An integrated environment for stereoscopic acquisition, off-line 3D elaboration, and visual presentation of biological actions

We present an integrated environment for stereoscopic acquisition, off-line 3D elaboration, and visual presentation of biological hand actions. The system is used in neurophysiological experiments aimed at the investigation of the parameters of the external stimuli that mirror neurons visually extract and match on their movement related activity.

Cortical mechanism for the visual guidance of hand grasping movements in the monkey: A reversible inactivation study

Picking up an object requires two basic motor operations: reaching and grasping. Neurophysiological studies in monkeys have suggested that the visuomotor transformations necessary for these two operations are carried out by separate parietofrontal circuits and that, for grasping, a key role is played by a specific sector of the ventral premotor cortex: area F5. The aim of the present study was to test the validity of this hypothesis by reversibly inactivating area F5 in monkeys trained to grasp objects of different shape, size and orientation. In separate sessions, the hand field of the primary motor cortex (area F1 or area 4) was also reversibly inactivated. The results showed that after inactivation of area F5 buried in the bank of the arcuate sulcus (the F5 sector where visuomotor neurones responding to object presentation are located), the hand shaping preceding grasping was markedly impaired and the hand posture was not appropriate for the object size and shape. The monkeys were eventually able to grasp the objects, but only after a series of corrections made under tactile control. With small inactivations the deficits concerned the contralesional hand, with larger inactivations the ipsilateral hand as well. In addition, there were signs of peripersonal neglect in the hemispace contralateral to the inactivation site. Following inactivation of area F5 lying on the cortical convexity (the F5 sector where visuomotor neurones responding to action observation, 'mirror neurones', are found) only a motor slowing was observed, the hand shaping being preserved. The inactivation of the hand field of area F1 produced a severe paralysis of contralateral finger movements with hypotonia. The results of this study indicate the crucial role of the ventral premotor cortex in visuomotor transformations for grasping movements. More generally, they provide strong support for the notion that distal and proximal movement organization relies upon distinct cortical circuits. Clinical data on distal movement deficits in humans are re-examined in the light of the present findings.

Modulation of spinal excitability during observation of hand actions in humans

There is growing evidence that observation of actions performed by other individuals activates observer's cortical motor areas. This matching of observed actions on the observer's motor repertoire could be at the basis of action recognition. Here we investigated if action observation, in addition to cortical motor areas, involves also low level motor structures mimicking the observed actions as if they were performed by the observer. Spinal cord excitability was tested by eliciting the H-reflex in a finger flexor muscle (flexor digitorum superficialis) in humans looking at goal-directed hand actions presented on a TV screen. We found that, in the absence of any detectable muscle activity, there was in the observers a significant modulation of the monosynaptic reflex size, specifically related to the different phases of the observed movement. The recorded H-reflex rapidly increased in size during hand opening, it was depressed during hand closing and quickly recovered during object lifting. This modulation pattern is, however, opposite to that occurring when the recorded muscles are actually executing the observed action [Lemon et al. (1995) J. Neurosci., 15, 6145-56]. Considering that, when investigated at cortical level the modulation pattern of corticospinal excitability replicates the observed movements [Fadiga et al. (1995) J. Neurophysiol., 73, 2608-2611], this spinal 'inverted mirror' behaviour might be finalised to prevent the overt replica of the seen action.

Action observation activates premotor and parietal areas in a somatotopic manner: an fMRI study

Functional magnetic resonance imaging (fMRI) was used to localize brain areas that were active during the observation of actions made by another individual. Object- and non-object-related actions made with different effectors (mouth, hand and foot) were presented. Observation of both object- and non-object-related actions determined a somatotopically organized activation of premotor cortex. The somatotopic pattern was similar to that of the classical motor cortex homunculus. During the observation of object-related actions, an activation, also somatotopically organized, was additionally found in the posterior parietal lobe. Thus, when individuals observe an action, an internal replica of that action is automatically generated in their premotor cortex. In the case of object-related actions, a further object-related analysis is performed in the parietal lobe, as if the subjects were indeed using those objects. These results bring the previous concept of an action observation/execution matching system (mirror system) into a broader perspective: this system is not restricted to the ventral premotor cortex, but involves several somatotopically organized motor circuits.

Visuomotor neurons: ambiguity of the discharge or 'motor' perception?

