Does fMRI repetition suppression reveal mirror neuron activity in the human brain? Insights from univariate and multivariate analysis

No Evidence for Cross-Modal fMRI Adaptation in Macaque Parieto-Premotor Mirror Neuron Regions

To probe the presence of mirror neurons in the human brain, cross-modal fMRI adaptation has been suggested as a suitable technique. The rationale behind this suggestion is that this technique allows making more accurate inferences about neural response properties underlying fMRI voxel activations, beyond merely showing shared voxels that are active during both action observation and execution. However, the validity of using cross-modal fMRI adaptation to demonstrate the presence of mirror neurons in parietal and premotor brain regions has been questioned given the inconsistent and weak results obtained in human studies. A better understanding of cross-modal fMRI adaptation effects in the macaque brain is required as the rationale for using this approach is based on several assumptions related to macaque mirror neuron response properties that still need validation. Here, we conducted a cross-modal fMRI adaptation study in macaque monkeys, using the same action execution and action observation tasks that successfully yielded mirror neuron region cross-modal action decoding in a previous monkey MVPA study. We scanned two male rhesus monkeys while they first executed a sequence of either reach-and-grasp or reach-and-touch hand actions and then observed a video of a human actor performing these motor acts. Both whole-brain and region-of-interest analyses failed to demonstrate cross-modal fMRI adaptation effects in parietal and premotor mirror neuron regions. Our results, in line with previous findings in non-human primates, show that cross-modal motor-to-visual fMRI adaptation is not easily detected in monkey brain regions known to house mirror neurons. Thus, our results advocate caution in using cross-modal fMRI adaptation as a method to infer whether mirror neurons can be found in the primate brain.

Distributed and hierarchical neural encoding of multidimensional biological motion attributes in the human brain

The human visual system can efficiently extract distinct physical, biological, and social attributes (e.g. facing direction, gender, and emotional state) from biological motion (BM), but how these attributes are encoded in the brain remains largely unknown. In the current study, we used functional magnetic resonance imaging to investigate this issue when participants viewed multidimensional BM stimuli. Using multiple regression representational similarity analysis, we identified distributed brain areas, respectively, related to the processing of facing direction, gender, and emotional state conveyed by BM. These brain areas are governed by a hierarchical structure in which the respective neural encoding of facing direction, gender, and emotional state is modulated by each other in descending order. We further revealed that a portion of the brain areas identified in representational similarity analysis was specific to the neural encoding of each attribute and correlated with the corresponding behavioral results. These findings unravel the brain networks for encoding BM attributes in consideration of their interactions, and highlight that the processing of multidimensional BM attributes is recurrently interactive.

Is there a relationship between EEG and sTMS neurophysiological markers of the putative human mirror neuron system?

The mirror neuron system (MNS) has been theorized to play a neurobiological role in a number of social cognitive abilities and is commonly indexed putatively in humans via interpersonal motor resonance (IMR) and mu suppression. Although both indices are thought to measure similar neuronal populations (i.e., "mirror neurons"), it has been suggested that these methods are unrelated, and therefore, incompatible. However, prior studies reporting no relationships were typically conducted in small and underpowered samples. Thus, we aimed to investigate this potential association in a large sample of neurotypical adults (N = 116; 72 females). Participants underwent transcranial magnetic stimulation (TMS), electromyography (EMG), and electroencephalography (EEG) during the observation of videos of actors performing grasping actions in order to index IMR and mu suppression (in beta, lower alpha, and upper alpha bandwidths). A series of linear regressions revealed no associations between IMR and each of the mu suppression bandwidths. Supplementary Bayesian analyses provided further evidence in favor of the null (B₀₁  = 8.85-8.93), providing further support for no association between the two indices of MNS activity. Our findings suggest that these two measures may indeed be unrelated indices that perhaps assess different neurophysiological aspects of the MNS. These results have important implications for future studies examining the MNS.

