Firing of inferior colliculus neurons in response to low-frequency sound stimulation during sleep and waking

Individualized Treatment of Tinnitus during Sleep Using Combined Tinnitus Signal and Music

INTRODUCTION

Tinnitus is a widely seen otological symptom that interferes with daily activities and causes discomfort. Tinnitus treatments can be classified into 4 main groups: pharmacological treatments, cognitive and behavioral therapy, psychological treatments, and combined treatment approaches made up of at least 2 of these 3 treatment methods.

OBJECTIVE

The aim of this study was to assess whether it would be possible to develop an individualized treatment method of tinnitus by application of a combined tinnitus signal and music during sleep.

METHODS

Forty-three ears of 30 patients who had subjective tinnitus were included. The patients were evaluated using Tinnitus Handicap Inventory, Visual Analogue Scale, and Beck Depression Inventory. The psychoacoustic parameters of tinnitus, such as tinnitus frequency and loudness, and minimal masking levels, were determined. The patients were asked to select musical melodies that they liked. The tinnitus frequency of each patient was taken as the central frequency according to ANSI 2004. All sound files were prepared as stereo channels, with 16-bit resolution and 44,100 Hz sampling rate. The root mean square power value of the music and the band noise's average root mean square power value were equalized with the "Amplification" command, and 70% of the music and 30% of wide/narrow-band noise were mixed as a stereo channel by the "Mix Paste" command. The patients were instructed to listen to that individualized music/narrow-band noise (tinnitus signal) for 2 h during sleep for a duration of 6 months.

RESULTS

Tinnitus frequencies of the patients measured prior to treatment and at the second, fourth, and sixth months of follow-up were not significantly different. A statistically significant decrease was seen in tinnitus loudness, minimal masking levels, and residual inhibition during the follow-up. Tinnitus Handicap Inventory scores decreased significantly during follow-up, and the number of patients who complained of tinnitus decreased (p < 0.05). The Visual Analogue Scale scores significantly decreased during follow-up (p < 0.05). Beck Depression Inventory scores decreased significantly during follow-up (p < 0.05).

CONCLUSION

Stimulation of the auditory and limbic systems during sleep by the tinnitus signal combined with individualized musical melodies seems an alternative, effective, and cheap method in the treatment of tinnitus.

State-dependent changes in cortical gain control as measured by auditory evoked responses to varying intensity stimuli

STUDY OBJECTIVES

Auditory evoked potential (AEP) components correspond to sequential activation of brain structures within the auditory pathway and reveal neural activity during sensory processing. To investigate state-dependent modulation of stimulus intensity response profiles within different brain structures, we assessed AEP components across both stimulus intensity and state.

DESIGN

We implanted adult female Sprague-Dawley rats (N = 6) with electrodes to measure EEG, EKG, and EMG. Intermittent auditory stimuli (6-12 s) varying from 50 to 75 dBa were delivered over a 24-h period. Data were parsed into 2-s epochs and scored for wake/sleep state.

RESULTS

All AEP components increased in amplitude with increased stimulus intensity during wake. During quiet sleep, however, only the early latency response (ELR) showed this relationship, while the middle latency response (MLR) increased at the highest 75 dBa intensity, and the late latency response (LLR) showed no significant change across the stimulus intensities tested. During rapid eye movement sleep (REM), both ELR and LLR increased, similar to wake, but MLR was severely attenuated.

CONCLUSIONS

Stimulation intensity and the corresponding AEP response profile were dependent on both brain structure and sleep state. Lower brain structures maintained stimulus intensity and neural response relationships during sleep. This relationship was not observed in the cortex, implying state-dependent modification of stimulus intensity coding. Since AEP amplitude is not modulated by stimulus intensity during sleep, differences between paired 75/50 dBa stimuli could be used to determine state better than individual intensities.

Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Neural representations during sleep: From sensory processing to memory traces

In the course of a day, the brain undergoes large-scale changes in functional modes, from attentive wakefulness to the deepest stage of sleep. The present paper evaluates how these state changes affect the neural bases of sensory and cognitive representations. Are organized neural representations still maintained during sleep? In other words, despite the absence of conscious awareness, do neuronal signals emitted during sleep contain information and have a functional relevance? Through a critical evaluation of the animal and human literature, neural representations at different levels of integration (from the most elementary sensory level to the most cognitive one) are reviewed. Recordings of neuronal activity in animals at presentation of neutral or significant stimuli show that some analysis of the external word remains possible during sleep, allowing recognition of behaviorally relevant stimuli. Event-related brain potentials in humans confirm the preservation of some sensory integration and discriminative capacity. Behavioral and neuroimaging studies in humans substantiate the notion that memory representations are reactivated and are reorganized during post-learning sleep; these reorganisations may account for the beneficial effects of sleep on behavioral performance. Electrophysiological results showing replay of neuronal sequences in animals are presented, and their relevance as neuronal correlates of memory reactivation is discussed. The reviewed literature provides converging evidence that structured neural representations can be activated during sleep. Which reorganizations unique to sleep benefit memory representations, and to what extent the operations still efficient in processing environmental information during sleep are similar to those underlying the non-conscious, automatic processing continually at work in wakefulness, are challenging questions open to investigation.

The thalamo-cortical auditory receptive fields: regulation by the states of vigilance, learning and the neuromodulatory systems

The goal of this review is twofold. First, it aims to describe the dynamic regulation that constantly shapes the receptive fields (RFs) and maps in the thalamo-cortical sensory systems of undrugged animals. Second, it aims to discuss several important issues that remain unresolved at the intersection between behavioral neurosciences and sensory physiology. A first section presents the RF modulations observed when an undrugged animal spontaneously shifts from waking to slow-wave sleep or to paradoxical sleep (also called REM sleep). A second section shows that, in contrast with the general changes described in the first section, behavioral training can induce selective effects which favor the stimulus that has acquired significance during learning. A third section reviews the effects triggered by two major neuromodulators of the thalamo-cortical system--acetylcholine and noradrenaline--which are traditionally involved both in the switch of vigilance states and in learning experiences. The conclusion argues that because the receptive fields and maps of an awake animal are continuously modulated from minute to minute, learning-induced sensory plasticity can be viewed as a "crystallization" of the receptive fields and maps in one of the multiple possible states. Studying the interplays between neuromodulators can help understanding the neurobiological foundations of this dynamic regulation.

In vivo approach to the cellular mechanisms for sensory processing in sleep and wakefulness

1. The present review analyzes sensory processing during sleep and wakefulness from a single neuronal viewpoint. Our premises are that processing changes throughout the sleep-wakefulness cycle may be at least partially evidenced in single neurons by (a) changes in the phase locking of the response to the hippocampal theta rhythm, (b) changes in the discharge rate and firing pattern of the response to sound, and (c) changes in the effects of the neurotransmitters involved in the afferent and efferent pathways. 2. The first part of our report is based on the hypothesis that the encoding of sensory information needs a timer in order to be processed and stored, and that the hippocampal theta rhythm could contribute to the temporal organization. We have demonstrated that the guinea pig's auditory and visual neuronal discharge exhibits a temporal relationship (phase locking) to the hippocampal theta waves during wakefulness and sleep phases. 3. The concept that the neural network organization during sleep versus wakefulness is different and can be modulated by sensory signals and vice versa, and that the sensory input may be influenced by the CNS state, i.e., asleep or awake, is introduced. During sleep the evoked firing of auditory units increases, decreases, or remains similar to that observed during quiet wakefulness. However, there has been no auditory unit yet that stops firing as the guinea pig enters sleep. Approximately half of the cortical neurons studied did not change firing rate when passing into sleep while others increased or decreased. Thus, the system is continuously aware of the environment. We postulate that those neurons that changed their evoked firing during sleep are also related to still unknown sleep processes. 4. Excitatory amino acid neurotransmitters participate in the synaptic transmission of the afferent and efferent pathways in the auditory system. In the inferior colliculus, however, the effects of glutamate's mediating the response to sound and the efferent excitation evoked by cortical stimulation failed to show differences in sleep and wakefulness. 5. Considering that neonates and also infants spend most of the time asleep, the continuous arrival of sensory information to the brain during both sleep phases may serve to "sculpt" the brain by activity-dependent mechanisms of neural development, as has been postulated for wakefulness.

