Injury- and use-related plasticity in the primary sensory cortex of adult mammals: possible relationship to perceptual learning

Dysfunction in the Interaction of Information Between and Within the Bilateral Primary Sensory Cortex

Background

Interhemispheric and intrahemispheric long-range synchronization and information communication are crucial features of functional integration between the bilateral hemispheres. Previous studies have demonstrated that disrupted functional connectivity (FC) exists in the bilateral hemispheres of patients with carpal tunnel syndrome (CTS), but they did not clearly clarify the phenomenon of central dysfunctional connectivity. This study aimed to further investigate the potential mechanism of the weakened connectivity of primary somatosensory cortex (S1) based on a precise template.

Methods

Patients with CTS (n = 53) and healthy control subjects (HCs) (n = 23) participated and underwent resting-state functional magnetic resonance imaging (rs-fMRI) scanning. We used FC to investigate the statistical dependency of the whole brain, effective connectivity (EC) to analyze time-dependent effects, and voxel-mirrored homotopic connectivity (VMHC) to examine the coordination of FC, all of which were adopted to explore the change in interhemispheric and intrahemispheric S1.

Results

Compared to the healthy controls, we significantly found a decreased strength of the two connectivities in the interhemispheric S1_hand, and the results of EC and VMHC were basically consistent with FC in the CTS. The EC revealed that the information output from the dominant hemisphere to the contralateral hemisphere was weakened.

Conclusion

This study found that maladjusted connections between and within the bilateral S1 revealed by these methods are present in patients with CTS. The dominant hemisphere with deafferentation weakens its effect on the contralateral hemisphere. The disturbance in the bilateral S1 provides reliable evidence to understand the neuropathophysiological mechanisms of decreased functional integration in the brains of patients with CTS.

Cortical pattern of reduced perfusion in hearing loss revealed by ASL-MRI

Age-related hearing loss (HL) can be related to brain dysfunction or structural damage and may result in cerebral metabolic/perfusion abnormalities. Arterial spin labeling (ASL) magnetic resonance imaging (MRI) allows investigating noninvasively brain perfusion changes. Pseudocontinuous ASL and T1-weighted MRI (at 3 T) and neuropsychological testing (Montreal Cognitive Assessment) were performed in 31 HL (age range = 47-77 years, mean age ± SD = 63.4 ± 8.4 years, pure-tone average [PTA] HL > 50 dB) and 28 normal hearing (NH; age range = 48-78 years, mean age ± SD = 59.7 ± 7.4 years) subjects. Cerebral blood flow (CBF) and gray matter volume (GMV) were analyzed in the cortical volume to assess perfusion and structural group differences. Two HL subjects showing cognitive impairment were excluded from group comparisons. No significant differences in either global or local atrophy were detected between groups but the HL group exhibited significant regional effects of reduced perfusion within the bilateral primary auditory cortex, with maximal CBF difference (-17.2%) in the right lateral Heschl's gyrus. For the whole sample of HL and NH subjects (n = 59 = 31 HL + 28 NH), the regional CBF was correlated positively to the regional GMV (p = 0.020). In HL subjects (n = 31), the regional CBF was correlated negatively to the audiogram steepness (frequency range: 2-4 kHz, right ear: p = 0.022, left ear: p = 0.015). The observed cortical pattern of perfusion reduction suggests that neuronal metabolism can be related to HL before the recognition of brain structural damage. This also illustrates the potential of ASL-MRI to contribute early functional markers of reduced central processing associated with HL.

Effects of Acoustic Environment on Tinnitus Behavior in Sound-Exposed Rats

Laboratory studies often rely on a damaging sound exposure to induce tinnitus in animal models. Because the time course and ultimate success of the induction process is not known in advance, it is not unusual to maintain sound-exposed animals for months while they are periodically assessed for behavioral indications of the disorder. To demonstrate the importance of acoustic environment during this period of behavioral screening, sound-exposed rats were tested for tinnitus while housed under quiet or constant noise conditions. More than half of the quiet-housed rats developed behavioral indications of the disorder. None of the noise-housed rats exhibited tinnitus behavior during 2 months of behavioral screening. It is widely assumed that the "phantom sound" of tinnitus reflects abnormal levels of spontaneous activity in the central auditory pathways that are triggered by cochlear injury. Our results suggest that sustained patterns of noise-driven activity may prevent the injury-induced changes in central auditory processing that lead to this hyperactive state. From the perspective of laboratory studies of tinnitus, housing sound-exposed animals in uncontrolled noise levels may significantly reduce the success of induction procedures. From a broader clinical perspective, an early intervention with sound therapy may reduce the risk of tinnitus in individuals who have experienced an acute cochlear injury.

Changes of metabolism and functional connectivity in late-onset deafness: Evidence from cerebral 18F-FDG-PET

Hearing loss is known to impact brain function. The aim of this study was to characterize cerebral metabolic Positron Emission Tomography (PET) changes in elderly patients fulfilling criteria for cochlear implant and investigate the impact of hearing loss on functional connectivity. Statistical Parametric Mapping-T-scores-maps comparisons of ¹⁸F-FDG-PET of 27 elderly patients fulfilling criteria for cochlear implant for hearing loss (best-aided speech intelligibility lower or equal to 50%) and 27 matched healthy subjects (p < 0.005, corrected for volume extent) were performed. Metabolic connectivity was evaluated through interregional correlation analysis. Patients were found to have decreased metabolism within the right associative auditory cortex, while increased metabolism was found in prefrontal areas, pre- and post-central areas, the cingulum and the left inferior parietal gyrus. The right associative auditory cortex was integrated into a network of increased metabolic connectivity that included pre- and post-central areas, the cingulum, the right inferior parietal gyrus, as well as the striatum on both sides. Metabolic values of the right associative auditory cortex and left inferior parietal gyrus were positively correlated with performance on neuropsychological test scores. These findings provide further insight into the reorganization of the connectome through sensory loss and compensatory mechanisms in elderly patients with severe hearing loss.

