Brain regions with mirror properties: a meta-analysis of 125 human fMRI studies

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.

Phasic alerting of neglect patients overcomes their spatial deficit in visual awareness

Patients with extensive damage to the right hemisphere of their brain often exhibit unilateral neglect of the left side of space. The spatial attention of these patients is strongly biased towards the right, so their awareness of visual events on the left is impaired. Extensive right-hemisphere lesions also impair tonic alertness (the ability to maintain arousal). This nonspatial deficit in alertness is often considered to be a different problem from spatial neglect, but the two impairments may be linked. If so, then phasically increasing the patients' alertness should temporarily ameliorate their spatial bias in awareness. Here we provide evidence to support this theory. Right-hemisphere-neglect patients judged whether a visual event on the left preceded or followed a comparable event on the right. They became aware of left events half a second later than right events on average. This spatial imbalance in the time course of visual awareness was corrected when a warning sound alerted the patients phasically. Even a warning sound on the right accelerated the perception of left visual events in this way. Nonspatial phasic alerting can thus overcome disabling spatial biases in perceptual awareness after brain injury.

Understanding the minds of others: A neuroimaging meta-analysis

Theory of mind (ToM) is an important skill that refers broadly to the capacity to understand the mental states of others. A large number of neuroimaging studies have focused on identifying the functional brain regions involved in ToM, but many important questions remain with respect to the neural networks implicated in specific types of ToM tasks. In the present study, we conducted a series of activation likelihood estimation (ALE) meta-analyses on 144 datasets (involving 3150 participants) to address these questions. The ALE results revealed common regions shared across all ToM tasks and broader task parameters, but also some important dissociations. In terms of commonalities, consistent activation was identified in the medial prefrontal cortex and bilateral temporoparietal junction. On the other hand, ALE contrast analyses on our dataset, as well as meta-analytic connectivity modelling (MACM) analyses on the BrainMap database, indicated that different types of ToM tasks reliably elicit activity in unique brain areas. Our findings provide the most accurate picture to date of the neural networks that underpin ToM function.

Consensus paper: the role of the cerebellum in perceptual processes

Various lines of evidence accumulated over the past 30 years indicate that the cerebellum, long recognized as essential for motor control, also has considerable influence on perceptual processes. In this paper, we bring together experts from psychology and neuroscience, with the aim of providing a succinct but comprehensive overview of key findings related to the involvement of the cerebellum in sensory perception. The contributions cover such topics as anatomical and functional connectivity, evolutionary and comparative perspectives, visual and auditory processing, biological motion perception, nociception, self-motion, timing, predictive processing, and perceptual sequencing. While no single explanation has yet emerged concerning the role of the cerebellum in perceptual processes, this consensus paper summarizes the impressive empirical evidence on this problem and highlights diversities as well as commonalities between existing hypotheses. In addition to work with healthy individuals and patients with cerebellar disorders, it is also apparent that several neurological conditions in which perceptual disturbances occur, including autism and schizophrenia, are associated with cerebellar pathology. A better understanding of the involvement of the cerebellum in perceptual processes will thus likely be important for identifying and treating perceptual deficits that may at present go unnoticed and untreated. This paper provides a useful framework for further debate and empirical investigations into the influence of the cerebellum on sensory perception.

Parietal neglect and visual awareness

The last decade has seen a resurgence of interest in the neural correlates of conscious vision, with most discussion focused on the 'blindsight' that can follow damage to primary visual cortex, in the occipital lobe. We suggest that new insights into the neural basis of visual awareness may be gleaned from a different neuropsychological phenomenon, namely visual 'neglect' after injury to regions in the parietal lobe. Neglect provides several revealing contrasts with occipital blindsight. Here we summarise four key findings. First, unlike the deficits caused by damage to primary visual cortex, the loss of awareness in parietal neglect is characteristically not strictly retinotopic. Second, visual segmentation processes are preserved in neglect, and can influence what will reach the patient's awareness. Third, extensive unconscious processing takes place for those stimuli on the neglected side which escape awareness, including some degree of object identification. Finally, parietal damage affects initial stages of motor planning as well as perception. These findings are consistent with recent data on single-cell activity in the monkey brain. They also suggest why areas in the inferior parietal lobe may play a prominent role in visual awareness.

