Biomedical subjects

G W Harding

Publications and source records attributed to G W Harding.

At least 19 recordsLinked to original sources

PubMed2000

Histopathological differences between temporary and permanent threshold shift.

The structural changes associated with noise-induced temporary threshold shift (TTS) were compared to the damage associated with permanent threshold shift (PTS). A within-animal paradigm involving survival-fixation was used to minimize problems with data interpretation from interanimal variability in response to noise. Auditory brainstem response thresholds for clicks and tone pips were determined pre- and 1-2 h post-exposure in 11 chinchillas. The animals were exposed for 24 h to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 86 dB. Three animals (0/0-day) had both cochleas terminal-fixed 2-3 h post-exposure. Two animals (27/27-day) had threshold shifts determined every other day for 1 week, every week thereafter, and underwent terminal-fixation of both cochleas 27 days after exposure. Six animals (0/n-day) had threshold shifts determined in both ears upon removal from the noise; their left cochlea was then survival-fixed 2-3 h post-exposure. Threshold shifts were determined in their right ear every 2-3 days until their hearing either returned to pre-exposure values or stabilized at a reduced level at which time their right cochlea was terminal-fixed (4-13 days post-exposure). All cochleas were prepared as plastic-embedded flat preparations. Missing hair cells were counted and supporting cells and nerve fibers were evaluated throughout the organ of Corti using phase-contrast microscopy. Post-exposure, all animals had moderate TTSs in their left and right ears which averaged 43 dB for 4-12 kHz. In the 0/0-day animals, the only abnormality which correlated with TTS was a buckling of the pillar bodies. In the 0/n-day animals, their left cochlea (survival-fixed 2-3 h post-exposure) had outer hair cell (OHC) stereocilia which were not embedded in the tectorial membrane in the region of the TTS whereas OHC stereocilia were embedded in the tectorial membrane throughout the cochleas of non-noise-exposed, survival-fixed controls. Three of six right cochleas (terminal-fixed 4-13 days post-exposure) from the 0/n-day animals developed a PTS and two of these cochleas had focal losses of inner and outer hair cells and afferent nerve fibers at the corresponding frequency location. The other cochlea with PTS had buckled pillars in the corresponding frequency region. These results suggest that with moderate levels of noise exposure, buckling of the supporting cells results in an uncoupling of the OHC stereocilia from the tectorial membrane which results in a TTS. The mechanisms resulting in TTS appear to be distinct from those that produce permanent hair cell damage and a PTS.

Acoustic Stimulation↗

PubMed1999

Survival-fixation of the cochlea: a technique for following time-dependent degeneration and repair in noise-exposed chinchillas.

To minimize problems with data interpretation due to interanimal variation in susceptibility to noise, we developed a survival-fixation paradigm which involves fixing one cochlea of an experimental chinchilla at one post-exposure time and fixing the second cochlea as much as 14-24 days later. This paradigm is analytically effective because there is a high correlation in the magnitude and pattern of damage in the left and right cochleas of binaurally exposed animals. Thus, each experimental animal provides two snapshots in the degeneration and repair continua in which it can be certain that both cochleas sustained equivalent amounts of damage during the exposure. Using this technique, the time course of degeneration of different structures and cells in the organ of Corti can be determined and primary damage can be distinguished from secondary effects. The present paper discusses the issues which had to be addressed to develop this technique and provides preliminary results from chinchillas exposed to a traumatic noise.

Animals↗

PubMed1998

Otoconial agenesis in tilted mutant mice.

The sense of balance is one of the phylogenetically oldest sensory systems. The vestibular organs, consisting of sensory hair cells and an overlying extracellular membrane, have been conserved throughout vertebrate evolution. To better understand mechanisms regulating vestibular development and mechanisms of vestibular pathophysiology, we have analyzed the mouse mutant, tilted (tlt), which has dysfunction of the gravity receptors. The tilted mouse arose spontaneously and has not been previously analyzed for a developmental or physiological deficit. Here we demonstrate that the tilted mouse, like the head tilt (het) mouse, specifically lacks otoconia and consequently does not sense spatial orientation relative to the force of gravity. Unlike other mouse mutations affecting the vestibular system (such as pallid, mocha and tilted head), the defect in the tilted mouse is highly penetrant, results in the nearly complete absence of otoconia, exhibits no degeneration of the sensory epithelium and has no apparent abnormal phenotype in other organ systems. We further demonstrate that protein expression in the macular sensory epithelium is qualitatively unaltered in tilted mutant mice.

Animals↗

PubMed1998

The role of micro-noise trauma in the etiology of aging-related changes in the inner ear.

