Already a new paper to appear in PNAS is generating a buzz in the press.
The study is by Alfonso Caramazza and colleagues who used an fMRI adaptation paradigm. Adaptation was assessed both for observing (O) and then executing (E) actions and executing and then observing (as well as O-O and E-E conditions). Assessing adaption in both directions, E->O and O->E, is critical because (i) if mirror neurons exist, adaptation should occur in both situations, and (ii) adaptation in the case of observing and then executing could be interpreted as motor priming during the observation event. The critical result was that in the regions they examined, fMRI adaptation was found for E-E conditions, showing that there is coding of information relevant to action execution, and also in O-E conditions suggesting prima facie that action observation and action execution are activating the same set of neurons in the ROIs. However, E-O trials did not exhibit adaption, which they should have if in fact there is a shared substrate for observation and execution (it shouldn’t matter what the order of presentation), and neither did O-O trials indicating that the ROIs were not coding perceptually driven information. This pattern of results can be explained if the ROIs are coding action execution information (E-E adaptation) and if observing an action that one might have to execute can prime these action coding regions (O-E adaptation).
This is a significant advance over previous attempts to find adaptation effects in the human mirror system because clear evidence of adaptation was identified, ruling out a power issue, and because they assessed observation-execution adaptation in both directions. This allows the authors to conclude with some degree of confidence that the direct matching hypothesis is incorrect.
So could it be possible that mirror neurons don't exist in humans? I have said that such an outcome would be surprising. But this new result makes me wonder whether there might be something funky about the training situation of macaques in which mirror neurons have been found that lead to the development of neurons with mirror properties. In other words, do mirror neurons even exist naturally in monkeys? ...