Thursday, April 16, 2009

Broca's area: It's a dessert topping! No it's a floor wax! No it's a cognitive control mechanism!

Debates over the function of Broca's area remind me of the old Saturday Night Live skit where a husband (Dan Aykroyd) and wife (Gilda Radner) are arguing about whether a product, "New Shimmer" is a dessert topping or a floor wax:

Wife: New Shimmer is a floor wax!
Husband: No, new Shimmer is a dessert topping!
Wife: It's a floor wax!
Husband: It's a dessert topping!
Wife: It's a floor wax, I'm telling you!

The spokesman (Chevy Chase) quickly enters at this point and says:
Hey, hey, hey, calm down, you two. New Shimmer is both a floor wax and a dessert topping!

With respect to Broca's area we're in the middle of an even more complicated argument.
Grodzinsky: It's syntactic movement!
Friederici: It's a hierarchical structure processor!
Rogalsky/Hickok: It's articulatory rehearsal (at least in the back part)!
Rizzolatti/Fadiga: It's action understanding!

To this argument we can add another view; one that doesn't get talked about as much.
Novick, Trueswell, Thompson-Schill: It's a cognitive control mechanism!

This is an interesting claim and I wonder to what extent "cognitive control" may be our Chevy Chase, telling us that Broca's area can do more than one thing.
Of course, I think at least some of these territorial claims to Broca's area will be resolved simply by mapping these various functions (within subjects) to the different subregions that comprise the foot of the third frontal convolution. But we won't worry about those details at this point. For now let's just see what Novick et al. suggest.

First, Novick et al. are addressing the claim that Broca's area is specifically involved in syntactic computation or in the temporary storage of syntactic information. They are not trying to address the many other claims. This is fine, but eventually we'll have to deal with the full range of data.

Now, what do they mean by cognitive control. Well, basically it is a mechanism for conflict resolution. Ok, what's that? The define conflict as:
...cases in which an individual receives incompatible information either about how best to characterize a stimulus or how best to respond to that stimulus. p. 265

They mention the Stroop task as a classic example. So cognitive control in this context would be the process of shifting attention toward task-relevant stimulus characteristics in order to override automatically generated but currently irrelevant representations (paraphrased from page 265).
What's the link to Broca's area? Well, incongruent trials on Stroop tasks activate Broca's area as does the processing or gardenpath sentences which requires resolution of syntactic ambiguity (conflict). Lesion and individual difference data are also presented along these lines. A subsequent empirical study by this group found that Stroop tasks and syntactic ambiguity resolution co-localize in Broca's area (January, Trueswell, & Thompson-Schill, in press, Journal of Cognitive Neuroscience).

I don't think necessarily that this hypothesis is going to solve the problem of what Broca's area is doing -- it's not going to be that simple. For example, it doesn't explain why portions of Broca's area activates during simple articulatory rehearsal. But there is enough evidence that this sort of claim needs to be included in the discussion.
Importantly, mechanisms such as this need to be considered when discussing the role of "motor areas" -- Broca's being a centerpiece in the mirror neuron "motor system" -- in speech perception. I suggested in my critique of D'Ausilio et al.'s paper that motor speech systems may influence perception in the following way:

motor and perceptual information may converge on higher-order executive processes where this information is used to color decision-making processes

This may be particularly relevant in situations where the speech stimuli are ambiguous, as in the noise degraded stimuli of D'Ausilio et al.
They responded to this suggestion by saying that this...
interpretation reminds [us of] 18-19th century models of the human mind, in the sense that requires an additional functional module

I'm no expert but my guess is that folks who study decision making for a living might argue (convincingly) that such as system is needed on independent grounds. In any case, the point is that "Broca's area" may be involved in certain functions that could be considered "executive" and one shouldn't be to hasty to ascribe a motor explanation to everything that happens in Broca's area.

NOVICK, J., TRUESWELL, J., & THOMPSON-SCHILL, S. (2005). Cognitive control and parsing: Reexamining the role of Broca's area in sentence comprehension Cognitive, Affective, & Behavioral Neuroscience, 5 (3), 263-281 DOI: 10.3758/CABN.5.3.263


Bill Idsardi said...

I've used that phrase in class and lab meetings ("it's a dessert topping and a floor wax!") only to get uncomprehending looks. It's nice to know somebody else really likes those early SNL skits.

gcogan said...

I think there's another function for Broca's Area that we're overlooking here. This is how Thomas Edison thought of memory according to a diary entry:

"We do not remember. A certain group of our little people do this for us. They live in the part of the brain which has become known as the ‘fold of Broca‘… There may be twelve of fifteen shifts that change about and are on duty at different times like men in a factory…. Therefore it seems likely that remembering a thing is all a matter of getting in touch with the shift that was on duty when the recording was done.”

Shall we call it the Edison-tiny person-shift-memory hypothesis?