The results presented here argue against a mirror system dysfunction in autism. Individuals diagnosed with autism exhibited robust responses in commonly reported mirror system areas aIPS and vPM both during observation (, green) and execution (, blue) of hand movements, which were equivalent to those of the control group. More importantly, autistic subjects exhibited visual and motor adaptation in right and left aIPS, which were indistinguishable in magnitude from those of the control subjects (Figures and ). We interpret these visual and motor adaptation effects as evidence of distinct neural populations that respond selectively to particular preferred movements and that adapt (decrease their response) when a movement is repeatedly observed or executed (Grill-Spector and Malach, 2001
). Such responses would be expected from neural populations that respond selectively to particular movements, including mirror neurons. These experiments, therefore, targeted a key feature of movement perception not addressed by previous studies - the ability of neural populations in mirror system areas to differentiate between different hand movements. Distinguishing between movements is a critical step for effectively mapping an observed movement onto the specific motor neuron population that encodes its execution and for determining the correct interpretation of the observed person’s intentions as hypothesized by mirror system theories (Dinstein, 2008
; Dinstein et al., 2008
In a previous study, we found similar movement-selective visual and motor adaptation in areas vPM and aIPS of control subjects (Dinstein et al., 2007
). In that study, we asked subjects to play the rock-paper-scissors game against a video-taped opponent while freely choosing their executed movement on each trial. We compared repeated versus non-repeated observed and executed movements to assess visual and motor adaptation respectively. These adaptation effects were very similar in both distribution and amplitude to those reported here, albeit using a different experimental design. This replicability confirms that visual and motor adaptation is a robust and reproducible phenomenon across different experimental protocols and demonstrates its successful use in assessing response selectivity in a population of autistic subjects. Future use of fMRI adaptation protocols in autism research offers many possibilities for precise characterization of neural population selectivity in different cortical systems of individuals with autism.
Previous studies that have examined mirror system responses in ASD during observation, execution, and imitation of movements have yielded inconsistent findings. While some studies have reported that individuals with autism exhibit weak fMRI (Dapretto et al., 2006
), EEG (Martineau et al., 2008
; Oberman et al., 2005
), MEG (Nishitani et al., 2004
), and TMS induced corticospinal excitability (Theoret et al., 2005
) responses, other fMRI (Williams et al., 2006
), EEG (Oberman et al., 2008
; Raymaekers et al., 2009
) and MEG (Avikainen et al., 1999
) studies have reported that individuals with autism exhibit equivalent responses to those of controls. There are numerous methodological issues that could have led to the disparate reports cited above. For example, it is difficult to control the behavior of subjects in an MRI scanner. When subjects are asked to imitate a movement, delays in the timing or length of the movement may greatly impact the estimated resulting brain response (e.g., if autistic subjects always imitate the movement later/slower than controls, their estimated brain response will seem weaker). Rather than trying to reconcile the results above, we simply suggest that the fact that individuals with autism can exhibit equally strong mirror system responses to those of controls argues against the claim of a generally dysfunctional mirror system in autism.
A far stronger argument against a mirror system dysfunction in autism lies in the finding that individuals with autism exhibit equivalent movement-selective adaptation to that of controls. Previous mirror system studies have used several different experimental tasks to assess mirror system responses, including passive observation and active imitation protocols using meaningless hand movements, hand-object interactions, symbolic hand movements, or emotional facial expressions. These different tasks recruit numerous neural populations (in mirror system and other cortical areas) that might include mirror neurons, but also include many other neural populations involved in vision, motor planning, motor execution, working memory, and emotion. Mirror neurons make up only about 10% of the neurons that respond during movement observation or execution in monkey mirror system areas (Fogassi et al., 2005
; Gallese et al., 1996
; Kohler et al., 2002
; Umilta et al., 2001
). Current neuroimaging techniques (fMRI, EEG, and MEG) sum over the responses of millions of neurons, thereby making it difficult to discern which of the many overlapping neural populations generated the responses in the ASD and control groups. Because of this limitation, neither previous mirror system studies of autism nor the current adaptation study are capable of isolating the responses of mirror neurons alone. Nevertheless, by assessing visual and motor adaptation in mirror system areas, we have isolated the responses of movement-selective neural populations important for movement perception, rather than summing across the responses of other neural populations that co-exist in these areas (Dinstein, 2008
). If mirror system theories of movement perception are indeed correct, one would expect sub-populations of mirror neurons to be “tuned” to the movement they encode. This means that mirror system areas would be expected to contain circuits of visual, mirror, and motor neurons that would be intimately inter-connected by their selectivity/preference for a particular movement. The fact that these movement-selective neural circuits respond normally (adapt in a movement-selective manner) in individuals with autism suggests that the functional integrity of their mirror system areas is intact. Characterizing neural selectivity offers a far more detailed assessment of the mirror system’s functional integrity, which was not possible in previous fMRI studies that summed over the responses of all neural populations within these areas.
