Monkey gaze behaviour during action observation and its relationship to mirror neuron activity

Non-shared coding of observed and executed actions prevails in macaque ventral premotor mirror neurons

According to the mirror mechanism the discharge of F5 mirror neurons of a monkey observing another individual performing an action is a motor representation of the observed action that may serve to understand or learn from the action. This hypothesis, if strictly interpreted, requires mirror neurons to exhibit an action tuning that is shared between action observation and execution. Due to insufficient data it remains contentious if this requirement is met. To fill in the gaps, we conducted an experiment in which identical objects had to be manipulated in three different ways in order to serve distinct action goals. Using three methods, including cross-task classification, we found that at most time points F5 mirror neurons did not encode observed actions with the same code underlying action execution. However, in about 20% of neurons there were time periods with a shared code. These time periods formed a distinct cluster and cannot be considered a product of chance. Population classification yielded non-shared coding for observed actions in the whole population, which was at times optimal and consistently better than shared coding in differentially selected subpopulations. These results support the hypothesis of a representation of observed actions based on a strictly defined mirror mechanism only for small subsets of neurons and only under the assumption of time-resolved readout. Considering alternative concepts and recent findings, we propose that during observation mirror neurons represent the process of a goal pursuit from the observer's viewpoint. Whether the observer's goal pursuit, in which the other's action goal becomes the observer's action goal, or the other's goal pursuit is represented remains to be clarified. In any case, it may allow the observer to use expectations associated with a goal pursuit to directly intervene in or learn from another's action.

Flexible constraint hierarchy during the visual encoding of tool-object interactions

Tools and objects are associated with numerous action possibilities that are reduced depending on the task-related internal and external constraints presented to the observer. Action hierarchies propose that goals represent higher levels of the hierarchy while kinematic patterns represent lower levels of the hierarchy. Prior work suggests that tool-object perception is heavily influenced by grasp and action context. The current study sought to evaluate whether the presence of action hierarchy can be perceptually identified using eye tracking during tool-object observation. We hypothesize that gaze patterns will reveal a perceptual hierarchy based on the observed task context and grasp constraints. Participants viewed tool-objects scenes with two types of constraints: task-context and grasp constraints. Task-context constraints consisted of correct (e.g., frying pan-spatula) and incorrect tool-object pairings (e.g., stapler-spatula). Grasp constraints involved modified tool orientations, which requires participants to understand how initially awkward grasp postures can help achieve the task. The visual scene contained three areas of interests (AOIs): the object, the functional tool-end (e.g., spoon handle) and the manipulative tool-end (e.g., spoon bowl). Results revealed two distinct processes based on stimuli constraints. Goal-oriented encoding, the attentional bias towards the object and manipulative tool-end, was demonstrated when grasp did not lead to meaningful tool-use. In images where grasp postures were critical to action performance, attentional bias was primarily between the object and functional tool-end, which suggests means-related encoding of the graspable properties of the object. This study expands from previous work and demonstrates a flexible constraint hierarchy depending on the observed task constraints.

Visual Attention and Motion Visibility Modulate Motor Resonance during Observation of Human Walking in Different Manners

To advance our knowledge on the motor system during cyclic gait observation, we aimed to explore the effects of gaze fixation on corticospinal excitability evaluated by single-pulse transcranial magnetic stimulation (TMS). Fourteen healthy adult volunteers watched a video of a demonstrator walking on a treadmill under three different conditions: (1) observing the right lower limb, (2) observing the right ankle joint, and (3) observing the right lower limb on a video focused on the area below the knee. In each condition, motor-evoked potentials elicited by TMS in the tibialis anterior (TA) muscle were measured synchronously with the demonstrator’s initial contact and toe-off points. Directing visual attention to the ankle joint and focusing on its movements caused corticospinal facilitation in the TA muscle compared with watching the video without any visual fixation. In addition, phase-dependent differences in corticospinal excitability between the initial contact and toe-off points were only detected when the visibility range was restricted to below the knee. Our findings indicated that motor resonance during cyclic gait observation is modulated by visual attention and motion visibility in different activation manners.

