BOTTOM-UP PROCESSING

An approach to the processing of spoken or written language which depends upon actual evidence in the speech signal or on the page. Smaller units of analysis are built into progressively larger ones. There is a contrast with top-down processing, the use of conceptual knowledge to inform or to reshape what is observed perceptually. The terms ‘bottom-up’ and ‘top-down’ are derived from computer science, where they refer respectively to processes that are data-driven and processes that are knowledge-driven.

Underlying the metaphors ‘top’ and ‘bottom’ is the idea that listening and reading proceed through levels of processing, with bottom-up information from the signal assembled into units of ever increasing size. In listening, the lowest level (i.e. the smallest unit) is the phonetic feature. The listener’s task might be portrayed as combining groups of features into phonemes, phonemes into syllables, syllables into words, words into clauses and clauses into propositions. At the ‘top’ is the global meaning of the utterance, into which new information is integrated as it emerges.

The truth is more complex. First, it is not certain that bottom-up processing involves all the levels described. Some researchers have argued that we process speech into syllables without passing through a phonemic level; others that we construct words directly from phonetic features. Nor does bottom-up processing deal with one level at a time. There is evidence that in listening it takes place at a delay of only a quarter of a second behind the speaker– which implies that the tasks of analysing the phonetic signal, identifying words and assembling sentences must all be going on in parallel.

A quarter of a second is roughly the length of an English syllable so the listener often begins the processing of a word before the speaker has finished saying it. Part of bottom-up processing therefore involves the listener forming hypotheses as to the identity of the word being uttered, which are activated to different degrees according to how closely they match the signal. The candidates compete with each other until, when the evidence is complete, one of them outstrips the rest. An important issue is the extent to which top-down evidence (from world knowledge or from knowledge of the text so far) can contribute to the activation of these word candidates.

 Non-psychological accounts sometimes refer to a conflict between ‘bottom-up models’ of reading and ‘top-down models’. This is misleading, as it implies that a choice has to be made. The issue is not to argue the case for one processing type over another, but to establish how the two interact and which one predominates in case of conflict.

The evidence is contradictory. Some commentators would say that top-down information is only used for checking bottom-up; some argue for bottom-up priority with contextual evidence only invoked once sufficient bottom-up evidence has become available. Those who favour a fully interactive model of listening or reading contend that both sources of evidence are available throughout. One argument for relying initially on bottom-up information is that bottom-up processing is more automatised than top-down, and therefore faster. Another is that multiple sources of information prevent rapid decision-making. Conversely, those who favour an interactive model argue that it is better to have all the information available at one time.

 Some commentators prefer to describe the processing of the letters, sounds and words of a message as lower-level processing (the opposite being higher-level). It is also referred to as perceptual processing (as against conceptual processing).

See also: Interactive activation, Interactive compensatory hypothesis, Modularity2, Reading: decoding, Speech perception, Top-down processing

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