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.     

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.     

Anatomical and physiological evidence for a relationship between the ‘cingular’ vocalization area and the auditory cortex in the squirrel monkey

With the aid of the autoradiographic tracing technique the projections from cortical limbic vocalization areas to the auditory cortex in the superior temporal gyrus were studied in the squirrel monkey. The vocalization areas were identified by exploring the anterior limbic cortex with moving electrodes until a site was found where electrical stimulation yielded vocalization. Projections from the region around the cingulate sulcus and supracallosal anterior cingulate gyrus have their terminal fields in the lower part of the superior temporal gyrus (STG) and upper bank of the superior temporal sulcus. Injections just in front of the genu of the corpus callosum and in the subcallosal gyrus and gyrus rectus lead to terminal fields in the middle part of STG. No projections were found in the upper part of STG, i.e. the primary auditory cortex.To test the functional properties of this pathway, action potentials of single neurons in the auditory cortex were recorded during electrical stimulation of the cingular vocalization area. From a total of 135 STG neurons, an effect on spontaneous activity was seen in 27 cells. All except one of these neurons also reacted to acoustic stimuli. In most cases, stimulation of the cingular area caused a decrease in the discharge rate of the STG neurons. In 4 neurons, stimulation of the vocalization area had an influence on the acoustic reactivity of the STG neurons. The results provide evidence that during phonation the ‘cingular’ vocalization area exerts a predominantly inhibitory influence on auditory cortex neurons. This effect probably is mediated via the extreme capsule. Its possible function is discussed.     

Neuronal activity in the medulla oblongata during vocalization. A single-unit recording study in the squirrel monkey

In six squirrel monkeys (Saimiri sciureus), the medulla oblongata was explored with microelectrodes, looking for vocalization-correlated activity. The vocalizations were elicited by microinjections of glutamate agonists into the periaqueductal grey of the midbrain. Vocalization-related cells were found in greater numbers in the nucl. ambiguus (Ab) and retroambiguus (RAb), in the parvocellular, magnocellular and central reticular formation as well as in the solitary tract nucleus and spinal trigeminal nucleus. Small numbers were also found in the vestibular complex, cuneate nuclei, inferior olive and lateral reticular nucleus. A differentiation of the neuronal responses into 12 reaction types reveals that the frequency of each reaction type varies from brain structure to brain structure, thus allowing a specification of the different vocalization-related areas. According to this specification, it is proposed that initiation of vocalization takes place via the parvocellular reticular formation; vocal pattern control is mainly brought about by the parvocellular reticular formation, Ab, solitary tract nucleus and spinal trigeminal nucleus; expiratory control and respiratory-laryngeal coordination is carried out by the RAb, Ab and central nucleus of the reticular formation; vocalization-specific postural adjustments are carried out via the vestibular and cuneate nuclei.     

Generating sexually differentiated vocal patterns: laryngeal nerve and EMG recordings from vocalizing male and female african clawed frogs (Xenopus laevis).

Male and female African clawed frogs (Xenopus laevis) produce sexually dimorphic vocalizations; for males these include advertisement, amplectant, and growling calls, whereas female calls include ticking. Previous studies have shown that the vocal organ, the larynx, of the sexes differs in physiological properties that parallel vocal differences. However, it was not clear whether these characteristics are sufficient to explain sex differences in vocal behavior. To examine the contribution of the CNS to generating vocal patterns, we developed a preparation in which both laryngeal nerve activity and electromyograms can be recorded from awake, vocalizing frogs. Recordings reveal that the CNS of the two sexes produces patterned activity that closely matches each vocalization whereas the larynx faithfully translates nerve activity into sound. Thus, the CNS is the source of sexually differentiated vocalizations in Xenopus laevis. Furthermore, detailed analyses of compound action potentials recorded from the nerve lead us to hypothesize that neuronal activity underlying different male call types is distinct; some calls are likely to be generated by synchronous firing of motoneuron populations of either constant size or progressively larger sizes, whereas others are generated by asynchronous activity of motoneurons, a pattern shared with vocal production in females. We suggest that these distinct neuronal activity patterns in males may be subserved by two populations of motor units in males that can be distinguished by the strength of the neuromuscular synapse.     

