Mathias S. Oechslin

White matter plasticity in the corticospinal tract of musicians: A diffusion tensor imaging study

With the advent of diffusion tensor imaging (DTI), the study of plastic changes in white matter architecture due to long-term practice has attracted increasing interest. Professional musicians provide an ideal model for investigating white matter plasticity because of their early onset of extensive auditory and sensorimotor training. We performed fiber tractography and subsequent voxelwise analysis, region of interest (ROI) analysis, and detailed slicewise analysis of diffusion parameters in the corticospinal tract (CST) on 26 professional musicians and a control group of 13 participants. All analyses resulted in significantly lower fractional anisotropy (FA) values in both the left and the right CST in the musician group. Furthermore, a right-greater-than-left asymmetry of FA was observed regardless of group. In the musician group, diffusivity was negatively correlated with the onset of musical training in childhood. A subsequent median split into an early and a late onset musician group (median=7 years) revealed increased diffusivity in the CST of the early onset group as compared to both the late onset group and the controls. In conclusion, these DTI-based findings might indicate plastic changes in white matter architecture of the CST in professional musicians. Our results imply that training-induced changes in diffusion characteristics of the axonal membrane may lead to increased radial diffusivity as reflected in decreased FA values.

The Plasticity of the Superior Longitudinal Fasciculus as a Function of Musical Expertise: A Diffusion Tensor Imaging Study

Previous neuroimaging studies have demonstrated that musical expertise leads to functional alterations in language processing. We utilized diffusion tensor imaging (DTI) to investigate white matter plasticity in musicians with absolute pitch (AP), relative pitch and non-musicians. Using DTI, we analysed the fractional anisotropy (FA) of the superior longitudinal fasciculus (SLF), which is considered the most primary pathway for processing and production of speech and music. In association with different levels of musical expertise, we found that AP is characterized by a greater left than right asymmetry of FA in core fibres of the SLF. A voxel-based analysis revealed three clusters within the left hemisphere SLF that showed significant positive correlations with error rates only for AP-musicians in an AP-test, but not for musicians without AP. We therefore conclude that the SLF architecture in AP musicians is related to AP acuity. In order to reconcile our observations with general aspects of development of fibre bundles, we introduce the Pioneer Axon Thesis, a theoretical approach to formalize axonal arrangements of major white matter pathways.

The multiple synaesthete E.S.: neuroanatomical basis of interval-taste and tone-colour synaesthesia.

Synaesthesia is the involuntary physical experience of a crossmodal linkage such as when hearing a tone evokes the additional sensation of seeing a colour. We previously described a professional musician with absolute pitch perception who experiences both different tastes in response to hearing different tone intervals (e.g., major third and sweet) and the more common tone-colour synaesthesia in which each particular tone is linked to a specific colour (e.g., C and red). One of the current theories of synaesthesia proposes that local crossactivation or disinhibition of feedback occurs because of increased connectivity between relevant brain areas. Based on diffusion tensor and T1-weighted magnetic resonance imaging we performed fractional anisotropy (FA) analysis, probabilistic fibre tractography, and voxel-based morphometry in the synaesthete E.S. compared with 17 professional musicians and 20 normal control subjects using voxel-wise z-score transformations. We report increased FA and volumetric white (WM) and grey matter (GM) peculiarities in E.S.s auditory and gustatory areas, hence explaining the interval-taste synaesthesia. Probabilistic fibre tractography revealed hyperconnectivity in bilateral perisylvian-insular regions in the synaesthete E.S. Differences in FA and volumetric WM and GM alterations in visual areas might represent the neuroarchitectural foundation of the tone-colour synaesthesia. Still unknown are the causes of the structural alterations, although an X-chromosomal linked dominant trait has been suggested. Whether hyperconnectivity occurs due to a failure in neural pruning or even synaptic sprouting remains to be shown. Our findings might have implications for the understanding of multimodal integration and may encourage similar research into dysfunctional perceptual phenomenon such as hallucinations in schizophrenics or in Charles Bonnet syndrome.