Emergence of virtual reality as a tool for upper limb rehabilitation: incorporation of motor control and motor learning principles

The primary focus of rehabilitation for individuals with loss of upper limb movement as a result of acquired brain injury is the relearning of specific motor skills and daily tasks. This relearning is essential because the loss of upper limb movement often results in a reduced quality of life. Although rehabilitation strives to take advantage of neuroplastic processes during recovery, results of traditional approaches to upper limb rehabilitation have not entirely met this goal. In contrast, enriched training tasks, simulated with a wide range of low- to high-end virtual reality-based simulations, can be used to provide meaningful, repetitive practice together with salient feedback, thereby maximizing neuroplastic processes via motor learning and motor recovery. Such enriched virtual environments have the potential to optimize motor learning by manipulating practice conditions that explicitly engage motivational, cognitive, motor control, and sensory feedback-based learning mechanisms. The objectives of this article are to review motor control and motor learning principles, to discuss how they can be exploited by virtual reality training environments, and to provide evidence concerning current applications for upper limb motor recovery. The limitations of the current technologies with respect to their effectiveness and transfer of learning to daily life tasks also are discussed.

Magnetic resonance spectroscopy features of Heschl's gyri in patients with unilateral acoustic neuroma: preliminary study

RATIONALE AND OBJECTIVES

To evaluate neurochemical alterations in Heschl's gyri and determine the most affected side in case of unilateral acoustic neuroma using magnetic resonance spectroscopy (MRS).

MATERIALS AND METHODS

Fifteen patients with unilateral acoustic neuroma were studied. Following routine cranial MRI sequences, MRS of Heschl's gyri on tumor and nontumor sides was obtained. MRS metabolite values of both Heschl's gyri were statistically compared.

RESULTS

The values of N-acetylaspartate (NAA) and Cr on nontumor side Heschl's gyrus (HG) were significantly lower than that on tumor side.

CONCLUSIONS

We found nontumor side HG more affected with lower NAA and Cr values, suggesting neuronal damage and decreased energy metabolism compared to the tumoral side.

Auditory frequency and intensity discrimination explained using a cortical population rate code

The nature of the neural codes for pitch and loudness, two basic auditory attributes, has been a key question in neuroscience for over century. A currently widespread view is that sound intensity (subjectively, loudness) is encoded in spike rates, whereas sound frequency (subjectively, pitch) is encoded in precise spike timing. Here, using information-theoretic analyses, we show that the spike rates of a population of virtual neural units with frequency-tuning and spike-count correlation characteristics similar to those measured in the primary auditory cortex of primates, contain sufficient statistical information to account for the smallest frequency-discrimination thresholds measured in human listeners. The same population, and the same spike-rate code, can also account for the intensity-discrimination thresholds of humans. These results demonstrate the viability of a unified rate-based cortical population code for both sound frequency (pitch) and sound intensity (loudness), and thus suggest a resolution to a long-standing puzzle in auditory neuroscience.

AMPA and GABA(A/B) receptor subunit expression in the cortex of adult squirrel monkeys during peripheral nerve regeneration

The primate somatosensory neuroaxis provides a highly translational model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for AMPA and GABAA/B receptor subunits in the area 3b cortex of adult squirrel monkeys one and five months after median nerve compression. This method of nerve injury was selected because it allows unique insight into how receptor expression changes during the regeneration of the peripheral nerve. One month after nerve compression, the pattern of subunit staining provides evidence that the cortex enters a state of reorganization. GABA α1 receptor subunits are significantly down-regulated in layer IV, V, and VI. Glur2/3 AMPA receptor subunits and postsynaptic GABABR1b receptor subunits are up and down regulated respectively across all layers of cortex. After five months of recovery from nerve compression, the pattern of AMPA and GABAA/B receptor subunits remain significantly altered in a layer specific manner. In layer II/III, GluR1, GluR2/3, and GABA α1 subunit expression is significantly up-regulated while post synaptic GABABR1b receptor subunits are significantly down regulated. In layer VI, V, and VI the GluR2/3 and presynaptic GABABR1a receptor subunits are significantly up-regulated, while the postsynaptic GABABR1b receptor subunits remain significantly down-regulated. Taken together, these results suggest that following nerve injury the cortex enters a state of reorganization that has persistent effects on cortical plasticity even after partial or total reinnervation of the peripheral nerve.

Speech discrimination after early exposure to pulsed-noise or speech

Early experience of structured inputs and complex sound features generate lasting changes in tonotopy and receptive field properties of primary auditory cortex (A1). In this study we tested whether these changes are severe enough to alter neural representations and behavioral discrimination of speech. We exposed two groups of rat pups during the critical period of auditory development to pulsed-noise or speech. Both groups of rats were trained to discriminate speech sounds when they were young adults, and anesthetized neural responses were recorded from A1. The representation of speech in A1 and behavioral discrimination of speech remained robust to altered spectral and temporal characteristics of A1 neurons after pulsed-noise exposure. Exposure to passive speech during early development provided no added advantage in speech sound processing. Speech training increased A1 neuronal firing rate for speech stimuli in naïve rats, but did not increase responses in rats that experienced early exposure to pulsed-noise or speech. Our results suggest that speech sound processing is resistant to changes in simple neural response properties caused by manipulating early acoustic environment.

Toward an epigenetic view of our musical mind

We are transient beings, in a world of constantly changing culture. At home in the fields of Art and Science, seemingly capable of magnificent abstractions, humans have an intense need to externalize their insights. Music is an art and a highly transmissible cultural product, but we still have an incomplete understanding of how our musical experience shapes and is vividly retained within our brain, and how it affects our behavior. However, the developing field of social epigenetics is now helping us to describe how communication and emotion, prime hallmarks of music, can be linked to a transmissible, biochemical change.

