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At least 307 records · Page 17Linked to original sources
PubMed2006

Conditioning pulse trains in cochlear implants: effects on loudness growth.

HYPOTHESIS: The addition of a high-rate (5 kpps) conditioning pulse train to the input signal of cochlear implants will result in shallower loudness growth across the dynamic range of cochlear implant patients. BACKGROUND: High-rate conditioning pulse trains have been shown to increase the dynamic range of sinusoidal stimuli for cochlear implant recipients in a manner consistent with stochastic resonance. This study further characterizes the effects of conditioning stimuli on loudness by examining the loudness growth functions for sinusoidal stimuli both with and without conditioning. METHODS: Seven post-lingually deafened adults using the Clarion CII cochlear implant participated in this study. The loudness growth functions of each subject were characterized using sinusoidal stimuli, both with and without the presence of a high-rate conditioner. Loudness was measured using magnitude estimation. RESULTS: The loudness growth functions of all seven subjects demonstrate an increase in dynamic range for sinusoidal stimuli with the addition of the conditioning pulse train. Shallower loudness growth is seen across the dynamic range with the addition of a conditioner. This result was shown for loudness growth fitted to exponential, power, and cumulative gaussian functions. CONCLUSION: The addition of high-rate conditioning pulse trains to sinusoidal stimuli presented to cochlear implant recipients results in larger dynamic ranges, with more gradual increases in loudness growth across the dynamic range. This suggests that signal-processing strategies incorporating conditioning may be clinically useful, requiring less compression of the input signal and leading to less distortion perceived by cochlear implant patients.

Adult↗
PubMedDate not supplied

Contrast constancy in natural scenes in shadow or direct light: A proposed role for contrast-normalisation (non-specific suppression) in visual cortex.

The range of contrasts in natural scenes is generally thought to far exceed the limited dynamic ranges of individual contrast-encoding neurons in the primary visual cortex. The visual system may employ gain-control mechanisms (Ohzawa et al. 1985) to compensate for the mismatch between the range of natural contrast energies and the limited dynamic range of visual neurons; one proposed mechanism is contrast normalisation or non-specific suppression (Heeger 1992a). This paper aims to evaluate the role of contrast normalisation in human contrast perception, using a computer model of primary visual cortex. The model uses orthogonal pairs of Gabor patches to simulate simple-cell receptive-fields to calculate local, band-limited contrast in a series of 50 digitised photographs of natural scenes. The average range of contrast energies in each image was 2.29 log units, while the "lifetime range" each model simple cell would see across all images was 2.98 log units. These ranges are greater than the dynamic range of real mammalian simple cells. Contrast normalisation (dividing contrast responses by the summed responses of all nearby neurons) reduces contrast ranges, perhaps sufficiently to match them to neurons' limited dynamic ranges. Comparison of images taken under diffuse and direct lighting conditions showed that contrast normalisation can sometimes match these conditions effectively. This may lead to perceptual contrast constancy in the face of spurious changes in contrast caused by natural environmental conditions.

Computer Simulation↗
PubMed2000

Speech recognition by normal-hearing and cochlear implant listeners as a function of intensity resolution.

The importance of intensity resolution in terms of the number of intensity steps needed for speech recognition was assessed for normal-hearing and cochlear implant listeners. In experiment 1, the channel amplitudes extracted from a six-channel continuous interleaved sampling (CIS) processor were quantized into 2, 4, 8, 16, or 32 steps. Consonant recognition was assessed for five cochlear implant listeners, using the Med-El/CIS-link device, as a function of the number of steps in the electrical dynamic range. Results showed that eight steps within the dynamic range are sufficient for reaching asymptotic performance in consonant recognition. These results suggest that amplitude resolution is not a major factor in determining consonant identification. In experiment 2, the relationship between spectral resolution (number of channels) and intensity resolution (number of steps) in normal-hearing listeners was investigated. Speech was filtered through 4-20 frequency bands, synthesized as a linear combination of sine waves with amplitudes extracted from the envelopes of the bandpassed waveforms, and then quantized into 2-32 levels to produce stimuli with varying degrees of intensity resolution. Results showed that the number of steps needed to achieve asymptotic performance was a function of the number of channels and the speech material used. For vowels, asymptotic performance was obtained with four steps, while for consonants, eight steps were needed for most channel conditions, consistent with our findings in experiment 1. For sentences processed though 4 channels, 16 steps were needed to reach asymptotic performance, while for sentences processed through 16 channels, 4 steps were needed. The results with normal-hearing listeners on sentence recognition point to an inverse relationship between spectral resolution and intensity resolution. When spectral resolution is poor (i.e., a small number of channels is available) a relatively fine intensity resolution is needed to achieve high levels of understanding. Conversely, when the intensity resolution is poor, a high degree of spectral resolution is needed to achieve asymptotic performance. The results of this study, taken together with previous findings on the effect of reduced dynamic range, suggest that the performance of cochlear implant subjects is primarily limited by the small number (four to six) of channels received, and not by the small number of intensity steps or reduced dynamic range.

