The dentate nucleus is a short-latency relay of a primary auditory transmission pathway

Reported Hearing Loss in Alzheimer's Disease Is Associated With Loss of Brainstem and Cerebellar Volume

Multiple epidemiological studies have revealed an association between presbycusis and Alzheimer’s Disease (AD). Unfortunately, the neurobiological underpinnings of this relationship are not clear. It is possible that the two disorders share a common, as yet unidentified, risk factor, or that hearing loss may independently accelerate AD pathology. Here, we examined the relationship between reported hearing loss and brain volumes in normal, mild cognitive impairment (MCI) and AD subjects using a publicly available database. We found that among subjects with AD, individuals that reported hearing loss had smaller brainstem and cerebellar volumes in both hemispheres than individuals without hearing loss. In addition, we found that these brain volumes diminish in size more rapidly among normal subjects with reported hearing loss and that there was a significant interaction between cognitive diagnosis and the relationship between reported hearing loss and these brain volumes. These data suggest that hearing loss is linked to brainstem and cerebellar pathology, but only in the context of the pathological state of AD. We hypothesize that the presence of AD-related pathology in both the brainstem and cerebellum creates vulnerabilities in these brain regions to auditory deafferentation-related atrophy. These data have implications for our understanding of the potential neural substrates for interactions between hearing loss and AD.

A static sound source can improve postural stability during walking

BACKGROUND

During walking, postural stability is controlled by visual, vestibular and proprioceptive input. The auditory system uses acoustic input to localize sound sources. For some static balance conditions, the auditory influence on posture was already proven. Little is known about the impact of auditory inputs on balance in dynamic conditions.

OBJECTIVE

This study is aimed at investigating postural stability of walking tasks in silence and sound on condition to better understand the impact of auditory input on balance in movement.

METHODS

Thirty participants performed: walking (eyes open), tandem steps, walking with turning head and walking over barriers. During each task, acoustic condition changed between silence and presented noise through an earth-fixed loudspeaker located at the end of the walking distance. Body sway velocity was recorded close to the body's center of gravity.

RESULTS

A decreased body sway velocity was significant for walking (eyes open), tandem steps and walking over barriers when noise was presented. Those auditory stimuli did not affect sway velocity while walking with turning head. The posture has probably improved due to the localization ability when walking with the head facing forward, while the localization ability was impaired when turning the head.

CONCLUSIONS

The localization ability of a fixed sound source through the auditory system has a significant but limited impact on posture while walking.

Dissociation between Cerebellar and Cerebral Neural Activities in Humans with Long-Term Bilateral Sensorineural Hearing Loss

Abnormal neural activity in the cerebellum has been implicated in hearing impairments, but the effects of long-term hearing loss on cerebellar function are poorly understood. To further explore the role of long-term bilateral sensorineural hearing loss on cerebellar function, we investigated hearing loss-induced changes among neural networks within cerebellar subregions and the changes in cerebellar-cerebral connectivity patterns using resting-state functional MRI. Twenty-one subjects with long-term bilateral moderate-to-severe sensorineural hearing loss and 21 matched controls with clinically normal hearing underwent MRI scanning and a series of neuropsychological tests targeting cognition and emotion. Voxel-wise functional connectivity (FC) analysis demonstrated decreased couplings between the cerebellum and other cerebral areas, including the temporal pole (TP), insula, supramarginal gyrus, inferior frontal gyrus (IFG), medial frontal gyrus, and thalamus, in long-term bilateral sensorineural hearing loss patients. An ROI-wise FC analysis found weakened interregional connections within cerebellar subdivisions. Moreover, there was a negative correlation between anxiety and FC between the left cerebellar lobe VI and left insula. Hearing ability and anxiety scores were also correlated with FC between the left cerebellar lobe VI and left TP, as well as the right cerebellar lobule VI and left IFG. Our results suggest that sensorineural hearing loss disrupts cerebellar-cerebral circuits, some potentially linked to anxiety, and interregional cerebellar connectivity. The findings contribute to a growing body showing that auditory deprivation caused by cochlear hearing loss disrupts not only activity with the classical auditory pathway but also portions of the cerebellum that communicates with other cortical networks.

