In a nutshell: In non-human primates, different areas of the auditory cortex process information about pitch, an important aspect of animal communication.

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Non-human primates communicate with each other by making sounds. To make sense of each sound they hear, animals need to recognise who is making it and where it is coming from. One feature that can be used to differentiate sounds is their pitch, or frequency.

Primates process sound in a part of the brain called the auditory cortex. Different areas of the auditory cortex have been linked to different aspects of sound processing, such as identifying the source of a sound and determining its location. One of these areas, the primary auditory cortex (A1), is involved in processing information about pitch, but little is known about what other areas – particularly those specialised in determining where a sound is coming from – might have a role.

Previous research showed that areas other than A1 might be involved, but the studies relied on experiments that used artificial sounds rather than real animal calls. To find out if animals responded differently to sounds from their own species, Brain Function CoE investigators Shuyu (Iris) Zhu, Leo Lui, Ramesh Rajan and Marcello Rosa and their colleagues at Monash University played a series of natural animal sounds to marmoset monkeys and measured how their brains responded. This study follows related research from the same researchers on how A1 processes pitch.

In this study, the researchers used recordings of four marmoset calls and created up to 13 pitch-shifted variations for each one. They played these sounds to a group of marmosets and measured the brain activity in their A1 and other areas of their auditory cortex.

The researchers found that brain cells across the auditory cortex respond to variations in pitch – even in areas that specialise in processing spatial information. The timing and duration of these responses was different in each area, suggesting that they may have different ways of processing pitch.

The timing of the responses also gives the researchers insight into how information flows from one region of the auditory cortex to the next, which gives them a better idea of how the auditory cortex might be structured.

Next steps:
The researchers plan to examine how representations of pitch and location are linked in the brain. In particular, they will investigate how brain areas dedicated to processing spatial information retain their functionality under the influence of pitch.

Zhu, S., Allitt, B., Samuel, A., Lui, L., Rosa, M. G. P., & Rajan, R. (2019). Sensitivity to vocalization pitch in the caudal auditory cortex of the marmoset: Comparison of core and belt areas. Frontiers in Systems Neuroscience, 13, 5. doi: 10.3389/fnsys.2019.00005

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