Biomedical subjects

Eric I Knudsen

Publications and source records attributed to Eric I Knudsen.

16 recordsLinked to original sources

PubMed2006

Auditory and visual space maps in the cholinergic nucleus isthmi pars parvocellularis of the barn owl.

The nucleus isthmi pars parvocellularis (Ipc) is a midbrain cholinergic nucleus that shares reciprocal, topographic connections with the optic tectum (OT). Ipc neurons project to spatially restricted columns in the OT, contacting essentially all OT layers in a given column. Previous research characterizes the Ipc as a visual processor. We found that, in the barn owl, the Ipc responds to auditory as well as to visual stimuli. Auditory responses were tuned broadly for frequency, but sharply for spatial cues. We measured the tuning of Ipc units to binaural sound localization cues, including interaural timing differences (ITDs) and interaural level differences (ILDs). Units in the Ipc were tuned to specific values of both ITD and ILD and were organized systematically according to their ITD and ILD tuning, forming a map of space. The auditory space map aligned with the visual space map in the Ipc. These results demonstrate that the Ipc encodes the spatial location of objects, independent of stimulus modality. These findings, combined with the precise pattern of projections from the Ipc to the OT, suggest that the role of the Ipc is to regulate the sensitivity of OT neurons in a space-specific manner.

Acoustic Stimulation↗

PubMed2006

Dynamic shifts in the owl's auditory space map predict moving sound location.

The optic tectum of the barn owl contains a map of auditory space. We found that, in response to moving sounds, the locations of receptive fields that make up the map shifted toward the approaching sound. The magnitude of the receptive field shifts increased systematically with increasing stimulus velocity and, therefore, was appropriate to compensate for sensory and motor delays inherent to auditory orienting behavior. Thus, the auditory space map is not static, but shifts adaptively and dynamically in response to stimulus motion. We provide a computational model to account for these results. Because the model derives predictive responses from processes that are known to occur commonly in neural networks, we hypothesize that analogous predictive responses will be found to exist widely in the central nervous system. This hypothesis is consistent with perceptions of stimulus motion in humans for many sensory parameters.

Adaptation, Physiological↗

PubMed2006

Economic, neurobiological, and behavioral perspectives on building America's future workforce.

A growing proportion of the U.S. workforce will have been raised in disadvantaged environments that are associated with relatively high proportions of individuals with diminished cognitive and social skills. A cross-disciplinary examination of research in economics, developmental psychology, and neurobiology reveals a striking convergence on a set of common principles that account for the potent effects of early environment on the capacity for human skill development. Central to these principles are the findings that early experiences have a uniquely powerful influence on the development of cognitive and social skills and on brain architecture and neurochemistry, that both skill development and brain maturation are hierarchical processes in which higher level functions depend on, and build on, lower level functions, and that the capacity for change in the foundations of human skill development and neural circuitry is highest earlier in life and decreases over time. These findings lead to the conclusion that the most efficient strategy for strengthening the future workforce, both economically and neurobiologically, and improving its quality of life is to invest in the environments of disadvantaged children during the early childhood years.

Adult↗

PubMed2006

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.

Acoustic Stimulation↗

PubMed2006

Top-down gain control of the auditory space map by gaze control circuitry in the barn owl.

High-level circuits in the brain that control the direction of gaze are intimately linked with the control of visual spatial attention. Immediately before an animal directs its gaze towards a stimulus, both psychophysical sensitivity to that visual stimulus and the responsiveness of high-order neurons in the cerebral cortex that represent the stimulus increase dramatically. Equivalent effects on behavioural sensitivity and neuronal responsiveness to visual stimuli result from focal electrical microstimulation of gaze control centres in monkeys. Whether the gaze control system modulates neuronal responsiveness in sensory modalities other than vision is unknown. Here we show that electrical microstimulation applied to gaze control circuitry in the forebrain of barn owls regulates the gain of midbrain auditory responses in an attention-like manner. When the forebrain circuit was activated, midbrain responses to auditory stimuli at the location encoded by the forebrain site were enhanced and spatial selectivity was sharpened. The same stimulation suppressed responses to auditory stimuli represented at other locations in the midbrain map. Such space-specific, top-down regulation of auditory responses by gaze control circuitry in the barn owl suggests that the central nervous system uses a common strategy for dynamically regulating sensory gain that applies across modalities, brain areas and classes of vertebrate species. This approach provides a path for discovering mechanisms that underlie top-down gain control in the central nervous system.