Paraflocculus plays a role in salicylate-induced tinnitus

Tinnitus impairs quality of life of about 1-2% of the whole population. In most severe situation, tinnitus may cause social isolation, depression and suicide. Drug treatments for tinnitus are generally ineffective, and the mechanisms of tinnitus are still undetermined. Accumulating evidence suggests that tinnitus is related to changes of widespread brain networks. Recent studies propose that paraflocculus (PFL), which is indirectly connected to various cortical regions, may be a gating zone of tinnitus. So we examined the electrophysiological changes and neurotransmitter alterations of the PFL in a rat model of sodium salicylate (SS)-induced tinnitus. We found that spontaneous firing rate (SFR) of the putative excitatory interneurons of the PFL was significantly increased. The level of glutamic acid, which is the main excitatory neurotransmitter in the nervous system, was also dramatically increased in the PFL after SS treatment. These results confirmed the hyperactivity of PFL in the rats with SS-treatment, which might be due to the increased glutamic acid. Then we examined the SFR of the auditory cortex (AC), the center for auditory perception, before and after electrical stimulation of the PFL. 71.4% (105/147) of the recorded neurons showed a response to the stimulation of the PFL. The result demonstrated that stimulation of the PFL could modulate the activity of the AC. Our study suggests a role of PFL in SS-induced tinnitus and AC as a potential target of PFL in the process of tinnitus.

The Role of Rhythm in Speech and Language Rehabilitation: The SEP Hypothesis

For thousands of years, human beings have engaged in rhythmic activities such as drumming, dancing, and singing. Rhythm can be a powerful medium to stimulate communication and social interactions, due to the strong sensorimotor coupling. For example, the mere presence of an underlying beat or pulse can result in spontaneous motor responses such as hand clapping, foot stepping, and rhythmic vocalizations. Examining the relationship between rhythm and speech is fundamental not only to our understanding of the origins of human communication but also in the treatment of neurological disorders. In this paper, we explore whether rhythm has therapeutic potential for promoting recovery from speech and language dysfunctions. Although clinical studies are limited to date, existing experimental evidence demonstrates rich rhythmic organization in both music and language, as well as overlapping brain networks that are crucial in the design of rehabilitation approaches. Here, we propose the “SEP” hypothesis, which postulates that (1) “sound envelope processing” and (2) “synchronization and entrainment to pulse” may help stimulate brain networks that underlie human communication. Ultimately, we hope that the SEP hypothesis will provide a useful framework for facilitating rhythm-based research in various patient populations.

The cerebellum generates motor-to-auditory predictions: ERP lesion evidence

Forward predictions are crucial in motor action (e.g., catching a ball, or being tickled) but may also apply to sensory or cognitive processes (e.g., listening to distorted speech or to a foreign accent). According to the "internal forward model," the cerebellum generates predictions about somatosensory consequences of movements. These predictions simulate motor processes and prepare respective cortical areas for anticipated sensory input. Currently, there is very little evidence that a cerebellar forward model also applies to other sensory domains. In the current study, we address this question by examining the role of the cerebellum when auditory stimuli are anticipated as a consequence of a motor act. We applied an N100 suppression paradigm and compared the ERP in response to self-initiated with the ERP response to externally produced sounds. We hypothesized that sensory consequences of self-initiated sounds are precisely predicted and should lead to an N100 suppression compared with externally produced sounds. Moreover, if the cerebellum is involved in the generation of a motor-to-auditory forward model, patients with focal cerebellar lesions should not display an N100 suppression effect. Compared with healthy controls, patients showed a largely attenuated N100 suppression effect. The current results suggest that the cerebellum forms not only motor-to-somatosensory predictions but also motor-to-auditory predictions. This extends the cerebellar forward model to other sensory domains such as audition.