Dynamic cooperation and competition between brain systems during cognitive control

The human brain is characterized by a remarkable ability to adapt its information processing based on current goals. This ability, which is encompassed by the psychological construct of cognitive control, involves activity throughout large-scale, specialized brain systems that support segregated functions at rest and during active task performance. Based on recent research, we propose an account in which control functions rely on transitory changes in patterns of cooperation and competition between neural systems. This account challenges current conceptualizations of control as relying on segregated or antagonistic activity of specialized brain systems. Accordingly, we argue that the study of transitory task-based interactions between brain systems is critical to understanding the flexibility of normal cognitive control and its disruption in pathological conditions.

Amygdala responses to fearful and happy facial expressions under conditions of binocular suppression

The human amygdala plays a crucial role in processing affective information conveyed by sensory stimuli. Facial expressions of fear and anger, which both signal potential threat to an observer, result in significant increases in amygdala activity, even when the faces are unattended or presented briefly and masked. It has been suggested that afferent signals from the retina travel to the amygdala via separate cortical and subcortical pathways, with the subcortical pathway underlying unconscious processing. Here we exploited the phenomenon of binocular rivalry to induce complete suppression of affective face stimuli presented to one eye. Twelve participants viewed brief, rivalrous visual displays in which a fearful, happy, or neutral face was presented to one eye while a house was presented simultaneously to the other. We used functional magnetic resonance imaging to study activation in the amygdala and extrastriate visual areas for consciously perceived versus suppressed face and house stimuli. Activation within the fusiform and parahippocampal gyri increased significantly for perceived versus suppressed faces and houses, respectively. Amygdala activation increased bilaterally in response to fearful versus neutral faces, regardless of whether the face was perceived consciously or suppressed because of binocular rivalry. Amygdala activity also increased significantly for happy versus neutral faces, but only when the face was suppressed. This activation pattern suggests that the amygdala has a limited capacity to differentiate between specific facial expressions when it must rely on information received via a subcortical route. We suggest that this limited capacity reflects a tradeoff between specificity and speed of processing.

Simple metric for scaling motor threshold based on scalp-cortex distance: application to studies using transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a unique method in neuroscience used to stimulate focal regions of the human brain. As TMS gains popularity in experimental and clinical domains, techniques for controlling the extent of brain stimulation are becoming increasingly important. At present, TMS intensity is typically calibrated to the excitability of the human motor cortex, a measure referred to as motor threshold (MT). Although TMS is commonly applied to nonmotor regions, most applications do not consider the effect of changes in distance between the stimulating device and underlying neural tissue. Here we show that for every millimeter from the stimulating coil, an additional 3% of TMS output is required to induce an equivalent level of brain stimulation at the motor cortex. This abrupt spatial gradient will have crucial consequences when TMS is applied to nonmotor regions because of substantial variance in scalp-cortex distances over different regions of the head. Stimulation protocols that do not account for cortical distance therefore risk substantial under- or overstimulation. We describe a simple method for adjusting MT to account for variations in cortical distance, thus providing a more accurate calibration than unadjusted MT for the safe and effective application of TMS in clinical and experimental neuroscience.

fMRI adaptation reveals mirror neurons in human inferior parietal cortex

Mirror neurons, as originally described in the macaque, have two defining properties [1, 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.

Unconscious priming eliminates automatic binding of colour and alphanumeric form in synaesthesia

Synaesthesia is an unusual perceptual phenomenon in which events in one sensory modality induce vivid sensations in another. Individuals may 'taste' shapes, 'hear' colours, or 'feel' sounds. Synaesthesia was first described over a century ago, but little is known about its underlying causes or its effects on cognition. Most reports have been anecdotal or have focused on isolated unusual cases. Here we report an investigation of 15 individuals with colour-graphemic synaesthesia, each of whom experiences idiosyncratic but highly consistent colours for letters and digits. Using a colour-form interference paradigm, we show that induced synaesthetic experiences cannot be consciously suppressed even when detrimental to task performance. In contrast, if letters and digits are presented briefly and masked, so that they are processed but unavailable for overt report, the synaesthesia is eliminated. These results show that synaesthetic experiences can be prevented despite substantial processing of the sensory stimuli that otherwise trigger them. We conclude that automatic binding of colour and alphanumeric form in synaesthesia arises after initial processes of letter and digit recognition are complete.

