Time evolution of noise induced oxidation in outer hair cells: role of NAD(P)H and plasma membrane fluidity.

BACKGROUND: Noise exposure impairs outer hair cells (OHCs). The common basis for OHC dysfunction and loss by acoustic over-stimulation is represented by reactive oxygen species (ROS) overload that may affect the membrane structural organization through generation of lipid peroxidation. METHODS: Here we investigated in OHC different functional zones the mechanisms linking metabolic functional state (NAD(P)H intracellular distribution) to the generation of lipid peroxides and to the physical state of membranes by two photon fluorescence microscopy. RESULTS: In OHCs of control animals, a more oxidized NAD(P)H redox state is associated to a less fluid plasma membrane structure. Acoustic trauma induces a topologically differentiated NAD(P)H oxidation in OHC rows, which is damped between 1 and 6h. Peroxidation occurs after ~4h from noise insult, while ROS are produced in the first 0.2h and damage cells for a period of time after noise exposure has ended (~7.5h) when a decrease of fluidity of OHC plasma membrane occurs. OHCs belonging to inner rows, characterized by a lower metabolic activity with respect to other rows, show less severe metabolic impairment. CONCLUSIONS: Our data indicate that plasma membrane fluidity is related to NAD(P)H redox state and lipid peroxidation in hair cells. GENERAL SIGNIFICANCE: Our results could pave the way for therapeutic intervention targeting the onset of redox umbalance.

Noise-induced hearing loss (NIHL) as a target of oxidative stress-mediated damage: cochlear and cortical responses after an increase in antioxidant defense.

This study addresses the relationship between cochlear oxidative damage and auditory cortical injury in a rat model of repeated noise exposure. To test the effect of increased antioxidant defenses, a water-soluble coenzyme Q10 analog (Qter) was used. We analyzed auditory function, cochlear oxidative stress, morphological alterations in auditory cortices and cochlear structures, and levels of coenzymes Q9 and Q10 (CoQ9 and CoQ10, respectively) as indicators of endogenous antioxidant capability. We report three main results. First, hearing loss and damage in hair cells and spiral ganglion was determined by noise-induced oxidative stress. Second, the acoustic trauma altered dendritic morphology and decreased spine number of II-III and V-VI layer pyramidal neurons of auditory cortices. Third, the systemic administration of the water-soluble CoQ10 analog reduced oxidative-induced cochlear damage, hearing loss, and cortical dendritic injury. Furthermore, cochlear levels of CoQ9 and CoQ10 content increased. These findings indicate that antioxidant treatment restores auditory cortical neuronal morphology and hearing function by reducing the noise-induced redox imbalance in the cochlea and the deafferentation effects upstream the acoustic pathway.

Cisplatin Chemotherapy and Cochlear Damage: Otoprotective and Chemosensitization Properties of Polyphenols.

Significance: Cisplatin is an important component of treatment regimens for different cancers. Notwithstanding that therapeutic success often results from partial efficacy or stabilizing the disease, chemotherapy failure is driven by resistance to drug treatment and occurrence of side effects, such as progressive irreversible ototoxicity. Cisplatin's side effects, including ototoxicity, are often dose limiting. Recent Advances: Cisplatin ototoxicity results from several mechanisms, including redox imbalance caused by reactive oxygen species production and lipid peroxidation, activation of inflammation, and p53 and its downstream pathways that culminate in apoptosis. Considerable efforts in research have targeted development of molecular interventions that can be concurrently administered with cisplatin or other chemotherapies to reduce side effect toxicities while preserving or enhancing the antineoplastic effects. Evidence from studies has indicated some polyphenols, such as curcumin, can help to regulate redox signaling and inflammatory effects. Furthermore, polyphenols can exert opposing effects in different types of tissues, that is, normal cells undergoing stressful conditions versus cancer cells. Critical Issues: This review article summarizes evidence of curcumin antioxidant effect against cisplatin-induced ototoxicity that is converted to a pro-oxidant activity in cisplatin-treated cancer cells, thus providing an ideal chemosensitivity combined with otoprotection. Polyphenols can modulate the adaptive responses to stress in the cisplatin-exposed cochlea. These adaptive effects can result from the interaction/cross talk between the cell's defenses, inflammatory molecules, and the key signaling molecules of signal transducers and activators of transcription 3 (STAT-3), nuclear factor κ-B (NF-κB), p53, and nuclear factor erythroid 2-related factor 2 (Nrf-2). Future Directions: We provide molecular evidence for alternative strategies for chemotherapy with cisplatin addressing the otoprotection and chemosensitization properties of polyphenols. Antioxid. Redox Signal. 36, 1229-1245.

