The right-hemispheric auditory cortex in humans is sensitive to degraded speech sounds

We investigated how degraded speech sounds activate the auditory cortices of the left and right hemisphere. To degrade the stimuli, we introduce uniform scalar quantization, a controlled and replicable manipulation, not used before, in cognitive neuroscience. Three Finnish vowels (/a/, /e/ and /u/) were used as stimuli for 10 participants in magnetoencephalography registrations. Compared with the original vowel sounds, the degraded sounds increased the amplitude of the right-hemispheric N1m without affecting the latency whereas the amplitude and latency of the N1m in the left hemisphere remained unaffected. Although the participants were able to identify the stimuli correctly, the increased degradation led to increased reaction times which correlated positively with the N1m amplitude. Thus, the auditory cortex of right hemisphere might be particularly involved in processing degraded speech and possibly compensates for the poor signal quality by increasing its activity.