporal areas, but they clearly differed in the sensory-specific cortices, affecting predominantly the sensory areas of the nondominant modality. Taken together, the results indicate that multisensory integration is mediated by flexible, highly adaptive physiological processes that can take place very early in the sensory processing chain and operate in both sensory-specific and nonspecific cortical structures in different ways.

Several attention-related wave forms in auditory areas: a topographic study.

The purpose of this study was to progress in the understanding of the electrogenesis of attention-related wave forms in order to highlight some of the underlying attentional processes. ERPs were recorded from 16 electrodes, from 12 subjects who attended selectively to either high or low pitch tones delivered at a constant inter-stimulus interval of 800 msec to either right or left ear, while ignoring a concurrent sequence of tones of the other pitch delivered to the other ear. The attention-related wave forms were obtained by subtracting ERPs to unattended tones from ERPs to the same tones when they were attended. These wave forms were topographically displayed by both potential maps and scalp current density maps and compared with the corresponding maps of the N1 component of the ERPs, to determine the similarity of their generators. It has been shown that the attention effect is expressed by at least two components in specific auditory areas, one of small amplitude, occurring during the ascending slope of the N1 component, sensitive to the pitch of the attended stimulus, and possibly originating in the supratemporal plane of the auditory cortex; another of large amplitude, peaking symmetrically over both hemispheres and having a different topography from that of the N1 component. As described by other authors, a third, later, component appears over frontal areas, but probably originates from deeper sources of the brain. Models of selective attention processes, particularly the 'attentional trace' concept, are discussed in the light of these results.