Time course of EEG complexity reflects attentional engagement during listening to speech in noise.

Auditory distractions are recognized to considerably challenge the quality of information encoding during speech comprehension. This study explores electroencephalography (EEG) microstate dynamics in ecologically valid, noisy settings, aiming to uncover how these auditory distractions influence the process of information encoding during speech comprehension. We examined three listening scenarios: (1) speech perception with background noise (LA), (2) focused attention on the background noise (BA), and (3) intentional disregard of the background noise (BUA). Our findings showed that microstate complexity and unpredictability increased when attention was directed towards speech compared with tasks without speech (LA > BA & BUA). Notably, the time elapsed between the recurrence of microstates increased significantly in LA compared with both BA and BUA. This suggests that coping with background noise during speech comprehension demands more sustained cognitive effort. Additionally, a two-stage time course for both microstate complexity and alpha-to-theta power ratio was observed. Specifically, in the early epochs, a lower level was observed, which gradually increased and eventually reached a steady level in the later epochs. The findings suggest that the initial stage is primarily driven by sensory processes and information gathering, while the second stage involves higher level cognitive engagement, including mnemonic binding and memory encoding.

Auditory Spatial Discrimination and Sound Localization in Single-Sided Deaf Participants Provided with a Cochlear Implant.

INTRODUCTION: Spatial hearing is most accurate using both ears, but accuracy decreases in persons with asymmetrical hearing between ears. In participants with deafness in one ear but normal hearing in the other ear (single-sided deafness [SSD]), this difference can be compensated by a unilateral cochlear implant (CI). It has been shown that a CI can restore sound localization performance, but it is still unclear to what extent auditory spatial discrimination can be improved. METHODS: The present study investigated auditory spatial discrimination using minimum audible angles (MAAs) in 18 CI-SSD participants. Results were compared to 120 age-matched normal-hearing (NH) listeners. Low-frequency (LF) and high-frequency (HF) noise bursts were presented from 4°, 30°, and 60° azimuth on the CI side and on the NH side. MAA thresholds were tested for correlation with localization performance in the same participants. RESULTS: There were eight good performers and ten poor performers. There were more poor performers for LF signals than for HF signals. Performance on the CI side was comparable to performance on the NH side. Most difficulties occurred at 4° and at 30°. Eight of the good performers in the localization task were also good performers in the MAA task. Only the localization ability at 4° on the CI side was positively correlated with the MAA at that location. CONCLUSION: Our data suggest that a CI can restore localization ability but not necessarily auditory spatial discrimination at the same time. The ability to discriminate between adjacent locations may be trainable during rehabilitation to enhance important auditory skills.

Common principles in the lateralization of auditory cortex structure and function for vocal communication in primates and rodents.

This review summarizes recent findings on the lateralization of communicative sound processing in the auditory cortex (AC) of humans, non-human primates and rodents. Functional imaging in humans has demonstrated a left hemispheric preference for some acoustic features of speech, but it is unclear to which degree this is caused by bottom-up acoustic feature selectivity or top-down modulation from language areas. Although non-human primates show a less pronounced functional lateralization in AC, the properties of AC fields and behavioural asymmetries are qualitatively similar. Rodent studies demonstrate microstructural circuits that might underlie bottom-up acoustic feature selectivity in both hemispheres. Functionally, the left AC in the mouse appears to be specifically tuned to communication calls, whereas the right AC may have a more 'generalist' role. Rodents also show anatomical AC lateralization, such as differences in size and connectivity. Several of these functional and anatomical characteristics are also lateralized in human AC. Thus, complex vocal communication processing shares common features among rodents and primates. We argue that a synthesis of results from humans, non-human primates and rodents is necessary to identify the neural circuitry of vocal communication processing. However, data from different species and methods are often difficult to compare. Recent advances may enable better integration of methods across species. Efforts to standardize data formats and analysis tools would benefit comparative research and enable synergies between psychological and biological research in the area of vocal communication processing.

The Encoding of Sound Source Elevation in the Human Auditory Cortex.