Nerve injury-induced changes in GABA(A) and GABA(B) sub-unit expression in area 3b and cuneate nucleus of adult squirrel monkeys: further evidence of developmental recapitulation

The primate somatosensory system provides an excellent model system with which to investigate adult neural plasticity. Here, we report immunohistochemical staining data for the GABA(A) α1, GABA(B)R1a, and GABA(B)R1b receptor subunits in somatosensory area 3b, and cuneate nucleus one week after median nerve compression in adult squirrel monkeys. We find a significant decrease in GABA(A) α1 subunit staining across all cortical layers and within both soma and neuropil of the deprived cortical and brainstem regions. The GABA(B) staining showed an opposing shift in deprived regions, with a significant increase in presynaptic GABA(B)R1a staining, and a significant decrease in postsynaptic GABA(B)R1b staining in deprived regions of the cortex and brainstem. These changes in receptor subunit expression generate patterns that are very similar to those reported in the neonate. Furthermore, the similarities between brainstem and cortical expression suggest conserved forms of adult plasticity in these two regions. Taken together these results, along with the results from our previous paper investigating AMPA subunit expression in these same animals, support the hypothesis that deprived neurons enter a previously hidden state of developmental recapitulation that serves to prime the brain for NMDA receptor mediated receptive field reorganization.

Linking topography to tonotopy in the mouse auditory thalamocortical circuit

The mouse sensory neocortex is reported to lack several hallmark features of topographic organization such as ocular dominance and orientation columns in primary visual cortex or fine-scale tonotopy in primary auditory cortex (AI). Here, we re-examined the question of auditory functional topography by aligning ultra-dense receptive field maps from the auditory cortex and thalamus of the mouse in vivo with the neural circuitry contained in the auditory thalamocortical slice in vitro. We observed precisely organized tonotopic maps of best frequency (BF) in the middle layers of AI and the anterior auditory field as well as in the ventral and medial divisions of the medial geniculate body (MGBv and MGBm, respectively). Tracer injections into distinct zones of the BF map in AI retrogradely labeled topographically organized MGBv projections and weaker, mixed projections from MGBm. Stimulating MGBv along the tonotopic axis in the slice produced an orderly shift of voltage-sensitive dye (VSD) signals along the AI tonotopic axis, demonstrating topography in the mouse thalamocortical circuit that is preserved in the slice. However, compared with BF maps of neuronal spiking activity, the topographic order of subthreshold VSD maps was reduced in layer IV and even further degraded in layer II/III. Therefore, the precision of AI topography varies according to the source and layer of the mapping signal. Our findings further bridge the gap between in vivo and in vitro approaches for the detailed cellular study of auditory thalamocortical circuit organization and plasticity in the genetically tractable mouse model.

Cortical map plasticity improves learning but is not necessary for improved performance

Cortical map plasticity is believed to be a key substrate of perceptual and skill learning. In the current study, we quantified changes in perceptual ability after pairing tones with stimulation of the cholinergic nucleus basalis to induce auditory cortex map plasticity outside of a behavioral context. Our results provide evidence that cortical map plasticity can enhance perceptual learning. However, auditory cortex map plasticity fades over weeks even though tone discrimination performance remains stable. This observation is consistent with recent reports that cortical map expansions associated with perceptual and motor learning are followed by a period of map renormalization without a decrement in performance. Our results indicate that cortical map plasticity enhances perceptual learning, but is not necessary to maintain improved discriminative ability.

Relationship between age of hearing-loss onset, hearing-loss duration, and speech recognition in individuals with severe-to-profound high-frequency hearing loss

The factors responsible for interindividual differences in speech-understanding ability among hearing-impaired listeners are not well understood. Although audibility has been found to account for some of this variability, other factors may play a role. This study sought to examine whether part of the large interindividual variability of speech-recognition performance in individuals with severe-to-profound high-frequency hearing loss could be accounted for by differences in hearing-loss onset type (early, progressive, or sudden), age at hearing-loss onset, or hearing-loss duration. Other potential factors including age, hearing thresholds, speech-presentation levels, and speech audibility were controlled. Percent-correct (PC) scores for syllables in dissyllabic words, which were either unprocessed or lowpass filtered at cutoff frequencies ranging from 250 to 2,000 Hz, were measured in 20 subjects (40 ears) with severe-to-profound hearing losses above 1 kHz. For comparison purposes, 20 normal-hearing subjects (20 ears) were also tested using the same filtering conditions and a range of speech levels (10-80 dB SPL). Significantly higher asymptotic PCs were observed in the early (<=4 years) hearing-loss onset group than in both the progressive- and sudden-onset groups, even though the three groups did not differ significantly with respect to age, hearing thresholds, or speech audibility. In addition, significant negative correlations between PC and hearing-loss onset age, and positive correlations between PC and hearing-loss duration were observed. These variables accounted for a greater proportion of the variance in speech-intelligibility scores than, and were not significantly correlated with, speech audibility, as quantified using a variant of the articulation index. Although the lack of statistical independence between hearing-loss onset type, hearing-loss onset age, hearing-loss duration, and age complicate and limit the interpretation of the results, these findings indicate that other variables than audibility can influence speech intelligibility in listeners with severe-to-profound high-frequency hearing loss.

Selective increase in representations of sounds repeated at an ethological rate

Exposure to sounds during early development causes enlarged cortical representations of those sounds, leading to the commonly held view that the size of stimulus representations increases with stimulus exposure. However, representing stimuli based solely on their prevalence may be inefficient, because many frequent environmental sounds are behaviorally irrelevant. Here, we show that cortical plasticity depends not only on exposure time but also on the temporal modulation rate of the stimulus set. We examined cortical plasticity induced by early exposure to 7 kHz tone pips repeated at a slow (2 Hz), fast (15 Hz), or ethological (6 Hz) rate. Certain rat calls are modulated near 6 Hz. We found that spectral representation of 7 kHz increased only in the ethological-rate-reared animals, whereas improved entrainment of cortical neurons was seen in animals reared in the slow- and fast-rate condition. This temporal rate dependence of spectral plasticity may serve as a filtering mechanism to selectively enlarge representations of species-specific vocalizations. Furthermore, our results indicate that spectral and temporal plasticity can be separately engaged depending on the statistical properties of the input stimuli.