Laryngeal afferent inputs during vocalization in the cat.

To reveal the kind of information about the larynx which is transmitted to the central nervous system during vocalization, we studied discharge patterns of single fibers of the laryngeal afferent nerve during electrically induced vocalization in ketamine-anesthetized cats. Recorded fibers were classified into four types based on their discharge patterns. Type A fibers responded to vocal fold vibration during vocalization. Type B fibers increased their activity during vocalization without synchronization with vocal fold vibration. Type C fibers decreased their activity during vocalization. Type D fibers discharged only at the onset of vocal fold adduction and abduction. We discuss the functional properties of these afferents and the possibility that these afferent inputs participate in the feedback control of vocalization.     

The presence of an audience modulates responses to familiar call stimuli in the male zebra finch forebrain

The ability to recognize familiar individuals is crucial for establishing social relationships. The zebra finch, a highly social songbird species that forms lifelong pair bonds, uses a vocalization, the distance call, to identify its mate. However, in males, this ability depends on social conditions, requiring the presence of an audience. To evaluate whether the presence of bystanders modulates the auditory processing underlying recognition abilities, we assessed, by using a lightweight telemetry system, whether electrophysiological responses driven by familiar and unfamiliar female calls in a high‐level auditory area [the caudomedial nidopallium (NCM)] were modulated by the presence of conspecific males. Males had experienced the call of their mate for several months and the call of a familiar female for several days. When they were exposed to female calls in the presence of two male conspecifics, NCM neurons showed greater responses to the playback of familiar female calls, including the mate's call, than to unfamiliar ones. In contrast, no such discrimination was observed in males when they were alone or when call‐evoked responses were collected under anaesthesia. Together, these results suggest that NCM neuronal activity is profoundly influenced by social conditions, providing new evidence that the properties of NCM neurons are not simply determined by the acoustic structure of auditory stimuli. They also show that neurons in the NCM form part of a network that can be shaped by experience and that probably plays an important role in the emergence of communication sound recognition.     

Expression of androgen receptor mRNA in the brain of Gekko gecko: implications for understanding the role of androgens in controlling auditory and vocal processes

The neuroanatomical distribution of androgen receptor (AR) mRNA-containing cells in the brain of a vocal lizard, Gekko gecko, was mapped using in situ hybridization. Particular attention was given to auditory and vocal nuclei. Within the auditory system, the cochlear nuclei, the central nucleus of the torus semicircularis, the nucleus medialis, and the medial region of the dorsal ventricular ridge contained moderate numbers of labeled neurons. Neurons labeled with the AR probe were located in many nuclei related to vocalization. Within the hindbrain, the mesencephalic nucleus of the trigeminal nerve, the vagal part of the nucleus ambiguus, and the dosal motor nucleus of the vagus nerve contained many neurons that exhibited strong expression of AR mRNA. Neurons located in the peripheral nucleus of the torus in the mesencephalon exhibited moderate levels of hybridization. Intense AR mRNA expression was also observed in neurons within two other areas that may be involved in vocalization, the medial preoptic area and the hypoglossal nucleus. The strongest mRNA signals identified in this study were found in cells of the pallium, hypothalamus, and inferior nucleus of the raphe. The expression patterns of AR mRNA in the auditory and vocal control nuclei of G. gecko suggest that neurons involved in acoustic communication in this species, and perhaps related species, are susceptible to regulation by androgens during the breeding season. The significance of these results for understanding the evolution of reptilian vocal communication is discussed.     

