The continued importance of comparative auditory research to modern scientific discovery

A rich history of comparative research in the auditory field has afforded a synthetic view of sound information processing by ears and brains. Some organisms have proven to be powerful models for human hearing due to fundamental similarities (e.g., well-matched hearing ranges), while others feature intriguing differences (e.g., atympanic ears) that invite further study. Work across diverse "non-traditional" organisms, from small mammals to avians to amphibians and beyond, continues to propel auditory science forward, netting a variety of biomedical and technological advances along the way. In this brief review, limited primarily to tetrapod vertebrates, we discuss the continued importance of comparative studies in hearing research from the periphery to central nervous system with a focus on outstanding questions such as mechanisms for sound capture, peripheral and central processing of directional/spatial information, and non-canonical auditory processing, including efferent and hormonal effects.

Diverse processing underlying frequency integration in midbrain neurons of barn owls

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

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

Natural ITD statistics predict human auditory spatial perception

A neural code adapted to the statistical structure of sensory cues may optimize perception. We investigated whether interaural time difference (ITD) statistics inherent in natural acoustic scenes are parameters determining spatial discriminability. The natural ITD rate of change across azimuth (ITDrc) and ITD variability over time (ITDv) were combined in a Fisher information statistic to assess the amount of azimuthal information conveyed by this sensory cue. We hypothesized that natural ITD statistics underlie the neural code for ITD and thus influence spatial perception. To test this hypothesis, sounds with invariant statistics were presented to measure human spatial discriminability and spatial novelty detection. Human auditory spatial perception showed correlation with natural ITD statistics, supporting our hypothesis. Further analysis showed that these results are consistent with classic models of ITD coding and can explain the ITD tuning distribution observed in the mammalian brainstem.

Multidimensional stimulus encoding in the auditory nerve of the barn owl

Auditory perception depends on multi-dimensional information in acoustic signals that must be encoded by auditory nerve fibers (ANF). These dimensions are represented by filters with different frequency selectivities. Multiple models have been suggested; however, the identification of relevant filters and type of interactions has been elusive, limiting progress in modeling the cochlear output. Spike-triggered covariance analysis of barn owl ANF responses was used to determine the number of relevant stimulus filters and estimate the nonlinearity that produces responses from filter outputs. This confirmed that ANF responses depend on multiple filters. The first, most dominant filter was the spike-triggered average, which was excitatory for all neurons. The second and third filters could be either suppressive or excitatory with center frequencies above or below that of the first filter. The nonlinear function mapping the first two filter outputs to the spiking probability ranged from restricted to nearly circular-symmetric, reflecting different modes of interaction between stimulus dimensions across the sample. This shows that stimulus encoding in ANFs of the barn owl is multidimensional and exhibits diversity over the population, suggesting that models must allow for variable numbers of filters and types of interactions between filters to describe how sound is encoded in ANFs.

Coding space-time stimulus dynamics in auditory brain maps

Sensory maps are often distorted representations of the environment, where ethologically-important ranges are magnified. The implication of a biased representation extends beyond increased acuity for having more neurons dedicated to a certain range. Because neurons are functionally interconnected, non-uniform representations influence the processing of high-order features that rely on comparison across areas of the map. Among these features are time-dependent changes of the auditory scene generated by moving objects. How sensory representation affects high order processing can be approached in the map of auditory space of the owl's midbrain, where locations in the front are over-represented. In this map, neurons are selective not only to location but also to location over time. The tuning to space over time leads to direction selectivity, which is also topographically organized. Across the population, neurons tuned to peripheral space are more selective to sounds moving into the front. The distribution of direction selectivity can be explained by spatial and temporal integration on the non-uniform map of space. Thus, the representation of space can induce biased computation of a second-order stimulus feature. This phenomenon is likely observed in other sensory maps and may be relevant for behavior.

