Tonotopic organization of auditory cortex in awake marmosets revealed by multi-modal wide-field optical imaging.

Tonotopic organization of the auditory cortex has been extensively studied in many mammalian species using various methodologies and physiological preparations. Tonotopy mapping in primates, however, is more limited due to constraints such as cortical folding, use of anesthetized subjects, and mapping methodology. Here we applied a combination of through-skull and through-window intrinsic optical signal imaging, wide-field calcium imaging, and neural probe recording techniques in awake marmosets (Callithrix jacchus), a New World monkey with most of its auditory cortex located on a flat brain surface. Coarse tonotopic gradients, including a recently described rostral-temporal (RT) to parabelt gradient, were revealed by the through-skull imaging of intrinsic optical signals and were subsequently validated by single-unit recording. Furthermore, these tonotopic gradients were observed with more detail through chronically implanted cranial windows with additional verifications on the experimental design. Moreover, the tonotopy mapped by the intrinsic-signal imaging methods was verified by wide-field calcium imaging in an AAV-GCaMP labeled subject. After these validations and with further effort to expand the field of view more rostrally in both windowed and through-skull subjects, an additional putative tonotopic gradient was observed more rostrally to the area RT, which has not been previously described by the standard model of tonotopic organization of the primate auditory cortex. Together, these results provide the most comprehensive data of tonotopy mapping in an awake primate species with unprecedented coverage and details in the rostral proportion and support a caudal-rostrally arranged mesoscale organization of at least three repeats of functional gradients in the primate auditory cortex, similar to the ventral stream of primate visual cortex.

Mesoscopic landscape of cortical functions revealed by through-skull wide-field optical imaging in marmoset monkeys.

The primate cerebral cortex is organized into specialized areas representing different modalities and functions along a continuous surface. The functional maps across the cortex, however, are often investigated a single modality at a time (e.g., audition or vision). To advance our understanding of the complex landscape of primate cortical functions, here we develop a polarization-gated wide-field optical imaging method for measuring cortical functions through the un-thinned intact skull in awake marmoset monkeys (Callithrix jacchus), a primate species featuring a smooth cortex. Using this method, adjacent auditory, visual, and somatosensory cortices are noninvasively parcellated in individual subjects with detailed tonotopy, retinotopy, and somatotopy. An additional pure-tone-responsive tonotopic gradient is discovered in auditory cortex and a face-patch sensitive to motion in the lower-center visual field is localized near an auditory region representing frequencies of conspecific vocalizations. This through-skull landscape-mapping approach provides new opportunities for understanding how the primate cortex is organized and coordinated to enable real-world behaviors.

A silent two-photon imaging system for studying in vivo auditory neuronal functions.

Two-photon laser-scanning microscopy has become an essential tool for imaging neuronal functions in vivo and has been applied to different parts of the neural system, including the auditory system. However, many components of a two-photon microscope, such as galvanometer-based laser scanners, generate mechanical vibrations and thus acoustic artifacts, making it difficult to interpret auditory responses from recorded neurons. Here, we report the development of a silent two-photon imaging system and its applications in the common marmoset (Callithrix Jacchus), a non-human primate species sharing a similar hearing range with humans. By utilizing an orthogonal pair of acousto-optical deflectors (AODs), full-frame raster scanning at video rate was achieved without introducing mechanical vibrations. Imaging depth can be optically controlled by adjusting the chirping speed on the AODs without any mechanical motion along the Z-axis. Furthermore, all other sound-generating components of the system were acoustically isolated, leaving the noise floor of the working system below the marmoset's hearing threshold. Imaging with the system in awake marmosets revealed many auditory cortex neurons that exhibited maximal responses at low sound levels, which were not possible to study using traditional two-photon imaging systems. This is the first demonstration of a silent two-photon imaging system that is capable of imaging auditory neuronal functions in vivo without acoustic artifacts. This capacity opens new opportunities for a better understanding of auditory functions in the brain and helps isolate animal behavior from microscope-generated acoustic interference.

Frequency discrimination in the common marmoset (Callithrix jacchus).

The common marmoset (Callithrix jacchus) is a highly vocal New World primate species that has emerged in recent years as a promising model system for studies of auditory and vocal processing. Our recent studies have examined perceptual mechanisms related to the pitch of harmonic complex tones in this species. However, no previous psychoacoustic work has measured marmosets' frequency discrimination abilities for pure tones across a broad frequency range. Here we systematically examined frequency difference limens (FDLs), which measure the minimum discriminable frequency difference between two pure tones, in marmosets across most of their hearing range. Results show that marmosets' FDLs are comparable to other New World primates, with lowest values in the frequency range of ∼3.5-14 kHz. This region of lowest FDLs corresponds with the region of lowest hearing thresholds in this species measured in our previous study and also with the greatest concentration of spectral energy in the major types of marmoset vocalizations. These data suggest that frequency discrimination in the common marmoset may have evolved to match the hearing sensitivity and spectral characteristics of this species' vocalizations.

Complex pitch perception mechanisms are shared by humans and a New World monkey.

