Research PaperPopulation responses in primary auditory cortex simultaneously represent the temporal envelope and periodicity features in natural speech

- Auditory cortex simultaneously represents the speech envelope and periodicity cues.
- These two temporal features are represented by distinct neural mechanisms.
- The envelope of population responses closely tracks the speech envelope.
- Periodicity cues in speech are represented by phase-locked multi-unit activity.
- Auditory cortex uses discrete coding mechanisms to track distinct features.

Speech perception relies on a listener's ability to simultaneously resolve multiple temporal features in the speech signal. Little is known regarding neural mechanisms that enable the simultaneous coding of concurrent temporal features in speech. Here we show that two categories of temporal features in speech, the low-frequency speech envelope and periodicity cues, are processed by distinct neural mechanisms within the same population of cortical neurons. We measured population activity in primary auditory cortex of anesthetized guinea pig in response to three variants of a naturally produced sentence. Results show that the envelope of population responses closely tracks the speech envelope, and this cortical activity more closely reflects wider bandwidths of the speech envelope compared to narrow bands. Additionally, neuronal populations represent the fundamental frequency of speech robustly with phase-locked responses. Importantly, these two temporal features of speech are simultaneously observed within neuronal ensembles in auditory cortex in response to clear, conversation, and compressed speech exemplars. Results show that auditory cortical neurons are adept at simultaneously resolving multiple temporal features in extended speech sentences using discrete coding mechanisms.