A pathway for predation in the brain of the barn owl (Tyto alba): projections of the gracile nucleus to the "claw area" of the rostral wulst via the dorsal thalamus

The Wulst of birds, which is generally considered homologous with the isocortex of mammals, is an elevation on the dorsum of the telencephalon that is particularly prominent in predatory species, especially those with large, frontally placed eyes, such as owls. The Wulst, therefore, is largely visual, but a relatively small rostral portion is somatosensory in nature. In barn owls, this rostral somatosensory part of the Wulst forms a unique physical protuberance dedicated to the representation of the contralateral claw. Here we investigate whether the input to this "claw area" arises from dorsal thalamic neurons that, in turn, receive their somatosensory input from the gracile nucleus. After injections of biotinylated dextran amine into the gracile nucleus and cholera toxin B chain into the claw area, terminations from the former and retrogradely labeled neurons from the latter overlapped substantially in the thalamic nucleus dorsalis intermedius ventralis anterior. These results indicate the existence in this species of a "classical" trisynaptic somatosensory pathway from the body periphery to the telencephalic Wulst, via the dorsal thalamus, one that is likely involved in the barn owl's predatory behavior. The results are discussed in the context of somatosensory projections, primarily in this and other avian species.

Functional outcome and subset identification in RA patients from meridional Europe: analysis of a Spanish cohort

The aim of this study was to study the short-term functional and anatomical prognosis of rheumatoid arthritis (RA) in a series of Spanish patients and to identify different subsets of patients as well as possible baseline factors associated with specific outcomes. All patients seen in our division who met the ACR criteria for RA and with disease duration between 2 and 7 years were eligible for the study. Available patients were further evaluated at the clinic for disease activity using biological tests and joint indices as joint counts and Thompson's index, functional capacity using the ACR functional classification (ACR-FC) and the modified Health Assessment Questionnaire (M-HAQ) and radiologic damage by the Sharp's radiologic scoring method. Cluster analysis was used to identify different clinical subsets of patients. One hundred and sixty-three patients were eligible for the study, 13 could not be located or refused to participate and 12 had died. Mean (+/-SD) age at disease onset and mean disease duration were, respectively, 56(+/-14) years and (55+/-20) months. Median (interquartile range) of M-HAQ was 0.4 (0.1-1.1) and 41% of patients were in III or IV ACR-FC. The majority of patients (93%) showed radiologic lesions and 65% had erosions. Cluster analysis identified three subsets: cluster I (70% of patients) was characterised by a good prognosis, cluster II (13%) by a high level of disease activity, and cluster III (17%) by a greater anatomic damage and longer disease duration. No baseline predictive markers were found for these different outcomes. We concluded that RA portends an overall poor short-term prognosis in a relative large percentage of our patients with significant anatomic and functional sequelae. Aggressive management is specially indicated in this subgroup of patients, although definitive prognostic markers for its early identification are still lacking.

n-3 Fatty acids plus oleic acid and vitamin supplemented milk consumption reduces total and LDL cholesterol, homocysteine and levels of endothelial adhesion molecules in healthy humans

BACKGROUND AND AIMS

Numerous studies suggest n -3 polyunsaturated fatty acids (n -3 PUFA) and oleic acid intake have beneficial effects on health including risk reduction of coronary heart disease. The purpose of this study was to evaluate the effect of a commercially available skimmed milk supplemented with n -3 PUFA, oleic acid, and vitamins E, B(6), and folic acid (Puleva Omega3) on risk factors for cardiovascular disease. (CVD).

METHODS

Thirty volunteers were given 500 ml/day of semi-skimmed milk for 4 weeks and then 500 ml/day of the n -3 enriched milk for 8 further weeks. Plasma and LDL lipoproteins were obtained from volunteers at the beginning of the study (T(pre)), and at 4, 8 and 12 weeks.

RESULTS

The consumption of n -3 enriched milk produced a significant decrease in plasma concentration of total and LDL cholesterol accompanied by a reduction in plasma levels of homocysteine. Plasma and LDL oxidability and vitamin E concentration remained unchanged throughout the study. A significant reduction in plasma levels of vascular cell adhesion molecule 1, and an increase in plasma concentration of folic acid were also observed.

CONCLUSION

Daily intake of n -3 PUFA and oleic acid supplemented skimmed milk plus folic acid and B-type vitamins has favourable effects on risk factors for CVD.

