An anatomical basis for visual calibration of the auditory space map in the barn owl's midbrain

Morphological examination and scanning electron microscopy of the barn owl's (Tyto alba) tongue

The beak structure changes according to the feeding patterns of birds. Further, the morphological and histological structures of their tongues vary. Therefore, the current study aimed to perform macroanatomical and histological examinations and scanning electron microscopy of the barn owl's (Tylo alba) tongue. Two dead barn owls were brought to the anatomy laboratory and were used as study material. The tongue of the barn owl was long, triangular-shaped with a bifurcated tip. There were no papillae in the anterior 1/3 of the tongue, and the lingual papillae were shaped toward the back. The radix linguae were surrounded by a single row of conical papillae. Irregular thread-like papillae were found on both sides of the tongue. The salivary gland ducts were on the lateral margin of the corpus linguae and the dorsal surface of the radix linguae. The lingual glands were in the lamina propria near the stratified squamous epithelium layer of the tongue. The dorsal surface of the tongue comprised non-keratinized stratified squamous epithelium, and the ventral surface and caudal part of the tongue had keratinized stratified squamous epithelium. Hyaline cartilages were detected in the connective tissue immediately below the non-keratinized stratified squamous epithelium on the dorsal surface of the root of the tongue. The study results can contribute to the current knowledge on the anatomical structure of birds. Further, they can be useful in managing the barn owl when used as companion animals and in research activity.

Diverse processing underlying frequency integration in midbrain neurons of barn owls

Emergent response properties of sensory neurons depend on circuit connectivity and somatodendritic processing. Neurons of the barn owl’s external nucleus of the inferior colliculus (ICx) display emergence of spatial selectivity. These neurons use interaural time difference (ITD) as a cue for the horizontal direction of sound sources. ITD is detected by upstream brainstem neurons with narrow frequency tuning, resulting in spatially ambiguous responses. This spatial ambiguity is resolved by ICx neurons integrating inputs over frequency, a relevant processing in sound localization across species. Previous models have predicted that ICx neurons function as point neurons that linearly integrate inputs across frequency. However, the complex dendritic trees and spines of ICx neurons raises the question of whether this prediction is accurate. Data from in vivo intracellular recordings of ICx neurons were used to address this question. Results revealed diverse frequency integration properties, where some ICx neurons showed responses consistent with the point neuron hypothesis and others with nonlinear dendritic integration. Modeling showed that varied connectivity patterns and forms of dendritic processing may underlie observed ICx neurons’ frequency integration processing. These results corroborate the ability of neurons with complex dendritic trees to implement diverse linear and nonlinear integration of synaptic inputs, of relevance for adaptive coding and learning, and supporting a fundamental mechanism in sound localization.

Neurons at higher stages of sensory pathways often display selectivity for properties of sensory stimuli that result from computations performed within the nervous system. These emergent response properties can be produced by patterns of neural connectivity and processing that occur within individual cells. Here we investigated whether neural connectivity and single-neuron computation may contribute to the emergence of spatial selectivity in auditory neurons in the barn owl’s midbrain. We used data from in vivo intracellular recordings to test the hypothesis from previous modeling work that these cells function as point neurons that perform a linear sum of their inputs in their subthreshold responses. Results indicate that while some neurons show responses consistent with the point neuron hypothesis, others match predictions of nonlinear integration, indicating a diversity of frequency integration properties across neurons. Modeling further showed that varied connectivity patterns and forms of single-neuron computation may underlie observed responses. These results demonstrate that neurons with complex morphologies may implement diverse integration of synaptic inputs, relevant for adaptive coding and learning.

Visual Signals in the Mammalian Auditory System

Coordination between different sensory systems is a necessary element of sensory processing. Where and how signals from different sense organs converge onto common neural circuitry have become topics of increasing interest in recent years. In this article, we focus specifically on visual-auditory interactions in areas of the mammalian brain that are commonly considered to be auditory in function. The auditory cortex and inferior colliculus are two key points of entry where visual signals reach the auditory pathway, and both contain visual- and/or eye movement-related signals in humans and other animals. The visual signals observed in these auditory structures reflect a mixture of visual modulation of auditory-evoked activity and visually driven responses that are selective for stimulus location or features. These key response attributes also appear in the classic visual pathway but may play a different role in the auditory pathway: to modify auditory rather than visual perception. Finally, while this review focuses on two particular areas of the auditory pathway where this question has been studied, robust descending as well as ascending connections within this pathway suggest that undiscovered visual signals may be present at other stages as well. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Silent Synapse-Based Mechanisms of Critical Period Plasticity

Critical periods are postnatal, restricted time windows of heightened plasticity in cortical neural networks, during which experience refines principal neuron wiring configurations. Here, we propose a model with two distinct types of synapses, innate synapses that establish rudimentary networks with innate function, and gestalt synapses that govern the experience-dependent refinement process. Nascent gestalt synapses are constantly formed as AMPA receptor-silent synapses which are the substrates for critical period plasticity. Experience drives the unsilencing and stabilization of gestalt synapses, as well as synapse pruning. This maturation process changes synapse patterning and consequently the functional architecture of cortical excitatory networks. Ocular dominance plasticity (ODP) in the primary visual cortex (V1) is an established experimental model for cortical plasticity. While converging evidence indicates that the start of the critical period for ODP is marked by the maturation of local inhibitory circuits, recent results support our model that critical periods end through the progressive maturation of gestalt synapses. The cooperative yet opposing function of two postsynaptic signaling scaffolds of excitatory synapses, PSD-93 and PSD-95, governs the maturation of gestalt synapses. Without those proteins, networks do not progress far beyond their innate functionality, resulting in rather impaired perception. While cortical networks remain malleable throughout life, the cellular mechanisms and the scope of critical period and adult plasticity differ. Critical period ODP is initiated with the depression of deprived eye responses in V1, whereas adult ODP is characterized by an initial increase in non-deprived eye responses. Our model proposes the gestalt synapse-based mechanism for critical period ODP, and also predicts a different mechanism for adult ODP based on the sparsity of nascent gestalt synapses at that age. Under our model, early life experience shapes the boundaries (the gestalt) for network function, both for its optimal performance as well as for its pathological state. Thus, reintroducing nascent gestalt synapses as plasticity substrates into adults may improve the network gestalt to facilitate functional recovery.