Appetitive conditioning-induced plasticity is expressed during paradoxical sleep in the medial geniculate, but not in the lateral amygdala

This study examined whether neurons in the medial division of the medial geniculate (MGm) and the dorsal part of the lateral amygdala (LAd) express learning-induced plasticity in paradoxical sleep (PS) after appetitive conditioning, as they do in PS after fear conditioning. Rats received tone-food pairings in 3 sessions. After each session, the tone was presented at a nonawakening intensity during PS. Multiunit activity was simultaneously recorded in MGm and LAd. During waking, increases in tone-evoked discharges developed in MGm and LAd; however, as training continued, they lessened in LAd, but not in MGm. During PS, conditioned tone responses were expressed in MGm, but not in LAd. Thus, these results demonstrate dissociation of MGm and LAd plasticity. Moreover, compared with fear conditioning results, they suggest that expression of amygdalar plasticity in PS depends on the emotional salience of the stimulus.

Effects of excitatory amino acid antagonists on the activity of inferior colliculus neurons during sleep and wakefulness

The contribution of N-methyl-D-aspartate to the response to sound of guinea pig inferior colliculus neurons was analyzed by recording single-unit activity before and after iontophoretic injection of a receptor specific antagonist, 2-amino-5-phosphonovaleric acid (AP5), during the sleep-waking cycle. The AP5 produced a significant firing decrease in most of the units recorded, while some neurons exhibited a particular decrease in the later part of the response. A latency reduction in one out of three units in paradoxical sleep was observed. A low proportion of them exhibited a significant firing increase. These actions were observed in wakefulness (W) as well as during sleep phases. We compared the action of kynurenic acid (Kyn) and the electrical stimulation of the auditory cortex on the same inferior colliculus neuron in anesthetized animals and during W. Both Kyn iontophoresis and cortical stimulation evoked similar changes, decreased firing rate in most inferior colliculus units, whereas a low proportion of them increased their discharge, in anesthetized guinea pigs and in W. Ascending as well as descending - efferent - glutamatergic fibers impinging on inferior colliculus neurons contribute to sound-evoked responses. The enhanced unitary activity observed in some neurons with after glutamatergic receptor blocking may indicate that polysynaptic pathways involving inhibitory neurons decreased their activity. These effects were observed in anesthetized and in behaving animals.

Inferior colliculus unitary activity in wakefulness, sleep and under barbiturates

The spontaneous unitary activity and the response to contralateral tone-burst were analyzed in the inferior colliculus (IC) of guinea pigs during the sleep-waking cycle and under the effects of pentobarbital anesthesia. Minor changes were observed in both spontaneous and evoked activity between wakefulness (W) and slow wave sleep (SWS). On the other hand, a consistent increase in the mean spontaneous firing rate and a significant decrement in the signal-to-noise ratio (S/N ratio) was observed during paradoxical sleep (PS). Pentobarbital anesthesia reduced the spontaneous and evoked firing rate, the duration of the excitatory response and increased the duration of the post-excitatory suppression. We conclude, that the processing of auditory information in the IC change markedly during PS. Because the IC is a compulsory station for almost all the ascending auditory pathways, the observed decrease in the S/N ratio may deeply affect the auditory perception during this behavioral state. Finally, the alteration of the neuronal activity induced by pentobarbital differs not only with the activity observed during W, but also with the activity observed during both SWS and PS.

Diversity of receptive field changes in auditory cortex during natural sleep

Twenty years ago, the study by Livingstone and Hubel [(1981) Nature, 291, 554] was viewed as a first step toward understanding how changes in state of vigilance affect sensory processing. Since then, however, very few attempts have been made to progress in this direction. In the present study, 56 cells were recorded in the auditory cortex of adult, undrugged guinea pigs, and the frequency tuning curves were tested during continuous and stable periods of wakefulness and of slow-wave sleep (SWS). Twelve cells were also tested during paradoxical sleep. Over the whole cell population, the response latency, the frequency selectivity and the size of the suprathreshold receptive field were not significantly modified during SWS compared with waking. However, this lack of global effects resulted from the heterogeneity of response changes displayed by cortical cells. During SWS, the receptive field size varied as a function of the changes in evoked responses: it was unchanged for the cells whose evoked responses were not modified (38% of the cells), reduced for the cells whose responses were decreased (48%) and enlarged for the cells whose responses were increased (14%). This profile of changes differs from the prevalent receptive field shrinkage that was observed in the auditory thalamus during SWS [Edeline et al. (2000), J. Neurophysiol., 84, 934]. It also contrasts with the receptive field enlargement that was described under anaesthesia when the EEG spontaneously shifted from a desynchronized to a synchronized pattern [Wörgötter et al. (1998), Nature, 396, 165]. Reasons for these differences are discussed.