Eleven chinchillas between 1 and 2.4 years of age had the malleus/incus complex removed from one middle ear and then lived in the Washington University animal facilities for 4 years post-surgery. Each animal had one ear (termed ambient-noise) in which the conductive apparatus was intact; the other ear (termed noise-protected) had a 50-60 dB conductive hearing loss. The background sound level in the animal facility was 59 dBA with periodic brief sounds up to 102 dBA. After the 4-year experimental period, both ears were fixed, embedded in plastic and dissected for microscopic examination as flat preparations. The quantitative and qualitative findings in the noise-protected ears were compared to those in the ambient-noise ears. Both groups of ears sustained losses of sensory and supporting cells throughout the organ of Corti. A variable amount of age pigment was found to have accumulated in the outer hair cells and all supporting cells. In the noise-protected ears, inner hair cell loss ranged from 1.0 to 3.1% and averaged 1.7 +/- 0.8%; outer hair cell loss ranged from 1.8 to 6.4% and averaged 3.6 +/- 1.2%. In the ambient-noise ears, inner hair cell loss ranged from 0.7 to 2.8% and averaged 1.6 +/- 0.7%; outer hair cell loss ranged from 1.3 to 5.4% and averaged 3.6 +/- 1.2%. Within-animal comparison of cell losses in the noise-protected and ambient-noise ears revealed no significant difference between the two groups. It is concluded that long-term exposure to micro-noise does not accelerate the spontaneous loss of sensory cells which occurs with aging. Although not quantified, there was no obvious difference in the amount or cellular distribution of age pigment in the two groups. Thus, it appears that the formation of age pigment in the ear is the result of the cells' basic metabolic processes rather than the wear and tear from sensory transduction.

Aging↗

PubMed1998

Spiral CT image deblurring for cochlear implantation.

Cochlear implantation is the standard treatment for profound hearing loss. Preimplantation and postimplantation spiral computed tomography (CT) is essential in several key clinical and research aspects. The maximum image resolution with commercial spiral CT scanners is insufficient to define clearly anatomical features and implant electrode positions in the inner ear. In this paper, we develop an expectation-maximization (EM)-like iterative deblurring algorithm to achieve spiral CT image super-resolution for cochlear implantation, assuming a spatially invariant linear spiral CT system with a three-dimensional (3-D) separable Gaussian point spread function (PSF). We experimentally validate the 3-D Gaussian blurring model via phantom measurement and profile fitting. The imaging process is further expressed as convolution of an isotropic 3-D Gaussian PSF and a blurred underlying volumetric image. Under practical conditions, an oblique reconstructed section is approximated as convolution of an isotropic two-dimensional (2-D) Gaussian PSF and the corresponding actual cross section. The spiral CT image deblurring algorithm is formulated with sieve and resolution kernels for suppressing noise and edge artifacts. A typical cochlear cross section is used for evaluation, demonstrating a resolution gain up to 30%40% according to the correlation criterion. Physical phantoms, preimplantation and postimplantation patients are reconstructed into volumes of 0.1-mm cubic voxels. The patient images are digitally unwrapped along the central axis of the cochlea and the implanted electrode array respectively, then oblique sections orthogonal to the central axis formed. After deblurring, representation of structural features is substantially improved in all the cases.

Algorithms↗

PubMed1997

Processing and analyzing the mouse temporal bone to identify gross, cellular and subcellular pathology.

A technique has been developed for preparing the mouse temporal bone for histopathological examination: first, as a whole mount to detect any gross malformations of the bony or membranous labyrinths; second, in dissected segments to localize damage in the different sensory organs and to quantify sensory- and supporting-cell losses; and finally, in semi-thick and thin sections to identify and characterize subcellular pathology. Examples are given of the successful application of this technique to mice with very different inner-ear problems, including those with an abnormally short cochlear spiral, a defective lateral semicircular canal, abnormal otoliths over the saccular macula, an increased susceptibility to noise damage and those which lack fibroblast growth factor receptor 3.

Animals↗

PubMed1997

Time course of nerve-fiber regeneration in the noise-damaged mammalian cochlea.

