This is a continuation of my commentary on the recently published Mirror Neuron Forum. The start of this thread can be found here.
I have suggested that the ventral stream supports action understanding. Here is Gallese’s response to that idea:
“GH’s statement that action understanding is a function ofthe ventral semantic or “what” stream can also be questioned. Where is the“what” of action in the ventral stream? Perhaps, GH would argue, it can be found in the STS. However, no evidence supports this argument…”
Sticking to the monkey data, let me quote from Rizzolatti and Craighero (2004):
“Neurons responding to the observation of actions done by others are present not only in area F5. A region in which neurons with these properties have been described is the cortex of the superior temporal sulcus (STS; Figure 1) (Perrett et al. 1989, 1990; Jellema et al. 2000; see Jellema etal. 2002). Movements effective in eliciting neuron responses in this region are walking, turning the head, bending the torso, and moving the arms. A small set of STS neurons discharge also during the observation of goal-directed hand movements (Perrett et al. 1990). If one compares the functional properties ofSTS and F5 neurons, … [the] STS appears to code a much larger number of movements than F5…” (p. 171).
This evidence is no less correlative than the evidence from F5 but it is evidence nonetheless, in contrast to VG’s statement, and no less valid than the correlative evidence from F5. Further, given that STS units appear to code a wider range of movements and do it with more specificity than mirror neurons, it is clear that the circumstantial evidence for STS being the seat of action understanding is stronger than that for the mirror system.
“and in contrast current evidence demonstrates quite the opposite. As recently shown by Cattaneo et al. (2010), only the motor system—and not the STS—can generalize a given motor goal independently from the effector accomplishing it.”
Now Gallese slips out of the monkey literature and into the human literature where according to VG we have no reason to assume that a “neural mechanism found in different species must necessarily preserve identical characteristics”. In any case, let’s consider this study, which involved stimulating the hand area of motor cortex and measuring TMS induced motor evoked potential (MEPs) in a hand muscle while subjects watched videos of actions. The main finding was that the goal of the action, rather than the particular movements themselves, modulated MEPs during observation. Does this mean that the motor system is “generalizing the goal”? No. Nor does this experiment even address that question. Notice that TMS was not applied to the mirror system (Broca’s area or PPC) or to the STS (the systems in question). It was applied to the output portion of the cortical machinery, M1. We can infer that something modulated M1, but we can’t tell what the source of that modulation is; it could be any network upstream to M1. So it could be that the abstract goal was coded in STS and this ultimately modulated M1. It could also be that Broca’s area coded the abstract goal. We simply can’t tell. Certainly we cannot conclude from this study that, “only the motor system—and not the STS—can generalize a given motor goal…”
So that’s the end of Gallese’s response to my answer toquestion #1. I see nothing in his rebuttal that carries even a hint of theoretical or empirical weight.
Cattaneo, L., Caruana, F., Jezzini, A., & Rizzolatti,G. (2009). Representation of goal and movements without overt motor behavior inthe human motor cortex: a transcranial magnetic stimulation study. Journal ofNeuroscience, 29(36), 11134-11138.
Rizzolatti, G., &Craighero, L. (2004). The mirror-neuron system. Annual Review of Neuroscience,27, 169-192.
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