Changes of acoustic nerve and cochlear nucleus evoked potentials due to repetitive stimulation

Neural adaptation to tone sequences in the songbird forebrain: patterns, determinants, and relation to the build-up of auditory streaming

Neural responses to tones in the mammalian primary auditory cortex (A1) exhibit adaptation over the course of several seconds. Important questions remain about the taxonomic distribution of multi-second adaptation and its possible roles in hearing. It has been hypothesized that neural adaptation could explain the gradual "build-up" of auditory stream segregation. We investigated the influence of several stimulus-related factors on neural adaptation in the avian homologue of mammalian A1 (field L2) in starlings (Sturnus vulgaris). We presented awake birds with sequences of repeated triplets of two interleaved tones (ABA-ABA-...) in which we varied the frequency separation between the A and B tones (DeltaF), the stimulus onset asynchrony (time from tone onset to onset within a triplet), and tone duration. We found that stimulus onset asynchrony generally had larger effects on adaptation compared with DeltaF and tone duration over the parameter range tested. Using a simple model, we show how time-dependent changes in neural responses can be transformed into neurometric functions that make testable predictions about the dependence of the build-up of stream segregation on various spectral and temporal stimulus properties.

Direct comparison between properties of adaptation of the auditory nerve and the ventral cochlear nucleus in response to repetitive clicks

The present study was designed to complete two previous reports [Loquet, G., Rouiller, E.M., 2002. Neural adaptation to pulsatile acoustical stimulation in the cochlear nucleus of the rat. Hear. Res. 171, 72-81; Loquet, G., Meyer, K., Rouiller, E.M., 2003. Effects of intensity of repetitive acoustic stimuli on neural adaptation in the ventral cochlear nucleus of the rat. Exp. Brain Res. 153, 436-442] on neural adaptation properties in the auditory system of the rat. Again, auditory near-field evoked potentials (ANEPs) were recorded in response to 250-ms trains of clicks from an electrode chronically implanted in the ventral cochlear nucleus (VCN). Up to now, our interest had focused on the adaptive behavior of the first one (N1) of the two negative ANEP components. A re-examination of our data for the second negative component (N2) was now undertaken. Results show that the adaptation time course observed for N2 displayed the same three-stage pattern previously reported for N1. Similarly, adaptation became more pronounced and occurred faster as stimulus intensity and/or repetition rate were increased. Based on latency data which suggest N1 and N2 to be mainly due to the activity of auditory-nerve (AN) fibers and cochlear nucleus neurons, respectively, it was concluded that neural adaptation assessed by gross-potentials was similar in the AN and VCN. This finding is meaningful in the context of our search to restore normal adaptation phenomena via electro-auditory hearing with an auditory brainstem implant on the same lines as our work in cochlear implants.

Matching the neural adaptation in the rat ventral cochlear nucleus produced by artificial (electric) and acoustic stimulation of the cochlea

To investigate neural adaptive properties, near-field evoked potentials were recorded from a chronically implanted electrode in the ventral cochlear nucleus in awake Long-Evans rats exposed to acoustic stimuli or receiving intracochlear electric stimulation. Stimuli were 250-ms trains of repetitive acoustic clicks (10, 30 and 50 dB SPL) or biphasic electric pulses (30, 50 and 70 microA) with intratrain pulse rates ranging from 100 to 1000 pulses per second (pps). The amplitude of the first negative (N(1)) to positive (P(1)) component of the average evoked potentials was measured for each consecutive individual pulse in the train. While a progressive exponential decrease in N(1)-P(1) amplitude was observed as a function of the position of the pulse within the train for both types of stimulation, the decrement of electric responses (adaptive pattern) was substantially less prominent than that observed for acoustic stimuli. Based on this difference, the present work was extended by modifying electric stimuli in order to try to restore normal adaptation phenomena. The results suggest the feasibility of mimicking acoustic adaptation by stimulation with exponentially decreasing electric pulse trains, which may be clinically applicable in the auditory implant field.

Effects of intensity of repetitive acoustic stimuli on neural adaptation in the ventral cochlear nucleus of the rat

To study neural adaptation as a function of stimulus intensity, auditory near-field evoked potentials were recorded from the ventral cochlear nucleus in awake Long Evans rats. Responses to 250-ms trains of repetitive clicks (pulse rates ranging from 100 to 1000 pulses per second) were collected at stimulus intensities of 5, 10, 30, 50 and 70 dB SPL. The amplitude of the first negative (N1) component of the average evoked potentials to individual pulses in the train was measured by using a subtraction method. The N1 responses were normalized with respect to the highest cochlear nucleus potential observed in the train, and then plotted as a function of click position in the train. As expected, the general trend of the curves was an exponential decay reaching a plateau more or less rapidly as a function of both intensity and rate of stimulation. Fitting these curves with exponential decay equations revealed that the rapid time constant decreased for increasing stimulus intensities whereas the short-term time constant is relatively independent of intensity. The amount of adaptation (expressed as the ratio of the plateau to the first peak amplitude) was substantially less prominent at low intensities (5-10 dB SPL) and low rates (100-200 pulses per second) than at higher intensities and high rates. These results indicate that adaptation patterns obtained in the ventral cochlear nucleus by using near-field evoked potentials exhibit properties comparable to those already present at the level of the auditory nerve.

