Selective modulation of interhemispheric connectivity by transcranial alternating current stimulation influences binaural integration

Brain connectivity plays a major role in the encoding, transfer, and integration of sensory information. Interregional synchronization of neural oscillations in the γ-frequency band has been suggested as a key mechanism underlying perceptual integration. In a recent study, we found evidence for this hypothesis showing that the modulation of interhemispheric oscillatory synchrony by means of bihemispheric high-density transcranial alternating current stimulation (HD-TACS) affects binaural integration of dichotic acoustic features. Here, we aimed to establish a direct link between oscillatory synchrony, effective brain connectivity, and binaural integration. We experimentally manipulated oscillatory synchrony (using bihemispheric γ-TACS with different interhemispheric phase lags) and assessed the effect on effective brain connectivity and binaural integration (as measured with functional MRI and a dichotic listening task, respectively). We found that TACS reduced intrahemispheric connectivity within the auditory cortices and antiphase (interhemispheric phase lag 180°) TACS modulated connectivity between the two auditory cortices. Importantly, the changes in intra- and interhemispheric connectivity induced by TACS were correlated with changes in perceptual integration. Our results indicate that γ-band synchronization between the two auditory cortices plays a functional role in binaural integration, supporting the proposed role of interregional oscillatory synchrony in perceptual integration.

Previously reward-associated sounds interfere with goal-directed auditory processing

Previously reward-associated stimuli have consistently been shown to involuntarily capture attention in the visual domain. Although previously reward-associated but currently task-irrelevant sounds have also been shown to interfere with visual processing, it remains unclear whether such stimuli can interfere with the processing of task-relevant auditory information. To address this question, we modified a dichotic listening task to measure interference from task-irrelevant but previously reward-associated sounds. In a training phase, participants were simultaneously presented with a spoken letter and number in different auditory streams and learned to associate the correct identification of each of three letters with high, low, and no monetary reward, respectively. In a subsequent test phase, participants were again presented with the same auditory stimuli but were instead instructed to report the number while ignoring spoken letters. In both the training and test phases, response time measures demonstrated that attention was biased in favour of the auditory stimulus associated with high value. Our findings demonstrate that attention can be biased towards learned reward cues in the auditory domain, interfering with goal-directed auditory processing.

Assessing the reliability of an online behavioural laterality battery: A pre-registered study.

Studies of cerebral lateralisation often involve participants completing a series of perceptual tasks under laboratory conditions. This has constrained the number of participants recruited in such studies. Online testing can allow for much larger sample sizes but limits the amount of experimental control that is feasible. Here we considered whether online testing could give valid and reliable results on four tasks: a rhyme decision visual half-field task, a dichotic listening task, a chimeric faces task, and a finger tapping task. We recruited 392 participants, oversampling left-handers, who completed the battery twice. Three of the tasks showed evidence of both validity and reliability, insofar as they showed hemispheric advantages in the expected direction and test-retest reliability of at least r= .75. The reliability of the rhyme decision task was less satisfactory (r= .62). We also confirmed a prediction that extreme left-handers were more likely to depart from typical lateralisation. Lateralisation across the two language tasks (dichotic listening and rhyme judgement) was weakly correlated, but unrelated to lateralisation on the chimeric faces task. We conclude that three of the tasks, dichotic listening, chimeric faces and finger tapping, show considerable promise for online evaluation of cerebral lateralisation.

What Does Language Have to Do With It? The Impact of Age and Bilingual Experience on Inhibitory Control in an Auditory Dichotic Listening Task

Purpose The purpose of the current study was to examine inhibition of irrelevant information in younger and older English monolingual and Spanish–English bilingual adults. Method Sixty-one participants divided into four groups: 15 younger English monolinguals, 16 younger Spanish–English bilinguals, 15 older English monolinguals, and 15 older Spanish–English bilinguals participated in this study. Younger participants were 18–25 years of age, and older participants were 47–62 years of age. Bilingual participants had learned Spanish from birth and began learning English by the age of 3 years old ( SD = 3.1). All participants had hearing thresholds of < 25 dB HL from 250 to 4000 Hz, bilaterally (American National Standards Institute [ANSI], 2004) and were right-handed. Inhibition was measured using a forced-attention dichotic consonant vowel listening task with a prime stimulus and the Simon task, a nonverbal visual test. Results Younger participants were better able to inhibit the irrelevant auditory and visual stimuli and modulate their attention according to the instructions that they were given compared to the older participants. However, no significant differences in inhibition performance or attention modulation were evidenced between the monolingual and bilingual groups in any of the dichotic listening conditions or on the Simon task. Conclusions No significant differences in performance on an auditory or a visual test of inhibition of irrelevant information was evidenced between the monolingual and bilingual participants in this study. These findings suggest that younger and older adult, early balanced Spanish–English bilinguals may not exhibit an advantage in the inhibition of irrelevant information compared to younger and older adult English monolinguals.