Photoacoustic determination of iron in corn meal

We report here on the use of the photoacoustic technique for the determination of low concentrations of total iron in corn meal samples. The determination of this element in food is of considerable interest because several foods are currently enriched with it at proper levels in order to increase the resistance of people, after consumation, to several diseases, such as anemia. The proposed technique is based on an open photoacoustic cell configuration in conjunction with a suitable colorimetric method. We applied it to a measurement of the total iron concentration in corn meal samples. The results agree very well with those obtained using a conventional spectrophotometric method, showing the possibilities of new experimental methodologies based on photothermal methods to perform this kind of study, with the advantage of a higher sensitivity and increment of the range of appreciable absorbance.

[Neurons that encode sound direction]

INTRODUCTION

In the auditory system, the inner ear breaks down complex signals into their spectral components, and encodes the amplitude and phase of each. In order to infer sound direction in space, a computation on each frequency component of the sound must be performed.

DEVELOPMENT

Space specific neurons in the owl s inferior colliculus respond only to sounds coming from a particular direction and represent the results of this computation. The interaural time difference (ITD) and interaural level difference (ILD define the auditory space for the owl and are processed in separate neural pathways. The parallel pathways that process these cues merge in the external nucleus of the inferior colliculus where the space specific neurons are selective to combinations of ITD and ILD. How do inputs from the two sources interact to produce combination selectivity to ITD ILD pairs? A multiplication of postsynaptic potentials tuned to ITD and ILD can account for the subthreshold responses of these neurons to ITD ILD pairs. Examples of multiplication by neurons or neural circuits are scarce, but many computational models assume the existence of this basic operation. The owl s auditory system uses such operation to create a 2 dimensional map of auditory space. The map of space in the owl s auditory system shows important similarities with representations of space in the cerebral cortex and other sensory systems. In encoding space or other stimulus features, individual neurons appear to possess analogous functional properties related to the synthesis of high order receptive fields.

Auditory spatial receptive fields created by multiplication

Examples of multiplication by neurons or neural circuits are scarce, although many computational models use this basic operation. The owl's auditory system computes interaural time (ITD) and level (ILD) differences to create a two-dimensional map of auditory space. Space-specific neurons are selective for combinations of ITD and ILD, which define, respectively, the horizontal and vertical dimensions of their receptive fields. A multiplication of separate postsynaptic potentials tuned to ITD and ILD, rather than an addition, can account for the subthreshold responses of these neurons to ITD-ILD pairs. Other nonlinear processes improve the spatial tuning of the spike output and reduce the fit to the multiplicative model.

Reciprocal actions between sensory signals and sleep

To the best of our knowledge, there is no simple way to induce neural networks to shift from waking mode into sleeping mode. Our best guess is that a whole group of neurons would be involved and that the process would develop in a period of time and a sequence which are mostly unknown. The quasi-total sensory deprivation elicits a new behavioral state called somnolence. Auditory stimulation as well as total auditory deprivation alter sleep architecture. Auditory units exhibiting firing shifts on passing to sleep (augmenting or diminishing) are postulated to be locked to sleep-related networks. Those ( approximately 50%) that did not change during sleep are postulated to continue informing the brain as in wakefulness. A rhythmic functional plasticity of involved networks is postulated. A number of auditory and visual cells have demonstrated a firing phase locking to the hippocampal theta rhythm. This phase locking occurs both during wakefulness and sleep phases. The theta rhythm may act as an organizer of sensory information in visual and auditory systems, in all behavioral states adding a temporal dimension to the sensory processing. Sensory information from the environment and body continuously modulates the central nervous system activity, over which sleep phenomenology must develop. It also produces a basal tonus during wakefulness and sleep, determining changes in the networks that contribute to sleep development and maintenance and, eventually, it also leads to sleep interruption.

Cellular mechanisms for resolving phase ambiguity in the owl's inferior colliculus

Both mammals and birds use the interaural time difference (ITD) for localization of sound in the horizontal plane. They may localize either real or phantom sound sources, when the signal consists of a narrow frequency band. This ambiguity does not occur with broadband signals. A plot of impulse rates or amplitude of excitatory postsynaptic potentials against ITDs (ITD curve) consists of peaks and troughs. In the external nucleus (ICX) of the owl's inferior colliculus, ITD curves show multiple peaks when the signal is narrow-band, such as tones. Of these peaks, one occurs at ITDi, which is independent of frequency, and others at ITDi +/- T, where T is the tonal period. The ITD curve of the same neuron shows a large peak (main peak) at ITDi and no or small peaks (side peaks) at ITDi +/- T, when the signal is broadband. ITD curves for postsynaptic potentials indicate that ICX neurons integrate the results of binaural cross-correlation in different frequency bands. However, the difference between the main and side peaks is small. ICX neurons further enhance this difference in the process of converting membrane potentials to impulse rates. Inhibition also appears to augment the difference between the main and side peaks.