Neural adaptation to pulsatile acoustical stimulation in the cochlear nucleus of the rat

This study, carried out in adult Long-Evans rats, was designed to investigate the adaptive properties of the cochlear nucleus to pulsatile acoustical stimuli. To achieve this purpose, near-field evoked potentials were picked up from the ventral cochlear nucleus in awake animals. Individual auditory thresholds were measured and responses to 250 ms trains of repetitive clicks with pulse rates ranging from 100 to 2000 pulses per second were collected. The amplitude of the first negative (N(1)) component of the evoked potentials to consecutive individual pulses in the train was measured by using a subtraction method. As expected, a rapid amplitude decrement of the responses in the train was obtained and a three phase adaptation was described. The decrease of individual N(1) component amplitude was fitted for each rate of stimulation with exponential decrease equations and time constants were calculated. Such an analysis allowed us to characterize three distinct adaptive processes which were discussed. The results were comparable to those obtained in previous studies in the auditory nerve and suggest that the adaptation recorded in the ventral cochlear nucleus by using near-field evoked potentials reflects the adaptive properties of auditory nerve fibers.

Rate and frequency interactions in the auditory brainstem response of the adult rat

Tone pip stimuli were used at different repetition rates to examine influences upon auditory brainstem response components. Rate increases to 80.1/s result in significant latency increases in waves IV and V at all test frequencies. Rate effects occur in rostral brainstem response components which show frequency related latencies different from wave I. Amplitude measures decline at higher repetition rates for waves I and IV at most test frequencies. Rate effects are most pronounced at 8 kHz stimulation which is around the optimal behavioral frequency in rat. At higher rates the second peak of wave I is enhanced at 8 kHz, whereas the third peak of wave I at 40 kHz disappears. These results demonstrate that tone-specific stimuli provide sensitive measures of latency, amplitude and wave morphology of components of the auditory brainstem response as a function of stimulus repetition.

Adaptive changes in the developing brain during intrauterine stress

Maturation of neurological performance in moderately to severely growth-retarded newborn infants (SGA) can be accelerated by 3 to 4 weeks or more when compared to the development of appropriately grown infants (AGA) of the same gestation. This is particularly the case in multiple pregnancies or pregnancies characterized by maternal hypertension. This clinical finding has been confirmed by neurophysiological studies on the maturation of brainstem auditory evoked responses (BAERs). The possible mechanisms which underly this phenomenon are not yet elucidated. Glucocorticoids, other steroid hormones and catecholamines are elevated in pregnancies with placental dysfunction, and it is known that these substances have multiple actions on neuronal maturation, particularly on mechanisms of release of neurotransmitters. These observations suggest that the acceleration of brain maturation, and lung maturation, in SGA infants reflects an adaptation of the fetus to early extrauterine life. However, if the placental dysfunction progresses, these mechanisms of adaptation will be overwhelmed by severe malnutrition and anoxia which result in cerebral lesions and risk of death. The clinical goal at the present time for obstetric management of these risk pregnancies is to distinguish between these two periods.

Electrophysiological mapping of the auditory areas in the cerebellum of the cat

We mapped the topographic distribution of auditory evoked potentials over the posterior cerebellum of the cat. Within the posterior vermis, positive-going evoked potentials were primarily recorded in lobule VI and negative ones in lobule VII. Peak latencies of these evoked potentials were between 8 and 18 ms. The amplitudes of evoked potentials from adjacent surface cortical sites often differed significantly. Thus, besides the segregation of positive and negative polarities in the evoked potentials from lobules VI and VII, the distribution of cerebellar auditory evoked potentials within a given lobule contained high- and low-amplitude sites arranged in a patchy pattern. Within the posterior vermis, the evoked potentials showed significant polarity and amplitude changes as a function of depth during microelectrode penetrations which were vertical to the surface of the cerebellar cortex. Cerebellar auditory evoked potentials were not detected over the paramedian lobules and most of the cerebellar hemispheres. There were no significant changes in the electrical activities as a function of depth in these areas. Auditory responses, however, were observed in the most lateral part of the cerebellar hemispheres.

Time constants of acoustic adaptation

Evoked potentials were recorded from cochlear nuclei of intact, awake cats through chronically implanted electrodes during stimulation with 10,000 c/sec at 35 dB above the visual detection threshold of evoked potentials. The stimulus rate was varied from 0.5 to 100/sec and the stimulus duration was varied from 1 to 1800 msec but no overlap occurred between successive stimuli. A model was proposed to explain the amplitude of evoked potentials as a function of the stimulus duration and repetition rate. The time constant for the exponential decrease of evoked potential amplitude during stimulation is 8.9 msec, whereas the time constant for the exponential recovery of evoked potentials following the cessation of the tone is 39.5 msec.