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

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.

Cortical processing of degraded speech sounds: effects of distortion type and continuity.

Human speech perception is highly resilient to acoustic distortions. In addition to distortions from external sound sources, degradation of the acoustic structure of the sound itself can substantially reduce the intelligibility of speech. The degradation of the internal structure of speech happens, for example, when the digital representation of the signal is impoverished by reducing its amplitude resolution.

Responsiveness of the human auditory cortex to degraded speech sounds: reduction of amplitude resolution vs. additive noise.

The cortical mechanisms underlying human speech perception in acoustically adverse conditions remain largely unknown. Besides distortions from external sources, degradation of the acoustic structure of the sound itself poses further demands on perceptual mechanisms. We conducted a magnetoencephalography (MEG) study to reveal whether the perceptual differences between these distortions are reflected in cortically generated auditory evoked fields (AEFs).

Temporal integration of vowel periodicity in the auditory cortex.

Cortical sensitivity to the periodicity of speech sounds has been evidenced by larger, more anterior responses to periodic than to aperiodic vowels in several non-invasive studies of the human brain. The current study investigated the temporal integration underlying the cortical sensitivity to speech periodicity by studying the increase in periodicity-specific cortical activation with growing stimulus duration. Periodicity-specific activation was estimated from magnetoencephalography as the differences between the N1m responses elicited by periodic and aperiodic vowel stimuli.

Sensitivity of the human auditory cortex to acoustic degradation of speech and non-speech sounds.

Background: Recent studies have shown that the human right-hemispheric auditory cortex is particularly sensitive to reduction in sound quality, with an increase in distortion resulting in an amplification of the auditory N1m response measured in the magnetoencephalography (MEG). Here, we examined whether this sensitivity is specific to the processing of acoustic properties of speech or whether it can be observed also in the processing of sounds with a simple spectral structure. We degraded speech stimuli (vowel /a/), complex non-speech stimuli (a composite of five sinusoidals), and sinusoidal tones by decreasing the amplitude resolution of the signal waveform.

Asymmetrical representation of auditory space in human cortex.

Recent single-neuron recordings in monkeys and magnetoencephalography (MEG) data on humans suggest that auditory space is represented in cortex as a population rate code whereby spatial receptive fields are wide and centered at locations to the far left or right of the subject. To explore the details of this code in the human brain, we conducted an MEG study utilizing realistic spatial sound stimuli presented in a stimulus-specific adaptation paradigm. In this paradigm, the spatial selectivity of cortical neurons is measured as the effect the location of a preceding adaptor has on the response to a subsequent probe sound.