Visual input shapes the auditory frequency responses in the inferior colliculus of mouse

The integration of visual and auditory information is important for humans or animals to build an accurate and coherent perception of the external world. Although some evidence has shown some principles of the audiovisual integration, little insight has been gained into its functional purpose. In this study, we investigated the functional influence of dynamic visual input on auditory frequency processing by recording single unit activity in the central nucleus of the inferior colliculus (ICc). Results showed that the auditory responses of ICc neurons to sound frequencies could be enhanced or suppressed by visual stimuli even though the same visual stimuli induced no neural responses when presented alone. For each ICc neuron, the most effective visual stimuli were located in the same azimuth as for auditory stimuli and preceded for ∼20 ms. Additionally, visual stimuli could steepen or flatten the frequency tuning curves (FTCs) of ICc neurons by various visual effects at each responsive frequency. The modulation degree of auditory FTCs was dependent on the minimal thresholds (MTs) of ICc neurons, i.e., with MTs increasing, the modulation degree decreased. Due to the non-homogeneous distribution of MTs which was lowest at 10 kHz, visual modulation of auditory FTCs exhibited a frequency-specific manner, the closer it reached the characteristic frequency (CF) of 10 kHz, the greater modulation. Thus, visual modulation of auditory frequency responses in ICc is dependent not only on the visual stimulus but also on the auditory characteristics of ICc neurons. These results suggest a moment-to-moment visual modulation of auditory frequency responses that in real time increase auditory frequency sensitivity to audiovisual stimuli. Furthermore, in the long term such modulation could serve to instruct auditory adaptive plasticity to maintain necessary and accurate auditory detection and perceptual behavior.

A novel relay nucleus between the inferior colliculus and the optic tectum in the chicken (Gallus gallus)

Processing multimodal sensory information is vital for behaving animals in many contexts. The barn owl, an auditory specialist, is a classic model for studying multisensory integration. In the barn owl, spatial auditory information is conveyed to the optic tectum (TeO) by a direct projection from the external nucleus of the inferior colliculus (ICX). In contrast, evidence of an integration of visual and auditory information in auditory generalist avian species is completely lacking. In particular, it is not known whether in auditory generalist species the ICX projects to the TeO at all. Here we use various retrograde and anterograde tracing techniques both in vivo and in vitro, intracellular fillings of neurons in vitro, and whole-cell patch recordings to characterize the connectivity between ICX and TeO in the chicken. We found that there is a direct projection from ICX to the TeO in the chicken, although this is small and only to the deeper layers (layers 13-15) of the TeO. However, we found a relay area interposed among the IC, the TeO, and the isthmic complex that receives strong synaptic input from the ICX and projects broadly upon the intermediate and deep layers of the TeO. This area is an external portion of the formatio reticularis lateralis (FRLx). In addition to the projection to the TeO, cells in FRLx send, via collaterals, descending projections through tectopontine-tectoreticular pathways. This newly described connection from the inferior colliculus to the TeO provides a solid basis for visual-auditory integration in an auditory generalist bird. J. Comp. Neurol. 525:513-534, 2017. © 2016 Wiley Periodicals, Inc.

Maps of interaural delay in the owl's nucleus laminaris

Axons from the nucleus magnocellularis form a presynaptic map of interaural time differences (ITDs) in the nucleus laminaris (NL). These inputs generate a field potential that varies systematically with recording position and can be used to measure the map of ITDs. In the barn owl, the representation of best ITD shifts with mediolateral position in NL, so as to form continuous, smoothly overlapping maps of ITD with iso-ITD contours that are not parallel to the NL border. Frontal space (0°) is, however, represented throughout and thus overrepresented with respect to the periphery. Measurements of presynaptic conduction delay, combined with a model of delay line conduction velocity, reveal that conduction delays can account for the mediolateral shifts in the map of ITD.

A review on auditory space adaptations to altered head-related cues

In this article we present a review of current literature on adaptations to altered head-related auditory localization cues. Localization cues can be altered through ear blocks, ear molds, electronic hearing devices, and altered head-related transfer functions (HRTFs). Three main methods have been used to induce auditory space adaptation: sound exposure, training with feedback, and explicit training. Adaptations induced by training, rather than exposure, are consistently faster. Studies on localization with altered head-related cues have reported poor initial localization, but improved accuracy and discriminability with training. Also, studies that displaced the auditory space by altering cue values reported adaptations in perceived source position to compensate for such displacements. Auditory space adaptations can last for a few months even without further contact with the learned cues. In most studies, localization with the subject's own unaltered cues remained intact despite the adaptation to a second set of cues. Generalization is observed from trained to untrained sound source positions, but there is mixed evidence regarding cross-frequency generalization. Multiple brain areas might be involved in auditory space adaptation processes, but the auditory cortex (AC) may play a critical role. Auditory space plasticity may involve context-dependent cue reweighting.

An integrative model of subcortical auditory plasticity

In direct conflict with the concept of auditory brainstem nuclei as passive relay stations for behaviorally-relevant signals, recent studies have demonstrated plasticity of the auditory signal in the brainstem. In this paper we provide an overview of the forms of plasticity evidenced in subcortical auditory regions. We posit an integrative model of auditory plasticity, which argues for a continuous, online modulation of bottom-up signals via corticofugal pathways, based on an algorithm that anticipates and updates incoming stimulus regularities. We discuss the negative implications of plasticity in clinical dysfunction and propose novel methods of eliciting brainstem responses that could specify the biological nature of auditory processing deficits.

Stimulus-specific adaptation, habituation and change detection in the gaze control system

This prospect article addresses the neurobiology of detecting and responding to changes or unexpected events. Change detection is an ongoing computational task performed by the brain as part of the broader process of saliency mapping and selection of the next target for attention. In the optic tectum (OT) of the barn owl, the probability of the stimulus has a dramatic influence on the neural response to that stimulus; rare or deviant stimuli induce stronger responses compared to common stimuli. This phenomenon, known as stimulus-specific adaptation, has recently attracted scientific interest because of its possible role in change detection. In the barn owl's OT, it may underlie the ability to orient specifically to unexpected events and is therefore opening new directions for research on the neurobiology of fundamental psychological phenomena such as habituation, attention, and surprise.