Sleep and wakefulness modulation of the neuronal firing in the auditory cortex of the guinea pig

Sleep-related changes-including modification in sensory processing-that influence brain and body functions, occur during both slow wave and paradoxical sleep. Our aim was to investigate how cortical auditory neurons behave during the sleep/waking cycle, and to study cell firing patterns in relation to the processing of auditory information without the interference of anesthetic drugs. We recorded single cells in the A region of the auditory cortex in restrained, chronically-implanted guinea pigs, and compared their evoked and spontaneous activity during sleep stages and quiet wakefulness. A new classification of the unit's responses to simple sound during wakefulness is presented. Moreover, a number of the neurons in the primary auditory cortex exhibited significant quantitative changes in their evoked or spontaneous firing rates. These changes could be correlated to sleep stages or wakefulness in 42.2% to 58.3% of the sampled neurons. A similar population did not show behavioral related changes in firing rates. Our results indicate that the responsiveness of the auditory system during sleep may be considered partially preserved. An important result was that spontaneous and evoked activity may vary in opposite directions, i.e. , the evoked activity could increase while the spontaneous activity decrease or vice versa. Then, a general question was proposed: is the increased spontaneous activity in the auditory cortex, particularly during PS, related to auditory hypnic 'images'? The studied cortical auditory neurons exhibit changes in their firing rates in correlation to stages of sleep and wakefulness. This is consistent with the hypothesis that a general shift in the neuronal networks involved in sensory processing occurs during sleep.

[Spontaneous acquired localized neuropathies in childhood]

Acquired non-surgical, non-traumatic localized neuropathies, excluding cranial nerve disorders, are rare in infancy. We review the clinical histories of six children, studied for this disorders amongst a total of 2,105 children seen in the Neuropediatric Department of the Hospital Miguel Servet in Zaragoza. Two were diagnosed as familial neuropathy with pressure sensitive paralysis. Two plexopathies were considered to be familial brachial plexopathy with minor dysmorphic features. One case was diagnosed as idiopathic radial neuropathy and a further case as idiopathic lumbosacral plexopathy. We emphasize that although rare in pediatrics, spontaneous localized neuropathies often show constitutional pathology, frequently hereditary. Idiophatic cases may also be hereditary, and it may be difficult to confirm the diagnosis if there is no family history or phenotypic characteristics. Diagnosis depends on the personal and family history, physical examination, neurophysiological study, absence of abnormal neuroimaging findings and awareness of the possibility of this diagnosis. These disorders probably occur more often than is generally believed.

Effects of interaural intensity difference on the processing of interaural time difference in the owl's nucleus laminaris

Interaural time and intensity differences (ITD and IID) are processed independently in the owl's auditory system. This paper examines whether this independence is established in nucleus laminaris (NL), the first site of ITD processing. A plot of discharge rate against time difference (ITD curve) is sinusoidal in NL. The ITDs that produce the peaks are called the most favorable ITDs, and those that produce the troughs are called the least favorable ITDs. IID had little effect on the discharge rates of laminaris neurons for the most and least favorable ITDs. The degree of peak-trough modulation changed slightly with variation in IID. In contrast, IID in tonal stimuli affected the temporal aspect of ITD curves depending on the difference between the stimulus frequency and the neuron's best frequency (BF). For frequencies below BF, IID caused large and systematic shifts in ITD toward the ear in which the sound was louder, whereas for frequencies above BF, IID caused small shifts in ITD toward the opposite ear. IID had little effect on ITD curves taken with BF or broadband noise. These results can be largely accounted for by the effects of frequency and intensity on the timing of impulses at the level of the cochlear nuclei. Thus, the processing of ITD by NL neurons is independent of IID for behaviorally relevant stimuli, because the timing of impulses is insensitive to sound level when the signal is broadband.

