A causal test of the motor theory of speech perception: a case of impaired speech production and spared speech perception

Abstract

The debate about the causal role of the motor system in speech perception has been reignited by demonstrations that motor processes are engaged during the processing of speech sounds. Here, we evaluate which aspects of auditory speech processing are affected, and which are not, in a stroke patient with dysfunction of the speech motor system. We found that the patient showed a normal phonemic categorical boundary when discriminating two non-words that differ by a minimal pair (e.g., ADA–AGA). However, using the same stimuli, the patient was unable to identify or label the non-word stimuli (using a button-press response). A control task showed that he could identify speech sounds by speaker gender, ruling out a general labelling impairment. These data suggest that while the motor system is not causally involved in perception of the speech signal, it may be used when other cues (e.g., meaning, context) are not available.

Keywords:

• apraxia of speech
• categorical perception
• motor theory of speech perception
• mirror neurons

Acknowledgements

The authors would like to thank AD for his dedicated participation in the study, without whom this research would not have been possible. They also would like to thank Stephen McAleavey and Gretchen Schlansker for help with ultrasound imaging.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. For this analysis, picture-naming accuracy was calculated as the number of correct words produced by the patient out of the 80 Snodgrass and Vandewart pictures. A similar analysis was carried out for spoken word repetition, assessed by the PALPA subtest “Syllable Length Repetition”. Note that no age-matched control data were available.

2. Eleven tokens were selected from the original set of 20, and were spaced two steps apart in the acoustic continuum, with the exception of the last two tokens (abd-19 and abd-20) which were one step apart.

Additional information

Funding

This research was supported by the National Science Foundation [grant 1349042] and National Institute of Health [grant NS076176] to B.Z.M. F.E.G. was supported by a University of Rochester Center for Visual Science pre-doctoral training fellowship [NIH training Grant 5T32EY007125-24]. This research was also supported, in part, by Norman and Arlene Leenhouts.