Spatial hearing is a crucial capacity of the auditory system. While the encoding of horizontal sound direction has been extensively studied, very little is known about the representation of vertical sound direction in the auditory cortex. Using high-resolution fMRI, we measured voxelwise sound elevation tuning curves in human auditory cortex and show that sound elevation is represented by broad tuning functions preferring lower elevations as well as secondary narrow tuning functions preferring individual elevation directions. We changed the ear shape of participants (male and female) with silicone molds for several days. This manipulation reduced or abolished the ability to discriminate sound elevation and flattened cortical tuning curves. Tuning curves recovered their original shape as participants adapted to the modified ears and regained elevation perception over time. These findings suggest that the elevation tuning observed in low-level auditory cortex did not arise from the physical features of the stimuli but is contingent on experience with spectral cues and covaries with the change in perception. One explanation for this observation may be that the tuning in low-level auditory cortex underlies the subjective perception of sound elevation.SIGNIFICANCE STATEMENT This study addresses two fundamental questions about the brain representation of sensory stimuli: how the vertical spatial axis of auditory space is represented in the auditory cortex and whether low-level sensory cortex represents physical stimulus features or subjective perceptual attributes. Using high-resolution fMRI, we show that vertical sound direction is represented by broad tuning functions preferring lower elevations as well as secondary narrow tuning functions preferring individual elevation directions. In addition, we demonstrate that the shape of these tuning functions is contingent on experience with spectral cues and covaries with the change in perception, which may indicate that the tuning functions in low-level auditory cortex underlie the perceived elevation of a sound source.

Neural bases of rhythmic entrainment in humans: critical transformation between cortical and lower-level representations of auditory rhythm.

The spontaneous ability to entrain to meter periodicities is central to music perception and production across cultures. There is increasing evidence that this ability involves selective neural responses to meter-related frequencies. This phenomenon has been observed in the human auditory cortex, yet it could be the product of evolutionarily older lower-level properties of brainstem auditory neurons, as suggested by recent recordings from rodent midbrain. We addressed this question by taking advantage of a new method to simultaneously record human EEG activity originating from cortical and lower-level sources, in the form of slow (< 20 Hz) and fast (> 150 Hz) responses to auditory rhythms. Cortical responses showed increased amplitudes at meter-related frequencies compared to meter-unrelated frequencies, regardless of the prominence of the meter-related frequencies in the modulation spectrum of the rhythmic inputs. In contrast, frequency-following responses showed increased amplitudes at meter-related frequencies only in rhythms with prominent meter-related frequencies in the input but not for a more complex rhythm requiring more endogenous generation of the meter. This interaction with rhythm complexity suggests that the selective enhancement of meter-related frequencies does not fully rely on subcortical auditory properties, but is critically shaped at the cortical level, possibly through functional connections between the auditory cortex and other, movement-related, brain structures. This process of temporal selection would thus enable endogenous and motor entrainment to emerge with substantial flexibility and invariance with respect to the rhythmic input in humans in contrast with non-human animals.

Perceptual grouping in the cocktail party: Contributions of voice-feature continuity.

Cocktail parties pose a difficult yet solvable problem for the auditory system. Previous work has shown that the cocktail-party problem is considerably easier when all sounds in the target stream are spoken by the same talker (the voice-continuity benefit). The present study investigated the contributions of two of the most salient voice features-glottal-pulse rate (GPR) and vocal-tract length (VTL)-to the voice-continuity benefit. Twenty young, normal-hearing listeners participated in two experiments. On each trial, listeners heard concurrent sequences of spoken digits from three different spatial locations and reported the digits coming from a target location. Critically, across conditions, GPR and VTL either remained constant or varied across target digits. Additionally, across experiments, the target location either remained constant (Experiment 1) or varied (Experiment 2) within a trial. In Experiment 1, listeners benefited from continuity in either voice feature, but VTL continuity was more helpful than GPR continuity. In Experiment 2, spatial discontinuity greatly hindered listeners' abilities to exploit continuity in GPR and VTL. The present results suggest that selective attention benefits from continuity in target voice features and that VTL and GPR play different roles for perceptual grouping and stream segregation in the cocktail party.

Enhanced visual adaptation in cochlear implant users revealed by concurrent EEG-fNIRS.

