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ABSTRACT: Vestibular paradigms are widely used for investigating mechanisms underlying cerebellar motor learning. These include adaptation of the vestibuloocular reflex (VOR) after visual-vestibular mismatch training and vestibular compensation after unilateral damage to the vestibular apparatus. To date, various studies have shown that VOR adaptation may be supported by long-term depression (LTD) at the parallel fiber to Purkinje cell synapse. Yet it is unknown to what extent vestibular compensation may depend on this cellular process. Here we investigated adaptive gain changes in the VOR and optokinetic reflex during vestibular compensation in transgenic mice in which LTD is specifically blocked in Purkinje cells via expression of a peptide inhibitor of protein kinase C (L7-PKCi mutants). The results demonstrate that neither the strength nor the time course of vestibular compensation are affected by the absence of LTD. In contrast, analysis of vestibular compensation in spontaneous mutants that lack a functional olivo-cerebellar circuit (lurchers) shows that this form of motor learning is severely impaired. We conclude that oculomotor plasticity during vestibular compensation depends critically on intact cerebellar circuitry but not on the occurrence of cerebellar LTD.
Journal of Neurophysiology 10/2006; 96(3):1187-95. · 3.32 Impact Factor
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ABSTRACT: The cochlear delay of the 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) was measured using the phase gradient method. With a constant f(2) and swept f(1), the resulting phase change of 2f(1)-f(2) was used to calculate the group delay for f(2) frequencies from 1 to 60 kHz. For f(2) frequencies between 2 and 60 kHz, the group delays were between 2.2 and 0.11 ms and continuously decreased for increasing f(2) and for increasing primary stimulus levels. For f(2) frequencies below 2 kHz, the group delay decreased to around 1 ms and was largely independent of stimulus level. The ratio curves resulting from the f(1) sweeps for high frequencies (f(2)16 kHz) displayed the typical mammalian shape with a peak in the level of 2f(1)-f(2) for a larger primary frequency separation (f(2)/f(1)1.15) and decreasing 2f(1)-f(2) level for smaller primary separation. In addition to this typical level maximum, for f(2) frequencies from about 1.8 to 16 kHz, the ratio curves displayed a second component in the form of an increase in the level of 2f(1)-f(2) for small primary separation at higher primary levels (level of f(2)30 dB SPL). For f(2) frequencies below 1.8 kHz, only the second component and no typical ratio peak as for higher f(2) could be observed and the associated group delay was always close to 0.8 ms. Several possible causes for this behavior are discussed, including different modes of DPOAE generation and modulation as well as changes in the nature of mechanical processing from base to apex in the gerbil cochlea. To evaluate the relative sensitivity of non-linear cochlear mechanics, an iso-distortion threshold curve was constructed from acoustical growth functions of the 2f(1)-f(2) DPOAE at optimum primary separation, by plotting the level of f(2) sufficient to evoke a distortion of -10 dB SPL as a function of f(2)2.5 kHz but failed to reflect the sensitivity for lower frequencies. This may be a consequence of more linear frequency processing in the apex.
Hearing Research 03/2000; 140(1-2):99-110. · 2.70 Impact Factor
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ABSTRACT: Mormoopid bat species have their echolocation system adapted to different hunting strategies. To study the corresponding mechanical properties of their inner ear, we measured distortion-product otoacoustic emissions to assess cochlear sensitivity and tuning. Mormoops blainvillii, Pteronotus macleayii and P. quadridens use frequency-modulated echolocation signals, sometimes preceded by a short narrowband signal component. Their distortion-product otoacoustic emission-threshold curves are most sensitive between 30 and 50 kHz and show no adaptation to the narrowband echolocation components. In contrast, the constant-frequency bat P. parnellii always uses long constant-frequency call components. Its inner ear is maximally sensitive at 62 kHz, the echo-frequency of the dominant constant-frequency component, and pronounced insensitivities at 61 and 93 kHz (CF2 and CF3 call frequency) are the major evolutionary change in comparison to its relatives. Furthermore, in P. parnellii, the optimum cochlear frequency separation is minimal at 62 and 93 kHz, associated with enhanced cochlear tuning, while for the other mormoopids there is no indication of enhanced tuning. The phylogeny of mormoopids, assessed by mitochondrial DNA analysis, shows a close relationship between the Pteronotus species. This suggests that major cochlear redesign, associated with the acquisition of echolocation-call specific cochlear processing in P. parnellii, has occurred within a relatively short evolutionary time scale.
