What's in Your Gamma? Activation of the Ventral Fronto-Parietal Attentional Network in Response to Distracting Sounds.

Auditory attention operates through top-down (TD) and bottom-up (BU) mechanisms that are supported by dorsal and ventral brain networks, respectively, with the main overlap in the lateral prefrontal cortex (lPFC). A good TD/BU balance is essential to be both task-efficient and aware of our environment, yet it is rarely investigated. Oscillatory activity is a novel method to probe the attentional dynamics with evidence that gamma activity (>30 Hz) could signal BU processing and thus would be a good candidate to support the activation of the ventral BU attention network. Magnetoencephalography data were collected from 21 young adults performing the competitive attention task, which enables simultaneous investigation of BU and TD attentional mechanisms. Distracting sounds elicited an increase in gamma activity in regions of the BU ventral network. TD attention modulated these gamma responses in regions of the inhibitory cognitive control system: the medial prefrontal and anterior cingulate cortices. Finally, distracting-sound-induced gamma activity was synchronous between the auditory cortices and several distant brain regions, notably the lPFC. We provide novel insight into the role of gamma activity 1) in supporting the activation of the ventral BU attention network and 2) in subtending the TD/BU attention balance in the PFC.

Fronto-central P3a to distracting sounds: An index of their arousing properties.

The P3a observed after novel events is an event-related potential comprising an early fronto-central phase and a late fronto-parietal phase. It has classically been considered to reflect the attention processing of distracting stimuli. However, novel sounds can lead to behavioral facilitation as much as behavioral distraction. This illustrates the duality of the orienting response which includes both an attentional and an arousal component. Using a paradigm with visual or auditory targets to detect and irrelevant unexpected distracting sounds to ignore, we showed that the facilitation effect by distracting sounds is independent of the target modality and endures more than 1500 ms. These results confirm that the behavioral facilitation observed after distracting sounds is related to an increase in unspecific phasic arousal on top of the attentional capture. Moreover, the amplitude of the early phase of the P3a to distracting sounds positively correlated with subjective arousal ratings, contrary to other event-related potentials. We propose that the fronto-central early phase of the P3a would index the arousing properties of distracting sounds and would be linked to the arousal component of the orienting response. Finally, we discuss the relevance of the P3a as a marker of distraction.

Brain dynamics of distractibility: interaction between top-down and bottom-up mechanisms of auditory attention.

Attention improves the processing of specific information while other stimuli are disregarded. A good balance between bottom-up (attentional capture by unexpected salient stimuli) and top-down (selection of relevant information) mechanisms is crucial to be both task-efficient and aware of our environment. Only few studies have explored how an isolated unexpected task-irrelevant stimulus outside the attention focus can disturb the top-down attention mechanisms necessary to the good performance of the ongoing task, and how these top-down mechanisms can modulate the bottom-up mechanisms of attentional capture triggered by an unexpected event. We recorded scalp electroencephalography in 18 young adults performing a new paradigm measuring distractibility and assessing both bottom-up and top-down attention mechanisms, at the same time. Increasing task load in top-down attention was found to reduce early processing of the distracting sound, but not bottom-up attentional capture mechanisms nor the behavioral distraction cost in reaction time. Moreover, the impact of bottom-up attentional capture by distracting sounds on target processing was revealed as a delayed latency of the N100 sensory response to target sounds mirroring increased reaction times. These results provide crucial information into how bottom-up and top-down mechanisms dynamically interact and compete in the human brain, i.e. on the precarious balance between voluntary attention and distraction.

Asymmetry of temporal auditory T-complex: right ear-left hemisphere advantage in Tb timing in children.

OBJECTIVE: To investigate brain asymmetry of the temporal auditory evoked potentials (T-complex) in response to monaural stimulation in children compared to adults. METHODS: Ten children (7 to 9 years) and ten young adults participated in the study. All were right-handed. The auditory stimuli used were tones (1100 Hz, 70 dB SPL, 50 ms duration) delivered monaurally (right, left ear) at four different levels of stimulus onset asynchrony (700-1100-1500-3000 ms). Latency and amplitude of responses were measured at left and right temporal sites according to the ear stimulated. RESULTS: Peaks of the three successive deflections (Na-Ta-Tb) of the T-complex were greater in amplitude and better defined in children than in adults. Amplitude measurements in children indicated that Na culminates on the left hemisphere whatever the ear stimulated whereas Ta and Tb culminate on the right hemisphere but for left ear stimuli only. Peak latency displayed different patterns of asymmetry. Na and Ta displayed shorter latencies for contralateral stimulation. The original finding was that Tb peak latency was the shortest at the left temporal site for right ear stimulation in children. Amplitude increased and/or peak latency decreased with increasing SOA, however no interaction effect was found with recording site or with ear stimulated. CONCLUSION: Our main original result indicates a right ear-left hemisphere timing advantage for Tb peak in children. The Tb peak would therefore be a good candidate as an electrophysiological marker of ear advantage effects during dichotic stimulation and of functional inter-hemisphere interactions and connectivity in children.