Auditory information processing during human sleep as revealed by event-related brain potentials

The main goal of this review is to elucidate up to what extent pre-attentive auditory information processing is affected during human sleep. Evidence from event-related brain potential (ERP) studies indicates that auditory information processing is selectively affected, even at early phases, across the different stages of sleep-wakefulness continuum. According to these studies, 3 main conclusions are drawn: (1) the sleeping brain is able to automatically detect stimulus occurrence and trigger an orienting response towards that stimulus if its degree of novelty is large; (2) auditory stimuli are represented in the auditory system and maintained for a period of time in sensory memory, making the automatic-change detection during sleep possible; and (3) there are specific brain mechanisms (sleep-specific ERP components associated with the presence of vertex waves and K-complexes) by which information processing can be improved during non-rapid eye movement sleep. However, the remarkably affected amplitude and latency of the waking-ERPs during the different stages of sleep suggests deficits in the building and maintenance of a neural representation of the stimulus as well as in the process by which neural events lead to an orienting response toward such a stimulus. The deactivation of areas in the dorsolateral pre-frontal cortex during sleep contributing to the generation of these ERP components is hypothesized to be one of the main causes for the attenuated amplitude of these ERPs during human sleep.

Auditory thalamus neurons during sleep: changes in frequency selectivity, threshold, and receptive field size

The present study describes how the frequency receptive fields (RF) of auditory thalamus neurons are modified when the state of vigilance of an unanesthetized animal naturally fluctuates among wakefulness (W), slow-wave sleep (SWS), and paradoxical sleep (PS). Systematic quantification of several RF parameters-including strength of the evoked responses, response latency, acoustic threshold, shape of rate-level function, frequency selectivity, and RF size-was performed while undrugged, restrained guinea pigs presented spontaneous alternances of W, SWS, and PS. Data are from 102 cells recorded during W and SWS and from 53 cells recorded during W, SWS, and PS. During SWS, thalamic cells behaved as an homogeneous population: as compared with W, most of them (97/102 cells) exhibited decreased evoked spike rates. The frequency selectivity was enhanced and the RF size was reduced. In contrast during PS, two populations of cells were identified: one (32/53 cells) showed the same pattern of changes as during SWS, whereas the other (21/53 cells) expressed values of evoked spike rates and RF properties that did not significantly differ from those in W. These two populations were equally distributed in the different anatomical divisions of the auditory thalamus. Last, during both SWS and PS, the responses latency was longer and the acoustic threshold was higher than in W but the proportion of monotonic versus nonmonotonic rate-level functions was unchanged. During both SWS and PS, no relationship was found between the changes in burst percentage and the changes of the RF properties. These results point out the dual aspect of sensory processing during sleep. On the one hand, they show that the auditory messages sent by thalamic cells to cortical neurons are reduced both in terms of firing rate at a given frequency and in terms of frequency range. On the other hand, the fact that the frequency selectivity and the rate-level function are preserved suggests that the messages sent to cortical cells are not deprived of informative content, and that the analysis of complex acoustic sounds should remain possible. This can explain why, although attenuated, reactivity to biologically relevant stimuli is possible during sleep.

The effect of sleep state on electroretinographic (ERG) activity during early human development

To assess the effects of sleep state on human retinal electric responses, full-field electroretinograms were obtained in a cross-sectional study from 123 preterm infants at 36, 40 and 56 weeks of post-gestational age. At each age, electroretinographic recordings were assigned to one of two groups according to whether the infants were in active sleep or quiet sleep. Both sleep states were determined behaviorally. Pure rod, maximal, 30 Hz flicker and light adapted single cone responses were evaluated when a sleep state was clearly established. Peak-to-peak amplitudes of most electroretinographic responses were significantly larger in active sleep relative to quiet sleep at 36 and 40 weeks of post-gestational age. We speculate that larger amplitudes during active sleep may play a role in the maturation of the visual system.