Acoustic Stimulation↗
PubMed2003

Improved temporal coding of sinusoids in electric stimulation of the auditory nerve using desynchronizing pulse trains.

Rubinstein et al. [Hearing Res. 127, 108-118 (1999)] suggested that the representation of electric stimulus waveforms in the temporal discharge patterns of auditory-nerve fiber (ANF) might be improved by introducing an ongoing, high-rate, desynchronizing pulse train (DPT). To test this hypothesis, activity of ANFs was studied in acutely deafened, anesthetized cats in response to 10-min-long, 5-kpps electric pulse trains that were sinusoidally modulated for 400 ms every second. Two classes of responses to sinusoidal modulations of the DPT were observed. Fibers that only responded transiently to the unmodulated DPT showed hyper synchronization and narrow dynamic ranges to sinusoidal modulators, much as responses to electric sinusoids presented without a DPT. In contrast, fibers that exhibited sustained responses to the DPT were sensitive to modulation depths as low as 0.25% for a modulation frequency of 417 Hz. Over a 20-dB range of modulation depths, responses of these fibers resembled responses to tones in a healthy ear in both discharge rate and synchronization index. This range is much wider than the dynamic range typically found with electrical stimulation without a DPT, and comparable to the dynamic range for acoustic stimulation. These results suggest that a stimulation strategy that uses small signals superimposed upon a large DPT to encode sounds may evoke temporal discharge patterns in some ANFs that resemble responses to sound in a healthy ear.

Animals↗
PubMed1996

Perception of detail and greyscale range in X-ray fluoroscopy images captured with a personal computer and frame-grabber.

OBJECTIVE: To assess the dynamic range of radiological images captured with a personal computer frame-grabbing system coupled to an X-ray fluoroscopy machine. METHODS: A 386DX-40 MHz, IBM compatible, computer with an SVGA monochrome graphics subsystem and a 387 co-processor, installed with a Screen Machine frame-grabber and controlled by a program specially written was used. Various systems were examined and the observer's perceptions of the results assessed. RESULTS: The dynamic range available to an ordinary X-ray fluoroscopy system was found to be restricted to about 750 mV. Similar measurements showed that the dynamic range was always restricted to 3/4-1/2 of the full available signal because of a high value of the dark voltage of the TV camera's target on all seven systems measured. The dynamic range of the computer-frame grabber system was found to be significantly wider than the Image Intensifier-TV camera chain but, surprisingly, it was affected by the type of file format used for image storing on disk. Clinical images from a barium meal examination as well as CT images captured after optimisation of the frame-grabber were found to contain large quantities of noise in the first two least significant bit planes making them redundant and limiting the grey levels needed for image display to less than 64. This number was also less than the 80 grey levels that could be discriminated by the human eye on the computer monitor. CONCLUSIONS: It was concluded that 6 bit digitisation would have been sufficient for image capture. The advantages of the wider dynamic range of the frame-grabber and the processing capabilities of the computer were tested for the possibility of improving the perception of detail. However, the results were negative. The limiting spatial resolution measured with a variable density bar pattern at all magnifications was about 0.4 lp/mm lower from that measured directly on the fluoroscopic screen. A detail perception test had the same result. The perception success was significantly lower with the digital images at all but the highest of the exposure rates and despite the use of image processing filters.

Artifacts↗
PubMed2002

Effects of stimulus level on speech perception with cochlear prostheses.

