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.     

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.     

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.     

C-fos Expression During Vocal Mobbing in the New World Monkey Saguinus fuscicollis

In order to find brain areas involved in the vocal expression of emotion, we compared c-fos expression in three groups of saddle-back tamarins (Saguinus fuscicollis). One group, consisting of three animals, was made to utter more than 800 mobbing calls by electrical stimulation of the periaqueductal grey of the midbrain (PAG). A second group, consisting of two animals, was stimulated in the PAG with the same intensity and for the same duration as the first group but at sites that did not produce vocalization. These sites lay somewhat medial to the vocalization-eliciting sites. A third group, consisting of two animals, was stimulated at vocalization-eliciting sites in the PAG but with an intensity below vocalization threshold. Fos-like immunoreactivity that was found in the vocalizing but not in the non-vocalizing animals was located in the dorsomedial and ventrolateral prefrontal cortex, anterior cingulate cortex, ventrolateral premotor cortex, sensorimotor face cortex, insula, inferior parietal cortex, superior temporal cortex, claustrum, entorhinal and parahippocampal cortex, basal amygdaloid nucleus, anterior and dorsomedial hypothalamus, nucleus reuniens, lateral habenula, Edinger-Westphal nucleus, ventral and dorsolateral midbrain tegmentum, nucleus cuneiformis, sagulum, pedunculopontine and laterodorsal tegmental nuclei, ventral raphe, periambigual reticular formation and solitary tract nucleus. For some of these structures (e.g. anterior cingulate cortex and periambigual reticular formation), there is evidence also from electrical stimulation, lesioning and single-unit recording studies that they are involved in vocal control. For other structures (e.g. lateral habenula, Edinger-Westphal nucleus), the available evidence speaks against such a role. Fos activation in these cases is probably related to non-vocal reactions accompanying the electrically elicited vocalizations. A third group of structures consists of areas for which a role in vocal control cannot be excluded but for which the present study presents the first evidence for such a role (e.g. claustrum and sagulum). These structures deserve further studies using more specific methods.     

Glutamate-induced vocalization in the squirrel monkey

In the squirrel monkey, 164 brain sites yielding vocalization when electrically stimulated were tested for their capability to produce vocalization when injected with mono-sodium-L-glutamate. The sites were located in the anterior limbic cortex, n. accumbens, substantia innominata, amygdala, n. striae terminalis, hypothalamus, midline thalamus, field H of Forel, substantia nigra, periventricular and periaqueductal gray, inferior colliculus, reticular formation of midbrain, pons and medulla, inferior olive, lateral reticular nucleus and nucleus of solitary tract. Of the 164 sites tested, 31 were positive. These were located in the substantia innominata, caudal periventricular and periaqueductal gray, lateral pontine and medullary reticular formation. While all the calls obtained from the forebrain and midbrain had a normal acoustic structure, most pontine and all medullary vocalizations had an artificial character. It is concluded that: the substantia innominata, caudal periventricular and periaqueductal gray, lateral pontine and medullary reticular formation represent relay stations of vocalization-controlling pathways; the periaqueductal gray represents the lowest relay station above the level of motor coordination; neurons responsible for motor coordination of vocalization lie in the reticular formation around the caudal brachium conjunctivum, the superior olive, n. facialis, n. ambiguus and below the n. solitarius; not all areas from which vocalization can be obtained by electrical stimulation of nerve cell bodies, dendrites and nerve endings (in contrast to fibers en passage) also yield vocalization when stimulated with glutamate.     

