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Alan Welsh coauthored, with William D. Marslen-Wilson, a 1978 experimental study of spoken word recognition in continuous speech. The experiments compared a shadowing task, in which listeners repeated speech, with a mispronunciation detection task performed while following a written text.^[ref]

Where an altered pronunciation occurred within a word affected performance, while context influenced the tasks differently. Comparing tasks helped separate an effect of incoming sound from an effect of instructions or expectations. The work contributed to the cohort model, which treats recognition as progressive evaluation of word candidates rather than waiting for an entire utterance.^[ref]

A cohort model explains spoken word recognition through an evolving set of candidate words, the lexical cohort. An initial stretch of speech activates plausible candidates; later acoustic evidence and, depending on the model version, contextual information constrain recognition. William D. Marslen-Wilson and Alan Welsh's 1978 study connected this account with experimental manipulations of continuous speech.^[ref]

For an illustrative English vocabulary, the opening sounds of “candle”, “candy” and “canister” initially overlap. Hearing the next consonant can remove “canister” while leaving the other two plausible; later sounds separate them. The candidates depend on pronunciation and the listener's vocabulary, not merely on matching written letters.

A deliberately simplified noise-free rule isHere $C_t$ is the candidate set after the evidence available at step $t$. This filtering illustration is not a fitted psychological model: noisy speech, graded activation and recovery from misleading evidence require additional assumptions. A uniqueness point concerns when the remaining acoustic prefix distinguishes a word within a specified vocabulary; it need not coincide with the listener's measured response time.

The original task comparisons motivate testing a model on intermediate behaviour and contextual effects, not just final accuracy.^[ref]

By William D. Marslen-Wilson and Alan Welsh. Cognitive Psychology 10(1), 29–63 (January 1978). DOI doi.org/10.1016/0010-0285(78)90018-X

Abstract: shadowing and mispronunciation-detection tasks; syllable-position and contextual-constraint effects, and a cohort account of continuous spoken-word recognition.

A shadowing task asks a participant to repeat incoming speech aloud while continuing to listen. The delay and accuracy of repetition provide behavioural measures of processing an unfolding speech stream. The task differs from repeating a whole sentence only after the speaker has finished.

William D. Marslen-Wilson and Alan Welsh used shadowing with altered pronunciations in continuous speech. Whether listeners repeated a changed form or fluently restored the expected word depended on aspects of the input and context. Comparison with a mispronunciation detection task helped show that task instructions change which influences appear in the measured response.^[ref]

A restoration is an observation about the spoken response. It does not alone establish that a listener consciously heard the original sound correctly, nor that every intervening processing stage had the same representation. This makes shadowing useful for testing the time course of spoken word recognition while keeping response production part of the explanation.

William D. Marslen-Wilson investigated the time course of spoken word recognition. Work with Alan Welsh helped develop the cohort model: early sounds make several words plausible, and subsequent evidence constrains that lexical cohort.^[ref]

Their experiments manipulated pronunciation within continuous speech rather than presenting only isolated written words. This made the timing of incoming information part of the question. A model can explain a listener's eventual correct answer yet still fail to explain how context and partial speech influence the response while the word unfolds.^[ref]