Human evoked cortical activity to signal-to-noise ratio and absolute signal level

The purpose of this study was to determine the effect of signal level and signal-to-noise ratio (SNR) on the latency and amplitude of evoked cortical activity to further our understanding of how the human central auditory system encodes signals in noise. Cortical auditory evoked potentials (CAEPs) were recorded from 15 young normal-hearing adults in response to a 1000 Hz tone presented at two tone levels in quiet and while continuous background noise levels were varied in five equivalent SNR steps. These 12 conditions were used to determine the effects of signal level and SNR level on CAEP components P1, N1, P2, and N2. Based on prior signal-in-noise experiments conducted in animals, we hypothesized that SNR, would be a key contributor to human CAEP characteristics. As hypothesized, amplitude increased and latency decreased with increasing SNR; in addition, there was no main effect of tone level across the two signal levels tested (60 and 75 dB SPL). Morphology of the P1-N1-P2 complex was driven primarily by SNR, highlighting the importance of noise when recording CAEPs. Results are discussed in terms of the current interest in recording CAEPs in hearing aid users.

Role of corticofugal feedback in hearing

The auditory system consists of the ascending and descending (corticofugal) systems. The corticofugal system forms multiple feedback loops. Repetitive acoustic or auditory cortical electric stimulation activates the cortical neural net and the corticofugal system and evokes cortical plastic changes as well as subcortical plastic changes. These changes are short-term and are specific to the properties of the acoustic stimulus or electrically stimulated cortical neurons. These plastic changes are modulated by the neuromodulatory system. When the acoustic stimulus becomes behaviorally relevant to the animal through auditory fear conditioning or when the cortical electric stimulation is paired with an electric stimulation of the cholinergic basal forebrain, the cortical plastic changes become larger and long-term, whereas the subcortical changes stay short-term, although they also become larger. Acetylcholine plays an essential role in augmenting the plastic changes and in producing long-term cortical changes. The corticofugal system has multiple functions. One of the most important functions is the improvement and adjustment (reorganization) of subcortical auditory signal processing for cortical signal processing.

Serotonergic modulation of plasticity of the auditory cortex elicited by fear conditioning

In the awake big brown bat, 30 min auditory fear conditioning elicits conditioned heart rate decrease and long-term best frequency (BF) shifts of cortical auditory neurons toward the frequency of the conditioned tone; 15 min conditioning elicits subthreshold cortical BF shifts that can be augmented by acetylcholine. The fear conditioning causes stress and an increase in the cortical serotonin (5-HT) level. Serotonergic neurons in the raphe nuclei associated with stress and fear project to the cerebral cortex and cholinergic basal forebrain. Recently, it has been shown that 5-HT(2A) receptors are mostly expressed on pyramidal neurons and their activation improves learning and memory. We applied 5-HT, an agonist (alpha-methyl-5-HT), or an antagonist (ritanserin) of 5-HT(2A) receptors to the primary auditory cortex and discovered the following drug effects: (1) 5-HT had no effect on the conditioned heart rate change, although it reduced the auditory responses; (2) 4 mm 5-HT augmented the subthreshold BF shifts, whereas 20 mm 5-HT did not; (3) 20 mm 5-HT reduced the long-term BF shifts and changed them into short-term; (4) alpha-methyl-5-HT increased the auditory responses and augmented the subthreshold BF shifts as well as the long-term BF shifts; (5) in contrast, ritanserin reduced the auditory responses and reversed the direction of the BF shifts. Our data indicate that the BF shift can be modulated by serotonergic neurons that augment or reduce the BF shift or even reverse the direction of the BF shift. Therefore, not only the cholinergic system, but also the serotonergic system, plays an important role in cortical plasticity according to behavioral demands.

Neuronavigierte repetitive transkranielle Magnetstimulation (rTMS)

BACKGROUND AND OBJECTIVE

Idiopathic tinnitus is a frequent and debilitating disorder of largely unknown pathophysiology. Focal brain activation in the auditory cortex has recently been demonstrated in chronic tinnitus. Low-frequency rTMS can reduce cortical hyperexcitability.

PATIENTS AND METHODS

In 12 patients with chronic tinnitus, fusion of [18F]deoxyglucose-PET and structural MRI (T1, MPRAGE) scans allowed the area of increased metabolic activity in the auditory cortex to be exactly identified; this area was selected as the target for rTMS. A neuronavigational system adapted for TMS positioning enabled the relative positions of the figure-8 coil and the target area to be monitored. Repetitive TMS (110% motor threshold; 1 Hz; 2000 stimuli per day over 5 days) was performed using a placebo-controlled crossover design. A sham coil system was used for the placebo stimulation. Treatment outcome was assessed with a specific tinnitus questionnaire (Goebel and Hiller).

RESULTS

In all 12 patients an asymmetrically increased metabolic activation of the gyrus of Heschl was detected. The tinnitus score was significantly improved after 5 days of active rTMS, an effect not seen after placebo stimulation.

CONCLUSION

These preliminary results show that neuronavigated rTMS may improve our understanding and treatment of chronic tinnitus.

Mechanisms underlying reorganization of fractured tactile cerebellar maps after deafferentation in developing and adult rats

Our previous studies showed that fractured tactile cerebellar maps in rats reorganize after deafferentation during development and in adulthood while maintaining a fractured somatotopy. Several months after deafferentation of the infraorbital branch of the trigeminal nerve, the missing upper lip innervation is replaced in the tactile maps in the granule cell layer of crus IIa. The predominant input into the denervated area is always the upper incisor representation. This study examined whether this reorganization was caused by mechanisms intrinsic to the cerebellum or extrinsic, i.e., occurring in somatosensory structures afferent to the cerebellum. We first compared normal and deafferented maps and found that the expansion of the upper incisor is not caused by a preexisting bias in the strength or abundance of upper incisor input in normal animals. We then mapped tactile representations before and immediately after denervation. We found that the pattern of reorganization observed in the cerebellum several months later is not caused by unmasking of a silent or weaker upper incisor representation. Both results indicate that the reorganization is not a result of subsequent growth or sprouting mechanism within the cerebellum itself. Finally, we compared postlesion maps in the cerebellum and the somatosensory cortex. We found that the upper incisor representation significantly expands in both regions and that this expansion is correlated, suggesting that reorganization in the cerebellum is a passive consequence of reorganization in afferent cerebellar pathways. This result has important developmental and functional implications.