A systematic, large-scale study of synaesthesia: implications for the role of early experience in lexical-colour associations

For individuals with synaesthesia, stimuli in one sensory modality elicit anomalous experiences in another modality. For example, the sound of a particular piano note may be 'seen' as a unique colour, or the taste of a familiar food may be 'felt' as a distinct bodily sensation. We report a study of 192 adult synaesthetes, in which we administered a structured questionnaire to determine the relative frequency and characteristics of different types of synaesthetic experience. Our data suggest the prevalence of synaesthesia in the adult population is approximately 1 in 1150 females and 1 in 7150 males. The incidence of left-handedness in our sample was within the normal range, contrary to previous claims. We did, however, find that synaesthetes are more likely to be involved in artistic pursuits, consistent with anecdotal reports. We also examined responses from a subset of 150 synaesthetes for whom letters, digits and words induce colour experiences ('lexical-colour' synaesthesia). There was a striking consistency in the colours induced by certain letters and digits in these individuals. For example, 'R' elicited red for 36% of the sample, 'Y' elicited yellow for 45%, and 'D' elicited brown for 47%. Similar trends were apparent for a group of non-synaesthetic controls who were asked to associate colours with letters and digits. Based on these findings, we suggest that the development of lexical-colour synaesthesia in many cases incorporates early learning experiences common to all individuals. Moreover, many of our synaesthetes experienced colours only for days of the week, letters or digits, suggesting that inducers that are part of a conventional sequence (e.g. Monday, Tuesday, Wednesday...; A, B, C...; 1, 2, 3...) may be particularly important in the development of synaesthetic inducer-colour pairs. We speculate that the learning of such sequences during an early critical period determines the particular pattern of lexical-colour links, and that this pattern then generalises to other words.

Motor role of human inferior parietal lobe revealed in unilateral neglect patients

The exact role of the parietal lobe in spatial cognition is controversial. One influential hypothesis proposes that it subserves spatial perception, whereas other accounts suggest that its primary role is to direct spatial movement. For humans, it has been suggested that these functions may be divided between inferior and superior parietal lobes, respectively. In apparent support of a purely perceptual function for the inferior parietal lobe (IPL), patients with lesions to this structure, particularly in the right hemisphere, exhibit unilateral spatial neglect (deficient awareness for the side of space opposite to that of their lesion). Here we show that patients with right IPL lesions also have a specific difficulty in initiating leftward movements towards visual targets on the left side of space. This motor impairment was not found in neglect patients with frontal lesions, contrary to previous proposals that motor aspects of neglect are particularly associated with anterior damage. Our results suggest that the human IPL operates as a sensorimotor interface, rather than subserving only perceptual functions.

Free-viewing perceptual asymmetries for the judgement of brightness, numerosity and size

Perceptual asymmetries under free-viewing conditions were investigated in 24 normal dextral adults. Three tasks were administered that required participants to chose between a pair of left/right reversed stimuli on the basis of their brightness, numerosity or size. These stimulus features were represented asymmetrically within the stimuli, so that each stimulus appeared darker, larger or more numerous on the left or right sides. Participants more often selected the stimulus with the relevant feature on the left-hand side for all three tasks. Response times for leftward responses were faster than rightward responses. Split-half reliabilities revealed a high level of consistency within the tasks. However, the correlation between tasks was low. These results suggest that the different tasks, while showing similar levels of perceptual asymmetry, engage distinct sets of lateralised processes.

Dissociable mechanisms of cognitive control in prefrontal and premotor cortex

Intelligent behavior depends on the ability to suppress inappropriate actions and resolve interference between competing responses. Recent clinical and neuroimaging evidence has demonstrated the involvement of prefrontal, parietal, and premotor areas during behaviors that emphasize conflict and inhibition. It remains unclear, however, whether discrete subregions within this network are crucial for overseeing more specific inhibitory demands. Here we probed the functional specialization of human prefrontal cortex by combining repetitive transcranial magnetic stimulation (rTMS) with integrated behavioral measures of response inhibition (stop-signal task) and response competition (flanker task). Participants undertook a combined stop-signal/flanker task after rTMS of the inferior frontal gyrus (IFG) or dorsal premotor cortex (dPM) in each hemisphere. Stimulation of the right IFG impaired stop-signal inhibition under conditions of heightened response competition but did not influence the ability to suppress a competing response. In contrast, stimulation of the right dPM facilitated execution but had no effect on inhibition. Neither of these results was observed during rTMS of corresponding left-hemisphere regions. Overall, our findings are consistent with existing evidence that the right IFG is crucial for inhibitory control. The observed double dissociation of neurodisruptive effects between the right IFG and right dPM further implies that response inhibition and execution rely on distinct neural processes despite activating a common cortical network.