Previous studies have observed lower visual cortex activation for visual processing in cochlear implant (CI) users compared to normal hearing controls, while others reported enhanced visual speechreading abilities in CI users. The present work investigated whether lower visual cortical activation for visual processing can be explained by a more efficient visual sensory encoding in CI users. Specifically, we investigated whether CI users show enhanced stimulus-specific adaptation for visual stimuli compared to controls. Auditory sensory adaptation was also investigated to explore the sensory specificity of the predicted effect. Twenty post-lingually deafened adult CI users and twenty age-matched controls were presented with repeated visual and auditory stimuli during simultaneous acquisition of electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS). By integrating EEG and fNIRS signals we found significantly enhanced visual adaptation and lower visual cortex activation in CI users compared to controls. That is, responses to repeated visual stimuli decreased more prominently in CI users than in controls. The results suggest that CI users process visual stimuli more efficiently than controls.

Frequency Selectivity of Voxel-by-Voxel Functional Connectivity in Human Auditory Cortex.

While functional connectivity in the human cortex has been increasingly studied, its relationship to cortical representation of sensory features has not been documented as much. We used functional magnetic resonance imaging to demonstrate that voxel-by-voxel intrinsic functional connectivity (FC) is selective to frequency preference of voxels in the human auditory cortex. Thus, FC was significantly higher for voxels with similar frequency tuning than for voxels with dissimilar tuning functions. Frequency-selective FC, measured via the correlation of residual hemodynamic activity, was not explained by generic FC that is dependent on spatial distance over the cortex. This pattern remained even when FC was computed using residual activity taken from resting epochs. Further analysis showed that voxels in the core fields in the right hemisphere have a higher frequency selectivity in within-area FC than their counterpart in the left hemisphere, or than in the noncore-fields in the same hemisphere. Frequency-selective FC is consistent with previous findings of topographically organized FC in the human visual and motor cortices. The high degree of frequency selectivity in the right core area is in line with findings and theoretical proposals regarding the asymmetry of human auditory cortex for spectral processing.

Enhanced brainstem and cortical encoding of sound during synchronized movement.

Movement to a steady beat has been widely studied as a model of alignment of motor outputs on sensory inputs. However, how the encoding of sensory inputs is shaped during synchronized movements along the sensory pathway remains unknown. To investigate this, we simultaneously recorded brainstem and cortical electro-encephalographic activity while participants listened to periodic amplitude-modulated tones. Participants listened either without moving or while tapping in sync on every second beat. Cortical responses were identified at the envelope modulation rate (beat frequency), whereas brainstem responses were identified at the partials frequencies of the chord and at their modulation by the beat frequency (sidebands). During sensorimotor synchronization, cortical responses at beat frequency were larger than during passive listening. Importantly, brainstem responses were also enhanced, with a selective amplification of the sidebands, in particular at the lower-pitched tone of the chord, and no significant correlation with electromyographic measures at tapping frequency. These findings provide first evidence for an online gain in the cortical and subcortical encoding of sounds during synchronized movement, selective to behavior-relevant sound features. Moreover, the frequency-tagging method to isolate concurrent brainstem and cortical activities even during actual movements appears promising to reveal coordinated processes along the human auditory pathway.

Parcellation of Human and Monkey Core Auditory Cortex with fMRI Pattern Classification and Objective Detection of Tonotopic Gradient Reversals.

Auditory cortex (AC) contains several primary-like, or "core," fields, which receive thalamic input and project to non-primary "belt" fields. In humans, the organization and layout of core and belt auditory fields are still poorly understood, and most auditory neuroimaging studies rely on macroanatomical criteria, rather than functional localization of distinct fields. A myeloarchitectonic method has been suggested recently for distinguishing between core and belt fields in humans (Dick F, Tierney AT, Lutti A, Josephs O, Sereno MI, Weiskopf N. 2012. In vivo functional and myeloarchitectonic mapping of human primary auditory areas. J Neurosci. 32:16095-16105). We propose a marker for core AC based directly on functional magnetic resonance imaging (fMRI) data and pattern classification. We show that a portion of AC in Heschl's gyrus classifies sound frequency more accurately than other regions in AC. Using fMRI data from macaques, we validate that the region where frequency classification performance is significantly above chance overlaps core auditory fields, predominantly A1. Within this region, we measure tonotopic gradients and estimate the locations of the human homologues of the core auditory subfields A1 and R. Our results provide a functional rather than anatomical localizer for core AC. We posit that inter-individual variability in the layout of core AC might explain disagreements between results from previous neuroimaging and cytological studies.

