The relatively clear description of a monkey ‘mirror system’ composed of two cortical areas that contain mirror neurons has, unfortunately, morphed into a rather vague concept in the search for an equivalent human ‘mirror system’. During the past ten years, dozens of studies have used different techniques, including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), electroencephalography (EEG), magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS), in an attempt to identify a human ‘mirror system’ homologue. We shall focus here mainly on the relevant fMRI studies, but note that studies using other techniques have adopted similar experimental protocols with essentially the same underlying logic and assumptions. In general, these studies have used three types of protocols to elicit mirror neuron responses in humans: passive movement observation, separate observation, and execution of movement, and imitation of movement. In the first protocol, subjects passively view images or video clips of movements, such as a smiling face or a hand grasping an object, and their fMRI responses are compared against a rest condition, following the logic that mirror neurons are active during movement observation and not during rest. In the second protocol, a movement execution condition is added to first isolate cortical areas that respond during execution; fMRI responses during movement observation are then analyzed only within these areas because mirror neurons are expected to respond both during observation and execution of a movement. In the third protocol, subjects passively observe movements, execute the same movements in the dark, or imitate the observed movements (that is, simultaneously observe and execute the movement). The fMRI responses during the imitation condition are compared with responses during observation and during execution with the logic that mirror neurons should be more active during simultaneous observation and execution than during execution or observation alone [5
There are two concerns with these protocols. The first is that they are unable to measure exclusive mirror neuron activity. For instance, the typical results of passive movement observation and imitation experiments reveal many cortical areas that exhibit larger fMRI responses during observation and imitation, including areas that are not believed to contain mirror neurons; primary visual cortex for example. This is clear evidence that there are many other neurons (in addition to mirror neurons) in diverse cortical areas that increase their responses during these two tasks. These neurons are likely involved in processes of visual recognition, visual motion perception, working memory, movement planning, and movement execution (in the case of imitation), which are all integral components of these tasks. Limiting the analysis to cortical areas that also respond during movement execution (by masking out areas that do not) does not solve this problem. Although this protocol identifies cortical areas that respond during both movement execution and observation, it does not isolate mirror neuron responses from the activity of other (possibly intermingled) visual, motor, and visuomotor neural populations that could underlie the measured fMRI responses.
How then can one know if the fMRI response exhibited by a particular brain area is generated by the activity of mirror neurons or by the activity of any of these other neural populations? Most studies have simply disregarded activity in all cortical areas except for ventral premotor (vPM) and anterior intraparietal sulcus (aIPS), because these two areas are assumed to be homologous to monkey areas F5 and PF/IPL and are, therefore, expected to contain mirror neurons. Using such circular reasoning, these studies have sidestepped the most important issue, which is to examine whether human mirror neurons actually exist and to characterize their physiology. This circular interpretation has been taken to such an extreme that some recent studies now interpret any fMRI response in areas vPM and aIPS as being due to mirror neuron activity. Such interpretations grossly ignore the fact that mirror neurons in the monkey account for only a small minority of the neurons in these areas and that the reported fMRI responses could easily be generated by activity of the many neighboring visual, motor, and visuomotor neurons that are not mirror neurons. The widespread cortical responses generated by the movement observation and imitation tasks have also created a vagueness regarding the exact location of the implicated vPM and aIPS areas, whose reported locations vary dramatically among different studies. For instance, the exact location of the implicated vPM area differs by up to 3 cm from one fMRI study to another (see Table 1 in [11
The second, and perhaps more important, concern with these studies lies in their lack of ability to assess movement selectivity. As mentioned above, movement selectivity is a defining physiological signature of mirror neurons in the monkey, and is of central importance for theories proposing that mirror neurons play a role in mapping perception to action. If mirror neurons indeed form a ‘dictionary’ of movements or movement goals, subpopulations of mirror neurons must respond selectively to particular movements or goals. Several ‘mirror system’ studies have attempted to assess selectivity by comparing fMRI responses to observed movements performed by different effectors (the foot, hand, and mouth). These studies have suggested that mirror neurons are distributed according to the classical somatotopic ‘homunculus’ organization in premotor and anterior parietal cortical areas [12
]. Similarly, several TMS studies have reported that primary motor cortex excitability is enhanced in an effector specific manner [14
]. Note, however, that these studies suffer from the same drawbacks described above; specifically, that it is unclear whether these somatotopically organized fMRI responses are due to mirror neuron activity or to that of other neural populations. Regardless, testing selectivity at the level of effectors is at a much grosser level of resolution than that of movements or movement goals, which is the level of resolution needed to support mirror system theories.
