Functional organization of forebrain pathways for song production and perception

This article reviews the organization of the forebrain nuclei of the avian song system. Particular emphasis is placed on recent physiologic recordings from awake behaving adult birds while they sing, call, and listen to broadcasts of acoustic stimuli. The neurons in the descending motor pathway (HVc and RA) are organized in a hierarchical arrangement of temporal units of song production, with HVc neurons representing syllables and RA neurons representing notes. The nuclei Uva and NIf, which are afferent to HVc, may help organize syllables into larger units of vocalization. HVc and RA are also active during production of all calls. The patterns of activity associated with calls differ between learned calls and those that are innately specified, and give insight into the interactions between the forebrain and midbrain during calling, as well as into the evolutionary origins of the song system. Neurons in Area X, the first part of the anterior forebrain pathway leading from HVc to RA, are also active during singing. Many HVc neurons are also auditory, exhibiting selectivity for learned acoustic parameters of the individual bird's own song (BOS). Similar auditory responses are also observed in RA and Area X in anesthetized birds. In contrast to HVc, however, auditory responses in RA are very weak or absent in awake birds under our experimental paradigm, but are uncovered when birds are anesthetized. Thus, the roles of both pathways beyond HVc in adult birds is under review. In particular, theories hypothesizing a role for the descending motor pathway (RA and below) in adult song perception do not appear to obtain. The data also suggest that the anterior forebrain pathway has a greater motor role than previously considered. We suggest that a major role of the anterior forebrain pathway is to resolve the timing mismatch between motor program readout and sensory feedback, thereby facilitating motor programming during birdsong learning. Pathways afferent to HVc may participate more in sensory acquisition and sensorimotor learning during song development than is commonly assumed. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 671–693, 1997     

黄喉鵐高级发声中枢及其壳区的神经投射和P物质在发声听觉中枢的分布

用生物素示踪法和P物质 (SP)免疫组化技术研究表明 :黄喉的高级发声中枢 (HVc)接受端脑听区 (L)、新纹状体中部界面核、新纹状体巨细胞核 (MAN)、丘脑葡萄形核、桥脑蓝斑核的传入 ,并有神经纤维投射到古纹状体栎核 (RA)和嗅叶X区 (X) ;HVc壳投射到RA壳并接受L的传入。听觉控制与学习通路与发声中枢之间有许多神经联系 ,提示黄喉发声学习依赖于听觉反馈。在HVc、RA和MAN有SP阳性细胞体 ,在X、中脑背内侧核和延髓舌下神经核气管鸣管部、丘脑卵圆核壳区、中脑背外侧核壳区及中脑丘间核有SP阳性纤维和终末。SP广泛分布于发声 -听觉中枢 ,可能参与了它们的活动     

黄喉鹀高级发声中枢及其壳区的神经投射和P物质在发声听觉中枢的分布

用生物素示踪法和P物质(SP)免疫组化技术研究表明:黄喉(巫鸟)的高级发声中枢(HVc)接受端脑听区(L)、新纹状体中部界面核、新纹状体巨细胞核(MAN)、丘脑葡萄形核、桥脑蓝斑核的传入,并有神经纤维投射到古纹状体栎核(RA)和嗅叶X区(X);HVc壳投射到RA壳并接受L的传入.听觉控制与学习通路与发声中枢之间有许多神经联系,提示黄喉(巫鸟)发声学习依赖于听觉反馈.在HVc、RA和MAN有SP阳性细胞体,在X、中脑背内侧核和延髓舌下神经核气管鸣管部、丘脑卵圆核壳区、中脑背外侧核壳区及中脑丘间核有SP阳性纤维和终末.SP广泛分布于发声-听觉中枢,可能参与了它们的活动.     

Dynamics of Vocalization-Induced Modulation of Auditory Cortical Activity at Mid-utterance

Background

Recent research has addressed the suppression of cortical sensory responses to altered auditory feedback that occurs at utterance onset regarding speech. However, there is reason to assume that the mechanisms underlying sensorimotor processing at mid-utterance are different than those involved in sensorimotor control at utterance onset. The present study attempted to examine the dynamics of event-related potentials (ERPs) to different acoustic versions of auditory feedback at mid-utterance.

