Perceptual decisions involve two processes, perceptual analysis and response selection, both of which can affect typical behavioral measures such as percent correct or reaction time. Crucially, response selection is strongly affected by response bias, the criterion that a given subject sets for responding one way or another under a particular set of task conditions. For example, if I ask you to tell me when you hear the syllable /ba/ in a noisy environment, and I tell you that I will give you $100 each time you say "yes" correctly (a hit), but will charge you $500 each time you say "yes" incorrectly (a false positive), you will be biased to be very conservative in saying "yes". I can push around your bias depending on how I set up the task. Presumably, brain damage or temporary modulation of neural activity (e.g., TMS) could also, in principle, change the task conditions for the subject and bias responses, so we have to be equally careful when we manipulate the system neurophysiologically.
Happily, there is a well-worked out method for measuring the perceptual and decision components of the task. It's called signal detection theory. Yet most speech/language neuroscientist fail to use SDT methods to control for response bias. The results of such studies are potentially contaminated.
For example, this is true of ALL of the TMS studies of the role of the motor system in speech perception/recognition. And it is true of the latest study in this line of experiments. I've pasted the abstract in below.
This study stimulated with theta-burst TMS the hand motor area and showed a change in response times in lexical decision to hand action verbs but not non-action verbs. They conclude that "premotor cortex has a functional role in action-language understanding."
Here's why you can't conclude this from the study: the dependent measure, RT, is susceptible to response bias and therefore can be modulated either by perceptual/recognition processes (the process under investigation) OR by biasing the decision process. The result is therefore ambiguous and as a result cannot lead to the conclusion the authors wish to make.