Assessing neural selectivity using non-invasive techniques in the human brain is difficult in situations where neurons with different preferences may be intermingled within a small volume of tissue (as seems to be the case in vPM and aIPS). Specifically, any particular fMRI voxel within these areas (usually 3 × 3 × 3 mm in size) will contain subpopulations of neurons selective for many different movements and will, therefore, respond when executing or observing many different movements. Showing an overall stronger fMRI response to one condition versus another (for example, to movements with ‘goals’ versus
movements without ‘goals’) does not mean that the underlying neurons are selective (for example, for a particular goal). Such an overall response difference could easily be generated by modulation of the whole neural population within each voxel by processes of attention, arousal, emotional valence and so on, regardless of whether these neurons are selective for movements/goals or not. By contrast, a selective increase in response of one subpopulation of neurons might be complemented by a decrease in the responses of other subpopulations, resulting in no change in the overall level of activity. This distinction between an overall increase in activity and a selective response by a subpopulation of neurons is well-understood in sensory systems [15
]. Using imitation and movement observation protocols to look for overall increases in fMRI responses cannot, therefore, be used to test theories regarding the function of human mirror neurons.
The critical challenge in studying the human mirror system is to devise new experimental protocols that can assess response selectivity to movements in the human brain. A common method for assessing neural selectivity using fMRI takes advantage of the fact that sensory neurons adapt/habituate when their preferred stimulus is presented repeatedly [16
]. Cortical areas containing neurons selective for a particular stimulus attribute are, therefore, expected to exhibit reduced fMRI responses when the preferred stimulus is repeated in comparison to when it is not repeated. This method can be applied to localize cortical areas that exhibit adaptation when the same movement is repeatedly observed, repeatedly executed, observed and then executed, or executed and then observed (cross-modal adaptation). In this way, one can assess the actual defining feature of mirror neurons: movement selectivity for observed and executed movements.
Three recent fMRI studies [11
] have used such ‘adaptation protocols’ to assess movement selectivity in the human brain. Two of these studies [17
] focused on movement observation and showed that several parietal areas exhibited reduced responses to movement repetition. One study [17
] attributed this adaptation to the goal of the observed movements (for example, grasp a cookie versus grasp a floppy disk), while the other [18
] attributed it to the identity of the movement (the type of grasp being observed). In the third study [11
], some of us attempted to isolate mirror neuron responses using a combined visual and motor adaptation protocol. We succeeded in finding five movement selective cortical areas that exhibited adaptation both when the same movement was observed repeatedly and when it was executed repeatedly. Unlike the dispersed imitation responses, our adaptation responses were limited to the anterior inferior frontal sulcus, ventral premotor, anterior intraparietal, superior intraparietal, and posterior intraparietal cortices. We suggest that all five areas should be considered as candidates for the human mirror system because they contained neurons selective for both observed and executed movements.
Despite our claim that the adaptation protocol is a superior way of identifying candidate mirror system areas, however, we still were unable to demonstrate the existence of mirror neurons in the human brain as we did not find any cortical areas exhibiting cross-modal adaptation [11
]. Such adaptation, in trials where the same movement was observed and then executed or executed and then observed, would have provided strong evidence that visual and motor adaptation were taking place in a single subpopulation of visuomotor mirror neurons. It is possible that the overlapping visual and motor adaptation effects that we did find were generated by two separate (possibly intermingled) subpopulations of visual and motor neurons that adapted independently during repeated observation and execution of the movements. Nonetheless, these results show for the first time that five specific areas of human cortex contain movement selective neurons both for observation and execution. If mirror neurons exist at all in the human brain, it is likely that they lie within these areas. We hope that future human mirror system studies use similar and novel protocols for assessing movement selective responses rather than relying on the circular reasoning commonly used to interpret imitation and passive movement observation experiment results.