Methodology/Principal findings

Subjects produced a vowel sound while hearing their pitch-shifted voice (100 cents), a sum of their vocalization and pure tones, or a sum of their vocalization and white noise at mid-utterance via headphones. Subjects also passively listened to playback of what they heard during active vocalization. Cortical ERPs were recorded in response to different acoustic versions of feedback changes during both active vocalization and passive listening. The results showed that, relative to passive listening, active vocalization yielded enhanced P2 responses to the 100 cents pitch shifts, whereas suppression effects of P2 responses were observed when voice auditory feedback was distorted by pure tones or white noise.

Conclusion/Significance

The present findings, for the first time, demonstrate a dynamic modulation of cortical activity as a function of the quality of acoustic feedback at mid-utterance, suggesting that auditory cortical responses can be enhanced or suppressed to distinguish self-produced speech from externally-produced sounds.     

Acute injections of brain‐derived neurotrophic factor in a vocal premotor nucleus reversibly disrupt adult birdsong stability and trigger syllable deletion

Behavioral variability serves an essential role in motor learning by enabling sensory feedback to select those motor patterns that minimize error. Birds use auditory feedback to learn how to sing, and their songs lose variability and become highly stereotyped, or crystallized, at the end of a sensitive period for sensorimotor learning. The molecular cues that regulate song variability are not well understood. In other systems, neurotrophins, and brain‐derived neurotrophic factor (BDNF) in particular, can mediate various forms of neural plasticity, including sensitive period neural circuit plasticity and activity‐dependent synapse formation, and may also influence learning and memory. Here, we have tested the hypothesis that neurotrophin expression in the robust nucleus of the arcopallium (RA), the telencephalic output controlling song, regulates song variability. BDNF and its receptor trkB are expressed in RA, and BDNF expression in RA appears to be highest in juveniles, when song is most variable and plastic, and synapse density highest. Thus, song variability and synaptic connectivity could be enhanced by augmented expression of BDNF in RA. In support of this idea, we found that BDNF injections into the adult RA induced the re‐expression of juvenile‐like phenotypes, including song variability and an increased synaptic density in RA. Furthermore, BDNF treatment also induced vocal plasticity, characterized by syllable deletions and persistent changes to the song patterns. These results suggest that endogenous BDNF could be a molecular regulator of the song variability essential to vocal plasticity and, ultimately, to song learning. © 2004 Wiley Periodicals, Inc. J Neurobiol, 2005     

Requirement of tryptophan hydroxylase during development for maturation of sensorimotor gating

Deficits in sensorimotor gating, a function to focus on the most salient stimulus, could lead to a breakdown of cognitive integrity, and could reflect the "flooding" by sensory overload and cognitive fragmentation seen in schizophrenia. Sensorimotor gating emerges at infancy, and matures during childhood. The mechanisms that underlie its development are largely unclear. Here, we screened the mouse genome, and found that tryptophan hydroxylase (TPH) is implicated in the maturation of sensorimotor gating. TPH, an enzyme involved in the biosynthesis of serotonin, proved to be required only during the weaning period for maturation of sensorimotor gating, but was dispensable for its emergence. Proper serotonin levels during development underlie the mature functional architecture for sensorimotor gating via appropriate actin polymerization. Thus, maintaining proper serotonin levels during childhood may be important for mature sensorimotor gating in adulthood.     

The participation of the cholinergic system of the rat sensorimotor cortex in regulating different types of movements

To study the role of the cholinergic system of the sensorimotor cortex in regulation of different manipulatory movements and locomotion of Wistar rats, the effects of injections of cholinergic drugs (a cholinergic agonist carbachol and an antagonist scopolamine) into the area of forepaw representation in the sensorimotor cortex on motor activity and performance of manipulatory movements (with prolonged and short pushing) were analyzed. The drugs were injected via special cannulae stereotaxically implanted into the cortex during surgery carried out under Nembutal anesthesia. Carbachol injections (0.03-3 micrograms in 1 microliter of physiologic solution) into the cortex resulted in a significant slowing down of both types of movements as well as an increase in locomotion in the open-field test. Injections of scopolamine (0.3-3 micrograms) into the same cortical area were accompanied by an increase in the number of fast manipulatory movements without significant changes in locomotor activity. The obtained evidence suggests that the cholinergic system of the sensorimotor cortex indifferent manners regulates the innate (locomotion) and acquired movements which require different periods of maintaining the muscle tone of the forepaw (short-time periods for the usual movements necessary for food taking from the narrow horizontal tube and prolonged periods for the learned slow movements with additional tactile and tonic components).     

