Lars Riecke

Parietal and superior frontal visuospatial maps activated by pointing and saccades

A recent study from our laboratory demonstrated that parietal cortex contains a map of visual space related to saccades and spatial attention and identified this area as the likely human homologue of the lateral intraparietal (LIP). A human homologue for the parietal reach region (PRR), thought to preferentially encode planned hand movements, has also been recently proposed. Both of these areas, originally identified in the macaque monkey, have been shown to encode space with eye-centered coordinates. Functional magnetic resonance imaging (fMRI) of humans was used to test the hypothesis that the putative human PRR contains a retinotopic map recruited by finger pointing but not saccades and to test more generally for differences in the visuospatial maps recruited by pointing and saccades. We identified multiple maps in both posterior parietal cortex and superior frontal cortex recruited for eye and hand movements, including maps not observed in previous mapping studies. Pointing and saccade maps were generally consistent within single subjects. We have developed new group analysis methods for phase-encoded data, which revealed subtle differences between pointing and saccades, including hemispheric asymmetries, but we did not find evidence of pointing-specific maps of visual space.

Hearing illusory sounds in noise: the timing of sensory-perceptual transformations in auditory cortex.

Constructive mechanisms in the auditory system may restore a fragmented sound when a gap in this sound is rendered inaudible by noise to yield a continuity illusion. Using combined psychoacoustic and electroencephalography experiments in humans, we found that the sensory-perceptual mechanisms that enable restoration suppress auditory cortical encoding of gaps in interrupted sounds. When physically interrupted tones are perceptually restored, stimulus-evoked synchronization of cortical oscillations at approximately 4 Hz is suppressed as if physically uninterrupted sounds were encoded. The restoration-specific suppression is induced most strongly in primary-like regions in the right auditory cortex during illusorily filled gaps and also shortly before and after these gaps. Our results reveal that spontaneous modulations in slow evoked auditory cortical oscillations that are involved in encoding acoustic boundaries may determine the perceived continuity of sounds in noise. Such fluctuations could facilitate stable hearing of fragmented sounds in natural environments.

4-Hz Transcranial Alternating Current Stimulation Phase Modulates Hearing

BACKGROUND Non-invasive brain stimulation with transcranial alternating currents (tACS) has been shown to entrain slow cortical oscillations and thereby influence various aspects of visual perception. Much less is known about its potential effects on auditory perception. OBJECTIVE In the present study, we apply a novel variant that enables near-equivalent stimulation of both auditory cortices to investigate the causal role of the phase of 4-Hz cortical oscillations for auditory perception. METHODS We measured detection performance for near-threshold auditory stimuli (4-Hz click trains) that were presented at various moments during ongoing tACS (two synchronous 4-Hz alternating currents applied transcranially to the two cerebral hemispheres). RESULTS We found that changes in the relative timing of acoustic and electric stimulation cause corresponding perceptual changes that oscillate predominantly at the 4-Hz frequency of the electric stimulation, which is consistent with previous results based on 10-Hz tACS. CONCLUSION TACS at various frequencies can affect auditory perception. Together with converging previous results based on acoustic stimulation (rather than tACS), this finding implies that fundamental aspects of auditory cognition are mediated by the temporal coherence of sound-induced cortical activity with ongoing cortical oscillations at multiple time scales.

The Auditory Continuity Illusion: A Parametric Investigation and Filter Model

A sound that is briefly interrupted by a silent gap is perceived as discontinuous. However, when the gap is filled with noise, the sound may be perceived as continuing through the noise. It has been shown that this continuity illusion depends on the masking of the omitted target sound, but the underlying mechanisms have yet to be quantified thoroughly. In this article, we systematically quantify the relation between perceived continuity and the duration, relative power, or notch width of the interrupting broadband noise for interrupted and noninterrupted amplitude-modulated tones at different frequencies. We fitted the psychometric results in order to estimate the range of the noise parameters that induced auditory grouping. To explain our results within a common theoretical framework, we applied a power spectrum model to the different masking results and estimated the critical bandwidth of the auditory filter that may be responsible for the continuity illusion. Our results set constraints on the spectral resolution of the mechanisms underlying the continuity illusion and provide a stimulus set that can be readily applied for neurophysiological studies of its neural correlates.