The cortical motor system has been classically considered as the unitary, output stage of the brain processing of sensory information. According to this idea, the motor cortex - the acting brain - receives the result of the perceptual processing (visual, acoustical, tactile, etc.) elaborated by the 'associative cortex'. During the last two decades this perspective has been challenged by a series of anatomical, hodological, and neurophysiological data. This converging evidence delineates a dramatically changed picture. Far from being unitary, the cortical motor system appears to be constituted by a constellation of distinct areas, each of those endowed with specific functional properties and linked by reciprocal connections with distinct sectors of the parietal cortex. Furthermore, several 'motor' neurons in addition to their motor discharge, are also activated by somatosensory and visual stimulation (somatomotor and visuomotor neurons). In the present paper we will discuss the functional properties of those sensorimotor neurons located in the ventral part of the monkey premotor cortex. On the basis of electrophysiological data, we will propose that the apparent parodox stemming from the coexistence within the same neuron of motor and sensory properties can be solved by postulating that the motor system not only executes actions but also internally represents them in terms of 'motor ideas'. These motor ideas may provide the neurobiological basis for space representation, understanding of actions made by others and, possibly, semantic categorization of objects.

Action for perception: a motor-visual attentional effect

Five experiments investigated whether preparation of a grasping movement affects detection and discrimination of visual stimuli. Normal human participants were required to prepare to grasp a bar and then to grasp it as fast as possible on presentation of a visual stimulus. On the basis of the degree of sharing of their intrinsic properties with those of the to-be-grasped bar, visual stimuli were categorized as "congruent" or "incongruent." Results showed that grasping reaction times to congruent visual stimuli were faster than reaction times to incongruent ones. These data indicate that preparation to act on an object produces faster processing of stimuli congruent with that object. The same facilitation was present also when, after the preparation of hand grasping, participants were suddenly instructed to inhibit the prepared grasping movement and to respond with a different motor effector. The authors suggest that these findings could represent an extension of the premotor theory of attention, from orienting of attention to spatial locations to orienting of attention to graspable objects.

Action for perception: a motor-visual attentional effect

Five experiments investigated whether preparation of a grasping movement affects detection and discrimination of visual stimuli. Normal human participants were required to prepare to grasp a bar and then to grasp it as fast as possible on presentation of a visual stimulus. On the basis of the degree of sharing of their intrinsic properties with those of the to-be-grasped bar, visual stimuli were categorized as "congruent" or "incongruent." Results showed that grasping reaction times to congruent visual stimuli were faster than reaction times to incongruent ones. These data indicate that preparation to act on an object produces faster processing of stimuli congruent with that object. The same facilitation was present also when, after the preparation of hand grasping, participants were suddenly instructed to inhibit the prepared grasping movement and to respond with a different motor effector. The authors suggest that these findings could represent an extension of the premotor theory of attention, from orienting of attention to spatial locations to orienting of attention to graspable objects.

Resonance behaviors and mirror neurons

This article is subdivided into two parts. In the first part we review the properties of a particular class of premotor neurons, the "mirror" neurons. With this term we define neurons that discharge both when the monkey makes a particular action and when it observes another individual (monkey or human) making a similar action. The second part is an attempt to give a neurophysiological account of the mechanisms underlying behaviors where an individual reproduces, overtly or internally, movements or actions made by another individual. We will refer to these behaviors as "resonance behaviors". We distinguish two types of resonance behavior. The first type is characterized by imitation, immediate or with delay, of movements made by other individuals. Examples of resonance behavior of this type are the "imitative" behaviors observed in birds, young infants and patients with frontal lesions. The second type of resonance behavior is characterized by the occurrence, at the observation of an action, of a neural pattern, which, when internally generated, determines the making of the observed action. In this type of resonance behavior the observed action is, typically, not repeated (overtly). We argue that resonance behavior of the second type is at the basis of the understanding of actions made by others. At the end of the article we review evidence of mirror mechanisms in humans and discuss their anatomical localizations.

Grasping objects and grasping action meanings: the dual role of monkey rostroventral premotor cortex (area F5)

Monkey area F5 consists of two main histochemical sectors, one buried inside the arcuate sulcus, the other located on the cortical convexity. Neurons of both sectors discharge during hand movements. Many of them also fire in response to the presentation of visual stimuli. However, the visual stimuli effective for triggering the neurons in each sector are markedly different. Neurons located in the bank of the arcuate sulcus respond to the observation of 3D objects, provided that object size and shape is congruent with the prehension type coded by the neuron ('canonical' F5 neurons). Neurons of the convexity discharge when the monkey observes hand actions performed by another individual, provided that they are similar to the motor action coded by the neuron ('mirror' neurons). What do the canonical F5 neurons and the surprising mirror neurons have in common? The interpretation we propose is that these two categories of F5 neurons both generate an internal copy of a potential hand action. In the case of 'canonical' neurons, this copy gives a description of how to grasp an object; in the case of mirror neurons it gives a description of an action made by another person. Because the individuals know the consequences of their actions, we propose that the internal motor copies of the observed actions represent the neural basis for understanding the meaning of actions made by others.

