Intracellular study of synaptic events related to phase-locking responses of cat cochlear nucleus cells to low frequency tones

Human inferior colliculus activity relates to individual differences in spoken language learning

A challenge to learning words of a foreign language is encoding nonnative phonemes, a process typically attributed to cortical circuitry. Using multimodal imaging methods [functional magnetic resonance imaging-adaptation (fMRI-A) and auditory brain stem responses (ABR)], we examined the extent to which pretraining pitch encoding in the inferior colliculus (IC), a primary midbrain structure, related to individual variability in learning to successfully use nonnative pitch patterns to distinguish words in American English-speaking adults. fMRI-A indexed the efficiency of pitch representation localized to the IC, whereas ABR quantified midbrain pitch-related activity with millisecond precision. In line with neural "sharpening" models, we found that efficient IC pitch pattern representation (indexed by fMRI) related to superior neural representation of pitch patterns (indexed by ABR), and consequently more successful word learning following sound-to-meaning training. Our results establish a critical role for the IC in speech-sound representation, consistent with the established role for the IC in the representation of communication signals in other animal models.

Neural synchrony in ventral cochlear nucleus neuron populations is not mediated by intrinsic processes but is stimulus induced: implications for auditory brainstem implants

The aim of this investigation was to elucidate if neural synchrony forms part of the spike time-based theory for coding of sound information in the ventral cochlear nucleus (VCN) of the auditory brainstem. Previous research attempts to quantify the degree of neural synchrony at higher levels of the central auditory system have indicated that synchronized firing of neurons during presentation of an acoustic stimulus could play an important role in coding complex sound features. However, it is unknown whether this synchrony could in fact arise from the VCN as it is the first station in the central auditory pathway. Cross-correlation analysis was conducted on 499 pairs of multiunit clusters recorded in the urethane-anesthetized rat VCN in response to pure tones and combinations of two tones to determine the presence of neural synchrony. The shift predictor correlogram was used as a measure for determining the synchrony owing to the effects of the stimulus. Without subtraction of the shift predictor, over 65% of the pairs of multiunit clusters exhibited significant correlation in neural firing when the frequencies of the tones presented matched their characteristic frequencies (CFs). In addition, this stimulus-evoked neural synchrony was dependent on the physical distance between electrode sites, and the CF difference between multiunit clusters as the number of correlated pairs dropped significantly for electrode sites greater than 800 microm apart and for multiunit cluster pairs with a CF difference greater than 0.5 octaves. However, subtraction of the shift predictor correlograms from the raw correlograms resulted in no remaining correlation between all VCN pairs. These results suggest that while neural synchrony may be a feature of sound coding in the VCN, it is stimulus induced and not due to intrinsic neural interactions within the nucleus. These data provide important implications for stimulation strategies for the auditory brainstem implant, which is used to provide functional hearing to the profoundly deaf through electrical stimulation of the VCN.

Brain micromotion around implants in the rodent somatosensory cortex

The magnitude of brain tissue micromotion relative to stationary brain implants and its impact on the viability and function of the surrounding brain tissue due to mechanical stresses is poorly understood. The central goal of this study is to characterize surface micromotion in the somatosensory cortex against stationary cylindrical implants. We used a differential variable reluctance transducer (DVRT) in adult rats (n = 6) to monitor micromotion normal to the somatosensory cortex surface. Experiments were performed both in the presence and in the absence of dura mater and displacement measurements were made at three different locations within craniotomies of two different sizes. In anesthetized rats, pulsatile surface micromotion was observed to be in the order of 10-30 microm due to pressure changes during respiration and 2-4 microm due to vascular pulsatility. Brain displacement values due to respiration were significantly lower in the presence of the dura compared to those without the dura. In addition, large inward displacements of brain tissue between 10-60 microm were observed in n = 3 animals immediately following the administration of anesthesia. Such significant micromotion can impact a wide variety of acute and chronic procedures involving any brain implants, precise neurosurgery or imaging and therefore has to be factored in the design of such procedures.

Intracellular marking of physiologically characterized cells in the ventral cochlear nucleus of the cat

In the cat ventral cochlear nucleus, separate neuronal classes have been defined based on morphological characteristics; physiologically defined unit types have also been described based on the shape of post-stimulus-time-histograms in response to tone bursts at characteristic frequency. The aim of the present study was to address directly the issue of how morphological cell types relate to physiological unit types. We used intracellular injections of horseradish peroxidase to stain individual neurons after their response characteristics were determined by intracellular recordings. The maintenance of a continuous negative resting potential, the correspondence of the calculated position of the electrode tip at the time of injection to the location of the stained neuron, and the similarity of response properties collected before and after the injection provide evidence that the injected, stained, and recovered neuron corresponds to the functionally defined unit. In the region around the nerve root in the anteroventral cochlear nucleus, two " primarylike " and one " primarylike with notch" units were "bushy" cells. "Bushy" cells are characterized by primary dendrites arising from one hemisphere of the soma and ramifying repeatedly to produce their bushy dendritic arbor. In this same region, the "chopper" and two "on" units were also "bushy" cells. In the posteroventral cochlear nucleus, the "chopper" unit was a "stellate" cell and the "on" unit was an "octopus" cell. These results are partially consistent with previous conclusions based on correlations established between the regional distribution of physiological unit types and morphological cell types. More importantly, they confirm and extend recent intracellular marking data (Rhode et al., ' 83b ). If our classification schemes have functional significance, we are left with the conclusion that the distinction between "bushy" and "stellate" cells in the auditory nerve root region of the ventral cochlear nucleus does not correspond in any simple way to distinctions between physiological unit types. More than one morphological cell type can exhibit the same particular response patter, and the same morphological cell type can exhibit several different response patterns.