Fast and persistent adaptation to new spectral cues for sound localization suggests a many-to-one mapping mechanism.

The adult human auditory system can adapt to changes in spectral cues for sound localization. This plasticity was demonstrated by changing the shape of the pinna with earmolds. Previous results indicate that participants regain localization accuracy after several weeks of adaptation and that the adapted state is retained for at least one week without earmolds. No aftereffect was observed after mold removal, but any aftereffect may be too short to be observed when responses are averaged over many trials. This work investigated the lack of aftereffect by analyzing single-trial responses and modifying visual, auditory, and tactile information during the localization task. Results showed that participants localized accurately immediately after mold removal, even at the first stimulus presentation. Knowledge of the stimulus spectrum, tactile information about the absence of the earmolds, and visual feedback were not necessary to localize accurately after adaptation. Part of the adaptation persisted for one month without molds. The results are consistent with the hypothesis of a many-to-one mapping of the spectral cues, in which several spectral profiles are simultaneously associated with one sound location. Additionally, participants with acoustically more informative spectral cues localized sounds more accurately, and larger acoustical disturbances by the molds reduced adaptation success.

Adaptation to shifted interaural time differences changes encoding of sound location in human auditory cortex.

The auditory system infers the location of sound sources from the processing of different acoustic cues. These cues change during development and when assistive hearing devices are worn. Previous studies have found behavioral recalibration to modified localization cues in human adults, but very little is known about the neural correlates and mechanisms of this plasticity. We equipped participants with digital devices, worn in the ear canal that allowed us to delay sound input to one ear, and thus modify interaural time differences, a major cue for horizontal sound localization. Participants wore the digital earplugs continuously for nine days while engaged in day-to-day activities. Daily psychoacoustical testing showed rapid recalibration to the manipulation and confirmed that adults can adapt to shifted interaural time differences in their daily multisensory environment. High-resolution functional MRI scans performed before and after recalibration showed that recalibration was accompanied by changes in hemispheric lateralization of auditory cortex activity. These changes corresponded to a shift in spatial coding of sound direction comparable to the observed behavioral recalibration. Fitting the imaging results with a model of auditory spatial processing also revealed small shifts in voxel-wise spatial tuning within each hemisphere.

Motor intention determines sensory attenuation of brain responses to self-initiated sounds.

One of the functions of the brain is to predict sensory consequences of our own actions. In auditory processing, self-initiated sounds evoke a smaller brain response than passive sound exposure of the same sound sequence. Previous work suggests that this response attenuation reflects a predictive mechanism to differentiate the sensory consequences of one's own actions from other sensory input, which seems to form the basis for the sense of agency (recognizing oneself as the agent of the movement). This study addresses the question whether attenuation of brain responses to self-initiated sounds can be explained by brain activity involved in movement planning rather than movement execution. We recorded ERPs in response to sounds initiated by button presses. In one condition, participants moved a finger to press the button voluntarily, whereas in another condition, we initiated a similar, but involuntary, finger movement by stimulating the corresponding region of the primary motor cortex with TMS. For involuntary movements, no movement intention (and no feeling of agency) could be formed; thus, no motor plans were available to the forward model. A portion of the brain response evoked by the sounds, the N1-P2 complex, was reduced in amplitude following voluntary, self-initiated movements, but not following movements initiated by motor cortex stimulation. Our findings demonstrate that movement intention and the corresponding feeling of agency determine sensory attenuation of brain responses to self-initiated sounds. The present results support the assumptions of a predictive internal forward model account operating before primary motor cortex activation.

Cortical brain states and corticospinal synchronization influence TMS-evoked motor potentials.