Local and system mechanisms for action execution and observation in parietal and premotor cortices

The action observation network (AON) includes a system of brain areas largely shared with action execution in both human and nonhuman primates. Yet temporal and tuning specificities of distinct areas and of physiologically identified neuronal classes in the encoding of self and others’ action remain unknown. We recorded the activity of 355 single units from three crucial nodes of the AON, the anterior intraparietal area (AIP), and premotor areas F5 and F6, while monkeys performed a Go/No-Go grasping task and observed an experimenter performing it. At the system level, during task execution, F6 displays a prevalence of suppressed neurons and signals whether an action has to be performed, whereas AIP and F5 share a prevalence of facilitated neurons and remarkable target selectivity; during task observation, F5 stands out for its unique prevalence of facilitated neurons and its stronger and earlier modulation than AIP and F6. By applying unsupervised clustering of spike waveforms, we found distinct cell classes unevenly distributed across areas, with different firing properties and carrying specific visuomotor signals. Broadly spiking neurons exhibited a balanced amount of facilitated and suppressed activity during action execution and observation, whereas narrower spiking neurons showed more mutually facilitated responses during the execution of one’s own and others’ action, particularly in areas AIP and F5. Our findings elucidate the time course of activity and firing properties of neurons in the AON during one’s own and others’ action, from the system level of anatomically distinct areas to the local level of physiologically distinct cell classes.

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F6 neurons show a prevalence of suppressed activity, encoding whether to act

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Area F5 and AIP share a prevalence of facilitated neurons and target selectivity

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Across-areas, waveform-based clustering distinguished three neuronal classes

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Narrow-spiking neurons exhibit mutual modulation during self and others’ action

Do Gaze Behaviours during Action Observation Predict Interpersonal Motor Resonance?

Interpersonal motor resonance (IMR) is a common putative index of the mirror neuron system (MNS), a network containing specialised cells that fire during both action execution and observation. Visual content inputs to the MNS, however, it is unclear whether visual behaviours mediate the putative MNS response. We aimed to examine gaze effects on IMR during action observation. Neurotypical adults (N = 99; 60 female) underwent transcranial magnetic stimulation, electromyography, and eye-tracking during the observation of videos of actors performing grasping actions. IMR was measured as a percentage change in motor evoked potentials (MEPs) of the first dorsal interosseous muscle during action observation relative to baseline. MEP facilitation was observed during action observation, indicating IMR (65.43%, SE = 11.26%, P < 0.001). Fixations occurring in biologically relevant areas (face/hand/arm) yielded significantly stronger IMR (81.03%, SE = 14.15%) than non-biological areas (63.92%, SE = 14.60, P = 0.012). This effect, however, was only evident in the first of four experimental blocks. Our results suggest that gaze fixation can modulate IMR, but this may be affected by the salience and novelty of the observed action. These findings have important methodological implications for future studies in both clinical and healthy populations.

Movement initiation and grasp representation in premotor and primary motor cortex mirror neurons

Pyramidal tract neurons (PTNs) within macaque rostral ventral premotor cortex (F5) and (M1) provide direct input to spinal circuitry and are critical for skilled movement control. Contrary to initial hypotheses, they can also be active during action observation, in the absence of any movement. A population-level understanding of this phenomenon is currently lacking. We recorded from single neurons, including identified PTNs, in (M1) (n = 187), and F5 (n = 115) as two adult male macaques executed, observed, or withheld (NoGo) reach-to-grasp actions. F5 maintained a similar representation of grasping actions during both execution and observation. In contrast, although many individual M1 neurons were active during observation, M1 population activity was distinct from execution, and more closely aligned to NoGo activity, suggesting this activity contributes to withholding of self-movement. M1 and its outputs may dissociate initiation of movement from representation of grasp in order to flexibly guide behaviour.

Anterior Intraparietal Area: A Hub in the Observed Manipulative Action Network

Current knowledge regarding the processing of observed manipulative actions (OMAs) (e.g., grasping, dragging, or dropping) is limited to grasping and underlying neural circuitry remains controversial. Here, we addressed these issues by combining chronic neuronal recordings along the anteroposterior extent of monkeys’ anterior intraparietal (AIP) area with tracer injections into the recorded sites. We found robust neural selectivity for 7 distinct OMAs, particularly in the posterior part of AIP (pAIP), where it was associated with motor coding of grip type and own-hand visual feedback. This cluster of functional properties appears to be specifically grounded in stronger direct connections of pAIP with the temporal regions of the ventral visual stream and the prefrontal cortex, as connections with skeletomotor related areas and regions of the dorsal visual stream exhibited opposite or no rostrocaudal gradients. Temporal and prefrontal areas may provide visual and contextual information relevant for manipulative action processing. These results revise existing models of the action observation network, suggesting that pAIP constitutes a parietal hub for routing information about OMA identity to the other nodes of the network.

