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Results for “Differential Threshold”

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PubMed1995

Effect of acoustic overstimulation on the hydropic ear.

The effect of acoustic overstimulation (2 kHz pure tone) on the compound action potential (CAP) threshold was investigated at frequencies ranging from 2 to 16 kHz using albino guinea pigs, both normal and with experimentally induced endolymphatic hydrops. The hydropic ears were less susceptible to acoustic overstimulation than the normal ears. As the CAP threshold was raised, the frequency exhibiting the greatest CAP threshold shift increased in both animal groups. The tendency was more noticeable in the hydropic ears than in the normal ears. These results are discussed from the aspect of cochlear hydrodynamics.

Action Potentials↗

PubMed1975

Threshold for pain from anterolateral quadrant stimulation as a predictor of success of percutaneous cordotomy for relief of pain.

A sensitive quantitative index for predicting optimal electrode position in percutaneous anterolateral cordotomy was determined by electrical stimulation through the lesioning electrode. If the threshold for pain elicited by the stimulation electrode was less than 300 muA, a 5-sec radiofrequency lesion of 50 mA would produce complete contralateral analgesia with no weakness. When the pain threshold exceeded 300 muA, incomplete or no analgesia would result with the standard single lesion. The results further suggested that the fibers in the anterolateral quadrant that transmit pain are discretely rather than diffusely localized.

Cordotomy↗

PubMed1992

Morphologic comparisons between rhodopsin-mediated and short-wavelength classes of retinal light damage.

The histologic manifestations of rhodopsin-mediated versus short-wavelength classes of retinal phototoxicity were compared after spectral exposures of the albino rat retina. Animals were exposed to wave-bands of light centered at the peak of rhodopsin absorbance (green, 500 nm) or in the ultraviolet A (UVA; 360 nm). Intensity-damage curves generated for each wave-band indicated that UVA light was 50-80 times more effective than green light at causing photoreceptor cell losses. Examination of early ultrastructural changes in rod inner segments, outer segments, and retinal pigment epithelium revealed a remarkable degree of similarity between UVA and green light-induced damage. Furthermore, the two classes of damage were indistinguishable in terms of post-exposure recovery from threshold damage and regional distribution of photoreceptor cell loss along the vertical meridian. The finding of essentially identical histologic manifestations for the two classes of damage raises the possibility that they share a common biochemical etiology or pathway of cell destruction.

Animals↗

PubMed1997

Prognostic factors in intraoperative facial nerve monitoring for acoustic neuroma.

OBJECTIVES: To determine the predictive value of intraoperative threshold stimulus for facial nerve outcome and the prevalence and prognostic value of persistent trains of activity and frequent spontaneous or mechanically induced contractions during acoustic neuroma surgery. STUDY DESIGN: Prospective recording and subsequent review of facial nerve activity. SETTING: Tertiary referral centre. PATIENTS AND METHODS: Consecutive patients undergoing acoustic neuroma surgery. Intraoperative facial nerve activity was digitised and stored on a personal computer for future analysis. Operative events were flagged. Recordings were available in 27 patients. MAIN OUTCOME MEASURES: Frequent mechanically induced contractions (< 20), prolonged trains of facial nerve activity (total time > 199 seconds), and facial nerve brainstem stimulus threshold were correlated with facial nerve outcome. RESULTS: A brainstem stimulus threshold > 0.1 mA was significantly associated with intermediate or poor facial nerve function (House-Brackmann grade > 2) on the sixth postoperative day, at 1 month and 6 months. Patients with normal or near-normal facial function on the first day and a threshold of > 0.1 mA were significantly more likely to develop a delayed facial nerve palsy. Frequent contractions were noted in 74% of patients and persistent train activity in 59%. Neither was predictive of facial nerve outcome. CONCLUSIONS: An elevated brainstem threshold is helpful in predicting delayed facial nerve palsy and suboptimal facial nerve outcome. Persistent train activity and frequent contractions, do not have major prognostic significance.

Adult↗

PubMed2001

Correlation between electrophysiological and morphological characteristics during maturation of rat supraoptic neurons.