Canadian Association of Neuroscience Review: development and plasticity of the auditory cortex

The functions of the cerebral cortex are predominantly established during the critical period of development. One obvious developmental feature is its division into different functional areas that systematically represent different environmental information. This is the result of interactions between intrinsic (genetic) factors and extrinsic (environmental) factors. Following this critical period, the cerebral cortex attains its adult form but it will continue to adapt to environmental changes. Thus, the cerebral cortex is constantly adapting to the environment (plasticity) from its embryonic stages to the last minute of life. This review details important factors that contribute to the development and plasticity of the auditory cortex. The instructive role of thalamocortical innervation, the regulatory role of cholinergic projection of the basal forebrain and the potential role of the corticofugal modulation are presented.

Suppression of cortical representation through backward conditioning

Temporal stimulus reinforcement sequences have been shown to determine the directions of synaptic plasticity and behavioral learning. Here, we examined whether they also control the direction of cortical reorganization. Pairing ventral tegmental area stimulation with a sound in a backward conditioning paradigm specifically reduced representations of the paired sound in the primary auditory cortex (AI). This temporal sequence-dependent bidirectional cortical plasticity modulated by dopamine release hypothetically serves to prevent the over-representation of frequently occurring stimuli resulting from their random pairing with unrelated rewards.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Gamma-aminobutyric acid circuits shape response properties of auditory cortex neurons

Neurons containing gamma aminobutyric acid (GABA) are widely distributed throughout the primary auditory cortex (AI). We investigated the effects of endogenous GABA by comparing response properties of 110 neurons in chinchilla AI before and after iontophoresis of bicuculline, a GABA(A) receptor antagonist, and/or CGP35348, a GABA(B) receptor antagonist. GABA(A) receptor blockade significantly increased spontaneous and driven discharge rates, dramatically decreased the thresholds of many neurons, and constricted the range of thresholds across the neural population. Some neurons with 'non-onset' temporal discharge patterns developed an onset pattern that was followed by a long pause. Interestingly, the excitatory response area typically expanded on both sides of the characteristic frequency; this expansion exceeded one octave in a third of the sample. Although GABA(B) receptor blockade had little effect alone, the combination of CGP35348 and bicuculline produced greater increases in driven rate and expansion of the frequency response area than GABA(A) receptor blockade alone, suggesting a modulatory role of local GABA(B) receptors. The results suggest that local GABA inhibition contributes significantly to intensity and frequency coding by controlling the range of intensities over which cortical neurons operate and the range of frequencies to which they respond. The inhibitory circuits that generate nonmonotonic rate-level functions are separate from those that influence other response properties of AI neurons.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Reorganization in awake rat auditory cortex by local microstimulation and its effect on frequency-discrimination behavior

In common with other sensory cortices, the mammalian primary auditory cortex (AI) demonstrates the capacity for large-scale reorganization following many experimental situations. For example, training animals in frequency-discrimination tasks has been shown to result in an increase in cortical frequency representation. Such central changes-most commonly, an increase in central representation of specific stimulus parameters-have been hypothesized to underlie the improvements in perceptual acuity (perceptual learning) seen in many learning situations. The actual behavioral relevance of central reorganizations, however, remains speculative. Here, we directly examine this issue. We first show that stimulating the AI cortex of the awake rat with a weak electric current (intracortical microstimulation or ICMS) has the effect of inducing central reorganizations similar to those accompanying the traditional plasticity experiments (a result previously noted only in anesthetized preparations). Depending on the site of AI stimulation, ICMS enlarged the cortical representation of certain frequencies. Next we examined the direct perceptual consequences of ICMS-induced AI reorganization for the rat's ability to discriminate frequencies. Over the course of the experiment, we also detailed, and made comparisons between, the frequency-response characteristics of rat AI cortex in the awake and ketamine-anesthetized animal. AI cells that responded to pure tones were divided into two categories--strongly and weakly responsive--based on the strength of their evoked discharge. Individual cells maintained their respective response strengths in both awake and anesthetized conditions. Strongly responsive cells showed at least four different temporal responses and tended to be narrowly tuned. Their responses were stable over the long term. In general frequency-response characteristics were qualitatively similar in the anesthetized and awake animal; bandwidths tended to be broader in awake animals. Although both strong and weak cell populations respond to tones, only the strongly responsive cells fit into a tonotopically organized scheme. By contrast, weakly responsive cells did not exhibit a frequency mapping and may represent a more diffuse input to AI than that underlying strongly responsive cells. In general, the overall frequency organization of AI was found to be equally well expressed in both the awake and anesthetized rat. ICMS reorganization of AI did not alter frequency-discrimination behavior in the rat--either signal detectability or response bias--suggesting that an increase in central representation, by itself, is insufficient to account for perceptual learning. It is likely that cortical reorganizations that accompany perceptual learning are strongly keyed to specific behavioral contexts.

Symptoms and signs caused by neural plasticity

Plastic changes in the central nervous system are associated with hyperactivity, hypersensitivity, and spread of activity including activation of brain regions that are not typically involved. Symptoms and signs such as neuropathic pain and tinnitus and hyperactive disorders such as muscle spasm and synkinesis may result from such changes in function. Plastic changes that cause symptoms of diseases can be initiated by novel stimulations, overstimulation, or deprivation of input and the induced changes in the function of central nervous system structures may persist and aggravate after these events have ceased if the condition is not reversed. Disorders that are caused by neural plasticity are potentially reversible with treatment. However, the absence of morphologic abnormalities makes diagnosis of these conditions difficult and their treatment has been hampered by lack of understanding of their pathophysiology. Here the role of neural plasticity in the pathophysiology of several disorders is reviewed.

