The nature of selective attention effects on auditory event-related potentials

The purpose of the research reported here was to examine a number of issues relating to the nature of selective attention effects on auditory event-related potentials (ERPs), namely, to determine the relative contribution of N1 and slow wave (SW) to the early and late components of Nd respectively, where Nd is defined as the negative shift of attended ERPs relative to unattended ERPs; to examine whether individual differences in Nd morphology are related to performance and the strategies that subjects use; and to determine the contribution of changes in the attended and unattended ERPs to Nd. Auditory ERPs were recorded from subjects as they carried out an auditory selective attention task and a visual target detection task. The auditory selective attention task was a multidimensional task in which stimuli varied on location, pitch and duration and in which the subject's task was to pay attention to a particular location/pitch combination and respond whenever they detected a long-duration target tone. In the visual target detection task, subjects were required to respond whenever they detected a colour change in a light-emitting diode which also acted as a fixation point. Auditory ERPs recorded during the visual task were used to provide a measure of exogenous components uncontaminated by differential effects of selective processing of auditory stimuli. The results suggested that early Nd and N1 are independently generated as Nd did not exhibit the contralateral scalp focus typical of N1, and that late Nd is independent of SW. While substantial differences in Nd morphology were observed over subjects, these differences showed no consistent relationships to performance or to task strategies. Comparison of auditory ERPs during active auditory attention with auditory ERPs recorded during the visual control task indicated that there was an early negative shift of the attended ERP, a later negative shift of the attended ERP which had a frontal focus and a later positive shift of the unattended ERP. These results suggest that there are active processes involved in the processing of stimuli from both the attended and unattended source.     

ERP indices of auditory selective attention in aging and Parkinson's disease

In the present study, we compared the performance of normal subjects in three age groups and of medicated Parkinson's disease patients on auditory selective attention processes. Two tone sequences were dichotically presented. Subjects responded to deviant tones in the attended location. Event-related potentials (ERPs) were recorded from nine scalp electrodes. The old group showed significant decline in hit rate, increase in N1 amplitude, and reduction in P3a, P3b, target negativity, and mismatch negativity amplitude. The amplitude and duration of late Nd increased with age, reflecting modulation of both attended and unattended standard ERPs. Although the middle group showed a similar pattern of ERP changes, the effects were generally nonsignificant. The Parkinson's disease group showed little further disruption of behavioral or ERP measures. However, Parkinson's disease affected late Nd in the direction opposite of that of aging, reflecting differential modulation of unattended standard positivity.     

Cross-Modal Selective Attention Effects on Retinal, Myogenic, Brainstem, and Cerebral Evoked Potentials

Short latency evoked potentials were recorded during a cross-modal selective attention task to evaluate recent proposals that sensory transmission in the peripheral auditory and visual pathways can be modified selectively by centrifugal mechanisms in humans. Twenty young adult subjects attended in turn to either left-ear tones or right-field flashes presented in a randomized sequence, in order to detect infrequent, lower-intensity targets. Attention-related enhancement of longer-latency components, including the visual P105 and the auditory N1/Nd waves and T-complex, showed that subjects were able to adopt a selective sensory set toward either modality. Neither the auditory evoked brainstem potentials nor the early visual components (electroretinogram, occipito-temporal N40, P50, N70 waves) were significantly affected by attention. Measures of retinal B-waves were significantly reduced in amplitude when attention was directed to the flashes, but concurrent recordings of eyelid electromyographic activity and the electro-oculogram indicated that this effect may have resulted from contamination of the retinal recordings by blink microreflex activity. A trend toward greater positivity in the 15-50 ms latency range for auditory evoked potentials to attended tones was observed. These results provide further evidence that the earliest levels of sensory transmission are unaffected by cross-modal selective attention, but that longer latency exogenous and endogenous potentials are enhanced to stimuli in the attended modality.     

The N1 effect of temporal attention is independent of sound location and intensity: Implications for possible mechanisms of temporal attention

It has been repeatedly shown that the auditory N1 is enhanced for sounds presented at an attended time point. The present study investigated the underlying mechanisms using a temporal cuing paradigm. In each trial, an auditory cue indicated at which time point a second sound could be relevant for response selection. Crucially, in addition to temporal attention, two physical sound features with known effects on the sensory N1 were manipulated: location and intensity. Positive evidence for conjoint effects of attention and location or attention and intensity would corroborate the notion that the sensory N1 was modulated by temporal attention, thus supporting a gain mechanism. However, the N1 effect of temporal attention was not similarly lateralized as the sensory N1, and, moreover, it was independent of sound intensity. Thus, the present results do not provide compelling evidence that temporal attention involves an increase in sensory gain.     

