Neuronal activity in the periaqueductal gray and bordering structures during vocal communication in the squirrel monkey

Neuroanatomical frameworks for volitional control of breathing and orofacial behaviors

Breathing is the only vital function that can be volitionally controlled. However, a detailed understanding how volitional (cortical) motor commands can transform vital breathing activity into adaptive breathing patterns that accommodate orofacial behaviors such as swallowing, vocalization or sniffing remains to be developed. Recent neuroanatomical tract tracing studies have identified patterns and origins of descending forebrain projections that target brain nuclei involved in laryngeal adductor function which is critically involved in orofacial behavior. These nuclei include the midbrain periaqueductal gray and nuclei of the respiratory rhythm and pattern generating network in the brainstem, specifically including the pontine Kölliker-Fuse nucleus and the pre-Bötzinger complex in the medulla oblongata. This review discusses the functional implications of the forebrain-brainstem anatomical connectivity that could underlie the volitional control and coordination of orofacial behaviors with breathing.

Midbrain node for context-specific vocalisation in fish

Vocalizations communicate information indicative of behavioural state across divergent social contexts. Yet, how brain regions actively pattern the acoustic features of context-specific vocal signals remains largely unexplored. The midbrain periaqueductal gray (PAG) is a major site for initiating vocalization among mammals, including primates. We show that PAG neurons in a highly vocal fish species (Porichthys notatus) are activated in distinct patterns during agonistic versus courtship calling by males, with few co-activated during a non-vocal behaviour, foraging. Pharmacological manipulations within vocally active PAG, but not hindbrain, sites evoke vocal network output to sonic muscles matching the temporal features of courtship and agonistic calls, showing that a balance of inhibitory and excitatory dynamics is likely necessary for patterning different call types. Collectively, these findings support the hypothesis that vocal species of fish and mammals share functionally comparable PAG nodes that in some species can influence the acoustic structure of social context-specific vocal signals.

Frontal cortex activity during the production of diverse social communication calls in marmoset monkeys

Vocal communication is essential for social behaviors in humans and non-human primates. While the frontal cortex is crucial to human speech production, its role in vocal production in non-human primates has long been questioned. It is unclear whether activities in the frontal cortex represent diverse vocal signals used in non-human primate communication. Here we studied single neuron activities and local field potentials (LFP) in the frontal cortex of male marmoset monkeys while the animal engaged in vocal exchanges with conspecifics in a social environment. We found that both single neuron activities and LFP were modulated by the production of each of the four major call types. Moreover, neural activities showed distinct patterns for different call types and theta-band LFP oscillations showed phase-locking to the phrases of twitter calls, suggesting a neural representation of vocalization features. Our results suggest important functions of the marmoset frontal cortex in supporting the production of diverse vocalizations in communication.

When laughter arrests speech: fMRI-based evidence

Who has not experienced that sensation of losing the power of speech owing to an involuntary bout of laughter? An investigation of this phenomenon affords an insight into the neuronal processes that underlie laughter. In our functional magnetic resonance imaging study, participants were made to laugh by tickling in a first condition; in a second one they were requested to produce vocal utterances under the provocation of laughter by tickling. This investigation reveals increased neuronal activity in the sensorimotor cortex, the anterior cingulate gyrus, the insula, the nucleus accumbens, the hypothalamus and the periaqueductal grey for both conditions, thereby replicating the results of previous studies on ticklish laughter. However, further analysis indicates the activity in the emotion-associated regions to be lower when tickling is accompanied by voluntary vocalization. Here, a typical pattern of activation is identified, including the primary sensory cortex, a ventral area of the anterior insula and the ventral tegmental field, to which belongs to the nucleus ambiguus, namely, the common effector organ for voluntary and involuntary vocalizations. During the conflictual voluntary-vocalization versus laughter experience, the laughter-triggering network appears to rely heavily on a sensory and a deep interoceptive analysis, as well as on motor effectors in the brainstem.

This article is part of the theme issue ‘Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience’.

Astrocytic modulation of central pattern generating motor circuits

Central pattern generators (CPGs) generate the rhythmic and coordinated neural features necessary for the proper conduction of complex behaviors. In particular, CPGs are crucial for complex motor behaviors such as locomotion, mastication, respiration, and vocal production. While the importance of these networks in modulating behavior is evident, the mechanisms driving these CPGs are still not fully understood. On the other hand, accumulating evidence suggests that astrocytes have a significant role in regulating the function of some of these CPGs. Here, we review the location, function, and role of astrocytes in locomotion, respiration, and mastication CPGs and propose that, similarly, astrocytes may also play a significant role in the vocalization CPG.