Cortical speech processing unplugged: a timely subcortico-cortical framework

Speech is inherently tied to time. This fundamental quality has long been deemed secondary, and has consequently not received appropriate recognition in speech processing models. We develop an integrative speech processing framework by synthesizing evolutionary, anatomical and neurofunctional concepts of auditory, temporal and speech processing. These processes converge in a network that extends cortical speech processing systems with cortical and subcortical systems associated with motor control. This subcortico-cortical multifunctional network is based on temporal processing and predictive coding of events to optimize interactions between the organism and the environment. The framework we outline provides a novel perspective on speech processing and has implications for future studies on learning, proficient use, and developmental and acquired disorders of speech production and perception.

Mice with the deleted neurofilament of low-molecular-weight (Nefl) gene: 1. Effects on regional brain metabolism

Neuronal intermediate filaments consist of the NFL subunit linked with NFM and NFH, and their alterations have been proposed as a pathogenesic cause in motor neuron diseases. Depletion of the Nefl gene in mice mimicks the reduced NFL mRNA levels seen in amyotrophic lateral sclerosis and causes perikaryal accumulation of neurofilament proteins and axonal hypotrophy in motoneurons. NFL -/- mice were evaluated for regional brain metabolism by means of quantitative histochemical estimation of cytochrome oxidase (COx) activity. The NFL null mice displayed enzymatic activity alterations in numerous hindbrain regions, mainly the cerebellum, connected regions of the brainstem (red nucleus, vestibular nuclei, and reticular formation), and cranial nerve nuclei. All of the affected regions presented elevated COx activity, except for the Purkinje cells of the cerebellum and the magnocellular red nucleus, where enzymatic activity was lower. NFL-disrupted mice displayed functional alterations in brainstem sensorimotor regions affected in amyotrophic lateral sclerosis.

The role of interpositus nucleus in eyelid conditioned responses

One of the most widely used experimental models for the study of learning processes in mammals has been the classical conditioning of nictitating membrane/eyelid responses, using both trace and delay paradigms. Mainly on the basis of permanent or transitory lesions of putatively-involved structures, and using other stimulation and recording techniques, it has been proposed that cerebellar cortex and/or nuclei could be the place/s where this elemental form of associative learning is acquired and stored. We have used here an output-to-input approach to review recent evidence regarding the involvement of the cerebellar interpositus nucleus in the acquisition of these conditioned responses (CRs). Eyelid CRs appear to be different in profile, duration, and peak velocity from reflexively-evoked blinks. In addition, CRs are generated in a quantum manner across conditioning sessions, suggesting a gradual neural process for their proper acquisition. Accessory abducens and orbicularis oculi motoneurons have different membrane properties and contribute differently to the generation of CRs, with significant species differences. In particular, facial motoneurons seem to encode eyelid velocity during reflexively-evoked blinks and eyelid position during CRs, two facts suggestive of a differential somatic versus dendritic arrival of specific motor commands for each type of movement. Identified interpositus neurons recorded in alert cats during classical conditioning of eyelid responses show firing properties suggestive of an enhancing role for CR performance. However, as their firing started after CR onset, and because they do not seem to encode eyelid position during the CR, the interpositus nucleus cannot be conclusively considered as the place where this acquired motor response is generated. More information is needed regarding neural signal transformations taking place in each involved neural center, and it its proposed that more attention should be paid to functional states (as opposed to neural sites) able to generate motor learning in mammals. The contribution of feedforward mechanisms normally involved in the processing activities of related centers and circuits, and the possible functional interactions within neural systems subserving the associative strength between the conditioned and unconditioned stimuli, are also considered.