Neural pathways underlying vocal control

Vocalization is a complex behaviour pattern, consisting of essentially three components: laryngeal activity, respiratory movements and supralaryngeal (articulatory) activity. The motoneurones controlling this behaviour are located in various nuclei in the pons (trigeminal motor nucleus), medulla (facial nucleus, nucl. ambiguus, hypoglossal nucleus) and ventral horn of the spinal cord (cervical, thoracic and lumbar region). Coordination of the different motoneurone pools is carried out by an extensive network comprising the ventrolateral parabrachial area, lateral pontine reticular formation, anterolateral and caudal medullary reticular formation, and the nucl. retroambiguus. This network has a direct access to the phonatory motoneurone pools and receives proprioceptive input from laryngeal, pulmonary and oral mechanoreceptors via the solitary tract nucleus and principal as well as spinal trigeminal nuclei. The motor-coordinating network needs a facilitatory input from the periaqueductal grey of the midbrain and laterally bordering tegmentum in order to be able to produce vocalizations. Voluntary control of vocalization, in contrast to completely innate vocal reactions, such as pain shrieking, needs the intactness of the forebrain. Voluntary control over the initiation and suppression of vocal utterances is carried out by the mediofrontal cortex (including anterior cingulate gyrus and supplementary as well as pre-supplementary motor area). Voluntary control over the acoustic structure of vocalizations is carried out by the motor cortex via pyramidal/corticobulbar as well as extrapyramidal pathways. The most important extrapyramidal pathway seems to be the connection motor cortex-putamen-substantia nigra-parvocellular reticular formation-phonatory motoneurones. The motor cortex depends upon a number of inputs for fulfilling its task. It needs a cerebellar input via the ventrolateral thalamus for allowing a smooth transition between consecutive vocal elements. It needs a proprioceptive input from the phonatory organs via nucl. ventralis posterior medialis thalami, somatosensory cortex and inferior parietal cortex. It needs an input from the ventral premotor and prefrontal cortex, including Broca's area, for motor planning of longer purposeful utterances. And it needs an input from the supplementary and pre-supplementary motor area which give rise to the motor commands executed by the motor cortex.     

All-or-nothing and graded slow waves in the lateral geniculate nucleus to stimulation of the midbrain reticular formation

The occurrence of slow negative potential changes in the lateral geniculate nucleus of the cat in response to stimulation of the midbrain reticular formation was studied. A method for distinguishing all-or-nothing from graded potentials in the presence of noise is described. Graded potential changes were obtained in the conscious and anesthetized states. All-or-nothing potential changes were obtained in the anesthetized animal only at times of spontaneous all-or-nothing waves (similar to ponto-geniculo-occipital waves). All-or-nothing waves could also be elicited in both lateral geniculate nuclei when a transection was made between the lateral geniculate nucleus and the stimulating site in the midbrain reticular formation on one side; this suggests the existence of a descending pathway to the ponto-geniculo-occipital generator in the pontine tegmentum. All-or-nothing waves could also be evoked in both lateral geniculate nuclei after a transection caudal to the stimulating site on one side. This suggests either that such stimulation excites a pathway which crosses to the other side of the brain and then descends to the ponto-geniculo-occipital generator or that the all-or-nothing mechanism is more rostral and stimulation of the midbrain reticular formation either excites this region directly or excites pathways projecting to this region.     

Brainstem afferents to the oculomotor omnipause neurons in monkey

To determine how saccade-related areas in the brainstem address the saccade generator, we examined the afferents to the nucleus raphe interpositus. This region contains the omnipause neurons, which are pivotal in the generation of saccades. Horseradish peroxidase injected iontophoretically into the nucleus raphe interpositus retrogradely labeled a variety of brainstem nuclei. The greatest numbers of labeled neurons were in the paramedian pontomedullary reticular formation, in the nuclei reticularis gigantocellularis, and paragigantocellularis lateralis. Labeling was more modest but consistent in the interstitial nucleus of Cajal and the adjacent mesencephalic reticular formation, the middle gray of the superior colliculi, the region dorsolateral to the nucleus reticularis tegmenti pontis, and the medial vestibular nucleus. A few neurons were labeled around the habenulopeduncular tract and in the medial portion of the nucleus of the fields of Forel, in the nucleus reticularis medullaris ventralis, and in the spinal nucleus of the trigeminal nerve, the cochlear nucleus, and the superior olivary complex. The distribution and density of labeling suggest that omnipause neurons in the monkey are more intimately connected with other oculomotor structures than those in the cat. In addition, the rhombencephalic reticular afferents to the monkey omnipause neurons are more concentrated in their immediate vicinity than in the cat. The label consistently found dorsolateral to the nucleus reticularis tegmenti pontis may be a newly discovered link in saccade generation.     