燕雀上纹状体腹侧尾核对发声和呼吸的调制效应及其纤维联系

在乌拉坦麻醉的鸣禽燕雀（Ｆｒｉｎｇｉｌｌａｍｏｎｔｉｆｒｉｎｇｉｌｌａ）上,观察电刺激上纹状体腹侧尾核（ＨＶｃ）对发声和呼吸的影响,随后在ＨＶｃ内注入ＣＢ－ＨＲＰ溶液,研究ＨＶｃ的中枢联系。结果如下：（１）电刺激ＨＶｃ的不同区域都引起鸣叫反应。（２）长串电脉冲刺激ＨＶｃ,产生明显的呼吸易化效应,表现为增频增幅的呼吸。（３）吸气期用短串电脉冲刺激ＨＶｃ,产生吸气切断效应;刺激落位于呼气相,可使该呼气时程明显延长,以配合鸣叫,然后转变为增频增幅的呼吸。（４）ＣＢＨＲＰ法表明,ＨＶｃ投射到古纹状体粗核和嗅叶Ｘ区,ＨＶｃ接受新纹状体前部大细胞核内侧部、新纹状体中部界面核、端脑听核－Ｌ区、丘脑葡萄形核及脑桥蓝斑核的传入投射。提示ＨＶｃ除控制发声外,尚参与呼吸易化的调制。ＨＶｃ对发声及呼吸的特异性影响,可能在鸣叫与呼吸的协调机制中起重要作用。     

Acute injections of brain-derived neurotrophic factor in a vocal premotor nucleus reversibly disrupt adult birdsong stability and trigger syllable deletion

Behavioral variability serves an essential role in motor learning by enabling sensory feedback to select those motor patterns that minimize error. Birds use auditory feedback to learn how to sing, and their songs lose variability and become highly stereotyped, or crystallized, at the end of a sensitive period for sensorimotor learning. The molecular cues that regulate song variability are not well understood. In other systems, neurotrophins, and brain-derived neurotrophic factor (BDNF) in particular, can mediate various forms of neural plasticity, including sensitive period neural circuit plasticity and activity-dependent synapse formation, and may also influence learning and memory. Here, we have tested the hypothesis that neurotrophin expression in the robust nucleus of the arcopallium (RA), the telencephalic output controlling song, regulates song variability. BDNF and its receptor trkB are expressed in RA, and BDNF expression in RA appears to be highest in juveniles, when song is most variable and plastic, and synapse density highest. Thus, song variability and synaptic connectivity could be enhanced by augmented expression of BDNF in RA. In support of this idea, we found that BDNF injections into the adult RA induced the re-expression of juvenile-like phenotypes, including song variability and an increased synaptic density in RA. Furthermore, BDNF treatment also induced vocal plasticity, characterized by syllable deletions and persistent changes to the song patterns. These results suggest that endogenous BDNF could be a molecular regulator of the song variability essential to vocal plasticity and, ultimately, to song learning.     

The relationship between rates of HVc neuron addition and vocal plasticity in adult songbirds

In adulthood, songbird species vary considerably in the extent to which they rely on auditory feedback to maintain a stable song structure. The continued recruitment of new neurons into vocal motor circuitry may contribute to this lack of resiliency in song behavior insofar as new neurons that are not privy to auditory instruction could eventually corrupt established neural function. In a first step to explore this possibility, we used a comparative approach to determine if species differences in the rate of vocal change after deafening in adulthood correlate positively with the extent of HVc neuron addition. We confirmed previous reports that deafening in adulthood changes syllable phonology much more rapidly in bengalese finches than in zebra finches. Using [(3)H]thymidine autoradiography to identify neurons generated in adulthood, we found that the proportion of new neurons in the HVc one month after labeling was nearly twice as great in bengalese than in zebra finches. Moreover, among the subset of HVc vocal motor neurons that project to the robust nucleus of the archistriatum, the incidence of [(3)H]thymidine-labeled neurons was nearly three times as great in bengalese than in zebra finches. This correlation between the proportion of newly added neurons and the rate of song deterioration supports the hypothesis that HVc neuron addition may disrupt stable adult song production if new neurons cannot be "trained" via auditory feedback.     