The neurohypophysial peptides oxytocin (OT) and vasopressin (AVP) are well known for their role in reproductive functions and fluid balance regulation, respectively. During development, these peptides are thought to act as trophic factors on both peripheral and central structures. However, despite this early developmental function, the maturation of their secreting neurons remains poorly investigated. In this study, we have characterized the electrical and morphological characteristics displayed by OT and AVP supraoptic (SO) neurons between embryonic day 21 and postnatal day 20. Transient changes in passive membrane properties, correlated with a transient increase in the dendritic arborization, were observed at the beginning of the second postnatal week (PW2). The action potential matured mostly during PW1 and its threshold progressively hyperpolarized in parallel with the resting membrane potential. During PW1, SO neurons displayed unique characteristics with a low-threshold Ca(2+)-dependent depolarizing potential and a prominent hyperpolarization-activated current (I(h) ). This latter is involved in a depolarizing sag during hyperpolarization and an after hyperpolarizing potential following a depolarization. During this period, maintaining E(Cl) unchanged by the use of gramicidin-perforated patch recordings revealed excitatory GABAergic potentials, that became inhibitory during PW2, whilst glutamatergic potential appeared. The electrical activity was very erratic in young neurons and progressively differentiated in the typical firing observed in mature neurons (tonic and phasic for OT and AVP neurons, respectively) during PW2--3. These results show that the development of electrical properties of SO neurons is correlated with the maturation of their dendritic arborization.

Action Potentials↗

PubMed1996

Rank-ordered regulation of motor units.

Myoelectric signals were detected from the tibialis anterior muscle of 5 subjects with a quadrifilar needle electrode while the subjects generated isometric forces that increased linearly with time (10% of maximal voluntary contraction/s) up to maximal voluntary level. Motor unit firing rates were studied as a function of force throughout the full range of muscle force output. The relationship between force and firing rate was found to contain three distinct regions. At recruitment and near maximal force levels, firing rates increased more rapidly with force than in the intermediate region. Furthermore, in the regions with rapid increases, the rate of change of firing rate was correlated to the recruitment threshold, with higher recruitment threshold motor units displaying greater rates of change. In the intermediate region, all motor units had similar rates of change of firing rate. A weak positive correlation was found between initial firing rate and recruitment threshold. Firing rates of motor units at any instant were found to be ordered according to the recruitment order: at any given time in the contraction motor units with lower recruitment thresholds had higher firing rates than units with higher recruitment thresholds. Firing rates of all motor units were observed to converge to the same value at maximal forces. Mechanisms underlying motor unit recruitment and firing rate modulation are discussed in the context of a conceptual model.

Action Potentials↗

PubMed2004

Apolipoprotein epsilon4 is associated with more rapid decline in odor identification than in odor threshold or Dementia Rating Scale scores.

Individuals with the apolipoprotein E epsilon4 genetic risk factor for Alzheimer's disease (AD) show deficits in olfactory function. The purpose of the present study was to examine longitudinally odor identification (odor ID), odor threshold, picture identification, and global cognitive status in allele positive (epsilon4+) and negative (epsilon4-) persons. Participants were initially given the San Diego Odor Identification test, an odor threshold test, and the Dementia Rating Scale (DRS). Participants were re-tested approximately four years later. The results indicate: (1) odor ID declined more rapidly in epsilon4+ than in epsilon4- normal elderly adults; (2) neither group exhibited a significant decline in odor threshold, picture identification or DRS scores. These results suggest that declines in odor identification occur before declines in other measures of dementia in persons at risk for AD because of their APOE allele genetic status.

Aged↗

PubMed2001

Changes in tooth pulpal detection and pain thresholds in relation to jaw movement in man.

The effect of jaw movements on pulpal sensory thresholds to electrical stimulation was studied in healthy humans. The movements consisted of repeated jaw opening and closing at two different frequencies (1 and 3 s(-1)). The detection/perception and pain thresholds of an upper or lower central incisor were determined by stimulation with monopolar constant current pulses at two different durations (0.5 and 5.0 ms). In the absence of jaw movement, the control (baseline) pain threshold was significantly higher than the detection threshold, and both thresholds were significantly decreased with an increase of the stimulus pulse duration. During jaw movement, pulpal detection and pain thresholds were significantly elevated, independent of the duration of the stimulus pulse. The jaw movement-related increase in detection thresholds was significantly dependent on the rate of cyclical jaw movements and on the site of stimulation. An increase in pulpal sensory thresholds was observed with stimulation of the lower incisor only; there was no change in thresholds for the upper incisor. Pulpal detection thresholds were significantly more elevated during jaw movement than pulpal pain thresholds. The results indicate that the reduction in pulpal sensitivity is related to the jaw movements. The effect of jaw movement on pulpal detection thresholds was segmentally restricted. In contrast, modulation of the pulpal pain thresholds was considerably weaker. The jaw movement-related suppression of pulpal sensitivity may be explained by activation of segmental afferent-induced inhibition, corollary efferent barrage from motor to sensory areas, or a combination of both.