Efferent projections of the cat oculomotor reticular omnipause neuron region: An autoradiographic study

Omnipause neurons (OPNs) are midline pontine neurons that are thought to be instrumental in the generation of saccadic eye movements. Inhibition of the tonically active OPNs is postulated to disinhibit the burst neurons that cause the saccadic discharge in motoneurons, leading to a sac-cade. To test whether the anatomical connections of OPNs are consistent with this scheme, we studied the efferent projections of the OPN region by using the technique of anterograde transport of tritiated amino acids. Injections into the OPN region yield a distinct pattern of labeled tracts and terminal fields that is different from the patterns following control injections in the surrounding reticular formation. Caudally, there are terminal fields over the paramedian reticular formation, the caudal part of the medial accessory nucleus of the inferior olivary complex, the nucleus prepositus hypoglossi, and the nucleus reticularis paragigantocellularis dorsalis caudal and ventromedial to the abducens nuclei. Rostrally, terminal label is distributed over parts of the nuclei reticularis pontis caudalis and oralis, the nucleus raphe pontis, the nucleus reticularis tegmenti pontis, the mesencephalic reticular nucleus, the central gray, and the nucleus of the H-field. Thus, there are direct projections from the OPN region to all areas known to contain burst neurons. In addition, OPNs also apparently have in-direct access to the spinal cord and cerebellum. Many of the latter connec-tions parallel the efferent projections of the superior colliculus.     

2-Deoxyglucose uptake during vocalization in the squirrel monkey brain

In the squirrel monkey (Saimiri sciureus), the cerebral 2-deoxyglucose uptake was compared between animals made to vocalize by electrical stimulation of the periaqueductal grey and animals stimulated in the same structure, but sub-threshold for vocalization. A significantly higher 2-deoxyglucose uptake in the vocalizers than the non-vocalizers was found in the dorsolateral prefrontal cortex, supplementary and pre-supplementary motor area, anterior and posterior cingulate cortex, primary motor cortex, claustrum, centrum medianum, perifornical hypothalamus, periaqueductal grey, intercollicular region, dorsal mesencephalic reticular formation, peripeduncular nucleus, substantia nigra, nucl. ruber, paralemniscal area, trigeminal motor, principal and spinal nuclei, solitary tract nucleus, nucl. ambiguus, nucl. retroambiguus, nucl. hypoglossus, ventral raphe and large parts of the medullary reticular formation. The study makes clear that vocalization, even in the case of genetically pre-programmed patterns, depends upon an extensive network, beyond the well-known periaqueductal grey, nucl. retroambiguus and cranial motor nuclei pathway.     

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.     

Significance of the Paralemniscal Tegmental Area for Audio-motor Control in the Moustached Bat, Pteronotus p. parnellii: The Afferent and Efferent Connections of the Paralemniscal Area

The paralemniscal tegmental area has been determined in the brain of the New World moustached bat, Pteronotus p. parnellii , by electrical microstimulation eliciting echolocation calls and pinna movements. It is located in the dorsal tegmentum rostral and medial to the dorsal nucleus of the lateral lemniscus and is characterized by medium sized and large neurons. Tracer injections (WGA-HRP) showed that the most intense input to the paralemniscal tegmental area originates in the intermediate and deep layers of the homolateral superior colliculus. The strong projections from the ipsi- and contralateral nucleus praepositus hypoglossus most probably contributes vestibular information. Further inputs in descending order of intensity are from the substantia nigra, the contralateral paralemniscal tegmental area, the putamen, the ventral reticular formation in its lateral portions, the medial cerebellar nucleus and the dorsal reticular formation. Efferent projections of the paralemniscal tegmental area reach the putamen bilaterally, the nucleus accumbens and other parts of the basal ganglia, the pretectal area, the substantia nigra, the intermediate and deep layers of the superior colliculus bilaterally and the tegmental area ventral to it. Connections to the dorsal part of the periaqueductal grey, the cuneiform nucleus and the parabrachial region are important in the context of vocal control, whereas projections to the medial portion of the contralateral facial nucleus may interfere with the control of pinna movement. The findings suggest that the paralemniscal tegmental area is involved in audio-motor control of vocalization and pinna movements in bats; connectional and functional similarities and disparities to tegmental regions described in other mammals are discussed.     