Corticospinal excitability is specifically modulated by motor imagery: a magnetic stimulation study

Transcranial magnetic stimulation (TMS) was used to investigate whether the excitability of the corticospinal system is selectively affected by motor imagery. To this purpose, we performed two experiments. In the first one we recorded motor evoked potentials from right hand and arm muscles during mental simulation of flexion/extension movements of both distal and proximal joints. In the second experiment we applied magnetic stimulation to the right and the left motor cortex of subjects while they were imagining opening or closing their right or their left hand. Motor evoked potentials (MEPs) were recorded from a hand muscle contralateral to the stimulated cortex. The results demonstrated that the excitability pattern during motor imagery dynamically mimics that occurring during movement execution. In addition, while magnetic stimulation of the left motor cortex revealed increased corticospinal excitability when subjects imagined ipsilateral as well as contralateral hand movements, the stimulation of the right motor cortex revealed a facilitatory effect induced by imagery of contralateral hand movements only. In conclusion, motor imagery is a high level process, which, however, manifests itself in the activation of those same cortical circuits that are normally involved in movement execution.

Premotor cortex activation during observation and naming of familiar tools

Positron emission tomography was used to investigate whether observation of real objects (tools of common use) activates premotor areas in the absence of any overt motor demand. Silent naming of the presented tools and silent naming of their use were also studied. Right-handed normal subjects were employed. Tool observation strongly activated the left dorsal premotor cortex. In contrast, silent tool naming activated Broca's area without additional activity in the dorsal premotor cortex. Silent tool-use naming, in addition to activating Broca's area, increased the activity in the left dorsal premotor cortex and recruited the left ventral premotor cortex and the left supplementary motor area. These data indicate that, even in the absence of any subsequent movement, the left premotor cortex processes objects that, like tools, have a motor valence. This dorsal premotor activation, which further augments when the subject names the tool use, should reflect the neural activity related to motor schemata for object use. The presence of an activation of both dorsal premotor cortex and ventral premotor cortex during tool-use naming suggests a role for these two areas in understanding object semantics.

Object representation in the ventral premotor cortex (area F5) of the monkey

Visual and motor properties of single neurons of monkey ventral premotor cortex (area F5) were studied in a behavioral paradigm consisting of four conditions: object grasping in light, object grasping in dark, object fixation, and fixation of a spot of light. The employed objects were six different three-dimensional (3-D) geometric solids. Two main types of neurons were distinguished: motor neurons (n = 25) and visuomotor neurons (n = 24). Motor neurons discharged in association with grasping movements. Most of them (n = 17) discharged selectively during a particular type of grip. Different objects, if grasped in similar way, determined similar neuronal motor responses. Visuomotor neurons also discharged during active movements, but, in addition, they fired also in response to the presentation of 3-D objects. The majority of visuomotor neurons (n = 16) showed selectivity for one or few objects. The response was present both in object grasping in light and in object fixation conditions. Visuomotor neurons that selectively discharged to the presentation of a given object discharged also selectively during grasping of that object. In conclusion, object shape is coded in F5 even when a response to that object is not required. The possible visual or motor nature of this object coding is discussed.

Evidence for visuomotor priming effect

'While seated, the patient took a glass, gave it to the examiner and then picked up a jug. He poured water into the glass and, having put down the jug, took the glass ...'. This compulsive behaviour, described by Lhermitte in patients with frontal lobe lesions, is an example of how, without any internal motivation, visual stimuli may impel a patient to act and 'grasp the objects presented and use them'. We investigated whether this behaviour is a pathological manifestation of a normal, automatic object to action transformation. To test this, we primed normal subjects, while ready to execute a grasping movement, by visually presenting them with drawings irrelevant to the task to be executed. Drawings visually congruent with the object to be grasped markedly reduced the reaction time for grasping. These data represent the first evidence for the existence of a visuomotor priming. Seeing an object facilitates an action congruent with the visual properties of that object.

