Neural pathways underlying vocal control

Communication of emotions in vocal expression and music performance: different channels, same code?

Many authors have speculated about a close relationship between vocal expression of emotions and musical expression of emotions. but evidence bearing on this relationship has unfortunately been lacking. This review of 104 studies of vocal expression and 41 studies of music performance reveals similarities between the 2 channels concerning (a) the accuracy with which discrete emotions were communicated to listeners and (b) the emotion-specific patterns of acoustic cues used to communicate each emotion. The patterns are generally consistent with K. R. Scherer's (1986) theoretical predictions. The results can explain why music is perceived as expressive of emotion, and they are consistent with an evolutionary perspective on vocal expression of emotions. Discussion focuses on theoretical accounts and directions for future research.

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.

Efferent subcortical projections of the laryngeal motorcortex in the rhesus monkey

In order to better understand the descending voluntary vocal control pathway, the efferent subcortical projections of the laryngeal motorcortex were studied in the rhesus monkey (Macaca mulatta). For this purpose, the left motorcortex was exposed in three animals under narcosis. By electrical brain stimulation, sites were identified yielding vocal fold adduction. Effective sites were injected with the anterograde tracer biotin dextran amine. Subcortical projections could be traced within the forebrain to the putamen, caudate nucleus, claustrum, zona incerta, field H of Forel and a number of thalamic nuclei, with the heaviest projections to the nuclei ventralis lateralis, ventralis posteromedialis, including its parvocellular part, medialis dorsalis, centralis medialis, centrum medianum and reuniens. In the midbrain, labeling was found in the deep mesencephalic nucleus. In the lower brainstem, fibers terminated in the pontine and medullary reticular formation, locus coeruleus, nucleus subcoeruleus, medial parabrachial nucleus, nucleus of the spinal trigeminal tract, solitary tract nucleus and facial nucleus. No projections were found to the nucl. ambiguus. The fact that monkeys, in contrast to humans, lack a direct connection of the motorcortex with the laryngeal motoneurons suggests that this connection has evolved in the last few million years and might represent one of the factors that made speech evolution possible.

The effect of amygdala lesions on conditional and unconditional vocalizations in rats

Electrolytic lesions centered on the amygdaloid central nucleus (ACe) resulted in the inability of rats to acquire a Pavlovian conditional vocalization response. Conditioning consisted of pairing a light conditional stimulus with a tailshock unconditional stimulus (US). The thresholds of three unconditional responses (URs) to tailshock were assessed prior to conditioning. These URs are organized at spinal (spinal motor reflexes), medullary (vocalizations during shock), and forebrain (vocalization afterdischarges, VADs) levels of the neuraxis. Compared to sham-lesioned controls, rats with amygdala lesions exhibited a selective elevation in the threshold of VADs. During conditioning the amplitude and duration of VADs were selectively reduced in amygdala-lesioned rats. These findings support earlier observations of that elicitation of VADs by tailshock correlates with the capacity of this US to support fear conditioning. The ACe may be involved in both associative and non-associative aspects of fear conditioning, but for progress in our understanding it is essential to evaluate its role in the generation of conditioning relevant URs.

Words in melody: an H(2)15O PET study of brain activation during singing and speaking

We used H(2)15O PET to characterize the interaction of words and melody by comparing brain activity measured while subjects spoke or sang the words to a familiar song. Relative increases in activity during speaking vs singing were observed in the left hemisphere, in classical perisylvian language areas including the posterior superior temporal gyrus, supramarginal gyrus, and frontal operculum, as well as in Rolandic cortices and putamen. Relative increases in activity during singing were observed in the right hemisphere: these were maximal in the right anterior superior temporal gyrus and contiguous portions of the insula; relative increases associated with singing were also detected in the right anterior middle temporal gyrus and superior temporal sulcus, medial and dorsolateral prefrontal cortices, mesial temporal cortices and cerebellum, as well as in Rolandic cortices and nucleus accumbens. These results indicate that the production of words in song is associated with activation of regions within right hemisphere areas that are not mirror-image homologues of left hemisphere perisylvian language areas, and suggest that multiple neural networks may be involved in different aspects of singing. Right hemisphere mechanisms may support the fluency-evoking effects of singing in neurological disorders such as stuttering or aphasia.