Impaired Facilitatory Mechanisms of Auditory Attention After Damage of the Lateral Prefrontal Cortex.

There is growing evidence that auditory selective attention operates via distinct facilitatory and inhibitory mechanisms enabling selective enhancement and suppression of sound processing, respectively. The lateral prefrontal cortex (LPFC) plays a crucial role in the top-down control of selective attention. However, whether the LPFC controls facilitatory, inhibitory, or both attentional mechanisms is unclear. Facilitatory and inhibitory mechanisms were assessed, in patients with LPFC damage, by comparing event-related potentials (ERPs) to attended and ignored sounds with ERPs to these same sounds when attention was equally distributed to all sounds. In control subjects, we observed 2 late frontally distributed ERP components: a transient facilitatory component occurring from 150 to 250 ms after sound onset; and an inhibitory component onsetting at 250 ms. Only the facilitatory component was affected in patients with LPFC damage: this component was absent when attending to sounds delivered in the ear contralateral to the lesion, with the most prominent decreases observed over the damaged brain regions. These findings have 2 important implications: (i) they provide evidence for functionally distinct facilitatory and inhibitory mechanisms supporting late auditory selective attention; (ii) they show that the LPFC is involved in the control of the facilitatory mechanisms of auditory attention.

Early neurophysiological correlates of vocal versus non-vocal sound processing in adults.

Electrophysiological correlates of voice processing were studied in twenty adults by comparing auditory evoked potentials in response to voice and environmental sounds in passive condition. Both categories of stimuli elicited similar cortical auditory responses (i.e. N1, P2, N2 peaks); however these peaks were overlapped by two components specifically elicited by voice. The first component was evidenced as a positive deflection recorded over the fronto-temporal sites, and lateralized on the right hemiscalp. This fronto-temporal positivity to voice (FTPV) may constitute the electrophysiological counterpart of the activation of the temporal voice areas previously described in neuroimaging studies. The second component was recorded at occipito-temporo-parietal sites. This occipito-temporo-parietal negativity to voice might correspond to visual mental imagery of the vocal sounds or to some form of mental simulation of the action sounds (e.g. coughing). Both components began as early as 70 ms post-stimulus onset indicating a rapid discrimination of voice in our auditory environment, which might be the basis of communication functions in humans.

Load effects in auditory selective attention: evidence for distinct facilitation and inhibition mechanisms.

It is unknown whether facilitation and inhibition of stimulus processing represent one or two mechanisms in auditory attention. We performed electrophysiological experiments in humans to address these two competing hypothesis. Participants performed an attention task under low or high memory load. Facilitation and inhibition were measured by recording electrophysiological responses to attended and ignored sounds and comparing them to responses to these same sounds when attention was considered to be equally distributed towards all sounds. We observed two late frontally distributed components: a negative one in response to attended sounds, and a positive one to ignored sounds. These two frontally distributed responses had distinct timing and scalp topographies and were differentially affected by memory load. Taken together these results provide evidence that attention-mediated top-down control reflects the activity of distinct facilitation and inhibition mechanisms.

Hemicraniectomy: a new model for human electrophysiology with high spatio-temporal resolution.

Human electrophysiological research is generally restricted to scalp EEG, magneto-encephalography, and intracranial electrophysiology. Here we examine a unique patient cohort that has undergone decompressive hemicraniectomy, a surgical procedure wherein a portion of the calvaria is removed for several months during which time the scalp overlies the brain without intervening bone. We quantify the differences in signals between electrodes over areas with no underlying skull and scalp EEG electrodes over the intact skull in the same subjects. Signals over the hemicraniectomy have enhanced amplitude and greater task-related power at higher frequencies (60-115 Hz) compared with signals over skull. We also provide evidence of a metric for trial-by-trial EMG/EEG coupling that is effective over the hemicraniectomy but not intact skull at frequencies >60 Hz. Taken together, these results provide evidence that the hemicraniectomy model provides a means for studying neural dynamics in humans with enhanced spatial and temporal resolution.

Visual activation and audiovisual interactions in the auditory cortex during speech perception: intracranial recordings in humans.

