Email updates

Keep up to date with the latest news and content from BMC Neuroscience and BioMed Central.

Open Access Highly Accessed Research article

Transient and sustained cortical activity elicited by connected speech of varying intelligibility

Hannu Tiitinen13*, Ismo Miettinen13, Paavo Alku2 and Patrick J C May13

Author affiliations

1 Department of Biomedical Engineering and Computational Science, Brain and Mind Laboratory, Aalto University School of Science, Espoo, Finland

2 Department of Signal Processing and Acoustics, Aalto University School of Electrical Engineering, Espoo, Finland

3 BioMag Laboratory, HUS Medical Imaging Center, Helsinki University Central Hospital, Helsinki, Finland

For all author emails, please log on.

Citation and License

BMC Neuroscience 2012, 13:157  doi:10.1186/1471-2202-13-157

Published: 31 December 2012

Abstract

Background

The robustness of speech perception in the face of acoustic variation is founded on the ability of the auditory system to integrate the acoustic features of speech and to segregate them from background noise. This auditory scene analysis process is facilitated by top-down mechanisms, such as recognition memory for speech content. However, the cortical processes underlying these facilitatory mechanisms remain unclear. The present magnetoencephalography (MEG) study examined how the activity of auditory cortical areas is modulated by acoustic degradation and intelligibility of connected speech. The experimental design allowed for the comparison of cortical activity patterns elicited by acoustically identical stimuli which were perceived as either intelligible or unintelligible.

Results

In the experiment, a set of sentences was presented to the subject in distorted, undistorted, and again in distorted form. The intervening exposure to undistorted versions of sentences rendered the initially unintelligible, distorted sentences intelligible, as evidenced by an increase from 30% to 80% in the proportion of sentences reported as intelligible. These perceptual changes were reflected in the activity of the auditory cortex, with the auditory N1m response (~100 ms) being more prominent for the distorted stimuli than for the intact ones. In the time range of auditory P2m response (>200 ms), auditory cortex as well as regions anterior and posterior to this area generated a stronger response to sentences which were intelligible than unintelligible. During the sustained field (>300 ms), stronger activity was elicited by degraded stimuli in auditory cortex and by intelligible sentences in areas posterior to auditory cortex.

Conclusions

The current findings suggest that the auditory system comprises bottom-up and top-down processes which are reflected in transient and sustained brain activity. It appears that analysis of acoustic features occurs during the first 100 ms, and sensitivity to speech intelligibility emerges in auditory cortex and surrounding areas from 200 ms onwards. The two processes are intertwined, with the activity of auditory cortical areas being modulated by top-down processes related to memory traces of speech and supporting speech intelligibility.

Keywords:
Acoustic distortion; Auditory evoked magnetic fields; Auditory cortex; Human; Intelligibility; Magnetoencephalography; N1m; P2m; Speech processing; Sustained field