Dynamic task observation: A gaze-mediated complement to traditional action observation treatment?

Action observation elicits changes in primary motor cortex known as motor resonance, a phenomenon thought to underpin several functions, including our ability to understand and imitate others' actions. Motor resonance is modulated not only by the observer's motor expertise, but also their gaze behaviour. The aim of the present study was to investigate motor resonance and eye movements during observation of a dynamic goal-directed action, relative to an everyday one - a reach-grasp-lift (RGL) action, commonly used in action-observation-based neurorehabilitation protocols. Skilled and novice golfers watched videos of a golf swing and an RGL action as we recorded MEPs from three forearm muscles; gaze behaviour was concurrently monitored. Corticospinal excitability increased during golf swing observation, but it was not modulated by expertise, relative to baseline; no such changes were observed for the RGL task. MEP amplitudes were related to participants' gaze behaviour: in the RGL condition, target viewing was associated with lower MEP amplitudes; in the golf condition, MEP amplitudes were positively correlated with time spent looking at the effector or neighbouring regions. Viewing of a dynamic action such as the golf swing may enhance action observation treatment, especially when concurrent physical practice is not possible.

Neurons in the Macaque Dorsal Premotor Cortex Respond to Execution and Observation of Actions

We identified neurons in dorsal premotor cortex (PMd) of the macaque brain that respond during execution and observation of reaching-to-grasp actions, thus fulfilling the mirror neuron (MirN) criterion. During observation, the percentage of grip-selective MirNs in PMd and area F5 were comparable, and the selectivity indices in the two areas were similar. During execution, F5-MirNs were more selective than PMd–MirNs for grip, which was reflected in the higher selectivity indices in F5 than in PMd. PMd displayed grip-related information earlier than F5 during both conditions. In both areas, the number of neurons exhibiting congruent visual and motor selectivity did not differ from that expected by chance. However, both the PMd and F5 neuronal ensembles provided observation–execution matching, suggesting that the congruency may be achieved in a distributed fashion across the selective elements of the population. Furthermore, representational similarity analysis revealed that grip encoding in PMd and F5 is alike during both observation and execution. Our study provides direct evidence of mirror activity in PMd during observation of forelimb movements, and suggests that PMd is a node of the MirN circuit.

The Role of Attention and Saccades on Parietofrontal Encoding of Contextual and Grasp-specific Affordances of Tools: An ERP Study

The ability to recognize a tool's affordances (how a spoon should be appropriately grasped and used), is vital for daily life. Prior research has identified parietofrontal circuits, including mirror neurons, to be critical in understanding affordances. However, parietofrontal action-encoding regions receive extensive visual input and are adjacent to parietofrontal attention control networks. It is unclear how eye movements and attention modulate parietofrontal encoding of affordances. To address this issue, scenes depicting tools in different use-contexts and grasp-postures were presented to healthy subjects across two experiments, with stimuli durations of 100 ms or 500 ms. The 100-ms experiment automatically restricted saccades and required covert attention, while the 500-ms experiment allowed overt attention. The two experiments elicited similar behavioral decisions on tool-use correctness and isolated the influence of attention on parietofrontal activity. Parietofrontal ERPs (P600) distinguishing tool-use contexts (e.g., spoon-yogurt vs. spoon-ball) were similar in both experiments. Conversely, parietofrontal ERPs distinguishing tool-grasps were characterized by posterior to frontal N130-N200 ERPs in the 100-ms experiment and by saccade-perturbed N130-N200 ERPs, frontal N400 and parietal P500 in the 500-ms experiment. Particularly, only overt gaze toward the hand-tool interaction engaged mirror neurons (frontal N400) when discerning grasps that manipulate but not functionally use a tool - (grasp bowl rather than stem of spoon). Results here detail the first human electrophysiological evidence on how attention selectively modulates multiple parietofrontal grasp-perception circuits, especially the mirror neuron system, while unaffecting parietofrontal encoding of tool-use contexts. These results are pertinent to neurophysiological models of affordances that typically neglect the role of attention in action perception.