Reciprocal connectivity of the periaqueductal gray with the ponto-medullary respiratory network in rat

Synaptic activities of the periaqueductal gray (PAG) can modulate or appropriate the respiratory motor activities in the context of behavior and emotion via descending projections to nucleus retroambiguus. However, alternative anatomical pathways for the mediation of PAG-evoked respiratory modulation via core nuclei of the brainstem respiratory network remains only partially described. We injected the retrograde tracer Cholera toxin subunit B (CT-B) in the pontine Kölliker-Fuse nucleus (KFn, n = 5), medullary Bötzinger (BötC, n = 3) and pre-Bötzinger complexes (pre-BötC; n = 3), and the caudal raphé nuclei (n = 3), and quantified the descending connectivity of the PAG targeting these brainstem respiratory regions. CT-B injections in the KFn, pre-BötC, and caudal raphé, but not in the BötC, resulted in CT-B-labeled neurons that were predominantly located in the lateral and ventrolateral PAG columns. In turn, CT-B injections in the lateral and ventrolateral PAG columns (n = 4) produced the highest numbers of CT-B-labeled neurons in the KFn and far fewer numbers of labeled neurons in the pre-BötC, BötC, and caudal raphé. Analysis of the relative projection strength revealed that the KFn shares the densest reciprocal connectivity with the PAG (ventrolateral and lateral columns, in particular). Overall, our data imply that the PAG may engage a distributed respiratory rhythm and pattern generating network beyond the nucleus retroambiguus to mediate downstream modulation of breathing. However, the reciprocal connectivity of the KFn and PAG suggests specific roles for synaptic interaction between these two nuclei that are most likely related to the regulation of upper airway patency during vocalization or other volitional orofacial behaviors.

Forebrain projection neurons target functionally diverse respiratory control areas in the midbrain, pons, and medulla oblongata

Eupnea is generated by neural circuits located in the ponto-medullary brainstem, but can be modulated by higher brain inputs which contribute to volitional control of breathing and the expression of orofacial behaviors, such as vocalization, sniffing, coughing, and swallowing. Surprisingly, the anatomical organization of descending inputs that connect the forebrain with the brainstem respiratory network remains poorly defined. We hypothesized that descending forebrain projections target multiple distributed respiratory control nuclei across the neuroaxis. To test our hypothesis, we made discrete unilateral microinjections of the retrograde tracer cholera toxin subunit B in the midbrain periaqueductal gray (PAG), the pontine Kölliker-Fuse nucleus (KFn), the medullary Bötzinger complex (BötC), pre-BötC, or caudal midline raphé nuclei. We quantified the regional distribution of retrogradely labeled neurons in the forebrain 12-14 days postinjection. Overall, our data reveal that descending inputs from cortical areas predominantly target the PAG and KFn. Differential forebrain regions innervating the PAG (prefrontal, cingulate cortices, and lateral septum) and KFn (rhinal, piriform, and somatosensory cortices) imply that volitional motor commands for vocalization are specifically relayed via the PAG, while the KFn may receive commands to coordinate breathing with other orofacial behaviors (e.g., sniffing, swallowing). Additionally, we observed that the limbic or autonomic (interoceptive) systems are connected to broadly distributed downstream bulbar respiratory networks. Collectively, these data provide a neural substrate to explain how volitional, state-dependent, and emotional modulation of breathing is regulated by the forebrain.

Laughter is in the air: involvement of key nodes of the emotional motor system in the anticipation of tickling

In analogy to the appreciation of humor, that of tickling is based upon the re-interpretation of an anticipated emotional situation. Hence, the anticipation of tickling contributes to the final outburst of ticklish laughter. To localize the neuronal substrates of this process, functional magnetic resonance imaging (fMRI) was conducted on 31 healthy volunteers. The state of anticipation was simulated by generating an uncertainty respecting the onset of manual foot tickling. Anticipation was characterized by an augmented fMRI signal in the anterior insula, the hypothalamus, the nucleus accumbens and the ventral tegmental area, as well as by an attenuated one in the internal globus pallidus. Furthermore, anticipatory activity in the anterior insula correlated positively with the degree of laughter that was produced during tickling. These findings are consistent with an encoding of the expected emotional consequences of tickling and suggest that early regulatory mechanisms influence, automatically, the laughter circuitry at the level of affective and sensory processing. Tickling activated not only those regions of the brain that were involved during anticipation, but also the posterior insula, the anterior cingulate cortex and the periaqueductal gray matter. Sequential or combined anticipatory and tickling-related neuronal activities may adjust emotional and sensorimotor pathways in preparation for the impending laughter response.