Examining cross-modal fMRI adaptation for observed and executed actions in the monkey brain

While mirror neurons have been found in several monkey brain regions, their existence in the human brain is still largely inferred from indirect non-invasive measurements like functional MRI. It has been proposed that, beyond showing overlapping brain responses during action observation and execution tasks, candidate mirror neuron regions should demonstrate cross-modal action specificity, in line with a defining physiological characteristic of these neurons in the monkey brain. Although cross-modal fMRI adaptation has been put forward as a suited method to test this key feature of cross-modal action specificity in the human brain, so far, the overall usefulness of this technique to demonstrate mirror neuron activity remains unclear. To date, it has never been tested to what extent monkey brain regions known to house mirror neurons, would yield uni- and/or cross-modal fMRI adaptation effects. We therefore performed an fMRI adaptation experiment while male rhesus macaques either performed or observed two different goal-directed hand actions. Executing grasp/lift or touch/press actions in the dark, as well as observing videos of these monkey hand actions, yielded robust responses throughout the brain, including overlapping fMRI responses in parietal and premotor mirror neuron regions. Uni-modal adaptation effects were mostly restricted to the visual modality and the early visual cortices. Both frequentist and Bayesian statistical analyses however suggested no evidence for cross-modal fMRI adaptation effects in monkey parietal and premotor mirror neuron regions. Overall, these findings suggest monkey mirror neuron activity does not readily translate into cross-modal repetition suppression effects that can be detected by fMRI.

The human mirror neuron system-A common neural basis for social cognition?

According to the theory of embodied simulation, mirror neurons (MN) in our brain's motor system are the neuronal basis of all social-cognitive processes. The assumption of such a mirroring process in humans could be supported by results showing that within one person the same region is involved in different social cognition tasks. We conducted an fMRI-study with 75 healthy participants who completed three tasks: imitation, empathy, and theory of mind. We analyzed the data using group conjunction analyses and individual shared voxel counts. Across tasks, across and within participants, we find common activation in inferior frontal gyrus, inferior parietal cortex, fusiform gyrus, posterior superior temporal sulcus, and amygdala. Our results provide evidence for a shared neural basis for different social-cognitive processes, indicating that interpersonal understanding might occur by embodied simulation.

Is the Putative Mirror Neuron System Associated with Empathy? A Systematic Review and Meta-Analysis

Theoretical perspectives suggest that the mirror neuron system (MNS) is an important neurobiological contributor to empathy, yet empirical support is mixed. Here, we adopt a summary model for empathy, consisting of motor, emotional, and cognitive components of empathy. This review provides an overview of existing empirical studies investigating the relationship between putative MNS activity and empathy in healthy populations. 52 studies were identified that investigated the association between the MNS and at least one domain of empathy, representing data from 1044 participants. Our results suggest that emotional and cognitive empathy are moderately correlated with MNS activity, however, these domains were mixed and varied across techniques used to acquire MNS activity (TMS, EEG, and fMRI). Few studies investigated motor empathy, and of those, no significant relationships were revealed. Overall, results provide preliminary evidence for a relationship between MNS activity and empathy. However, our findings highlight methodological variability in study design as an important factor in understanding this relationship. We discuss limitations regarding these methodological variations and important implications for clinical and community translations, as well as suggestions for future research.

Early motor resonance differentiates schizophrenia patients from healthy subjects and predicts social cognition performance

BACKGROUND

Diminished motor resonance (facilitation of motor potentials during action observation) is possibly related to social cognition deficits in schizophrenia. Adequate social cognition requires the successful moment-to-moment appraisal of social stimuli over a temporal window. However, similar changes in motor resonance with successive action observation stimuli are unknown. We compared the time-course of motor resonance evoked during successive action observation stimuli between schizophrenia patients (antipsychotic-naïve and medicated) and healthy subjects and examined its association with social cognition performance.

METHOD

Fifty-four schizophrenia patients (33 antipsychotic-naive) and 45 healthy subjects underwent 10-recordings (T1 to T10) of cortical reactivity, using two single (sp)- and two paired-pulse (pp) transcranial magnetic stimulation (TMS) paradigms, while they observed goal-directed actions and a static image. They also underwent comprehensive social cognition assessments.

RESULTS

Sp-motor resonance revealed a significant quadratic time effect (initial fall and then rise) in patients and healthy subjects [F=12.21, P=0.001]. Such a pattern was not observed for pp-motor resonance. We categorized motor resonance as early (T1-T3), middle (T4-T7) and late (T8-T10) based on pair-wise comparisons. Early, but not middle or late sp-motor resonance was reduced in antipsychotic naïve patients compared to the medicated patients and healthy subjects (F=3.41, P=0.037). Social cognition composite score had significant correlations with both early sp-motor resonance (r=0.34, P=0.01) and early pp-motor resonance (r=0.314, P=0.02) in the combined patient group.

CONCLUSIONS

Motor resonance time-courses did not vary across groups. The magnitude of early motor resonance was reduced in the antipsychotic-naïve schizophrenia group, compared to healthy subjects. Early phase motor resonance was associated with social cognition deficits in patients.