Telemetrically recorded neuronal activity in the inferior colliculus and bordering tegmentum during vocal communication in squirrel monkeys (Saimiri sciureus)

In order to find out whether the inferior colliculus, in addition to its auditory decoding function, also has an auditory gating function in the sense that it treats self-produced sounds differently from external ones, we have explored the inferior colliculus and bordering tegmentum for neurones reacting differently to self-produced vocalizations and vocalizations produced by conspecifics. The experiments were made in the squirrel monkey (Saimiri sciureus), using a telemetric extracellular recording technique which allowed to register neuronal activity in freely moving animals during natural vocal communication. The results show that the neurones of the central nucleus of the inferior colliculus do not react differently to self-produced and group mate vocalizations of the same type. In the external nucleus of the inferior colliculus, in addition to classical auditory neurones, neurones are found which react to the vocalizations of group mates, but not to self-produced vocalizations. In the paralemniscal area just below the inferior colliculus, there are neurones which are active during self-produced vocalization, but not during vocalization produced by other animals. The results suggest that the external nucleus of the inferior colliculus and bordering tegmentum are involved in vocalization-dependent auditory gating processes.     

On the search for the vocal pattern generator. A single-unit recording study

In the squirrel monkey (Saimiri sciureus), single-unit activity was compared between the midbrain periaqueductal grey and the parvocellular and central nuclei of the medullary reticular formation during the production of species-specific vocalization. It was found that all three areas contain neurones with vocalization-related activity. The relative number of specific reactions types differed between areas, however. While the majority of periaqueductal cells fired just before, but not during vocalization, most cells in the reticular formation fired before and during vocalization. Modulation of discharge rate with changing fundamental frequency was only found in the reticular formation, not the periaqueductal grey. It is concluded that the parvocellular and central nuclei of the reticular formation, but not the periaqueductal grey are involved in vocal pattern generation.     

Periaqueductal gray and the region of the paralemniscal area have different functions in the control of vocalization in the neotropical bat,Phyllostomus discolor

The periaqueductal gray matter and the region of the paralemniscal area were neuroanatomically delineated in the brain of the neotropical bat Phyllostomus discolor[Wagner (1843) Arch. Naturgesch., 9, 365-368] and were probed with electrical microstimulation for eliciting vocalizations. In a well-delimited rostral portion of the periaqueductal gray exclusively, communication calls could be triggered at low stimulation currents. Communication calls as well as echolocation calls could be elicited at the dorsal and ventral edges of this area. Pharmacological stimulation with microdialysed kainic acid in this particular periaqueductal gray area demonstrated that neurons and not fibres of passage are activated for triggering vocalization. Solely echolocation calls were emitted upon electrical microstimulation or with microdialysed kainic acid in the region of the paralemniscal area. The periaqueductal gray appears to be involved in vocal pathways that control both communication calls and echolocation calls, while the region of the paralemniscal area seems to be specialized for control of echolocation calls only. Respiration is similarly influenced by stimulation in the periaqueductal gray and the region of the paralemniscal area. Periaqueductal gray and paralemniscal area interact differently with the final common pathway for vocalization, and may represent different functional organization in the vocal controlling pathways for communication calls and echolocation calls.     

Feeding and contact call stimulation both induce zenk and cfos expression in a higher order telencephalic area necessary for vocal learning in budgerigars

Stimulation with natural contact calls and feeding were used to assess zenk and fos protein expression in budgerigars (Melopsittacus undulatus), a vocal learning parrot species in which feeding and physical contact often occur in conjunction with vocalization. Although only calls induced gene expression in Field L, the primary telencephalic auditory area, both calls and feeding induced gene expression in the frontal lateral nidopallium (NFl), a brain area in receipt of input from Field L which projects to areas afferent to vocal control nuclei and which is necessary for new call learning. NFl thus appears poised to provide both non-auditory as well as auditory feedback to the vocal system.     