The time course of events which are essential for nerve-fiber regeneration in the mammalian cochlea was determined using a group of chinchillas that had been exposed for 3.5 hr to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. The animals recovered from 40 min (0 days) to 100 days at which times their inner ears were fixed and the organs of Corti prepared for phase-contrast and bright-field microscopy as plastic-embedded flat preparations. Selected areas identified in the flat preparations were semi-thick and thin sectioned at radial or tangential angles for examination by bright-field and transmission electron microscopy. The following time-ordered events appeared critical for nerve-fiber regeneration: (1) The area of the basilar membrane in which regeneration had a possibility of occurring showed signs of severe injury. Outer hair cells degenerated first followed by outer pillars, inner pillars, inner hair cells and other supporting cells; (2) Myelinated nerve fibers in the osseous spiral lamina became fragmented, starting at the distal ends of the fibers. This degeneration gradually extended back to Rosenthal's canal; (3) Fibrous processes, originating from Schwann-like cells in the osseous spiral lamina, extended laterally on the basilar membrane; (4) Schwann cells lined up medial to the habenulae perforata in the areas of severest damage, apparently ready to migrate through the habenulae onto the basilar membrane; (5) Schwann-cell nuclei appeared on the basilar membrane beneath the developing layer of squamous epithelium which was in the process of replacing the degenerated portion of the organ of Corti; (6) Regenerated nerve fibers with thin myelin sheaths or a simple investment of Schwann cell cytoplasm appeared in areas of total loss of the organ of Corti; and (7) The myelin sheaths on the regenerated nerve fibers gradually became thicker.

Animals↗

PubMed1996

Skeletal overgrowth and deafness in mice lacking fibroblast growth factor receptor 3.

Fibroblast growth factor receptor 3 (Fgfr3) is a tyrosine kinase receptor expressed in developing bone, cochlea, brain and spinal cord. Achondroplasia, the most common genetic form of dwarfism, is caused by mutations in FGFR3. Here we show that mice homozygous for a targeted disruption of Fgfr3 exhibit skeletal and inner ear defects. Skeletal defects include kyphosis, scoliosis, crooked tails and curvature and overgrowth of long bones and vertebrae. Contrasts between the skeletal phenotype and achondroplasia suggest that activation of FGFR3 causes achondroplasia. Inner ear defects include failure of pillar cell differentiation and tunnel of Corti formation and result in profound deafness. Our results demonstrate that Fgfr3 is essential for normal endochondral ossification and inner ear development.

Achondroplasia↗

PubMed1995

Regenerated nerve fibers in the noise-damaged chinchilla cochlea are not efferent.

Nerve-fiber regeneration in the chinchilla cochlea following a traumatic noise exposure was systematically described by Bohne and Harding (1992). However, their study did not determine the origin of the regenerated nerve fibers (RNFs). In the present study, 23 chinchillas were exposed for 12 h to a 0.5 kHz octave band of noise at 120 dB SPL. After a 3-month or 1-year recovery period, their right cochleas were incubated to demonstrate acetylcholinesterase (AChE) activity and then briefly counterstained with Neutral Red or OsO4. Their left cochleas were fixed with OsO4 and dissected using a combined organ of Corti (OC)/modiolus technique that preserved both structures for high-resolution microscopy. All cochleas were prepared as plastic-embedded flat preparations. Damage was located in the basal two-thirds of the cochlea and generally consisted of multiple lesions in the OC, often involving total degeneration of one or more OC segments (i.e., OC wipeouts). The OC wipeouts were separated from one another by areas which contained some identifiable cells of the OC (i.e., OC remnants). Most RNFs were found in OC wipeouts adjacent to OC remnants. In those animals (83%) with significant OC damage, 13 (100%) 3-month-recovery chinchillas had 1-96 RNFs while 6 (86%) 1-year-recovery chinchillas had 7-62 RNFs. In the AChE-stained cochleas, none of the RNFs were AChE-positive, but normal AChE-positive fibers were found in the undamaged apical turn. A variable number of surviving spiral ganglion cells was present in those regions of Rosenthal's canal that had originally innervated the missing hair cells in the OC wipeouts and remnants. It is concluded that RNFs are not part of the efferent cochlear system and therefore, most likely belong to the afferent system.

Acetylcholinesterase↗

PubMed1995

Patterns of synaptic activity in forward and feedback pathways within rat visual cortex.