Adult↗

PubMed1991

Experimental and clinical main forms of hypoxic-ischaemic brain damage and their monitoring.

A short review of main pathogenetic forms of hypoxic-ischaemic brain damage and its consequences for causally orientated therapy and monitoring is given. Different pathogenetic components act in several combinations. In these processes disturbances of the cardio-vascular system, of the haemorheology mainly at the level of microvessels, and disturbances of the blood brain barrier are involved. Furthermore, there are relevant disturbances of the neuronal metabolism such as accumulation of cytosolic Ca++ or disturbances caused by the effect of radicals, lipid peroxidation and changes of protein synthesis. Nowadays, chances of a cause-related therapy are increased by improved evaluation of main pathogenetic components especially regarding secondary brain damage. To evaluate these components in detail, animal models must be more clinically related, especially regarding long-term studies. The increasing knowledge about critical thresholds of reversible and irreversible brain changes also favours effective cerebral function monitoring. The main conclusions from previous experimental as well as clinically related studies indicate the necessity of very early therapeutical interventions.

Brain↗

PubMed1977

Somatic and autonomic indexes of recovery from electroconvulsive shock-induced amnesia in rats.

Autonomic response indexes of experimental amnesia have recently been found to have higher electroconvulsive shock (ECS) intensity thresholds and steeper retrograde gradients than have traditional somatic indexes. The present studies examined the hypothesis that recovery from somatically indexed experimental amnesia depends upon the existence of autonomically available residual memory. In a between-subjects design, a 200-mA ECS was used to produce amnesia for a tone-footshock pairing as indicated by lick suppression, defection, and bradycardia. The next day, these amnesic animals received a reminder footshock outside of the training apparatus, which was found to restore memory on a test trial 24 hr later. The behavior of control groups indicated that this reminder effect was due to the restoration of specific memory rather than systemic consequences of treatment. With a within-subjects design, a second experiment obtained a reminder effect in animals individually shown to be "fully" amnesic by all three response indexes monitored. A third experiment varied the intensity of the reminder footshock and revealed that the different memory indexes examined do not have reminder-footshock thresholds inversely related to their initial resistance to amnesia. The results support a retrieval-failure view of experimental amnesia and suggest that the same fundamental physiological processes underlie both autonomically indexed memory and somatically indexed memory.

Acoustic Stimulation↗

PubMed2007

Wavelength dependence of ocular damage thresholds in the near-ir to far-ir transition region: proposed revisions to MPES.

This report summarizes the results of a series of infrared (IR) laser-induced ocular damage studies conducted over the past decade. The studies examined retinal, lens, and corneal effects of laser exposures in the near-IR to far-IR transition region (wavelengths from 1.3-1.4 mum with exposure durations ranging from Q-switched to continuous wave). The corneal and retinal damage thresholds are tabulated for all pulsewidth regimes, and the wavelength dependence of the IR thresholds is discussed and contrasted to laser safety standard maximum permissible exposure limits. The analysis suggests that the current maximum permissible exposure limits could be beneficially revised to (1) relax the IR limits over wavelength ranges where unusually high safety margins may unintentionally hinder applications of recently developed military and telecommunications laser systems; (2) replace step-function discontinuities in the IR limits by continuously varying analytical functions of wavelength and pulsewidth which more closely follow the trends of the experimental retinal (for point-source laser exposures) and corneal ED50 threshold data; and (3) result in an overall simplification of the permissible exposure limits over the wavelength range from 1.2-2.6 mum. A specific proposal for amending the IR maximum permissible exposure limits over this wavelength range is presented.

Animals↗

PubMed2004

Mechanism for neuronal spike generation by small and large ion channel clusters.

Neuronal action potentials are generated by clusters of ion channels between the Hillock and the first segment. If the clusters comprise a large number of sodium and potassium channels, action potentials are generated if the membrane potential exceeds a threshold of about -55 mV. Such behavior is well described by an excitable model such as, for example, the Hodgkin-Huxley equations. In this paper we show through stochastic modeling that if the size of the generating ion channel cluster is small, action potentials are generated regardless of whether the membrane potential is below or above the excitation threshold. Action potential generation is then determined by single-channel kinetics. We further show that this switch in generation mechanism manifests itself in peculiar statistical properties of the generated spike trains at small cluster sizes.

