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

Abstract

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. 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.

Results: We 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.

Conclusions: We propose that the increased activity of AEFs reflects 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.

Figure 1

The effect of stimulus manipulation using uniform scalar quantization. Left column (A-F): the 16-bit and 1-bit waveform of each stimulus type. Right column (G-L): the power spectrum of 16-bit and 1-bit mode of each stimulus type.

Figure 2

The effect of stimulus degradation on the auditory evoked fields. Grand-averaged waveforms for all stimuli from MEG gradiometer channels with maximum response amplitude in the left and right hemisphere.

Figure 3

The effect of stimulus degradation on the N1m ECD amplitude and latency. The effects of bit mode on the average ECD amplitude and latency of N1m for each stimulus category (/a/: vowel /a/; SC: sine-wave composite; SW: sine-wave). Error bars indicate SEM.

Figure 4

The N1m ECD location. Mean N1m ECD location (± SEM) for all subjects and stimuli in the left and right temporal plane (/a/: vowel /a/; SC: sine-wave composite; SW: sine-wave).

Figure 5

The effects of stimulus degradation on the P1m, N1m and P2m amplitudes and latencies. The effects of bit mode on the average P1m, N1m and P2m amplitudes and latencies for each stimulus category (/a/: vowel /a/; SC: sine-wave composite; SW: sine-wave). Error bars indicate SEM.

Figure 6

The behavioral recognition task. Left: Grand-averaged reaction times for each stimulus and bit mode. Right: Grand-averaged identification accuracy for each stimulus and bit mode. (/a/, /e/, /i/, /o/, /u/: vowel to be recognized; SC: sine-wave composite; SW: sine-wave). Error bars indicate SEM.