Fast and slow parietal pathways mediate spatial attention

Mechanisms of selective attention are vital for guiding human behavior. The parietal cortex has long been recognized as a neural substrate of spatial attention, but the unique role of distinct parietal subregions has remained unclear. Using single-pulse transcranial magnetic stimulation, we found that the angular gyrus of the right parietal cortex mediates spatial orienting during two distinct time periods after the onset of a behaviorally relevant event. The biphasic involvement of the angular gyrus suggests that both fast and slow visual pathways are necessary for orienting spatial attention.

Is the mirror neuron system involved in imitation? A short review and meta-analysis

It has been suggested that the mirror neuron system provides an important neural substrate for humans' ability to imitate. Mirror neurons have been found during single-cell recordings in monkeys in area F5 and PF. It is believed that the human equivalent of this mirror system in humans is the pars opercularis of the inferior frontal gyrus (area 44) and the rostral part of the inferior parietal lobule. This article critically reviews published fMRI studies that examined the role of frontal and parietal brain regions in imitation. A meta-analysis using activation likelihood estimation (ALE) revealed that the superior parietal lobule, inferior parietal lobule, and the dorsal premotor cortex but not the inferior frontal gyrus, are all commonly involved in imitation. An additional meta-analysis using a label-based review confirmed that in the frontal lobe, the premotor cortex rather than the inferior frontal gyrus is consistently active in studies investigating imitation. In the parietal region the superior and inferior parietal lobules are equally activated during imitation. Our results suggest that parietal and frontal regions which extend beyond the classical mirror neuron network are crucial for imitation.

Executive "brake failure" following deactivation of human frontal lobe

In the course of daily living, humans frequently encounter situations in which a motor activity, once initiated, becomes unnecessary or inappropriate. Under such circumstances, the ability to inhibit motor responses can be of vital importance. Although the nature of response inhibition has been studied in psychology for several decades, its neural basis remains unclear. Using transcranial magnetic stimulation, we found that temporary deactivation of the pars opercularis in the right inferior frontal gyrus selectively impairs the ability to stop an initiated action. Critically, deactivation of the same region did not affect the ability to execute responses, nor did it influence physiological arousal. These findings confirm and extend recent reports that the inferior frontal gyrus is vital for mediating response inhibition.

An evaluation of the role of internal cues in the pathogenesis of parkinsonian hypokinesia

Our animal studies suggest that the basal ganglia provide an internal non-specific cue to trigger movement and imply that Parkinson's disease involves a deficiency in this cueing mechanism. Indeed parkinsonian patients typically rely upon external visual cues. To assess the effects of such non-specific cueing mechanisms on movement, we examined patients' utilization of a variety of auditory cues. Ten patients suffering from Parkinson's disease, and their matched controls, pressed buttons at a series of two-way choice points sequentially down a pathway, both when the latter remained illuminated throughout its length, and when it had to be followed from memory alone. In other experimental conditions, auditory cues were also provided, either contingent upon the previous response, at its initiation (a medium level of advance information) or at its completion (a low level of advance information), or as a series of regularly paced (non-contingent) auditory cues (from a metronome). In addition to error data, we recorded down time (DT, time to initiate each next response) and movement time (MT, time to execute each next response). However, both DT and MT measurements showed that parkinsonian patients were enormously disadvantaged by the absence of external cues. While contingent auditory cues were of some help, the performance of patients with Parkinson's disease was dramatically improved by the provision of non-contingent auditory information. Moreover, parkinsonian patients, unlike controls, were greatly affected by the length of individual sub-movements, especially in the absence of external cues. When the pathway to be followed remained illuminated, sub-movement length had little effect. We conclude that for well-learnt, predictable sequences the basal ganglia provide a non-specific internal cue that is necessary for switching between one movement and the next in a movement sequence, and also for development of preparatory activity for each sub-movement in the sequence.

Distance-adjusted motor threshold for transcranial magnetic stimulation

OBJECTIVE

To examine the relationship between coil-cortex distance and effective cortical stimulation using transcranial magnetic stimulation (TMS) in the left and right motor cortex. We also compare the effect of coil-cortex distance using 50 and 70 mm figure-eight stimulating coils.