Vocalization after periaqueductal grey inactivation with the GABA agonist muscimol in the squirrel monkey

In order to specify the role of the periaqueductal grey (PAG) in vocal production, we tested the effect of PAG inactivation on the electrical elicitability of vocalization from various brain structures in the squirrel monkey. For this purpose, we implanted stimulation electrodes at 64 vocalization-eliciting sites throughout the brain and compared the elicitability of vocalization before and after muscimol injection into the PAG. It turned out that only vocalization sites in the forebrain (cingulate cortex, hypothalamus) and rostralmost mesencephalic reticular formation could be blocked by PAG inactivation, whereas all vocalization sites in the caudal midbrain, pons and medulla remained unaffected. It is concluded that the PAG is not the site of vocal pattern generation, but rather serves gating functions.

Spontaneous vocal differentiation of coo-calls for tools and food in Japanese monkeys

Vocal production and its usage in nonhuman primates may share common features with primitive human language. We trained two Japanese monkeys to use a rake-shaped tool to retrieve distant food. After the training, the monkeys spontaneously began vocalizing coo-calls in the tool-using context. We then trained one of the monkeys to vocalize to request food or the tool. Three independent acoustic parameters were measured and each parameter was independently analyzed across conditions using a multiple comparison test. We found that the monkey spontaneously differentiated their coo-calls to ask for either food or tool during the course of this training. This process might involve a change from emotional vocalizations into intentionally controlled ones by associating them with consciously planned tool use. We thus established a novel hypothesis about the origin of voluntary vocal control that could be approached from neurophysiological procedures.

Mirror neurons responding to the observation of ingestive and communicative mouth actions in the monkey ventral premotor cortex

In the ventral premotor cortex (area F5) of the monkey there are neurons that discharge both when the monkey performs specific motor actions and when it observes another individual performing a similar action (mirror neurons). Previous studies on mirror neurons concerned hand actions. Here, we describe the mirror responses of F5 neurons that motorically code mouth actions. The results showed that about one-third of mouth motor neurons also discharge when the monkey observes another individual performing mouth actions. The majority of these 'mouth mirror neurons' become active during the execution and observation of mouth actions related to ingestive functions such as grasping, sucking or breaking food. Another population of mouth mirror neurons also discharges during the execution of ingestive actions, but the most effective visual stimuli in triggering them are communicative mouth gestures (e.g. lip smacking). Some also fire when the monkey makes communicative gestures. These findings extend the notion of mirror system from hand to mouth action and suggest that area F5, the area considered to be the homologue of human Broca's area, is also involved in communicative functions.

A neural basis for auditory feedback control of vocal pitch

Hearing one's own voice is essential for the production of correct vocalization patterns in many birds and mammals, including humans. Bats, for instance, adjust temporal, spectral, and intensity parameters of their echolocation calls by precisely monitoring the characteristics of the returning echo signals. However, neuronal substrates and mechanisms for auditory feedback control of vocalizations are still mostly unknown in any vertebrate. We used echolocating horseshoe bats to investigate the role of the midbrain and hindbrain tegmentum for the control of call frequencies in response to changing auditory feedback. These bats accurately control the frequency of their echolocation calls through auditory feedback both when the bat is at rest [resting frequency (RF)] and when it is flying and compensating for changes in echo frequency caused by flight-induced Doppler shifts [Doppler shift compensation (DSC)]. We iontophoretically injected various GABAergic and glutamatergic transmitter agonists and antagonists into the brainstem tegmentum. We found that within the parabrachial nuclei and the immediately adjacent tegmentum, excitatory effects caused by application of the glutamate agonist AMPA or the GABA(A) antagonist bicuculline raised RF and the frequency of calls emitted during DSC. Bicuculline application routinely blocked DSC altogether. Alternately, inhibitory effects caused by application of either the GABA(A) agonist muscimol or the AMPA antagonist CNQX lowered call frequencies emitted at rest and during DSC. Such an audio-vocal feedback mechanism might share basic aspects with audio-vocal feedback controlling the pitch of vocalizations in other mammals, including the involuntary response to "pitch-shifted feedback" in humans.

Respiratory and telencephalic modulation of vocal motor neurons in the zebra finch

