Tuesday, November 4, 2008

Mirror neurons in humans revealed by fMRI adaptation

Riitta Salmelin alerted me to this study which used an fMRI adaptation paradigm to identify mirror neurons in the human brain. Mirror neurons have previously been assumed to exist in humans, but without direct evidence. Here is the abstract for the paper, FYI:

Chong, T. T., R. Cunnington, et al. (2008). "FMRI adaptation reveals mirror neurons in human inferior parietal cortex." Curr Biol 18(20): 1576-80.
Mirror neurons, as originally described in the macaque, have two defining properties [1, 2]: They respond specifically to a particular action (e.g., bringing an object to the mouth), and they produce their action-specific responses independent of whether the monkey executes the action or passively observes a conspecific performing the same action. In humans, action observation and action execution engage a network of frontal, parietal, and temporal areas. However, it is unclear whether
these responses reflect the activity of a single population that represents both observed and executed actions in a common neural code or the activity of distinct but overlapping populations of exclusively perceptual and motor neurons [3]. Here, we used fMRI adaptation to show that the right inferior parietal lobe (IPL) responds independently to specific actions regardless of whether they are observed or executed. Specifically, responses in the right IPL were attenuated when participants observed a recently executed action relative to one that had not previously been performed. This adaptation across action and perception demonstrates that the right IPL responds selectively to the motoric and perceptual representations of actions and is the first evidence for a neural response in humans that shows both defining properties of mirror neurons

This is a very cool and cleverly designed study. Basically, they were looking for areas that showed adaptation (decreased BOLD amplitude) for observed actions that followed the same executed actions relative to observed actions that were not previously executed. Here is the result:

They observed adaption in one of their ROIs in the right parietal lobe (ROIs included IFG, IPL, and STS). If you buy the adaptation logic -- it seems reasonable to me -- this means that mirror neurons live in the right parietal lobe of humans. So we finally have some direct evidence for the existence of mirror neurons in humans. Cool. I knew someday we'd have decent evidence. It is surprising, though, that no mirror neurons were found in the frontal lobe or the left hemisphere (where damage can lead to disorders of action production and recognition), but let's not get bogged down in details.

A couple of points are relevant. One is that if this result holds, it means that human mirror neurons and monkey mirror neurons are different. Chong et al. used pantomimed gestures. Classic F5 mirror neurons don't respond to pantomime. In effect, we have a new animal that needs to be studied in its own right. Who knows, maybe the function of these human mirror neurons are completely different! Another relevant point is that just because some form of mirror neuron exists in humans doesn't mean that this system supports action understanding. The Chong et al. study has nothing to say about this question. So all previous critiques of the action understanding portion of the mirror neuron doctrine still hold.

T CHONG, R CUNNINGTON, M WILLIAMS, N KANWISHER, J MATTINGLEY (2008). fMRI Adaptation Reveals Mirror Neurons in Human Inferior Parietal Cortex Current Biology, 18 (20), 1576-1580 DOI: 10.1016/j.cub.2008.08.068


Greig said...

Very cool result if it holds. Interesting, though, how the adaptation effect observed was not in the anterior intraparietal sulcus (aIPS) where a host of previous studies had reported mirror neuron activity, as Trevor et al. note. I just wish these pesky little critters would decide where they are so we can test their language properties.

The Vlad said...

This smells fishy to me. In fact, the very notion of the 'mirror system' or 'mirror neurons' has no meaning to me. I think what's needed now is a clear computational statement about the functional properties of neurons encoding sensory, motor and sensory-motor information within a well-defined behavioral context.

tom said...

Interesting paper. fMRI is perfect for these adaptation/priming type of paradigms. The one thing about these results that worries me is the lack of an adaptation effect for the 'Observe/Execute' task. In the paper, the authors argue that this is because the SMG is a 'motor planning area' therefore any adaptation effects in this condition are 'offset' by increases in activation resulting from the covert motor planning that occurs during the 'Observe' part of the task when subjects know they will shortly be performing an action. To support this, they point to a table in the supplementary material that shows higher activation in 'motor preparation areas' (SMA, PMd, CMA)during the 'Observe' part of the 'Observe-Execute' condition relative to the 'Execute-Observe' condition (collapsed across the Novel/Repeated dimension. I don't find this a particularly convincing argument. Firstly because, weirdly, data from the right SMG is not shown on this graph, so we have no idea exactly what was happening there during observation. Secondly, even if the SMG is involved in motor planning, any planning related increases during the 'Observe' period should be common to both the Novel and Repeated 'Execute' stimuli - i.e. an adaptation effect should still be visible in the contrast of interest (Novel vs Repeated for Observe/Execute), if this is really a 'mirror' area.

Greg Hickok said...

Good point Tom, I hadn't thought that part of their argument through. Seems there are several funky things about the study. Having played with adaption effects in fMRI though, if there is one thing I've learned, it is that these effects are extremely finicky and not very strong. Adaptation could be occurring in other areas (IFG for example) and in other conditions, but weren't detected in this study. At the same time, if these effects are really that weak, I wonder how meaningful they are.

mrG said...

Since that post links to here, it seemed only fair that this post should link to there:

" in a piece published in Current Biology based on data published in the Journal of Neurosciences, I. Dinstein suggested that [mirror neurons] can hardly be found in humans, and J. Neurosciences editors seem to concur."

Stuart said...

Mirror neurons may be incredibly difficult to pinpoint specifically. There are several areas of the brain that they are thought to, or have been known to exist. However if the theory that epigenetics and neuroplasticity has a major role to play in development is correct, then it may be purely a random assignment of totipotent cells.

For example in the study by Neville & Lawson (1987) people who were born deaf had adapted the primary audio area of their brains so that instead of being redundant, the region became specialised in vision (particularly peripheral). This evidence of brain plasticity has been supported in pathological reports of young (and some older) stroke victims, and even more convincingly in contraversial rewiring ferrits experiments (Surr et al 2000).

If this is the case that the brain areas only develop and specialise as a result of the information being pumped into each region, then mirror neurons may be incredibly unreliable in location, which would explain why the exact location in which they reside remains ambiguous.