Possible roles for GABAergic inhibition in the vocal control system of the zebra finch

The final common output from the telencephalic vocal control system in songbirds is the projection from nucleus RA, which drives respiratory and syringeal muscles via medullary nuclei. We examined the possible role of GABAergic inhibition in RA of adult male zebra finches by micro-injecting bicuculline, an antagonist of inhibitory GABA(A) receptors, while recording simultaneously with multiple microelectrodes. Following bicuculline injection, the normally high spontaneous activity of RA neurons exhibited a pattern of rhythmic bursting lasting up to 30 min. The bursts were often accompanied by involuntary vocalizations: monosyllabic notes resembling calls. Other experiments used microinjections that were below threshold for involuntary vocalization. When the bird sang to a female during a period after the injection, song structure was degraded: song duration was lengthened, noisiness increased, and novel syllables appeared. The results suggest that GABA normally contributes to regulating excitability in RA. When this regulation is blocked, activity increases sufficiently to engage the respiratory and vocal musculature. Synaptic inputs that affect GABAergic interneurons in RA could thus play a role in initiation and control of vocalization. The abnormal vocalizations produced in the presence of bicuculline suggest that GABAergic inhibition may normally help to shape the pattern of learned vocalizations, as well as to regulate overall RA activity.     

Cortical lesion effects and vocalization in the squirrel monkey

The effects of bilateral destruction of the cortical face area, anterior and posterior supplementary motor area and anterior cingular cortex on spontaneous vocalization were studied in 16 squirrel monkeys (Saimiri sciureus). Each type of lesion was made in two groups of two animals each. Both animals of a group received the same type of lesion at the same day. Each group was recorded for 10 sessions of one hour before operation and 10 sessions after operation. Pre- and post-operative vocalizations were compared in respect to total number and acoustic structure. It was found that none of the lesions affected acoustic structure as judged by a sonagraphic analysis. However, lesions in the anterior supplementary motor area (at the level of the callosal genu) reduced the total vocalization number significantly. This decrease was essentially due to a drastic reduction of the so-called isolation peep, a long-distance contact call. The results suggest: (i) that the cortical face area is only involved in the control of learnt vocal utterances (such as human speech and song) but not in the production of genetically preprogrammed utterances (such as monkey calls and human pain groans); (ii) that the anterior cingulate cortex is necessary for the volitional initiation of vocalization but not for the initiation of calls in an emotional situation; (iii) that the posterior supplementary motor area does not play any role in vocal behaviour of monkeys; and (iv) that the anterior supplementary motor area is involved in the production of vocalizations which are not triggered directly by external events.     

Repetitive vocalizations evoked by electrical stimulation of avian brains. IV. Evoked and spontaneous activity in expiratory and inspiratory nerves and muscles of the chicken (Gallus gallus).

The activity in respiratory nerves and muscles in response to electrical stimulation of vocal substrates in the brain and to CO2 stimulation of the respiratory centers was studied in 28 adult chickens. It was found that the same nerves and muscles were active during both vocalization and respiration. Stimulation of vocal substrates resulted in short latency bursting in the expiratory nerves and muscles. As stimulation intensity increased, progressively longer duration bursts composed of numerous subbursts were produced. By relating muscle activity with sound production , such bursting was shown to underlie evoked vocalizations. Background activity in inspiratory nerves and muscles continued uninterruptedly past stimulus onset only stopping when expiratory activity began. Thereafter inspiratory bursting reciprocated with expiratory bursting and was shown to underlie the intervals between vocalizations. The pattern of activity which was evoked by stimulating vocal substrates was found to strongly interact with the pattern of activity evoked by CO2 stimulation of the respiratory system. Simultaneous records of respiratory and tracheal muscles demonstrated that the same information was sent to both groups of muscles during evoked vocalization. Activity in the respiratory muscles was recorded during spontaneous vocalization of a free-moving bird and was found to resemble that recorded from anesthetized birds. Finally the activity of single units in the obex region of the medulla was recorded during electrical stimulation of vocal substrates and during CO2 stimulation of the respiratory system. Rhythmically active units were found only in the medulla. Unit activity paralleled that found in the nerves and muscles. On the basis on the data accumulated, two models of the chicken vocal system are presented. The first is a model of the sound-producing structures of the chicken. The second is a model of the neural machinery which controls the sound-producing structures. The two models are used as a basis for an explanation of the production of voclizations by the chick of the same species.     