This study is one of a series that examines stimulus features important for cochlear implant function. Here, we examine effects of stimulus level. In subjects with cochlear implants, a number of psychophysical tests of temporal discrimination (pulse rate discrimination, gap detection, etc.) show marked improvement as a function of stimulus level through most or all of the dynamic range, while electrode-place discrimination can improve or degrade as a function of level. In this study, effects of these combined potential influences were studied by examining the effects of stimulus level on syllable identification. We tested two hypotheses: that syllable identification varies as a function of stimulus level and that level and electrode configuration interact in affecting syllable identification. We examined vowel and consonant identification as a function of stimulus level for bipolar and monopolar electrode configurations. We used experimental processor maps where upper and lower stimulation limits of each electrode pair were equated to eliminate confounding effects of dynamic range, which varies across subjects and electrodes. For each channel, stimulation amplitude was set to a fixed percentage of its dynamic range. Eight adult subjects with Nucleus CI24M implants were tested using the SPEAK processing strategy. With each electrode configuration, stimulus levels were tested from 0% to 90% of the dynamic range in nine steps. The effects on consonant and vowel identification were similar. Phoneme identification was usually better for monopolar than for bipolar stimulation. In the lower half of the dynamic range, syllable identification usually increased as a function of stimulus level. In the upper half of the dynamic range, syllable identification continued to increase as a function of level to 90% of the dynamic range for some subjects, while for others there was no appreciable change or a decrease as a function of level. Decreases in performance at high levels were more common with monopolar than bipolar stimulation. These results suggest that if speech processors are programmed to optimize level for each individual, speech perception performance could be improved.

Acoustic Stimulation↗
PubMed1992

Identification of nociceptive neurons in the medial thalamus: morphological studies of nociceptive neurons with intracellular injection of horseradish peroxidase.

Somatosensory neurons including nociceptive ones in the medial thalamus have been studied extracellularly, and are classified into three types: nociceptive-specific, wide dynamic range, and tap neurons. However, the morphological characteristics of these three neurons have not yet been clarified. We studied the morphological characteristics of the neurons by iontophoretic injection of HRP into single neurons in 32 cats. Nine wide dynamic range neurons and two tap neurons were electrophysiologically identified and successfully stained with HRP in and around the parafascicular and subparafascicular nuclei, and in the mediodorsal nucleus. All nine wide dynamic range neurons had fewer dendrites which formed scanter tufts, whereas the two tap neurons had many more dendrites which formed denser tufts: i.e. wide dynamic range neurons were of the isodendritic type, and the tap neurons were of the allodendritic type, according to Ramón-Moliner's classification. The axons of the two tap neurons ran antero-laterally whereas those of wide dynamic range neurons ran in many directions. These morphological differences suggest that tap neurons may have more specialized and fixed functions than wide dynamic range neurons.

Animals↗
PubMed1985

Temporal response features of cat auditory cortex neurons contributing to sensitivity to tones delivered in the presence of continuous noise.

Single cat auditory cortex neurons have limited intensity dynamic ranges for characteristic frequency (CF) tones. In the presence of continuous wide-spectrum noise, these cells' tone responses undergo a dynamic range shift towards higher SPLs. In the present study, the mechanisms underlying this dynamic range shift were examined by probing the sensitivity of the cells to CF tones delivered at various delays after the onset and/or offset of a long duration noise mask. Fifty cells were studied in the cortex of 7 anesthetized cats using acoustically mixed tonal and noise stimuli presented monaurally to the contralateral ear through a calibrated, sealed stimulating system. For most neurons, the dynamic range shift induced by continuous noise was fully developed in the responses to CF tones delivered 100-250 ms after the onset of a noise mask. For nonmonotonic cells, shorter delays between noise and tone onsets resulted in a profound suppression of tone responses that was consistent with the view that noise stimuli evoke a short latency, but transient, inhibitory response in these neurons. Studies of monotonic cells with short tone delays revealed that the usual excitatory response to noise onset was sometimes followed by a period of inhibition. In most cells, as soon after mask onset that CF tones were able to evoke spike discharges, those responses had latent periods comparable to those of responses to tones of the same SPL delivered in continuous noise. After the offset of an 800 ms noise mask effecting a 15-25 dB dynamic range shift for CF tones, recovery of tone sensitivity to within 5 dB of control levels typically took 50-200 ms. On the basis of these observations, it is argued that in order for a CF tone to excite a cortical neuron after the onset of a noise mask, the tone amplitude must be sufficient to overcome both the transient central neural consequences of noise onset, and a short-term adaptation that is probably peripheral in origin. The implications of these data for the sensitivity of cortical cells to temporally varying stimuli are discussed.