Neuronal morphology in subdivisions of the inferior colliculus of chicken (Gallus gallus)

The avian inferior colliculus (IC), also referred to as the nucleus mesencephalicus lateralis pars dorsalis (MLd), is an auditory midbrain nucleus that converges auditory cues from tonotopically organized brainstem nuclei. This information is relayed onto the optic tectum on the one hand and to nucleus ovoidalis on the other hand. Morphologically, there has been considerable debate about the number and nomenclature of the subnuclei within the IC. Here, we provide morphological characteristics of single cells in five IC subnuclei in chicken. The cellular structure within the IC was studied by whole-cell patch technique and biocytin iontophoresis. In addition, histological staining was performed, to delineate the borders between subnuclei of the IC. We were able to discriminate between 5 subnuclei: the core of the central nucleus (ICCc), the medial and lateral shell of the central nucleus (ICCms and ICCls), the external nucleus (ICX) and the superficial nucleus (ICS) of the IC. Our findings suggest the existence of at least two different morphologies of neurons with two subtypes each. The IC in chicken is a largely homogenous nucleus in terms of neuronal anatomy on a cellular level. However, its compartmentation into diversified subnuclei with different neurophysiological characteristics suggests a complex system to process auditory information. The auditory system in chicken is not as hypertrophied as in specialists such as the barn owl, but appears to have comparable connectivity and cellular morphology.

Distribution of eye position information in the monkey inferior colliculus

The inferior colliculus (IC) is thought to have two main subdivisions, a central region that forms an important stop on the ascending auditory pathway and a surrounding shell region that may play a more modulatory role. In this study, we investigated whether eye position affects activity in both the central and shell regions. Accordingly, we mapped the location of eye position-sensitive neurons in six monkeys making spontaneous eye movements by sampling multiunit activity at regularly spaced intervals throughout the IC. We used a functional map based on auditory response patterns to estimate the anatomical location of recordings, in conjunction with structural MRI and histology. We found eye position-sensitive sites throughout the IC, including at 27% of sites in tonotopically organized recording penetrations (putatively the central nucleus). Recordings from surrounding tissue showed a larger proportion of sites indicating an influence of eye position (33-43%). When present, the magnitude of the change in activity due to eye position was often comparable to that seen for sound frequency. Our results indicate that the primary ascending auditory pathway is influenced by the position of the eyes. Because eye position is essential for visual-auditory integration, our findings suggest that computations underlying visual-auditory integration begin early in the ascending auditory pathway.

Early life exposure to noise alters the representation of auditory localization cues in the auditory space map of the barn owl

The auditory space map in the optic tectum (OT) (also known as superior colliculus in mammals) relies on the tuning of neurons to auditory localization cues that correspond to specific sound source locations. This study investigates the effects of early auditory experiences on the neural representation of binaural auditory localization cues. Young barn owls were raised in continuous omnidirectional broadband noise from before hearing onset to the age of ∼ 65 days. Data from these birds were compared with data from age-matched control owls and from normal adult owls (>200 days). In noise-reared owls, the tuning of tectal neurons for interaural level differences and interaural time differences was broader than in control owls. Moreover, in neurons from noise-reared owls, the interaural level differences tuning was biased towards sounds louder in the contralateral ear. A similar bias appeared, but to a much lesser extent, in age-matched control owls and was absent in adult owls. To follow the recovery process from noise exposure, we continued to survey the neural representations in the OT for an extended period of up to several months after removal of the noise. We report that all the noise-rearing effects tended to recover gradually following exposure to a normal acoustic environment. The results suggest that deprivation from experiencing normal acoustic localization cues disrupts the maturation of the auditory space map in the OT.

How the owl tracks its prey--II

Barn owls can capture prey in pitch darkness or by diving into snow, while homing in on the sounds made by their prey. First, the neural mechanisms by which the barn owl localizes a single sound source in an otherwise quiet environment will be explained. The ideas developed for the single source case will then be expanded to environments in which there are multiple sound sources and echoes--environments that are challenging for humans with impaired hearing. Recent controversies regarding the mechanisms of sound localization will be discussed. Finally, the case in which both visual and auditory information are available to the owl will be considered.

Experience, cortical remapping, and recovery in brain disease

Recovery of motor function in brain and spinal cord disorders is an area of active research that seeks to maximize improvement after an episode of neuronal death or dysfunction. Recovery likely results from changes in structure and function of undamaged neurons, and this plasticity is a target for rehabilitative strategies. Sensory and motor function are mapped onto brain regions somatotopically, and these maps have been demonstrated to change in response to experience, particularly in development, but also in adults after injury. The map concept, while appealing, is limited, as the fine structure of the motor representation is not well-ordered somatotopically. But after stroke, the spared areas of the main cortical map for movement appear to participate in representing affected body parts, expanding representation in an experience-dependent manner. This occurs in both animal models and human clinical trials, although one must be cautious in comparing the results of invasive electrophysiological techniques with non-invasive ones such as transcranial magnetic stimulation. Developmental brain disorders, such as cerebral palsy, and embryonic abnormalities, such as dysmelia, demonstrate the potential of the human brain to remap the motor system. Future therapies may be able to use that potential to maximize recovery.

Micro-rewiring as a substrate for learning

How does the brain encode life experiences? Recent results derived from vital imaging, computational modeling, cellular physiology and systems neuroscience have pointed to local changes in synaptic connectivity as a powerful substrate, here termed micro-rewiring. To examine this hypothesis, I first review findings on micro-structural dynamics with focus on the extension and retraction of dendritic spines. Although these observations demonstrate a biological mechanism, they do not inform us of the specific changes in circuit configuration that might occur during learning. Here, computational models have made testable predictions for both the neuronal and circuit levels. Integrative approaches in the mammalian neocortex and the barn owl auditory localization pathway provide some of the first direct evidence in support of these 'synaptic-clustering' mechanisms. The implications of these data and the challenges for future research are discussed.

