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B M Clopton

Publications and source records attributed to B M Clopton.

At least 19 recordsLinked to original sources
PubMed1995

Electrode configuration and spread of neural excitation: compartmental models of spiral ganglion cells.

A compartmental model of spiral ganglion cells in a potential field was used to predict spike discharges to electrical stimuli. The field was generated by monopolar, bipolar, or quadrupolar point source electrode geometries. The discharges of a population of afferent neurons were modeled to compare the relative spread of excitation. At the same stimulus intensity the spread of excitation was least for the quadrupolar and greatest for the radial bipolar, and this ordering held after equating for maximal spike discharge rates over the three geometries. Excitation spread from quadrupolar stimulation was highly dependent on stimulus intensity.

Electric Stimulation↗
PubMed1995

Effects of electrical current configuration on stimulus detection.

Psychophysical detection of electrical stimulation of the cochlea was studied as a function of electrical-current configuration. Subjects were postlingually deaf humans with Nucleus 20 + 2, Nucleus 22, and Ineraid cochlear implants and nonhuman primates unilaterally deafened and implanted with a multielectrode array similar to the Nucleus implant. In nonhuman primate and human Ineraid subjects, which had percutaneous connectors, we compared threshold functions for sinusoids and pulse trains for quadrupolar, bipolar, monopolar, and parallel multipolar stimulation. Thresholds decreased across this set of configurations. In some cases, the effects of current configuration were dependent on sinusoidal frequency and pulse duration. Pulse duration-dependent effects were also seen when comparing bipolar, monopolar, and common-ground configurations. Bipolar and monopolar stimulation were compared in Nucleus subjects using pulse trains at 50 microseconds per phase. For bipolar stimulation, thresholds decreased as a function of electrode separation, reaching a level near that for monopolar stimulation at separations of 3.5 to 6.5 mm in most cases. These results may be interpreted in terms of effects of current configuration on the magnitude and shape of electrical-potential fields produced in the cochlea, although more central factors also play a role in determining psychophysical detection thresholds.

Animals↗
PubMed1995

Effects of electrical current configuration on potential fields in the electrically stimulated cochlea: field models and measurements.

Potential distributions measured within the scala tympani of the anesthetized guinea pig support the assertion that focusing is possible when currents are appropriately delivered to the electrodes in the scala tympani. Results obtained with a lumped-element model agree with measurements made in the inner ears of monkeys during monopolar and bipolar stimulation. The predictions are closer for potential distributions apical to the stimulating electrode than they are for basal distributions. In one monkey, in which electrodes were implanted in the middle ear as well as in the inner ear, we obtained measurements of the impedance from inside the scala tympani to points within the middle ear. These impedances are smaller that those initially used in the model, in which the round window membrane was assumed to have a relatively high impedance. A model of the common ground configuration was developed using finite electrode impedances. Finite impedances broaden the potential distributions in this model. Potential distributions from the lumped element model are compared with those obtained with an analytical model, to suggest ways in which focused and unfocused stimuli can affect the excitation of neurons in the implanted ear.

Animals↗
PubMed1991

Spectrotemporal receptive fields of neurons in cochlear nucleus of guinea pig.

Spectrotemporal receptive fields (STRFs) [Hermes et al., Hear. Res. 5, 147-178, 1981] for neurons in the cochlear nuclei (CN) of guinea pig were estimated. Sixteen periodic segments of bandlimited, synthesized noise evoked replicable, distinctive period histograms for spike discharges. All driven units in the major divisions of the CN having their characteristic frequency (CF) within the noise bandlimits had unique STRFs for a given intensity of noise stimulation. The STRF maximum corresponded to the unit's CF, and details of the STRF patterns differed over CN divisions and response classes derived from tonebursts. The sizes of features in STRFs from this mammal appeared significantly smaller in their temporal and spectral extents than those reported in the torus semicircularis of an amphibian and were roughly comparable to the few units reported from cat ventral CN [Eggermont et al., Quart. Rev. Biophys. 16, 341-414, 1983]. STRFs, as they are presently obtained, provide useful insight into some aspects of afferent processing and perhaps connectivity, but their interpretation is specific to the level of stimulation and limited by the need to choose a specific energy distribution to represent the stimulus.

Acoustic Stimulation↗
PubMed1991

Shared-stimulus driving and connectivity in groups of neurons in the dorsal cochlear nucleus.