Microstimulation in Different Parts of the Periaqueductal Gray Generates Different Types of Vocalizations in the Cat

In the cat four different types of vocalization, mews, howls, cries, and hisses were generated by microstimulation in different parts of the periaqueductal gray (PAG). While mews imply positive vocal expressions, howls, hisses, and cries represent negative vocal expressions. In the intermediate PAG, mews were generated in the lateral column, howls, and hisses in the ventrolateral column. Cries were generated in two other regions, the lateral column of the rostral PAG and the ventrolateral column of the caudal PAG. In order to define the specific motor patterns of the mews, howls, and cries, the following muscles were recorded during these vocalizations; larynx (cricothyroid, thyroarytenoid, and posterior cricoarytenoid), tongue (genioglossus), jaw (digastric), and respiration muscles (diaphragm, internal intercostal, external, and internal abdominal oblique). During these mews, howls, and cries we analyzed the frequency, intensity, activation cascades power density, turns, and amplitude analysis of the electromyograms (EMGs). It appeared that each type of vocalization consists of a specific circumscribed motor coordination. The nucleus retroambiguus (NRA) in the caudal medulla is known to serve as the final premotor interneuronal output system for vocalization. Although neurochemical microstimulation in the NRA itself also generated vocalizations, they only consisted of guttural sounds, the EMGs of which involved only small parts of the EMGs of the mews, howls, and cries generated by neurochemical stimulation in the PAG. These results demonstrate that positive and negative vocalizations are generated in different parts of the PAG. These parts have access to different groups of premotoneurons in the NRA, that, in turn, have access to different groups of motoneurons in the brainstem and spinal cord, resulting in different vocalizations. The findings would serve a valuable model for diagnostic assessment of voice disorders in humans.

Tauopathy in the periaqueductal gray, kölliker-fuse nucleus and nucleus retroambiguus is not predicted by ultrasonic vocalization in tau-P301L mice

Upper airway and vocalization control areas such as the periaqueductal gray (PAG), kölliker-fuse nucleus (KF) and nucleus retroambiguus (NRA) are prone to developing tauopathy in mice expressing the mutant human tau P301L protein. Consequently, impaired ultrasonic vocalization (USV) previously identified in tau-P301L mice at the terminal disease stage of 8-9 months of age, was attributed to the presence of tauopathy in these regions. Our aim was to establish whether the onset of USV disorders manifest prior to the terminal stage, and if USV disorders are predictive of the presence of tauopathy in the PAG, KF and NRA. USVs produced by tau-P301L and wildtype mice aged 3-4, 5-6 or 8-9 months were recorded during male-female interaction. Immunohistochemistry was then performed to assess the presence or degree of tauopathy in the PAG, KF and NRA of mice displaying normal or abnormal USV patterns. Comparing various USV measurements, including the number, duration and frequency of calls, revealed no differences between tau-P301L and wildtype mice across all age groups, and linear discriminant analysis also failed to identify separate USV populations. Finally, the presence of tauopathy in the PAG, KF and NRA in individual tau-P301L mice did not reliably associate with USV disorders. Our findings that tauopathy in designated mammalian vocalization centres, such as the PAG, KF and NRA, did not associate with USV disturbances in tau-P301L mice questions whether USV phenotypes in this transgenic mouse are valid for studying tauopathy-related human voice and speech disorders.

Development of snake-directed antipredator behavior by wild white-faced capuchin monkeys: III. the signaling properties of alarm-call tonality

In many primates, the acoustic properties of alarm calls can provide information on the level of perceived predatory threat as well as influence the antipredator behavior of nearby conspecifics. The present study examined the harmonics-to-noise ratio (tonality of spectral structure) of alarm calls emitted by white-faced capuchin monkeys (Cebus capucinus) in trees directed at photographic models of a boa constrictor, neotropical rattlesnake, scorpion eater snake, and white snake-shaped control presented on the ground. The average and peak harmonics-to-noise ratios of initial alarm calls by infants, juveniles, and adults and those of nearby second callers were analyzed using PRAAT software. Averaged for age class, the peak harmonics-to-noise ratio of alarm calls directed at the boa constrictor model, characterizing a primary capuchin predator, was reliably higher than the peak harmonics-to-noise ratio of alarm calls directed at the harmless scorpion eater model. This effect was influenced by the higher harmonics-to-noise ratio of infant alarm calls and it disconfirmed our prediction, based on primate vocalization research, that snake perception would increase arousal and alarm-call noisiness. Levels of call tonality did not distinguish the boa and rattlesnake or rattlesnake and scorpion eater models for any age class. Higher alarm-call tonality appeared contagious to nearby perceivers, with focal alarm calling influencing the level of tonality of the first calls of second callers. Together, these findings suggest that the higher peak harmonics-to-noise ratio of capuchin alarm calling directed at snakes is contagious and possibly conveys information about the level of perceived predatory threat.

