The electrophysiological correlates of sublexical speech perception: A combined electrophysiological/cognitive neuropsychology approach
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1
Rice University, Department of Psychology, United States
Electrophysiology provides a window into neural processes of speech perception as they unfold over time. Previous research has focused on the N100 component which has been assumed to index sublexical speech processing (Toscano et al., 2010). While it has been demonstrated that the N100 is sensitive to phonetic information (i.e. voice onset time or VOT), experiments with healthy participants alone cannot determine whether the N100 is indexing sublexical processing proper or indexing more general acoustic processes that are correlated with later sublexical speech processing stages. Accessing a phonological representation from an acoustic input depends on a series of processes that overlap in time. The electrical activity from any of these component processing steps could be generating the N100. To address this limitation of previous research, we investigate N100 effects in individuals with speech perception deficits that can be isolated to sublexical processing. If the N100 is indexing sublexical processing, then we would expect abnormalities in the N100 component in these individuals. If the N100 is indexing a process that precedes sublexical processing, then the N100 in these individuals should be normal. By combining electrophysiological and cognitive neuropsychological methods, we can better link ERP components to specific cognitive processes.
Specifically, we investigated the N100 in a single individual (FW) with an acquired speech perception deficit following focal brain damage to the left superior temporal lobe. FW’s stroke did not impair his hearing but left him extremely impaired on several measures of speech perception (single word-single picture matching, consonant and vowel discrimination, auditory lexical decision). FW also shows a relative preservation of non-phonological acoustic processing, suggesting that his speech perception deficit can be isolated to a language-specific level of sublexical processing. He and 21 healthy control participants had EEG continuously recorded while they underwent a categorical perception experiment with four 9-step VOT continua. All control participants showed categorical perception in this task, while FW’s categorization curve was essentially flat, indicating at chance discrimination of phonemes at all VOTs. Strikingly, FW’s N100 waveform shows sensitivity to VOT consistent with what is observed with control participants from this study and consistent with what has previously been reported in the N100 literature. However, starting around 150ms post-stimulus onset, his electrophysiological response starts to track what he reports hearing rather than the acoustic properties of the stimulus itself. The dissociation of an intact N100 with impaired phoneme perception suggests the N100 is generated by acoustic representations that correlate with VOT and precede sublexical processing, but not by the sublexical processes impaired in FW. The fact that his EEG waveform starts to track his eventual response, which is uncorrelated with acoustic properties of the stimulus, around 150ms, suggests that sublexical categorization is beginning at this later time point. Our study demonstrates that the N100 reflects early encoding of phonetic cues, possibly even before language specific representations are accessed. Thus, electrophysiological studies of patients with selective language impairments can be used to inform which cognitive processes are indexed by ERP components.
Acknowledgements
This research was supported by an NSF IGERT fellowship to C.N.
References
Toscano, J. C., McMurray, B., Dennhardt, J., & Luck, S. J. (2010). Continuous perception and graded categorization: Electrophysiological evidence for a linear relationship between the acoustic signal and perceptual encoding of speech. Psychological Science, 21(10), 1532-1540.
Keywords:
Speech Perception,
Sublexical processing,
N100,
EEG/ERP,
Aphasia
Conference:
Academy of Aphasia 56th Annual Meeting, Montreal, Canada, 21 Oct - 23 Oct, 2018.
Presentation Type:
oral presentation
Topic:
Eligible for a student award
Citation:
Noe
CM and
Fischer-Baum
S
(2019). The electrophysiological correlates of sublexical speech perception: A combined electrophysiological/cognitive neuropsychology approach.
Conference Abstract:
Academy of Aphasia 56th Annual Meeting.
doi: 10.3389/conf.fnhum.2018.228.00106
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Received:
01 May 2018;
Published Online:
27 Sep 2019.
*
Correspondence:
Mr. Colin M Noe, Rice University, Department of Psychology, Houston, United States, colinmichaelnoe@gmail.com