Extracellular spike discharges were recorded from ensembles of up to five neurons simultaneously in the DCN of guinea pig using solid-state, thin-film, multichannel electrodes having up to five recording sites spanning up to 600 microns. Responses from 73 unit pairs were collected of which 54 had both units responding to pseudorandom wideband noise stimulation. Shared-stimulus driving was present in 78% (42/54) of the unit pairs and could be attributed to an overlap in their spectral sensitivities. Effective connectivity was indicated for 87% (47/54) of the unit pairs. Wideband noise proved more useful than tonebursts for investigating shared-stimulus driving and connectivity because it evoked widespread, but not overly synchronous, responses in the ensembles.

Acoustic Stimulation↗
PubMed1991

A spectrotemporal analysis of DCN single unit responses to wideband noise in guinea pig.

Spectrotemporal receptive fields (STRFs) were estimated for chopper and pauser units recorded in guinea pig dorsal cochlear nucleus (DCN). Sixteen wideband, periodic noise stimuli, represented as time-frequency surfaces of energy density, were cross correlated in time with the unit's corresponding period histograms to determine if specific energy patterns tended to precede spike occurrence. The STRFs obtained were unique to the DCN, as compared to the ventral cochlear nucleus (VCN) [Clopton and Backoff, 1991, Hear. Res. 52, 329-344] in their degree of temporal and spectral complexity. Certain unit response types, classified from their peristimulus-time histograms (PSTHs) to tonebursts, were associated with distinctive patterns in the STRFs. All STRFs had at least one region of elevated energy density (peak region) closely preceding spike occurrence, which may reflect a short-pathway, primary excitatory input (or inputs) to the neuron. In addition, some units displayed low-energy regions (troughs) with greater temporal precedences on their STRFs, particularly when higher stimulus intensities were used. This analysis approach appears to have potential for investigating functional neural connectivity and predicting responses to novel complex stimuli, although specific implementations of the technique impose limitations on the interpretation of results.

Acoustic Stimulation↗
PubMed1989

Effect of electrical pulse shape on AVCN unit responses to cochlear stimulation.

Electrical stimulation of the cochlea with a multiple-electrode array is best accomplished using pulsatile instead of continuous stimulation. The optimum shapes of electrical pulses for this purpose are still uncertain due to a lack of knowledge about their stimulation efficiency and requirements of the encoding strategy. We presented an extensive set of charge-balanced, rectangular pulse shapes to the guinea pig cochlea. Durations per phase for these constant-current pulses ranged from 20 microseconds to 900 microseconds with initially positive and initially negative polarities. Spike counts from single units in the anteroventral cochlear nucleus differed significantly for different pulse shapes, as did their initial latencies. Implications for stimulation efficiency and encoding strategies are discussed.

Animals↗
PubMed1987

Radial current flow and source density in the basal scala tympani.

Ionic movement between the scala media and scala tympani is modulated by acoustic stimulation. It underlies electrical currents in the fluids of these compartments and produces voltage gradients from which partial current-flow densities can be estimated. Radial voltage gradients were sampled in the first turn of the scala tympani of the guinea pig cochlea at known distances from the basilar membrane. Large potential gradients indicated significant current-flow densities near the organ of Corti with less flow near the lateral wall of the cochlea and over the spiral lamina. Current-source densities were highest within 100 micron of the organ of Corti. Current-source density analysis suggested that source-sink pairs can be detected from field observations in the scala tympani.

Acoustic Stimulation↗
PubMed1987

Unit responses in ventral cochlear nucleus reflect cochlear coding of rapid frequency sweeps.

This study examines the encoding of rapid frequency sweeps in single units of the ventral cochlear nucleus (VCN). Sweeps were designed to explore the role of cochlear mechanics in shaping the temporal responses across cells in the VCN. The time course of frequency change for rapidly rising frequency sweeps theoretically produced simultaneous displacement maxima by cancelling travel time along the cochlear partition. Rising sweeps with longer time courses only partially canceled travel time, while falling sweeps had time courses of frequency change equal to or greater than travel time. Falling sweeps thus augmented normal travel time. Latency of unit firing to sweeps across unit characteristic frequency (CF) reflected cochlear delay-line mechanics. The latency-CF functions agreed with predictions from travel-time estimates for rising-frequency sweeps, but responses to falling sweeps were less predictable.