Effects of acetylcholine and atropine on plasticity of central auditory neurons caused by conditioning in bats

In the big brown bat (Eptesicus fuscus), conditioning with acoustic stimuli followed by electric leg-stimulation causes shifts in frequency-tuning curves and best frequencies (hereafter BF shifts) of collicular and cortical neurons, i.e., reorganization of the cochleotopic (frequency) maps in the inferior colliculus (IC) and auditory cortex (AC). The collicular BF shift recovers 180 min after the conditioning, but the cortical BF shift lasts longer than 26 h. The collicular BF shift is not caused by conditioning, as the AC is inactivated during conditioning. Therefore it has been concluded that the collicular BF shift is caused by the corticofugal auditory system. The collicular and cortical BF shifts both are not caused by conditioning as the somatosensory cortex is inactivated during conditioning. Therefore it has been hypothesized that the cortical BF shift is mostly caused by both the subcortical (e.g., collicular) BF shift and the activity of nonauditory systems such as the somatosensory cortex excited by an unconditioned leg-stimulation and the cholinergic basal forebrain. The main aims of our present studies are to examine whether acetylcholine (ACh) applied to the AC augments the collicular and cortical BF shifts caused by the conditioning and whether atropine applied to the AC abolishes the cortical BF shift but not the collicular BF shift, as expected from the preceding hypothesis. In the awake bat, we made the following findings. ACh applied to the AC augments not only the cortical BF shift but also the collicular BF shift through the corticofugal system. Atropine applied to the AC reduces the collicular BF shift and abolishes the cortical BF shift which otherwise would be caused. ACh applied to the IC significantly augments the collicular BF shift but affects the cortical BF shift only slightly. ACh makes the cortical BF shift long-lasting beyond 4 h, but it cannot make the collicular BF shift long-lasting beyond 3 h. Atropine applied to the IC abolishes the collicular BF shift. It reduces the cortical BF shift but does not abolish it. Our findings favor the hypothesis that the BF shifts evoked by the corticofugal system, and an increased ACh level in the AC evoked by the basal forebrain are both necessary to evoke a long-lasting cortical BF shift.

Variation in shoulder position sense at mid and extreme range of motion

OBJECTIVE

To determine the effect of different joint positions on position sense of asymptomatic shoulders.

DESIGN

Repeated-measures design.

SETTING

Laboratory in an educational institution.

PARTICIPANTS

Thirty-four asymptomatic, right-handed men.

INTERVENTIONS

The ability of subjects to replicate 3 criterion positions was examined on subjects' right shoulders by using an isokinetic dynamometer. Three criterion positions were the 50th, 75th, and 90th percentiles of each individual's total passive shoulder rotation range measured from the full internal rotation position.

MAIN OUTCOME MEASURE

Repositioning accuracy, indicating the difference in degrees between the criterion and matching positions.

RESULTS

All subjects were able to reproduce the criterion position near the end of external rotation range more accurately and consistently than in the middle range of motion (ROM).

CONCLUSIONS

Position sense acuity at the shoulder complex varied across the ROM and may be enhanced near the end of rotation range where there is more tension on the restraints to movement. Therefore, an individual's ROM should be factored into any attempt to assess or rehabilitate shoulder proprioception.

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.

Plasticity of bat's central auditory system evoked by focal electric stimulation of auditory and/or somatosensory cortices

Recent findings indicate that the corticofugal system would play an important role in cortical plasticity as well as collicular plasticity. To understand the role of the corticofugal system in plasticity, therefore, we studied the amount and the time course of plasticity in the inferior colliculus (IC) and auditory cortex (AC) evoked by focal electrical stimulation of the AC and also the effect of electrical stimulation of the somatosensory cortex on the plasticity evoked by the stimulation of the AC. In adult big brown bats (Eptesicus fuscus), we made the following major findings. 1) Electric stimulation of the AC evokes best frequency (BF) shifts, i.e., shifts in frequency-response curves of collicular and cortical neurons. These BF shifts start to occur within 2 min, reach a maximum (or plateau) at 30 min, and then recover approximately 180 min after a 30-min-long stimulus session. When the stimulus session is lengthened from 30 to 90 min, the plateau lasts approximately 60 min, but BF shifts recover approximately 180 min after the session. 2) The electric stimulation of the somatosensory cortex delivered immediately after that of the AC, as in fear conditioning, evokes a dramatic lengthening of the recovery period of the cortical BF shifts but not that of the collicular BF shift. The electric stimulation of the somatosensory cortex delivered before that of the AC, as in backward conditioning, has no effect on the collicular and cortical BF shifts. 3) Electric stimulation of the AC evokes BF shifts not only in the ipsilateral IC and AC but also in the contralateral IC and AC. BF shifts are smaller in amount and shorter in recovery time for contralateral collicular and cortical neurons than for ipsilateral ones. Our findings support the hypothesis that the AC and the corticofugal system have an intrinsic mechanism for reorganization of the IC and AC, that the reorganization is highly specific to a value of an acoustic parameter (frequency), and that the reorganization is augmented by excitation of nonauditory sensory cortex that makes the acoustic stimulus behaviorally relevant to the animal through associative learning.

The corticofugal system for hearing: recent progress

Peripheral auditory neurons are tuned to single frequencies of sound. In the central auditory system, excitatory (or facilitatory) and inhibitory neural interactions take place at multiple levels and produce neurons with sharp level-tolerant frequency-tuning curves, neurons tuned to parameters other than frequency, cochleotopic (frequency) maps, which are different from the peripheral cochleotopic map, and computational maps. The mechanisms to create the response properties of these neurons have been considered to be solely caused by divergent and convergent projections of neurons in the ascending auditory system. The recent research on the corticofugal (descending) auditory system, however, indicates that the corticofugal system adjusts and improves auditory signal processing by modulating neural responses and maps. The corticofugal function consists of at least the following subfunctions. (i) Egocentric selection for short-term modulation of auditory signal processing according to auditory experience. Egocentric selection, based on focused positive feedback associated with widespread lateral inhibition, is mediated by the cortical neural net working together with the corticofugal system. (ii) Reorganization for long-term modulation of the processing of behaviorally relevant auditory signals. Reorganization is based on egocentric selection working together with nonauditory systems. (iii) Gain control based on overall excitatory, facilitatory, or inhibitory corticofugal modulation. Egocentric selection can be viewed as selective gain control. (iv) Shaping (or even creation) of response properties of neurons. Filter properties of neurons in the frequency, amplitude, time, and spatial domains can be sharpened by the corticofugal system. Sharpening of tuning is one of the functions of egocentric selection.

