Brain networks of bottom-up triggered and top-down controlled shifting of auditory attention

Stimulus-related independent component and voxel-wise analysis of human brain activity during free viewing of a feature film

Understanding how the brain processes stimuli in a rich natural environment is a fundamental goal of neuroscience. Here, we showed a feature film to 10 healthy volunteers during functional magnetic resonance imaging (fMRI) of hemodynamic brain activity. We then annotated auditory and visual features of the motion picture to inform analysis of the hemodynamic data. The annotations were fitted to both voxel-wise data and brain network time courses extracted by independent component analysis (ICA). Auditory annotations correlated with two independent components (IC) disclosing two functional networks, one responding to variety of auditory stimulation and another responding preferentially to speech but parts of the network also responding to non-verbal communication. Visual feature annotations correlated with four ICs delineating visual areas according to their sensitivity to different visual stimulus features. In comparison, a separate voxel-wise general linear model based analysis disclosed brain areas preferentially responding to sound energy, speech, music, visual contrast edges, body motion and hand motion which largely overlapped the results revealed by ICA. Differences between the results of IC- and voxel-based analyses demonstrate that thorough analysis of voxel time courses is important for understanding the activity of specific sub-areas of the functional networks, while ICA is a valuable tool for revealing novel information about functional connectivity which need not be explained by the predefined model. Our results encourage the use of naturalistic stimuli and tasks in cognitive neuroimaging to study how the brain processes stimuli in rich natural environments.

Neural events leading to and associated with detection of sounds under high processing load

The neural events that lead to successful or failed detection of suprathreshold sounds are not well established. In this experiment, event-related potentials (ERPs) and functional magnetic resonance imaging (fMRI) were recorded while participants performed two tasks: a primary difficult duration judgment task on a sequence of tones presented to one ear, and a secondary target detection task on an auditory oddball stream presented to the other ear. The paradigm was designed to elicit competition and variability in detection of auditory targets despite identical input. Successful detection of auditory targets was associated mainly with greater fMRI activity in superior parietal cortex and thalamus. In the ERPs, successful detection was linked with a larger fronto-central negativity at 200-400 ms, and a later centro-posterior positivity. Failure to detect targets was associated with greater fMRI signal in the default mode network, a significantly smaller electrical fronto-central negativity and no late positivity. These findings demonstrate that variability in auditory detection is related to modulation of activity in multimodal parietal and frontal networks active ∼ 200 ms after target onset. Results are consistent with a limited capacity and late selection view of attention.

Lateralization of frequency-specific networks for covert spatial attention to auditory stimuli

We conducted a cued spatial attention experiment to investigate the time-frequency structure of human EEG induced by attentional orientation of an observer in external auditory space. Seven subjects participated in a task in which attention was cued to one of two spatial locations at left and right. Subjects were instructed to report the speech stimulus at the cued location and to ignore a simultaneous speech stream originating from the uncued location. EEG was recorded from the onset of the directional cue through the offset of the inter-stimulus interval (ISI), during which attention was directed toward the cued location. Using a wavelet spectrum, each frequency band was then normalized by the mean level of power observed in the early part of the cue interval to obtain a measure of induced power related to the deployment of attention. Topographies of band specific induced power during the cue and inter-stimulus intervals showed peaks over symmetric bilateral scalp areas. We used a bootstrap analysis of a lateralization measure defined for symmetric groups of channels in each band to identify specific lateralization events throughout the ISI. Our results suggest that the deployment and maintenance of spatially oriented attention throughout a period of 1,100 ms is marked by distinct episodes of reliable hemispheric lateralization ipsilateral to the direction in which attention is oriented. An early theta lateralization was evident over posterior parietal electrodes and was sustained throughout the ISI. In the alpha and mu bands punctuated episodes of parietal power lateralization were observed roughly 500 ms after attentional deployment, consistent with previous studies of visual attention. In the beta band these episodes show similar patterns of lateralization over frontal motor areas. These results indicate that spatial attention involves similar mechanisms in the auditory and visual modalities.

Electrical neuroimaging of voluntary audiospatial attention: evidence for a supramodal attention control network

Previous attempts to investigate the supramodal nature of attentional control have focused primarily on identifying neuroanatomical overlap in the frontoparietal systems activated during voluntary shifts of spatial attention in different sensory modalities. However, the activation of the same neural structures is insufficient evidence for a supramodal system, as the same brain regions could interact with one another in very different ways during shifts of attention in different modalities. Thus, to explore the similarity of the functional networks, it is necessary to identify the neural structures involved and to examine the timing and sequence of activities within the network. To this end, we used an electrical neuroimaging technique to localize the neural sources of electroencephalographic signals recorded from human subjects during audiospatial shifts of attention and to examine the timing and sequence of activities within several regions of interest. We then compared the results to an analogous study of visuospatial attention shifts. Similar frontal and parietal regions were activated during visual and auditory shifts of attention, and the timing of activities within these regions was nearly identical. Following this modality-independent sequence of attention-control activity, activity in the relevant sensory cortex was enhanced in anticipation of the response-relevant target. These results are consistent with the hypothesis that a single supramodal network of frontal and parietal regions mediates voluntary shifts of spatial attention and controls the flow of sensory information in modality-specific sensory pathways.