Birdsong, like speech, involves coordinated vocal and respiratory activity achieved under telencephalic control. The avian vocal organ, or syrinx, is innervated by motor neurons (MNs) in the tracheosyringeal part of the hypoglossal nucleus (XIIts) that receive their synaptic input from medullary respiratory areas and telencephalic song control areas. Despite the importance of XIIts MNs to learned vocalizations, little is known about their intrinsic electrical properties or their synaptic inputs. Therefore, we made in vitro and in vivo intracellular recordings from XIIts MNs in adult male zebra finches to characterize their intrinsic properties and their synaptic modulation by respiratory and telencephalic areas. In vitro, electrical stimulation of ipsilateral or contralateral medullary respiratory areas (RAm) routinely evoked glycine receptor-mediated inhibition in XIIts. With inhibition blocked, similar stimulation evoked excitatory synaptic responses capable of driving sustained MN firing that was mediated partly by NMDA receptors. These inhibitory and excitatory inputs likely arise from RAm neurons, because chemical or electrical stimulation of RAm evoked similar responses in XIIts. In vivo, XIIts MNs displayed rhythmical, expiratory-related activity. EPSPs were pronounced at expiratory onset, but IPSPs were not apparent during inspiration, although XIIts MN firing was suppressed. However, hyperpolarizations as well as excitation were evoked by playback of the bird's own song, a stimulus that potently excites the telencephalic song nucleus that innervates XIIts. These findings illuminate functional properties of the songbird's brainstem circuitry and its specific activation by telencephalic inputs, which could coordinate vocal and respiratory activity during singing.

Stuttering: a dynamic motor control disorder

The purpose of this review is to determine what neural mechanisms may be dysfunctional in stuttering. Three sources of evidence are reviewed. First, studies of dynamic inter-relationships among brain regions during normal speech and in persons who stutter (PWS) suggest that the timing of neural activity in different regions may be abnormal in PWS. Second, the brain lesions associated with acquired stuttering are reviewed. These indicate that in a high percentage of cases, the primary speech and language regions are not affected but lesions involve other structures, such as the basal ganglia, which may modulate the primary speech and language regions. Third, to characterize the motor control disorder in stuttering, similarities and differences from focal dystonias such as spasmodic dysphonia (SD) and Tourette's syndrome (TS) are reviewed. This review indicates that the central control abnormalities in stuttering are not due to disturbance in one particular brain region but rather a system dysfunction that interferes with rapid and dynamic speech processing for production.

EDUCATIONAL OBJECTIVES

The reader will be able to describe: (1) the similarities and differences between stuttering and other speech motor control disorders, (2) which brain lesions are most likely to produce acquired stuttering in adults, and (3) what type of brain abnormality most likely underlies stuttering.

Towards a functional neural systems model of developmental stuttering

This paper overviews recent developments in an ongoing program of brain imaging research on developmental stuttering that is being conducted at the University of Texas Health Science Center, San Antonio. This program has primarily used H(2)15O PET imaging of different speaking tasks by right-handed adult male and female persistent stutterers, recovered stutterers and controls in order to isolate the neural regions that are functionally associated with stuttered speech. The principal findings have emerged from studies using condition contrasts and performance correlation techniques. The emerging findings from these studies are reviewed and referenced to a neural model of normal speech production recently proposed by Jürgens [Neurosci. Biobehav. Rev. 26 (2002) 235]. This paper will report (1) the reconfiguration of previous findings within the Jürgens Model; (2) preliminary findings of an investigation with late recovered stutterers; (3) an investigation of neural activations during a treatment procedure designed to produce a sustained improvement in fluency; and (4) an across-studies comparison that seeks to isolate neural regions within the Jürgens Model that are consistently associated with stuttering. Two regions appear to meet this criterion: right anterior insula (activated) and anterior middle and superior temporal gyri (deactivated) mainly in right hemisphere. The implications of these findings and the direction of future imaging investigations are discussed.

EDUCATIONAL OBJECTIVES

The reader will learn about (1) recent uses of H(2)15O PET imaging in stuttering research; (2) the use of a new neurological model of speech production in imaging research on stuttering; and (3) initial findings from PET imaging investigations of treated and recovered stutterers.

Modulation of the auditory cortex during speech: an MEG study

Several behavioral and brain imaging studies have demonstrated a significant interaction between speech perception and speech production. In this study, auditory cortical responses to speech were examined during self-production and feedback alteration. Magnetic field recordings were obtained from both hemispheres in subjects who spoke while hearing controlled acoustic versions of their speech feedback via earphones. These responses were compared to recordings made while subjects listened to a tape playback of their production. The amplitude of tape playback was adjusted to match the amplitude of self-produced speech. Recordings of evoked responses to both self-produced and tape-recorded speech were obtained free of movement-related artifacts. Responses to self-produced speech were weaker than were responses to tape-recorded speech. Responses to tones were also weaker during speech production, when compared with responses to tones recorded in the presence of speech from tape playback. However, responses evoked by gated noise stimuli did not differ for recordings made during self-produced speech versus recordings made during tape-recorded speech playback. These data suggest that during speech production, the auditory cortex (1) attenuates its sensitivity and (2) modulates its activity as a function of the expected acoustic feedback.

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.

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.