Articulating: The Neural Mechanisms of Speech Production

Speech production is a highly complex sensorimotor task involving tightly coordinated processing across large expanses of the cerebral cortex. Historically, the study of the neural underpinnings of speech suffered from the lack of an animal model. The development of non-invasive structural and functional neuroimaging techniques in the late 20^th century has dramatically improved our understanding of the speech network. Techniques for measuring regional cerebral blood flow have illuminated the neural regions involved in various aspects of speech, including feedforward and feedback control mechanisms. In parallel, we have designed, experimentally tested, and refined a neural network model detailing the neural computations performed by specific neuroanatomical regions during speech. Computer simulations of the model account for a wide range of experimental findings, including data on articulatory kinematics and brain activity during normal and perturbed speech. Furthermore, the model is being used to investigate a wide range of communication disorders.

Functional Specialization of the Primate Frontal Lobe during Cognitive Control of Vocalizations

Cognitive vocal control is indispensable for human language. Frontal lobe areas are involved in initiating purposeful vocalizations, but their functions remain elusive. We explored the respective roles of frontal lobe areas in initiating volitional vocalizations. Macaques were trained to vocalize in response to visual cues. Recordings from the ventrolateral prefrontal cortex (vlPFC), the anterior cingulate cortex (ACC), and the pre-supplementary motor area (preSMA) revealed single-neuron and population activity differences. Pre-vocal activity appeared first after the go cue in vlPFC, showing onset activity that was tightly linked to vocal reaction times. However, pre-vocal ACC onset activity was not indicative of call timing; instead, ramping activity reaching threshold values betrayed call onset. Neurons in preSMA showed weakest correlation with volitional call initiation and timing. These results suggest that vlPFC encodes the decision to produce volitional calls, whereas downstream ACC represents a motivational preparatory signal, followed by a general motor priming signal in preSMA.

Development of an optogenetic toolkit for neural circuit dissection in squirrel monkeys

Optogenetic tools have opened a rich experimental landscape for understanding neural function and disease. Here, we present the first validation of eight optogenetic constructs driven by recombinant adeno-associated virus (AAV) vectors and a WGA-Cre based dual injection strategy for projection targeting in a widely-used New World primate model, the common squirrel monkey Saimiri sciureus. We observed opsin expression around the local injection site and in axonal projections to downstream regions, as well as transduction to thalamic neurons, resembling expression patterns observed in macaques. Optical stimulation drove strong, reliable excitatory responses in local neural populations for two depolarizing opsins in anesthetized monkeys. Finally, we observed continued, healthy opsin expression for at least one year. These data suggest that optogenetic tools can be readily applied in squirrel monkeys, an important first step in enabling precise, targeted manipulation of neural circuits in these highly trainable, cognitively sophisticated animals. In conjunction with similar approaches in macaques and marmosets, optogenetic manipulation of neural circuits in squirrel monkeys will provide functional, comparative insights into neural circuits which subserve dextrous motor control as well as other adaptive behaviors across the primate lineage. Additionally, development of these tools in squirrel monkeys, a well-established model system for several human neurological diseases, can aid in identifying novel treatment strategies.

Motor organization of positive and negative emotional vocalization in the cat midbrain periaqueductal gray

Neurochemical microstimulation in different parts of the midbrain periaqueductal gray (PAG) in the cat generates four different types of vocalization, mews, howls, cries, and hisses. Mews signify positive vocal expression, whereas howls, hisses, and cries signify negative vocal communications. Mews were generated in the lateral column of the intermediate PAG and howls and hisses in the ventrolateral column of the intermediate PAG. Cries were generated in two regions, the lateral column of the rostral PAG and the ventrolateral column of the caudal PAG. To define the specific motor patterns belonging to mews, howls, and cries, the following muscles were recorded during these vocalizations: larynx (cricothyroid, thyroarytenoid, and posterior cricoarytenoid), tongue (genioglossus), jaw (digastric), and respiration (diaphragm, internal intercostal, external abdominal oblique, and internal abdominal oblique) muscles. Furthermore, the frequency, intensity, activation cascades, and turns and amplitude analyses of the electromyograms (EMGs) during these vocalizations were analyzed. The results show that each type of vocalization consists of a specific, circumscribed motor coordination. The nucleus retroambiguus (NRA) in the caudal medulla serves as the final premotor interneuronal output system for vocalization. NRA neurochemical microstimulation also generated vocalizations (guttural sounds). Analysis of the EMGs demonstrated that these vocalizations consist of only small parts of the emotional voalizations generated by neurochemical stimulation in the PAG. These results demonstrate that motor organization of positive and negative emotional vocal expressions are segregated in the PAG and that the PAG uses the NRA as a tool to gain access to the motoneurons generating vocalization.