Interstimulus interval and the selective-attention effect on auditory ERPs:

The attention effect on the auditory event-related potential (ERP) in dichotic conditions was studied as a function of the interstimulus interval (ISI). Subjects attended to stimuli delivered to a designated ear and responded to infrequent pitch deviants in this input. The mean ISI was either 80, 160, 480, or 800 ms. Negative difference waves (Nds) were computed by subtracting ERPs to unattended standards from ERPs to the same stimuli when attended. The exogenous Nl, as estimated from unattended standard ERPs, was larger contralaterally to the stimulus and inverted in polarity at mastoids. With decreasing ISIs, Nl diminished in amplitude much faster than did the Nd. In addition, Nl latency remained stable, whereas Nd peaked markedly earlier with shorter ISIs, almost perfectly coinciding with the exogenous Nl. However, this temporal coincidence found in grand averages proved to be illusory in single subjects. The early Nd showed no contralateral asymmetry at its peak, but asymmetry was apparent during the ascending slope. These lateral asymmetries resembled those of the exogenous Nl but occurred later. The early Nd peak was, at least mainly, caused by an endogenous attention effect, the processing negativity (PN), even with very short ISIs, but an effect on the exogenous Nl could not be excluded.     

Cognitive resources are distributed among the entire auditory landscape in auditory scene analysis

Although attention has been shown to enhance neural representations of selected inputs, the fate of unselected background sounds is still debated. The goal of the current study was to understand how processing resources are distributed among attended and unattended sounds during auditory scene analysis. We used a three-stream paradigm with four acoustic features uniquely defining each sound stream (frequency, envelope shape, spatial location, tone quality). We manipulated task load by having participants perform a difficult auditory task and an easy movie-viewing task with the same set of sounds in separate conditions. The mismatch negativity (MMN) component of event-related brain potentials (ERPs) was measured to evaluate sound processing in both conditions. We found no effect of task demands on unattended sound processing: MMNs were elicited by unattended deviants during both low- and high-load task conditions. A key factor of this result was the use of unique tone feature combinations to distinguish each of the three sound streams, strengthening the segregation of streams. In the auditory task, the P3b component demonstrates a two-stage process of target evaluation. Thus, these results, in conjunction with results of previous studies, suggest that stimulus-driven factors that strengthen stream segregation can free up processing capacity for higher-level analyses. The results illustrate the interactive nature of top-down and stimulus-driven processes in stream formation, supporting a distributive theory of attention that balances the strength of the bottom-up input with perceptual goals in analyzing the auditory scene.     

Auditory selective attention modulates activation of human inferior colliculus

Selective auditory attention powerfully modulates neural activity in the human auditory cortex (AC). In contrast, the role of attention in subcortical auditory processing is not well established. Here, we used functional MRI (fMRI) to examine activation of the human inferior colliculus (IC) during strictly controlled auditory attention tasks. The IC is an obligatory midbrain nucleus of the ascending auditory pathway with diverse internal and external connections. The IC also receives a massive descending projection from the AC, suggesting that cortical processes affect IC operations. In this study, 21 subjects selectively attended to left-ear or right-ear sounds and ignored sounds delivered to the other ear. IC activations depended on the direction of attention, indicating that auditory processing in the human IC is not only determined by acoustic input but also by the current behavioral goals.     

Attentional modulation of human auditory cortex

Attention powerfully influences auditory perception, but little is understood about the mechanisms whereby attention sharpens responses to unattended sounds. We used high-resolution surface mapping techniques (using functional magnetic resonance imaging, fMRI) to examine activity in human auditory cortex during an intermodal selective attention task. Stimulus-dependent activations (SDAs), evoked by unattended sounds during demanding visual tasks, were maximal over mesial auditory cortex. They were tuned to sound frequency and location, and showed rapid adaptation to repeated sounds. Attention-related modulations (ARMs) were isolated as response enhancements that occurred when subjects performed pitch-discrimination tasks. In contrast to SDAs, ARMs were localized to lateral auditory cortex, showed broad frequency and location tuning, and increased in amplitude with sound repetition. The results suggest a functional dichotomy of auditory cortical fields: stimulus-determined mesial fields that faithfully transmit acoustic information, and attentionally labile lateral fields that analyze acoustic features of behaviorally relevant sounds.     