Multiple representations of information in the primary auditory cortex of cats. I. Stability and change in slow components of unit activity after conditioning with a click conditioned stimulus

Recordings of activity were made from 647 single units of the A(I) cortex of awake cats to evaluate behavioral state-dependent changes in the population response to a 70-dB click. Averages of PST histograms of unit activity were used to assess the changes in response. This report focuses on slow components of the responses disclosed by averages employing bin widths of 16 ms. Responses were compared before and after a Pavlovian blink CR was produced by forward pairing of click conditioned stimuli (CSs) with USs. A backward-paired 70-dB hiss was presented as a discriminative stimulus. Studies were also done after backward pairing of the click CSs (backward conditioning) that produced weak sensitization instead of a conditioned response. There were four main findings. First, components of activity elicited 32-160 ms after presenting the hiss decreased significantly after conditioning and after backward conditioning. The decreases after conditioning represented the most pronounced changes in activity evoked by either clicks or hisses in this behavioral state. Second, baseline firing decreased after both conditioning and backward conditioning. The direction of baseline change was opposite that found in adjacent cortical regions and in A(I) cortex after operant conditioning employing an acoustic cue. Third, prior to conditioning, unit activity in response to the hiss declined before the sound of the hiss reached its peak or terminated. This decrease was thought to represent a habituatory adaptation of response to a prolonged acoustic stimulus. This type of habituation to a lengthy stimulus has been recognized, behaviorally, but has not been observed previously in the activity of units of the auditory receptive cortex. Fourth, the percentage of click responsive units did not change significantly after the click was used as a CS for conditioning, and despite the accompanying changes in baseline activity, the absolute levels of activity summed in the first 16 ms after click delivery remained stable across behavioral states in which the motor response to the click was altered profoundly. The onset of the conditioned motor response began 20 ms after the click, and was shown earlier to depend on rapid, potentiated transmission through the cochlear nucleus and motor cortex for its generation. Thus the stability of the response to the click in the primary auditory receptive cortex was unexpected. This led us to make further analyses of the data with 2- and 4-ms bin widths (see companion report) that eventually disclosed a potentiated response to the click. The findings show stability and change in the response to the click as a CS, depending on the band pass (bin width) used for analysis of spike activity. In the representation disclosed by low pass filtering in this study, the response was stable. This representation provided information suitable for identifying commonalties of the click signals across varying behavioral states. The representations of the click and hiss contained in the slow components of the population response in the A(I) cortex were uncorrelated with the selective potentiation of activity in motor cortex and behavioral performance in response to click as a CS after conditioning. Although changes in the activity evoked by hisses occurred after conditioning, the changes also occurred after backward conditioning when only small, sensitized behavioral responses to clicks and hisses were observed. Basic theoretical considerations about information transmission in complex neural networks plus clinical observations comparing derangements of linguistic and non-linguistic cortical functions in humans suggest that multiple representations of conditioned stimulus inputs may exist in local populations of cortical neurons. Together, our studies provide evidence for two different, concurrent representations of information about a click CS encoded in the spike activity of the A(I) cortex.

Multiple representations of information in the primary auditory cortex of cats. II. Stability and change in early (<32 ms), rapid components of activity after conditioning with a click conditioned stimulus