Vocal-motor and auditory connectivity of the midbrain periaqueductal gray in a teleost fish

The midbrain periaqueductal gray (PAG) plays a central role in the descending control of vocalization across vertebrates. The PAG has also been implicated in auditory-vocal integration, although its precise role in such integration remains largely unexplored. Courtship and territorial interactions in plainfin midshipman fish depend on vocal communication, and the PAG is a central component of the midshipman vocal-motor system. We made focal neurobiotin injections into the midshipman PAG to both map its auditory-vocal circuitry and allow evolutionary comparisons with tetrapod vertebrates. These injections revealed an extensive bidirectional pattern of connectivity between the PAG and known sites in both the descending vocal-motor and the ascending auditory systems, including portions of the telencephalon, dorsal thalamus, hypothalamus, posterior tuberculum, midbrain, and hindbrain. Injections in the medial PAG produced dense label within hindbrain auditory nuclei, whereas those confined to the lateral PAG preferentially labeled hypothalamic and midbrain auditory areas. Thus, the teleost PAG may have functional subdivisions playing different roles in vocal-auditory integration. Together the results confirm several pathways previously identified by injections into known auditory or vocal areas and provide strong support for the hypothesis that the teleost PAG is centrally involved in auditory-vocal integration.

Stereotypic laryngeal and respiratory motor patterns generate different call types in rat ultrasound vocalization

Rodents produce highly variable ultrasound whistles as communication signals unlike many other mammals, who employ flow-induced vocal fold oscillations to produce sound. The role of larynx muscles in controlling sound features across different call types in ultrasound vocalization (USV) was investigated using laryngeal muscle electromyographic (EMG) activity, subglottal pressure measurements and vocal sound output in awake and spontaneously behaving Sprague-Dawley rats. Results support the hypothesis that glottal shape determines fundamental frequency. EMG activities of thyroarytenoid and cricothyroid muscles were aligned with call duration. EMG intensity increased with fundamental frequency. Phasic activities of both muscles were aligned with fast changing fundamental frequency contours, for example in trills. Activities of the sternothyroid and sternohyoid muscles, two muscles involved in vocal production in other mammals, are not critical for the production of rat USV. To test how stereotypic laryngeal and respiratory activity are across call types and individuals, sets of ten EMG and subglottal pressure parameters were measured in six different call types from six rats. Using discriminant function analysis, on average 80% of parameter sets were correctly assigned to their respective call type. This was significantly higher than the chance level. Since fundamental frequency features of USV are tightly associated with stereotypic activity of intrinsic laryngeal muscles and muscles contributing to build-up of subglottal pressure, USV provide insight into the neurophysiological control of peripheral vocal motor patterns.

Mouse vocal communication system: are ultrasounds learned or innate?

Mouse ultrasonic vocalizations (USVs) are often used as behavioral readouts of internal states, to measure effects of social and pharmacological manipulations, and for behavioral phenotyping of mouse models for neuropsychiatric and neurodegenerative disorders. However, little is known about the neurobiological mechanisms of rodent USV production. Here we discuss the available data to assess whether male mouse song behavior and the supporting brain circuits resemble those of known vocal non-learning or vocal learning species. Recent neurobiology studies have demonstrated that the mouse USV brain system includes motor cortex and striatal regions, and that the vocal motor cortex sends a direct sparse projection to the brainstem vocal motor nucleus ambiguous, a projection previously thought be unique to humans among mammals. Recent behavioral studies have reported opposing conclusions on mouse vocal plasticity, including vocal ontogeny changes in USVs over early development that might not be explained by innate maturation processes, evidence for and against a role for auditory feedback in developing and maintaining normal mouse USVs, and evidence for and against limited vocal imitation of song pitch. To reconcile these findings, we suggest that the trait of vocal learning may not be dichotomous but encompass a broad spectrum of behavioral and neural traits we call the continuum hypothesis, and that mice possess some of the traits associated with a capacity for limited vocal learning.

The neural control of vocalization in mammals: a review

The review describes a model of vocal control, based mainly on research in the squirrel monkey, which consists of two hierarchically organized pathways. One runs from the anterior cingulate cortex via the periaqueductal gray (PAG) into the reticular formation of pons and medulla oblongata, and from there to the phonatory motoneurons. This pathway controls the readiness to vocalize. Although the anterior cingulate cortex in this pathway plays a role in voluntary initiation of vocal behavior, the PAG is involved in vocal gating at a more elementary level. The second pathway runs from the motor cortex via the reticular formation to the phonatory motoneurons and includes two feedback loops providing the motor cortex with preprocessed information needed by the latter to generate the final motor commands. One of these feedback loops involves the basal ganglia and the other the cerebellum. The motor cortex together with its feedback loops is involved in the production of learned vocal patterns. These structures seem to be dispensable, however, for the production of innate vocal patterns, such as the nonverbal emotional vocal utterances of humans and most nonhuman mammalian vocalizations. These innate vocal patterns seem to be generated in the pontine and medullary reticular formation.

