Tuesday, January 8, 2008

Semantics and Brain course - 1st meeting summary

Our Winter quarter started this week, and the first meeting of our grad seminar on Semantics and the Brain was yesterday. In addition to a number of fabulous grad students, we have the participation of several faculty members, including Kent Johnson (whose got a very cool "world clock" on his homepage), and new UCI Cog Sci faculty, Lisa Pearl and Jon Sprouse.

Here are the high points of our discussion:

1. Semantics is complicated. Ok, not the deepest of insights, but it's worth acknowledging. The neuroscience community (Talking Brains included!) is notorious for oversimplifying "semantics." It is not hard to find references to terms such as "semantic knowledge" as if it were a single unitary thing. David and I talk about "mapping between sound and meaning" as if this were a simple computational task. In fact "mapping between sound and meaning" encompasses pretty much the totality of the field of linguistics. A good reminder of how little we know!

2. Semantics is complicated. Here's some of the ways:

(i) The meaning of words (lexical concepts) is hard (impossible) to define in terms of necessary and sufficient conditions. This is true even for concrete objects. What's a "chair"? Can't be defined by shape (think, beanbag), can't be defined a something you can sit on (think, swing or horse).

(ii) The meaning behind verbs includes a lot of structure (i.e., argument structure). Sleep requires that there be a sleeper, kick requires that there be a kicker and a kickee, and put requires that there be a putter, a thing that is put, and a place that the thing is put. The "arguments" that a verb takes can themselves vary in complexity. For example, you can kick a football, but you can't think a football, you have to think something more complex, I though ABOUT footballs, etc.

(iii) Word meanings are likely decomposable. To get an intuitive sense of this, consider the difference in meaning between walk, strut, run, swagger, and stagger. All contain a bit of meaning roughly equal to ambulate, but differ in the manner of ambulation. So we might characterize these words as ambulate+manner. (The semantics literature is full of arguments for decomposition of meaning.)

(iv) There may be multiple dimensions of meaning. For example, "The BEAR chased the lion" (there are multiple possible chasers) means something different than "The bear chased the LION" (there are multiple possible chasees), even though the propositional structure is the same.

We could go on to discuss quantifier scope, fuzzy boundaries and family resemblance, part-whole relations, reference transfer ("Chomsky is on the shelf next to Plato" means "the book by Chomsky..."), etc.

The point is that when us neuroscience types think we've identified the neural substrate of "semantics" we are barely scratching the surface, AND we're probably wrong about what we've scratched. Consider the claim that the meaning of action words is encoded in motor cortex (e.g., Hauk et al. 2004). Just because motor cortex for the hand area lights up when people read the word throw, this does not mean that this chunk of tissue is coding anywhere near the depth of the meaning of throw.

3. Semantics is complicated. When thinking about the neural basis of "semantics" we have to be careful about which aspect(s) of semantics we are talking about.

Hauk O, Johnsrude I, Pulvermüller F.
Somatotopic representation of action words in human motor and premotor cortex.
Neuron. 2004 Jan 22;41(2):301-7.

No comments: