The effect of repetition on intersubject synchrony assessed with fMRI.

We investigated how repeated exposure to a stimulus affects intersubject synchrony in the brains of young and older adults. We used functional magnetic resonance imaging (fMRI) to measure brain responses to familiar and novel stimuli. Young adults participated in a familiarization paradigm designed to mimic 'natural' exposure while older adults were presented with stimuli they had known for more than 50 years. Intersubject synchrony was calculated to detect common stimulus-driven brain activity across young and older adults as they listened to the novel and familiar stimuli. Contrary to our hypotheses, synchrony was not related to the amount of stimulus exposure; both young and older adults showed more synchrony to novel than to familiar stimuli regardless of whether the stimuli had been heard once, known for a few weeks, or known for more than 50 years. In young adults these synchrony differences were found across the brain in the bilateral temporal lobes, and in the frontal orbital cortex. In older adults the synchrony differences were found only in the bilateral temporal lobes. This reduction may be related to an increase in idiosyncratic responses after exposure to a stimulus but does not seem to be related to how well the stimuli are learned or to differences in attention. Until the effects of repeated exposure on synchrony are fully understood, future studies using intersubject synchrony, where the novelty of the stimuli cannot be guaranteed, may consider exposing all of their participants to the stimuli once before data are collected to mitigate the effects of any systematic differences in stimulus exposure.

Pitch discrimination is better for synthetic timbre than natural musical instrument timbres despite familiarity.

Pitch discrimination is better for complex tones than pure tones, but how pitch discrimination differs between natural and artificial sounds is not fully understood. This study compared pitch discrimination thresholds for flat-spectrum harmonic complex tones with those for natural sounds played by musical instruments of three different timbres (violin, trumpet, and flute). To investigate whether natural familiarity with sounds of particular timbres affects pitch discrimination thresholds, this study recruited non-musicians and musicians who were trained on one of the three instruments. We found that flautists and trumpeters could discriminate smaller differences in pitch for artificial flat-spectrum tones, despite their unfamiliar timbre, than for sounds played by musical instruments, which are regularly heard in everyday life (particularly by musicians who play those instruments). Furthermore, thresholds were no better for the instrument a musician was trained to play than for other instruments, suggesting that even extensive experience listening to and producing sounds of particular timbres does not reliably improve pitch discrimination thresholds for those timbres. The results show that timbre familiarity provides minimal improvements to auditory acuity, and physical acoustics (e.g., the presence of equal-amplitude harmonics) determine pitch discrimination thresholds more than does experience with natural sounds and timbre-specific training.

The Effect of Familiarity on Neural Representations of Music and Language.

We investigated how familiarity alters music and language processing in the brain. We used fMRI to measure brain responses before and after participants were familiarized with novel music and language stimuli. To manipulate the presence of language and music in the stimuli, there were four conditions: (1) whole music (music and words together), (2) instrumental music (no words), (3) a capella music (sung words, no instruments), and (4) spoken words. To manipulate participants' familiarity with the stimuli, we used novel stimuli and a familiarization paradigm designed to mimic "natural" exposure, while controlling for autobiographical memory confounds. Participants completed two fMRI scans that were separated by a stimulus training period. Behaviorally, participants learned the stimuli over the training period. However, there were no significant neural differences between the familiar and unfamiliar stimuli in either univariate or multivariate analyses. There were differences in neural activity in frontal and temporal regions based on the presence of language in the stimuli, and these differences replicated across the two scanning sessions. These results indicate that the way we engage with music is important for creating a memory of that music, and these aspects, over and above familiarity on its own, may be responsible for the robust nature of musical memory in the presence of neurodegenerative disorders such as Alzheimer disease.

Beat perception ability and instructions to synchronize influence gait when walking to music-based auditory cues.

Synchronizing gait to music-based auditory cues (rhythmic auditory stimulation) is a strategy used to manage gait impairments in a variety of neurological conditions, including Parkinson's disease. However, knowledge of how to individually optimize music-based cues is limited. The purpose of this study was to investigate how instructions to synchronize with auditory cues influences gait outcomes among healthy young adults with either good or poor beat perception ability. 65 healthy adults walked to metronome and musical stimuli with high and low levels of perceived groove (how much it induces desire to move) and familiarity at a tempo equivalent to their self-selected walking pace. Participants were randomized to instruction conditions: (i) synchronized: match footsteps with the beat, or (ii) free-walking: walk comfortably. Participants were classified as good or poor beat perceivers using the Beat Alignment Test. In this study, poor beat perceivers show better balance-related parameters (stride width and double-limb support time) when they are not instructed to synchronize their gait with cues (versus when synchronization was required). Good beat perceivers, in contrast, were better when instructed to synchronize gait (versus when no synchronization was required). Changes in stride length and velocity were influenced by musical properties, in particular the perceived 'groove' (greater stride length and velocity with high- versus low-groove cues) and, in some cases, this interacted with beat perception ability. The results indicate that beat perception ability and instructions to synchronize indeed influence spatiotemporal gait parameters when walking to music- and metronome-based rhythmic auditory stimuli. Importantly, these results suggest that both low groove cues and instructing poor beat perceivers to synchronize may interfere with performance while walking, thus potentially impacting both empirical and clinical outcomes.