Effect of Different Movement Speed Modes on Human Action Observation: An EEG Study

Action observation (AO) generates event-related desynchronization (ERD) suppressions in the human brain by activating partial regions of the human mirror neuron system (hMNS). The activation of the hMNS response to AO remains controversial for several reasons. Therefore, this study investigated the activation of the hMNS response to a speed factor of AO by controlling the movement speed modes of a humanoid robot's arm movements. Since hMNS activation is reflected by ERD suppressions, electroencephalography (EEG) with BCI analysis methods for ERD suppressions were used as the recording and analysis modalities. Six healthy individuals were asked to participate in experiments comprising five different conditions. Four incremental-speed AO tasks and a motor imagery (MI) task involving imaging of the same movement were presented to the individuals. Occipital and sensorimotor regions were selected for BCI analyses. The experimental results showed that hMNS activation was higher in the occipital region but more robust in the sensorimotor region. Since the attended information impacts the activations of the hMNS during AO, the pattern of hMNS activations first rises and subsequently falls to a stable level during incremental-speed modes of AO. The discipline curves suggested that a moderate speed within a decent inter-stimulus interval (ISI) range produced the highest hMNS activations. Since a brain computer/machine interface (BCI) builds a path-way between human and computer/mahcine, the discipline curves will help to construct BCIs made by patterns of action observation (AO-BCI). Furthermore, a new method for constructing non-invasive brain machine brain interfaces (BMBIs) with moderate AO-BCI and motor imagery BCI (MI-BCI) was inspired by this paper.

Primate social attention: Species differences and effects of individual experience in humans, great apes, and macaques

When viewing social scenes, humans and nonhuman primates focus on particular features, such as the models' eyes, mouth, and action targets. Previous studies reported that such viewing patterns vary significantly across individuals in humans, and also across closely-related primate species. However, the nature of these individual and species differences remains unclear, particularly among nonhuman primates. In large samples of human and nonhuman primates, we examined species differences and the effects of experience on patterns of gaze toward social movies. Experiment 1 examined the species differences across rhesus macaques, nonhuman apes (bonobos, chimpanzees, and orangutans), and humans while they viewed movies of various animals' species-typical behaviors. We found that each species had distinct viewing patterns of the models' faces, eyes, mouths, and action targets. Experiment 2 tested the effect of individuals' experience on chimpanzee and human viewing patterns. We presented movies depicting natural behaviors of chimpanzees to three groups of chimpanzees (individuals from a zoo, a sanctuary, and a research institute) differing in their early social and physical experiences. We also presented the same movies to human adults and children differing in their expertise with chimpanzees (experts vs. novices) or movie-viewing generally (adults vs. preschoolers). Individuals varied within each species in their patterns of gaze toward models' faces, eyes, mouths, and action targets depending on their unique individual experiences. We thus found that the viewing patterns for social stimuli are both individual- and species-specific in these closely-related primates. Such individual/species-specificities are likely related to both individual experience and species-typical temperament, suggesting that primate individuals acquire their unique attentional biases through both ontogeny and evolution. Such unique attentional biases may help them learn efficiently about their particular social environments.

Evidence for the representation of movement kinematics in the discharge of F5 mirror neurons during the observation of transitive and intransitive actions

Mirror neurons (MirNs) are sensorimotor neurons that fire both when an animal performs a goal-directed action and when the same animal observes another agent performing the same or a similar transitive action. It has been claimed that the observation of intransitive actions does not activate MirNs in a monkey’s brain. Prompted by recent evidence indicating that the discharge of MirNs is modulated also by non-object-directed actions, we investigated thoroughly the efficacy of intransitive actions to trigger MirNs’ discharge. Using representational similarity analysis, we also studied whether the elements constituting the visual scene presented to the monkey during the observation of actions (both transitive and intransitive) are represented in the discharge of MirNs. For this purpose, the moving hand was modeled by its kinematics and the object by features of its geometry. We found that MirNs respond to the observation of both transitive and intransitive actions and that the discharge differences evoked by the observation of object- and non-object-directed actions are correlated more with the kinematic differences of these actions than with the differences of the objects’ features. These findings support the view that observed action kinematics contribute to action mirroring.

