title: Previous exposure to intact speech increases intelligibility of its digitally degraded counterpart as a function of stimulus complexity.
abstract: Recent studies have shown that acoustically distorted sentences can be perceived as either unintelligible or intelligible depending on whether one has previously been exposed to the undistorted, intelligible versions of the sentences. This allows studying processes specifically related to speech intelligibility since any change between the responses to the distorted stimuli before and after the presentation of their undistorted counterparts cannot be attributed to acoustic variability but, rather, to the successful mapping of sensory information onto memory representations. To estimate how the complexity of the message is reflected in speech comprehension, we applied this rapid change in perception to behavioral and magnetoencephalography (MEG) experiments using vowels, words and sentences. In the experiments, stimuli were initially presented to the subject in a distorted form, after which undistorted versions of the stimuli were presented. Finally, the original distorted stimuli were presented once more. The resulting increase in intelligibility observed for the second presentation of the distorted stimuli depended on the complexity of the stimulus: vowels remained unintelligible (behaviorally measured intelligibility 27%) whereas the intelligibility of the words increased from 19% to 45% and that of the sentences from 31% to 65%. This increase in the intelligibility of the degraded stimuli was reflected as an enhancement of activity in the auditory cortex and surrounding areas at early latencies of 130-160ms. In the same regions, increasing stimulus complexity attenuated mean currents at latencies of 130-160ms whereas at latencies of 200-270ms the mean currents increased. These modulations in cortical activity may reflect feedback from top-down mechanisms enhancing the extraction of information from speech. The behavioral results suggest that memory-driven expectancies can have a significant effect on speech comprehension, especially in acoustically adverse conditions where the bottom-up information is decreased.
authors.affiliation: Brain and Mind Laboratory, Department of Neuroscience and Biomedical Engineering (NBE), School of Science, Aalto University, PO Box 12200, FI-00076 AALTO, Finland; BioMag Laboratory, PO Box 340, FI-00029 HUS, Helsinki University Central Hospital, Finland. Electronic address: maria.hakonen@aalto.fi.
authors.forename: Maria
authors.initials: M
authors.lastname: Hakonen
authors.affiliation: Special Laboratory Non-Invasive Brain Imaging, Leibniz Institute for Neurobiology, Brenneckestraße 6, D-39118 Magdeburg, Germany.
authors.forename: Patrick J C
authors.initials: PJC
authors.lastname: May
authors.affiliation: Brain and Mind Laboratory, Department of Neuroscience and Biomedical Engineering (NBE), School of Science, Aalto University, PO Box 12200, FI-00076 AALTO, Finland.
authors.forename: Jussi
authors.initials: J
authors.lastname: Alho
authors.affiliation: Department of Signal Processing and Acoustics, School of Electrical Engineering, Aalto University, PO Box 13000, FI-00076 AALTO, Finland.
authors.forename: Paavo
authors.initials: P
authors.lastname: Alku
authors.affiliation: Department of Signal Processing and Acoustics, School of Electrical Engineering, Aalto University, PO Box 13000, FI-00076 AALTO, Finland.
authors.forename: Emma
authors.initials: E
authors.lastname: Jokinen
authors.affiliation: Brain and Mind Laboratory, Department of Neuroscience and Biomedical Engineering (NBE), School of Science, Aalto University, PO Box 12200, FI-00076 AALTO, Finland.
authors.forename: Iiro P
authors.initials: IP
authors.lastname: Jääskeläinen
authors.affiliation: Brain and Mind Laboratory, Department of Neuroscience and Biomedical Engineering (NBE), School of Science, Aalto University, PO Box 12200, FI-00076 AALTO, Finland; BioMag Laboratory, PO Box 340, FI-00029 HUS, Helsinki University Central Hospital, Finland; Department of Signal Processing and Acoustics, School of Electrical Engineering, Aalto University, PO Box 13000, FI-00076 AALTO, Finland.
authors.forename: Hannu
authors.initials: H
authors.lastname: Tiitinen
mesh_terms: D000161:Acoustic Stimulation; D000328:Adult; D001921:Brain / Q000502:physiology*; D032882:Comprehension / Q000502:physiology*; D005260:Female; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D012815:Signal Processing, Computer-Assisted; D013065:Speech Intelligibility / Q000502:physiology; D013067:Speech Perception / Q000502:physiology*; D055815:Young Adult
keywords: Acoustic distortion; Auditory neuroscience; Comprehension; Intelligibility; Magnetoencephalography; Speech