Animals↗
PubMed2002

Exogenous tumor necrosis factor-alpha induces abnormal discharges in rat dorsal horn neurons.

STUDY DESIGN: An electrophysiologic study to examine effects of exogenous application of tumor necrosis factor-alpha (TNF-alpha ) activities and nociresponses of dorsal horn neurons in the spinal cord at L5. OBJECTIVES: To investigate the role of TNF-alpha in the induction and development of hyperalgesia in neural mechanisms responsible for a radicular pain. SUMMARY OF BACKGROUND DATA: TNF-alpha is found in the herniated disc and known to play a pivotal role in the development of inflammatory hyperalgesia; however, it is not known whether TNF-alpha causes abnormal discharge in the dorsal horn neurons and enhances nociresponse. METHODS: Single-unit activities of neurons in the L5 superficial dorsal horn were extracellularly recorded, using 28 urethane-anesthetized rats. The wide dynamic range and nociceptive-specific neurons activated by stimulation of the hind paw were selected. Effects of exogenous TNF-alpha were examined regarding 1) spontaneous discharges of wide dynamic range and nociceptive-specific neurons, 2) responses of wide dynamic range neurons to noxious stimulation, and 3) morphologic changes in the dorsal root ganglion. RESULTS: Application of TNF-alpha to the nerve root induced 1) a significant increase in spikes/sec in spontaneous discharges of wide dynamic range and nociceptive-specific neurons, 2) enhanced responses of wide dynamic range neurons to noxious stimulation, and 3) inflammatory changes in the ganglion. CONCLUSION: These results suggest the possibility that TNF-alpha produced in the vicinity of nerve roots due to disc herniation might cause ectopic discharges in primary afferent fibers and thereby induce the prolonged excitation in pain-processing neurons responsible for radicular pain.

Action Potentials↗
PubMed1999

Amplitude mapping and phoneme recognition in cochlear implant listeners.

OBJECTIVE: Speech and other environmental sounds must be compressed to accommodate the small electric dynamic range in cochlear implant listeners. The objective of this paper is to study whether and how amplitude compression and dynamic range reduction affect phoneme recognition in quiet and in noise for cochlear implant listeners. DESIGN: Four implant listeners using the Nucleus-22 SPEAK speech processor participated in this study. The amount of compression was varied by manipulating the Q-value in the SPEAK processor. The size of the dynamic range was systematically reduced by increasing the threshold level and decreasing the comfortable level in the processor. Both female- and male-talker vowel and consonant materials were used to evaluate speech recognition performance in quiet and in noise. Speech-spectrum-shaped noise was mixed with the speech signal and presented continuously to the speech processor through a direct electric connection. Signal to noise ratios were changed over a 30 to 40 dB range, within which phoneme recognition increased from chance to asymptotic performance. Phoneme recognition scores were obtained as the number of active electrodes was reduced from 20 to 10 to 4. For purposes of comparison, phoneme recognition data also were collected in four normal-hearing listeners under comparable laboratory conditions. RESULTS: In both quiet and noise, the amount of amplitude compression did not significantly affect phoneme recognition. The reduction of dynamic range marginally affected phoneme recognition in quiet, but significantly degraded phoneme recognition in noise. Generally, the 20- and 10-electrode processors produced similar performance, whereas the 4-electrode processor produced significantly poorer performance. Compared with normal-hearing listeners, cochlear-implant listeners required higher signal to noise ratios to achieve comparable recognition performance and produced significantly lower recognition scores at the same signal to noise ratios. CONCLUSIONS: The amount of amplitude compression does not significantly affect phoneme recognition, whereas reducing dynamic range significantly lowers phoneme recognition, particularly in noise and for vowels. Because the SPEAK processor extracts mostly spectral peaks, the present conclusions may not be applied to other types of processors extracting temporal envelope cues. The present results also suggest that more than four electrodes are required to optimize speech recognition in multiple-talker and noise conditions. A significant performance gap in speech recognition still remains between cochlear implant and normal-hearing listeners at the same signal to noise ratios. Improved cochlear implant designs and fitting procedures are required to narrow and, hopefully, close this performance gap.

Adult↗
PubMed1997

Electrophysiological evidence that neurokinin A acts via NK-1 receptors in the cat dorsal horn.