NEW & NOTEWORTHY Mirror neurons in the monkey brain are thought to respond exclusively to the observation of object-directed actions. Here, we show that mirror neurons also respond to the observation of intransitive actions and that the kinematics of the observed movements are represented in their discharge. This finding supports the view that mirror neurons provide also a kinematics-based representation of actions.

Cortical and subcortical connections of parietal and premotor nodes of the monkey hand mirror neuron network

Mirror neurons (MNs) are a class of cells originally discovered in the monkey ventral premotor cortex (PMv) and inferior parietal lobule (IPL). They discharge during both action execution and action observation and appear to play a crucial role in understanding others' actions. It has been proposed that the mirror mechanism is based on a match between the visual description of actions, encoded in temporal cortical regions, and their motor representation, provided by PMv and IPL. However, neurons responding to action observation have been recently found in other cortical regions, suggesting that the mirror mechanism relies on a wider network. Here we provide the first description of this network by injecting neural tracers into physiologically identified IPL and PMv sectors containing hand MNs. Our results show that these sectors are reciprocally connected, in line with the current view, but IPL MN sectors showed virtually no direct connection with temporal visual areas. In addition, we found that PMv and IPL MN sectors share connections with several cortical regions, including the dorsal and mesial premotor cortex, the primary motor cortex, the secondary somatosensory cortex, the mid-dorsal insula and the ventrolateral prefrontal cortex, as well as subcortical structures, such as motor and polysensory thalamic nuclei and the mid-dorsal claustrum. We propose that each of these regions constitutes a node of an "extended network", through which information relative to ongoing movements, social context, environmental contingencies, abstract rules, and internal states can influence MN activity and contribute to several socio-cognitive functions.

Motor resonance during action observation is gaze-contingent: A TMS study

When we observe others performing an action, visual input to our mirror neuron system is reflected in the facilitation of primary motor cortex (M1), a phenomenon known as 'motor resonance'. However, it is unclear whether this motor resonance is contingent upon our point-of-gaze. In order to address this issue, we collected gaze data from participants as they viewed an intransitive action - thumb abduction/adduction - under four conditions: with natural gaze behaviour (free viewing) and with their gaze fixated on each of three predetermined loci at various distances from the prime mover. In a control condition, participants viewed little finger movements, also with a fixated gaze. Transcranial magnetic stimulation (TMS) was delivered to M1 and motor evoked potentials (MEPs) were recorded from the right abductor pollicis brevis (APB) and right abductor digiti minimi (ADM). Results showed that, relative to a free viewing condition, a fixated point-of-gaze which maximized transfoveal motion facilitated MEPs in APB. Moreover, during free viewing, saccade amplitudes and APB MEP amplitudes were negatively correlated. These findings indicate that motor resonance is contingent on the observer's gaze behaviour and that, for simple movements, action observation effects may be enhanced by employing a fixed point-of-gaze.

Eye tracking uncovered great apes' ability to anticipate that other individuals will act according to false beliefs

Using a novel eye-tracking test, we recently showed that great apes anticipate that other individuals will act according to false beliefs. This finding suggests that, like humans, great apes understand others' false beliefs, at least in an implicit way. One key question raised by our study is why apes have passed our tests but not previous ones. In this article, we consider this question by detailing the development of our task. We considered 3 major differences in our task compared with the previous ones. First, we monitored apes' eye movements, and specifically their anticipatory looks, to measure their predictions about how agents will behave. Second, we adapted our design from an anticipatory-looking false belief test originally developed for human infants. Third, we developed novel test scenarios that were specifically designed to capture the attention of our ape participants. We then discuss how each difference may help explain differences in performance on our task and previous ones, and finally propose some directions for future studies.

The Extended Mirror Neuron Network

Mirror neurons (MNs) are a fascinating class of cells originally discovered in the ventral premotor cortex (PMv) and, subsequently, in the inferior parietal lobule (IPL) of the macaque, which become active during both the execution and observation of actions. In this review, I will first highlight the mounting evidence indicating that mirroring others’ actions engages a broad system of reciprocally connected cortical areas, which extends well beyond the classical IPL-PMv circuit and might even include subcortical regions such as the basal ganglia. Then, I will present the most recent findings supporting the idea that the observation of one’s own actions, which might play a role in the ontogenetic origin and tuning of MNs, retains a particular relevance within the adult MN system. Finally, I will propose that both cortical and subcortical mechanisms do exist to decouple MN activity from the motor output, in order to render it exploitable for high-order perceptual, cognitive, and even social functions. The findings reviewed here provide an original framework for envisaging the main challenges and experimental directions of future neurophysiological and neuroanatomical studies of the monkey MN system.