Effects of intermodal attention on the auditory steady-state response and the event-related potential

The aim of the present study was to simultaneously measure and compare intermodal attention effects in event-related brain potentials (ERPs) and auditory steady-state responses (ASSRs). For this purpose, 40-Hz amplitude modulated tones and a visual fixation cross were presented concurrently. By means of target detection tasks either on the sounds or on the fixation cross, participants' attention was directed to the respective modality. Attended sounds elicited a negative difference (Nd) in the ERP relative to unattended sounds. Nd was divided into an early and a late part as often observed for intramodal attention. Moreover, attention to the sounds led to a significant enhancement of the ASSR. This modulation of the ASSR by intermodal attention is demonstrated for the first time in the EEG. The present data suggest that ASSRs could provide a useful tool for the investigation of the neural dynamics of intermodal attentional processes.     

Processing of novel sounds and frequency changes in the human auditory cortex: Magnetoencephalographic recordings

Whole-head magnetoencephalographic (MEG) responses to repeating standard tones and to infrequent slightly higher deviant tones and complex novel sounds were recorded together with event-related brain potentials (ERPs). Deviant tones and novel sounds elicited the mismatch negativity (MMN) component of the ERP and its MEG counterpart (MMNm) both when the auditory stimuli were attended to and when they were ignored. MMNm generators were located bilateral to the superior planes of the temporal lobes where preattentive auditory discrimination appears to occur. A subsequent positive P3a component was elicited by deviant tones and with a larger amplitude by novel sounds even when the sounds were to be ignored. Source localization for the MEG counterpart of P3a (P3am) suggested that the auditory cortex in the superior temporal plane is involved in the neural network of involuntary attention switching to changes in the acoustic environment.     

Processing of auditory stimuli during auditory and visual attention as revealed by event-related potentials

Auditory event-related brain potentials (ERPs) were recorded during auditory and visual selective attention tasks. Auditory stimuli consisted of frequent standard tones (1000 Hz) and infrequent deviant tones (1050 Hz and 1300 Hz) delivered randomly to the left and right ears. Visual stimuli were vertical line gratings randomly presented on a video monitor at mean intervals of 6 s. During auditory attention, the subject attended to the stimuli in a designated ear and responded to the 1300-Hz deviants occurring among the attended tones. During visual attention, the subject responded to the occasional visual stimuli. ERPs for tones delivered to the attended ear were negatively displaced relative to ERPs elicited by tones delivered to the unattended ear and to ERPs elicited by auditory stimuli during visual attention. This attention effect consisted of negative difference waves with early and late components. Mismatch negativities (MMNs) were elicited by 1300-Hz and 1050-Hz deviants irrespective of whether they occurred among attended or unattended tones. MMN amplitudes were unaffected by attention, supporting the proposal that the MMN is generated by an automatic cerebral discrimination process.     

Sound-identity processing in early areas of the auditory ventral stream in the macaque

Auditory cortical processing is thought to be accomplished along two processing streams. The existence of a posterior/dorsal stream dealing, among others, with the processing of spatial aspects of sound has been corroborated by numerous studies in several species. An anterior/ventral stream for the processing of nonspatial sound qualities, including the identification of sounds such as species-specific vocalizations, has also received much support. Originally discovered in anterolateral belt cortex, most recent work on the anterior/ventral pathway has been performed on far anterior superior temporal (ST) areas and on ventrolateral prefrontal cortex (VLPFC). Regions of the anterior/ventral stream near its origin in early auditory areas have been less explored. In the present study, we examined three early auditory regions with different anteroposterior locations (caudal, middle, and rostral) in awake rhesus macaques. We analyzed how well classification based on sound-evoked activity patterns of neuronal populations replicates the original stimulus categories. Of the three regions, the rostral region (rR), which included core area R and medial belt area RM, yielded the greatest classification success across all stimulus classes or between classes of natural sounds. Starting from ～80 ms past stimulus onset, clustering based on the population response in rR became clearly more successful than clustering based on responses from any other region. Our study demonstrates that specialization for sound-identity processing can be found very early in the auditory ventral stream. Furthermore, the fact that this processing develops over time can shed light on underlying mechanisms. Finally, we show that population analysis is a more sensitive method for revealing functional specialization than conventional types of analysis.     

N2ac: an ERP component associated with the focusing of attention within an auditory scene

Humans must often focus attention onto relevant sensory signals in the presence of simultaneous irrelevant signals. This type of attention has been explored in vision with the N2pc component, and the present study sought to find an analogous auditory effect. In Experiment 1, two 750-ms sounds were presented simultaneously, one from each of two lateral speakers. On each trial, participants indicated whether one of the two sounds was a pre-defined target. We found that targets elicited an N2ac component: a negativity in the N2 latency range at anterior contralateral electrodes. We also observed a later and more posterior contralateral positivity. Experiment 2 replicated these effects and demonstrated that they arose from competition between attended and unattended tones rather than reflecting lateralized effects of attention for individual tones. The N2ac component may provide a useful tool for studying selective attention within auditory scenes.     

