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PubMed1995

Stereopsis, spatial frequency and retinal eccentricity.

Stereoscopic depth discrimination thresholds increase with retinal eccentricity and distance from the horopter. However, in contrast to spatial resolution, the effects of spatial frequency on stereo-thresholds in the periphery are unknown. For spatial vision, it is generally assumed that the retina is comprised of a series of overlapping spatial filter mechanisms and that there is a commensurate increase in spatial scale as a function of retinal eccentricity. If the same holds true for mechanisms sensitive to stereoscopic depth, then stereo-thresholds for low spatial frequency stereoscopic stimuli may remain relatively invariant across the visual field, while thresholds for relatively high spatial frequency stimuli would increase. To further understand the role of the disparity sensitive mechanisms involved in depth discrimination, increment depth discrimination thresholds for both crossed and uncrossed disparities were measured as a function of eccentricity for retinal locations up to 10.0 deg along the horizontal meridian. We found that stereoscopic depth discrimination thresholds, as a function of distance from the horopter, increased in an exponential manner irrespective of spatial frequency. Stereo-thresholds also increased as a function of retinal eccentricity, however, the rate of increase depended on the spatial frequency composition of the stimuli. Best stereo-thresholds for stimuli composed of low spatial frequencies remained relatively invariant for retinal eccentricities up to 10.0 deg, while thresholds for the high spatial frequency stimuli increased with eccentricity.

Depth Perception↗
PubMed2006

Threshold behavior in the initiation of hippocampal population bursts.

Hippocampal population discharges such as sharp waves, epileptiform firing, and GDPs recur at long and variable intervals. The mechanisms for their precise timing are not well understood. Here, we show that population bursts in the disinhibited CA3 region are initiated at a threshold level of population firing after recovery from a previous event. Each population discharge follows an active buildup period when synaptic traffic and cell firing increase to threshold levels. Single-cell firing can advance burst onset by increasing population firing to suprathreshold values. Population synchrony is suppressed when threshold frequencies cannot be reached due to reduced cellular excitability or synaptic efficacy. Reducing synaptic strength reveals partially synchronous population bursts that are curtailed by GABA(B)-mediated conductances. Excitatory glutamatergic transmission and delayed GABA(B)-mediated signals have opposing feedback effects on CA3 cell firing and so determine threshold behavior for population synchrony.

Animals↗
PubMed2001

Inverse relation between input resistance and threshold current in canine cardiac syncytium.

INTRODUCTION: The present study was designed to examine the extent to which differences in input resistance contribute to the heterogeneity of excitability in canine endocardium. METHODS AND RESULTS: The experiments were performed in isolated canine right anterior papillary muscles. The preparations were superfused with oxygenated Tyrode's solution at 36 degrees to 37 degrees C. Conventional methods were used for recording and stimulation. To obtain pertinent data, two microelectrodes were inserted into the same cell or into two contiguous cells. One microelectrode injected intracellular hyperpolarizing current to polarize the membrane; the second microelectrode recorded the changes in transmembrane potential. The two micreoelectrodes were separated by 8 to 10 microm. The procedure used provided input resistance and threshold current values for the doubly impaled cell or the two contiguous cell units. The same procedure was repeated at different sites of the preparation. The plot of input resistance versus threshold current showed a good fit (R2 = 0.97) between the experimental data and the curve for the rectangular hyperbola XY = 30 mV; therefore, the input resistance and the threshold current are inversely related. CONCLUSION: The results indicate that the canine cardiac syncytium is nonhomogeneous with respect to input resistance and that input resistance is inversely related to the minimal current needed to reach threshold. Accordingly, the electrical excitability of the cells studied also is inhomogeneous.

Animals↗
PubMed1995

Effects of furosemide on distortion product otoacoustic emissions and on neuronal responses in the anteroventral cochlear nucleus.