At the interface of the auditory and vocal motor systems: NIf and its role in vocal processing,production and learning

Communication between auditory and vocal motor nuclei is essential for vocal learning. In songbirds, the nucleus interfacialis of the nidopallium (NIf) is part of a sensorimotor loop, along with auditory nucleus avalanche (Av) and song system nucleus HVC, that links the auditory and song systems. Most of the auditory information comes through this sensorimotor loop, with the projection from NIf to HVC representing the largest single source of auditory information to the song system. In addition to providing the majority of HVC’s auditory input, NIf is also the primary driver of spontaneous activity and premotor-like bursting during sleep in HVC. Like HVC and RA, two nuclei critical for song learning and production, NIf exhibits behavioral-state dependent auditory responses and strong motor bursts that precede song output. NIf also exhibits extended periods of fast gamma oscillations following vocal production. Based on the converging evidence from studies of physiology and functional connectivity it would be reasonable to expect NIf to play an important role in the learning, maintenance, and production of song. Surprisingly, however, lesions of NIf in adult zebra finches have no effect on song production or maintenance. Only the plastic song produced by juvenile zebra finches during the sensorimotor phase of song learning is affected by NIf lesions. In this review, we carefully examine what is known about NIf at the anatomical, physiological, and behavioral levels. We reexamine conclusions drawn from previous studies in the light of our current understanding of the song system, and establish what can be said with certainty about NIf’s involvement in song learning, maintenance, and production. Finally, we review recent theories of song learning integrating possible roles for NIf within these frameworks and suggest possible parallels between NIf and sensorimotor areas that form part of the neural circuitry for speech processing in humans.     

Neuron loss and addition in developing zebra finch song nuclei are independent of auditory experience during song learning

In zebra finches early auditory experience is critical for normal song development. Young males first listen to and memorize a suitable song model and then use auditory feedback from their own vocalizations to mimic that model. During these two phases of vocal learning, song-related brain regions exhibit large, hormone-induced changes in volume and neuron number. Overlap between these neural changes and auditory-based vocal learning suggests that processing and acquiring auditory input may influence cellular processes that determine neuron number in the song system. We addressed this hypothesis by measuring neuron density, nuclear volume, and neuron number within the song system of normal male zebra finches and males deafened prior to song learning (10 days of age). Measures were obtained at 25, 50, 65, and 120 days of age, and included four song nuclei: the hyperstriatum ventralis pars caudalis or higher vocal center (HVc), Area X, the robust nucleus of the archistriatum (RA), and the lateral magnocellular nucleus of the anterior neostriatum (IMAN). In both HVc and Area X, nuclear volume and neuron number increased markedly with age in both normal and deafened birds. The volume of RA also increased with age and was not affected by early deafening. In IMAN, deafening also did not affect the overall age-related loss of neurons, although at 25 days neuron number was slightly less in deafened than in normal birds. We conclude that while the addition and loss of neurons in the developing song system may provide plasticity essential for song learning, these changes do not reflect learning.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lesions of HVc block the developmental masculinizing effects of estradiol in the female zebra finch song system

The neural song control system of female zebra finches is permanently masculinized if the females are given estradiol within 1 month after hatching. One hypothesis is that estradiol acts on neurons in the caudal nucleus of the ventral hyperstriatum (HVc) to cause developmental changes that lead to masculinizing influences in other song control regions. To test whether lesions of HVc block the masculinizing effects of estradiol elsewhere in the song system, we gave 20-day-old females either a Silastic pellet containing estradiol or no implant, and they received either a unilateral lesion of HVc or no lesion. At 60 days of age, they were sacrificed. The volumes of brain regions and sizes of neurons were measured in four song nuclei: HVc, robust nucleus of the archistriatum (RA), lateral magnocellular nucleus of the neostriatum (lMAN), and Area X. Lesions of HVc blocked the masculinizing effects of estradiol on RA and Area X on the side of the lesion. Thus, HVc must be intact in order for estradiol to masculinize these two nuclei. This observation is compatible with the hypothesis that estradiol acts on or near HVc to masculinize several song nuclei, although other interpretations are also possible.     

Functional MRI of auditory responses in the zebra finch forebrain reveals a hierarchical organisation based on signal strength but not selectivity

Background

Male songbirds learn their songs from an adult tutor when they are young. A network of brain nuclei known as the ‘song system’ is the likely neural substrate for sensorimotor learning and production of song, but the neural networks involved in processing the auditory feedback signals necessary for song learning and maintenance remain unknown. Determining which regions show preferential responsiveness to the bird''s own song (BOS) is of great importance because neurons sensitive to self-generated vocalisations could mediate this auditory feedback process. Neurons in the song nuclei and in a secondary auditory area, the caudal medial mesopallium (CMM), show selective responses to the BOS. The aim of the present study is to investigate the emergence of BOS selectivity within the network of primary auditory sub-regions in the avian pallium.