The Effects of Channel-Selective Attention on the Mismatch Negativity Wave Elicited by Deviant Tones

The mismatch negativity (MMN) is an event-related brain potential elicited by infrequent, physically deviant sounds in a sequence of repetitive auditory stimuli. Two dichotic listening experiments that were designed to optimize the selective focusing of attention provided a strong test of Näätänen's proposal that the MMN is unaffected by attention and reflects the operation of a strongly automatic mismatch detection system. In Experiment 1, tones were presented at intervals of 120-320 ms, and the deviant tones (intensity decrements) in both the attended and unattended ears elicited negative waves consistent in waveshape, latency, and distribution with previously described MMNs. In contrast to previous reports, however, the MMN elicited by the unattended-channel deviant was markedly reduced (peak amplitude of less than 1 μV) relative to the corresponding negative wave elicited by the attended-channel deviants (3–4 μV), as well as relative to previously reported MMNs (3–6 μV) elicited by comparable deviations in stimulus intensity. In Experiment 2, which employed interstimulus intervals of 65–205 ms, the unattended-channel MMN elicited by the deviant fainter tones was barely discernible, whereas the corresponding attended-channel negativity was again about 3-4 μV.
These findings call into question the assertion that the auditory mismatch detection process and the associated MMN wave are wholly independent of attentional influence. Rather, these data provide evidence that the processing of stimuli in unattended channels can be attenuated or gated at an early sensory level under conditions of highly focused auditory selective attention.     

Did You Listen to the Beat? Auditory Steady-State Responses in the Human Electroencephalogram at 4 and 7 Hz Modulation Rates Reflect Selective Attention

The acoustic envelope of human speech correlates with the syllabic rate (4–8 Hz) and carries important information for intelligibility, which is typically compromised in multi-talker, noisy environments. In order to better understand the dynamics of selective auditory attention to low frequency modulated sound sources, we conducted a two-stream auditory steady-state response (ASSR) selective attention electroencephalogram (EEG) study. The two streams consisted of 4 and 7 Hz amplitude and frequency modulated sounds presented from the left and right side. One of two streams had to be attended while the other had to be ignored. The attended stream always contained a target, allowing for the behavioral confirmation of the attention manipulation. EEG ASSR power analysis revealed a significant increase in 7 Hz power for the attend compared to the ignore conditions. There was no significant difference in 4 Hz power when the 4 Hz stream had to be attended compared to when it had to be ignored. This lack of 4 Hz attention modulation could be explained by a distracting effect of a third frequency at 3 Hz (beat frequency) perceivable when the 4 and 7 Hz streams are presented simultaneously. Taken together our results show that low frequency modulations at syllabic rate are modulated by selective spatial attention. Whether attention effects act as enhancement of the attended stream or suppression of to be ignored stream may depend on how well auditory streams can be segregated.     

Sensorial suppression of self-generated sounds and its dependence on attention

The auditory processing of self-generated sounds is characterized by an attenuated vertex N1-component of the event-related potential (ERP) compared to the responses elicited by externally generated sounds. Typically, a motor condition where sounds are actively produced by button presses is compared with a passive listening condition. While this effect is usually interpreted as reflection of an internal forward model system, the impact of attention and arousal on the so called self-generation effect has not been systematically controlled in these studies: Is the auditory stimulation more attended during the active task compared to passive listening, e.g., caused by a higher arousal level? Or is it rather attended less and attention is drawn away from the task-irrelevant stimulation to the motor task? Accordingly, the self-generation effects reported in the literature can easily be over- or underestimated. In the present study we disentangled attention from the self-generation effect by introducing an active listening condition, in which attention is focused to the same feature as in the self-generation condition — the stimulus onset-to-onset interval. We observed a classical ‘self-generation effect’, i.e. attenuated amplitudes for self-generated compared to passive listened sounds at frontocentral electrodes. As expected this effect was overlapped by attention effects in space and time. However, topographical and tomographical analyses allowed us to clearly disentangle both effects. Our results argue for the existence of a genuine self-generation effect, but emphasize the problem of possible over- or underestimation caused by attentional confounds.     

