Quantification of inter-brain coupling: A review of current methods used in haemodynamic and electrophysiological hyperscanning studies.

Hyperscanning is a form of neuroimaging experiment where the brains of two or more participants are imaged simultaneously whilst they interact. Within the domain of social neuroscience, hyperscanning is increasingly used to measure inter-brain coupling (IBC) and explore how brain responses change in tandem during social interaction. In addition to cognitive research, some have suggested that quantification of the interplay between interacting participants can be used as a biomarker for a variety of cognitive mechanisms aswell as to investigate mental health and developmental conditions including schizophrenia, social anxiety and autism. However, many different methods have been used to quantify brain coupling and this can lead to questions about comparability across studies and reduce research reproducibility. Here, we review methods for quantifying IBC, and suggest some ways moving forward. Following the PRISMA guidelines, we reviewed 215 hyperscanning studies, across four different brain imaging modalities: functional near-infrared spectroscopy (fNIRS), functional magnetic resonance (fMRI), electroencephalography (EEG) and magnetoencephalography (MEG). Overall, the review identified a total of 27 different methods used to compute IBC. The most common hyperscanning modality is fNIRS, used by 119 studies, 89 of which adopted wavelet coherence. Based on the results of this literature survey, we first report summary statistics of the hyperscanning field, followed by a brief overview of each signal that is obtained from each neuroimaging modality used in hyperscanning. We then discuss the rationale, assumptions and suitability of each method to different modalities which can be used to investigate IBC. Finally, we discuss issues surrounding the interpretation of each method.

Walking on a 'peg leg': extensor muscle activities and sensory feedback after distal leg denervation in cockroaches.

Previous studies in insects demonstrated that leg coordination changes following complete ablation of distal limb segments. However, normal coordination was restored when small 'peg leg' prostheses were attached to leg stumps to permit substrate contact. We have adapted this paradigm to preserve appropriate leg mass and inertia by severing all nerves and muscle tendons in the femur of the cockroach hind leg and converting the animal's own limb into a peg leg. Recordings of muscle activities and leg movements before and after denervation showed that: (1) the 'peg leg' is actively used in walking and regular bursts occur in motoneurons to leg extensor muscles; (2) driving of motoneuron activity is sufficient to produce 'fictive' bursting in a muscle whose tendon (apodeme) is cut in the ablation; and (3) similar motoneuron activities are found in walking on an oiled glass surface, when the effects of body weight and mechanical coupling are minimized. When distal segments were completely severed in these preparations, leg use and muscle bursting were disrupted but could be restored if the stumps were pressed against the substrate. These results support the hypothesis that feedback from receptors in proximal leg segments indicating forces allows for active leg use in walking.