Transcriptome changes associated with instructed learning in the barn owl auditory localization pathway

Owls reared wearing prismatic spectacles learn to make adaptive orienting movements. This instructed learning depends on re-calibration of the midbrain auditory space map, which in turn involves the formation of new synapses. Here we investigated whether these processes are associated with differential gene expression, using longSAGE. Newly fledged owls were reared for 8-36 days with prism or control lenses at which time the extent of learning was quantified by electrophysiological mapping. Transciptome profiles were obtained from the inferior colliculus (IC), the major site of synaptic plasticity, and the optic tectum (OT), which provides an instructive signal that controls the direction and extent of plasticity. Twenty-two differentially expressed sequence tags were identified in IC and 36 in OT, out of more than 35,000 unique tags. Of these, only four were regulated in both structures. These results indicate that regulation of two largely independent gene clusters is associated with synaptic remodeling (in IC) and generation of the instructive signal (in OT). Real-time PCR data confirmed the changes for two transcripts, ubiquitin/polyubiquitin and tyrosine 3-monooxgenase/tryotophan 5-monooxygenase activation protein, theta subunit (YWHAQ; also referred to as 14-3-3 protein). Ubiquitin was downregulated in IC, consistent with a model in which protein degradation pathways act as an inhibitory constraint on synaptogenesis. YWHAQ was up-regulated in OT, indicating a role in the synthesis or delivery of instructive information. In total, our results provide a path towards unraveling molecular cascades that link naturalistic experience with synaptic remodeling and, ultimately, with the expression of learned behavior.

Visual- and saccade-related signals in the primate inferior colliculus

The inferior colliculus (IC) is normally thought of as a predominantly auditory structure because of its early position in the ascending auditory pathway just before the auditory thalamus. Here, we show that a majority of IC neurons (64% of 180 neurons) in awake monkeys carry visual- and/or saccade-related signals in addition to their auditory responses (P < 0.05). The response patterns involve primarily excitatory visual responses, but also increased activity time-locked to the saccade, slow rises in activity time-locked to the onset of the visual stimulus, and inhibitory responses. The presence of these visual-related signals suggests that the IC plays a role in integrating visual and auditory information. More broadly, our results show that interactions between sensory pathways can occur at very early points in sensory processing streams, which implies that multisensory integration may be a low-level rather than an exclusively high-level process.

Adaptive auditory plasticity in developing and adult animals

Enormous progress has been made in our understanding of adaptive plasticity in the central auditory system. Experiments on a range of species demonstrate that, in adults, the animal must attend to (i.e., respond to) a stimulus in order for plasticity to be induced, and the plasticity that is induced is specific for the acoustic feature to which the animal has attended. The requirement that an adult animal must attend to a stimulus in order for adaptive plasticity to occur suggests an essential role of neuromodulatory systems in gating plasticity in adults. Indeed, neuromodulators, particularly acetylcholine (ACh), that are associated with the processes of attention, have been shown to enable adaptive plasticity in adults. In juvenile animals, attention may facilitate plasticity, but it is not always required: during sensitive periods, mere exposure of an animal to an atypical auditory environment can result in large functional changes in certain auditory circuits. Thus, in both the developing and mature auditory systems substantial experience-dependent plasticity can occur, but the conditions under which it occurs are far more stringent in adults. We review experimental results that demonstrate experience-dependent plasticity in the central auditory representations of sound frequency, level and temporal sequence, as well as in the representations of binaural localization cues in both developing and adult animals.

Adaptation in the Auditory Space Map of the Barn Owl

Auditory neurons in the owl’s external nucleus of the inferior colliculus (ICX) integrate information across frequency channels to create a map of auditory space. This study describes a powerful, sound-driven adaptation of unit responsiveness in the ICX and explores the implications of this adaptation for sensory processing. Adaptation in the ICX was analyzed by presenting lightly anesthetized owls with sequential pairs of dichotic noise bursts. Adaptation occurred in response even to weak, threshold-level sounds and remained strong for more than 100 ms after stimulus offset. Stimulation by one range of sound frequencies caused adaptation that generalized across the entire broad range of frequencies to which these units responded. Identical stimuli were used to test adaptation in the lateral shell of the central nucleus of the inferior colliculus (ICCls), which provides input directly to the ICX. Compared with ICX adaptation, adaptation in the ICCls was substantially weaker, shorter lasting, and far more frequency specific, suggesting that part of the adaptation observed in the ICX was attributable to processes resident to the ICX. The sharp tuning of ICX neurons to space, along with their broad tuning to frequency, allows ICX adaptation to preserve a representation of stimulus location, regardless of the frequency content of the sound. The ICX is known to be a site of visually guided auditory map plasticity. ICX adaptation could play a role in this cross-modal plasticity by providing a short-term memory of the representation of auditory localization cues that could be compared with later-arriving, visual–spatial information from bimodal stimuli.

Seeing sounds: visual and auditory interactions in the brain

Objects and events can often be detected by more than one sensory system. Interactions between sensory systems can offer numerous benefits for the accuracy and completeness of the perception. Recent studies involving visual-auditory interactions have highlighted the perceptual advantages of combining information from these two modalities and have suggested that predominantly unimodal brain regions play a role in multisensory processing.

Why seeing is believing: merging auditory and visual worlds

Vision may dominate our perception of space not because of any inherent physiological advantage of visual over other sensory connections in the brain, but because visual information tends to be more reliable than other sources of spatial information, and the central nervous system integrates information in a statistically optimal fashion. This review discusses recent experiments on audiovisual integration that support this hypothesis. We consider candidate neural codes that would enable optimal integration and the implications of optimal integration for perception and plasticity.

The "other" transformation required for visual-auditory integration: representational format

Multisensory integration of spatial signals requires not only that stimulus locations be encoded in the same spatial reference frame, but also that stimulus locations be encoded in the same representational format. Previous studies have addressed the issue of spatial reference frame, but representational format, particularly for sound location, has been relatively overlooked. We discuss here our recent findings that sound location in the primate inferior colliculus is encoded using a "rate" code, a format that differs from the place code used for representing visual stimulus locations. Possible mechanisms for transforming signals from rate-to-place or place-to-rate coding formats are considered.

