Reversibility of abnormal auditory findings in cerebral hemisphere lesions

Three cases are presented to illustrate the reversibility of abnormal central auditory function in patients with cerebral hemisphere lesions. In 1 patient, abnormal scores with dichotic simultaneous-binaural and competing sentence discrimination tests in the ear contralateral to the affected hemisphere returned to normal after evacuation of an intracerebral hematoma of the right temporal lobe. In the other 2 cases, abnormal dichotic speech discrimination test scores in the left ear improved for a period of time after decompression and subtotal removal of astrocytomas deep in the parietal lobe of the right hemisphere. Eventually, central auditory test findings became abnormal again in these 2 cases which was consistent with the deteriorating condition of the patients. Findings in these 3 cases indicate that distorted and dichotic speech discrimination tests yield information which may be useful in monitoring the course of patients with reversible as well as progressive lesions of the brain.     

Right hemisphere specialization for intensity discrimination of musical and speech sounds

Sound intensity is the primary and most elementary feature of auditory signals. Its discrimination plays a fundamental role in different behaviours related to auditory perception such as sound source localization, motion detection, and recognition of speech sounds. This study was aimed at investigating hemispheric asymmetries for processing intensity of complex tones and consonant-vowel syllables. Forty-four right-handed non-musicians were presented with two dichotic matching-to-sample tests with focused attention: one with complex tones of different intensities (musical test) and the other with consonant-vowel syllables of different intensities (speech test). Intensity differences (60, 70, and 80 dBA) were obtained by altering the gain of a synthesized harmonic tone (260 Hz fundamental frequency) and of a consonant-vowel syllable (/ba/) recorded from a natural voice. Dependent variables were accuracy and reaction time. Results showed a significant clear-cut left ear advantage in both tests for both dependent variables. A monaural control experiment ruled out possible attentional biases. This study provides behavioural evidence of a right hemisphere specialization for the perception of the intensity of musical and speech sounds in healthy subjects.     

Psychoacoustic and electrophysiologic correlates of central hearing disorders in man

Summary Evaluation of central hearing disorders in neuropsychologic patients is handicapped by their insufficient ability to describe auditory deficits and by the lack of easily applicable audiological tests. A novel psychoacoustic discrimination test (PDT) was developed to determine ear asymmetries in the discrimination of changes in intensity, frequency, or temporal structure of regularly presented dichotic stimuli. In 19 of 21 patients with lesions of the auditory cortex or the acoustic radiation according to CT scan evaluation a higher error score was observed for target stimuli presented at the ear contralateral to the side of brain infarction (6 right, 15 left). In the remaining 2 and in 3 other patients with lesions sparing auditory structures no significant ear asymmetries were seen. This may indicate that auditory perception is reduced in patients with only one intact auditory cortex or one intact acoustic radiation, possibly because of a limitation in information processing capacity. Auditory evoked potential results are presented for a normal subject and two patients to illustrate electrophysiologic correlates of central hearing disorders. Using a transformation of scalp into dipole source activity (Scherg and von Cramon 1986), a unilateral loss of middle latency activity was found in case A, who had a lesion of the left acoustic radiation. The extended lesion of the right auditory cortex in case B resulted in a loss of both middle and late latency dipole source potentials of the right temporal lobe. In both cases a corresponding increase in the PDT error score on the contralateral ear was found.     

Altered processing of communication signals in the subcortical auditory sensory pathway in autism

Autism spectrum disorder (ASD) is characterised by social communication difficulties. These difficulties have been mainly explained by cognitive, motivational, and emotional alterations in ASD. The communication difficulties could, however, also be associated with altered sensory processing of communication signals. Here, we assessed the functional integrity of auditory sensory pathway nuclei in ASD in three independent functional magnetic resonance imaging experiments. We focused on two aspects of auditory communication that are impaired in ASD: voice identity perception, and recognising speech‐in‐noise. We found reduced processing in adults with ASD as compared to typically developed control groups (pairwise matched on sex, age, and full‐scale IQ) in the central midbrain structure of the auditory pathway (inferior colliculus [IC]). The right IC responded less in the ASD as compared to the control group for voice identity, in contrast to speech recognition. The right IC also responded less in the ASD as compared to the control group when passively listening to vocal in contrast to non‐vocal sounds. Within the control group, the left and right IC responded more when recognising speech‐in‐noise as compared to when recognising speech without additional noise. In the ASD group, this was only the case in the left, but not the right IC. The results show that communication signal processing in ASD is associated with reduced subcortical sensory functioning in the midbrain. The results highlight the importance of considering sensory processing alterations in explaining communication difficulties, which are at the core of ASD.     