Endogenous Delta/Theta Sound-Brain Phase Entrainment Accelerates the Buildup of Auditory Streaming

In many natural listening situations, meaningful sounds (e.g., speech) fluctuate in slow rhythms among other sounds. When a slow rhythmic auditory stream is selectively attended, endogenous delta (1‒4 Hz) oscillations in auditory cortex may shift their timing so that higher-excitability neuronal phases become aligned with salient events in that stream [1, 2]. As a consequence of this stream-brain phase entrainment [3], these events are processed and perceived more readily than temporally non-overlapping events [4-11], essentially enhancing the neural segregation between the attended stream and temporally noncoherent streams [12]. Stream-brain phase entrainment is robust to acoustic interference [13-20] provided that target stream-evoked rhythmic activity can be segregated from noncoherent activity evoked by other sounds [21], a process that usually builds up over time [22-27]. However, it has remained unclear whether stream-brain phase entrainment functionally contributes to this buildup of rhythmic streams or whether it is merely an epiphenomenon of it. Here, we addressed this issue directly by experimentally manipulating endogenous stream-brain phase entrainment in human auditory cortex with non-invasive transcranial alternating current stimulation (TACS) [28-30]. We assessed the consequences of these manipulations on the perceptual buildup of the target stream (the time required to recognize its presence in a noisy background), using behavioral measures in 20 healthy listeners performing a naturalistic listening task. Experimentally induced cyclic 4-Hz variations in stream-brain phase entrainment reliably caused a cyclic 4-Hz pattern in perceptual buildup time. Our findings demonstrate that strong endogenous delta/theta stream-brain phase entrainment accelerates the perceptual emergence of task-relevant rhythmic streams in noisy environments.

Hearing an Illusory Vowel in Noise: Suppression of Auditory Cortical Activity

Human hearing is constructive. For example, when a voice is partially replaced by an extraneous sound (e.g., on the telephone due to a transmission problem), the auditory system may restore the missing portion so that the voice can be perceived as continuous (Miller and Licklider, 1950; for review, see Bregman, 1990; Warren, 1999). The neural mechanisms underlying this continuity illusion have been studied mostly with schematic stimuli (e.g., simple tones) and are still a matter of debate (for review, see Petkov and Sutter, 2011). The goal of the present study was to elucidate how these mechanisms operate under more natural conditions. Using psychophysics and electroencephalography (EEG), we assessed simultaneously the perceived continuity of a human vowel sound through interrupting noise and the concurrent neural activity. We found that vowel continuity illusions were accompanied by a suppression of the 4 Hz EEG power in auditory cortex (AC) that was evoked by the vowel interruption. This suppression was stronger than the suppression accompanying continuity illusions of a simple tone. Finally, continuity perception and 4 Hz power depended on the intactness of the sound that preceded the vowel (i.e., the auditory context). These findings show that a natural sound may be restored during noise due to the suppression of 4 Hz AC activity evoked early during the noise. This mechanism may attenuate sudden pitch changes, adapt the resistance of the auditory system to extraneous sounds across auditory scenes, and provide a useful model for assisted hearing devices.