M187. A MULTIMODAL, NEUROIMAGING STUDY OF TDCS MECHANISMS RELATED TO AUDITORY VERBAL HALLUCINATIONS IN HEALTHY INDIVIDUALS

Background Transcranial direct current stimulation (tDCS) is a brain stimulation method which is growing in popularity in both research and clinical settings, especially as a treatment of auditory verbal hallucinations (AVH) in patients with schizophrenia. However, the underlying neural mechanisms of this tDCS treatment are poorly understood. Current AVH models propose that AVH arise from hyperactivation in the left temporo parietal (LTPC), causing AVH, and from hypoactivation in the left dorsolateral prefrontal cortex (LDLPFC), leading to diminished control over AVH. We aimed to “mimic” this hyper-/hypoactivation pattern in healthy individuals with tDCS by placing the excitatory anode above the LTPC and the inhibitory cathode over the LDLPFC and then to study the effects of tDCS on these brain areas. Previous studies examined either brain activation, neurochemistry, or behavior, with other electrode montages, but few looked at those aspects together. The present study therefore examined tDCS effects with fMRI (functional magnetic resonance imaging), MR spectroscopy, behavioral tasks and simulation of the electric field in a multimodal approach. We hypothesized that tDCS would (a) lead to similar behavioral deficits in healthy individuals as in schizophrenia patients and (b) induce changes in the stimulated areas on neurotransmitter and functional activation level. Methods Thirty-two healthy participants (18 males, mean age=26 yrs) were tested twice, ca. one week apart, with either real or sham (control) 2mA tDCS for 20 min while in a GE 750, 3T MRI scanner. The order of real/sham stimulation was counterbalanced in a double-blind design. During fMRI, participants completed a dichotic listening task in a block design, in order to measure behavior and brain activation changes. Before and after fMRI/tDCS, MR spectroscopy was carried out in two voxels placed under the electrodes. The data was analyzed with repeated measures ANOVAs. After data-collection, the structural T1 sequence was used to simulate the electric field of tDCS stimulation. Results Glx (Glutamate and glutamine combined) showed a trend (F(1,31)=3.35, p=.077, η2p=.098) to increase after tDCS stimulation compared to before, however this was not electrode specific. Neither fMRI, nor the dichotic listening task (all F≤1.64, p≥.203, η2p≤.052) showed any stimulation specific differences between real and sham stimulation. The tDCS simulation revealed large individual differences in the electric field induced. Discussion In the present study, tDCS seemed to have little effect on the measured brain parameters and little validation for the AVH model was found. The mechanisms of tDCS and how it affects the underlying brain tissue are poorly understood and seem to be affected by different stimulation parameters like stimulation duration, current strength and electrode montage. To use tDCS most effectively in schizophrenia research and treatment of auditory hallucinations, it should be validated with a multitude of methods, similar to the approach described here.

Noradrenergic responsiveness preserves selective attention across the adult life span

ABSTRACTSelectively attending to relevant information while blocking out distractors is crucial for goal-directed behavior, yet with advancing age, deficits emerge in attentional selectivity. Decrements in attention have been associated with altered noradrenergic activity in animals. However, research linking noradrenergic functioning to attention in aging humans is scarce, likely reflecting long-standing methodological challenges in non-invasive assessments. We studied whether age-related differences in the noradrenergic system predict differences in attention. We measured pupil dilation, a non-invasive marker of phasic norepinephrine (NE) release, while concurrently recording the electroencephalogram (EEG), of younger (N=39♂; 25.2±3.2 years) and older adults (N=38♂; 70.6±2.7 years). NE release was triggered on a trial-by-trial basis using fear-conditioned (CS+) stimuli. During conditioning, pupil and EEG markers related to heightened NE activity were identified. Afterwards, in a dichotic listening task, participants were cued to direct attention to either the left or right ear while highly similar syllable pairs were presented simultaneously to both ears.During the dichotic listening task, presentation of fear-conditioned stimuli reinstated the acquired fear response, as reflected in pupil and EEG alpha–beta-band responses. Critically, pupil dilation to CS+ was correlated with stronger EEG alpha–beta desynchronization, suggesting a common dependence on NE release. On a behavioral level, NE release facilitated attention. In particular, structural equation modeling revealed that the responsiveness of the NE system is associated with attention on a latent construct level, measured by several indicator tasks. Overall, our results suggest that the responsiveness of the NE system supports attention across the life span.Significance statementIn old age the ability to selectively process relevant aspects of the environment fades. Animal research suggests that the neuromodulator norepinephrine helps to maintain selective attention. We tested younger and older adults across a variety of attention tasks. In addition, we used arousing stimuli to experimentally activate participants’ noradrenergic system while recording pupillometry and electroencephalography (EEG) to infer its functional capacity. Older adults showed compromised attention and reduced noradrenergic responsiveness as indicated by interrelated pupil and EEG markers. Crucially, in both age groups a more responsive noradrenergic system was strongly associated to attention. Our findings link animal and human studies on the neural underpinning of attention in aging and underscore the importance of the noradrenergic system in late life cognition.