Perception of dichotic chords by normal and commisurotomized human subjects

A perceptual suppression of an ipsilateral by a concurrent contralateral auditory signal occurs in commissurotomized subjects and probably in normal subjects. This suppression of the ipsilateral signal depends on the nature of the auditory stimuli. For dichotic speech sounds the suppression of the ipsilateral signal is overwhelming; for dichotically presented pure tones it is not present. For dichotically presented pure tones, unlike speech signals, a subcortical central pitch processor determines the contribution to the perceptual experience of the right and left ear signals in both normal as well as commissurotomized subjects.     

Temporal and speech processing deficits in auditory neuropathy

Auditory neuropathy affects the normal synchronous activity in the auditory nerve, without affecting the amplification function in the inner ear. Patients with auditory neuropathy often complain that they can hear sounds, but cannot understand speech. Here we report psychophysical tests indicating that these patients' poor speech recognition is due to a severe impairment in their temporal processing abilities. We also simulate this temporal processing impairment in normally hearing listeners and produce similar speech recognition deficits. This study demonstrates the importance of neural synchrony for auditory perceptions including speech recognition in humans. The results should contribute to better diagnosis and treatment of auditory neuropathy.     

Side of the stimulated ear influences the hemispheric balance in coding tonal stimuli

OBJECTIVE: To evaluate whether the side of stimulated ear affects the hemispheric asymmetry of auditory evoked cortical activations. METHODS: Using a whole-head neuromagnetometer, we recorded neuromagnetic approximately 100 ms responses (N100m) in 21 healthy right-handers to 100 ms 1 kHz tones delivered alternatively to left and right ear. RESULTS: Although the peak latencies of N100m were shorter in contralateral than in ipsilateral hemisphere, the difference was significant only for the left ear stimulation. Based on the relative N100m amplitudes across hemispheres, the laterality evaluation showed a rightward predominance of N100m activation to tone stimuli, but the lateralization toward the right hemisphere was more apparent by the left than by the right ear stimulation (laterality index: -0.27 versus -0.10, p=0.008). Within the right hemisphere, the N100m was 2-4 mm more posterior for left ear than for right ear stimulation. CONCLUSIONS: The hemispheric asymmetry in auditory processing depends on the side of the stimulated ear. The more anterior localization of right N100m responses to ipsilateral than to contralateral ear stimulation suggests that there might be differential neuronal populations in the right hemisphere for processing spatially different auditory inputs.     

Neuro-audiological abnormalities in patients with temporal lobe tumors

A battery of neuro-audiological tests was given to 11 patients with unilateral tumors of the temporal lobe. Tests of central auditory function included low-pass filtered speech discrimination, binaural resynthesis of segmented speech, alternate and simultaneous-binaural speech discrimination, and competing sentence discrimination tests. Findings with these tests demonstrated abnormal scores in the ear contralateral to the affected temporal lobe in either the left or right hemisphere. Patients with tumors located in the posterior region of the temporal lobe yielded abnormal scores with the monotic and dichotic tests in the battery. Patients with tumors located in the antero-inferior regions of the temporal lobe demonstrated abnormal test scores in the contralateral ear with low-pass filtered discrimination tests and experienced little or no difficulty with alternate and simultaneous-binaural discrimination or competing sentence discrimination. Binaural resynthesis of segmented speech was abnormal in 4 patients with documented brain stem involvement secondary to temporal lobe tumor. A hypothesis is offered to explain the contralateral ear effect and the differences in test results obtained in patients with posterior and antero-inferior temporal lobe tumors.     

Speech-in-noise perception deficit in adults with dyslexia: effects of background type and listening configuration

Developmental dyslexia is associated with impaired speech-in-noise perception. The goal of the present research was to further characterize this deficit in dyslexic adults. In order to specify the mechanisms and processing strategies used by adults with dyslexia during speech-in-noise perception, we explored the influence of background type, presenting single target-words against backgrounds made of cocktail party sounds, modulated speech-derived noise or stationary noise. We also evaluated the effect of three listening configurations differing in terms of the amount of spatial processing required. In a monaural condition, signal and noise were presented to the same ear while in a dichotic situation, target and concurrent sound were presented to two different ears, finally in a spatialised configuration, target and competing signals were presented as if they originated from slightly differing positions in the auditory scene. Our results confirm the presence of a speech-in-noise perception deficit in dyslexic adults, in particular when the competing signal is also speech, and when both signals are presented to the same ear, an observation potentially relating to phonological accounts of dyslexia. However, adult dyslexics demonstrated better levels of spatial release of masking than normal reading controls when the background was speech, suggesting that they are well able to rely on denoising strategies based on spatial auditory scene analysis strategies.     

