How’s Your Auditory Cortex Upon Degradation of Sound?

The research by Miettinen et. Al (2010) examined whether sensitivity of the right-hemispheric auditory cortex 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. The stimuli used were a Finnish vowel /a/, a complex non-speech sound that is a combination of sine wave tones, and a sine wave tone. The researchers degraded speech by decreasing the amplitude resolution of the signal waveform. The amplitude resolution was impoverished by reducing the number of bits to represent the signal samples. Auditory evoked magnetic fields (AEFs) were measured in the left and right hemisphere of sixteen healthy subjects. AEFs were recorded in a magnetically shielded room. Unrestricted equivalent current dipoles (ECD) were determined using planar gradiometers over each temporal region of the left and right hemispheres.

The auditory N1m’s amplitude, latency, and source location were analyzed using the repeated-measures analyses of variance (ANOVA) for the effects of hemisphere, stimulus type, and bit mode. The reaction times and identification accuracy for the behavioral experiment were analyzed with ANOVA for effects of stimulus type and bit mode. In all post-hoc comparisons, the Newman-Keuls was used. The behavioral experiment involved the ability of the participants to identify whether the degraded stimuli were vowel or not. These included /a/, /e/, /i/, /o/, /u/.

The researchers found that the AEF amplitudes increased significantly with stimulus distortion for all stimulus types, which indicates that the right-hemispheric N1m sensitivity is not related exclusively to degradation of acoustic properties of speech. In addition, the P1m and P2m responses were amplified with increasing distortion similarly in both hemispheres. The AEF latencies were not systematically affected by the distortion.

Data suggested that the activation of the human auditory cortex is greatly affected by the acoustic degradation of speech and non-speech sounds. The reduction of the stimulus waveform amplitude resolution resulted in amplified AEFs non-systematic responses in latency. The N1m amplitudes were asymmetric because there was a great difference between the left and the right hemispheres.

The researchers proposed that the increased activity of AEFs reflected cortical processing of acoustic properties common to both speech and non-speech stimuli. More specifically, the enhancement is most likely caused by spectral changes brought about by the decrease of amplitude resolution, in particular the introduction of periodic, signal-dependent distortion to the original sound. Converging evidence suggests that the observed AEF amplification could reflect cortical sensitivity to periodic sounds (Miettinen et al. , 2010)

The article is highly technical that would not be understood by a person who has no expertise in the field of perception and the human auditory system.


Source: Miettinen, I. et al. (2010). Sensitivity of the human auditory cortex to acoustic degradation of speech and non-speech sounds. BMC Neuroscience. Pp. 11-24 retrieved from ESCOHOST