Individual listening success explained by synergistic interaction of two distinct neural filters

Successful speech comprehension requires the listener to differentiate relevant from irrelevant sounds. Recent neurophysiological studies have typically addressed one of two candidate neural filter solutions for this problem: the selective neural tracking of speech in auditory cortex via the modulation of phase-locked cortical responses, or the suppression of irrelevant inputs via alpha power modulations in parieto-occipital cortex. However, empirical evidence on their relationship and direct relevance to behavior is scarce. Here, a large, age-varying sample (N=76, 39–70 years) underwent a challenging dichotic listening task. Irrespective of listeners’ age, measures of behavioral performance, neural speech tracking, and alpha power lateralization all increased in response to spatial-attention cues. Under most challenging conditions, individual listening success was predicted best by the synergistic interaction of these two distinct neural filter strategies. Trial-by-trial fluctuations of lateralized alpha power and ignored-speech tracking did not co-vary, which demonstrates two neurobiologically distinct filter mechanisms.

Auditory Attention Causes Gain Enhancement and Frequency Sharpening at Successive Stages of Cortical Processing—Evidence from Human Electroencephalography

Previous findings have suggested that auditory attention causes not only enhancement in neural processing gain, but also sharpening in neural frequency tuning in human auditory cortex. The current study was aimed to reexamine these findings. Specifically, we aimed to investigate whether attentional gain enhancement and frequency sharpening emerge at the same or different processing levels and whether they represent independent or cooperative effects. For that, we examined the pattern of attentional modulation effects on early, sensory-driven cortical auditory-evoked potentials occurring at different latencies. Attention was manipulated using a dichotic listening task and was thus not selectively directed to specific frequency values. Possible attention-related changes in frequency tuning selectivity were measured with an adaptation paradigm. Our results show marked disparities in attention effects between the earlier N1 deflection and the subsequent P2 deflection, with the N1 showing a strong gain enhancement effect, but no sharpening, and the P2 showing clear evidence of sharpening, but no independent gain effect. They suggest that gain enhancement and frequency sharpening represent successive stages of a cooperative attentional modulation mechanism that increases the representational bandwidth of attended versus unattended sounds.

CEREBRAL LATERALITY, EMOTION, AND CARDIOPULMONARY FUNCTIONS: AN INVESTIGATION OF LEFT AND RIGHT CVA PATIENTS

Background: It has been evidenced that the outcome of a CVA patient differs as a function of the cerebral hemisphere that is damaged by the stroke, especially in terms of emotional changes. In contrast, the Bi-Hemispheric Model of Emotion posits that each hemisphere has its own emotional specialization. The current experiment tested the competing predictions of the two theoretical perspectives in a mixed sample of left cerebrovascular accident (LCVA) patients and right cerebrovascular accident (RCVA) patients using a Dichotic Listening task and the Affective Auditory Verbal Learning Test (AAVLT). Heart Rate (HR) and Pulse Oxygen Saturation (SpO2) were recorded as sympathetic measures. It was expected that the predictions of the Bi-Hemispheric Model would be supported. A series of mixed design ANOVAs were used to analyze the data. Material/Methods: Participants consisted of 21 patients grouped into either post-acute status left cerebrovascular accident (LCVA) or right cerebrovascular accident (RCVA). Tests included the The Dichotic Listening test, The Affective Auditory Verbal Learning Test (AAVLT), HR and Sp02 measurement using a Fingertip Pulse Oximeter and the Mood Assessment Scale for depression. Results: Results revealed that both groups exhibited decreased auditory detection abilities in the ear contralateral to CVA location. Additionally, CVA patients recalled significantly more positive words than negative or neutral words and exhibited a significant learning curve. LCVA patients exhibited a recency effect, while RCVA patients exhibited a heigh tened primacy effect. Findings from the HR and Sp02 measures suggested a parasympathetic response to emotionally neutral information as well as an impaired sympathetic response to emotionally negative information in RCVA patients. Conclusions: The results lend partial support to the hypothesis drawn from the Bi-Hemispheric Model of Emotion, as evidenced by the diametrically opposite effects in these groups, which reflects opposing cerebral processes.