Acoustic Stimulation↗
PubMed1984

Unit responses at cochlear nucleus to electrical stimulation through a cochlear prosthesis.

Afferent auditory fibers of the guinea pig cochlea were electrically stimulated with current introduced through electrodes in the scala tympani. Thresholds were determined for unit responses recorded in the ventral cochlear nuclei to a sinusoid of 98 Hz from response-rate growth functions versus stimulus intensity. Suprathreshold response rates for most units grew rapidly from threshold to saturation at 2-15 dB above threshold. Peristimulus time histograms were collected for responses to single sinusoids and combinations of two and five sinusoids ranging from 86 to 134 Hz. Spike occurrences were highly synchronous with individual cycles of the pure sinusoids, but responses to the more complex waveforms occurred primarily to the more intense peaks. The amplitude envelope was thus a major contributor to responses to multiple sinusoids. Destruction of cochlear structures with neomycin increased unit thresholds and produced some changes in waveform encoding.

Action Potentials↗
PubMed1978

Changes in latency and duration of neural responding following developmental auditory deprivation.

The initial latency of spikes evoked by click stimulation and the duration over which spiking occurred were observed in the inferior colliculi of rats. One ear of these animals had been deprived of early auditory stimulation by ligation of the external meatus. Clicks presented to the normally experienced ears evoked spikes in the opposite colliculus with latencies that depended on the characteristic frequency of the unit. Low-frequency (less than 5 kHz) units had latencies from 6-10 msec. Latencies declined to 3-4 msec for high frequency (greater than 20 kHz) units. After an ear had been deprived of sound from 10 days after birth, response latencies of units in the opposite colliculus with characteristic frequencies below about 10 kHz were comparable to controls, but most units above 10 kHz had latencies 2-3 times control latencies. Spike activity evoked in these units did not continue as long as that for most comparable control units. Ears sound deprived for an equal period from 60 days after birth also had changes in latencies and response durations, but these were much less than in the developmentally deprived. Latencies of gross potentials at the auditory nerve were not affected by early deprivation, indicating a central origin for the latency changes.

Acoustic Stimulation↗
PubMed1977

Effects of lesioning the dorsal and intermediate acoustic striae on binaural interaction at the inferior colliculus.

Contralateral clicks normally activate units in the inferior colliculus of rats. Ipsilateral clicks usually inhibit this activity when their intensity just exceeds the contralateral intensity. When the contralateral dorsal and intermediate acoustic striae are lesioned as they pass over the restiform body, ipsilateral clicks inhibit even if they are substantially less intense than contralateral clicks. Contralateral click thresholds for unit activation are elevated by about 10 dB, but this shift in sensitivity cannot account for the marked advantage gained by the ipsilateral inhibitory input. These findings suggest that one or both of these pathways contributes heavily to binaural interaction at the inferior colliculus.

Animals↗
PubMed1976

Effect of early exposure to patterned sound on unit activity in rat inferior colliculus.

1. Young rats were exposed to one of two patterns of sounds for 5 h daily during the first 4 mo of life. The up pattern consisted of a tone swept from 6 to 9 kHz in 1 s alternated with a 1-s noise burst. The down pattern differed in that the sweeps were from 9 to 6 kHz. 2. A pattern evoked more spikes, on the average, from units in the inferior colliculus of rats exposed to that pattern than from units in animals exposed to the other pattern. 3. The exposure effect was most pronounced in the unit responses to the noise-burst segment within a pattern suggesting a long-lasting, malleable influence of the tone sweep which defined the pattern. Responses to pattern noise components were less for both exposed groups than for the unexposed controls, suggesting that inhibitory mechanisms were responsible for the pattern discrimination evident in the responses of the exposed unit population. 4. Unit responses in unexposed rats were somewhat more selective for the down pattern so that the resulting shift in response selectivity was relatively more apparent for exposure to the up pattern. 5. While control and down-exposed units generally responded more to both the tone sweep and noise burst in one pattern, a large proportion of the up-exposed unit population continued to favor the down over the up-swept tone, but responded more to the up-pattern noise bursts. This suggests that the unit responses to the noise bursts did not simply reflect prolonged responses to the tone sweeps. 6. No similar effects were seen for units from mothers similarly exposed to the same patterns.

Acoustic Stimulation↗