Grasping others' movements: Rapid discrimination of object size from observed hand movements

During reach-to-grasp movements, the hand is gradually molded to conform to the size and shape of the object to be grasped. Yet the ability to glean information about object properties by observing grasping movements is poorly understood. In this study, we capitalized on the effect of object size to investigate the ability to discriminate the size of an invisible object from movement kinematics. The study consisted of 2 phases. In the first action execution phase, to assess grip scaling, we recorded and analyzed reach-to-grasp movements performed toward differently sized objects. In the second action observation phase, video clips of the corresponding movements were presented to participants in a two-alternative forced-choice task. To probe discrimination performance over time, videos were edited to provide selective vision of different periods from 2 viewpoints. Separate analyses were conducted to determine how the participants' ability to discriminate between stimulus alternatives (Type I sensitivity) and their metacognitive ability to discriminate between correct and incorrect responses (Type II sensitivity) varied over time and viewpoint. We found that as early as 80 ms after movement onset, participants were able to discriminate object size from the observation of grasping movements delivered from the lateral viewpoint. For both viewpoints, information pickup closely matched the evolution of the hand's kinematics, reaching an almost perfect performance well before the fingers made contact with the object (60% of movement duration). These findings suggest that observers are able to decode object size from kinematic sources specified early on in the movement. (PsycINFO Database Record

Exploring associations between gaze patterns and putative human mirror neuron system activity

The human mirror neuron system (MNS) is hypothesized to be crucial to social cognition. Given that key MNS-input regions such as the superior temporal sulcus are involved in biological motion processing, and mirror neuron activity in monkeys has been shown to vary with visual attention, aberrant MNS function may be partly attributable to atypical visual input. To examine the relationship between gaze pattern and interpersonal motor resonance (IMR; an index of putative MNS activity), healthy right-handed participants aged 18–40 (n = 26) viewed videos of transitive grasping actions or static hands, whilst the left primary motor cortex received transcranial magnetic stimulation. Motor-evoked potentials recorded in contralateral hand muscles were used to determine IMR. Participants also underwent eyetracking analysis to assess gaze patterns whilst viewing the same videos. No relationship was observed between predictive gaze and IMR. However, IMR was positively associated with fixation counts in areas of biological motion in the videos, and negatively associated with object areas. These findings are discussed with reference to visual influences on the MNS, and the possibility that MNS atypicalities might be influenced by visual processes such as aberrant gaze pattern.

A human homologue of monkey F5c

Area F5c is a monkey premotor area housing mirror neurons which responds more strongly to grasping observation when the actor is visible than when only the actor's hand is visible. Here we used this characteristic fMRI signature of F5c in seven imaging experiments – one in macaque monkeys and six in humans – to identify the human homologue of monkey F5c. By presenting the two grasping actions (actor, hand) and varying the low level visual characteristics, we localized a putative human homologue of area F5c (phF5c) in the inferior part of precentral sulcus, bilaterally. In contrast to monkey F5c, phF5c is asymmetric, with a right-sided bias, and is activated more strongly during the observation of the later stages of grasping when the hand is close to the object. The latter characteristic might be related to the emergence, in humans, of the capacity to precisely copy motor acts performed by others, and thus imitation.

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We use parallel fMRI to identify the human homologue of macaque F5c.

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In premotor cortex only F5c reacts more to observing grasping with the actor visible.

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Two bilateral inferior precentral sulcus sites respond similarly for many stimuli.

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The human homologues of F5c are asymmetric and require fixation near the target.

Mirror neuron activation prior to action observation in a predictable context

A fundamental capacity of social animals consists in the predictive representation of upcoming events in the outside world, such as the actions of others. Here, we tested the activity of ventral premotor area F5 mirror neurons (MNs) while monkeys observed an experimenter performing (Action condition) or withholding (Inaction condition) a grasping action, which could be predicted on the basis of previously presented auditory instructions. Many of the recorded MNs discharged only during action observation (Action MNs), but one-third also encoded the experimenter's withheld action (Inaction MNs). Interestingly, while most of Action MNs exhibited reactive discharge during action observation, becoming active after the go signal, the majority of Inaction MNs showed predictive discharge. MN population activity as a whole displayed an overall predictive activation pattern, becoming active, on average, 340 ms before the go signal. Furthermore, MNs became active earlier when the observed action was performed in the monkeys' extrapersonal rather than peripersonal space, suggesting that context-based neural prediction of others' actions plays different roles depending on the monkeys' ability to interact with the observed agent.