Activity was recorded from single units of the A(I) cortex of awake animals to identify early (<32 ms) components of the population response to a 70 dB click and establish if they changed after using the click as a CS for conditioning. A 70 dB hiss was used as a discriminative stimulus. Responses to these stimuli were compared before and after a forward order of pairing that produced conditioning and a backward order of pairing that produced weak sensitization (backward conditioning). Averages of discharges in 2 and 4 ms bins distinguished primary (8-12 ms) from secondary (12-16 ms) temporal components of response to the click, and confirmed that the onset of the response was shorter in A(I) (8 ms, mean of 647 units) than in the adjacent, A(II) cortex (16 ms, mean of 95 units). (All times include a 1.6 ms transmission delay in sound arrival.) Primary and secondary components of A(I) responses to click did not change uniformly after changes in behavioral state, and were affected differently by both conditioning and backward conditioning. The percentage of cells with onsets of response to the click at secondary latencies (and to the hiss at tertiary latencies) increased after backward conditioning but not after conditioning, as did the magnitude of activity in response to the click. (The latter had a lesser degree of increase after conditioning.) The primary response to the click did not show these increases. The non-uniform changes suggested that temporal processing of the click was conducted differently in the 8-12 ms post stimulus period than in the 12-16 ms period. Within the total population of cells, it was possible to identify a small subgroup (13%) of highly auditory-responsive units that showed an increased primary response to the click as a CS selectively after conditioning and not after backward conditioning. The secondary component of response in these cells increased after both conditioning and backward conditioning. The percentages of cells responding to the click and hiss at primary latencies did not change significantly after conditioning, even in the subgroup of highly responsive cells. The results characterize differently timed components of rapid responses to acoustic stimuli in the A(I) cortex, disclose significant temporal differences in primary, secondary and tertiary information processing that affect the representations of the transmitted acoustic message across different behavioral states, and find one representation in a small subgroup of cells that supports the hypothesis that cells of the A(I) cortex have a selectively potentiated response to the CS after conditioning.

Differences in responses to 70 dB clicks of cerebellar units with simple versus complex spike activity: (i) in medial and lateral ansiform lobes and flocculus; and (ii) before and after conditioning blink conditioned responses with clicks as conditioned stimuli

Activity was recorded from 554 cerebellar units in eleven conscious cats to determine if responses to 70 dB clicks differed in units with simple and complex spike discharges. Effects of region of recording and behavioral state (with click used as a conditioned stimulus for conditioning) were also assessed. Cells with only simple spikes were distinguished from cells that had the following types of complex spike events: Type I-simple or initial spike followed > 1 ms by multiple spikes with baseline displacement (classical complex spikes), Type II--followed < or = 1 ms by spikes with or without baseline displacement (spikes in the absolute refractory period should arise from a separate site of initiation), and Type III-followed by spikes and displacement too close to the baseline noise to distinguish as Type I or II. Among the groups mean baseline activity was greatest in cells with Type I complex spikes, least in cells with Type III complex spikes, and greater in Type II cells than simple cells. Significant increases in activity within 32 ms of presenting clicks were found in the groups of Type II cells and simple cells. These appear to be the main auditory responsive cells of the cerebellar regions studied. Activity of Type II cells best reflected the temporal properties of the click; responses of simple cells had slower onsets (except in flocculus) and longer durations. Responses to click in Type II and simple cells differed in recordings from: (i) lateral ansiform lobe (lateral crus I and portions of crus II), (ii) medial ansiform lobe (medial crus I), and (iii) flocculus. The largest mean responses above baseline in the first 32 ms after click were found in Type II cells of the lateral ansiform lobe with onsets of 8-16 ms. Magnitudes of response differed before and after conditioning and backward conditioning. In the lateral ansiform lobe, the < 32 ms response to click was greater in Type II than simple cells in each state, but showed a greater increase above baseline after backward conditioning when conditioned responses were not produced than after conditioning. The onset of increased activity to click conditioned stimuli in Type II cells of the lateral ansiform region preceded the onset of the blink conditioned response after conditioning, consisted almost entirely of simple spikes, and reflected an increase in magnitude of response as opposed to an increased number of responsive units. After conditioning, an increased number of units in the flocculus responded to click conditioned stimuli in the 16-24 ms post stimulus period. Of the 16 cells with an onset of increased activity at this time, eight showed only simple spike activity. Seven of the remaining eight cells (all Type II) showed a significant increase in conditioned stimulus-evoked complex spiking above the low (usually < 1/s) baseline level of complex spike discharges. The findings support the conclusions that cerebellar units can respond rapidly enough to acoustic stimuli to play a role in auditory as well as motor processing and that the responses to 70 dB clicks differ among cells with simple and complex spike discharges. The differences are influenced substantially by the region of cerebellar recording and the behavioral state. The findings in cells of the flocculus offer the first evidence that complex as well as simple spike activity can contribute to an increased probability of discharge to click as a conditioned stimulus after conditioning.