Experience-dependent plasticity in the auditory cortex and the inferior colliculus of bats: role of the corticofugal system

In the big brown bat, Eptesicus fuscus, the response properties of neurons and the cochleotopic (frequency) maps in the auditory cortex (AC) and inferior colliculus can be changed by auditory conditioning, weak focal electric stimulation of the AC, or repetitive delivery of weak, short tone bursts. The corticofugal system plays an important role in information processing and plasticity in the auditory system. Our present findings are as follows. In the AC, best frequency (BF) shifts, i.e., reorganization of a frequency map, slowly develop and reach a plateau approximately 180 min after conditioning with tone bursts and electric-leg stimulation. The plateau lasts more than 26 h. In the inferior colliculus, on the other hand, BF shifts rapidly develop and become the largest at the end of a 30-min-long conditioning session. The shifted BFs return (i. e., recover) to normal in approximately 180 min. The collicular BF shifts are not a consequence of the cortical BF shifts. Instead, they lead the cortical BF shifts. The collicular BF shifts evoked by conditioning are very similar to the collicular and cortical BF shifts evoked by cortical electrical stimulation. Therefore, our working hypothesis is that, during conditioning, the corticofugal system evokes subcortical BF shifts, which in turn boost cortical BF shifts. The cortical BF shifts otherwise would be very small. However, whether the cortical BF shifts are consequently boosted depends on nonauditory systems, including nonauditory sensory cortices, amygdala, basal forebrain, etc., which determine the behavioral relevance of acoustic stimuli.

Conceptualizing functional neuroplasticity

There are at least four major forms of functional neuroplasticity that can be studied in humans: homologous area adaptation, cross-modal reassignment, map expansion, and compensatory masquerade. Homologous area adaptation is the assumption of a particular cognitive process by a homologous region in the opposite hemisphere. Cross-modal reassignment occurs when structures previously devoted to processing a particular kind of sensory input now accepts input from a new sensory method. Map expansion is the enlargement of a functional brain region on the basis of performance. Compensatory masquerade is a novel allocation of a particular cognitive process to perform a task. By focusing on these four forms of functional neuroplasticity, several fundamental questions about how functional cooperation between brain regions is achieved can be addressed.

Modulation of responses and frequency tuning of thalamic and collicular neurons by cortical activation in mustached bats

In the Jamaican mustached bat, Pteronotus parnellii parnellii, one of the subdivisions of the primary auditory cortex is disproportionately large and over-represents sound at approximately 61 kHz. This area, called the Doppler-shifted constant frequency (DSCF) processing area, consists of neurons extremely sharply tuned to a sound at approximately 61 kHz. We found that a focal activation of the DSCF area evokes highly specific corticofugal modulation in the inferior colliculus and medial geniculate body. Namely a focal activation of cortical DSCF neurons tuned to, say, 61. 2 kHz with 0.2-ms-long, 100-nA electric pulses drastically increases the excitatory responses of thalamic and collicular neurons tuned to 61.2 kHz without shifting their best frequencies (BFs). However, it decreases the excitatory responses of subcortical neurons tuned to frequencies slightly higher or lower than 61.2 kHz and shifts their BFs away from 61.2 kHz. The BF shifts are symmetrical and centrifugal around 61.2 kHz. These corticofugal effects are larger on thalamic neurons than on collicular neurons. The cortical electrical stimulation sharpens the frequency-tuning curves of subcortical neurons. These corticofugal effects named "egocentric selection" last </=2.5 h after the cessation of a 7-min-long cortical electrical stimulation. In the mustached bat, corticofugal modulation serves to increase the contrast in neural representation of sound at approximately 61 kHz, which is an important component of an echo bearing velocity information. It is also most likely that the corticofugal system plays an important role in plasticity of the central auditory system. Another subdivision of the auditory cortex of the mustached bat is called the FM-FM area. This area consists of delay-tuned combination-sensitive neurons, called FM-FM neurons, and has the echo-delay axis for the systematic representation of target distances. A focal electric stimulation of the FM-FM area evokes changes in the responses of collicular and thalamic FM-FM neurons. These changes are basically the same as those described in the present paper. Therefore corticofugal modulation takes place for frequency domain analysis in exactly the same way as it does in time domain analysis.

Tonotopic mapping in auditory cortex of the adult chinchilla with amikacin-induced cochlear lesions

We have found a reorganization of tonotopic maps (based on neuron response thresholds) in primary auditory cortex of the adult chinchilla after amikacin-induced basal cochlear lesions. We find an over-representation of a frequency that corresponds to the border area of the cochlear lesion. The reorganization observed is similar in extent to that previously seen in a developmental model. The properties of neurons within the over-represented area were investigated in order to determine whether their responses originated from a common input (an indication of true plasticity) or represented only the result of truncating the activity of the sensory epithelium ("pseudo-plasticity"). Some aspects of our data fit with a true plasticity model and indicate the potential for the deafferented cortex of the mature cortex to regain connections with the surviving sensory epithelium.