Space-dependent representation of objects and other's action in monkey ventral premotor grasping neurons

The macaque ventral premotor area F5 hosts two types of visuomotor grasping neurons: "canonical" neurons, which respond to visually presented objects and underlie visuomotor transformation for grasping, and "mirror" neurons, which respond during the observation of others' action, likely playing a role in action understanding. Some previous evidence suggested that canonical and mirror neurons could be anatomically segregated in different sectors of area F5. Here we investigated the functional properties of single neurons in the hand field of area F5 using various tasks similar to those originally designed to investigate visual responses to objects and actions. By using linear multielectrode probes, we were able to simultaneously record different types of neurons and to precisely localize their cortical depth. We recorded 464 neurons, of which 243 showed visuomotor properties. Canonical and mirror neurons were often present in the same cortical sites; and, most interestingly, a set of neurons showed both canonical and mirror properties, discharging to object presentation as well as during the observation of experimenter's goal-directed acts (canonical-mirror neurons). Typically, visual responses to objects were constrained to the monkey peripersonal space, whereas action observation responses were less space-selective. Control experiments showed that space-constrained coding of objects mostly relies on an operational (action possibility) rather than metric (absolute distance) reference frame. Interestingly, canonical-mirror neurons appear to code object as target for both one's own and other's action, suggesting that they could play a role in predictive representation of others' impending actions.

Do monkey F5 mirror neurons show changes in firing rate during repeated observation of natural actions?

Mirror neurons were first discovered in area F5 of macaque monkeys. In humans, noninvasive studies have demonstrated an increased blood oxygen level-dependent (BOLD) signal in homologous motor areas during action observation. One approach to demonstrating that this indicates the existence of mirror neurons in humans has been to employ functional (f)MRI adaptation to test whether the same population of neurons is active during both observation and execution conditions. Although a number of human studies have reported fMRI adaptation in these areas, a recent study has shown that macaque mirror neurons do not attenuate their firing rate with two repetitions. Here we investigated whether mirror neurons modulate their firing rate when monkeys observed the same repeated natural action multiple times. We recorded from 67 mirror neurons in area F5 of two macaque monkeys while they observed an experimenter perform a reach-to-grasp action on a small food reward using a precision grip. Although no changes were detectable for the first two repetitions, we show that both the firing rate and the latency at which mirror neurons discharged during observation were subtly modulated by the repetition of the observed action over 7-10 trials. Significant adaption was mostly found in the period immediately before the grasp was performed. We also found that the local field potential activity in F5 (beta-frequency range, 16-23 Hz), which is attenuated during action observation, also showed systematic changes with repeated observation. These LFP changes occurred well in advance of the mirror neuron adaptation. We conclude that macaque mirror neurons can show intra-modal adaptation, but whether this is related to fMRI adaptation of the BOLD signal requires further investigation.

Corticospinal mirror neurons

Here, we report the properties of neurons with mirror-like characteristics that were identified as pyramidal tract neurons (PTNs) and recorded in the ventral premotor cortex (area F5) and primary motor cortex (M1) of three macaque monkeys. We analysed the neurons' discharge while the monkeys performed active grasp of either food or an object, and also while they observed an experimenter carrying out a similar range of grasps. A considerable proportion of tested PTNs showed clear mirror-like properties (52% F5 and 58% M1). Some PTNs exhibited 'classical' mirror neuron properties, increasing activity for both execution and observation, while others decreased their discharge during observation ('suppression mirror-neurons'). These experiments not only demonstrate the existence of PTNs as mirror neurons in M1, but also reveal some interesting differences between M1 and F5 mirror PTNs. Although observation-related changes in the discharge of PTNs must reach the spinal cord and will include some direct projections to motoneurons supplying grasping muscles, there was no EMG activity in these muscles during action observation. We suggest that the mirror neuron system is involved in the withholding of unwanted movement during action observation. Mirror neurons are differentially recruited in the behaviour that switches rapidly between making your own movements and observing those of others.