Large-scale functional connectivity in associative learning: interrelations of the rat auditory, visual, and limbic systems

Large-scale functional connectivity in associative learning: interrelations of the rat auditory, visual, and limbic systems. J. Neurophysiol. 80: 3148-3162, 1998. Functional relations between specialized parts of the brain may be important determinants of learned behaviors. To study this, we examined the interrelations of the auditory system with several extraauditory structures in two groups of rats having different behavioral histories. Both groups were trained to associate a tone conditional stimulus (CS) with an aversive unconditional stimulus (US). For one group, a light presented with the tone predicted the absence of the US (group TL-). In the other group, the light was a neutral stimulus (group TL0). Fluorodeoxyglucose (FDG) incorporation was measured in the presence of the tone-light compound. Because the tone-light compound was physically identical for both groups, neural differences between groups reflected differences in the learned associative properties of the stimuli. Covariances of FDG uptake in the auditory system and extraauditory structures were examined using partial least squares. Three strong covariance or functional connectivity patterns were identified. The first pattern mainly reflected similarities between groups, with strong interrelations between the subcortical auditory system and the thalamocortical visual system, cerebellum, deep cerebellar nuclei, and midline thalamus. This pattern of interactions may represent part of a common circuit for relaying the associative value of the tone CS to the cerebellum and the midline thalamus. The external nucleus of the inferior colliculus and medial division of the medial geniculate nucleus were associated more strongly with this pattern for group TL-, which was interpreted as representing the change of the associative value of the tone by the light, mediated through extraauditory influences on these two regions. A second pattern involved midbrain auditory regions, superior colliculus, zona incerta, and subiculum and was stronger for group TL0. The relations between midbrain structures may represent the excitatory conditioned response (CR) evoked by the tone in this group. The final pattern was strongest in group TL- and involved interrelations of the thalamocortical auditory system with hippocampus, basolateral amygdala, and hypothalamus. This pattern may represent the learned inhibition of the CR to the tone in the presence of the light. These findings are consistent with behavioral studies suggesting that at least two types of associations are formed during associative learning. One is the sensory relation of the stimuli and another is the relation between the CS and the affective components of the US. These behavioral associations are mapped to the patterns of functional connectivity between auditory and extraauditory regions.

Acoustic transmission in the dentate nucleus. I. Patterns of activity to click and hiss; II. Changes in activity and excitability after conditioning

Recordings were made from 95 units of the dentate nucleus of naive cats to determine if patterns of response to 70 dB clicks could be distinguished from those to another acoustic stimulus (a hiss) of approximately equal sound pressure level. Further studies of an additional 309 units were conducted to determine if unit excitability and the response to clicks changed after Pavlovian conditioning in which blink responses were elicited by the clicks as conditioned stimuli. Over 50% of units tested before conditioning responded to click or hiss with increased activity, and 8% responded in the first 4-8 ms after the onset of the rapidly rising click. Cross-correlation of the respective 160 ms poststimulus histogram averages of mean activity showed dissimilar patterns of response to clicks and hisses (Pearson product-moment correlation coefficient + 0.02). Thus the averaged population responses distinguished these stimuli. In addition, individual cells were found in each behavioral state that responded selectively to either click or hiss. After conditioning with click as the conditioned stimulus, the number of units responding in the first 4-8 ms to click increased to 23%. The mean magnitude of activity 4-8 ms after presenting the click increased after conditioning but not after sensitization produced by backward pairing of the stimuli used for conditioning. After backward pairing only 6% of the units responded in the first 4-8 ms to click. The changes in activity after conditioning were accompanied by increases in neural excitability to intracellularly applied depolarizing current. In contrast with the changes in activity, the increases in neural excitability were also found after backward pairing. We conclude that short as well as long latency acoustic transmissions to click change in the dentate nucleus after conditioning, that changes in response to click are expressed in 4-8 ms responsive cells, and that many of these cells have different patterns of spike activity in response to click and hiss. The findings support the hypothesis that the dentate nucleus can play a significant role in short as well as long latency, adaptive acoustic transmission that can enhance the response to an acoustic signal used as a Pavlovian conditioned stimulus.