Deficits in the Mimicry of Facial Expressions in Parkinson's Disease.

BACKGROUND: Humans spontaneously mimic the facial expressions of others, facilitating social interaction. This mimicking behavior may be impaired in individuals with Parkinson's disease, for whom the loss of facial movements is a clinical feature. OBJECTIVE: To assess the presence of facial mimicry in patients with Parkinson's disease. METHOD: Twenty-seven non-depressed patients with idiopathic Parkinson's disease and 28 age-matched controls had their facial muscles recorded with electromyography while they observed presentations of calm, happy, sad, angry, and fearful emotions. RESULTS: Patients exhibited reduced amplitude and delayed onset in the zygomaticus major muscle region (smiling response) following happy presentations (patients M = 0.02, 95% confidence interval [CI] -0.15 to 0.18, controls M = 0.26, CI 0.14 to 0.37, ANOVA, effect size [ES] = 0.18, p < 0.001). Although patients exhibited activation of the corrugator supercilii and medial frontalis (frowning response) following sad and fearful presentations, the frontalis response to sad presentations was attenuated relative to controls (patients M = 0.05, CI -0.08 to 0.18, controls M = 0.21, CI 0.09 to 0.34, ANOVA, ES = 0.07, p = 0.017). The amplitude of patients' zygomaticus activity in response to positive emotions was found to be negatively correlated with response times for ratings of emotional identification, suggesting a motor-behavioral link (r = -0.45, p = 0.02, two-tailed). CONCLUSIONS: Patients showed decreased mimicry overall, mimicking other peoples' frowns to some extent, but presenting with profoundly weakened and delayed smiles. These findings open a new avenue of inquiry into the "masked face" syndrome of PD.

The role of the extended MNS in emotional and nonemotional judgments of human song.

In the present study, we examined the involvement of the extended mirror neuron system (MNS)-specifically, areas that have a strong functional connection to the core system itself-during emotional and nonemotional judgments about human song. We presented participants with audiovisual recordings of sung melodic intervals (two-tone sequences) and manipulated emotion and pitch judgments while keeping the stimuli identical. Mu event-related desynchronization (ERD) was measured as an index of MNS activity, and a source localization procedure was performed on the data to isolate the brain sources contributing to this ERD. We found that emotional judgments of human song led to greater amounts of ERD than did pitch distance judgments (nonemotional), as well as control judgments related to the singer's hair, or pitch distance judgments about a synthetic tone sequence. Our findings support and expand recent research suggesting that the extended MNS is involved to a greater extent during emotional than during nonemotional perception of human action.

Audio-visual facilitation of the mu rhythm.

Previous studies demonstrate that perception of action presented audio-visually facilitates greater mirror neuron system (MNS) activity in humans (Kaplan and Iacoboni in Cogn Process 8(2):103-113, 2007) and non-human primates (Keysers et al. in Exp Brain Res 153(4):628-636, 2003) than perception of action presented unimodally. In the current study, we examined whether audio-visual facilitation of the MNS can be indexed using electroencephalography (EEG) measurement of the mu rhythm. The mu rhythm is an EEG oscillation with peaks at 10 and 20 Hz that is suppressed during the execution and perception of action and is speculated to reflect activity in the premotor and inferior parietal cortices as a result of MNS activation (Pineda in Behav Brain Funct 4(1):47, 2008). Participants observed experimental stimuli unimodally (visual-alone or audio-alone) or bimodally during randomized presentations of two hands ripping a sheet of paper, and a control video depicting a box moving up and down. Audio-visual perception of action stimuli led to greater event-related desynchrony (ERD) of the 8-13 Hz mu rhythm compared to unimodal perception of the same stimuli over the C3 electrode, as well as in a left central cluster when data were examined in source space. These results are consistent with Kaplan and Iacoboni's (in Cogn Process 8(2):103-113, 2007), findings that indicate audio-visual facilitation of the MNS; our left central cluster was localized approximately 13.89 mm away from the ventral premotor cluster identified in their fMRI study, suggesting that these clusters originate from similar sources. Consistency of results in electrode space and component space support the use of ICA as a valid source localization tool.