Auditory deprivation modifies sleep in the guinea-pig

After destruction of both cochleae, a significant enhancement of both paradoxical sleep and slow wave sleep together with decreased wakefulness, were observed for up to 45 days. The sleep augmentation consisted of an increment in the number of episodes of both slow wave and paradoxical sleep rather than in the duration of single episodes. The partial isolation provoked by deafness is postulated as explanation. We suggest that the suppression of one input to a complex set of networks related to the sleep-waking cycle, introduce an imbalance that leads to sleep enhancement.

Tolerance to sound intensity of binaural coincidence detection in the nucleus laminaris of the owl

Neurons of the owl's nucleus laminaris serve as coincidence detectors for measurement of interaural time difference. The discharge rate of nucleus laminaris neurons for both monaural and binaural stimulation increased with sound intensity until they reached an asymptote. Intense sounds affected neither the ratio between binaural and monaural responses nor the interaural time difference for which nucleus laminaris neurons were selective. Theoretical analysis showed that high afferent discharge rates cause coincidence detectors with only excitatory input to lose their selectivity for interaural time difference when coincidence of impulses from the same side becomes as likely as that of impulses from the two sides. We hypothesize that inhibitory input whose strength increases with sound intensity protects nucleus laminaris neurons from losing their sensitivity to interaural time difference with intense sounds.

Firing of inferior colliculus auditory neurons is phase-locked to the hippocampus theta rhythm during paradoxical sleep and waking

The activity of 52 single auditory units in the central nucleus of the inferior colliculus (IC) was recorded along with cortical and hippocampal (CA1) electrograms and neck muscle electromyograms in behaving, head-restrained guinea pigs during paradoxical sleep (PS) and wakefulness. Sixteen (30%) of the IC auditory units showed positive correlation with the hippocampal theta (theta) rhythm: 8 (15%) were theta rhythmic with theta phase-locking (type 1), 8 (15%) showed only theta phase-locking with no rhythmicity (type 2), while 70% did not show any correlation to hippocampal theta rhythm (type 3). During wakefulness IC neurons (4 of 13) showed a higher synchrony with hippocampal theta when sound-stimulated at the unit's characteristic frequency. During PS all IC auditory neurons recorded presented some hippocampal theta correlation: 40% were rhythmic and phase-locked to the theta frequency and 60% were nonrhythmic maintaining the theta phase-locking. Shifts in the angle of phase-locking to the theta rhythm were observed during PS. It is suggested that the hippocampal theta rhythm may play the part of an internal clock, adding a temporal dimension to the processing of auditory sensory information.

Effects of sleep on the responses of single cells in the lateral superior olive

The effects of behavioral shifts on auditory lateral superior olive neurons were analyzed in guinea-pigs during the sleep-waking cycle with single unit extracellular recordings at the unit characteristic frequency and with low sound intensity. Shifts in the number of spikes in response to pure tones and in spontaneous firing proved to be closely related to waking, slow wave and paradoxical sleep. All of the recorded lateral superior olive (LSO) auditory neurons showed sleep-related firing shifts. Moreover, changes in the pattern of discharge over time were observed in 15% of the LSO cells on passing from waking to sleep. Sleep may determine either an increase or a decrease of the firing number in response to sound. The most important change observed in decreasing firing units was the near-absence of units responding to sound in the paradoxical sleep phase during the last 40 ms of the response. The waking cues for binaural detection, studied with our experimental paradigm, disappeared during slow wave sleep. We thus conclude that the binaural function of some lateral superior olive neurons (11.5%) was impaired during this sleep period in the present experimental conditions. Auditory efferent pathways are postulated to impinge on the auditory processing at LSO nucleus level during the sleep-waking cycle. Thus, auditory unitary activity appears to be dependent on both incoming information, and a CNS descending action closely related to the waking and sleep periods. Functional interactions between pontine sleep-related groups of neurons and auditory system units are suggested.

alpha-L-arabinofuranosidase production by Aspergillus nidulans

The effects of some physico-chemical parameters on production of extracellular alpha-L-arabinofuranosidase by Aspergillus nidulans were examined. Highest levels of alpha-L-arabinofuranosidase were generated with cultures grown on 1% (w/v) purified beet pulp arabinan at 30 degrees C and at an initial pH of 7.0. The enzyme was shown to be very sensitive to the action of proteases. Zymogram overlay of a protein profile obtained by SDS-PAGE revealed the occurrence of a band (M(r) 36,000) exhibiting alpha-L-arabinofuranosidase activity. The isoelectric pH of the enzyme lay near 4.3. Temperature and pH optima for the activity of crude alpha-L-arabinofuranosidase preparations were 55 degrees C and 5.5, respectively. Enzyme activity was greatly reduced by thiol reagents such as Hg2+ and p-hydroxymercuribenzoate and showed a Km value of 2.7 mM on p-nitrophenyl alpha-L-arabinofuranoside as substrate.