1. The laminar and temporal distribution of synaptic activity supplied by forward and feedback connections between different areas of rat visual cortex was determined with the use of current source density (CSD) analysis in in vitro slices. In forward connections, synaptic potentials were evoked by electrically stimulating area 17 and recording in the extrastriate area LM (lateromedial), that ranks at the second hierarchical level, one step above primary visual cortex. For activating feedback connections, the location of stimulating and recording electrodes was reversed. 2. The synaptic interactions in reciprocal intracortical circuits are excitatory, and they are mediated through glutamate receptors that are blocked by kynurenic acid. 3. Forward connections from area 17 to area LM provide input to all layers including a strong input to layer 4. In contrast, feedback input to layer 4 is weak and is mainly directed to superficial and deep layers. This laminar distribution closely resembles that seen anatomically. 4. Both forward and feedback connections evoke distinct temporal patterns of synaptic activation in different layers. Although onset and peak latencies are slightly shorter in the forward than in the feedback pathway, the difference is not statistically significant. 5. The spatiotemporal distribution of synaptic activation by forward connections resembles the pattern evoked by geniculocortical inputs. Feedback connections show greater similarities to long-range connections within area 17, although they are not identical. Our results support the notion derived from anatomic and in vivo physiological studies that forward and feedback pathways belong to functionally distinct cortical circuits.

Animals↗

PubMed1994

Is the older ear more susceptible to noise damage?

Eight chinchillas aged 8.9 to 12.8 years were used to examine the effect of noise on the aging ear. The left malleus/incus complex was removed to produce a 50-dB conductive hearing loss which protected those ears from noise damage. The animals were then exposed for 36 days to an octave band of noise with a center frequency of 0.5 kHz and a sound pressure level of 95 dB. After 1 hour (n = 2) or 1 month (n = 6) of recovery, their cochleas were prepared for microscopic examination. The percentages of missing inner hair cells (IHCs) were 7.4 +/- 6.0% and 7.8 +/- 5.1% for their protected and exposed ears, respectively. Outer hair cell (OHC) loss was 12.8 +/- 8.7% and 20.6 +/- 7.8% for their protected and exposed ears, respectively. A paired sample Student's t test revealed that OHC loss was significantly greater (P = .003) in the older-exposed compared to the older-protected ears whereas IHC loss was not significantly different. For younger-exposed ears (i.e., 1 to 3 years), the percentages of missing IHCs and OHCs averaged 2.6 +/- 2.0% and 12.3 +/- 4.6%, respectively. When the aging-related cell loss was subtracted from total loss in the younger- and older-exposed ears, the noise-induced loss of sensory cells in the older ears was not significantly different from that in the younger ears. Therefore, it is concluded that older chinchilla ears are not more susceptible to noise damage than younger ears.

Aging↗

PubMed1994

Distinctive electrophysiological characteristics of functionally discrete brain areas: a tenable approach to functional localization.

The direct cortical response (DCR), an electrical potential recorded in the immediate vicinity of a surface cortical stimulus, shows a configuration in the primary sensory areas of animals that is different from the one observed in association cortex. This suggested the possibility that systematic study of the DCR in the human brain might reveal a profile of configurations in which the form of the response provides functional information about the gyri being tested. Studies were carried out in subhuman primates and in patients undergoing surgery for tumors, occult vascular malformations, and epilepsy. In the animals, DCR's from somatosensory, motor, and association cortex are distinguishable; however, there are no differences in configuration between motor and premotor responses, or between association responses from prefrontal and parietal cortex. In patients with epilepsy due to nonspace-occupying pathology, the responses did not show distinguishing features. In contrast, in the patients with tumors or occult vascular malformations, DCR's from somatosensory, motor, and premotor cortex could be readily distinguished from each other. Responses along the mid and posterior sylvian fissure of the dominant hemisphere also had distinctive features, but more data are needed before the significance of this finding with respect to language function can be assessed. The accumulating results suggest that analysis of DCR's may prove to be a useful method for functional localization in individuals with focal space-taking pathology.

Animals↗

PubMed1993

An automated seizure monitoring system for patients with indwelling recording electrodes.

An automated monitoring system has been developed to record from indwelling electrode arrays in patients undergoing evaluation for surgical treatment of intractable seizures. The functional aspects of this system's design are discussed and the range of electrical seizure patterns, other epileptiform events and artifacts that the system must handle are described. The system includes a flowing graphics image of as many as 32 channels of real-time ECoG, automatic seizure detection, recording of the ECoG for up to 6 min prior to seizure onset, and EEG for 792 clinical and subclinical seizures during 1578 h of monitoring with an artifact rate of 28% for all events recorded. However, system performance was judged upon the 86% accuracy for detection and recording of patient-specific seizure patterns (multiple seizures with the same pattern counted as 1) with 1.26 brief spike bursts and 0.67 artifacts/h.

Adolescent↗

PubMed1993

Combined organ of Corti/modiolus technique for preparing mammalian cochleas for quantitative microscopy.