Identification and discrimination disorders in auditory perception: a report on two cases

Auditory perception was investigated in two brain-damaged subjects. The first patient had a left temporoparietal ischaemic lesion. He presented a right-ear extinction in dichotic tasks, as well as difficulties in understanding and repeating verbal material and impaired identification of melodies. All discrimination tests were well performed. The second patient had a right capsulolenticular and frontal ischaemic lesion. He presented a left ear dichotic extinction and severe difficulties in discrimination of environmental sounds and melodies but no major difficulty in naming and identification. From these results, it is hypothesized that identification and discrimination involve distinct mechanisms within the processing of auditory stimuli, and that they may be selectively disrupted in brain-damaged subjects.     

Sex and Hemispheric Differences for Rapid Auditory Processing in Normal Adults

Previous research suggests that left hemisphere specialisation for processing speech may specifically depend on rate-specific parameters, with rapidly successive or faster changing acoustic stimuli (e.g. stop consonant-vowel syllables) processed preferentially by the left hemisphere. The current study further investigates the involvement of the left hemisphere in processing rapidly changing auditory information, and examines the effects of sex on the organisation of this function. Twenty subjects participated in an auditory discrimination task involving the target identification of a two-tone sequence presented to one ear, paired with white noise to the contralateral ear. Analyses demonstrated a right ear advantage for males only at the shorter interstimulus interval durations (mean = 20msec) whereas no ear advantage was observed for women. These results suggest that the male brain is more lateralised for the processing of rapidly presented auditory tones, specifically at shorter stimulus durations.     

Sex and hemispheric differences for rapid auditory processing in normal adults

Previous research suggests that left hemisphere specialisation for processing speech may specifically depend on rate-specific parameters, with rapidly successive or faster changing acoustic stimuli (e.g. stop consonant-vowel syllables) processed preferentially by the left hemisphere. The current study further investigates the involvement of the left hemisphere in processing rapidly changing auditory information, and examines the effects of sex on the organisation of this function. Twenty subjects participated in an auditory discrimination task involving the target identification of a two-tone sequence presented to one ear, paired with white noise to the contralateral ear. Analyses demonstrated a right ear advantage for males only at the shorter interstimulus interval durations (mean = 20 msec) whereas no ear advantage was observed for women. These results suggest that the male brain is more lateralised for the processing of rapidly presented auditory tones, specifically at shorter stimulus durations.     

Left hemisphere specialization for rapid temporal processing: a study with auditory 40 Hz steady-state responses.

OBJECTIVE: To investigate rapid temporal processing in the auditory cortex by using auditory 40 Hz steady-state responses (SSRs). METHODS: A 40 Hz tone-burst at 500 Hz spectral frequency was presented monaurally to record SSRs in 10 normal subjects. The recording electrodes were placed over C1, C2, C3, C4, C5, C6, T3, T4, Fz, Cz and Pz, referring to an electrode at the 7th cervical spinous process. For comparison, unstimulated SSRs were recorded. A total of 200 responses of 1s epoch were averaged and subjected to discrete fast Fourier transforms to yield the amplitude and phase of the 40 Hz component. The coherence (Coh) values of the 40 Hz component between homologous electrodes were also calculated. RESULTS: At the temporal electrodes contralateral to the stimulated ear, the amplitude was significantly larger and its phase was significantly smaller than those of the ipsilateral side. The interhemispheric Coh between T3 and T4 in response to right ear stimulation was significantly greater than those of left ear stimulation or the unstimulated condition. CONCLUSIONS: Our results suggest that 40 Hz auditory information is predominantly processed in the left auditory cortex, interacting with the right hemisphere. This finding is consistent with the fact that the left auditory cortex plays an important role in rapid temporal processing. SIGNIFICANCE: Auditory 40 Hz SSRs with Coh analysis are useful for investigating the left hemisphere specialization for rapid temporal processing.     