Methods and Findings

Using blood oxygen level-dependent (BOLD) fMRI, we investigated neural responsiveness to natural and manipulated self-generated vocalisations and compared the selectivity for BOS and conspecific song in different sub-regions of the thalamo-recipient area Field L. Zebra finch males were exposed to conspecific song, BOS and to synthetic variations on BOS that differed in spectro-temporal and/or modulation phase structure. We found significant differences in the strength of BOLD responses between regions L2a, L2b and CMM, but no inter-stimuli differences within regions. In particular, we have shown that the overall signal strength to song and synthetic variations thereof was different within two sub-regions of Field L2: zone L2a was significantly more activated compared to the adjacent sub-region L2b.

Conclusions

Based on our results we suggest that unlike nuclei in the song system, sub-regions in the primary auditory pallium do not show selectivity for the BOS, but appear to show different levels of activity with exposure to any sound according to their place in the auditory processing stream.     

A Real-Time Brain-Machine Interface Combining Motor Target and Trajectory Intent Using an Optimal Feedback Control Design

Real-time brain-machine interfaces (BMI) have focused on either estimating the continuous movement trajectory or target intent. However, natural movement often incorporates both. Additionally, BMIs can be modeled as a feedback control system in which the subject modulates the neural activity to move the prosthetic device towards a desired target while receiving real-time sensory feedback of the state of the movement. We develop a novel real-time BMI using an optimal feedback control design that jointly estimates the movement target and trajectory of monkeys in two stages. First, the target is decoded from neural spiking activity before movement initiation. Second, the trajectory is decoded by combining the decoded target with the peri-movement spiking activity using an optimal feedback control design. This design exploits a recursive Bayesian decoder that uses an optimal feedback control model of the sensorimotor system to take into account the intended target location and the sensory feedback in its trajectory estimation from spiking activity. The real-time BMI processes the spiking activity directly using point process modeling. We implement the BMI in experiments consisting of an instructed-delay center-out task in which monkeys are presented with a target location on the screen during a delay period and then have to move a cursor to it without touching the incorrect targets. We show that the two-stage BMI performs more accurately than either stage alone. Correct target prediction can compensate for inaccurate trajectory estimation and vice versa. The optimal feedback control design also results in trajectories that are smoother and have lower estimation error. The two-stage decoder also performs better than linear regression approaches in offline cross-validation analyses. Our results demonstrate the advantage of a BMI design that jointly estimates the target and trajectory of movement and more closely mimics the sensorimotor control system.     

Chronic morphine treatment decreases acoustic startle response and prepulse inhibition in rats

The reward-related effects of addictive drugs primarily act via the dopamine system, which also plays an important role in sensorimotor gating. The mesolimbic dopamine system is the common pathway of drug addiction and sensorimotor gating. However, the way in which addictive drugs affect sensorimotor gating is currently unclear. In previous studies, we examined the effects of morphine treatment on sensory gating in the hippocampus. The present study investigated the effects of morphine on sensorimotor gating in rats during chronic morphine treatment and withdrawal. Rats were examined during treatment with morphine for 10 successive days, followed by a withdrawal period. Acoustic startle responses to a single startle stimulus (115 dB SPL) and prepulse inhibition responses were recorded. The results showed that acoustic startle responses were attenuated during morphine treatment, but not during withdrawal. PPI was impaired in the last 2 morphine treatment days, but returned to a normal level during withdrawal.     

Microglial regulation of cholinergic differentiation in the basal forebrain

Because inflammation during pregnancy can lead to neurodevelopmental anomalies, we investigated the role of inflamed microglia on cholinergic precursors in the rat embryonic basal forebrain (BF) cultured on embryonic day 15. Conditioned medium (CM) taken from microglia stimulated variously (microglial CM; MCM) increased activity of choline acetyltransferase (ChAT), the enzyme responsible for acetylcholine biosynthesis and a phenotypic hallmark of the cholinergic neuron. There was a concomitant decline in glutamic acid decarboxylase expression. Of stimulators tested, only β-amyloid failed to produce effective MCM. Infection with a Lac-Z-containing retrovirus revealed that MCM promoted cholinergic differentiation from undifferentiated precursors in the population. Several candidates were tested for their ability to mimic MCM. Mature nerve growth factor (NGF) did not mimic MCM, but acted synergistically with it to promote enormous increases in ChAT activity. However, a microglial cell line produced high-molecular weight forms of NGF (pro-NGF) that were lethal to mature cholinergic neurons. Although bone morphogenetic proteins (BMP) 2, 4, and 9 increased ChAT activity dose-dependently, noggin did not inhibit the effects of the MCM, suggesting that BMPs were not the only active factor(s) in the MCM. Embryonic microglia isolated following maternal inflammation produced a variety of immune system cytokines and chemokines. One of these, interleukin-6 (IL-6), was tested for its ability to promote cholinergic differentiation. Although IL-6 alone did not mimic the action of MCM, neutralization of it inhibited MCM effectiveness. Thus, following maternal inflammation, a complex microglial-derived cocktail of factors can promote excess cholinergic differentiation in the embryonic BF.     