Do auditory responses recorded from awake animals reflect the anatomical parcellation of the auditory thalamus?

Previous studies performed in anesthetized animals have shown differences between the acoustic responses of neurons recorded from the different divisions of the medial geniculate body (MGB). This study aimed at determining whether or not such differences are also expressed when neurons are recorded from awake animals. The auditory responses of 130 neurons of the auditory thalamus were determined in awake, restrained guinea pigs while the state of vigilance of the animals was continuously monitored. There were significantly more 'on' phasic evoked responses and significantly fewer 'non-responsive' or 'labile' cells in the ventral division of the MGB (MGv) than in the other divisions. The response latencies and the variability of the latencies were smaller in the MGv than in the other divisions. The tuning of the neurons obtained from MGv and from the lateral part of the posterior complex were significantly sharper than those coming from the dorsal division of the MGB and the medial division. The mean threshold and the percentage of monotonic vs. non-monotonic intensity functions were not different in the subdivisions of the auditory thalamus. When compared with previous studies, the quantifications of the acoustic responses obtained in the present study gave values that differed from those reported under deep anesthesia, but were close to those reported under light anesthesia. Lastly, even if none of the physiological characteristic makes it possible, by itself, to determine the locus of recordings in the auditory thalamus, we conclude that the physiological characteristics of the evoked responses obtained in MGv differ from those of other divisions.

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.

Auditory cortical efferent actions upon inferior colliculus unitary activity in the guinea pig

Differential actions on inferior colliculus central nucleus (ICc) single cells spontaneous activity were observed with both ipsilateral and contralateral auditory cortical electrical stimulation (ACx stimulation). Following ACx stimulation, a firing depression of the spontaneous activity was obtained using contralateral or ipsilateral cortical stimulation, although a greater effect was elicited by the contralateral cortex. In contrast, ipsilateral ACx stimulation elicited more excitation with a shorter latency than contralateral stimulation. In units that failed to show spontaneous firing, the sound-evoked responses and ACx stimulation were studied; approximately 50% of them demonstrated firing depression to ACx stimulation on either side with either clicks or tone-bursts. Thirty percent of the units failed to show changes in response to any cortical stimulation. A temporary disruption of ICc-evoked neuronal discharge was elicited during contralateral cortex stimulation, as previously reported to occur during sleep. The demonstration that auditory cortices may differentially affect the same ICc unit activity, i.e. spontaneous and evoked, suggests that auditory processing may depend on the ongoing spontaneous activity plus the effects exerted from each auditory cortex activation.

In vivo intracellular characteristics of inferior colliculus neurons in guinea pigs

Intracellular in vivo recordings of physiologically identified inferior colliculus central nucleus (ICc) auditory neurons (n = 71) were carried out in anesthetized guinea pigs. The neuronal membrane characteristics are described showing mainly quantitative differences with a previous report [Nelson, P.G. and Erulkar, S.D., J. Neurophysiol., 26 (1963) 908-923]. The spontaneous spike activity was consistent with the discharge pattern of most extracellularly recorded units. The action potentials showed different spike durations, short and long, and some of them exhibited hyperpolarizing post-potentials. There were also differences in firing rate. The ICc neurons exhibited irregular activity producing spike trains as well as long silent periods (without spikes). Intracellular current injection revealed membrane potential adaptation and shifts that outlasted the electrical stimuli by 20-30 ms. Both evoked synaptic potentials and the spike activity in response to click and tone-burst stimulation were analyzed. Depolarizing-hyperpolarizing synaptic potentials were found in response to contralateral and binaural sound stimulation that far outlasted the stimulus (up to 90 ms). When ipsilaterally stimulated, inhibitory responses and no-responses were also recorded. Although few cells were studied, a similar phenomenon was observed using tone-burst stimulation; moreover, a good correlation was obtained between membrane potential shifts and the triggered spikes (input-output relationship). These in vivo results demonstrate the synaptic activity underlying many of the extracellularly recorded discharge patterns. The data are consistent with the known multi-synaptic ascending pathway by which signals arrive at the ICc as well as the descending corticofugal input that may contribute to the generation of long duration post-synaptic potentials.

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.