Mismatch brain response to speech sound changes in rats

Abstract

Understanding speech is based on neural representations of individual speech sounds. In humans, such representations are capable of supporting an automatic and memory-based mechanism for auditory change detection, as reflected by the mismatch negativity (MMN) of event-related potentials. There are also findings of neural representations of speech sounds in animals, but it is not known whether these representations can support the change detection mechanism analogous to that underlying the MMN in humans. To this end, we presented synthesized spoken syllables to urethane-anesthetized rats while local field potentials were epidurally recorded above their primary auditory cortex. In an oddball condition, a deviant stimulus /ga/ or /ba/ (probability 1:12 for each) was rarely and randomly interspersed between frequently presented standard stimulus /da/ (probability 10:12). In an equiprobable condition, 12 syllables, including /da/, /ga/, and /ba/, were presented in a random order (probability 1:12 for each). We found evoked responses of higher amplitude to the deviant /ba/, albeit not to /ga/, relative to the standard /da/ in the oddball condition. Furthermore, the responses to /ba/ were higher in amplitude in the oddball condition than in the equiprobable condition. The findings suggest that anesthetized rat's brain can form representations of human speech sounds, and that these representations can support the memory-based change detection mechanism analogous to that underlying the MMN in humans. Our findings show a striking parallel in speech processing between humans and rodents and may thus pave the way for feasible animal models of memory-based change detection.

Keywords: auditory; equiprobable condition; local field potentials; oddball condition; rat; speech sound.

Figure 1

Waveforms (the upper row) and spectrograms (the lower row) of the speech syllables used in the experiments. In the spectrograms, the dark areas indicate the time and frequency points where the acoustic energy is the highest.

Figure 2

Grand-averaged responses to syllables in the oddball and equiprobable conditions. (A) Responses to the standard syllable /da/ and deviant syllable /ba/ presented in the oddball condition; (B) responses to the standard syllable /da/ and deviant syllable /ga/ presented in the oddball condition; (C) responses to the deviant syllable /ba/ in the oddball condition and control–deviant syllable /ba/ in the equiprobable condition. The latency ranges that indicated significant differences between the responses in t-tests are marked with a rectangle. The y-axis indicates the stimulus onset.