SIMATIC WINCC

This chapter briefly reviews the main findings of lexical ambiguity research which sheds further light on the relationship of different processing systems involved in sentence processing. Overall, the findings indicate that all word senses of a homonym (such as ’bug’, ’port’) are activated fast and independently of contextual and syntactic factors, which seems to be relevant in the light of claims that semantic information is not processed fast enough to be available to the parser. Processing lexical ambiguities has often been compared to processing syntactic ambiguity, especially in order to undermine the claim that maintaining multiple outputs of a parser are too cost­

intensive in terms of short-term memory load. The findings that multiple meanings of lexically ambiguous words are output supported the view that "multiple outputs of modules do not necessarily lead to significantly greater processing load" (Tanenhaus, Carlson & Seidenberg, 1985, p. 375). However, as Frazier & Rayner (1987) point out, the similarities in processing lexical and syntactic ambiguities might be limited, given that the former involves accessing prestored representations, whereas the latter requires the computation of multiple representations.

In lexical ambiguity research, the semantic priming effect is used as the main tool to detect which meaning or meanings of the ambiguous word are activated. Subjects have to perform a lexical decision or a naming task on a word related to one of the

meanings of the ambiguous word (eg. after the ambiguous word ’bug’, either ’ant’ or ’spy’ is presented). Performance on this word should be facilitated due to priming from the activated meaning/s of the ambiguous word. This is usually studied with a cross-modal paradigm, in which the context sentence and the ambiguous noun are presented auditorily and the word for the naming or lexical decision task is presented visually. This technique was pioneered by Conrad (1974), Swinney (1979) and Tanenhaus, Leiman & Seidenberg (1979). These studies presuppose that there is semantic priming, and focus on the investigation of whether or not sentential context can function as a sort of prime, in the sense that a sentence which is related to one of

the meanings of the ambiguous noun might facilitate access to this meaning of the ambiguous word. Since the focus of these studies is on context effects, not much attention is given to the semantic priming effect itself, which here merely serves as a welcome instrument to measure meaning activation of the ambiguous noun.

Diagram 3.1. shows in summary form the entities involved in ambiguity studies.

lexical decision or naming task

I I sentence .... ambiguous noun . TARGET ... sentence ..

D iagram 3.1: Schematic representation of entities involved in lexical ambiuguity studies

The timing of presenting the target is crucial. There is no debate that after some time following the presentation of the ambiguous noun only the contextually-appropriate meaning is activated (and performance is facilitated only on the target related to that meaning). Controversy exists regarding the nature of meaning activation prior to this point in time: is there multiple or selective (autonomous or context-dependent) activation of the meaning of homonyms?

3.3.2. M ultiple access hypothesis

A number of studies converge in their findings that all senses are activated rapidly for a short period. Apart from the speed of sense activation, the fact that context does not affect the initial access is regarded as evidence for a modular view of sense activation. Neither the syntactic structure of a sentence preceding the prime (Seidenberg,

Tanenhaus, Leiman & Bienkowski, 1982; Tanenhaus, Leiman & Seidenberg, 1979) nor the previous textual context (Swinney, 1979) led to the selection of only the

contextually appropriate sense. Onifer and Swinney (1981) showed that even the less dominant meaning of an ambiguous noun is initially activated despite a context which was biased towards the more dominant meaning. There is some variation in the details about the time when context begins to have an influence. Swinney (1979) showed that context has an effect after 750 ms or less after the occurrence of the ambiguous noun: at that time, only the contextually appropriate meaning is activated. Till, Mross & Kintsch (1988) found that the inappropriate sense becomes deactivated after 400 msec. In the Tanenhaus et al. study (1979), a syntactic bias (such as the sentence ’John began to watch’) led to the selective facilitation of the correct target (’look’ as opposed to ’time’) after an initial period of 200 ms, during which both targets were equally facilitated.

