Mirror neurons in macaque area F5 fire when an animal performs an action, such as a mouth or limb movement, and also when the animal passively observes an identical or similar action performed by another individual. Brain-imaging studies in humans conducted over the last 20 years have repeatedly attempted to reveal analogous brain regions with mirror properties in humans, with broad and often speculative claims about their functional significance across a range of cognitive domains, from language to social cognition. Despite such concerted efforts, the likely neural substrates of these mirror regions have remained controversial, and indeed the very existence of a distinct subcategory of human neurons with mirroring properties has been questioned. Here we used activation likelihood estimation (ALE), to provide a quantitative index of the consistency of patterns of fMRI activity measured in human studies of action observation and action execution. From an initial sample of more than 300 published works, data from 125 papers met our strict inclusion and exclusion criteria. The analysis revealed 14 separate clusters in which activation has been consistently attributed to brain regions with mirror properties, encompassing 9 different Brodmann areas. These clusters were located in areas purported to show mirroring properties in the macaque, such as the inferior parietal lobule, inferior frontal gyrus and the adjacent ventral premotor cortex, but surprisingly also in regions such as the primary visual cortex, cerebellum and parts of the limbic system. Our findings suggest a core network of human brain regions that possess mirror properties associated with action observation and execution, with additional areas recruited during tasks that engage non-motor functions, such as auditory, somatosensory and affective components.

A key aspect of higher cognitive function is the ability to switch rapidly and efficiently between alternative modes of response where this is appropriate behaviourally. Such suppression appears to be highly dependent upon the integrity of the prefrontal cortex, yet other cortical areas are likely to be necessary to implement response switching. Language switching in bilingual speakers is a clear example of a task in which response switching is required. Functional brain imaging studies have demonstrated parietal cortex activation during repeated language switching within a translation task. Here we used event-related dense-sensor EEG recording techniques to examine the time course of language switching during a visually cued naming task in which bilingual participants named digits in either their first or second language. Switch-related modulation of ERP components was evident over parietal and frontal cortices, and in the latter case showed an asymmetry across first and second languages. Correspondence with a frontal ERP component found when suppressing manual responding in a Go/No-Go reaction time task may imply that similar inhibitory mechanisms are involved in both response suppression and language switching.

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Current evidence suggests that attention deficit hyperactivity disorder (ADHD) involves dysfunction in wide functional networks of brain areas associated with attention and cognition. This study examines the structural integrity of white-matter neural pathways, which underpin these functional networks, connecting fronto-striatal and fronto-parietal circuits, in children with ADHD. Fifteen right-handed 8 to 18-year-old males with ADHD-combined type and 15 right-handed, age, verbal, and performance IQ-matched, healthy males underwent diffusion tensor imaging. A recent method of tract-based spatial statistics was used to examine fractional anisotropy (FA) and mean diffusivity within major white-matter pathways throughout the whole-brain. White-matter abnormalities were found in several distinct clusters within left fronto-temporal regions and right parietal-occipital regions. Specifically, participants with ADHD showed greater FA in white-matter regions underlying inferior parietal, occipito-parietal, inferior frontal, and inferior temporal cortex. Secondly, eigenvalue analysis suggests that the difference in FA in ADHD may relate to a lesser degree of neural branching within key white-matter pathways. Tractography methods showed these regions to generally form part of white-matter pathways connecting prefrontal and parieto-occipital areas with the striatum and the cerebellum. Our findings demonstrate anomalous white-matter development in ADHD in distinct cortical regions that have previously been shown to be dysfunctional or hypoactive in fMRI studies of ADHD. These data add to an emerging picture of abnormal development within fronto-parietal cortical networks that may underpin the cognitive and attentional disturbances associated with ADHD.

Summary Mirror neurons, as originally described in the macaque, have two defining properties [1] and [2]: They respond specifically to a particular action (e.g., bringing an object to the mouth), and they produce their action-specific responses independent of whether the monkey executes the action or passively observes a conspecific performing the same action. In humans, action observation and action execution engage a network of frontal, parietal, and temporal areas. However, it is unclear whether these responses reflect the activity of a single population that represents both observed and executed actions in a common neural code or the activity of distinct but overlapping populations of exclusively perceptual and motor neurons [3]. Here, we used fMRI adaptation to show that the right inferior parietal lobe (IPL) responds independently to specific actions regardless of whether they are observed or executed. Specifically, responses in the right IPL were attenuated when participants observed a recently executed action relative to one that had not previously been performed. This adaptation across action and perception demonstrates that the right IPL responds selectively to the motoric and perceptual representations of actions and is the first evidence for a neural response in humans that shows both defining properties of mirror neurons.

Exact timing is essential for functional MRI data analysis. Datasets are commonly measured using repeated 2D imaging methods, resulting in a temporal offset between slices. To compensate for this timing difference, slice-timing correction (i.e. temporal data interpolation) has been used as an fMRI pre-processing step for more than fifteen years. However, there has been an ongoing debate about the effectiveness and applicability of this method. This paper presents the first elaborated analysis of the impact of the slice-timing effect on simulated data for different fMRI paradigms and measurement parameters, taking into account data noise and smoothing effects. Here we show, depending on repetition time and paradigm design, slice-timing effects can significantly impair fMRI results and slice-timing correction methods can successfully compensate for these effects and therefore increase the robustness of the data analysis. In addition, our results from simulated data were supported by empirical in vivo datasets. Our findings suggest that slice-timing correction should be included in the fMRI pre-processing pipeline.

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