Beta‐band activity in auditory pathways reflects speech localization and recognition in bilateral cochlear implant users

In normal‐hearing listeners, localization of auditory speech involves stimulus processing in the postero‐dorsal pathway of the auditory system. In quiet environments, bilateral cochlear implant (CI) users show high speech recognition performance, but localization of auditory speech is poor, especially when discriminating stimuli from the same hemifield. Whether this difficulty relates to the inability of the auditory system to translate binaural electrical cues into neural signals, or to a functional reorganization of auditory cortical pathways following long periods of binaural deprivation is unknown. In this electroencephalography study, we examined the processing of auditory syllables in postlingually deaf adults with bilateral CIs and in normal‐hearing adults. Participants were instructed to either recognize (“recognition” task) or localize (“localization” task) the syllables. The analysis focused on event‐related potentials and oscillatory brain responses. N1 amplitudes in CI users were larger in the localization compared with recognition task, suggesting an enhanced stimulus processing effort in the localization task. Linear beamforming of oscillatory activity in CI users revealed stronger suppression of beta‐band activity after 200 ms in the postero‐dorsal auditory pathway for the localization compared with the recognition task. In normal‐hearing adults, effects for longer latency event‐related potentials were found, but no effects were observed for N1 amplitudes or beta‐band responses. Our study suggests that difficulties in speech localization in bilateral CI users are not reflected in a functional reorganization of cortical auditory pathways. New signal processing strategies of cochlear devices preserving unambiguous binaural cues may improve auditory localization performance in bilateral CI users. Hum Brain Mapp 35:3107–3121, 2014. © 2013 Wiley Periodicals, Inc .     

Late auditory evoked potentials asymmetry revisited.

OBJECTIVE: To investigate brain asymmetries of the auditory evoked potential (AEP) N100, T-complex, and P200 in response to monaural stimulation. METHODS: Electroencephalographic (EEG) recordings from 68 channels were used to record auditory cortex responses to monaural stimulation from normal hearing participants (N=16). White-noise stimuli and 1000Hz tones were repeatedly presented to either the left or right ear. Source localization of the AEP N100 response was carried out with two symmetric regional sources placed into left and right auditory cortex. Regional source waveform amplitude and latency asymmetries were analyzed for tangential and radial activity explaining the N100, T-complex and P200 AEP components. RESULTS: Regional source waveform analysis showed that early tangential activity in the N100 latency range exhibited larger contralateral amplitudes and shorter latencies for both tone and noise monaural stimuli. Lateralized activity was significantly greater when tones or noise was presented to the left compared to the right ear (p<.001). The ear difference in the degree of lateralization arose due to hemispheric asymmetry. Significantly more tangential activity in the N100 latency range was recorded in the right compared to the left hemisphere in response to stimulation by either tones or noise (p<.001). Neither the radial activity modelling the T-complex, nor activity modelling the P200, showed robust ear or hemisphere differences. CONCLUSIONS: Regional source waveform analysis revealed that the extent of auditory evoked potential asymmetries depends on the ear and hemisphere examined. These findings have implications for future studies utilizing AEP asymmetries to examine normal auditory function or experience-related changes in the auditory cortex. SIGNIFICANCE: The right compared to the left auditory cortex may be more involved in processing monaurally presented tone and noise stimuli.     

Right unilateral auditory agnosia following left lenticular hemorrhage

A 33-year old patient who had had left lenticular hemorrhage presented with an inability to understand with the right ear oral language and, in a less dramatic way, nonverbal sounds. This unilateral auditory agnosia was first associated with a right motor underutilization and right motor, sensitive, visual and auditive extinctions. Speech discrimination scores were 100% with the left ear and 15% with the right ear, even less in dichotic conditions. Tonal audiogram, as well as early and late components of the auditory evoked potentials were normal. Cerebral regional perfusion and metabolism were impaired over the left parietotemporal area. There was severe hypoactivation of the left hemisphere with right monaural verbal stimulations. Rehabilitation consisting of non-specific attention tasks and repetitions of words reaching only the right ear was undertaken 15 months after the stroke. The oral language comprehension improved, as did the left hemisphere activation, and the extinction phenomena disappeared, except for the auditory one. The unilaterality of the auditory agnosia could be due, in part, to a peculiar physiological processing in this patient, such as poor performance of his right ipsilateral auditory pathway which could be improved with practice. A striatal lesion could induce a spatial hemi-inattention as reflected by the multimodal extinction in this case. Besides, a lack of selective activation for verbal stimulation of the left hemisphere is suggested.     