Gemsbacher and Faust (1991, G & F for short) replicated Swinney’s classic fmding with a different technique, using a within-modality priming paradigm. The subjects’ primary task was to read short sentences, presented one word at a time, with a 150 ms gap between each word. The words were presented in the centre of a VDU screen. 150 ms after the prime word disappeared, the target word was presented in capital letters at the top of the screen. The subject had been instructed to make a rapid lexical decision for a word occurring in this position. After the lexical decision was made, the

remaining words of the sentence were presented in the same way as the words before the prime word. Two testing points were introduced, one immediate and one delayed, which were created by varying the presentation rate of the sentences. At the fast rate, a 5 letter word appeared for 233 ms, and at the slow rate, for 700 ms. In the first

experiment reported in G & F (1991), the prime words were ambiguous because they could be read as nouns or verbs (eg. punch). In the second experiment, the prime word was ambiguous since it had two possible noun readings (eg. quack). The results are the same in both experiments: they replicate the classic findings, i.e. at the immediate test point, both the contextually appropriate and the inappropriate meanings were activated, whereas at the delayed test point, only the contextually appropriate one was activated. G & F also attempted to evaluate different mechanisms put forward to explain how the

inappropriate meanings become less activated after the delay. One such mechanism is ’compensatory inhibition’, i.e. the suggestion that multiple meanings share a fixed amount of activation, and that at the late point in processing, when activation for the appropriate meaning is increased, activation for the inappropriate one has to decrease because there is only this limited, fixed amount of activation available. Other

mechanisms involve the decay due to lack of stimulation, and suppression from the activated meaning. They tried to decide between these proposals in their second experiment. This time, the prime was a noun which was ambiguous due to the two possible noun readings it has (eg. ’quack’). Primes were part of a sentence which was biased to one or the other of the meanings of quack (eg. ’Pam was diagnosed by a quack...’ and ’Pam heard a sound like a quack...’). In the Neutral Condition, the preceding sentence did not bias either reading of the noun (eg. ’Pam was annoyed by the quack...’). The target words for this example were ’doctor’ and ’duck’. The control condition against which facilitation was compared was the RT to ’duck’ or ’doctor’ following an unrelated prime (eg. ’pupil’), (the same Control procedure had been used in Experiment I). The crucial manipulation in experiment 2 is the Neutral Sentence Condition (sentences are not biased to either meaning of the ambiguous word). How activated will the meanings of the ambiguous word be at the late test? The two explanations of the decrease in activation of the inappropriate meaning at the late test point make different predictions about the neutral sentences. According to the decay proposal, inappropriate meanings decay because they do not receive any stimulation from the sentence context. In the neutral sentences, neither meaning of the prime word receives any contextual stimulation: appropriate and inappropriate meanings should both decay and be less activated at the late test point. This does not seem to be a very likely suggestion because it entails that context is necessarv to maintain activation of meaning. It would seem to be very inefficient of a system to allow for such rapid decay of the meaning of a particular word due to lack of support from the previous context since it would give up information that might be very valuable in processing the very next word. On the other hand, according to the suppression hypothesis, inappropriate meanings are less activated because the context suppresses their

activation. For the crucial test case of the Neutral Sentence Condition, this implies the prediction that appropriate and inappropriate meanings should remain as activated as

they were at the early test point since the neutral context does not suppress either meaning. G & F’s data clearly show that in neutral sentences in the late condition both meanings of the ambiguous noun are activated. Thus the suppression hypothesis is supported: context suppresses the inappropriate meaning after a short delay.

As a further, stronger test of the suppression hypothesis one could introduce an Anomalous Condition, in which the sentential context rules out both meanings of the prime (eg. Tam was eaten by the quack’). The prediction would be that at the early test point, lexical decision on both ’doctor’ and ’duck’ would be facilitated relative to a control condition (since ’quack’ produced facilitation in G & F’s Neutral Sentences). At the late test point, no facilitation should be evidenced: context should have

suppressed activation for either meaning of ’quack’.