Early Noise-Induced Hearing Loss Accelerates Presbycusis Altering Aging Processes in the Cochlea.

Several studies identified hearing loss as a risk factor for aging-related processes, including neurodegenerative diseases, as dementia and age-related hearing loss (ARHL). Although the association between hearing impairment in midlife and ARHL has been widely documented by epidemiological and experimental studies, the molecular mechanisms underlying this association are not fully understood. In this study, we used an established animal model of ARHL (C57BL/6 mice) to evaluate if early noise-induced hearing loss (NIHL) could affect the onset or progression of age-related cochlear dysfunction. We found that hearing loss can exacerbate ARHL, damaging sensory-neural cochlear epithelium and causing synaptopathy. Moreover, we studied common pathological markers shared between hearing loss and ARHL, demonstrating that noise exposure can worsen/accelerate redox status imbalance [increase of reactive oxygen species (ROS) production, lipid peroxidation, and dysregulation of endogenous antioxidant response] and vascular dysfunction [increased expression of hypoxia-inducible factor-1alpha (HIF-1α) and vascular endothelial growth factor C (VEGFC)] in the cochlea. Unveiling the molecular mechanisms underlying the link between hearing loss and aging processes could be valuable to identify effective therapeutic strategies to limit the effect of environmental risk factors on age-related diseases.

Post-processing analysis of transient-evoked otoacoustic emissions to detect 4 kHz-notch hearing impairment--a pilot study.

BACKGROUND: To identify a parameter to distinguish normal hearing from hearing impairment in the early stages. The parameter was obtained from transient-evoked otoacoustic emissions (TEOAEs), overcoming the limitations of the usually adopted waveform descriptive parameters which may fail in standard clinical screenings. MATERIAL/METHODS: Audiometric examinations and TEOAE analysis were conducted on 15 normal ears and on 14 hearing-impaired ears that exhibited an audiometric notch around 4 kHz. TEOAE signals were analyzed through a multivariate technique to filter out the individual variability and to highlight the dynamic structure of the signals. The new parameter (named radius 2-dimension--RAD2D) was defined and evaluated for simulated TEOAE signals modeling a different amount of hearing impairment. RESULTS: Audiometric examinations indicated 14 ears as impaired-hearing (IH), while the TEOAE ILO92 whole reproducibility parameter (WWR) indicated as IH 7 signals out of 14 (50%). The proposed new parameter indicated as IH 9 signals out of 14 (64%), reducing the number of false negative cases of WWR. CONCLUSIONS: In this preliminary study there is evidence that the new parameter RAD2D defines the topology and the quantification of the damage in the inner ear. The proposed protocol can be useful in hearing screenings to identify hearing impairments much earlier than conventional pure tone audiometry and TEOAE pass/fail test.

The work by Giulio Ceradini in explaining the mechanism of semilunar cardiac valve function.

Using an excised pig heart preparation with tubes, a manometer, and a visualizing apparatus, Giulio Ceradini, an Italian physiologist working in the years of 1871-1872 in Carl Ludwig's famous laboratory in Leipzig, Germany, illustrated the mechanism of closure of the semilunar valves. He was the first to conceive that the closure of the heart valves depends not on a static back pressure nor upon eddies but is primarily the consequence of the decelerated systolic efflux. This pioneer research of Ceradini was first published in German in 1872 (4). The purpose of the present report is to revisit Ceradini's pioneering experiments and his interpretation of heart valve closure, which remains as true as it was in 1872.

Noise-Induced Cochlear Damage Involves PPAR Down-Regulation through the Interplay between Oxidative Stress and Inflammation.

