Wednesday, September 11, 2013

Gunter, Wagner, and Friederici: "Working Memory and Lexical Ambiguity Resolution as Revealed by ERPs" (2003)

When a word is unexpected in its context, it often elicits a measurable N400 response. This is the case even if the unexpected word is not impossible in the context, only surprising. An example of such a sentence might possibly be
  • The plants flourished in the spring water.
But such shifts between hypotheses are difficult, and it would be reasonable to expect individual differences between how good people are at making such retrospective reinterpretations.

Somewhat surprisingly, this paper by Thomas C. Gunter, Susanne Wagner, and Angela Friederici presents some evidence that seems to suggest that people who score high on working memory tests tend to be worse at this kind of switching than people who score low.

Half Full or Half Empty?

The authors interpret this finding as evidence that the process of disambiguation is a matter of inhibition as opposed to activation.

But this seems rather nonsensical; there is no real-world difference between having too many feet and having too few shoes. Activating meaning A could just as well be described as inhibiting meaning B, unless some more precise anatomical theory is intended, and that doesn't seem to be the case in this paper.

But it seems fair enough to say that the test subjects who find the switching difficult must in some sense have been more "successful" in selecting a single hypothesis and getting rid of all the others. As far as I can see, the data in the paper tells us nothing about what the process behind this polarization is at the neurological level, only that it correlates positively with large working memory spans.


The materials that Gunter, Wagner, and Friederici used consisted in sentences with meaning oscillations of the following four kinds:
The most difficult switch is the one where the evidence first favors the dominant (more frequent) meaning, and then afterwards the subordinate (less frequent). Those are the cases that provoke larger N400 responses in people with long working memory spans.

A graph showing the averaged ERPs of the low-memory group (top) and the high-memory group (bottom) immediately after reading the disambiguating final verb (from experiment 1, p. 647).

This is not a matter of timing, the authors argue. Even when they added several intervening words, the effect persisted (experiments 2 and 3). It is thus not that the subjects with short working memory spans were slower at making a decision, but rather that they never made up their mind quite as forcefully and irreversibly as the subjects with long memory spans.

The authors report having used 88 different items in the experiment (p. 653), but they only quote two: The clay/tone example above, and the sentence
  • Der Ball wurde vom Spieler/Tänzer geworfen/eröffnet
which they use to explain their experimental paradigm throughout the paper.

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