Electroencephalography Measurements in Awake Marmosets Listening to Conspecific Vocalizations

Primates use species-specific vocalizations to convey biologically important information, such as the caller's emotional state or intention to maintain social bonds, the presence of predators, or other dangerous situations. Investigation of the neural mechanisms underlying the perception of vocalization in vocal-rich nonhuman primates may provide us with critical clues to better understand the evolutionary origins of human language.

Common marmosets are small primates native to South America. In recent years, marmosets have been increasingly used as model animals, alongside macaque monkeys, because of their high reproductivity, ease of use owing to their small size, and the development of useful transgenic techniques^1,2,3. In addition to their utility as disease models, rich vocal communication within groups is another unique characteristic of this species^4,5,6,7. Marmosets routinely exchange vocal signals to communicate with invisible conspecifics in the forest. By examining the brain activity involved in vocal perception and production in marmosets, we can determine how they process the auditory information of their own or conspecific calls in the brain and identify which neural circuits are involved. Previous studies have demonstrated neural activity in the primary auditory cortex^8,9,10,11,12 and frontal cortex^13,14 involved in vocal production in marmosets. Furthermore, these excited and suppressed neuronal responses were modulated by auditory-vocal interactions in the primary auditory cortex^8,10. These studies provided detailed neural activity data at the single-neuron level using invasive recording methods. Numerous studies have further examined the neural activity involved in marmoset vocal production; however, vocal perception remains poorly understood^15,16.

Several noninvasive brain imaging studies have elucidated the neural mechanisms of vocal processing in marmosets^17,18,19; their high spatial resolution is an advantage, however, keeping animals in the awake state during scanning requires advanced techniques. However, more recently, Jafari et al. identified frontotemporal regions involved in vocal perception in awake marmosets using functional magnetic resonance imaging (fMRI)¹⁹. Almost all experiments to elucidate the brain functions involved in vocal perception and production in humans have been conducted using noninvasive methods, such as scalp electroencephalography (EEG), magnetoencephalography (MEG)^20,21, and fMRI^22,23,24. Numerous studies in humans have investigated brain activity related to vocal perception using EEG. Most of these studies have focused on emotional information^25,26,27 and the saliency of emotional words²⁸, with the results revealing changes in event-related potentials during vocal perception²⁹. Electrocorticography (ECoG) and single-neuron recordings using intracranially implanted electrodes in humans have only been conducted in a limited number of experiments in patients undergoing neurosurgical treatment^30,31.

An evolutionary perspective comparing humans with monkeys is important when understanding the unique neural mechanisms underlying vocal perception and production that have developed in humans. To directly compare the neural mechanisms involved in speech perception and vocalization in vocal-rich nonhuman primates, such as the marmoset, with humans, it is important to compare data between the two species using the same method. Functional MRI allows whole-brain imaging and has a high spatial resolution. It has the advantage of recording activity perpendicular to the skull or in deep regions that are difficult to record with EEG or MEG. However, the MRI machine is expensive to install and maintain, and there are many restrictions on the stimuli that can be presented due to the nature of the device. In comparison, EEG, event-related potentials (ERPs), and MEG have a high temporal resolution, making them useful for analyzing time-series vocal processing. In particular, EEG has the advantages of high mobility and the ability to be used in a variety of experimental settings, relatively low cost, and the requirement for just a single operator.

Since a large amount of EEG data has already been obtained in humans, EEG measurement methods using non-invasive paradigms are needed for non-human primates. Our research group developed a unique noninvasive EEG recording method using tubes³² for macaques and marmosets. Here, we report several novel findings regarding auditory processing in nonhuman primates^{33,34,35,36,37}. To characterize brain activity in response to species-specific vocalizations in marmosets, we constructed an experimental system to noninvasively record brain activity using electrodes placed on the scalp. In this study, we describe the EEG measurement method for marmosets.