The aim of the present study was to investigate the effects of the non-peptide NK-2 receptor antagonist, SR 48968 on the responses of dorsal horn neurons to iontophoretic application of the endogenous NK-2 receptor ligand, neurokinin A, and on synaptically elicited responses in chloralose-anaesthetized cats. The effect of iontophoretic application of neurokinin A was tested on 51 dorsal horn neurons. Of these, 43 were wide dynamic range and the rest non-nociceptive neurons. Neurokinin A induced a slow, prolonged excitation of 25 of the wide dynamic range neurons. All remaining neurons were unaffected. SR 48968 (50 microg to 1.0 mg/kg, i.v.) did not affect the on-going basal activity (n = 8) or the slow excitation induced by neurokinin A in any of the nine wide dynamic range neurons tested. To eliminate the possibility that systemically administered SR 48968 may not be reaching central sites, SR 48968 was also applied iontophoretically (70-120 nA) to five neurons and tested against excitatory responses to iontophoretically applied neurokinin A. The on-going activity of these cells were unaffected by SR 48968. The responses to neurokinin A were also unaffected suggesting that neurokinin A did not mediate its effects via NK-2 receptors. SR 48968 also had no effect on the excitatory responses of seven neurons to iontophoretic application of the NK-1 receptor agonist, substance P indicating that substance P actions are not mediated via NK-2 receptors and that SR 48968 did not react with NK-1 receptors. Responses of the neurons to non-noxious (hair) stimulation (n = 10), noxious mechanical (n = 5) and noxious thermal (n = 8) stimulation of the receptive field were also unaffected by SR 48968, suggesting a lack of participation of NK-2 receptors in these responses. However, responses of wide dynamic range neurons to neurokinin A were totally blocked by i.v. administration (0.5 mg/kg) of the NK-1 receptor antagonists CP-96,345 (n = 7) and CP-99,994 (n = 5) but not by CP-96,344 (n = 4), the inactive enantiomer of CP-96,345. These data suggest that neurokinin A, like substance P may be acting via NK-1, rather than NK-2 receptors, to produce excitation of wide dynamic range neurons in the dorsal horn of the cat spinal cord.

Animals↗
PubMed1997

A digital filterbank hearing aid: predicting user preference and performance for two signal processing algorithms.

OBJECTIVE: In a series of experiments with a wearable binaural digital hearing aid, two hearing aid processing algorithms were compared. Both algorithms provided individual frequency shaping via a seven-band filterbank with compression limiting in the high-frequency channel. They differed in the processing of the low-frequency channel, using dynamic range compression for one (DynEar) and linear processing with compression limiting for the other (LinEar). In a pilot field test we found that LinEar/ DynEar preference based on use time could be predicted from auditory dynamic range data. For the subjects who preferred DynEar, the mean dynamic range was broader for low and mid frequencies and narrower for high frequencies, as compared with the LinEar preference subjects. These groupings were tested as predictors of user preference and performance in a main field test. DESIGN: The main study included 26 hearing aid users with symmetrical sensorineural losses. The algorithms were compared in a one-mo-long blind field test. A data logger function was included for objective recording of the total time each algorithm was used and how the volume controls were used. The preference was based on the time used for each algorithm and on subjective statements. Threshold signal-to-noise ratio (S/N-threshold) for speech was tested, and sound quality ratings were obtained through a questionnaire. We also tested the S/N-thresholds for the subjects' conventional (own) aids. RESULTS: The preference was correctly predicted by the dynamic range data on 12 out of 15 new cases. S/N-thresholds were lower for the preferred fittings compared with the nonpreferred fittings and with the subjects' own aids. In the questionnaire the preferred fittings were rated significantly higher in terms of overall impression and clearness. Because of the systematic way the DynEar-preference subjects adjusted the high-frequency DynEar gain, we speculate that upward spread of masking may have been a factor in preference and performance. Additionally, LinEar-preference subjects' preference and performance might have been influenced by excessive compression ratios with the DynEar processing in these cases. CONCLUSIONS: 1. Preference for DynEar versus LinEar depends on the auditory dynamic range. 2. S/N-thresholds for speech were better for the preferred fittings, which also were rated higher in terms of overall impression of sound quality and clearness.