Reorganization of the frequency map of the auditory cortex evoked by cortical electrical stimulation in the big brown bat

In a search phase of echolocation, big brown bats, Eptesicus fuscus, emit biosonar pulses at a rate of 10/s and listen to echoes. When a short acoustic stimulus was repetitively delivered at this rate, the reorganization of the frequency map of the primary auditory cortex took place at and around the neurons tuned to the frequency of the acoustic stimulus. Such reorganization became larger when the acoustic stimulus was paired with electrical stimulation of the cortical neurons tuned to the frequency of the acoustic stimulus. This reorganization was mainly due to the decrease in the best frequencies of the neurons that had best frequencies slightly higher than those of the electrically stimulated cortical neurons or the frequency of the acoustic stimulus. Neurons with best frequencies slightly lower than those of the acoustically and/or electrically stimulated neurons slightly increased their best frequencies. These changes resulted in the over-representation of repetitively delivered acoustic stimulus. Because the over-representation resulted in under-representation of other frequencies, the changes increased the contrast of the neural representation of the acoustic stimulus. Best frequency shifts for over-representation were associated with sharpening of frequency-tuning curves of 25% of the neurons studied. Because of the increases in both the contrast of neural representation and the sharpness of tuning, the over-representation of the acoustic stimulus is accompanied with an improvement of analysis of the acoustic stimulus.

Perceptual consequences of peripheral hearing loss: do edge effects exist for abrupt cochlear lesions?

There is a growing body of research that shows evidence of central neural reorganization in response to lesions in the auditory periphery, even if the lesions occur in maturity. This reorganization consists of an increased neural representation of frequencies corresponding to the edge frequency of the lesion. Data were collected to determine whether this over-representation might have consequences for human perception. The hypothesis was that increased central representation might increase acuity on some psychophysical tasks performed at the edge frequency. Tasks included frequency sweep detection (for tones), intensity discrimination (for 100-Hz-wide bands of noise and tones), gap detection and gap discrimination (both for 100-Hz-wide bands of noise). Results from observers with steeply sloping hearing losses were compared with results from normal-hearing observers performing these tasks with masking noise generated to simulate steeply sloping hearing loss. None of these data provide compelling evidence for the hypothesized edge effect. A 40-Hz following response to tone bursts was collected from a subset of the hearing-impaired observers in an attempt to confirm the animal physiology findings of neural over-representation of the edge frequency. No edge-frequency effect was noted in the results, though there was a non-significant tendency for one of the hearing-impaired observers to show shorter latency of response.

Experience-dependent corticofugal adjustment of midbrain frequency map in bat auditory system

Recent studies of corticofugal modulation of auditory information processing indicate that cortical neurons mediate both a highly focused positive feedback to subcortical neurons "matched" in tuning to a particular acoustic parameter and a widespread lateral inhibition to "unmatched" subcortical neurons. This cortical function for the adjustment and improvement of subcortical information processing is called egocentric selection. Egocentric selection enhances the neural representation of frequently occurring signals in the central auditory system. For our present studies performed with the big brown bat (Eptesicus fuscus), we hypothesized that egocentric selection adjusts the frequency map of the inferior colliculus (IC) according to auditory experience based on associative learning. To test this hypothesis, we delivered acoustic stimuli paired with electric leg stimulation to the bat, because such paired stimuli allowed the animal to learn that the acoustic stimulus was behaviorally important and to make behavioral and neural adjustments based on the acquired importance of the acoustic stimulus. We found that acoustic stimulation alone evokes a change in the frequency map of the IC; that this change in the IC becomes greater when the acoustic stimulation is made behaviorally relevant by pairing it with electrical stimulation; that the collicular change is mediated by the corticofugal system; and that the IC itself can sustain the change evoked by the corticofugal system for some time. Our data support the hypothesis.

Loudness perception and frequency discrimination in subjects with steeply sloping hearing loss: possible correlates of neural plasticity

Loudness functions and frequency difference limens (DLFs) were measured in five subjects with steeply sloping high-frequency sensorineural hearing loss. The stimuli were pulsed pure tones encompassing a range of frequencies. Loudness data were obtained using a 2AFC matching procedure with a 500-Hz reference presented at a number of levels. DLFs were measured using a 3AFC procedure with intensities randomized within 6 dB around an equal-loudness level. Results showed significantly shallower loudness functions near the cutoff frequency of the loss than at a lower frequency, where hearing thresholds were near normal. DLFs were elevated, on average, relative to DLFs measured using the same procedure in five normally hearing subjects, but showed a local reduction near the cutoff frequency in most subjects with high-frequency loss. The loudness data are generally consistent with recent models that describe loudness perception in terms of peripheral excitation patterns that are presumably restricted by a steeply sloping hearing loss. However, the DLF data are interpreted with reference to animal experiments that have shown reorganization in the auditory cortex following the introduction of restricted cochlear lesions. Such reorganization results in an increase in the spatial representation of lesion-edge frequencies, and is comparable with the functional reorganization observed in animals following frequency-discrimination training. It is suggested that similar effects may occur in humans with steeply sloping high-frequency hearing loss, and therefore, the local reduction in DLFs in our data may reflect neural plasticity.

Corticofugal modulation of the midbrain frequency map in the bat auditory system

The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.

Injury-induced reorganization of frequency maps in adult auditory cortex: the role of unmasking of normally-inhibited inputs

Restricted cochlear lesions in adult animals, causing partial deafness, result in a reorganization of primary auditory cortex (AI) such that the region deprived of its normal input by the lesion is occupied by an expanded representation of peri-lesion cochlear regions, and hence of peri-lesion frequencies. One possible mechanism underlying the change in frequency responsiveness involved in such reorganization is that inputs to the cortical neurons at frequencies at and near their "new" post-lesion characteristic frequencies (CFs) are normally present but suppressed by inhibition, and are "unmasked" by the effects of the lesion. Evidence in support of this explanation is provided by two-tone forward-masking experiments which reveal that many AI neurons receive surround inhibitory input. When input to such neurons at their CF is reduced by an intense temporary-threshold-shift (TTS)-inducing stimulus, the response areas of some neurons expand into the region of their inhibitory surrounds, the effect that would be expected if unmasking were involved in cortical reorganization. In other neurons, however, response areas contracted after the TTS-inducing stimulation. Although unmasking of normally-inhibited inputs is likely to contribute to auditory cortical reorganization, the immediate unmasking that is seen in visual and somatosensory systems is unlikely to play a major role in auditory cortical reorganization, as no evidence of immediate unmasking was seen following acute cochlear lesions in guinea pigs.