Hemodynamic studies have shown that the auditory cortex can be activated by visual lip movements and is a site of interactions between auditory and visual speech processing. However, they provide no information about the chronology and mechanisms of these cross-modal processes. We recorded intracranial event-related potentials to auditory, visual, and bimodal speech syllables from depth electrodes implanted in the temporal lobe of 10 epileptic patients (altogether 932 contacts). We found that lip movements activate secondary auditory areas, very shortly (approximately equal to 10 ms) after the activation of the visual motion area MT/V5. After this putatively feedforward visual activation of the auditory cortex, audiovisual interactions took place in the secondary auditory cortex, from 30 ms after sound onset and before any activity in the polymodal areas. Audiovisual interactions in the auditory cortex, as estimated in a linear model, consisted both of a total suppression of the visual response to lipreading and a decrease of the auditory responses to the speech sound in the bimodal condition compared with unimodal conditions. These findings demonstrate that audiovisual speech integration does not respect the classical hierarchy from sensory-specific to associative cortical areas, but rather engages multiple cross-modal mechanisms at the first stages of nonprimary auditory cortex activation.

Effects of selective attention on the electrophysiological representation of concurrent sounds in the human auditory cortex.

In noisy environments, we use auditory selective attention to actively ignore distracting sounds and select relevant information, as during a cocktail party to follow one particular conversation. The present electrophysiological study aims at deciphering the spatiotemporal organization of the effect of selective attention on the representation of concurrent sounds in the human auditory cortex. Sound onset asynchrony was manipulated to induce the segregation of two concurrent auditory streams. Each stream consisted of amplitude modulated tones at different carrier and modulation frequencies. Electrophysiological recordings were performed in epileptic patients with pharmacologically resistant partial epilepsy, implanted with depth electrodes in the temporal cortex. Patients were presented with the stimuli while they either performed an auditory distracting task or actively selected one of the two concurrent streams. Selective attention was found to affect steady-state responses in the primary auditory cortex, and transient and sustained evoked responses in secondary auditory areas. The results provide new insights on the neural mechanisms of auditory selective attention: stream selection during sound rivalry would be facilitated not only by enhancing the neural representation of relevant sounds, but also by reducing the representation of irrelevant information in the auditory cortex. Finally, they suggest a specialization of the left hemisphere in the attentional selection of fine-grained acoustic information.

Listening in silence activates auditory areas: a functional magnetic resonance imaging study.

Directing attention to some acoustic features of a sound has been shown repeatedly to modulate the stimulus-induced neural responses. On the contrary, little is known about the neurophysiological impact of auditory attention when the auditory scene remains empty. We performed an experiment in which subjects had to detect a sound emerging from silence (the sound was detectable after different durations of silence). Two frontal activations (right dorsolateral prefrontal and inferior frontal) were found, regardless of the side where sound was searched for, consistent with the well established role of these regions in attentional control. The main result was that the superior temporal cortex showed activations contralateral to the side where sound was expected to be present. The area extended from the vicinity of Heschl's gyrus to the surrounding areas (planum temporale/anterior lateral areas). The effect consisted of both an increase in the response to a sound delivered after attention was directed to detect its emergence and a baseline shift during the silent period. Thus, in absence of any acoustic stimulus, the search for an auditory input was found to activate the auditory cortex.

Dynamics of a temporo-fronto-parietal network during sustained spatial or spectral auditory processing.

Animal and human studies have suggested that posterior temporal, parietal, and frontal regions are specifically involved in auditory spatial (location and motion) processing, forming a putative dorsal "where" pathway. We used scalp EEG and current density mapping to investigate the dynamics of this network in human subjects presented with a varying acoustic stream in a two-factor paradigm: spatial versus pitch variations, focused versus diverted attention. The main findings were: (i) a temporo-parieto-frontal network was activated during the whole duration of the stream in all conditions and modulated by attention; (ii) the left superior temporal cortex was the only region showing different activations for pitch and spatial variations. Therefore, parietal and frontal regions would be involved in task-related processes (attention and motor preparation), whereas the differential processing of acoustic spatial and object-related features seems to take place at the temporal level.

Listening to a walking human activates the temporal biological motion area.

A vivid perception of a moving human can be evoked when viewing a few point-lights on the joints of an invisible walker. This special visual ability for biological motion perception has been found to involve the posterior superior temporal sulcus (STSp). However, in everyday life, human motion can also be recognized using acoustic cues. In the present study, we investigated the neural substrate of human motion perception when listening to footsteps, by means of a sparse sampling functional MRI design. We first showed an auditory attentional network that shares frontal and parietal areas previously found in visual attention paradigms. Second, an activation was observed in the auditory cortex (Heschl's gyrus and planum temporale), likely to be related to low-level sound processing. Most strikingly, another activation was evidenced in a STSp region overlapping the temporal biological motion area previously reported using visual input. We thus propose that a part of the STSp region might be a supramodal area involved in human motion recognition, irrespective of the sensory modality input.