The cross-talk between oxidative stress and inflammation seems to play a key role in noise-induced hearing loss. Several studies have addressed the role of PPAR receptors in mediating antioxidant and anti-inflammatory effects and, although its protective activity has been demonstrated in several tissues, less is known about how PPARs could be involved in cochlear dysfunction induced by noise exposure. In this study, we used an in vivo model of noise-induced hearing loss to investigate how oxidative stress and inflammation participate in cochlear dysfunction through PPAR signaling pathways. Specifically, we found a progressive decrease in PPAR expression in the cochlea after acoustic trauma, paralleled by an increase in oxidative stress and inflammation. By comparing an antioxidant (Q-ter) and an anti-inflammatory (Anakinra) treatment, we demonstrated that oxidative stress is the primary element of damage in noise-induced cochlear injury and that increased inflammation can be considered a consequence of PPAR down-regulation induced by ROS production. Indeed, by decreasing oxidative stress, PPARs returned to control values, reactivating the negative control on inflammation in a feedback loop.

Styrene targets sensory and neural cochlear function through the crossroad between oxidative stress and inflammation.

BACKGROUND: Although styrene is an established ototoxic agent at occupational exposure levels, the mechanisms of styrene toxicity in the auditory system are still unclear. OBJECTIVES: The aim of this study was to identify the consequences of styrene chronic exposure in cochlear structures, looking for the mechanisms of ototoxicity of this organic compound and focusing on cell targets and oxidative stress/inflammatory processes. METHODS: Male adult Wistar rats were exposed to styrene (400 mg/kg by gavage for 5 days/week, 3 consecutive weeks). Hearing loss was evaluated by measuring auditory brainstem responses (ABR), morphological analysis were performed to evaluate hair cell and spiral ganglion neuron survival, as well as synaptic damage. Analysis of apoptotic (p53) and inflammatory (NF-κB, TNF-α, IL-1ß and IL-10) mediators were performed by immunofluorescence analysis and western blot. RESULTS: Styrene ototoxic effects induced a hearing loss of about 35-40 dB. Immunofluorescence and western blotting analyses demonstrated that styrene administration induced redox imbalance and activated inflammatory processes, targeting sensory hair cell and neural dysfunction by a cross-talk between oxidative and inflammatory mediators. DISCUSSION: Major findings connect styrene ototoxicity to an interplay between redox imbalance and inflammation, leading to the intriguing assumption of a mixed sensory and neural styrene-induced ototoxicity. Thus, in a clinical perspective, data reported here have important implications for styrene risk assessment in humans.

The dual role of curcumin and ferulic acid in counteracting chemoresistance and cisplatin-induced ototoxicity.

Platinum-based agents, such as cisplatin, form the mainstay of currently used chemotherapeutic regimens for several malignancies; however, the main limitations are chemoresistance and ototoxic side effects. In this study we used two different polyphenols, curcumin and ferulic acid as adjuvant chemotherapeutics evaluating (1) in vivo their antioxidant effects in protecting against cisplatin ototoxicity and (2) in vitro the transcription factors involved in tumor progression and cisplatin resistance. We reported that both polyphenols show antioxidant and oto-protective activity in the cochlea by up-regulating Nrf-2/HO-1 pathway and downregulating p53 phosphorylation. However, only curcumin is able to influence inflammatory pathways counteracting NF-κB activation. In human cancer cells, curcumin converts the anti-oxidant effect into a pro-oxidant and anti-inflammatory one. Curcumin exerts permissive and chemosensitive properties by targeting the cisplatin chemoresistant factors Nrf-2, NF-κB and STAT-3 phosphorylation. Ferulic acid shows a biphasic response: it is pro-oxidant at lower concentrations and anti-oxidant at higher concentrations promoting chemoresistance. Thus, polyphenols, mainly curcumin, targeting ROS-modulated pathways may be a promising tool for cancer therapy. Thanks to their biphasic activity of antioxidant in normal cells undergoing stressful conditions and pro-oxidant in cancer cells, these polyphenols probably engage an interplay among the key factors Nrf-2, NF-κB, STAT-3 and p53.

Publisher Correction: The dual role of curcumin and ferulic acid in counteracting chemoresistance and cisplatin-induced ototoxicity.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Anti-oxidant and anti-inflammatory effects of caffeic acid: in vivo evidences in a model of noise-induced hearing loss.