Development of the projection from the nucleus of the brachium of the inferior colliculus to the superior colliculus in the ferret

Neurons in the deeper layers of the superior colliculus (SC) have spatially tuned receptive fields that are arranged to form a map of auditory space. The spatial tuning of these neurons emerges gradually in an experience-dependent manner after the onset of hearing, but the relative contributions of peripheral and central factors in this process of maturation are unknown. We have studied the postnatal development of the projection to the ferret SC from the nucleus of the brachium of the inferior colliculus (nBIC), its main source of auditory input, to determine whether the emergence of auditory map topography can be attributed to anatomical rewiring of this projection. The pattern of retrograde labeling produced by injections of fluorescent microspheres in the SC on postnatal day (P) 0 and just after the age of hearing onset (P29), showed that the nBIC-SC projection is topographically organized in the rostrocaudal axis, along which sound azimuth is represented, from birth. Injections of biotinylated dextran amine-fluorescein into the nBIC at different ages (P30, 60, and 90) labeled axons with numerous terminals and en passant boutons throughout the deeper layers of the SC. This labeling covered the entire mediolateral extent of the SC, but, in keeping with the pattern of retrograde labeling following microsphere injections in the SC, was more restricted rostrocaudally. No systematic changes were observed with age. The stability of the nBIC-SC projection over this period suggests that developmental changes in auditory spatial tuning involve other processes, rather than a gross refinement of the projection from the nBIC.

Anatomical traces of juvenile learning in the auditory system of adult barn owls

Early experience plays a powerful role in shaping adult neural circuitry and behavior. In barn owls, early experience markedly influences sound localization. Juvenile owls that learn new, abnormal associations between auditory cues and locations in visual space as a result of abnormal visual experience can readapt to the same abnormal experience in adulthood, when plasticity is otherwise limited. Here we show that abnormal anatomical projections acquired during early abnormal sensory experience persist long after normal experience has been restored. These persistent projections are perfectly situated to provide a physical framework for subsequent readaptation in adulthood to the abnormal sensory conditions experienced in early life. Our results show that anatomical changes that support strong learned neural connections early in life can persist even after they are no longer functionally expressed. This maintenance of silenced neural circuitry that was once adaptive may represent an important mechanism by which the brain preserves a record of early experience.

Sequential Learning From Multiple Tutors and Serial Retuning of Auditory Neurons in a Brain Area Important to Birdsong Learning

Songbirds hear many vocal models during a juvenile sensitive period, transiently imitating some while retaining imitations of others in their repertoires. Despite subsequent conflicting experiences, early experience can exert lasting effects on neural structure and function, raising the possibility that transiently expressed vocalizations or their relevant models are stored in the adult songbird's brain. One site where learned song representations could be stored is the lateral magnocellular nucleus of the anterior nidopallium (LMAN), which in the adult songbird contains neurons responsive to playback of the bird's own song (BOS) and the tutor song (TS). To test whether LMAN neurons develop and retain responses to transiently learned songs, we exposed zebra finch hatchlings [posthatch day 0 (PHD0)] to a first TS (TS1) for about 30 days, isolated them for about 30 days, then exposed them to a second TS (TS2) for 30 days starting at PHD 60. Behavioral analysis showed that PHD 60 juveniles had started to copy TS1, although this copying was transient, because the adult BOS resembled TS2 and not TS1. We found that LMAN auditory responses paralleled these behavioral changes: LMAN neurons at PHD 60 responded strongly and selectively to both the juvenile BOS and TS1, whereas LMAN neurons in adults responded to the adult BOS and TS2, but not to the transiently learned song or its model. Therefore LMAN auditory responses can be lost or overwritten as the juvenile copies a new song, suggesting that the adult LMAN does not store information about transiently learned songs or their models.

Local neurons play key roles in the mammalian olfactory bulb

Over the past few decades, research exploring how the brain perceives, discriminates, and recognizes odorant molecules has received a growing interest. Today, olfaction is no longer considered a matter of poetry. Chemical senses entered the biological era when an increasing number of scientists started to elucidate the early stages of the olfactory pathway. A combination of genetic, biochemical, cellular, electrophysiological and behavioral methods has provided a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. Our group is exploring the physiology of the main olfactory bulb, the first processing relay in the mammalian brain. From different electrophysiological approaches, we are attempting to understand the cellular rules that contribute to the synaptic transmission and plasticity at this central relay. How olfactory sensory inputs, originating from the olfactory epithelium located in the nasal cavity, are encoded in the main olfactory bulb remains a crucial question for understanding odor processing. More importantly, the persistence of a high level of neurogenesis continuously supplying the adult olfactory bulb with newborn local neurons provides an attractive model to investigate how basic olfactory functions are maintained when a large proportion of local neurons are continuously renewed. For this purpose, we summarize the current ideas concerning the molecular mechanisms and organizational strategies used by the olfactory system to encode and process information in the main olfactory bulb. We discuss the degree of sensitivity of the bulbar neuronal network activity to the persistence of this high level of neurogenesis that is modulated by sensory experience. Finally, it is worth mentioning that analyzing the molecular mechanisms and organizational strategies used by the olfactory system to transduce, encode, and process odorant information in the olfactory bulb should aid in understanding the general neural mechanisms involved in both sensory perception and memory. Due to space constraints, this review focuses exclusively on the olfactory systems of vertebrates and primarily those of mammals.

Anatomical markers for the subdivisions of the barn owl's inferior-collicular complex and adjacent peri- and subventricular structures

The anatomy of the inferior-collicular complex of the barn owl, situated below the fourth ventricle in the tectal lobe, was studied by determining the distribution of antigens with antibodies directed against tyrosine hydroxylase, gamma-aminobutyric acid (GABA)(Abeta), dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32), calretinin, and calbindin. Additionally, the somata were stained with cresyl violet, and fibers were marked according to the Gallyas procedure. These markers were chosen to allow for an easy delineation of the boundaries between the subnuclei of the inferior colliculus. We could discriminate eight structures that belong to the three subnuclei of the inferior colliculus [the central nucleus (ICC), the superficial nucleus (ICS), the external nucleus (ICX)] and to the optic tectum. Periventricular tectal layers 15a and 15b stained well with all the antibodies used. The ICS, embedded in tectal layer 15a, may be divided into a dorsal and a ventral lamina. It does not have direct contact with the other nuclei of the inferior colliculus. The border between tectal layer 15a and ICX was well marked by all antibodies, but less so in Gallyas and cresyl violet stains. The ICC consists of a core and a medial and lateral shell. The core was clearly demarcated with antibodies against calretinin and calbindin. The border between the lateral shell and the ICX was marked less well than the borders between ICX and 15a, but the somata were much more darkly labeled with the DARPP-32 antibody in ICX than in the lateral shell of ICC. None of the markers delineated the border between the medial and lateral shell of ICC.