Attention and cognitive control networks assessed in a dichotic listening fMRI study

A meaningful interaction with our environment relies on the ability to focus on relevant sensory input and to ignore irrelevant information, i.e. top-down control and attention processes are employed to select from competing stimuli following internal goals. In this, the demands for the recruitment of top-down control processes depend on the relative perceptual salience of the competing stimuli. In the present functional magnetic resonance imaging (fMRI) study, we investigated the recruitment of top-down control processes in response to varying degrees of control demands in the auditory modality. For this purpose, we tested 20 male and 20 female subjects with a dichotic listening paradigm, in which the relative perceptual salience of two simultaneously presented stimuli was systematically manipulated by varying the inter-aural intensity difference (IID) and asking the subjects to selectively attend to either ear. The analysis showed that the interaction between IID and attentional direction involves two networks in the brain. A fronto-parietal network, including the pre-supplementary motor area, anterior cingulate cortex, inferior frontal junction, insula and inferior parietal lobe, was recruited during cognitively demanding conditions and can thus be seen as a top-down cognitive control network. In contrast, a second network including the superior temporal and the post-central gyri was engaged under conditions with low cognitive control demands. These findings demonstrate how cognitive control is achieved through the interplay of distinct brain networks, with their differential engagement determined as a function of the level of competition between the sensory stimuli.

Brain bases for auditory stimulus-driven figure-ground segregation

Auditory figure-ground segregation, listeners' ability to selectively hear out a sound of interest from a background of competing sounds, is a fundamental aspect of scene analysis. In contrast to the disordered acoustic environment we experience during everyday listening, most studies of auditory segregation have used relatively simple, temporally regular signals. We developed a new figure-ground stimulus that incorporates stochastic variation of the figure and background that captures the rich spectrotemporal complexity of natural acoustic scenes. Figure and background signals overlap in spectrotemporal space, but vary in the statistics of fluctuation, such that the only way to extract the figure is by integrating the patterns over time and frequency. Our behavioral results demonstrate that human listeners are remarkably sensitive to the appearance of such figures. In a functional magnetic resonance imaging experiment, aimed at investigating preattentive, stimulus-driven, auditory segregation mechanisms, naive subjects listened to these stimuli while performing an irrelevant task. Results demonstrate significant activations in the intraparietal sulcus (IPS) and the superior temporal sulcus related to bottom-up, stimulus-driven figure-ground decomposition. We did not observe any significant activation in the primary auditory cortex. Our results support a role for automatic, bottom-up mechanisms in the IPS in mediating stimulus-driven, auditory figure-ground segregation, which is consistent with accumulating evidence implicating the IPS in structuring sensory input and perceptual organization.

Is my mobile ringing? Evidence for rapid processing of a personally significant sound in humans

Anecdotal reports and also empirical observations suggest a preferential processing of personally significant sounds. The utterance of one's own name, the ringing of one's own telephone, or the like appear to be especially effective for capturing attention. However, there is a lack of knowledge about the time course and functional neuroanatomy of the voluntary and the involuntary detection of personally significant sounds. To address this issue, we applied an active and a passive listening paradigm, in which male and female human participants were presented with the SMS ringtone of their own mobile and other's ringtones, respectively. Enhanced evoked oscillatory activity in the 35-75 Hz band for one's own ringtone shows that the brain distinguishes complex personally significant and nonsignificant sounds, starting as early as 40 ms after sound onset. While in animals it has been reported that the primary auditory cortex accounts for acoustic experience-based memory matching processes, results from the present study suggest that in humans these processes are not confined to sensory processing areas. In particular, we found a coactivation of left auditory areas and left frontal gyri during passive listening. Active listening evoked additional involvement of sensory processing areas in the right hemisphere. This supports the idea that top-down mechanisms affect stimulus representations even at the level of sensory cortices. Furthermore, active detection of sounds additionally activated the superior parietal lobe supporting the existence of a frontoparietal network of selective attention.

Sound processing hierarchy within human auditory cortex

Both attention and masking sounds can alter auditory neural processes and affect auditory signal perception. In the present study, we investigated the complex effects of auditory-focused attention and the signal-to-noise ratio of sound stimuli on three different auditory evoked field components (auditory steady-state response, N1m, and sustained field) by means of magnetoencephalography. The results indicate that the auditory steady-state response originating in primary auditory cortex reflects the signal-to-noise ratio of physical sound inputs (bottom-up process) rather than the listener's attentional state (top-down process), whereas the sustained field, originating in nonprimary auditory cortex, reflects the attentional state rather than the signal-to-noise ratio. The N1m was substantially influenced by both bottom-up and top-down neural processes. The differential sensitivity of the components to bottom-up and top-down neural processes, contingent on their level in the processing pathway, suggests a stream from bottom-up driven sensory neural processing to top-down driven auditory perception within human auditory cortex.

Identification of attention and cognitive control networks in a parametric auditory fMRI study

In the competition for limited processing resources, top-down attention and cognitive control processes are needed to separate relevant from irrelevant sensory information and to interact with the environment in a meaningful way. The demands for the recruitment of top-down control processes depend on the relative salience of the competing stimuli. In the present event-related functional magnetic resonance imaging (fMRI) study we investigated the dynamics of neuronal networks during varying degrees of top-down control demands. We tested 20 participants with a dichotic auditory discrimination task in which the relative perceptual salience of two simultaneously presented syllables was parametrically varied by manipulating the inter-aural intensity differences (IIDs) and instructing the subjects to selectively attend to either the louder or weaker of the two stimuli. A significant interaction of IID manipulation and attentional instruction was detected bilaterally in the inferior parietal lobe and pre-supplementary motor area, and in the precentral gyrus, anterior cingulate cortex, and inferior frontal gyrus of the right hemisphere. The post hoc analysis of the interaction pattern allowed for an assignment of these regions to either of two sets of regions which can be interpreted to constitute two different brain networks: a fronto-parietal attention control network, involved in the integration of saliency-based and instruction-based processing preferences, and a medial-lateral frontal cognitive control network, involved in the processing of the conflicts arising in the attempt to follow the attentional instruction in face of the varying inter-aural stimulus salience.