Transcranial magnetic stimulation (TMS) influences cortical processes. Recent findings indicate, however, that, in turn, the efficacy of TMS depends on the state of ongoing cortical oscillations. Whereas power and phase of electromyographic (EMG) activity recorded from the hand muscles as well as neural synchrony between cortex and hand muscles are known to influence the effect of TMS, to date, no study has shown an influence of the phase of cortical oscillations during wakefulness. We applied single-pulse TMS over the motor cortex and recorded motor-evoked potentials along with the electroencephalogram (EEG) and EMG. We correlated phase and power of ongoing EEG and EMG signals with the motor-evoked potential (MEP) amplitude. We also investigated the functional connectivity between cortical and hand muscle activity (corticomuscular coherence) with the MEP amplitude. EEG and EMG power and phase in a frequency band around 18 Hz correlated with the MEP amplitude. High beta-band (∼34 Hz) corticomuscular coherence exhibited a positive linear relationship with the MEP amplitude, indicating that strong synchrony between cortex and hand muscles at the moment when TMS is applied entails large MEPs. Improving upon previous studies, we demonstrate a clear dependence of TMS-induced motor effects on the state of ongoing EEG phase and power fluctuations. We conclude that not only the sampling of incoming information but also the susceptibility of cortical communication flow depends cyclically on neural phase.

Sensation of agency and perception of temporal order.

After adaptation to a fixed temporal delay between actions and their sensory consequences, stimuli delivered during the delay are perceived to occur prior to actions. Temporal judgments are also influenced by the sensation of agency (experience of causing our own actions and their sensory consequences). Sensory consequences of voluntary actions are perceived to occur earlier in time than those of involuntary actions. However, it is unclear whether temporal order illusions influence the sensation of agency. Thus, we tested how the illusionary reversal of motor actions and sound events affect the sensation of agency. We observed an absence of the sensation of agency in the auditory modality in a condition in which sounds were falsely perceived as preceding motor acts relative to the perceived temporal order in the control condition. This finding suggests a strong association between the sensation of agency and the temporal order perception of actions and their consequences.

Mapping human pitch representation in a distributed system using depth-electrode recordings and modeling.

Depth-electrode recordings from the auditory cortex of humans undergoing presurgical evaluation for epilepsy allow the recording of ensemble responses to pitch in the form of local field potentials. These recordings allow another test of the hypothesis that there is a specialized neural ensemble for pitch within auditory cortex. Moreover, the technique allows recordings from multiple sites with millisecond temporal resolution to allow modeling of the effective connectivity between these sites. Here we argue that this takes the form of a hierarchical network of pitch-sensitive regions. Activity can be understood as reflecting predictive coding, in which perceptual predictions and error messages are continuously exchanged between a higher pitch center and lower-level auditory cortex.

Spectro-temporal modulation transfer function of single voxels in the human auditory cortex measured with high-resolution fMRI.

Are visual and auditory stimuli processed by similar mechanisms in the human cerebral cortex? Images can be thought of as light energy modulations over two spatial dimensions, and low-level visual areas analyze images by decomposition into spatial frequencies. Similarly, sounds are energy modulations over time and frequency, and they can be identified and discriminated by the content of such modulations. An obvious question is therefore whether human auditory areas, in direct analogy to visual areas, represent the spectro-temporal modulation content of acoustic stimuli. To answer this question, we measured spectro-temporal modulation transfer functions of single voxels in the human auditory cortex with functional magnetic resonance imaging. We presented dynamic ripples, complex broadband stimuli with a drifting sinusoidal spectral envelope. Dynamic ripples are the auditory equivalent of the gratings often used in studies of the visual system. We demonstrate selective tuning to combined spectro-temporal modulations in the primary and secondary auditory cortex. We describe several types of modulation transfer functions, extracting different spectro-temporal features, with a high degree of interaction between spectral and temporal parameters. The overall low-pass modulation rate preference of the cortex matches the modulation content of natural sounds. These results demonstrate that combined spectro-temporal modulations are represented in the human auditory cortex, and suggest that complex signals are decomposed and processed according to their modulation content, the same transformation used by the visual system.

Neural Substrates and Models of Omission Responses and Predictive Processes.