Anxiogenic effects of activation of NK-1 receptors of the dorsal periaqueductal gray as assessed by the elevated plus-maze, ultrasound vocalizations and tail-flick tests

Ultrasound vocalizations (USVs) known as 22kHz are usual components of the defensive responses of rats exposed to threatening conditions. The amount of emission of 22kHz USVs depends on the intensity of the aversive stimuli. While moderate fear causes an anxiolytic-sensitive enhancement of the defensive responses, high fear tended to reduce the defensive performance of the animals to aversive stimuli. The dorsal periaqueductal gray (dPAG) is an important vocal center and a crucial structure for the expression of defensive responses. Substance P (SP) is involved in the modulation of the defensive response at this midbrain level, but the type of neurokinin receptors involved in this action is not completely understood. In this study we examined whether local injections of the selective NK-1 agonist SAR-MET-SP (10-100 pmol/0.2microL) into the dPAG (i) cause anxiogenic effects in the elevated plus-maze (EPM) (Exp. I), (ii) influence the novelty-induced 22kHz USVs recorded within the frequency range of 20-26kHz (Exp. II) and (iii) change the nociceptive reactivity to heat applied to the rat's tail (Exp III). The data obtained showed that SAR-MET-SP elicited significant "anxiety-like" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. These anxiogenic effects were accompanied with antinociception and disruption of the novelty-induced increase in the number and duration of 22kHz USVs. These findings are in agreement with the notion that NK-1 receptors of the dPAG may be an important neurochemical target for new selective drugs aimed at the control of pathological anxiety states.

Neural systems for vocal learning in birds and humans: a synopsis

I present here a synopsis on a hypothesis that I derived on the similarities and differences of vocal learning systems in vocal learning birds for learned song and in humans for spoken language. This hypothesis states that vocal learning birds-songbirds, parrots, and hummingbirds-and humans have comparable specialized forebrain regions that are not found in their close vocal non-learning relatives. In vocal learning birds, these forebrain regions appear to be divided into two sub-pathways, a vocal motor pathway mainly used to produce learned vocalizations and a pallial-basal-ganglia-thalamic loop mainly used to learn and modify the vocalizations. I propose that humans have analogous forebrain pathways within and adjacent to the motor and pre-motor cortices, respectively, used to produce and learn speech. Recent advances have supported the existence of the seven cerebral vocal nuclei in the vocal learning birds and the proposed brain regions in humans. The results in birds suggest that the reason why the forebrain regions are similar across distantly related vocal learners is that the vocal pathways may have evolved out of a pre-existing motor pathway that predates the ancient split from the common ancestor of birds and mammals. Although this hypothesis will require the development of novel technologies to be fully tested, the existing evidence suggest that there are strong genetic constraints on how vocal learning neural systems can evolve.

Substance P injected into the dorsal periaqueductal gray causes anxiogenic effects similar to the long-term isolation as assessed by ultrasound vocalizations measurements

Housing conditions change the emotional state of the animals. Ultrasound vocalizations (USVs) termed as 22 kHz are the usual components of the defensive responses of rats exposed to threatening conditions such as isolation. The amount of emission of 22 kHz USVs depends on the intensity of the aversive stimuli. While short periods of isolation caused an anxiolytic-sensitive enhancement of the defensive responses, long-term isolation tended to reduce the defensive performance of the animals to aversive stimuli. The dorsal periaqueductal gray (dPAG) is an important vocal center and a crucial structure for the expression of defensive response. While it has been shown that Substance P (SP) at this midbrain level is involved in the modulation of the defensive response, its role in the emission of ultrasound vocalizations has not been evaluated. In this study we examined whether the resocialization and local injections of SP into the dPAG have an influence on the isolation-induced 22 kHz USVs recorded within the frequency range of 18-26 kHz. Rats isolated for 1 day showed a significant increase in the number and duration of USVs, which were reversed by resocialization. On the other hand, 2-week isolation reduced the number and duration of 22 kHz USVs, which could not be reversed by resocialization. SP injections into the dPAG (35 pmol/0.2 microL) caused a reduction in the 22 kHz USVs. Pretreatment with the NK-1 receptor antagonist spantide (100 pmol/0.2 microL) blocked these effects but exhibited no effect when given alone. These findings suggest that 1-day and 2-week isolation recruit distinct brain defensive systems. Also, in agreement with the notion that intense fear is associated with the neural substrates of fear of the dPAG, activation of NK-1 receptors of this midbrain structure reduces the 22 kHz USVs.

On the role of the reticular formation in vocal pattern generation

This review is an attempt to localize the brain region responsible for pattern generation of species-specific vocalizations. A catalogue is set up, listing the criteria considered to be essential for a vocal pattern generator. According to this catalogue, a vocal pattern generator should show vocalization-correlated activity, starting before vocal onset and reflecting specific acoustic features of the vocalization. Artificial activation by electrical or glutamatergic stimulation should produce artificially sounding vocalization. Lesioning is expected to have an inhibitory or deteriorating effect on vocalization. Anatomically, a vocal pattern generator can be assumed to have direct or, at least, oligosynaptic connections with all the motoneuron pools involved in phonation. A survey of the literature reveals that the only area meeting all these criteria is a region, reaching from the parvocellular pontine reticular formation just above the superior olive through the lateral reticular formation around the facial nucleus and nucleus ambiguus down to the caudalmost medulla, including the dorsal and ventral reticular nuclei and nucleus retroambiguus. It is proposed that vocal pattern generation takes place within this whole region.