Cytochrome oxidase activity in the auditory system of the mouse: a qualitative and quantitative histochemical study

Detailed qualitative and quantitative determinations of cytochrome oxidase activity in the central auditory system of BALB/cJ mice were obtained at the light microscopic level. Cytochrome oxidase activity was determined using quantitative densitometry calibrated with standards of spectrophotometrically assayed enzymatic activity. This was done together with a cobalt-intensified histochemical procedure using fresh-frozen brains without perfusion-fixation. The resulting method showed improved sensitivity and allowed quantification of histochemical labeling as actual enzyme activity units. Adjacent sections were processed for either Nissl, fiber or Golgi stains to correlate the histochemical labeling with tissue morphology. The more peripheral auditory nuclei showed primarily somatic labeling with specific cell types showing predominant reactivity. However, higher auditory structures, including the inferior colliculus, medial geniculate and auditory cortex, showed predominantly neuropil reactivity. Comparison of mean cytochrome oxidase activities for the 27 auditory regions quantified revealed a trend for decreasing activity from the brainstem to the forebrain in central lemniscal structures. The extra-lemniscal auditory regions at each level showed lower activity than the corresponding lemniscal regions. The regions with the higher activity values showed around 10 times the labeling density of the white matter, indicating the high sensitivity of the method. The darkly labeling auditory structures were clearly delineated from surrounding neural regions, supporting the concept that basal levels of oxidative metabolic capacity are larger for the auditory system. It was concluded that the quantitative approach to cytochrome oxidase histochemistry may be applied successfully to the mouse brain. The normative data presented may be used as a starting point for other investigations of the effects of experimental manipulations on the metabolic activity of the auditory system.

Network analysis of functional auditory pathways mapped with fluorodeoxyglucose: associative effects of a tone conditioned as a Pavlovian excitor or inhibitor

The purpose of this study was to examine how opposite learned associative properties of the same auditory stimulus are represented by the pattern of network interactions between auditory system structures. [14C(U)]2-fluoro-2-deoxyglucose (FDG) autoradiography was used to compare mean auditory system activity and interregional correlations resulting from the presentation of a tone trained as either a Pavlovian conditioned excitor or inhibitor. Rats were trained with reinforced trials of the conditioned excitor (A+) intermixed with non-reinforced trials of a tone-light compound (AX-). For the Conditioned Excitor group, the tone was the excitor (A+), while for the Conditioned Inhibitor group the tone was the inhibitor (X-). After conditioning, both groups were injected with FDG and presented with the same tone. Structural equation models, constructed from the anatomical connections between auditory regions and their interregional correlations in FDG uptake, were used to calculate path coefficients representing the network interactions. The opposite associative significance of the tone was reflected as functional changes in the interactions between parallel auditory pathways. Direct covariance effects through lemniscal pathways from the ventral cochlear nucleus were similar in absolute magnitude but differed in sign between the Excitor and Inhibitor network models. Extra-auditory influences on the dorsal cochlear nucleus were greater for the tone-inhibitor, reflecting possible interactions of this nucleus with extra-auditory regions. The different associative effects of the tone suggest that central auditory pathways can code not only the physical qualities, but also the associative significance of auditory stimuli. These findings demonstrate that neural network interactions differentiate the associative effects of tones in the brain. It is proposed that associative learning is a distributed property of neural networks and that such a property can be understood by considering the interactions between component parts of the network.