Morphology of primary axosomatic endings in the anteroventral cochlear nucleus of the cat: a study of the endbulbs of Held

The central axons of Type I spiral ganglion neurons travel in the auditory nerve and terminate in the cochlear nucleus. The ascending branches of these axons innervate the anteroventral cochlear nucleus and give rise to large axosomatic endings, called the endbulbs of Held, and smaller boutons. This paper reports a study of the endbulbs of Held, stained by horseradish peroxidase and variants of the Golgi method in kittens 2, 5, 10, 20, and 45 days postnatal and adult cats. Endbulbs tend to fall into two extreme groups with some endbulbs having an intermediate appearance; consequently, we have defined three descriptive stages of endbulbs that are conceived of as representing a developmental sequence. One group of endbulbs is found mostly in kittens younger than 10 days postnatal and is similar to the classic description of endbulbs by Ramón y Cajal ('09). The other extreme group of endbulbs is found mostly in adult cats. In these cases, the parent axonal trunk divides into several thick, gnarled branches that in turn branch again, sometimes repeatedly. These branches display irregular varicosities and form a cup-shaped arborization into which the postsynaptic cell body nestles. A chronology of postnatal endbulb development has been inferred from the relative proportions of the different endbulb stages at various ages. Maturation transforms the endbulb of Held from a large, spoon-shaped swelling having many filipodia into an elaborate tree with broad trunks and many smaller branches. Some implications of the proposed development sequence are discussed.

Quantitative analysis of methods for reducing physiological brain pulsations

Normal movements of the mammalian brain, caused by the arterial and venous pressure fluctuations of each cardiac and respiratory cycle, have made obtaining stable intracellular recordings from neurons difficult. This study quantitated the movements of the cats' brainstem and examined the effects of traditional neurophysiological techniques used to reduce pulsation. Two components of brain movement were recorded: (1) an arterial component--relatively low amplitude (110-266 micrometers) and short duration (330-400 ms) excursions corresponding to the pressure wave of each cardiac systole [A-wave]; and (2) a pulmonary component--slower (10-12/min), high amplitude plateau-like displacement (300-950 micrometers) lasting for a time (2.4-5.1 s) corresponding to the inspiration of each respiratory cycle [P-wave]. Pneumothoraces and mechanical ventilation combined with elevating the animal's head reduced the pulmonary component by an average of 68% and the arterial component by 40%. Cerebrospinal fluid drainage could reduce the P-wave component of movement by as much as 50%. To reduce arterial pulsations below 100 micrometers, the mean arterial pressure (MAP) had to be lowered to less than 40 mm Hg, which was not compatible with maintaining normal brainstem auditory evoked responses. Residual movements at MAPs greater than 50 mm Hg were still sufficient to make stable intracellular penetration of small neurons difficult. The authors suggest the solution to this problem is the development of a cardiopulmonary bypass system which generates a non-pulsatile flow of oxygenated blood, described in a companion paper.

Neural phase-locking properties in the absence of cochlear outer hair cells

A combined regimen of kanamycin sulfate treatment (175 mg/kg/day) and behavioral evaluation of resulting audiometric threshold shifts was used to produce selective outer hair cells (OHC) loss in chinchillas. This protocol resulted in a 3-7 mm region in the cochlear base in which OHCs were completely absent and inner hair cells (IHCs) were largely resent and normal at both light and electron microscopic levels. Partial OHC loss was associated with audiometric threshold shifts in excess of 15 dB, while complete OHC loss was associated with audiometric threshold shifts in excess of 40 dB. After recovery periods of at least three weeks, phase-locking was examined across frequency for auditory nerve (VIIIth nerve) and ventral cochlear nucleus (VCN) neurons. The frequency range for neural phase-locking in normal subjects extended up to approximately 4 kHz for VIIIth nerve fibers and 3 kHz for VCN neurons. Following kanamycin intoxication, however, the frequency range for neural phase-locking in both of these auditory regions varied with characteristic frequency (CF): neurons whose CF corresponded to normal cochlear regions exhibited phase-locking throughout the normal frequency range; neurons whole CF corresponded to cochlear regions with selective OHC loss exhibited a marked reduction in the frequency range over which they could phase-lock.

Transmission delay of phase-locked cells in the medial geniculate body

Over 4000 single unit recordings were obtained from the medial geniculate body (MGB) of nitrous oxide anaesthetized cats. Out of 1600 cells sensitive to tone bursts below 4 kHz, 10% were responding in a sustained manner. From these, 121 were tested for phase-locked responses. The general characteristics of these units have been described in a previous report. The central tendency of the discharges distribution within the period or mean phase angle was studied for many frequencies in 24 phase-locked units. For each of them, the mean phase angle shifts linearly with the frequency. The slope of these phase versus frequency lines is an accurate measure of the transmission delay from the cochlea to the MGB. This delay is a function of the unit's characteristic frequency and shows that the time spread introduced by the cochlea between the high and low frequency components of an acoustic signal is preserved up to the MGB. Subtracting the cochlear delay from this overall delay, the neural delay from the eighth nerve to the MGB was found to be 6.4 ms for neurons having a CF above 300 Hz; it was greater by 3 ms for cells with a CF below that frequency.

Survey of intracellular recording in the cochlear nucleus of the cat

Intracellular recordings were made in the cochlear nucleus of anesthetized cats. In anterior passes, one never obtained sustained depolarizations from 'primary-like' units. For 'chopper' units, however, it was possible to record sustained depolarizations accompaneid by spikes that lasted as long as the tone burst. 'Pauser, 'buildup' and 'on' units also had spike responses that could be accompanied by sustained depolarizations. For 'pauser', 'buildup' and 'on' units, hyperpolarization was not seen during the times when no spike discharges appeared so long as the tone bursts were at the characteristic frequency of the units.