Characteristics of rapid eye movement sleep behavior disorder in narcolepsy

The basal forebrain (BF) plays an important role in regulating cortical activity and sleep/wake states. Both cholinergic and non-cholinergic neurons of the BF project to the cerebral cortex and hippocampus, whereas the hypothalamus and brainstem nuclei are mostly innervated by non-cholinergic BF neurons. Neurons in the BF show various discharge profiles in relation to cortical activity and behavioral states and are differentially modulated by neurotransmitters of other sleep/wake regulatory neurons. Recent technical advances have made it possible to correlate discharge profiles of single BF neurons during sleep/wake states with their neurochemical phenotypes, and to make selective lesions of certain cell types. The goal of this review is to summarize the current knowledge of the anatomy and sleep/wake regulatory functions of cholinergic and non-cholinergic BF neurons. We will first review the anatomical heterogeneity of BF neurons, and then discuss recent evidence for the firing patterns of BF cholinergic and non-cholinergic neurons during natural sleep–wake patterns, and finally, discuss their roles in sleep homeostasis. It is proposed that through different neurotransmitters, projections, and state-regulated activity, the cholinergic and non-cholinergic BF neurons collectively and differently regulate cortical activity and sleep-wake states.

     

Joint hormonal and sensory stimulation modulate neuronal number in adult canary brains

Treatment of adult female canaries with testosterone (T) causes them to produce male-typical vocalizations and results in striking growth of brain nuclei that control song behavior (Nottebohm, 1980). The song-control nucleus HVc (caudal nucleus of the ventral hyperstriatum) contains cells that concentrate testosterone or its metabolites, suggesting that steroid hormones may induce the growth of HVc directly by regulating the expression of specific genes in those HVc neurons that have steroid receptors. However, we have previously provided evidence that is inconsistent with the idea that steroids promote growth of HVc solely via a direct action on hormone receptors: testosterone treatment of deafened adult females results in very little growth of HVc, relative to T-treated hearing birds (Bottjer et al., 1986b). Thus, birds in the former group undergo very little overall growth of HVc despite high circulating levels of hormone. We show here that the slightly increased size of HVc in T-treated deaf birds is attributable to an increase in neuronal spacing; the greatly increased size of HVc in T-treated hearing birds is due to an increase in neuronal number as well as spacing. There was virtually no increase in number of HVc neurons in T-treated deafened birds relative to control groups, whereas T-treated hearing birds showed a marked increase in neuron number. The song-control nucleus RA (robust nucleus of the archistriatum), which receives direct afferent input from HVc, also increases in size in response to testosterone treatment. However, the volume of RA increases in both hearing and deafened birds; this increase is primarily due to an increase in neuronal spacing as well as a small increase in neuron number. These results demonstrate that the number of neurons in a specific vocal-control nucleus (HVc) can change dramatically in adult canaries and suggest that some synergistic action of hormonal and sensory stimulation is necessary to induce such a change.     

A Comparison of Sensorimotor Adaptation in the Visual and in the Auditory Modality

We compared sensorimotor adaptation in the visual and the auditory modality. Subjects pointed to visual targets while receiving direct spatial information about fingertip position in the visual modality, or they pointed to visual targets while receiving indirect information about fingertip position in the visual modality, or they pointed to auditory targets while receiving indirect information about fingertip position in the auditory modality. Feedback was laterally shifted to induce adaptation, and aftereffects were tested with both target modalities and both hands. We found that aftereffects of adaptation were smaller when tested with the non-adapted hand, i.e., intermanual transfer was incomplete. Furthermore, aftereffects were smaller when tested in the non-adapted target modality, i.e., intermodal transfer was incomplete. Aftereffects were smaller following adaptation with indirect rather than direct feedback, but they were not smaller following adaptation with auditory rather than visual targets. From this we conclude that the magnitude of adaptive recalibration rather depends on the method of feedback delivery (indirect versus direct) than on the modality of feedback (visual versus auditory).