Development of output connections from the inferior colliculus to the optic tectum in barn owls

We studied the development of the projection from the external nucleus of the inferior colliculus (ICX) to the optic tectum (OT) in the barn owl. The projection was labeled by tracer application in vitro to either the OT or the ICX, or by staining ICX cells intracellularly with biocytin. The axons of ICX neurons bifurcated into an ascending branch that projected toward the OT and a descending branch that coursed caudally to an unknown target in the brainstem. Axons of the ICX were observed to grow into the OT from embryonic day 16 (E16) on. From E22 on, side branches of the axonal projections could be found within the OT. At the day of hatching (E32), the projection displayed a dorsoventral topography comparable to the adult owl; however, atopically projecting cells remained. The complexity of the axonal arborization in the adult barn owl was found to be slightly increased compared with the hatchling. The terminal area of individual ICX cells in the OT of the adult barn owl was still broad, a finding that had not been expected from the sharply defined physiological response properties of the bimodal neurons in the space map of the OT. However, the width of the termination zone was in accordance with the large dendritic tree of the adult ICX cells, because both spanned comparable angles in their respective maps. Our data suggest that a coarse projection from the ICX to the OT can develop without coherent sensory input and may, therefore, be innately determined.

Cytoarchitecture of the avian optic tectum: neuronal substrate for cellular computation

To analyse cellular computation in the vertebrate brain, a thorough knowledge of the underlying anatomy, physiology and connectivity of the neuronal substrate is essential. This review compiles data on one of the best known structures of the vertebrate brain, the optic tectum of birds. The functions of this structure are multifold, but can be attributed largely to orientation and the basic analysis of sensory data in a spatial context. In the tectum, a wealth of data on physiology and anatomy has been gathered over more than a century and provides an excellent background for computational studies. The analysis of the optic tectum is facilitated by several principles of organisation, including the retinotopic input and the highly laminated layout with separated input and output layers. Moreover, the molecular mechanisms guiding the development and connectivity have been analysed in detail. As the avian tectum and the mammalian superior colliculus are partly homologous, the cellular mechanisms unraveled in the tectum can also be transferred to the colliculus and thus contribute to the understanding of the vertebrate visual system in general.

Rapid adaptation to auditory-visual spatial disparity

The so-called ventriloquism aftereffect is a remarkable example of rapid adaptative changes in spatial localization caused by visual stimuli. After exposure to a consistent spatial disparity of auditory and visual stimuli, localization of sound sources is systematically shifted to correct for the deviation of the sound from visual positions during the previous adaptation period. In the present study, this aftereffect was induced by presenting, within 17 min, 1800 repetitive noise or pure-tone bursts in combination with synchronized, and 20 degrees disparate flashing light spots, in total darkness. Post-adaptive sound localization, measured by a method of manual pointing, was significantly shifted 2.4 degrees (noise), 3.1 degrees (1 kHz tones), or 5.8 degrees (4 kHz tones) compared with the pre-adaptation condition. There was no transfer across frequencies; that is, shifts in localization were insignificant when the frequencies used for adaptation and the post-adaptation localization test were different. It is hypothesized that these aftereffects may rely on shifts in neural representations of auditory space with respect to those of visual space, induced by intersensory spatial disparity, and may thus reflect a phenomenon of neural short-term plasticity.

Synaptic modification in neurons of the central nucleus of the inferior colliculus

Whole-cell patch clamp recordings were made from neurons in the central nucleus of the inferior colliculus (ICC) in brain slices from rat (8-13 days old). ICC neurons were classified by their discharge pattern in response to depolarizing and hyperpolarizing current injection. Excitatory postsynaptic currents (EPSCs) were elicited by stimulation of synaptic inputs under the condition that the synaptic inhibition was suppressed by strychnine and picrotoxin. EPSCs in all tested types of ICC neurons showed posttetanic, long-term potentiation (LTP) and long-term depression with tetanic stimulation. The potentiated EPSCs consisted of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and NMDA receptor mediated components. The magnitude of LTP was larger when the intracellular concentration of the calcium buffer ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetracetic acid (EGTA) was lower and stimulation frequency was higher in cells with rebound firing patterns. Blocking N-methyl-D-aspartate (NMDA) receptors in rebound cells prevented generation of LTP. These results suggest that excitatory synaptic transmission in ICC neurons can be modified. LTP in the auditory midbrain may be important for activity-dependent, adaptive changes in response to normal and pathological stimulus conditions.

Instructed learning in the auditory localization pathway of the barn owl

A bird sings and you turn to look at it a process so automatic it seems simple. But is it? Our ability to localize the source of a sound relies on complex neural computations that translate auditory localization cues into representations of space. In barn owls, the visual system is important in teaching the auditory system how to translate cues. This example of instructed plasticity is highly quantifiable and demonstrates mechanisms and principles of learning that may be used widely throughout the central nervous system.

The role of auditory experience in the formation of neural circuits underlying vocal learning in zebra finches

The initial establishment of topographic mapping within developing neural circuits is thought to be shaped by innate mechanisms and is primarily independent of experience. Additional refinement within topographic maps leads to precise matching between presynaptic and postsynaptic neurons and is thought to depend on experiential factors during specific sensitive periods in the animal's development. In male zebra finches, axonal projections of the cortical lateral magnocellular nucleus of the anterior neostriatum (lMAN) are critically important for vocal learning. Overall patterns of topographic organization in the majority of these circuits are adult-like throughout the sensitive period for vocal learning and remain stable despite large-scale functional and morphological changes. However, topographic organization within the projection from the core subregion of lMAN (lMAN(core)) to the motor cortical robust nucleus of the archistriatum (RA) is lacking at the onset of song development and emerges during the early stages of vocal learning. To study the effects of song-related experience on patterns of axonal connectivity within different song-control circuits, we disrupted song learning by deafening juvenile zebra finches or exposing them to loud white noise throughout the sensitive period for song learning. Depriving juvenile birds of normal auditory experience delayed the emergence of topographic specificity within the lMAN(core)-->RA circuit relative to age-matched controls, whereas topographic organization within all other projections to and from lMAN was not affected. The projection from lMAN(core) to RA therefore provides an unusual example of experience-dependent modification of large-scale patterns of brain circuitry, in the sense that auditory deprivation influences the development of overall topographic organization in this pathway.