Audio-vocal interaction in the pontine brainstem during self-initiated vocalization in the squirrel monkey

The adjustment of the voice by auditory input happens at several brain levels. The caudal pontine brainstem, though rarely investigated, is one candidate area for such audio-vocal integration. We recorded neuronal activity in this area in awake, behaving squirrel monkeys (Saimiri sciureus) during vocal communication, using telemetric single-unit recording techniques. We found audio-vocal neurons at locations not described before, namely in the periolivary region of the superior olivary complex and the adjacent pontine reticular formation. They showed various responses to external sounds (noise bursts) and activity increases (excitation) or decreases (inhibition) to self-produced vocalizations, starting prior to vocal onset and continuing through vocalizations. In most of them, the responses to noise bursts and self-produced vocalizations were similar, with the only difference that neuronal activity started prior to vocal onset. About one-third responded phasically to noise bursts, independent of whether they increased or decreased their activity to vocalization. The activity of most audio-vocal neurons correlated with basic acoustic features of the vocalization, such as call duration and/or syllable structure. Auditory neurons near audio-vocal neurons showed significantly more frequent phasic response patterns than those in areas without audio-vocal activity. Based on these findings, we propose that audio-vocal neurons showing similar activity to external acoustical stimuli and vocalization play a role in olivocochlear regulation. Specifically, audio-vocal neurons with a phasic response to external auditory stimuli are candidates for the mediation of basal audio-vocal reflexes such as the Lombard reflex. Thus, our findings suggest that complex audio-vocal integration mechanisms exist in the ventrolateral pontine brainstem.

Localization of a vocal pattern generator in the pontine brainstem of the squirrel monkey

Very little is known about the coordination of muscles involved in mammalian vocalization at the level of single neurons. In the present study, a telemetric single-unit recording technique was used to explore the ventrolateral pontine brainstem for vocalization-correlated activity in the squirrel monkey during vocal communication. We found a discrete area in the reticular formation just above the superior olivary complex showing vocalization-correlated activity. These neurons showed an increase in neuronal activity exclusively just before and during vocalization; none of them was active during mastication, swallowing or quiet respiration. Furthermore, the neuronal activity of these neurons reflected acoustic features, such as call duration or syllable structure of frequency-modulated vocalization, directly. Based on these findings and previously reported anatomical data, we propose that this area serves as a vocal pattern generator for frequency-modulated call types.

Midbrain periaqueductal gray and vocal patterning in a teleost fish

Midbrain structures, including the periaqueductal gray (PAG), are essential nodes in vertebrate motor circuits controlling a broad range of behaviors, from locomotion to complex social behaviors such as vocalization. Few single-unit recording studies, so far all in mammals, have investigated the PAG's role in the temporal patterning of these behaviors. Midshipman fish use vocalization to signal social intent in territorial and courtship interactions. Evidence has implicated a region of their midbrain, located in a similar position as the mammalian PAG, in call production. Here, extracellular single-unit recordings of PAG neuronal activity were made during forebrain-evoked fictive vocalizations that mimic natural call types and reflect the rhythmic output of a known hindbrain-spinal pattern generator. The activity patterns of vocally active PAG neurons were mostly correlated with features related to fictive call initiation. However, spike trains in a subset of neurons predicted the duration of vocal output. Duration is the primary feature distinguishing call types used in different social contexts and these cells may play a role in directly establishing this temporal dimension of vocalization. Reversible, lidocaine inactivation experiments demonstrated the necessity of the midshipman PAG for fictive vocalization, whereas tract-tracing studies revealed the PAG's connectivity to vocal motor centers in the fore- and hindbrain comparable to that in mammals. Together, these data support the hypotheses that the midbrain PAG of teleosts plays an essential role in vocalization and is convergent in both its functional and structural organization to the PAG of mammals.

Telemetric recording of neuronal activity

A telemetric system is described which allows the wireless registration of extracellular neuronal activity and vocalization-associated skull vibrations in freely moving, socially living squirrel monkeys (Saimiri sciureus). The system consists of a carrier platform with numerous guiding tubes implanted on the skull. Custom-made microdrives are mounted on the platform, allowing the exploration of two electrode tracks at the same time. Commercially available quartz-insulated platinum-tungsten microelectrodes are used. The electrodes can be moved over a distance of 8-10 mm by turning a screw on the microdrive. Vocalization-associated skull vibrations are recorded with a piezo-ceramic element. Skull vibration signal and the signals from the two microelectrodes are fed into separate transmitters having different carrier frequencies. The signals are picked up by an antenna in the animal cage and are sent to three receivers in the central laboratory. Here, the signals are transferred via an analog/digital interface to a personal computer for data analysis and to a video recorder for long-term storage. The total weight of the head mount including carrier platform, microdrive, electrodes, skull vibration sensor, three transmitters, and protection cap is 32 g. The transmitters are powered with two rechargeable lithium batteries, allowing about 8 h of continuous recording. Reliable signal transmission is obtained over a distance of about 2 m. Recording stability allows to follow the activity of specific neurons up to several hours, with no movement artefacts during locomotion.