Localization of grasp representations in humans by positron emission tomography. 2. Observation compared with imagination

Positron emission tomography imaging of cerebral blood flow was used to localize brain areas involved in the representation of hand grasping movements. Seven normal subjects were scanned under three conditions. In the first, they observed precision grasping of common objects performed by the examiner. In the second, they imagined themselves grasping the objects without actually moving the hand. These two tasks were compared with a control task of object viewing. Grasp observation activated the left rostral superior temporal sulcus, left inferior frontal cortex (area 45), left rostral inferior parietal cortex (area 40), the rostral part of left supplementary motor area (SMA-proper), and the right dorsal premotor cortex. Imagined grasping activated the left inferior frontal (area 44) and middle frontal cortex, left caudal inferior parietal cortex (area 40), a more extensive response in left rostral SMA-proper, and left dorsal premotor cortex. The two conditions activated different areas of the right posterior cerebellar cortex. We propose that the areas active during grasping observation may form a circuit for recognition of hand-object interactions, whereas the areas active during imagined grasping may be a putative human homologue of a circuit for hand grasping movements recently defined in nonhuman primates. The location of responses in SMA-proper confirms the rostrocaudal segregation of this area for imagined and real movement. A similar segregation is also present in the cerebellum, with imagined and observed grasping movements activating different parts of the posterior lobe and real movements activating the anterior lobe.

Localization of grasp representations in humans by PET: 1. Observation versus execution

Positron emission tomography (PET) was used to localize brain regions that are active during the observation of grasping movements. Normal, right-handed subjects were tested under three conditions. In the first, they observed grasping movements of common objects performed by the experimenter. In the second, they reached and grasped the same objects. These two conditions were compared with a third condition consisting of object observation. On the basis of monkey data, it was hypothesized that during grasping observation, activations should be present in the region of the superior temporal sulcus (STS) and in inferior area 6. The findings in humans demonstrated that grasp observation significantly activates the cortex of the middle temporal gyrus including that of the adjacent superior temporal sulcus (Brodmann's area 21) and the caudal part of the left inferior frontal gyrus (Brodmann's area 45). The possible functional homologies between these areas and the monkey STS region and frontal area F5 are discussed.

Coding of peripersonal space in inferior premotor cortex (area F4)

1. We studied the functional properties of neurons in the caudal part of inferior area 6 (area F4) in awake monkeys. In agreement with previous reports, we found that the large majority (87%) of neurons responded to sensory stimuli. The responsive neurons fell into three categories: somatosensory neurons (30%); visual neurons (14%); and bimodal, visual and somatosensory neurons (56%). Both somatosensory and bimodal neurons typically responded to light touch of the skin. Their RFs were located on the face, neck, trunk, and arms. Approaching objects were the most effective visual stimuli. Visual RFs were mostly located in the space near the monkey (peripersonal space). Typically they extended in the space adjacent to the tactile RFs. 2. The coordinate system in which visual RFs were coded was studied in 110 neurons. In 94 neurons the RF location was independent of eye position, remaining in the same position in the peripersonal space regardless of eye deviation. The RF location with respect to the monkey was not modified by changing monkey position in the recording room. In 10 neurons the RF's location followed the eye movements, remaining in the same retinal position (retinocentric RFs). For the remaining six neurons the RF organization was not clear. We will refer to F4 neurons with RF independent of eye position as somatocentered neurons. 3. In most somatocentered neurons (43 of 60 neurons) the background level of activity and the response to visual stimuli were not modified by changes in eye position, whereas they were modulated in the remaining 17. It is important to note that eye deviations were constantly accompanied by a synergic increase of the activity of the ipsilateral neck muscles. It is not clear, therefore, whether the modulation of neuron discharge depended on eye position or was a consequence of changes in neck muscle activity. 4. The effect of stimulus velocity (20-80 cm/s) on neuron response intensity and RF extent in depth was studied in 34 somatocentered neurons. The results showed that in most neurons the increase of stimulus velocity produced an expansion in depth of the RF. 5. We conclude that space is coded differently in areas that control somatic and eye movements. We suggest that space coding in different cortical areas depends on the computational necessity of the effectors they control.

Action recognition in the premotor cortex

We recorded electrical activity from 532 neurons in the rostral part of inferior area 6 (area F5) of two macaque monkeys. Previous data had shown that neurons of this area discharge during goal-directed hand and mouth movements. We describe here the properties of a newly discovered set of F5 neurons ("mirror neurons', n = 92) all of which became active both when the monkey performed a given action and when it observed a similar action performed by the experimenter. Mirror neurons, in order to be visually triggered, required an interaction between the agent of the action and the object of it. The sight of the agent alone or of the object alone (three-dimensional objects, food) were ineffective. Hand and the mouth were by far the most effective agents. The actions most represented among those activating mirror neurons were grasping, manipulating and placing. In most mirror neurons (92%) there was a clear relation between the visual action they responded to and the motor response they coded. In approximately 30% of mirror neurons the congruence was very strict and the effective observed and executed actions corresponded both in terms of general action (e.g. grasping) and in terms of the way in which that action was executed (e.g. precision grip). We conclude by proposing that mirror neurons form a system for matching observation and execution of motor actions. We discuss the possible role of this system in action recognition and, given the proposed homology between F5 and human Brocca's region, we posit that a matching system, similar to that of mirror neurons exists in humans and could be involved in recognition of actions as well as phonetic gestures.

