Tuesday, July 23, 2013

Hogaboam and Perfetti: "Lexical Ambiguity and Sentence Comprehension" (1975)

Along with the 1979 paper by David Swinney, this paper by Thomas Hogaboam and Charles Perfetti seems to be one of the most cited early papers on the psychology of ambiguity resolution. It is notable for pointing out the prominent function of word sense frequency.

The Language Machine

In a distinctly 1950s cognitive psychology style, the paper contrasts a number of hypotheses about ambiguity resolution, each formulated as a little block of virtual computer code:
[According to the prior decision model] the processes that provide access to lexical items may operate in such a way as to provide access only to the contextually correct meaning. (p. 265)
In [the exhaustive computation model] both meanings of an ambiguous word are accessed and further processed to determine which meaning is appropriate to the context. (p. 265)
[The one meaning hypothesis] holds that one meaning is accessed and checked against the context. If a match occurs the other meanings are not accessed, but if a match does not occur the process is repeated until a match is found. (p. 265)
When the "one meaning hypothesis" makes the additional assumption that the meanings are looked up in decreasing order of frequency (rather than randomly), it is called the "ordered search hypothesis":
In the ordered search model when an ambiguous word occurs in a sentence, an ordered lexical search takes place. The order of the search is determined by frequency of usage of the lexical entries, the most frequent being first. The search is self-terminating, so that as soon as an acceptable match occurs, no other lower entries will be checked, and all higher entries will have already been checked. (p. 266)
Since the prior decision model and the random-search version of the one meaning hypothesis are quickly dispatched, this leaves only the exhaustive computation hypothesis and the ordered search hypothesis in the field.

The Reduced Field

Hogaboam and Perfetti do, however, consider the option that exhaustive computation may occur differentiated in time. This leads in practice to the following set of competing hypotheses (p. 272):

The two leftmost drawings represent an exhaustive and an ordered search, respectively. The rightmost panel represents a modified version of the exhaustive search:
According to this model a search initiated at a token node activates both senses, but the primary sense becomes activated prior to the secondary sense. […] That is, both senses could be processed in parallel, but the primary sense may take less to to process and would thus be available for other processes sooner than the secondary sense. (p. 272)
So while parallel processing and time differences are in principle conceivable in the algorithmic world of Hogaboam and Perfetti, differences in activation are not: You either move stuff from the disk drive to the working memory, or you don't.

Serial or Skewed Parallel?

Interestingly, they do raise the concern that all of this speculation about mental algorithms may multiply entities beyond necessity. Specifically, there is a risk that the models only differ as to whether they hypothesize a computation in the working memory or in the long-term storage:
That is, under the conceptualization represented by the panel on the right, an ordering effect would be expected at a working memory level. This possibility indicates that contrasting the various models of the disambiguation process in effect may be setting up straw men. (p. 272)
However, their experiment (discussed below) does show observable differences between words, and these have to be ascribed some cause or other. From the perspective of 1975 cognitive science, the safest best seems to be word senses queuing up for serial processing:
For the present it is most parsimonious to propose, as a hypothesis, that the effect is a true order-of-processing effect and not an artifact of parallel processing. (p. 272)
So while they do briefly flirt with the idea of differentiated activation in a parallel network, they quickly return home to the comfortable world of Turing machines plodding from discrete state to discrete state.

The Big Deal

The experiment itself is set up as follows: A tape recording of a sentence is played to you, and you then have to decide whether its last word is ambiguous or not. If it is ambiguous, you must come up with an example of a different sense the word can be used in.

Here are some example sentences that can illustrate the task:
  1. The antique typewriter was missing a letter.
  2. The anti-pollution campaign created interest.
  3. The gun collector displayed the arms.
  4. The tired hiker rested his feet.
If you're like the average test subject, you should find this task more difficult for sentences 2 and 4, but easier for sentences 1 and 3.

This is because the words in the difficult sentences are used in their dominant meaning; you thus have to retrieve a relatively rare word sense in order to come up with a response. For the easy sentences, however, the word is used in a less frequent sense, and you can cite the frequent and readily available word sense as a response.

As indicated above, there is a number of ways that one can interpret this result. However, it should be clear that at the very least, it demonstrates that frequency plays a key role in word comprehension.

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