Abstract
BackgroundRecent 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.ResultsWe 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.ConclusionsWe 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.
Highlights
Recent studies have shown that the human right-hemispheric auditory cortex is 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)
The results show that degradation of both speech and non-speech sounds amplifies the auditory evoked field (AEF) bilaterally, indicating that the auditory cortices of both hemispheres are highly sensitive to distortion
The data presented here suggest that the activation of human auditory cortex as indexed by the amplitude of the AEFs is highly dependent on acoustic degradation of speech and non-speech sounds
Summary
Recent studies have shown that the human right-hemispheric auditory cortex is 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). Speech perception is an intricate process exceptionally resilient to distortions of almost any kind, whether occurring in natural environments or caused by manipulation of particular stimulus features in laboratory conditions This extraordinary robustness enables successful communication under acoustically adverse conditions. The basis of the robustness of human speech perception lies in the fact that there does not seem to be a single indispensable property within the acoustic signal upon which the entire recognition process relies. This view is supported by a wealth of behavioral research aimed at delineating the boundary conditions of speech intelligibility. Saberi and Perrott [6] demonstrated that continuous speech is highly resistant to time reversal of local segments up to a duration of 50 ms, which indicates that a detailed analysis of the shortterm acoustic spectrum is not essential for speech comprehensibility
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