Physiological characterization, localization and synaptic inputs of bursting and nonbursting neurons in the trigeminal principal sensory nucleus of the rat

A population of neurons in the trigeminal principal sensory nucleus (NVsnpr) fire rhythmically during fictive mastication induced in the in vivo rabbit. To elucidate whether these neurons form part of the central pattern generator (CPG) for mastication, we performed intracellular recordings in brainstem slices taken from young rats. Two cell types were defined, nonbursting (63%) and bursting (37%). In response to membrane depolarization, bursting cells, which dominated in the dorsal part of the NVsnpr, fired an initial burst followed by single spikes or recurring bursts. Non-bursting neurons, scattered throughout the nucleus, fired single action potentials. Microstimulation applied to the trigeminal motor nucleus (NVmt), the reticular border zone surrounding the NVmt, the parvocellular reticular formation or the nucleus reticularis pontis caudalis (NPontc) elicited a postsynaptic potential in 81% of the neurons tested for synaptic inputs. Responses obtained were predominately excitatory and sensitive to glutamatergic antagonists DNQX and/or APV. Some inhibitory and biphasic responses were also evoked. Bicuculline methiodide or strychnine blocked the IPSPs indicating that they were mediated by GABA(A) or glycinergic receptors. About one-third of the stimulations activated both types of neurons antidromically, mostly from the masseteric motoneuron pool of NVmt and dorsal part of NPontc. In conclusion, our new findings show that some neurons in the dorsal NVsnpr display both firing properties and axonal connections which support the hypothesis that they may participate in masticatory pattern generation. Thus, the present data provide an extended basis for further studies on the organization of the masticatory CPG network.     

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. I. Excitatory burst neurons

Saccadic burst neurons in the pontine reticular formation have been implicated in the generation of saccades in the horizontal plane on the basis of lesion and extracellular recording studies in the cat and monkey. In the present study, saccadic burst neurons were anatomically and physiologically characterized with intraaxonal recording and injection of horseradish peroxidase in the alert squirrel monkey. A population of burst neurons were found that appear analogous to the excitatory burst neurons (EBNs) described previously in the cat. All neurons are located in the caudal pontine reticular formation and have a major axonal projection to the ipsilateral abducens nucleus. Additional projections were found to the medial vestibular nucleus, the nucleus prepositus, and regions of the pontine and medullary reticular formation rostral, ventral, and caudal to the abducens. All neurons fire exclusively during saccades and have a discharge pattern similar to that of medium-lead burst neurons described previously in the cat and monkey. In most neurons the saccadic burst begins 5-15 msec before saccade onset. Linear relationships exist between burst duration and saccade duration, number of spikes in the burst and saccade amplitude, and firing frequency and instantaneous velocity. Physiological activity of each neuron shows the closest relationship with the amplitude of the saccade component in a particular direction. For all neurons, this on-direction is in the ipsilateral hemifield and is predominantly horizontal, but may have either an upward or downward vertical component. These results support a major role for the EBNs in the monkey in generating the saccadic burst in abducens motoneurons, as well as in contributing to the oculomotor activity in other classes of premotor neurons.     

Anatomical basis for audio-vocal integration in echolocating horseshoe bats

Neurophysiological recordings suggest that audio-vocal neurons located in the paralemniscal tegmentum of the midbrain in horseshoe bats provide an interface between the pathways for auditory sensory processing and those for the motor control of vocalization. To verify these physiological results anatomically, the projection pattern of the audio-vocally active area in the paralemniscal tegmentum was investigated by using extracellular tracer injections of wheat germ agglutinin conjugated to horseradish peroxidase. Several nuclei of the lemniscal auditory pathway (dorsal nucleus of the lateral lemniscus, central nucleus of the inferior colliculus, lateral superior olive) as well as the nucleus of the central acoustic tract appear to project to the paralemniscal tegmentum. Other possible sources of afferent projections are a small but distinctly labeled structure within the lateral hypothalamic area, the substantia nigra pars compacta, the deep mesencephalic nucleus, the rostral portion of the inferior colliculus, the deep and intermediate layers of the superior colliculus, and several small areas in the rhombencephalic reticular formation. No direct efferent projection from the audio-vocally active area of the paralemniscal tegmentum to primarily auditory structures was found. Instead, the main targets were structures that are involved in the control of different motor patterns. These targets include the deep and intermediate layers of the superior colliculus and the dorsomedial portion of the facial nucleus, both of which most probably control pinna movements in cats, and the reticular formation medial and caudal to the facial nucleus and rostral to the nucleus ambiguus, which represents an area involved in the control of vocalization. Hence, the anatomical projection pattern suggests that the paralemniscal tegmentum in horseshoe bats serves as a link between the processing of auditory information and the control of vocalization and related motor patterns. © 1996 Wiley-Liss, Inc.     