Predictive coding theories argue that deviance detection phenomena, such as mismatch responses and omission responses, are generated by predictive processes with possibly overlapping neural substrates. Molecular imaging and electrophysiology studies of mismatch responses and corollary discharge in the rodent model allowed the development of mechanistic and computational models of these phenomena. These models enable translation between human and non-human animal research and help to uncover fundamental features of change-processing microcircuitry in the neocortex. This microcircuitry is characterized by stimulus-specific adaptation and feedforward inhibition of stimulus-selective populations of pyramidal neurons and interneurons, with specific contributions from different interneuron types. The overlap of the substrates of different types of responses to deviant stimuli remains to be understood. Omission responses, which are observed both in corollary discharge and mismatch response protocols in humans, are underutilized in animal research and may be pivotal in uncovering the substrates of predictive processes. Omission studies comprise a range of methods centered on the withholding of an expected stimulus. This review aims to provide an overview of omission protocols and showcase their potential to integrate and complement the different models and procedures employed to study prediction and deviance detection.This approach may reveal the biological foundations of core concepts of predictive coding, and allow an empirical test of the framework's promise to unify theoretical models of attention and perception.

Sound Localization in Single-Sided Deaf Participants Provided With a Cochlear Implant.

Spatial hearing is crucial in real life but deteriorates in participants with severe sensorineural hearing loss or single-sided deafness. This ability can potentially be improved with a unilateral cochlear implant (CI). The present study investigated measures of sound localization in participants with single-sided deafness provided with a CI. Sound localization was measured separately at eight loudspeaker positions (4°, 30°, 60°, and 90°) on the CI side and on the normal-hearing side. Low- and high-frequency noise bursts were used in the tests to investigate possible differences in the processing of interaural time and level differences. Data were compared to normal-hearing adults aged between 20 and 83. In addition, the benefit of the CI in speech understanding in noise was compared to the localization ability. Fifteen out of 18 participants were able to localize signals on the CI side and on the normal-hearing side, although performance was highly variable across participants. Three participants always pointed to the normal-hearing side, irrespective of the location of the signal. The comparison with control data showed that participants had particular difficulties localizing sounds at frontal locations and on the CI side. In contrast to most previous results, participants were able to localize low-frequency signals, although they localized high-frequency signals more accurately. Speech understanding in noise was better with the CI compared to testing without CI, but only at a position where the CI also improved sound localization. Our data suggest that a CI can, to a large extent, restore localization in participants with single-sided deafness. Difficulties may remain at frontal locations and on the CI side. However, speech understanding in noise improves when wearing the CI. The treatment with a CI in these participants might provide real-world benefits, such as improved orientation in traffic and speech understanding in difficult listening situations.

Blindsight mediated by an S-cone-independent collicular pathway: an fMRI study in hemispherectomized subjects.

The purpose of our study was to investigate the ability to process achromatic and short-wavelength-sensitive cone (S-cone)-isolating (blue-yellow) stimuli in the blind visual field of hemispherectomized subjects and to demonstrate that blindsight is mediated by a collicular pathway that is independent of S-cone inputs. Blindsight has been described as the ability to respond to visual stimuli in the blind visual field without conscious awareness [Weiskrantz, L., Warrington, E. K., Sanders, M. D., & Marshall, J. Visual capacity in the hemianopic field following a restricted occipital ablation. Brain, 97, 709-728, 1974]. The roles of the subcortical neural structures in blindsight, such as the pulvinar and the superior colliculus, have been debated and an underlying neural correlate has yet to be confirmed. Using fMRI, we tested the ability to process visual stimuli that isolated the achromatic and short-wavelength-sensitive (S-)-cone pathways in three subjects: one control subject, one hemispherectomized subject with blindsight, and one hemispherectomized subject without blindsight. We demonstrated that (1) achromatic and S-cone-isolating stimuli presented to the normal visual hemifield of hemispherectomized subjects and to both visual hemifields of the control subject activated contralateral visual areas (V1/V2), as expected; (2) achromatic stimulus presentation but not S-cone-isolating stimulus presentation to the blind hemifield of the subject with blindsight activated visual areas FEF/V5; (3) whereas the cortical activation of the control subject was enhanced by an additional stimulus (achromatic and S-cone isolating) presented in the contralateral visual field, activation pattern of the subject with blindsight was enhanced by achromatic stimuli only. We conclude that the human superior colliculus is blind to the S-cone-isolating stimuli, and blindsight is mediated by an S-cone-independent collicular pathway.