An improved cochlear preparation technique is described with the following key features: 1) Preservation of the organ of Corti (OC) and the spiral ganglion cells (SGCs) in the same cochlea for quantitative, high-resolution microscopic evaluation; 2) Dissection of the plastic-embedded cochlea so that the entire OC can be prepared as whole mounts for quantitative study while the modiolus remains in a single block; 3) Decalcification and serial sectioning of the modiolus so that all SGC bodies are available for microscopic examination. This technique will be valuable for correlating the condition of the OC, nerve terminals, nerve fibers and the SGC bodies in normal and damaged cochleas.

Animals↗

PubMed1993

Temporal lobectomy that spares the amygdala for temporal lobe epilepsy.

Rationale, surgical techniques, and results in 70 patients with complex partial seizures who underwent temporal lobectomy with sparing of the amygdala are discussed. Removal of entorhinal cortex may be the common denominator that explains the similar results obtained with different types of temporal lobectomies for epilepsy.

Adolescent↗

PubMed1992

The currents that flow in the somatosensory cortex during the direct cortical response.

A current-flow and current-source-density analysis of the sensory evoked response (SER) and the direct cortical response (DCR) in the somatosensory cortex of rats was performed to determine the origin of these potentials. The SER was found to originate in layers II and III, as in cats, with a single excitatory neuronal circuit component. The DCR, on the other hand, has five components, three inhibitory and two excitatory. The activation and magnitude of these components vary with stimulus strength and frequency. During the second and fourth ms of the response, two inhibitory currents flow in layers V and VI; 2 ms later, excitatory current flows in layers II and III. This excitatory current appears to be the same one involved in the SER. Five ms later, the superficial excitatory current is replaced by an inhibitory one in the neighborhood of the DCR's negative peak. At strong stimulus strengths, this is followed by an excitatory current in layer V. The early inhibitory and excitatory components step up through layers upper-VI, V and III over time, implying that inhibition followed by excitation moves upward through cortex. The currents associated with the DCR in somatosensory cortex are compared with those for the DCR in motor and association cortex.

Animals↗

PubMed1992

Height changes in the organ of Corti after noise exposure.

To determine whether or not exposure to noise causes an alteration in the height of the organ of Corti (OC), 16 cochleas which had been exposed for one or two hours to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB were examined microscopically as whole mounts. These specimens were divided into four groups: early ears (N = 3) recovered less than 0.6 hours following the exposure; intermediate ears (N = 5) recovered 0.6-4.0 hours; 1-day ears (N = 3) recovered 24 hours; and late ears (N = 5) recovered 2-21 days. Height was measured at three positions across the OC and at multiple percentage locations from apex to base. The OC-height data from the noise-exposed cochleas were compared statistically to those from ten control cochleas. A significant reduction (P < or = 0.01) in OC height at the third outer hair cell (OHC) was first evident in the early ears in the region 65-95% distance from the apex. The height was reduced even further in the intermediate ears and included a region from 15-25% distance from the apex as well as the 65-95% region. In the late ears, heights had returned to control values, except within focal OC lesions. Height at the first row of OHCs was less affected than at the third row, and height at the inner hair cell (IHC) was least affected. These height changes were accompanied by distortion of the shape and position of OHCs, the shape of Deiters' cells and buckling of inner and outer pillar bodies. Sometimes IHCs had distorted shapes and were displaced from their usual positions. Although no functional measures were obtained from these ears, data from the literature indicate that the exposure described above would have produced a sizable threshold shift. Transient reduction in OC height likely accounts for some portion of noise-induced threshold shifts.

Acoustic Stimulation↗

PubMed1992

Neural regeneration in the noise-damaged chinchilla cochlea.

Recent studies in the bird ear have shown that degenerated hair cells are sometimes replaced by regenerated receptor cells. The present study evaluated the adult mammalian cochlea for evidence of hair-cell and nerve-fiber regeneration. Eighty-eight noise-damaged chinchilla cochleas were examined as plastic-embedded whole mounts by phase-contrast and bright-field microscopy. No signs of hair-cell regeneration were found. However, 32 (70%) of 46 cochleas damaged by high-intensity noise and 20 (48%) of 42 cochleas damaged by moderate-intensity noise contained a variable number of nerve fibers which appeared to be regenerated. These fibers, which were located in severely damaged areas of organ of Corti, differed from residual fibers with respect to their diameters, the degree and pattern of myelination, and by the abnormal paths they followed within the osseous spiral lamina and on the basilar membrane. The number of regenerated fibers varied with type of exposure and length of recovery. The strongest response was found in ears exposed to a high-intensity, low-frequency noise. The results described here indicate that a potential exists for the biological restoration of the mammalian inner ear.

Animals↗