Wednesday, March 25, 2009

Talking Brains is inching towards a useful medium! -- Comment from Luciano Fadiga

When David and I started this blog we hoped it would be a forum for discussion of issues in language science, that would include PIs as well as junior scientists. Up until recently when we seem to be getting an increase in commentary, it has been just the David and Greg show. In this light, I can't tell you how happy I am to hear from Luciano Fadiga, whose work has been a topic of this blog on occasion. I've had very interesting, open, and informative email discussion with Luciano, and now I'm thrilled to see he is willing to discuss some of these issues publicly so that we all can benefit.

Although we've never met personally, I can tell you that I like Luciano quite a lot and learn much from his insights and ideas. So welcome Luciano! By the way, you noted in your comment that the blog is unbalanced in that we get to post on the main page and others are relegated to the virtual basement a click, and maybe a scroll, below. When we started the blog we offered to post commentary from other investigators on the main level -- a kind of "from the lab of" feature -- but no one took us up on it. The offer still stands! So in that spirit, I'm re-posting Luciano's comment here. I will respond in the basement. :-)

Dear Greg,

I see that now we agree on almost everything!

1) We think that the addition of noise to speech is important to make the task difficult enough (now we know from a new experiment that this is definitely true and we are trying to understand whether it is a problem of ceiling effect or if it reflects other mechanisms).

2) We both think that the motor involvement may reflect an attentional-like mechanism (never heard the so called "premotor theory of attention"?).

3) We agree now on the fact that 80% of misunderstanding is a relevant deficit and not the proof that the lesion of Broca’s area has nothing to do with speech perception.

If you remember, however, the fact that Broca’s area plays a marginal role in phoneme discrimination is exactly the title of our contribution to the special issue on mirror neurons that you are editing now. I would say ‘surprisingly’ because I completely ignored your so deep anti-mirror position when I accepted to contribute to this issue.

However, here the weather is sunny, we are in the most beautiful country of the world, we are very happy of how things are going on, and now we are even happier because you agree with us!
Unfortunately, we don't have enough time now to go in depth in this debate (we are supposed to stay in the lab to make experiments to answer to yours - and ours - doubts) and, as you probably know, I am quite refractory to formulate “highly theoretical theories”. I have the impression that sometimes you make some confusion between what I write and what they write some much more intelligent and ‘multidisciplinary’ colleagues of mine.

I confirm, however, that my intention of the last five years was to investigate the CAUSAL role of the motor/premotor system in perception. Now we have interesting results, such as this on CB and (please, prepare a lot of space on your blog!) another paper coming soon, showing that frontal aphasics do have problems in pragmatically representing others’ actions. However, also if I had found the opposite of what I am finding, it would have been for me a precious information. My goal is to understand how the brain works by keeping a constant distinction between data and speculations.

What I definitely disagree is the fact that your posts on the blog are so evident and visible, while the comments from others can be seen only by clicking on a small link. But this is a secondary issue.
Have a nice day (I hope sunny as well, in California, surrounded by palm trees)!
Friendly yours,



Greg Hickok said...

Hi Luciano, it is sunny indeed -- a comfortable 74F in Irvine today.

Thank you very much for your post. If there is one question I could settle, it is the question of what you are actually proposing as the role of motor systems in speech perception. I have to say, given some of the wording in your papers, e.g., speech perception is "grounded" in motor circuits, I have interpreted your position as being essentially a motor theory of speech perception but without the speech-is-special assumption of the standard MT theory. But it seems based on your comment, that you don't believe this. At the same time, you don't seem to believe that the auditory system is where speech perception happens (modulated by the motor system, of course).

So how does speech recognition happen in the context of understanding a word? Here's some alternatives; let me know if any of these is close to your view.

1. Acoustic input generates a pattern of motor-speech activity in frontal motor-related cortices and this pattern of activity makes contact with a lexical-semantic representation. I would call this a motor theory in the sense of Liberman, in that the speech representations that allow for recognition are in motor speech systems.

2. Acoustic input generates a pattern of acoustic-speech activity in the temporal lobe(s) and this pattern of activity makes contact with lexical-semantic representations. The acoustic-speech activity can be modulated by input from the motor system in the form of forward models or something like it. I would call this an auditory theory of speech recognition. This is what I believe to be the case.

3. Acoustic input generates a pattern of activity both in acoustic-speech systems in the temporal lobe and in motor-speech systems in the frontal lobe and this JOINT activity makes contact with lexical-semantic representations. This is NOT a motor theory or a sensory theory since neither system has privileged access to the lexical-semantic system. Let's call this a sensory-motor theory.

If I can understand which of these is closer to your position, then we can start talking about some of the details.


Luciano Fadiga said...

Hi Greg,
I would add another possibility:

4. Normally, acoustic input generates a pattern of activity in the temporal lobe that may match or not with a predictive temporal modulation imposed by motor/premotor centers. Thus, understanding means matching. In the case of a sudden, unexpected, acoustic input, this is matched with its best “audiomotor” approximation which is the result of a joined fronto-temporal activity. The fact that we then report the percept in coincidence with the instant of the acoustic stimulation, does not mean that we really understood the stimulus at that time. Time means nothing in the brain as Libet (but before Ernst Mach) said. The problem that cooks the brain of several colleagues is the following: “speech is acoustic or motor?” I would say: “It is neither acoustic nor motor. It is “sensorimotor” a new tassonomic category that we are forced to consider if we take into account data from monkey electrophysiology (Fadiga et al. 2001 Int. J. Psychophysiol.) This view puts mirror neurons (which someone considers a miracle) within the more general category of sensorimotor neurons.

Sensorimotor neurons are so common in the brain, also in primary cortices, that we are forced to think that they represent something new with respect to the old three categories (sensory, motor and associative). One example? The visuomotor neurons located in FEF, responding (the same neuron) when the monkey makes a saccade towards a given location AND/OR when a spot of light is switched on in that same location. What they serve for? To orient visuospatial attention towards a given location by using the recurrent circuits linking parietal to frontal cortex. This is, in few words, what I think.

Thank you for the very nice hospitality you offered me in your blog.

Greg Hickok said...

Ok, getting closer, but my brain might still be a bit "cooked" about your perspective (I like that phrase!). So let me tell you what I think you are saying, and you tell me if I've got it right or not.

First, I agree completely that the old idea that there is a clear separation between sensory and motor systems is invalid. The two interact extensively and rapidly.

I also fully agree that mirror neurons fit with the more general category of sensorimotor neurons.

At the same time, I believe that the sensory systems alone are capable of generating sensory percepts (recognition of visual objects or the meaning of words); motor systems (as well as other sources of information) can modulate the operations of these sensory systems, but are not necessary for generating recognition.

It sounds like you believe the motor system IS necessary for recognition: "understanding means matching" between sensory input and motor predictions. Even when there is no motor prediction as in the case of a "sudden unexpected, acoustic input" this is still matched with the closest "audimotor approximation." So while the motor system is *critical* for recognition, your's is not a "motor theory" in the sense of Liberman, but as you state a "sensorimotor" theory.

Do I have it right? I hope so, because then we can start talking about ways to distinguish between a sensorimotor theory and an auditory theory.

For example, wouldn't you predict that without the availability of a motor system to match with (generate audiomotor fronto-temporal activity), you shouldn't be able to understand/recognize sounds? And what about sounds that have no motor correlate? Does your model only apply to sensory events that we can reproduce in the motor system?

Thanks much.