Action Potentials↗

PubMed2004

Predicting the threshold of pulse-train electrical stimuli using a stochastic auditory nerve model: the effects of stimulus noise.

The incorporation of low levels of noise into an electrical stimulus has been shown to improve auditory thresholds in some human subjects (Zeng et al., 2000). In this paper, thresholds for noise-modulated pulse-train stimuli are predicted utilizing a stochastic neural-behavioral model of ensemble fiber responses to bi-phasic stimuli. The neural refractory effect is described using a Markov model for a noise-free pulse-train stimulus and a closed-form solution for the steady-state neural response is provided. For noise-modulated pulse-train stimuli, a recursive method using the conditional probability is utilized to track the neural responses to each successive pulse. A neural spike count rule has been presented for both threshold and intensity discrimination under the assumption that auditory perception occurs via integration over a relatively long time period (Bruce et al., 1999). An alternative approach originates from the hypothesis of the multilook model (Viemeister and Wakefield, 1991), which argues that auditory perception is based on several shorter time integrations and may suggest an NofM model for prediction of pulse-train threshold. This motivates analyzing the neural response to each individual pulse within a pulse train, which is considered to be the brief look. A logarithmic rule is hypothesized for pulse-train threshold. Predictions from the multilook model are shown to match trends in psychophysical data for noise-free stimuli that are not always matched by the long-time integration rule. Theoretical predictions indicate that threshold decreases as noise variance increases. Theoretical models of the neural response to pulse-train stimuli not only reduce calculational overhead but also facilitate utilization of signal detection theory and are easily extended to multichannel psychophysical tasks.

Algorithms↗

PubMed1976

Photoreceptor thresholds and visual pigment levels in normal and vitamin A-deprived Xenopus tadpoles.

1. Measurements of the aspartate-isolated PIII component of the electroretinogram (ERG) were used to estimate photoreceptor threshold in dark-adapted tadpoles of the clawed toad, Xenopus laevis raised on a normal diet. Spectral sensitivity functions established that the rod mechanism governed absolute dark thresholds from stage 40 to the end of premetamorphic development. 2. Parallel measures of rod outer-segment dimensions and visual pigment levels demonstrated a) that visual pigment concentration remained constant at all tadpole stages, and b) that the fall in threshold over the course of premetamorphic development depended exclusively on the increased probability of quantal absorption that accompanied the growth of the rod outer segments. 3. When tadpoles were obtained from vitamin A-deficient females and raised subsequently on a vitamin A-free diet, the first appearance of the ERG was delayed and its absolute threshold raised, with respect to controls. 4. Histological examination of vitamin A-deprived retinas indicated that rod outer segments retained their structural integrity in spite of up to a 75% loss of visual pigment. 5. The threshold rise which accompanied a fall in visual pigment levels, whether effected by dietary deficiency or by bleaching, was greater than could be attributed solely to a reduction in the probability of quantal absorption.

Age Factors↗

PubMed1993

Circadian variations in ischemic threshold and their relation to the occurrence of ischemic episodes.