Adult↗
PubMed2002

The effect of sound duration on rate-amplitude functions of inferior collicular neurons in the big brown bat, Eptesicus fuscus.

During echolocation, the amplitude and duration of echo pulses of the big brown bat, Eptesicus fuscus, covary throughout the entire course of hunting. The purpose of this study was to examine if variation in sound duration might affect the amplitude selectivity of inferior collicular (IC) neurons of this bat species under free-field stimulation conditions. A family of rate-amplitude functions of each IC neuron was obtained with different sound durations. The effect of sound duration on the neuron's amplitude selectivity was then studied by examining the type, best amplitude, dynamic range and slope of each rate-amplitude function. The rate-amplitude functions of 83 IC neurons determined with different sound durations were either monotonic, saturated or non-monotonic. Neurons with monotonic rate-amplitude functions had the highest best amplitude, largest dynamic range but smallest slope. Neurons with non-monotonic rate-amplitude functions had the lowest best amplitude, smallest dynamic range but largest slope. The best amplitude, dynamic range and slope of neurons with saturated rate-amplitude functions were intermediate between these two types. Rate-amplitude functions of one group (47, 57%) of IC neurons changed from one type to another with sound duration and one-third of these neurons were tuned to sound duration. As a result, the best amplitude, dynamic range, and slope also varied with sound duration. However, rate-amplitude functions of the other group (36, 43%) of IC neurons were hardly affected by sound duration and two-thirds of these neurons were tuned to sound duration. Biological relevance of these findings in relation to bat echolocation is discussed.

Acoustic Stimulation↗
PubMed2001

Wide dynamic dose range of VIPAR polymer gel dosimetry.

In this work the extent of the linear dose response and the dynamic dose range of N-vinylpyrrolidone-argon based (VIPAR) polymer gels were investigated. VIPAR gels were irradiated using a 6 MV linear accelerator up to 60 Gy and a Nucletron microSelectron 192Ir HDR brachytherapy source to much higher doses to cover a dose range of two orders of magnitude. They were then MR scanned at 1.5 T to obtain T2-maps. VIPAR gel measurements obtained from the two irradiation regimes were calibrated against ion chamber measurements and dose calculations derived using the AAPM TG-43 protocol respectively. A satisfying agreement between the calibration results derived using the 6 MV x-rays and the 192Ir source was found for doses up to 60 Gy, implying that the response of the VIPAR gels is independent of photon energy and dose rate. A linear R2 dose response up to approximately 40 Gy and a dynamic dose range up to at least approximately 250 Gy were observed. VIPAR gel dose measurements derived using the monoexponentially fitted brachytherapy calibration data were found to be quite accurate.

Algorithms↗
PubMed1996

Coding of odor intensity in a steady-state deterministic model of an olfactory receptor neuron.

The coding of odor intensity by an olfactory receptor neuron model was studied under steady-state stimulation. Our model neuron is an elongated cylinder consisting of the following three components: a sensory dendritic region bearing odorant receptors, a passive region consisting of proximal dendrite and cell body, and an axon. First, analytical solutions are given for the three main physiological responses: (1) odorant-dependent conductance change at the sensory dendrite based on the Michaelis-Menten model, (2) generation and spreading of the receptor potential based on a new solution of the cable equation, and (3) firing frequency based on a Lapicque model. Second, the magnitudes of these responses are analyzed as a function of odorant concentration. Their dependence on chemical, electrical, and geometrical parameters is examined. The only evident gain in magnitude results from the activation-to-conductance conversion. An optimal encoder neuron is presented that suggests that increasing the length of the sensory dendrite beyond about 0.3 space constant does not increase the magnitude of the receptor potential. Third, the sensitivities of the responses are examined as functions of (1) the concentration at half-maximum response, (2) the lower and upper concentrations actually discriminated, and (3) the width of the dynamic range. The overall gain in sensitivity results entirely from the conductance-to-voltage conversion. The maximum conductance at the sensory dendrite appears to be the main tuning constant of the neuron because it determines the shift toward low concentrations and the increase in dynamic range. The dynamic range of the model cannot exceed 5.7 log units, for a sensitivity increase at low odor concentration is compensated by a sensitivity decrease at high odor concentration.

Models, Neurological↗
PubMed1992

Regulating ATP turnover rates over broad dynamic work ranges in skeletal muscles.