Journal of Comparative Physiology 10/1999; 185(3):217-28. · 2.01 Impact Factor
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ABSTRACT: To receive information on the intracochlear magnitude and propagation properties of cochlear distortion products, the neuronal response of AVCN cells to distortion stimuli and the acoustical correlates in the ear canal (DPOAE) were measured for frequencies between 0.7 and 45.3 kHz. Comparison of the growth of neuronal response to a distortion stimulus and the neuronal rate-versus-level function for an externally applied pure tone of equal frequency allowed for an assessment of the intracochlear magnitude of the distortions. AVCN neurons with a characteristic frequency (CF) > 1.8 kHz started to respond to the intracochlear distortion stimulus, at primary levels for which the ear-canal level of the corresponding DPOAE was close to the pure-tone threshold of the units. This finding suggests that transmission of sound energy is comparable in the forward and reverse direction, and that mechanical distortions of the cochlea are fully encoded by neurons in the AVCN. For neurons with a CF < 1.8 kHz, the intracochlear magnitude of the distortion stimulus appeared to be about 15 to 30 dB higher than the corresponding DPOAE, at the threshold of neuronal response. This discrepancy between intracochlear magnitude of cochlear distortions and their acoustical expression may be explained by high-pass filter action of the middle ear during DPOAE re-emission from the cochlea. A contribution to the observed discrepancy of the type of distortion (cubic versus quadratic) used as stimulus, and possible differences in mechanical frequency processing between the apex and base of the gerbil cochlea, are also discussed. The delay of the neuronal response to an intracochlear distortion stimulus was on average 1.1 ms longer than the neuronal delay to an external pure tone of equal frequency and intensity, most likely stemming from the activation delay of the DPOAE generating mechanisms.
The Journal of the Acoustical Society of America 02/1999; 105(1):491-502. · 1.55 Impact Factor
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ABSTRACT: The acoustic distortion product 2f1-f2 was measured in the ear canal of the gray short-tailed opossum. Monodelphis domestica, during stimulation with two pure tone stimuli of the frequencies f1 and f2 (f1 < f2). From 2f1-2f growth functions for different primary frequencies from 4 to 85 kHz, a distortion threshold curve was obtained describing the relative threshold of non-linear mechanical processes in the cochlea. Distortion products could be measured over the entire investigated frequency range and the distortion audiogram proved sensitive (thresholds < 20 dB SPL) over a wide range from 8 to 55 kHz with a broad minimum around 17.5 kHz. Thresholds steeply increased for frequencies below 8 kHz which can be regarded as an original mammalian feature (Heffner and Masterton, 1980). To assess the tuning characteristics of the cochlea, suppression tuning curves of 2f1-2f were measured. The shape and tuning sharpness of these curves resembled that of modern eutherian mammals with a shallower low-frequency and a steeper high-frequency flank and Q10dB values between 2.4 and 7.12. The optimum primary ratio f2/f1 (best-ratio) was on average 1.172. For f2 frequencies between 20 and 30 kHz the ration curves displayed multiple peaks in periodic intervals. The same peaks were also evident for higher-order distortions (3f1-2f2, 4f1-3f2). It is discussed to which extent this finding is consistent with the second filter theory of Brown et al. (1992) and Allen and Fahey (1993) and how in Monodelphis domestica this filter could work differently, possibly representing a less evolved stage, than in eutherian mammals.
Hearing Research 06/1996; 94(1-2):47-53. · 2.70 Impact Factor
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ABSTRACT: The cochlear delay of the 2f1−f2 distortion product otoacoustic emission (DPOAE) was measured using the phase gradient method. With a constant f2 and swept f1, the resulting phase change of 2f1−f2 was used to calculate the group delay for f2 frequencies from 1 to 60 kHz. For f2 frequencies between 2 and 60 kHz, the group delays were between 2.2 and 0.11 ms and continuously decreased for increasing f2 and for increasing primary stimulus levels. For f2 frequencies below 2 kHz, the group delay decreased to around 1 ms and was largely independent of stimulus level. The ratio curves resulting from the f1 sweeps for high frequencies (f2>16 kHz) displayed the typical mammalian shape with a peak in the level of 2f1−f2 for a larger primary frequency separation (f2/f1>1.15) and decreasing 2f1−f2 level for smaller primary separation. In addition to this typical level maximum, for f2 frequencies from about 1.8 to 16 kHz, the ratio curves displayed a second component in the form of an increase in the level of 2f1−f2 for small primary separation at higher primary levels (level of f2>30 dB SPL). For f2 frequencies below 1.8 kHz, only the second component and no typical ratio peak as for higher f2 could be observed and the associated group delay was always close to 0.8 ms. Several possible causes for this behavior are discussed, including different modes of DPOAE generation and modulation as well as changes in the nature of mechanical processing from base to apex in the gerbil cochlea. To evaluate the relative sensitivity of non-linear cochlear mechanics, an iso-distortion threshold curve was constructed from acoustical growth functions of the 2f1−f2 DPOAE at optimum primary separation, by plotting the level of f2 sufficient to evoke a distortion of −10 dB SPL as a function of f2. This distortion audiogram resembled the neuronal and behavioral audiogram for frequencies >2.5 kHz but failed to reflect the sensitivity for lower frequencies. This may be a consequence of more linear frequency processing in the apex.
Hearing Research.