BACKGROUND: The occurrence of ischemic episodes during daily activity has been reported to exhibit a bimodal circadian distribution, yet its relation to the ischemic threshold (heart rate at 1-mm ST segment depression) has not been explored. METHODS AND RESULTS: To determine whether the ischemic threshold during daily activity exhibits a circadian pattern that might relate to the frequency of occurrence of ischemic episodes, we studied the time of occurrence and the heart rate at onset of ischemia in 1,371 ischemic episodes recorded in 41 patients with stable coronary disease, positive exercise testing, and repeated ischemic episodes during ambulatory ECG monitoring (AEM). All patients had 7 days of AEM; 23 were off any anti-ischemic therapy, while 18 were on low dose of beta-blockers. The occurrence of ischemic episodes exhibited the typical bimodal circadian distribution with a prominent peak between 7:00 and 11:00 AM and a second less prominent peak between 6:00 and 9:00 PM. The threshold of myocardial ischemia exhibited a different single-peaked circadian distribution; it was lowest between 1:00 and 3:00 AM and highest between 10 AM and 1 PM. Time series analyses indicated a strong hour-by-hour trend of each of the two circadian distributions, whereas the two series cross-correlated maximally at a lag of zero hours (p < 0.01), indicating a complex interplay between myocardial oxygen demand and supply in determining the occurrence of ischemic episodes during daily activity. The morning increase in the frequency of ischemic episodes could not be attributed to a reduced threshold but rather to an increase in demand. The low threshold at night-time might probably indicate that the mechanism of ischemia during these hours is reduced coronary flow due to increased coronary tone. Secondary analyses for several predefined patients' subsets gave similar results. Patients who received low-dose beta-blockers maintained the bimodal circadian distribution of the occurrence of ischemic episodes, whereas the ischemic threshold exhibited a constant pattern with no circadian changes. CONCLUSIONS: Our results demonstrate that myocardial oxygen demand is a major determinant of daily ischemia, yet changes in the ischemic threshold that probably reflect dynamic changes in coronary tone play also an important role. The relative contribution of increased demand and decreased threshold to the genesis of ischemic episodes during daily activity can be assessed by AEM and may help to optimize medical therapy.

Adrenergic beta-Antagonists↗

PubMed1999

Electrically evoked potentials recorded in adult and pediatric CLARION implant users.

The purpose of this study was to record electrical auditory brain stem responses (EABRs) and electrical middle latency responses (EMLRs) in the clinic from 3 adult CLARION Multi-Strategy Cochlear Implant subjects and to record EABRs in the operating room from 3 pediatric subjects. For 2 of the 3 adult subjects, EABR thresholds were within the subject's behavioral dynamic range, whereas 1 adult subject's EABR thresholds were either absent or, when present, exceeded the upper limit of the dynamic range. For this subject, EMLRs were absent or of poor morphology for the channels tested. Moreover, this subject was not able to understand speech in an open-set, auditory-only format. The EABR thresholds obtained with children were within the behavioral dynamic range for 2 of the 3 subjects, but exceeded comfortable loudness levels for 1 subject. Although the EABR thresholds were measured at stimulus levels that were audible for all subjects, the relationship of the EABR threshold levels to behavioral measures of loudness varied. Evoked potentials that originate more centrally, such as the EMLR, should be investigated further to determine the possible relationship to postimplant performance.

Adult↗

PubMed1996

Persistent increased threshold of electrical stimulation selective to motor nerve in multifocal motor neuropathy.

In multifocal motor neuropathy (MMN) the threshold of electrical stimulation showed a persistent, marked increase for the motor nerve which decreased after treatment with intravenous immunoglobulin or oral cyclophosphamide; whereas, the threshold was normal for the sensory nerve. This discrepancy of the thresholds for motor and sensory nerves indicates that the increased threshold for motor nerve is not caused by change in perineural capacitance, such as subperi- and endoneural edema or perineural thickening. Inching studies showed that the site of the elevated motor nerve threshold was closely associated with conduction slowing and block. For the cause of the increased threshold, therefore, we suppose the presence of a factor which interferes with reorganization of the nodal property in the remyelinative process or which directly blocks sodium channels where the blood-nerve barrier is impaired in MMN.

Action Potentials↗

PubMed1994

Ventilatory responses to exercise performed below and above the first ventilatory threshold.

These experiments examined the changes in ventilation at the start and end of exercise. Six subjects walked on a treadmill at two work rates above and two below that corresponding to their first ventilatory thresholds, for three durations. The subjects also exercised at the lowest and highest work rates while inspiring oxygen-enriched air. The group mean results showed that the abrupt increases in ventilation at the start of exercises at work rates above that of the first ventilatory threshold were greater than those below, but did not vary with duration or work rate either above or below. The abrupt falls in ventilation at the end of the exercises were less than the increases at the start. At work rates above that of the first ventilatory threshold, increases in work rate and duration were found to reduce the abrupt falls. The time constants of exponential curves fitted to the post-exercise declines in ventilation increased with work rate, and also with duration for work rates above that of the first ventilatory threshold. Finally, breathing oxygen enriched air did not alter any of these variables. These findings were interpreted as showing that the fast neural exercise drive is enhanced at work rates above that of the first ventilatory threshold, and becomes progressively less as exercise continues, a process exaggerated at higher work rates. In addition, the time course of the decline in ventilation following exercise, although altered by work rate and duration, was independent of the level of oxygenation.

Adult↗