Absolute Pitch—Functional Evidence of Speech-Relevant Auditory Acuity

Absolute pitch (AP) has been shown to be associated with morphological changes and neurophysiological adaptations in the planum temporale, a cortical area involved in higher-order auditory and speech perception processes. The direct link between speech processing and AP has hitherto not been addressed. We provide first evidence that AP compared with relative pitch (RP) ability is associated with significantly different hemodynamic responses to complex speech sounds. By systematically varying the lexical and/or prosodic information of speech stimuli, we demonstrated consistent activation differences in AP musicians compared with RP musicians and nonmusicians. These differences relate to stronger activations in the posterior part of the middle temporal gyrus and weaker activations in the anterior mid-part of the superior temporal gyrus. Furthermore, this pattern is considerably modulated by the auditory acuity of AP. Our results suggest that the neural underpinnings of pitch processing expertise exercise a strong influence on propositional speech perception (sentence meaning).

Musical training intensity yields opposite effects on grey matter density in cognitive versus sensorimotor networks

Using optimized voxel-based morphometry, we performed grey matter density analyses on 59 age-, sex- and intelligence-matched young adults with three distinct, progressive levels of musical training intensity or expertise. Structural brain adaptations in musicians have been repeatedly demonstrated in areas involved in auditory perception and motor skills. However, musical activities are not confined to auditory perception and motor performance, but are entangled with higher-order cognitive processes. In consequence, neuronal systems involved in such higher-order processing may also be shaped by experience-driven plasticity. We modelled expertise as a three-level regressor to study possible linear relationships of expertise with grey matter density. The key finding of this study resides in a functional dissimilarity between areas exhibiting increase versus decrease of grey matter as a function of musical expertise. Grey matter density increased with expertise in areas known for their involvement in higher-order cognitive processing: right fusiform gyrus (visual pattern recognition), right mid orbital gyrus (tonal sensitivity), left inferior frontal gyrus (syntactic processing, executive function, working memory), left intraparietal sulcus (visuo-motor coordination) and bilateral posterior cerebellar Crus II (executive function, working memory) and in auditory processing: left Heschl’s gyrus. Conversely, grey matter density decreased with expertise in bilateral perirolandic and striatal areas that are related to sensorimotor function, possibly reflecting high automation of motor skills. Moreover, a multiple regression analysis evidenced that grey matter density in the right mid orbital area and the inferior frontal gyrus predicted accuracy in detecting fine-grained incongruities in tonal music.

Long-term exposure to music enhances the sensitivity of the auditory system in children

This event‐related brain potential study aims to contribute to the present debate regarding the effect of musical training on the maturation of the human auditory nervous system. To address this issue, we recorded the mismatch negativity (MMN) evoked by violin and pure sine‐wave tones in a group of 7.5‐ to 12‐year‐old children who had either several years of musical experience with Suzuki violin lessons, or no musical training. The strength of the MMN responses to violin tones evident in the Suzuki students clearly surpassed responses in controls; the reverse pattern was observed for sine‐wave tones. Suzuki students showed significantly shorter MMN latencies to violin tones than to pure tones; the MMN latency did not differ significantly between pure tones and violin sounds in the control group. Thus, our data provide general evidence of how and to what extent extensive musical experience affects the maturation of human auditory function at multiple levels, namely, accuracy and speed of auditory discrimination processing. Our findings add to the present understanding of neuroplastic organization and function of the mammalian nervous system. Furthermore, behavioural recordings obtained from the participating children provide corroborating evidence for a relationship between the duration and intensity of training, the specific sensitivity to instrumental timbre, and pitch recognition abilities.