A further important test of mirror system integrity is cross-modal adaptation. Cross-modal fMRI adaptation has been reported in mirror system areas as subjects observe a movement they have just executed or execute a movement they have just observed (Chong et al., 2008
; Kilner et al., 2009
; Lingnau et al., 2009
). Such adaptation is a signature of mirror neuron populations responding repeatedly to their preferred movement regardless of whether it is being observed or executed. The current study was not designed to assess cross-modal adaptation although future studies could do so building on the results reported here.
In further analyses, we noticed that individual autistic subjects exhibited larger block-by-block response variability/unreliability than individual control subjects (, error bars). It is well known that different individuals with autism exhibit distinct and unique behavioral symptoms. Such behavioral variability may be expected to generate between-subject cortical response variability and, indeed, several studies have reported that brain responses during different motor and visual tasks are more variable across autistic individuals than across control individuals (Hasson et al., 2009
; Humphreys et al., 2008
; Muller et al., 2003
; Muller et al., 2001
). Here, however, we describe a different type of variability; variability in the brain responses of single subjects across different blocks of an experiment. This is a measure of the consistency or reliability of a single subject’s neural responses across different trials/blocks of the experiment (within-subject variability). Despite exhibiting equivalent cortical response amplitudes on average, individuals with autism exhibited significantly larger within-subject variability than controls in early visual and ventral premotor areas during movement observation and in several motor areas during movement execution (Figures and ). This difference in response variability was not due to a general difference in the hemodynamic response which was nearly identical in the two groups (Supplementary Figure 7
There may be several sources for the greater within-subject response variability found in the autism group. One possibility is that individuals with autism behave more variably (with less consistency) throughout an experiment than control subjects. For example, subjects with autism may have exhibited ”noisy” eye movements across blocks, which may have generated more variable visual system responses during the movement observation experiment. A more exciting (yet speculative) possibility is that larger within-subject response variability is a measure of increased neural “noise”, which may be a general characteristic of neural networks in autism. Several theories have proposed that ASD may be caused by early development of abnormally connected, “noisy”, and “hyper-plastic” cortical networks (Markram et al., 2007
; Rubenstein and Merzenich, 2003
) that are more prone to epilepsy; a common co-morbidity in autism (Tuchman and Rapin, 2002
). These theories suggest that noisy neural responses may cause the environment to be perceived as inconsistent and noisy, making it difficult for the child to cope with the outside world, and driving him/her to develop autistic behavioral symptoms in response. Further studies assessing within-subject response variability, while controlling for within-subject behavioral variability, across age, IQ, and gender matched subject groups are urgently needed to investigate this hypothesis.
Regardless of the source of fMRI response variability, our results clearly show that this variability is not equal across the two subject groups, as is commonly assumed when interpreting fMRI studies of autism. An implication of this difference in variability is that one should exercise caution when comparing activations using statistical parameter maps (SPM) across the two groups (as done in Figures and ). Differences in statistical significance (p values) may be caused by differences in either the average response amplitude or by differences in the variability of the response across trials/blocks. For example, observing a statistically significant “activation” in the control group SPM, which is absent in the autism group SPM, might not be due to a weaker response in the autism group. The responses might be of equal strength across groups, on average, but with larger variability in the autism group.
Finally, if the mirror system of ASD individuals responds in a normal movement-selective manner, why do these individuals have problems imitating and understanding the movements/intentions of others? First, it is unclear whether individuals with autism actually do have such behavioral impairments (Hamilton et al., 2007
). But even if we accept that they do, this question further assumes that our ability to imitate and understand one another socially is dependent only on the activity of mirror neurons. There is little evidence to support such an assumption (see (Dinstein et al., 2008
; Hickok, 2009
; Southgate and Hamilton, 2008
). Even in monkey studies, where mirror neurons have been successfully isolated (Fogassi et al., 2005
; Gallese et al., 1996
; Umilta et al., 2001
), there is no evidence for a causal relationship between mirror neuron activity and the ability of the monkey to understand the meaning of an observed movement. Proof of such a relationship would require showing that the removal (ablation, inactivation) of mirror neurons impairs the monkey’s ability to understand the meaning of observed movements. As yet, this experiment has not been performed. There is also no evidence for a connection between mirror neuron activity and imitation of movements. This issue has not been studied in monkeys although there have been reports that macaque monkeys do imitate, at least during infancy (Ferrari et al., 2006
). Numerous imaging studies have concluded that imitation and action understanding in humans are abilities that depend on mirror system responses. However, imaging studies do not test causality, but rather report brain responses that are associated with the performance of a particular task. Moreover, these studies clearly show that activities of numerous visual and motor neural populations (not just mirror system areas) are correlated with imitation and action understanding tasks. There is, therefore, no concrete evidence to suggest that a dysfunction in mirror neurons would cause impairments in imitation or understanding the intentions of others. Similarly, there is no reason to expect that individuals with difficulties imitating or understanding actions necessarily have dysfunctional mirror neurons, rather than dysfunctions in numerous other neural populations that play integral roles in these abilities.