Acoustic Stimulation↗

PubMed2005

Auditory-visual fusion in speech perception in children with cochlear implants.

Speech, for most of us, is a bimodal percept whenever we both hear the voice and see the lip movements of a speaker. Children who are born deaf never have this bimodal experience. We tested children who had been deaf from birth and who subsequently received cochlear implants for their ability to fuse the auditory information provided by their implants with visual information about lip movements for speech perception. For most of the children with implants (92%), perception was dominated by vision when visual and auditory speech information conflicted. For some, bimodal fusion was strong and consistent, demonstrating a remarkable plasticity in their ability to form auditory-visual associations despite the atypical stimulation provided by implants. The likelihood of consistent auditory-visual fusion declined with age at implant beyond 2.5 years, suggesting a sensitive period for bimodal integration in speech perception.

Adult↗

PubMed2005

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.

Animals↗

PubMed2005

Hunting increases adaptive auditory map plasticity in adult barn owls.

The optic tectum (OT) of barn owls contains topographic maps of auditory and visual space. Barn owls reared with horizontally displacing prismatic spectacles (prisms) acquire a novel auditory space map in the OT that restores alignment with the prismatically displaced visual map. Although juvenile owls readily acquire alternative maps of auditory space as a result of experience, this plasticity is reduced greatly in adults. We tested whether hunting live prey, a natural and critically important behavior for barn owls, increases auditory map plasticity in adult owls. Two groups of naive adult owls were fit with prisms. The first group was fed dead mice during 10 weeks of prism experience, while the second group was required to hunt live prey for an identical period of time. When the owls hunted live prey, auditory maps shifted substantially farther (five times farther, on average) and the consistency of tuning curve shifts within each map increased. Only a short period of time in each day, during which the two groups experienced different conditions, accounts for this effect. In addition, increased map plasticity correlated with behavioral improvements in the owls' ability to strike and capture prey. These results indicate that the experience of hunting dramatically increases adult adaptive plasticity in this pathway.

Acoustic Stimulation↗

PubMed2004

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.

Animals↗

PubMed2004

Multiple sites of adaptive plasticity in the owl's auditory localization pathway.

In the midbrain auditory localization pathway of the barn owl, a map of auditory space is relayed from the external nucleus of the inferior colliculus (ICX) to the deep and intermediate layers of the optic tectum (OT) and from these layers to the superficial layers. Within the OT, the auditory space map aligns with a visual map of space. Raising young barn owls with a prismatic displacement of the visual field leads to progressive changes in auditory tuning in the OT that tend to realign the auditory space map with the prismatically displaced visual space map. The only known site of this adaptive plasticity is in the ICX, in which the auditory system first creates a map of space. In this study, we identified an additional site of plasticity in the OT. In owls that experienced prisms beginning late in the juvenile period, adaptive shifts in auditory tuning in the superficial layers of the OT exceeded the adaptive shifts that occurred in the deep layers of the OT or in the ICX. Anatomical results from these owls demonstrated that the topography of intrinsic OT connections was systematically altered in the adaptive direction. In juvenile owls, plasticity in the OT increased as plasticity in the ICX decreased. Because plasticity at both sites has been shown to decline substantially in adults, these results suggest that an age-dependent decrease in auditory map plasticity occurs first in the ICX and later at the higher level, in the OT.

Acoustic Stimulation↗

PubMed2004

Sensitive periods in the development of the brain and behavior.