Processing of voiced and unvoiced acoustic stimuli in musicians

Past research has shown that musical training induces changes in the processing of supra-segmental aspects of speech, such as pitch and prosody. The aim of the present study was to determine whether musical expertise also leads to an altered neurophysiological processing of sub-segmental information available in the speech signal, in particular the voice-onset-time. Using high-density EEG-recordings we analyzed the neurophysiological responses to voiced and unvoiced consonant-vowel-syllables and noise-analogs in 26 German speaking adult musicians and non-musicians. From the EEG the N1 amplitude of the event-related potential and two microstates from the topographical EEG analysis (one around the N1 amplitude and one immediately preceding the N1 microstate) were calculated to the different stimuli. Similar to earlier studies the N1 amplitude was different to voiced and unvoiced stimuli in non-musicians with larger amplitudes to voiced stimuli. The more refined microstate analysis revealed that the microstate within the N1 time window was shorter to unvoiced stimuli in non-musicians. For musicians there was no difference for the N1 amplitudes and the corresponding microstates between voiced and unvoiced stimuli. In addition, there was a longer very early microstate preceding the microstate at the N1 time window to non-speech stimuli only in musicians. Taken together, our findings suggest that musicians process unvoiced stimuli (irrespective whether these stimuli are speech or non-speech stimuli) differently than controls. We propose that musicians utilize the same network to analyze unvoiced stimuli as for the analysis of voiced stimuli. As a further explanation it is also possible that musicians devote more neurophysiological resources into the analysis of unvoiced segments.

Effects of prior information on decoding degraded speech: An fMRI study

Expectations and prior knowledge are thought to support the perceptual analysis of incoming sensory stimuli, as proposed by the predictive‐coding framework. The current fMRI study investigated the effect of prior information on brain activity during the decoding of degraded speech stimuli. When prior information enabled the comprehension of the degraded sentences, the left middle temporal gyrus and the left angular gyrus were activated, highlighting a role of these areas in meaning extraction. In contrast, the activation of the left inferior frontal gyrus (area 44/45) appeared to reflect the search for meaningful information in degraded speech material that could not be decoded because of mismatches with the prior information. Our results show that degraded sentences evoke instantaneously different percepts and activation patterns depending on the type of prior information, in line with prediction‐based accounts of perception. Hum Brain Mapp 35:61–74, 2014.

Hippocampal volume predicts fluid intelligence in musically trained people

Recently, age‐related hippocampal (HP) volume loss could be associated with a decrease in general fluid intelligence (gF). In the present study we investigated whether and how extensive musical training modulates human HP volume and gF performance. Previously, some studies demonstrated positive effects of musical training on higher cognitive functions such as learning and memory, associated with neural adaptations beyond the auditory domain. In order to detect possible associations between musical training and gF, we bilaterally segmented the HP formation and assessed the individual gF performance of people with different levels of musical expertise. Multiple regression analyses revealed that HP volume predicts gF in musicians but not in nonmusicians; in particular, bilaterally enhanced HP volume is associated with increased gF exclusively in musically trained people (amateurs and experts). This result suggests that musical training facilitates the recruitment of cognitive resources, which are essential for gF and linked to HP functioning. Musical training, even at a moderate level of intensity, can thus be considered as a potential strategy to decelerate age‐related effects of cognitive decline.

40Hz-Transcranial alternating current stimulation (tACS) selectively modulates speech perception.

The present study investigated the functional relevance of gamma oscillations for the processing of rapidly changing acoustic features in speech signals. For this purpose we analyzed repetition-induced perceptual learning effects in 18 healthy adult participants. The participants received either 6Hz or 40Hz tACS over the bilateral auditory cortex, while repeatedly performing a phoneme categorization task. In result, we found that 40Hz tACS led to a specific alteration in repetition-induced perceptual learning. While participants in the non-stimulated control group as well as those in the experimental group receiving 6Hz tACS considerably improved their perceptual performance, the application of 40Hz tACS selectively attenuated the repetition-induced improvement in phoneme categorization abilities. Our data provide causal evidence for a functional relevance of gamma oscillations during the perceptual learning of acoustic speech features. Moreover, we demonstrate that even less than twenty minutes of alternating current stimulation below the individual perceptual threshold is sufficient to affect speech perception. This finding is relevant in that this novel approach might have implications with respect to impaired speech processing in dyslexics and older adults.