The continuity illusion adapts to the auditory scene

The human auditory system is efficient at restoring sounds of interest. In noisy environments, for example, an interrupted target sound may be illusorily heard as continuing smoothly when a loud noise masks the interruptions. In quiet environments, however, sudden interruptions might signal important events. In that case, restoration of the target sound would be disadvantageous. Achieving useful perceptual stability may require the restoration mechanism to adapt its output to current perceptual demands, a hypothesis which has not yet been fully evaluated. In this study, we investigated whether auditory restoration depends on preceding auditory scenes, and we report evidence that restoration adapts to the perceived continuity of target sounds and to the loudness of interrupting sounds. In the first experiment, listeners adapted to illusory and non-illusory tone sweeps (targets) and interrupting noise, and we observed that the perceived continuity of the target and the loudness of the interrupting noise influenced the extent of subsequent restorations. A second experiment revealed that these adaptation effects were unrelated to the adapted spectra, indicating that non-sensory representations of the perceived auditory scene were involved. We argue that auditory restoration is a dynamic illusory phenomenon which recalibrates continuity hearing to different acoustic environments.

Frequency-Selective Attention in Auditory Scenes Recruits Frequency Representations Throughout Human Superior Temporal Cortex

&NA; A sound of interest may be tracked amid other salient sounds by focusing attention on its characteristic features including its frequency. Functional magnetic resonance imaging findings have indicated that frequency representations in human primary auditory cortex (AC) contribute to this feat. However, attentional modulations were examined at relatively low spatial and spectral resolutions, and frequency‐selective contributions outside the primary AC could not be established. To address these issues, we compared blood oxygenation level‐dependent (BOLD) responses in the superior temporal cortex of human listeners while they identified single frequencies versus listened selectively for various frequencies within a multifrequency scene. Using best‐frequency mapping, we observed that the detailed spatial layout of attention‐induced BOLD response enhancements in primary AC follows the tonotopy of stimulus‐driven frequency representations—analogous to the “spotlight” of attention enhancing visuospatial representations in retinotopic visual cortex. Moreover, using an algorithm trained to discriminate stimulus‐driven frequency representations, we could successfully decode the focus of frequency‐selective attention from listeners’ BOLD response patterns in nonprimary AC. Our results indicate that the human brain facilitates selective listening to a frequency of interest in a scene by reinforcing the fine‐grained activity pattern throughout the entire superior temporal cortex that would be evoked if that frequency was present alone.

Stimulus Presentation at Specific Neuronal Oscillatory Phases Experimentally Controlled with tACS: Implementation and Applications

In recent years, it has become increasingly clear that both the power and phase of oscillatory brain activity can influence the processing and perception of sensory stimuli. Transcranial alternating current stimulation (tACS) can phase-align and amplify endogenous brain oscillations and has often been used to control and thereby study oscillatory power. Causal investigation of oscillatory phase is more difficult, as it requires precise real-time temporal control over both oscillatory phase and sensory stimulation. Here, we present hardware and software solutions allowing temporally precise presentation of sensory stimuli during tACS at desired tACS phases, enabling causal investigations of oscillatory phase. We developed freely available and easy to use software, which can be coupled with standard commercially available hardware to allow flexible and multi-modal stimulus presentation (visual, auditory, magnetic stimuli, etc.) at pre-determined tACS-phases, opening up a range of new research opportunities. We validate that stimulus presentation at tACS phase in our setup is accurate to the sub-millisecond level with high inter-trial consistency. Conventional methods investigating the role of oscillatory phase such as magneto-/electroencephalography can only provide correlational evidence. Using brain stimulation with the described methodology enables investigations of the causal role of oscillatory phase. This setup turns oscillatory phase into an independent variable, allowing innovative, and systematic studies of its functional impact on perception and cognition.

Global not local masker features govern the auditory continuity illusion

When an acoustic signal is temporarily interrupted by another sound, it is sometimes heard as continuing through, even when the signal is actually turned off during the interruption—an effect known as the “auditory continuity illusion.” A widespread view is that the illusion can only occur when peripheral neural responses contain no evidence that the signal was interrupted. Here we challenge this view using a combination of psychophysical measures from human listeners and computational simulations with a model of the auditory periphery. The results reveal that the illusion seems to depend more on the overall specific loudness than on the peripheral masking properties of the interrupting sound. This finding indicates that the continuity illusion is determined by the global features, rather than the fine-grained temporal structure, of the interrupting sound, and argues against the view that the illusion arises in the auditory periphery.