Lateral response dynamics and hemispheric dominance for speech perception

In this study, we investigated the mechanism for the left cerebral hemisphere's dominance for speech perception. We utilized the crossover of auditory pathways in the central nervous system to present speech stimuli more directly to the left hemisphere (via the right ear) and right hemisphere (via the left ear). Using functional MRI, we found that estimated duration of neural response in the left auditory cortex increased as more speech information was directly received from the right ear. Conversely, response duration in the right auditory cortex was not modulated when more speech information was directly received from the left ear. These data suggest that selective temporal responding distinguishes the dominant from nondominant hemisphere of the human brain during speech perception.     

Auditory brain-stem potentials with unilateral pontine hemorrhage

Although there have been extensive anatomical and physiological studies in animals, the actual neural sources, or even the laterality, of some components of auditory brain-stem evoked potentials in humans are uncertain. We studied these responses in a 56-year-old patient who had a clearly demarcated pontine hemorrhage on the right side. The patient was somnolent, with dense left hemiplegia and signs of involvement of right cranial nerves V, VI, and VII. Stimulation of the left ear (ie, contralateral to the lesion) evoked a normal series of waves with clearly resolved positive components peaking at 2.0, 3.3, 4.8 (wave IV), and 6.0 ms (wave V). Stimulation of the right ear (ie, ipsilateral to the lesion) evoked only waves I, III, and IV. These results suggest that a pathway ipsilateral to the stimulated ear is necessary and sufficient for generation of auditory wave V and that wave IV is generated in bilateral pathways.     

Auditory neglect.

Auditory neglect was investigated in normal controls and in patients with a recent unilateral hemispheric lesion, by requiring them to detect the interruptions that occurred in one ear in a sound delivered through earphones either mono-aurally or binaurally. Control patients accurately detected interruptions. One left brain damaged (LBD) patient missed only once in the ipsilateral ear while seven of the 30 right brain damaged (RBD) patients missed more than one signal in the monoaural test and nine patients did the same in the binaural test. Omissions were always more marked in the left ear and in the binaural test with a significant ear by test interaction. The lesion of these patients was in the parietal lobe (five patients) and the thalamus (four patients). The relation of auditory neglect to auditory extinction was investigated and found to be equivocal, in that there were seven RBD patients who showed extinction, but not neglect and, more importantly, two patients who exhibited the opposite pattern, thus challenging the view that extinction is a minor form of neglect. Also visual and auditory neglect were not consistently correlated, the former being present in nine RBD patients without auditory neglect and the latter in two RBD patients without visual neglect. The finding that in some RBD patients with auditory neglect omissions also occurred, though with less frequency, in the right ear, points to a right hemisphere participation in the deployment of attention not only to the contralateral, but also to the ipsilateral space.     

Neuromagnetic functional coupling during dichotic listening of speech sounds

The present magnetoencephalography (MEG) study tested the hypothesis of a phase synchronization (functional coupling) of cortical alpha rhythms (about 6-12 Hz) within a "speech" cortical neural network comprising bilateral primary auditory cortex and Wernicke's areas, during dichotic listening (DL) of consonant-vowel (CV) syllables. Dichotic stimulation was done with the CV-syllable pairs /da/-/ba/ (true DL, yielded by stimuli having high spectral overlap) and /da/-/ka/ (sham DL, obtained with stimuli having poor spectral overlap). Whole-head MEG activity (165 sensors) was recorded from 10 healthy right-handed non-musicians showing right ear advantage in a speech DL task. Functional coupling of alpha rhythms was defined as the spectral coherence at the following bands: alpha 1 (about 6-8 Hz), alpha 2 (about 8-10 Hz), and alpha 3 (about 10-12) with respect to the peak of individual alpha frequency. Results showed an inverse pattern of functional coupling: during DL of speech sounds, spectral coherence of the high-band alpha rhythms increased between left auditory and Wernicke's areas with respect to sham DL, whereas it decreased between left and right auditory areas. The increase of functional coupling within the left hemisphere would underlie the processing of the syllable presented to the right ear, which arrives to the left auditory cortex without the interference of the other syllable presented to the left ear. Conversely, the decrease of inter-hemispherical coupling of the high-band alpha might be due to the fact that the two auditory cortices do not receive the same information from the ears during DL. These results suggest that functional coupling of alpha rhythms can constitute a neural substrate for the lateralization of auditory stimuli during DL.