SCOPE: The imbalance of cellular redox status, in conjunction with the activation of inflammatory processes, have been considered common predominant mechanisms of noise-induced hearing loss. The identification of novel natural products as potential therapeuticstargeting oxidative stress and inflammatory pathways is an emerging field. Here, we focused on the polyphenol caffeic acid (CA), the major representative of hydroxycinnamic acids and phenolic acid, in order to investigate its protective capacity in a model of sensorineural hearing loss induced by noise. METHODS AND RESULTS: Hearing loss was induced by exposing animals (Wistar rats) to a pure tone, 120 dB, 10 kHz for 60 min. By using auditory brainstem responses (ABRs) and immunofluorescence analysis, we found that CA protects auditory function and limits cell death in the cochlear middle/basal turn, damaged by noise exposure. Immunofluorescence analysis provided evidence that CA mediates multiple cell protection mechanisms involving both anti-inflammatory and anti-oxidant effects by decreasing NF-κB and IL-1ß expression in the cochlea and opposing the oxidative/nitrosative damage induced by noise insult. CONCLUSIONS: These results demonstrate that the supplementation of polyphenol CA can be considered a valid therapeutic strategy for attenuating noise-induced hearing loss and cochlear damage targeting both inflammatory signalling and cochlear redox balance.

Noise induced hearing loss and vestibular dysfunction in the guinea pig.

This study analysed the acoustic and vestibular functional and morphological modifications in guinea pigs after acoustic trauma. Animals were exposed to noise (6 kHz, at 120 dB SPL for 60 minutes) and then auditory brainstem responses (ABR) and vestibuloocular reflex (VOR) were measured at 6 hours, 1 day, 3, 7, and 21 days after noise. Western blotting and immunostaining for 4-hydroxy-2-noneal (4-HNE) and vascular endothelial growth factor (VEGF) were performed in the cochlear and vestibular regions at 1 and 7 days after noise exposure. A significant decrease of VOR gain was observed on day 1 and the recovery was completed at day 21. ABR threshold values reached a level of 80 dB at day 1 after trauma reaching a value of about 50 dB SPL on day 21. 4-HNE expression, a marker of lipid peroxidation was strongly increased in the cochlea. In the vestibule, 4-HNE immunoreactivity was faint. However, VEGF was up-regulated both in the cochlea and vestibule. In conclusion, the expression of VEGF in both cochlear and vestibular structures suggests a reparative role with potentially therapeutic implications.

Targeting dysregulation of redox homeostasis in noise-induced hearing loss: Oxidative stress and ROS signaling.

Hearing loss caused by exposure to recreational and occupational noise remains a worldwide disabling condition and dysregulation of redox homeostasis is the hallmark of cochlear damage induced by noise exposure. In this review we discuss the dual function of ROS to both promote cell damage (oxidative stress) and cell adaptive responses (ROS signaling) in the cochlea undergoing a stressful condition such as noise exposure. We focus on animal models of noise-induced hearing loss (NIHL) and on the function of exogenous antioxidants to maintaining a physiological role of ROS signaling by distinguishing the effect of exogenous "direct" antioxidants (i.e. CoQ10, NAC), that react with ROS to decrease oxidative stress, from the exogenous "indirect" antioxidants (i.e. nutraceutics and phenolic compounds) that can activate cellular redox enzymes through the Keap1-Nrf2-ARE pathway. The anti-inflammatory properties of Nrf2 signaling are discussed in relation to the ROS/inflammation interplay in noise exposure. Unveiling the mechanisms of ROS regulating redox-associated signaling pathways is essential in providing relevant targets for innovative and effective therapeutic strategies against NIHL.

Antioxidant protection against acoustic trauma by coadministration of idebenone and vitamin E.

Idebenone, a synthetic analogue of coenzyme Q, attenuates noise-induced hearing loss by virtue of its antioxidant properties. This study involves a guinea pig model of acoustic trauma where the effectiveness of idebenone is analyzed in comparison with Vitamin E (alpha-tocopherol) that exhibits a potent antioxidant activity in the inner ear. Idebenone and vitamin E were injected intraperitoneally 1 h before noise exposure and once daily for three days; functional and morphological studies were then carried out, respectively, by auditory brainstem responses evaluation, scanning electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay identification of missing and apoptotic cells was also performed. The results showed that the protective effects of idebenone and vitamin E were not additive implying that the two antioxidants may share competitive mechanisms.

Pioglitazone Represents an Effective Therapeutic Target in Preventing Oxidative/Inflammatory Cochlear Damage Induced by Noise Exposure.