Experience exerts a profound influence on the brain and, therefore, on behavior. When the effect of experience on the brain is particularly strong during a limited period in development, this period is referred to as a sensitive period. Such periods allow experience to instruct neural circuits to process or represent information in a way that is adaptive for the individual. When experience provides information that is essential for normal development and alters performance permanently, such sensitive periods are referred to as critical periods. Although sensitive periods are reflected in behavior, they are actually a property of neural circuits. Mechanisms of plasticity at the circuit level are discussed that have been shown to operate during sensitive periods. A hypothesis is proposed that experience during a sensitive period modifies the architecture of a circuit in fundamental ways, causing certain patterns of connectivity to become highly stable and, therefore, energetically preferred. Plasticity that occurs beyond the end of a sensitive period, which is substantial in many circuits, alters connectivity patterns within the architectural constraints established during the sensitive period. Preferences in a circuit that result from experience during sensitive periods are illustrated graphically as changes in a ''stability landscape,'' a metaphor that represents the relative contributions of genetic and experiential influences in shaping the information processing capabilities of a neural circuit. By understanding sensitive periods at the circuit level, as well as understanding the relationship between circuit properties and behavior, we gain a deeper insight into the critical role that experience plays in shaping the development of the brain and behavior.

Animals↗

PubMed2003

Adaptive plasticity in the auditory thalamus of juvenile barn owls.

Little is known about the capacity of the thalamus for experience-dependent plasticity. Here, we demonstrate adaptive changes in the tuning of auditory thalamic neurons to a major category of sound localization cue, interaural time differences (ITDs), in juvenile barn owls that experience chronic abnormal hearing. Abnormal hearing was caused by a passive acoustic filtering device implanted in one ear that altered the timing and level of sound differently at different frequencies. Experience with this device resulted in adaptive, frequency-dependent shifts in the tuning of thalamic neurons to ITD that mimicked the acoustic effects of the device. Abnormal hearing did not alter ITD tuning in the central nucleus of the inferior colliculus, the primary source of input to the auditory thalamus. Therefore, the thalamus is the earliest stage in the forebrain pathway in which this plasticity is expressed. A visual manipulation, chronic prismatic displacement of the visual field, which causes adaptive changes in ITD tuning at higher levels in the forebrain, had no effect on thalamic ITD tuning. The results demonstrate that, during the juvenile period, auditory experience shapes neuronal response properties in the thalamus in a frequency-specific manner and suggest that this thalamic plasticity is driven by self-organizational forces and not by visual instruction.

Acoustic Stimulation↗

PubMed2002

Incremental training increases the plasticity of the auditory space map in adult barn owls.

The plasticity in the central nervous system that underlies learning is generally more restricted in adults than in young animals. In one well-studied example, the auditory localization pathway has been shown to be far more limited in its capacity to adjust to abnormal experience in adult than in juvenile barn owls. Plasticity in this pathway has been induced by exposing owls to prismatic spectacles that cause a large, horizontal shift of the visual field. With prisms, juveniles learn new associations between auditory cues, such as interaural time difference (ITD), and locations in visual space, and acquire new neurophysiological maps of ITD in the optic tectum, whereas adults do neither. Here we show that when the prismatic shift is experienced in small increments, maps of ITD in adults do change adaptively. Once established through incremental training, new ITD maps can be reacquired with a single large prismatic shift. Our results show that there is a substantially greater capacity for plasticity in adults than was previously recognized and highlight a principled strategy for tapping this capacity that could be applied in other areas of the adult central nervous system.

Acoustic Stimulation↗

PubMed2002

Gated visual input to the central auditory system.

The central auditory system translates sound localization cues into a map of space guided, in part, by visual experience. In barn owls, this process takes place in the external nucleus of the inferior colliculus (ICX). However, to date, no trace of visual activity has been observed in this auditory nucleus. Here we show that strong visual responses, which are appropriate to guide auditory plasticity, appear in the ICX when inhibition is blocked in the optic tectum. Thus, visual spatial information is gated into the auditory system by an inhibitory mechanism that operates at a higher level in the brain.

Acoustic Stimulation↗

PubMed2002

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.

Aging↗

PubMed2002

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.

Adaptation, Physiological↗