The optic tectum controls visually guided adaptive plasticity in the owl's auditory space map

The midbrain contains an auditory map of space that is shaped by visual experience. When barn owls are raised wearing spectacles that horizontally displace the visual field, the auditory space map in the external nucleus of the inferior colliculus (ICX) shifts according to the optical displacement of the prisms. Topographic visual activity in the optic tectum could serve as the template that instructs the auditory space map. We studied the effects of a restricted, unilateral lesion in the portion of the optic tectum that represents frontal space. Here we show that such a lesion eliminates adaptive adjustments specifically in the portion of the auditory map that represents frontal space on the same side of the brain, while the rest of the map continues to adjust adaptively. Thus, activity in the tectum calibrates the auditory space map in a location-specific manner. Because the site of adaptive changes is the ICX, the results also indicate that the tectum provides a topographic instructive signal that controls adaptive auditory plasticity in the ICX.

Light and electron microscopic observations of a direct projection from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat

A direct projection from rat mesencephalic trigeminal nucleus (Vme) neurons to the hypoglossal nucleus (XII) motoneurons was studied using a double labeling method of anterogradely biotinylated dextran amine (BDA) tracing combined with retrogradely horseradish peroxidase (HRP) transport at both light and electron microscopic levels. BDA was iontophoresed unilaterally into the caudal Vme, and 7 days later HRP was injected into the ipsilateral tongue to label hypoglossal motoneurons. The BDA-labeled fibers were seen descended along Probst' tract and were traced to the caudal medulla. In this course, the fibers gave off axon collaterals bearing varicosities in the trigeminal motor nucleus (Vmo), the parvicellular reticular formation (PCRt), the dorsomedial portions of the subnuclei of oralis (Vodm) and interpolaris (Vidm) and in the XII ipsilaterally. The labeling of terminals was most dense in the PCRt at the levels of caudal pons and rostral medulla, which displayed a "dumbbell-shaped" form in the transverse planes. In the XII, labeled terminals were distributed mainly in the dorsal compartment of the nucleus. One hundred sixty-eight appositions made by BDA-labeled terminals on HRP-labeled motoneurons were seen in the dorsal compartment (71%) and in the lateral subcompartment (24%) of the ventral XII. Under electron microscopy BDA-labeled boutons containing clear, spherical synaptic vesicles were found to form synaptic contacts with the somata and dendrites of hypoglossal motoneurons with asymmetric specializations. The present study provides new evidence that the trigeminal proprioceptive afferent neurons terminate in the XII and make synaptic contacts with their motoneurons.

The neural basis of perceptual learning

Perceptual learning is a lifelong process. We begin by encoding information about the basic structure of the natural world and continue to assimilate information about specific patterns with which we become familiar. The specificity of the learning suggests that all areas of the cerebral cortex are plastic and can represent various aspects of learned information. The neural substrate of perceptual learning relates to the nature of the neural code itself, including changes in cortical maps, in the temporal characteristics of neuronal responses, and in modulation of contextual influences. Top-down control of these representations suggests that learning involves an interaction between multiple cortical areas.

Adaptive axonal remodeling in the midbrain auditory space map

The auditory space map in the external nucleus of the inferior colliculus (ICX) of barn owls is highly plastic, especially during early life. When juvenile owls are reared with prismatic spectacles (prisms) that displace the visual field laterally, the auditory spatial tuning of neurons in the ICX adjusts adaptively to match the visual displacement. In the present study, we show that this functional plasticity is accompanied by axonal remodeling. The ICX receives auditory input from the central nucleus of the inferior colliculus (ICC) via topographic axonal projections. We used the anterograde tracer biocytin to study experience-dependent changes in the spatial pattern of axons projecting from the ICC to the ICX. The projection fields in normal adults were sparser and more restricted than those in normal juveniles. The projection fields in prism-reared adults were denser and broader than those in normal adults and contained substantially more bouton-laden axons that were appropriately positioned in the ICX to convey adaptive auditory spatial information. Quantitative comparison of results from juvenile and prism-reared owls indicated that prism experience led to topographically appropriate axonal sprouting and synaptogenesis. We conclude that this elaboration of axons represents the formation of an adaptive neuronal circuit. The density of axons and boutons in the normal projection zone was preserved in prism-reared owls. The coexistence of two different circuits encoding alternative maps of space may underlie the ability of prism-reared owls to readapt to normal conditions as adults.

Adaptive adjustment of connectivity in the inferior colliculus revealed by focal pharmacological inactivation

In the midbrain sound localization pathway of the barn owl, a map of auditory space is synthesized in the external nucleus of the inferior colliculus (ICX) and transmitted to the optic tectum. Early auditory experience shapes these maps of auditory space in part by modifying the tuning of the constituent neurons for interaural time difference (ITD), a primary cue for sound-source azimuth. Here we show that these adaptive modifications in ITD tuning correspond to changes in the pattern of connectivity within the inferior colliculus. We raised owls with an acoustic filtering device in one ear that caused frequency-dependent changes in sound timing and level. As reported previously, device rearing shifted the representation of ITD in the ICX and tectum but not in the primary source of input to the ICX, the central nucleus of the inferior colliculus (ICC). We applied the local anesthetic lidocaine (QX-314) iontophoretically in the ICC to inactivate small populations of neurons that represented particular values of frequency and ITD. We measured the effect of this inactivation in the optic tecta of a normal owl and owls raised with the device. In the normal owl, inactivation at a critical site in the ICC eliminated responses in the tectum to the frequency-specific ITD value represented at the site of inactivation in the ICC. The location of this site was consistent with the known pattern of ICC-ICX-tectum connectivity. In the device-reared owls, adaptive changes in the representation of ITD in the tectum corresponded to dramatic and predictable changes in the locations of the critical sites of inactivation in the ICC. Given that the abnormal representation of ITD in the tectum depended on frequency and was likely conveyed directly from the ICX, these results suggest that experience causes large-scale, frequency-specific adjustments in the pattern of connectivity between the ICC and the ICX.