It has long been appreciated that rates of ATP utilization and production need to be extremely closely balanced. To put it in molecular rather than molar terms, in human muscle engaged in a 15-min work protocol, approximately 3.3 x 10(20) ATP/g are used and resynthesized at approximately 100 times the resting cycling rates before fatigue, during which time only a 20-25% decrease in the ATP pool is sustained. Analysis of how such remarkable regulatory precision is achieved suggests that in resting muscle myosin behaves as a latent catalyst whose full catalytic potential 1) is realized with the arrival of an activator signal (Ca2+) and 2) is tempered with reaction products; such proactive control, initiated at ATP utilization, sets the required flux through ATP-producing pathways. For any given enzyme step in ATP-producing pathways, reaction velocity (v) becomes the independent parameter, with substrate concentration ([S], the dependent parameter) being adjusted accordingly. Because the dynamic range for muscles (change from resting to maximum ATP turnover rates) can exceed 100-fold, in many studies of working muscle the percent change in ATP turnover rate exceeds (sometimes by very large margins) the percent change in [S]. These observations are not easily explained by current metabolic regulation models but are consistent with pathway enzymes behaving as latent catalysts in resting muscle. In this view, the unmasking of such latent catalytic potential is the main explanation for how large changes in v can be achieved with modest (sometimes immeasurable) changes in [S].(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed2004

Effects of pulse rate and electrode array design on intensity discrimination in cochlear implant users.

The effects of pulse rate on intensity discrimination were evaluated in 14 subjects with Clarion C-I cochlear implants. Subjects had a standard [Clarion spiral electrode array (SPRL group)] or perimodiolar electrode array [Clarion HiFocus electrode array with electrode positioning system (HF+EPS group)]. Weber fractions for intensity discrimination [ Wf(dB)= 10 log deltaI/I] were evaluated at five levels over dynamic range at each of three pulse rates (200, 1625 and 6500 pps) using monopolar stimulation. Weber fractions were smaller for 200 pps stimuli than for 1625 or 6500 pps stimuli in both groups. Weber fractions were significantly smaller for SPRL subjects (mean Wf(dB) = -9.1 dB) than for HF+EPS subjects (mean Wf(dB) = -6.7 dB). Intensity difference limens (DLs) expressed as a percentage of dynamic range (DR) (deltaI%DR= deltaI/DRdB* 100) did not vary systematically with pulse rate in either group. Larger intensity DLs combined with smaller dynamic ranges led to fewer intensity steps over the dynamic range for HF+EPS subjects (average 9 steps) compared to SPRL subjects (average 23 steps). The observed effects of pulse rate and electrode array design may stem primarily from an inverse relationship between absolute current amplitude and the size of intensity DLs. The combination of smaller dynamic ranges and larger Weber fractions in HF+EPS subjects could be the result of increased variability of neural outputs in these subjects.

Adult↗
PubMed1993

Activity of deep dorsal horn neurons in the anaesthetized rat during hyperalgesia of the hindpaw induced by ultraviolet irradiation.

Thermal hyperalgesia was induced by UV irradiation of the glabrous skin of the hindpaw of adult female Sprague-Dawley rats. We have recorded single cell activity and studied excitability changes in wide dynamic range neurons in the lumbar spinal segments during the early phase (days 1-3) and late phase (days 5-7) of thermal hyperalgesia in animals under urethane anaesthesia. The proportion of spontaneously active wide dynamic range cells was increased following UV irradiation and the degree of spontaneous activity was enhanced during the course of hyperalgesia. In addition there was a significant increase in the total number of spikes evoked by standardized mechanical and noxious heat stimuli when tested at days 1-3 and days 5-7. The duration of the evoked responses was also significantly prolonged in both UV-treated groups. The noxious temperature threshold to radiant heat stimulation was significantly decreased on the UV-treated but not on the contralateral hindpaw. The average size of the receptive fields on the UV-treated paws was expanded in comparison to control. To differentiate between possible central and peripheral components of the hyperactivity of wide dynamic range cells we performed in situ dorsal rhizotomy during the recording. Cutting the dorsal roots (L2-5) evoked a significantly larger and more prolonged discharge in wide dynamic range cells in both UV-treated groups in comparison to control. Spontaneous activity in spinal wide dynamic range neurons was reduced after rhizotomy in each group. However, the decrease was only significant at days 1-3 (P < 0.05) but not at days 5-7.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia↗