Premotor cortex and the recognition of motor actions

In area F5 of the monkey premotor cortex there are neurons that discharge both when the monkey performs an action and when he observes a similar action made by another monkey or by the experimenter. We report here some of the properties of these 'mirror' neurons and we propose that their activity 'represents' the observed action. We posit, then, that this motor representation is at the basis of the understanding of motor events. Finally, on the basis of some recent data showing that, in man, the observation of motor actions activate the posterior part of inferior frontal gyrus, we suggest that the development of the lateral verbal communication system in man derives from a more ancient communication system based on recognition of hand and face gestures.

Motor facilitation during action observation: a magnetic stimulation study

1. We stimulated the motor cortex of normal subjects (transcranial magnetic stimulation) while they 1) observed an experimenter grasping 3D-objects, 2) looked at the same 3D-objects, 3) observed an experimenter tracing geometrical figures in the air with his arm, and 4) detected the dimming of a light. Motor evoked potentials (MEPs) were recorded from hand muscles. 2. We found that MEPs significantly increased during the conditions in which subjects observed movements. The MEP pattern reflected the pattern of muscle activity recorded when the subjects executed the observed actions. 3. We conclude that in humans there is a system matching action observation and execution. This system resembles the one recently described in the monkey.

Understanding motor events: a neurophysiological study

Neurons of the rostral part of inferior premotor cortex of the monkey discharge during goal-directed hand movements such as grasping, holding, and tearing. We report here that many of these neurons become active also when the monkey observes specific, meaningful hand movements performed by the experimenters. The effective experimenters' movements include among others placing or retrieving a piece of food from a table, grasping food from another experimenter's hand, and manipulating objects. There is always a clear link between the effective observed movement and that executed by the monkey and, often, only movements of the experimenter identical to those controlled by a given neuron are able to activate it. These findings indicate that premotor neurons can retrieve movements not only on the basis of stimulus characteristics, as previously described, but also on the basis of the meaning of the observed actions.

Space coding by premotor cortex

Many neurons in inferior area 6, a cortical premotor area, respond to visual stimuli presented in the space around the animal. We were interested to learn whether the receptive fields of these neurons are coded in retinotopic or in body-centered coordinates. To this purpose we recorded single neurons from inferior area 6 (F4 sector) in a monkey trained to fixate a light and detect its dimming. During fixation visual stimuli were moved towards the monkey both within and outside the neuron's receptive field. The fixation point was then moved and the neuron retested with the monkey's gaze deviated to the new location. The results showed that most inferior area 6 visual neurons code the stimulus position in spatial and not in retinal coordinates. It is proposed that these visual neurons are involved in generating the stable body-centered frame of reference necessary for programming visually guided movements.

Ultradian and circadian changes in the cAMP concentration in the preoptic region of the rat

The concentration of adenosine 3',5'-cyclic monophosphate was measured, during the wake-sleep cycle, in the preoptic region and the cerebral cortex of rats kept in normal laboratory conditions (ambient temperature 22 +/- 0.5 degrees C, 12 h:12 h light-dark cycle) and, during wakefulness, in the preoptic region of rats exposed to extended light and dark periods (i.e. dark in the light hours of the normal photoperiod, and light in the dark hours of the normal photoperiod). The results show that the concentration of cAMP in the preoptic region changes according to the ultradian wake-sleep cyclic, decreasing from wakefulness, through synchronized sleep and to desynchronized sleep. This pattern of change was found to occur both in light and dark hours, however, in the dark hours the levels of preoptic cAMP are higher than those observed in the light hours. In contrast, no significant modification in cAMP concentration was found in the cerebral cortex. In the extended light and dark periods preoptic cAMP concentration increases above the levels found during wakefulness in normal photoperiods. These results show that preoptic cAMP concentration is influenced by ultradian and circadian factors which also appear to be related to sleep processes.

Relationship between cAMP concentration in anterior hypothalamic-preoptic region and the ultradian wake-sleep cycle

In the rat anterior hypothalamic-preoptic region adenosine 3':5'-cyclic monophosphate concentration changes during the ultradian wake-sleep cycle. The administration of DL-propranolol and the exposure to low ambient temperature decreased the nucleotide concentration and also modified the wake-sleep cycle. This suggests that in this region a biochemical correlation exists with different functional states.