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The 'Wug' test is more commonly used to test children's use of the plural which they master before the past tense. Both types of 'inflectional' morphemes, past tense and plural, are used creatively with novel words to produce inflected words which children could never have heard before.
In fact, the /ed/ morpheme occurs in three forms - /d/, /t/ or /^d/, as in ‘rammed’, ‘talked’ and ‘pitted’, respectively. These different forms are allomorphic variations of each other, and are conditioned by the process of voicing assimilation. Verb stems that end in a voiced phoneme take the voiced /d/ allomorph, unvoiced verbs stem endings take the unvoiced /t/ allomorph, and those ending in a dental consonant (/t/ or /d/) take the epenthesised allomorph /^d/. Voicing assimilation is itself a linguist rule that is well understood by phonologists.
The software/hardware distinction does not do away with the need to understand how symbolic systems are implemented in the human brain. In fact, it is likely that the brain is limited in the types of symbol systems it can use. Eventually, we need to understand how the structure of the brain leads to a symbolic capacity. This is often called the ‘symbol grounding problem’.

Symbols and Rules:

Cognitive psychology emerged in the 1950s as an important new approach to the study of the mind. Information theory had already had an important impact on theories of memory and attention. The idea that the mind is an information processing system akin to a computer programme began to take hold. In a neighbouring discipline, structural linguistics described language in terms of symbols (like nouns, adjectives, verbs and prepositions) manipulated by a set of rules, just like conventional computer programming languages. Cognitive psychologists viewed information processing in the mind in terms of the same kind of vocabulary that linguists use: The mind is a symbol manipulation machine, operating according to a well-defined set of rules. The goal of the cognitive psychologist was to discover the operating system of the mind. The scientific work involved specifying the exact nature of the symbols and rules involved.
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Noam Chomsky (b. 1928) played a key role in the 'cognitive revolution' of the 1950s and 60s after the publication of his ground breaking monograph Syntactic Structures (1957) which ushered in detailed experimental investigations of the mental structures involved in language processing. He is probably better known amongst psychologists for his damning rebuke, in 1959, of B. F. Skinner's Verbal Behavior, considered by many to 'nail the coffin' on behaviourism. He remains an influential player both in linguistic theory and political thinking.
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Dave Rumelhart, Jay McClelland and Geoff Hinton laid much of the foundational work for modern day connectionism in the mid 1980s.
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Performance on task A may be linearly related to the resources available. Task B may show a non-linear relationship. Assuming that an individual has a set amount of resources to devote to each of the 2 tasks, then their level of performance on each task is likely to be different. Moreover, since individuals (particularly, those suffering from brain damage) may have differing levels of resources available, then one patient may show better performance on task A than task B whilst another patient shows better performance on task B than task A. (After Shallice, 1988)
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Joanisse & Seidenberg's (1999) structured neural network model of English past tense formation
In fact, there is some dispute as to the identity of the default plural in German. For example, Hahn and Nakisa (2000) have argued that German speakers generalise to novel words and produce errors in a much wider variety of ways than other commentators (Marcus et al., 1995) have suggested.
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