Recent progress in hearing loss research has provided strong evidence for the imbalance of cellular redox status and inflammation as common predominant mechanisms of damage affecting the organ of Corti including noise induced hearing loss. The discovery of a protective molecule acting on both mechanisms is challenging. The thiazolidinediones, a class of antidiabetic drugs including pioglitazone and rosiglitazone, have demonstrated diverse pleiotrophic effects in many tissues where they exhibit anti-inflammatory, anti-proliferative, tissue protective effects and regulators of redox balance acting as agonist of peroxisome proliferator-activated receptors (PPARs). They are members of the family of ligand regulated nuclear hormone receptors that are also expressed in several cochlear cell types, including the outer hair cells. In this study, we investigated the protective capacity of pioglitazone in a model of noise-induced hearing loss in Wistar rats and the molecular mechanisms underlying this protective effects. Specifically, we employed a formulation of pioglitazone in a biocompatible thermogel providing rapid, uniform and sustained inner ear drug delivery via transtympanic injection. Following noise exposure (120 dB, 10 kHz, 1 h), different time schedules of treatment were employed: we explored the efficacy of pioglitazone given immediately (1 h) or at delayed time points (24 and 48 h) after noise exposure and the time course and extent of hearing recovery were assessed. We found that pioglitazone was able to protect auditory function at the mid-high frequencies and to limit cell death in the cochlear basal/middle turn, damaged by noise exposure. Immunofluorescence and western blot analysis provided evidence that pioglitazone mediates both anti-inflammatory and anti-oxidant effects by decreasing NF-κB and IL-1ß expression in the cochlea and opposing the oxidative damage induced by noise insult. These results suggest that intratympanic pioglitazone can be considered a valid therapeutic strategy for attenuating noise-induced hearing loss and cochlear damage, reducing inflammatory signaling and restoring cochlear redox balance.

Anodal transcranial direct current stimulation affects auditory cortex plasticity in normal-hearing and noise-exposed rats.

BACKGROUND: Transcranial direct current stimulation (tDCS) is a non-invasive tool capable to modulate cortical functions by affecting neuronal excitability and synaptic plasticity. OBJECTIVE: Here we investigated the effects of anodal tDCS on auditory cortex (ACx) in normal-hearing rats and following a paradigm of noise-induced hearing loss (NIHL), that causes morphological alterations in ACx pyramidal neurons. METHODS: Male rats exposed to intense pure tone (10 kHz) were subsequently subjected to unilateral anodal tDCS of ACx and changes in dendritic morphology and spines were assessed by Golgi-Cox staining 30 days after the onset of the acoustic trauma. Molecular and functional changes were investigated by Western immunoblotting, immunofluorescence experiments and electrophysiological recordings in brain slices. RESULTS: We found that NIHL altered dendritic morphology by decreasing spine density, mostly in layer 2/3 pyramidal neurons. Interestingly, tDCS increased ACx spine density, targeting apical dendrites of layer 2/3 and 5/6 pyramidal neurons in rats with normal auditory function and both apical and basal arborizations in layer 2/3 of NIHL rats. Twenty-four hours after tDCS, Bdnf and synaptophysin levels in ACx increased both in normal-hearing and noise-exposed rats. Field recordings showed that basal synaptic transmission at layer 2/3 horizontal connections was significantly reduced in noise-exposed rats compared to normal-hearing animals and, notably, input-output curves of noise-exposed animals subjected to tDCS were similar to those of normal-hearing rats. CONCLUSIONS: Our findings provide novel evidence that anodal tDCS affects structural plasticity in the ACx suggesting that it might be beneficial in treating cortical alterations due to cochlear damage.

The Antioxidant Effect of Rosmarinic Acid by Different Delivery Routes in the Animal Model of Noise-Induced Hearing Loss.