Role of nucleus retroambigualis in respiratory reflexes evoked by superior laryngeal and vestibular nerve afferents and in emesis

An ascending projection from the medullary nucleus retroambigualis (NRA) has recently been described as important for the control of the upper airway during vocalization. We evaluated the importance of this projection in other behaviors by making localized injections of the neurotoxin kainic acid in the NRA in decerebrate cats, most of which were paralyzed and artificially ventilated. In contrast to its importance for vocalization, the NRA is not essential for activation of upper airway musculature during respiration, swallowing, vomiting, or reflexes elicited by superior laryngeal or vestibular nerve afferents. However, kainic acid injections in the NRA and adjacent reticular formation prolonged the inhibitory phrenic motoneuronal response to superior laryngeal nerve stimulation and abolished or reduced abdominal motoneuronal responses during respiration, vomiting, and superior laryngeal nerve stimulation. Thus, of the behaviors we investigated, the importance of the ascending projection from the NRA appears to be limited to vocalization, while descending projections from the NRA region are important in a number of behaviors.     

Respiration-related neurons in the ventral medulla of newborn rats in vitro

In brainstem-spinal cord preparations isolated from newborn rats, we examined functions of the ventral medulla in respiratory rhythm generation, and located respiratory neurons in that region. Removal of the dorsal half of the medulla caused only modest reduction of the rate of inspiratory bursts from the cervical (C4 or C5) ventral root and moderate changes in the burst pattern. We describe here two types of respiratory neurons; Pre-I neurons that are presumably crucial in primary rhythm generation, and inspiratory (I) neurons that we presume to be important in inspiratory pattern generation. Pre-I neurons were located close to phenylethanolamine N-methyltransferase (PNMT)-immunoreactive (IR) neurons that are common in the reticular formation of the rostral ventrolateral medulla (RVL). Distributions of I neurons and Pre-I neurons overlapped in the RVL, and I neurons were also near the nucleus ambiguus in the more caudal part of the ventrolateral medulla. The results indicate that the ventral medulla is essential to inspiratory pattern generation as well as rhythm generation. It is suggested that the RVL is an important site in rhythm generation. The region of inspiratory pattern generation may extend more caudally in the ventral medulla.     

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.     

Projection fibers from the main sensory trigeminal nucleus and the supratrigeminal region

Fiber projections from the main sensory trigeminal nucleus and the supratrigeminal region (reticular formation dorsal and rostrodorsal to the motor trigeminal nucleus) in the cat and monkey have been studied by the Nauta, Fink-Heimer and Marchi methods. Following experimental lesions in the main sensory trigeminal nucleus and its vicinity, the crossed ventral and the uncrossed dorsal trigeminal tracts were traced rostrally to the posteromedial ventral nucleus of the thalamus. The uncrossed dorsal trigeminal tract ascended through Forel's tegmental fascicle (tractus fasciculorum tegmenti Foreli). However, Forel's tegmental fascicle was composed mainly of the ascending reticular fibers terminating in the intralaminar nuclei of the thalamus and in the subthalamus. Commissural components distributing chidfly to the motor trigeminal nucleus of the opposite side appeared to arise from the supratrigeminal region as well as from the main sensory trigeminal nucleus. A fiber bundle, arising from the reticular zones surrounding the motor trigeminal nucleus, descended through the lateral reticular formation immediately ventromedial to the spinal trigeminal nucleus down to lower levels of the medulla oblongata. The functional significance of the supratrigeminal region as an interneuron system intercalated between the trigeminal sensory and the cranial motor systems is discussed.     

Neuronal activity in the inferior colliculus and bordering structures during vocalization in the squirrel monkey

In four squirrel monkeys (Saimiri sciureus), the inferior colliculus, together with the neighboring superior colliculus, reticular formation, cuneiform nucleus and parabrachial area, were explored with microelectrodes, looking for neurons that might be involved in the discrimination between self-produced and external sounds. Vocalization was elicited by kainic acid injections into the periaqueductal gray of the midbrain. Acoustic tests were carried out with ascending and descending narrow-band noise sweeps spanning virtually the whole hearing range of the squirrel monkey. Altogether 577 neurons were analyzed. Neurons that both were audiosensitive and fired in advance of self-produced vocalization were found almost exclusively in the pericentral nuclei of the inferior colliculus and the adjacent reticular formation. Only the latter, however, contained, in addition, neurons that fired during external acoustic stimulation, but remained quiet during self-produced vocalization. These findings suggest that the reticular formation bordering the inferior colliculus is involved in the discrimination between self-produced and foreign vocalization on the basis of a vocalmotor feedforward mechanism.