Prey targeting by the infrared-imaging snake Python molurus: effects of experimental and congenital visual deprivation

Boid and crotaline snakes possess two distinct types of organ evolved to image radiant electromagnetic energy: the lateral eye, which responds to visible light, and the pit organ, which responds to infrared radiation. While infrared imaging may allow accurate predatory targeting in complete absence of visual information, both infrared and visual information are probably normally involved in prey targeting. We examined the roles of vision and infrared imaging in Python molurus predatory performance under conditions of (1) high visual contrast; (2) very low visual contrast; (3) complete blinding; (4) experimental monocular occlusion; and (5) congenital monocularity. Normally sighted pythons were equally successful at targeting white (BALB/c) and black (C57BL6/J) mice (Mus domesticus) against a black background. Binocularly occluded snakes exhibited strike angles and distances similar to non-occluded snakes, but exhibited lower strike success, suggesting that high visible contrast is not required for accurate targeting, but that precise targeting depends to some degree upon visual information. Strike angles, distances and latencies were indistinguishable between snakes subjected to experimental monocular occlusion and normally sighted snakes. However, snakes congenitally lacking one eye preferentially targeted on the sighted side. Thus, accurate targeting of highly mobile homeothermic prey by Python can be accomplished with little or no visual information, but performance can be affected by complete visual deprivation or by alteration of visual input during development. The developmental effects of early visual deprivation in this system provide a novel opportunity to investigate the neural integration of two electromagnetic radiation-imaging systems in a single animal.

Axonal projections and synapses from the supratrigeminal region to hypoglossal motoneurons in the rat

Neural circuits from the supratrigeminal region (Vsup) to the hypoglossal motor nucleus were studied in rats using anterograde and retrograde neuroanatomical tracing methodologies. Iontophoretic injection of 10% biotinylated dextran amine (BDA) unilaterally into the Vsup anterogradely labeled axons and axon terminals bilaterally in the hypoglossal nucleus (XII) as well as other regions of the brainstem. In the ipsilateral XII, the highest density of BDA labeling was found in the dorsal compartment and the ventromedial subcompartment of the ventral compartment, where BDA labeling formed a dense, patchy distribution. Microinjection of 20% horseradish peroxidase (HRP) ipsilaterally or bilaterally into the tongue resulted in retrograde labeling of XII motoneurons confined to the dorsal and ventral compartments of the hypoglossal motor nucleus. Under light microscopical examination, BDA-labeled terminals were observed closely apposing the somata and primary dendrites of HRP-labeled hypoglossal motoneurons. Two hundred and sixty-five of these BDA-labeled terminals were examined at the ultrastructural level. One hundred and twelve BDA-labeled axon terminals were observed synapsing with either the somata (39%, 44/112) or the large or medium-size dendrites (61%, 68/112) of retrogradely labeled hypoglossal motoneurons. Axon terminals containing spherical vesicles (S-type) formed asymmetric synapses with HRP-labeled hypoglossal motoneuron dendrites. In contrast to this, F(F)-type axon terminals, containing flattened vesicles, formed symmetric synapses with both the somata and dendrites of HRP-labeled hypoglossal motoneurons with a preponderance of the contacts on their somata. Axon terminals containing pleomorphic vesicles (F(P)-type) were noted forming both symmetric and asymmetric synapses with HRP-labeled hypoglossal motoneuron somata and dendrites. The present study provides anatomical evidence of neuronal projections and synaptic connections from the supratrigeminal region to hypoglossal motoneurons. These data suggest that the supratrigeminal region, as one of the premotor neuronal pools of the hypoglossal nucleus, may coordinate and modulate the activity of tongue muscles during oral motor behaviors.

Traces of learning in the auditory localization pathway

One of the fascinating properties of the central nervous system is its ability to learn: the ability to alter its functional properties adaptively as a consequence of the interactions of an animal with the environment. The auditory localization pathway provides an opportunity to observe such adaptive changes and to study the cellular mechanisms that underlie them. The midbrain localization pathway creates a multimodal map of space that represents the nervous system's associations of auditory cues with locations in visual space. Various manipulations of auditory or visual experience, especially during early life, that change the relationship between auditory cues and locations in space lead to adaptive changes in auditory localization behavior and to corresponding changes in the functional and anatomical properties of this pathway. Traces of this early learning persist into adulthood, enabling adults to reacquire patterns of connectivity that were learned initially during the juvenile period.

Neural adaptation in the generation of rhythmic behavior

Motor systems can adapt rapidly to changes in external conditions and to switching of internal goals. They can also adapt slowly in response to training, alterations in the mechanics of the system, and any changes in the system resulting from injury. This article reviews the mechanisms underlying short- and long-term adaptation in rhythmic motor systems. The neuronal networks underlying the generation of rhythmic motor patterns (central pattern generators; CPGs) are extremely flexible. Neuromodulators, central commands, and afferent signals all influence the pattern produced by a CPG by altering the cellular and synaptic properties of individual neurons and the coupling between different populations of neurons. This flexibility allows the generation of a variety of motor patterns appropriate for the mechanical requirements of different forms of a behavior. The matching of motor output to mechanical requirements depends on the capacity of pattern-generating networks to adapt to slow changes in body mechanics and persistent errors in performance. Afferent feedback from body and limb proprioceptors likely plays an important role in driving these long-term adaptive processes.

The development of abnormal axon trajectories after rotation of one eye in Xenopus

The targeting of isthmotectal axons in the Xenopus binocular pathway is guided by both activity-dependent cues and activity-independent cues. Abnormal visual activity induced by unilateral eye rotation overrides activity-independent cues and causes isthmotectal axons to arborize at new locations during a critical period of development that ends approximately 3 months postmetamorphosis (PM). Horseradish peroxidase staining of isthmotectal axons reveals that they normally run rostrocaudally in the tectum; in contrast, those axons in animals with early eye rotation have circuitous trajectories. In this paper, by studying the trajectories and branching patterns of isthmotectal axons at different times after eye rotation, we aimed to investigate when and how activity cues determine the projection pattern of isthmotectal axons. As suggested by electrophysiological recording, isthmotectal axons initially grow normally and make arbors according to activity-independent cues despite the presence of abnormal visual input. Our findings demonstrate that the development of abnormal trajectories starts by 2 weeks PM in response to eye rotation and is a protracted process. It begins in the tectal regions in which the initial connections of isthmotectal axons are first formed according to activity-independent cues. At transitional stages (5 and 10 weeks), axons with arbors at two different locations are observed, with locations corresponding to the old and new termination sites, respectively. Later, at 10 weeks of age, the fainter horseradish peroxidase staining in arbors at old termination sites suggests that the older arbors are undergoing withdrawal.