Highlights: Marmosets offer novel avenues to study the intricacies of the primate social brain; Humans and marmosets share notable similarities in aspects of their sociality; The challenges of the primate social landscape has shaped primate brain evolution; Active social paradigms in marmosets lend unique insight into social brain function. (Miller et al., 2016) Figure 1. Primate Evolutionary Tree Cladogram showing the evolutionary divergence between humans, rodents, and each of the main primate taxonomic groups and estimated time points of divergence (MYA, millions of years ago).
(Wang et al., 2013) Harmonicity is an essential component of music found in all cultures. It is also a unique feature of vocal communication sounds such as human speech and animal vocalizations. Harmonics in sounds are produced by a variety of acoustic generators and reflectors in the natural environment, including vocal apparatuses of humans and animal species as well as music instruments of many types. We live in an acoustic world full of harmonicity. Given the widespread existence of the harmonicity in many aspects of the hearing environment, it is natural to expect that it be reflected in the evolution and development of the auditory systems of both humans and animals, in particular the auditory cortex. Recent neuroimaging and neurophysiology experiments have identified regions of non-primary auditory cortex in humans and non-human primates that have selective responses to harmonic pitches. Accumulating evidence has also shown that neurons in many regions of the auditory cortex exhibit characteristic responses to harmonically related frequencies beyond the range of pitch. Together, these findings suggest that a fundamental organizational principle of auditory cortex is based on the harmonicity. Such an organization likely plays an important role in music processing by the brain. It may also form the basis of the preference for particular classes of music and voice sounds.
Experiments in animals have provided an important complement to human studies of pitch perception by revealing how the activity of individual neurons represents harmonic complex and periodic sounds. Such studies have shown that the acoustical parameters associated with pitch are represented by the spiking responses of neurons in A1 (primary auditory cortex) and various higher auditory cortical fields. The responses of these neurons are also modulated by the timbre of sounds. In marmosets, a distinct region on the low-frequency border of primary and non-primary auditory cortex may provide pitch tuning that generalizes across timbre classes. (Wang and Walker, 2012) Figure 1: Schematics of auditory cortex across four species: A, Macaque; B, marmoset; C, cat; and D, ferret.
(Wang et al., 2008) Fig. 2. Two distinct types of cortical responses to periodic click trains. (A) An example of stimulus-synchronized responses to click trains recorded from A1 of awake marmosets. Top: Dot raster. The horizontal bar below x axis indicates the stimulus duration (1000 ms). Bottom: Vector strengths (dashed line) and Rayleigh statistics (solid line) analyzed for the stimulus-synchronized responses shown in the top plot. The dotted line (at the Rayleigh statistics of 13.8) indicates the threshold for statistically significant stimulus-synchronized activity. A synchronization boundary is calculated and indicated by an arrow. (B) An example of non-synchronized responses to click trains recorded from A1 of awake marmosets. Top: Dot raster. Bottom: Driven discharge rate is plotted versus ICI for the non-synchronized responses shown in the top plot. Vertical bars represent standard errors of the means (S.E.M.). The arrow indicates calculated rate-response boundary. (C) Response of a synchronized neuron to a sequence of ramped or damped sinusoids with different repetition periods (3–100 ms) and a fixed half-life ( Lu et al., 2001a). This neuron responded more strongly to damped sinusoids across different repetition periods. Average discharge rates for ramped (dashed line) and ramped (solid line) stimuli are plotted as functions of stimulus repetition period. Vertical bars represent standard errors of the means (S.E.M.). (D) Example of a non-synchronized neuron that responded more strongly to ramped sinusoids across different repetition periods. The format is the same as in C. Adapted from Lu et al. (2001b). The inset above C and D shows examples of ramped and damped sinusoids.
(Wang et al., 2007) Abstract: In contrast to the visual system, the auditory system has longer subcortical pathways and more spiking synapses between the peripheral receptors and the cortex. This unique organization reflects the needs of the auditory system to extract behaviorally relevant information from a complex acoustic environment using strategies different from those used by other sensory systems. The neural representations of acoustic information in auditory cortex can be characterized by three types: (1) isomorphic (faithful) representations of acoustic structures; (2) non-isomorphic transformations of acoustic features and (3) transformations from acoustical to perceptual dimensions. The challenge facing auditory neurophysiologists is to understand the nature of the latter two transformations. In this article, I will review recent studies from our laboratory regarding temporal discharge patterns in auditory cortex of awake marmosets and cortical representations of time-varying signals. Findings from these studies show that (1) firing patterns of neurons in auditory cortex are dependent on stimulus optimality and context and (2) the auditory cortex forms internal representations of sounds that are no longer faithful replicas of their acoustic structures.