Monday, October 19, 2009

What's fundamental about the motor system's role in speech perception? Two surprises from the NLC debate

Far from being extremists in radically supporting the original formulation of the motor theory of speech perception, our hypothesis is that the motor system provides fundamental information to perceptual processing of speech sounds and that this contribution becomes fundamental to focus attention on others’ speech, particularly under adverse listening conditions or when coping with degraded stimuli. -Fadigo, NLC abstracts, 2009

Two surprises from the NLC debate between myself and Luciano Fadiga.

1. After reading his talk abstract and talking to him before the session, I thought he was going to agree that motor information at best can modulate auditory speech processing. Instead, he strongly defended a "fundamental" role for the motor system in the processing of speech sounds.

2. A majority of his arguments were not based on speech perception but came from data regarding the role of frontal systems in word-level processing ("in Broca's area there are only words"), comprehension of action semantics, syntactic processing ("Broca's region is an 'action syntax' area"), and action sequence processing.

I was expecting a more coherent argument.

The very first question during the discussion period was from someone (don't know who) who defended Luciano saying something to the effect that of course the auditory system is involved but it doesn't mean that the motor system is not fundamental. I again pointed to the large literature indicating that you don't need a motor system to perceive speech and this argues against some fundamental process. This in turn prompted the questioner to raise the dreaded Mork and Mindy argument -- something about how Mork sits by putting his head on the couch and that we understand this to be sitting but know it is not correct... I, of course, was completely defenseless and conceded my case immediately.

But seriously, when confronted with evidence that damage to the motor system doesn't produce the expected perceptional deficits, or that we can understanding actions that we cannot produce with our own motor system, it is a common strategy among mirror neuron theorists to retreat to the claim that of course many areas are involved (can you see the hands waving?). You see this all over the place in Rizzolatti's writings for example. But somehow only the motor system's involvement is "fundamental" or provides "grounding" to these perceptual processes:

“speech comprehension is grounded in motor circuits…”
-D’Ausilio, … Fadiga et al. 2009

So here is a question I would like to pose to Fadiga (or any of his co-authors):
Is speech perception grounded in auditory circuits?


Bill Idsardi said...

Overall, I find "grounded" just too wishy-washy a term. As you said in your remarks, they start out with a strong position, but their back-off position doesn't even seem testable.

Marc Sato said...

Hi Greg,

I think you extrapolate Luciano's view on the role of the motor system in speech perception.

Although I worked two years in Rizzolatti's group and I was always fascinated/impressed by the work done in Parma, I fully agree with you that mirror neurons in humans (or, as would better say Luciano, the action-observation matching system) cannot explain everything.

I also agree that there are many issues that have to be better/deeply discuss, rather than only arguing for a possible 'causal', 'mediating' 'essential' role of the motor system in speech perception. Mainly, the fact that speech perception is preserved following lesions to frontal-motor regions. Note that I would not say, as in your talk, 'phonemic perception' - see e.g. Blumstein et al., 1977, Basso et al., 1977, Miceli et al., 1980; Caplan et al., 1995 as quoted by Stephen Wilson and Marco Iacoboni. And the fact that speech perception is a complex process, involving multi-step, multi-process tasks (e.g., see our tms study with Vince Gracco and Pascale Tremblay: Brain Lang. 2009 Oct;111(1):1-7).

But Luciano's view (together with those of Leïla and Alessandro) on speech perception are far from being so extremist as what you imply in your blog (and perhaps as what you think). To convince you, I invite you to read a short paper (published in a French linguistic journal, without impact factor ;-) ) we wrote with Luciano and Jean-Luc Schwartz:

As quoted in the final discussion of this paper : "Altogether, and whatever the position, there seems to be indeed a common language of perception and action, shaping speech communication and human language. ... Progress in most dimensions of speech
research, including perception, production, development, phylogeny, and technology, should derive from this fascinating perspective".
Of course this quote is simple or even naive, but I really think with many colleagues that sensorimotor interactions in speech perception ARE fascinating perspectives which deserves more work, more debate (here, thanks to you and David Poeppel for your blog!). And, surely, more collaborative projects and scientific but friendly discussions...

Another point, my own view is that the motor system is likely to be involved in some (late) phonetic-decision or -categorization stage (especially under adverse listening conditions, e.g. acoustic noise, foreign language or incongruent AV speech inputs).
Apparently, you prefer the term: 'response selection'. But, don't you think this process is not an integral part of speech perception?

And to finish with a joke, a final quote from your own paper with Kayoko Okada on auditory-visual speech perception:
'This shows that visual speech activates sensory–motor integration networks and therefore is consistent with the view that visual speech can influence heard speech via this system, perhaps in the
form of efferent copies of motor predictions'.
But, correct me if I am wrong, motor-to-sensory discharges in speech perception means some prior sensory-to-motor projections? So, do you mean that, at least, AV speech perception is partly 'grounded' in motor circuits ;-) ?



Greg Hickok said...

Bill: that's the problem isn't it? "Grounded", "fundamental", "basis of", "critical"... all undefined and therefore meaningless. I thought it was very funny when Luciano said something like, "I never said 'basis of', just 'grounded'" -- as if either term has any meaning. :-)

Marc: I enjoy your comments. It is fair to say that my critiques go beyond Luciano's position. But then I'm arguing against a set of theoretical proposals and not specifically Fadiga.

I would question whether mirror neurons in humans can explain *anything* nevermind everything. How can you make any explanatory conclusions regarding a class of cells that whose functional properties have never been assessed?

I would not try to defend my use of the term "phonemic perception" -- point well taken.

I don't doubt that Luciano's beliefs are less extreme than the position I'm arguing strongly against. However, publishing statements like "the ultimate constituents of speech are not sounds but articulatory gestures" can mislead a lot of young scientists (and those outside of the field) who don't know the subtleties or long history of the debate. Because of this, it is important to hold people accountable not only for what they believe, but what they choose to put in the literature.

I agree, we should spend more time studying sensory-motor integration in speech. This is a major component of my own research program. But because I try to understand the role of sensory systems in speech production rather than the reverse, my papers get published in JoCN and J. Neurophys. and not Neuron and Current Biology.

Let me comment though, on this statement from your paper: "there seems to be indeed a common language of perception and action". I think you are muddling perception for the control of action (where there has to be "parity" and a common "language") and perception for understanding which involves a different set of computations. This is where most people are mislead on these issues. If your measures of speech "perception" invoke motor processes, it is no surprise that the motor system appears to be involved in speech "perception".

I would agree with you that processing speech in noise is probably more then normal than under sound booth conditions. However, if you are referring to phonetic categorization as it is implemented in syllable discrimination or identification tasks, then I disagree: these tasks involve processes that are not involved in ecologically valid conditions.

Finally, thanks for checking out out poster and for actually reading it! Yes, I believe there are strong sensory-motor connections that operate in both directions. AV speech, I believe, is mostly a sensory-sensory phenomenon with some minor contribution from the motor system.