Spatial and cross-modal attention alter responses to unattended sensory information in early visual and auditory human cortex

Attending to a visual or auditory stimulus often requires irrelevant information to be filtered out, both within the modality attended and in other modalities. For example, attentively listening to a phone conversation can diminish our ability to detect visual events. We used functional magnetic resonance imaging (fMRI) to examine brain responses to visual and auditory stimuli while subjects attended visual or auditory information. Although early cortical areas are traditionally considered unimodal, we found that brain responses to the same ignored information depended on the modality attended. In early visual area V1, responses to ignored visual stimuli were weaker when attending to another visual stimulus, compared with attending to an auditory stimulus. The opposite was true in more central visual area MT+, where responses to ignored visual stimuli were weaker when attending to an auditory stimulus. Furthermore, fMRI responses to the same ignored visual information depended on the location of the auditory stimulus, with stronger responses when the attended auditory stimulus shared the same side of space as the ignored visual stimulus. In early auditory cortex, responses to ignored auditory stimuli were weaker when attending a visual stimulus. A simple parameterization of our data can describe the effects of redirecting attention across space within the same modality (spatial attention) or across modalities (cross-modal attention), and the influence of spatial attention across modalities (cross-modal spatial attention). Our results suggest that the representation of unattended information depends on whether attention is directed to another stimulus in the same modality or the same region of space.     

The late Nd reflects a memory trace containing amodal spatial information

The early Nd reflects the analysis of simple features of selectively attended auditory stimuli, but the precise nature of the more complex processing reflected by the late Nd is unclear. The late but not the early Nd is sensitive to interference from a concurrently presented visual spatial attention switching task. This experiment investigated whether the late Nd is also sensitive to deeper visual attention switching. Twenty-one subjects performed a dichotic listening task concurrently with either visual spatial or visual letter matching attention switching tasks. Late Nd amplitude was reduced by the spatial but not the letter matching task, indicating insensitivity to deeper attention switching. P300 amplitude was reduced by both tasks. Reductions in N100 and P200 were uncorrelated. We propose that, in part, the late Nd reflects an amodal memory trace containing spatial information, possibly involving a "where" rather than a "what" auditory pathway.     

Vision enhances selective attention to body-related information

Viewing ones’ own hands while directing attention to one of the hands leads to earlier attentional modulations of somatosensory processing than when hands are not visible. This effect of vision on tactile-spatial selection could be explained by vision providing additional information about the location of the hands in external space. The present study investigated whether vision of the hands also affected tactile-attentional mechanisms when the relative locations of the hands were irrelevant. Participants silently counted infrequent tactile or auditory deviants in an alternating stream of tactile and auditory stimuli while ignoring stimuli in the other modality, when their hands were either visible or covered from view. Modality-selective attentional modulations of ERPs to tactile stimuli (when touches vs. tones were attended) were already present for the time range of the N80 component when hands were visible, but there were only later modulations (starting at N140) when hands were covered. This suggests that, rather than being restricted to tasks requiring spatial selection between body parts, vision of the hands can facilitate attention toward the body in far more general terms. In contrast to tactile stimuli, attentional modulations of ERPs to auditory stimuli (when tones vs. touches were attended) were not reliably affected by viewing the hands. This suggests that the primary purpose of visual facilitation may be to enhance the processing of body-related information only.     

Combined Use of Microreflexes and Event-Related Brain Potentials as Measures of Auditory Selective Attention

Two versions of a selective listening experiment were conducted in an attempt to specify the level of the nervous system at which selective attention first influences auditory information processing. Post-auricular reflexes (PARs), preputse inhibition of reflexes, and the N1 component of the event-related potential (ERP) were measured concurrently to assess auditory processing in the hindbrain. midbrain. and forebrain. respectively. Sequences of intense, reflex-eliciting tones were presented to the two ears in random order and at a rapid rate as subjects listened to a designated ear to detect rare tones of slightly lower intensity. The ERPs showed the typical enhancement of early and late negativity (Nd) to attended car tunes, with the early Nd overlapping the evoked N1 component. Although there was an overall effect of attention direction on the efferent limb of the post-auricular reflex, no effect of selective attention on the afferent limb was observed. A selective sensory attentional effect was found for prepulse inhibition of the post-auricular reflex: Inhibition of PAR amplitude was enhanced when the immediately preceding tone was in the attended ear relative to when it was in the unattended ear. These results, together with findings from prior studies of reflexes and ERPs. indicate that evoked auditorv activity in the lower brainstem is obligatory and invariant with attention, whereas later activity mediated in the upper brainstem can be modulated by attention.