The perception of the pitch of harmonic complex sounds is a crucial function of human audition, especially in music and speech processing. Whether the underlying mechanisms of pitch perception are unique to humans, however, is unknown. Based on estimates of frequency resolution at the level of the auditory periphery, psychoacoustic studies in humans have revealed several primary features of central pitch mechanisms. It has been shown that (i) pitch strength of a harmonic tone is dominated by resolved harmonics; (ii) pitch of resolved harmonics is sensitive to the quality of spectral harmonicity; and (iii) pitch of unresolved harmonics is sensitive to the salience of temporal envelope cues. Here we show, for a standard musical tuning fundamental frequency of 440 Hz, that the common marmoset (Callithrix jacchus), a New World monkey with a hearing range similar to that of humans, exhibits all of the primary features of central pitch mechanisms demonstrated in humans. Thus, marmosets and humans may share similar pitch perception mechanisms, suggesting that these mechanisms may have emerged early in primate evolution.

The role of harmonic resolvability in pitch perception in a vocal nonhuman primate, the common marmoset (Callithrix jacchus).

Pitch is one of the most fundamental percepts in the auditory system and can be extracted using either spectral or temporal information in an acoustic signal. Although pitch perception has been extensively studied in human subjects, it is far less clear how nonhuman primates perceive pitch. We have addressed this question in a series of behavioral studies in which marmosets, a vocal nonhuman primate species, were trained to discriminate complex harmonic tones differing in either spectral (fundamental frequency [f0]) or temporal envelope (repetition rate) cues. We found that marmosets used temporal envelope information to discriminate pitch for acoustic stimuli with higher-order harmonics and lower f0 values and spectral information for acoustic stimuli with lower-order harmonics and higher f0 values. We further measured frequency resolution in marmosets using a psychophysical task in which pure tone thresholds were measured as a function of notched noise masker bandwidth. Results show that only the first four harmonics are resolved at low f0 values and up to 16 harmonics are resolved at higher f0 values. Resolvability in marmosets is different from that in humans, where the first five to nine harmonics are consistently resolved across most f0 values, and is likely the result of a smaller marmoset cochlea. In sum, these results show that marmosets use two mechanisms to extract pitch (harmonic templates [spectral] for resolved harmonics, and envelope extraction [temporal] for unresolved harmonics) and that species differences in stimulus resolvability need to be taken into account when investigating and comparing mechanisms of pitch perception across animals.

First-spike rank order as a reliable indicator of burst initiation and its relation with early-to-fire neurons.

In this paper, we study the spontaneous cortical neuronal network in hopes of finding a reliable indicator of burst initiation pathway, which would allow us to study burst initiation in conjunction with burst propagation in future research. Electrical activity is recorded using a 96-electrode microelectrode array on a weekly batch culture (half of the medium was replaced twice every week). We hypothesize that the first-spike onset sequence, which we call first-spike rank order (FSRO) is a reliable indicator of burst initiation, and verified our hypothesis by studying evoked bursts using rearranged rank probability matrices. Under similar conditions, stimulating the same site reliably reproduces the same FSRO. Spontaneous bursts can be classified based on their FSRO using dendrogram clustering. Bursts with different first-spike sequences showed evidence of sharing common early-to-fire neurons, but early-to-fire neurons only consist of a minority of neuronal activity during burst initiation, which is in partial accordance with existing literature. In the study of early-to-fire neurons, we also noticed that our batch-cultured network did not show clear preburst activity, which may indicate fundamental difference compared to continuous perfusion culture.

Microelectrode array-based system for neuropharmacological applications with cortical neurons cultured in vitro.

Microelectrode arrays (MEAs) provide a means to investigate the electrophysiological behavior of neuronal systems through the measurements from neuronal culture preparations. Changes in activity patterns of neuronal networks are usually detected by applying neural chemicals. Because of the difficulties of fabricating the arrays, and the delicate and less reliable properties of cortical neurons, MEA-based systems with cortical neuronal networks for neurophamacological applications are technically difficult, therefore restricting their utility. Here, we report a new approach to the development of such MEA-based system with sensitive and durable MEAs conveniently fabricated and the culture conditions optimized. Upon growth differentiation, cortical neurons, cultured directly on MEAs, reach a developmentally stable and reliable activity state. With this system, we monitored the global spontaneous activities of neuronal networks and demonstrated the fine discrimination for specific substances and unique property of cortical neurons, which validated both the applicability and necessity of such system in pharmacological bioassay.

Design proposal of imaging activities of cultured neural network on a silicon substrate with neural-electronic-optical integrated microsystem.

Multielectrode Arrays have currently been widely used as Neural-Network research tools. In this paper, we propose a novel mixed integrated sensor array microsystem combining cultured biological neurons, silicon integrated circuits and optical components together on a single silicon substrate. Using the light emitting property of GaAs-LED dependent on current passing from the recorded action potentials signals by a Multielectrode-Array in situ, we are able to characterize the neural activities of the cultured neural networks directly with a visible image displayed by a monitoring CCD Camera. Researchers may use various ready image process tools for experimental data analysis to directly observe results.