HYPOTHESIS: Trans-tympanic Rosmarinic Acid (RA), as compared with the systemic administration, protects against noise-induced auditory hair cell and hearing losses in rats in vivo. BACKGROUND: ROS production, lipoperoxidative damage, and an imbalance of antioxidant defences play a significant role in noise-induced hearing loss. Several molecules with antioxidant properties have been tested to restore redox homeostasis; however, drug delivery system represents a challenge for their effectiveness. In our model, acute and intense noise exposure induces hearing loss, hair cell death, and oxidative stress, with an increase in superoxide production and over-expression of lipid peroxidation in cochlear structures. METHODS: RA was administrated in male Wistar rats by trans-tympanic (20 µl) and systemic (10 mg/kg) modality. In systemic administration, RA was injected 1 hour before noise exposure and once daily for the following 3 days. ABRs were measured before and at days 1, 3, 7, and 30 after noise exposure. Rhodamine-phalloidin staining, dihydroethidium and 8-isoprostane immunostainings were performed to assess and quantify outer hair cells loss, superoxide production, and lipid peroxidation in the different experimental groups. RESULTS: Systemic RA administration significantly decreased noise-induced hearing loss and the improvement of auditory function was paralleled by a significant reduction in cochlear oxidative stress. The trans-tympanic modality of drug administration showed a similar degree of protection both at the functional and morphological levels. CONCLUSION: The effectiveness of RA given via trans-tympanic injection could be interesting for the future application of this minimally-invasive procedure in the treatment of ROS-induced hearing loss.

A tribute to Italian physiologists of Jewish descent evicted during the persecution ordered by the Fascist Regime in 1938.

The present report commemorates the persecution of five renowned Italian physiologists of Jewish descent that lost their chairs in medical schools because of the anti-semitic policies of the fascist regime. In 1938, Mussolini promulgated the Racial Laws, officially with the aim of safeguarding the purity of the Italian race in conquered African colonies. However, their true intent was to persecute the Italian Jewish community in agreement with the policy of Nazi Germany. In accordance with the Racial Laws, all non-Aryans were banished from professional activities and were evicted from public, social, and academic life. As a result, 98 full professors in Italian universities were removed from their academic positions. In medical schools, physiology, more than other discipline, lost the most prominent faculty members. Of the 17 full Professors of Human Physiology, five were of Jewish descent, and all were evicted: they were Camillo Artom from Palermo, Mario Camis from Bologna, Carlo Foà from Milan, Amedeo Herlitzka from Turin, and Ugo Lombroso from Genoa. All were talented and famous scientists who were forced to leave Italy and take refuge in foreign countries. At the end of World War II, Camis, Foà, Herlitzka, and Lombroso returned to Italy and resumed their previous academic positions, whereas Artom remained in the United States. Unfortunately, Camis died later that year. During the postwar period, some of the fascists responsible for the Jewish persecution were killed or committed suicide while the survivors were imprisoned and prosecuted. However, all were soon released and resumed their former positions.

Protective properties of idebenone in noise-induced hearing loss in the guinea pig.

Idebenone is a synthetic analogue of coenzyme Q10 with antioxidant properties. The present study investigated the antioxidant activity of idebenone in the rescue of acoustic trauma. Noise-induced hearing loss was induced by exposing guinea pigs to a continuous pure tone and idebenone was injected intraperitoneally 1 h before noise exposure and once daily for 3 days. Guinea pigs treated with idebenone showed significantly smaller auditory threshold shifts than unprotected control animals. Missing and apoptotic cells were identified with scanning electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay. Protected animals presented a lesser extent of both apoptotic activation and hair cell loss in the organ of Corti. Our results suggest an antioxidant function of idebenone in protection from noise-induced hearing loss and provide a rationale for exploring therapeutic strategies in humans.

Spatial orientation of periodic alternating drift (PAD).

Sinusoidal vestibular stimulation induces in the intact rabbit in prone position a periodic alternating drift (PAD), evident in the earth horizontal plane when the animal is rotated about the vertical axis but weak in the vertical one when the animal is rotated about the longitudinal axis. It has been hypothesized that these oscillations are related to an intrinsic instability of the velocity storage, due to the length of its time constant. The velocity storage has the longest time constant aligned with the vertical axis, and it changes its orientation with the gravity vector. The present research examined the spatial orientation of PAD in relation to changes of the animal position with respect to gravity. Normal pigmented rabbits were sinusoidally oscillated about their longitudinal axes to evoke vertical eye responses. The stimulation was carried out with the animal in prone position and with the animal in nose-up condition. With the animal in prone position, PAD had a weak vertical component, but an evident horizontal component was visible. When the animal was in nose-up position, the horizontal component of PAD was clearly visible, while the vertical component was negligible. In both stimulation conditions PAD period and peak velocity were not modulated by the stimulus characteristics. These results are consistent with a model of PAD based on an interaction between velocity storage and the cerebellar adaptation-habituation circuit.