Vocalization and stapedius muscle activity evoked by local electrical stimulation of midbrain in the chicken (Gallus gallus)

The organization of chicken mesencephalic areas from which stapedius muscle activity and vocalization can be differently elicited was studied. Our results show the existence of an area, around the mesencephalic ‘calling area’, from which stapedius muscle activity can be evoked independently of vocalization. Furthermore, low threshold ‘vocalization loci’ stimulation evokes field potentials in the stapedius-controlling area, due to the activation of stapedius-controlling neurons by vocalization neurons.     

Neurogenesis and plasticity in the CNS of adult birds.

Neurogenesis, typically a developmental phenomenon, continues into adult life in song birds. Cells born in the walls of the lateral ventricle migrate and differentiate throughout the adult telencephalon. I will argue here that birds take advantage of these new neurons as a form of plasticity. Most of the neurons connecting the different song control nuclei are born early in development. One important exception is the central efferent motor pathway for learned vocalization. This pathway is formed by projection neurons born during juvenile and adult life. Recruitment of new projection neurons at different times of the year and in different species correlates with vocal learning. Adult neurogenesis as a form of plasticity may serve learning and it may also teach us how to repair the damaged brain.     

Vocalization and stapedius muscle activity evoked by local electrical stimulation of midbrain in the chicken (Gallus gallus)

The organization of chicken mesencephalic areas from which stapedius muscle activity and vocalization can be differently elicited was studied. Our results show the existence of an area, around the mesencephalic ‘calling area’, from which stapedius muscle activity can be evoked independently of vocalization. Furthermore, low threshold ‘vocalization loci’ stimulation evokes field potentials in the stapedius-controlling area, due to the activation of stapedius-controlling neurons by vocalization neurons.     

Reward-related neuronal activity in the subthalamic nucleus of the monkey

The subthalamic nucleus is a key structure for motor information processing in the basal ganglia. Little is known about its involvement in other aspects of behavior such as motivation. We investigated neuronal activity in the subthalamic nucleus while a monkey performed arm-reaching movements to obtain a liquid reward. Most neurons were modulated both during the movement and reward phases of the task. The changes in activity occurring after or just before the delivery of reward consisted of either increases or decreases in firing and were not directly related to mouth movements. These findings indicate that STN neurons are involved in the detection and expectation of reward, consistent with a role for these neurons in the processing of motivational information.     

Activity of cholinergic neurons in the laterodorsal tegmental nucleus during emission of 22kHz vocalization in rats

It has been postulated that the ascending cholinergic tegmental system is responsible for the initiation of the aversive emotional state with a concomitant alarm vocalization in the rat. It is assumed that the activity of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) will cause release of acetylcholine in the target areas and will initiate the emission of 22 kHz vocalizations. The goal of the present study was to test the hypothesis that the cholinergic neurons of the LDT increase their activity during emission of 22 kHz alarm calls. Vocalizations were induced by an air puff or by intrahypothalamic-preoptic injection of carbachol. The activity of the LDT cholinergic neurons was studied by a double histochemical labelling for choline acetyltransferase, as a marker of cholinergic somata, and for c-Fos protein, as a marker of cells with heighten metabolic activity. Both air puff stimulation and intracerebral carbachol induced comparable 22 kHz alarm vocalizations. The activity of neurons in the LDT was significantly higher during prolonged emission of 22 kHz alarm calls induced by air puff or injection of carbachol than in the non-vocalizing or low-vocalizing controls. There were approximately two times more of all c-Fos-labelled cells in the LDT of vocalizing animals and 2.5 times more active cholinergic neurons during prolonged 22 kHz vocalization than in the control conditions without vocalization. However, the active cholinergic neurons constituted only a small proportion of all active LDT cells (2.3%). At the same time, there were no significant increases in the number of c-Fos-labelled cells in the neighbouring pedunculopontine nucleus (PPT). These findings lead to the conclusion that the neurons of the LDT, including cholinergic neurons, but not those of the PPT, significantly increased their activity during prolonged emission of alarm vocalizations, as evidenced by the c-Fos immunoreactivity.