Internally-generated sound stimulates cochlear nucleus units

The body generates many physiological sounds. One of the most prominent is that produced by the blood flowing inside the vessels with each heart beat. On the other hand, the cochlea is a very sensitive receptor with a low threshold. Given the anatomical close proximity of the carotid artery and other vessels to the inner ear, the possibility of its being stimulated is very high. Cochlear nucleus spontaneous as well sound-responding auditory units were studied. A close relationship between the heart beat, that is the blood flow, and the cochlear nucleus firing was demonstrated, in anesthetized and awake guinea-pigs. Temporary mechanical interruption of the blood flow through the ipsilateral carotid artery abolished firing increments at the cochlear nucleus time-locked to the heart beat. We conclude that one component of the so called 'spontaneous' firing in the auditory system is actually evoked activity due to normal body-generated sounds or noises.

Single unit activity in the guinea-pig cochlear nucleus during sleep and wakefulness

The effects of waking and sleep on the response properties of auditory units in the ventral cochlear nucleus (CN) were explored by using extracellular recordings in chronic guinea-pigs. Significant increases and decreases in firing rate were detected in two neuronal groups, a) the "sound-responding" and b) the "spontaneous" (units that do not show responses to any acoustic stimuli controlled by the experimenter). The "spontaneous" may be considered as belonging to the auditory system because the corresponding units showed a suppression of their discharge when the receptor was destroyed. The auditory CN units were characterized by their PSTH in response to tones at their characteristic frequency and also by the changes in firing rate and probability of discharge evaluated during periods of waking, slow wave and paradoxical sleep. The CNS performs functions dependent on sensory inputs during wakefulness and sleep phases. By studying the auditory input at the level of the ventral CN with constant sound stimuli, it was shown that, in addition to the firing rate shifts, some units presented changes in the temporal probability of discharge, implying central actions on the corresponding neurons. The mean latency of the responses, however, did not show significant changes throughout the sleep-waking cycle. The auditory efferent pathways are postulated to modulate the auditory input at CN level during different animal states. The probability of firing and the changes in the temporal pattern, as shown by the PSTH, are thus dependent on both the auditory input and the functional brain state related to the sleep-waking cycle.

Periaqueductal gray influence on anteroventral cochlear nucleus unitary activity and naloxone effects

The effect of periaqueductal gray (PAG) electrical stimulation on the response properties of auditory and 'spontaneously' firing units (abolished when the cochlea is destroyed) in the anteroventral cochlear nucleus (AVCN) was explored using extracellular recordings in acute guinea-pigs. Significant increases and decreases in firing rate were detected in both neuronal groups: only 4% of the sound-responding units were insensitive to PAG stimulation while the 'spontaneous' units showed significantly smaller changes in firing rate in response to PAG stimulation. The auditory AVCN neurons were categorized both by their sound post stimulus time (PST) histograms at their characteristic frequency (CF) and the changes in the probability of discharge after PAG stimulation while the tone burst was maintained constant. PAG was implicated in pain input modulation through enkephalin actions. Because enkephalins have been also observed at the CN level, a pharmacological approach administering naloxone was carried out. We observed that 1) naloxone abolished the unit discharge shifts observed after PAG stimulation and 2) when the drug was injected without PAG stimulation, it produced changes in the firing, increasing or decreasing, and shifts in the probability of discharge versus time, even in cases in which the firing rate was not altered. An involvement of the auditory efferent pathways to CN is postulated and a possible enkephalinergic factor is suggested as a modulator of the auditory input at this level. The probability of discharge observed in the PSTH at the AVCN is dependent on the auditory input plus the central efferent action to its neurons.