Facilitatory and interfering effects of neighbourhood density on speech production: Evidence from aphasic errors

Abstract

In a system where tens of thousands of words are made up of a limited number of phonemes, many words are bound to sound alike. This similarity of the words in the lexicon as characterized by phonological neighbourhood density (PhND) has been shown to affect speed and accuracy of word comprehension and production. Whereas there is a consensus about the interfering nature of neighbourhood effects in comprehension, the language production literature offers a more contradictory picture with mainly facilitatory but also interfering effects reported on word production. Here we report both of these two types of effects in the same study. Multiple regression mixed models analyses were conducted on PhND effects on errors produced in a naming task by a group of 21 participants with aphasia. These participants produced more formal errors (interfering effect) for words in dense phonological neighbourhoods, but produced fewer nonwords and semantic errors (a facilitatory effect) with increasing density. In order to investigate the nature of these opposite effects of PhND, we further analysed a subset of formal errors and nonword errors by distinguishing errors differing on a single phoneme from the target (corresponding to the definition of phonological neighbours) from those differing on two or more phonemes. This analysis confirmed that only formal errors that were phonological neighbours of the target increased in dense neighbourhoods, while all other errors decreased. Based on additional observations favouring a lexical origin of these formal errors (they exceeded the probability of producing a real-word error by chance, were of a higher frequency, and preserved the grammatical category of the targets), we suggest that the interfering effect of PhND is due to competition between lexical neighbours and target words in dense neighbourhoods.

Keywords::

• Speech production
• Lexicon
• Speech errors
• Phonological neighbourhood
• Aphasia.

ACKNOWLEDGEMENTS

This research was supported by the Swiss National Science Foundation [grant number PP001-118969 and grant number PP00P1_ 140796] to Marina Laganaro.

Notes

1 See, for instance, the following definitions of formal errors: “matched minimally on one phoneme occurring in the same syllable and word position or two phonemes occurring in any position” (Gordon, 2002); “A single phoneme, with the exception of schwa, shared at any position in the target and response” (Best, 1996); “one of the following criteria . . . (1) overlap of first or last phonemes, (2) overlap of at least two phonemes in any syllable or word position, or (3) overlap of at least one phoneme in the same syllable and word positions, aligning words left to right” (Gagnon et al., 1997).

2 Among the different neighbourhood measures, Levenshtein's measure correlated with the neighbourhood measure following Luce and Pisoni (r = −.762 on monosyllabic items and r = .−892 on disyllabic); we decided to use the latter measure whose operationalization is more straightforward (high values corresponding to dense neighbourhood); the number of phonological neighbours sharing the first phoneme and number of neighbours of higher frequency also had high correlations (above .620) with the neighbourhood measure.