Call type-specific differences in vocalization-related afferents to the periaqueductal gray of squirrel monkeys (Saimiri sciureus)

In a recent retrograde tracing study in the squirrel monkey, we found that regions in the midbrain periaqueductal gray (PAG) producing different call types when pharmacologically stimulated, receive their input largely from the same structures. The aim of the present study was to find out, whether there are quantitative differences in this input. For this reason, we counted retrogradely labeled neurons in various brain regions after injections of wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP) into three different vocalization-eliciting PAG sites: one site producing non-aversive contact calls (clucking); a second site producing slightly aversive social mobbing calls (cackling); and a third site producing highly aversive defensive threat calls (shrieking). Cell counting was carried out by the help of the optical fractionator technique. Six squirrel monkeys were used, two for each call type. In some regions, marked differences in the number of retrogradely labeled cells between the three call type groups occured. Such regions are the nucl. accumbens, preoptic area, posterior hypothalamus, anterior cingulate cortex, subcallosal gyrus and the nucl. striae terminalis. In some of these regions, the number of retrogradely labeled cells correlated positively (posterior hypothalamus) or negatively (preoptic area, nucl. striae terminalis) with the "aversiveness" of the elicited call type. Other regions of interest, e.g., the dorsomedial prefrontal and precallosal cortex, amygdala and hypothalamic regions surrounding the fornix, revealed no clear differences in their afferent projections to the different vocalization-eliciting PAG sites. The results make clear that distinct vocalization-controlling regions in the PAG receive a qualitatively similar but quantitatively differentiated input.

Afferents of vocalization-controlling periaqueductal regions in the squirrel monkey

In order to determine the input of vocalization-controlling regions of the midbrain periaqueductal gray (PAG), wheat germ agglutinin-horseradish peroxidase was injected in six squirrel monkeys (Saimiri sciureus) at PAG sites yielding vocalization when injected with the glutamate agonist homocysteic acid. Brains were scanned for retrogradely labeled areas common to all six animals. The results show that the vocalization-eliciting sites receive a widespread input, with the heaviest projections coming from the surrounding PAG, dorsomedial and ventromedial hypothalamus, medial preoptic region, substantia nigra pars diffusa, zona incerta and reticular formation of the mesencephalon, pons, and medulla. The heaviest cortical input reaches the PAG from the mediofrontal cortex. Moderate to weak projections come from the insula, lateral prefrontal, and premotor cortex as well as the superior and middle temporal cortex. Subcortical moderate to weak projections reach the PAG from the central and medial amygdala, nucleus of the stria terminalis, septum, nucleus accumbens, lateral preoptic region, lateral and posterior hypothalamus, globus pallidus, pretectal area, deep layers of the superior colliculus, the pericentral inferior colliculus, mesencephalic trigeminal nucleus, locus coeruleus, substantia nigra pars compacta, dorsal and ventral raphe, vestibular nuclei, spinal trigeminal nucleus, solitary tract nucleus, and nucleus gracilis. The input of the periaqueductal vocalization-eliciting regions thus is dominated by limbic, motivation-controlling afferents; input, however, also comes from sensory, motor, arousal-controlling, and cognitive brain areas.

Learned birdsong and the neurobiology of human language

Vocal learning, the substrate for human language, is a rare trait found to date in only three distantly related groups of mammals (humans, bats, and cetaceans) and three distantly related groups of birds (parrots, hummingbirds, and songbirds). Brain pathways for vocal learning have been studied in the three bird groups and in humans. Here I present a hypothesis on the relationships and evolution of brain pathways for vocal learning among birds and humans. The three vocal learning bird groups each appear to have seven similar but not identical cerebral vocal nuclei distributed into two vocal pathways, one posterior and one anterior. Humans also appear to have a posterior vocal pathway, which includes projections from the face motor cortex to brainstem vocal lower motor neurons, and an anterior vocal pathway, which includes a strip of premotor cortex, the anterior basal ganglia, and the anterior thalamus. These vocal pathways are not found in vocal non-learning birds or mammals, but are similar to brain pathways used for other types of learning. Thus, I argue that if vocal learning evolved independently among birds and humans, then it did so under strong genetic constraints of a pre-existing basic neural network of the vertebrate brain.