METHODS

Coil-cortex distance was manipulated within each participant using 5 and 10 mm acrylic separators placed between the coil and scalp surface. The effect of cortical stimulation was indexed by resting motor threshold (MT).

RESULTS

Increasing distance between the coil and underlying cortex was associated with a steep linear increase in MT. For each additional millimetre separating the stimulating coil from the scalp surface, an additional approximately 2.8% of absolute stimulator output (approximately 0.062 T) was required to reach MT. The gradient of the observed distance effect did not differ between hemispheres, and no differences were observed between the 50 and 70 mm TMS coils.

CONCLUSIONS

Coil-cortex distance directly influences the magnitude of cortical stimulation in TMS. The relationship between TMS efficacy and coil-cortex distance is well characterised by a linear function, providing a simple and effective method for scaling stimulator output to a distance adjusted MT.

SIGNIFICANCE

MT measured at the scalp-surface is dependent on the underlying scalp-cortex distance, and therefore does not provide an accurate index of cortical excitability. Distance-adjusted MT provides a more accurate index of cortical excitability, and improves the safety and efficacy of MT-calibrated TMS.

Preattentive filling-in of visual surfaces in parietal extinction

Unilateral brain damage frequently produces "extinction," in which patients can detect brief single visual stimuli on either side but are unaware of a contralesional stimulus if presented concurrently with an ipsilesional stimulus. Explanations for extinction have invoked deficits in initial processes that operate before the focusing of visual attention or in later attentive stages of vision. Preattentive vision was preserved in a parietally damaged patient, whose extinction was less severe when bilateral stimuli formed a common surface, even if this required visual filling-in to yield illusory Kanizsa figures or completion of partially occluded figures. These results show that parietal extinction arises only after substantial processing has generated visual surfaces, supporting recent claims that visual attention is surface-based.

Visual extinction and prior entry: impaired perception of temporal order with intact motion perception after unilateral parietal damage

Two patients with left-sided visual extinction after right parietal damage were each given two 'prior entry' tasks that have recently been used to study attentional biases in normals. The first task presented two unconnected bars, one in each visual field, with the patients asked to judge which appeared sooner. Both patients reported that the right bar preceded the left unless the latter led by over 200 msec, suggesting a severe bias to the right affecting the time-course of visual awareness. The second task presented one continuous line in a scrolling format across the same spatial extent, with the patients asked to judge which direction the line moved in. The patients now performed normally. Thus, the perception of temporal order for separate events was impaired by the lesions, but without disrupting motion perception within single events. The implications are discussed for theories of normal and pathological attention, visual awareness, and motion perception.

Reduction in external cues and movement sequencing in Parkinson's disease

To identify the focus of impairment in the performance of sequential movements of patients with Parkinson's disease, the extent of their reliance on external cues was examined. Eighteen patients with idiopathic Parkinson's disease and their matched controls performed a series of button presses at sequential choice points along a response board. The illuminated pathway to be followed successively extinguished ahead of each move according to three levels of reduction of external cues. Patients with Parkinson's disease were particularly disadvantaged with high levels of reduction of external cueing in terms both of movement preparation time (button down time) and movement execution time (movement time between buttons). Moreover, with high levels of reduction of external cueing, patients with Parkinson's disease were particularly subject to progressive slowing (movement time, not down time) further down the sequence. The basal ganglia may help generate internal cues for releasing successive stages of a predefined movement sequence.

Functional brain networks related to individual differences in human intelligence at rest

Intelligence is a fundamental ability that sets humans apart from other animal species. Despite its importance in defining human behaviour, the neural networks responsible for intelligence are not well understood. The dominant view from neuroimaging work suggests that intelligent performance on a range of tasks is underpinned by segregated interactions in a fronto-parietal network of brain regions. Here we asked whether fronto-parietal interactions associated with intelligence are ubiquitous, or emerge from more widespread associations in a task-free context. First we undertook an exploratory mapping of the existing literature on functional connectivity associated with intelligence. Next, to empirically test hypotheses derived from the exploratory mapping, we performed network analyses in a cohort of 317 unrelated participants from the Human Connectome Project. Our results revealed a novel contribution of across-network interactions between default-mode and fronto-parietal networks to individual differences in intelligence at rest. Specifically, we found that greater connectivity in the resting state was associated with higher intelligence scores. Our findings highlight the need to broaden the dominant fronto-parietal conceptualisation of intelligence to encompass more complex and context-specific network dynamics.