1. The objective of this study was to precisely evaluate the relationship between the threshold of neurons in the anteroventral cochlear nucleus (AVCN) and the properties of distortion product otoacoustic emissions (DPOAEs). Response areas of multiunit clusters in the AVCN and DPOAEs in the ear canal were measured alternately in the adult gerbil during furosemide-induced changes of the endocochlear potential. Stimulus frequencies of the probe tones for DPOAE measurement were in the range of f1 = 1.7-7.6 kHz and f2 = 2.0-9.0 kHz; the ratio f2:f1 was always 1.18. Stimulus amplitudes were varied in 5-dB steps from 30 to 80 dB SPL, with either equal amplitudes (L1 = L2) or unequal, with L1 set 10 dB above L2. Multiunit response areas were determined from cluster responses to a series of 100-ms tone bursts presented with a pseudo-random sequence in frequency and intensity. 2. Changes in the multiunit discharge properties after 50-75 mg/kg furosemide injection were as follows: the best frequency (BF) threshold increased from initial values in the range of 20-30 dB SPL to 50-80 dB SPL at 10-20 min postinjection and then recovered fully by 60-90 min. The spontaneous discharge activity decreased to zero before any changes in the frequency threshold curve were observed and did not return to initial values for several hours. Likewise, total discharge rates of stimulus elicited responses were reduced and tended to stay reduced even after BF threshold had fully recovered. 3. From the DPOAE measurements, the changes observed in the cubic distortion tone (CDT, 2f1-f2) emission after furosemide injection were as follows: at high levels of the probe tones, changes in the emission intensities generally stayed within a 10-dB range. The CDT amplitudes for low stimulus levels, however, were typically reduced by up to 40 dB, but recovered (depending on the furosemide dosage) by approximately 60-90 min. 4. At low to moderate stimulus levels of 40-60 dB SPL, there was a near perfect, minute-by-minute covariation of the ear canal CDT amplitude and the BF threshold measured in the AVCN. A 10-dB increase in threshold was associated with a 5- to 7-dB decrease in the CDT emission. 5. The optimum stimulus parameter set for the noninvasive estimation of cochlear performance from the CDT response was for stimulus amplitudes L1 = 50, L2 = 40 dB SPL. 6. This experiment demonstrates that CDT emissions at low stimulus levels are very good predictors of the thresholds of cochlear afferents, but this validity is lost for BF thresholds greater than approximately 60-70 dB SPL. 7. The ear canal CDT amplitude is better correlated with the BF threshold sensitivity of neuronal response areas in the AVCN than with the spontaneous discharge rate or absolute above-threshold discharge rates.

Animals↗
PubMed1996

Reduced arrhythmogenicity of biphasic versus monophasic T-wave shocks. Implications for defibrillation efficacy.

BACKGROUND: Biphasic waveforms defibrillate more effectively than monophasic waveforms; however, the mechanism remains unknown. The "upper-limit-of-vulnerability" hypothesis of defibrillation suggests that unsuccessful defibrillation is due to reinduction of ventricular fibrillation (VF). Thus, VF induction mechanisms may be important for the understanding of defibrillation mechanisms. We therefore compared myocardial VF vulnerability for monophasic versus biphasic shocks. METHODS AND RESULTS: In 10 Langendorff-perfused rabbit hearts, monophasic and biphasic T-wave shocks were randomly administered over a wide range of shock coupling intervals and shock strengths, and the two-dimensional coordinates within which VF was induced were used to calculate the area of vulnerability (AOV) for both shock waveforms. The arrhythmic response to biphasic shocks differed from that to monophasic shocks in three distinct ways: (1) the AOV was smaller (8.9 +/- 4.2 versus 13.9 +/- 6.0 area units, P < .02), (2) the transition zone between VF-inducing and nonarrhythmogenic shocks was narrower (14.7 +/- 4.8 versus 29.9 +/- 6.4 area units, P < .001), and (3) the entire AOV shifted toward longer coupling intervals (by 11.0 +/- 8.8 ms at the left border [P < .005] and 6.0 +/- 5.2 ms at the right border [P = .005] of the AOV). CONCLUSIONS: Biphasic shocks encounter a smaller AOV than monophasic shocks, a narrower transition zone from VF to no arrhythmia induction, and a lesser effectiveness in inducing VF at short coupling intervals. In keeping with the upper-limit-of-vulnerability hypothesis, these waveform-dependent differences in VF inducibility might help explain the lower defibrillation threshold for biphasic shocks.

Animals↗
PubMed1984

Oxygen deficit and anaerobic threshold in the incremental exercise in normoxia and hypoxia.

The differences between VO2 at the 5th min in steady state exercise and VO2 in incremental exercise were consecutively added from start of incremental exercise to a given time. These added values were defined as oxygen deficit. Anaerobic threshold was determined as the breakpoint of blood lactate during incremental exercise. The oxygen deficit at the anaerobic threshold in normoxia was almost identical to that in hypoxia.

Adult↗
PubMed1981

[Detection of the non-simultaneity of the tones in a 2-tone signal].

In the middle frequency range (250--4870 Hz) the threshold of non-simultaneity of the abruptly "switched on" tone does not depend on the difference of tone frequencies and slightly depends on the sequence of tones. Increasing time of tones rising increases the threshold and causes obvious dependence of the threshold on the sequence of tones. In case of recognition of the tone sequence, the interval between their onsets must considerably exceed the threshold of non-simultaneity of "switched on" tones.

Auditory Perception↗
PubMed2000

A ligand-receptor signaling threshold model of stem cell differentiation control: a biologically conserved mechanism applicable to hematopoiesis.

A major limitation to the widespread use of hematopoietic stem cells (HSC) is the relatively crude level of our knowledge of how to maintain these cells in vitro without loss of the long-term multilineage growth and differentiation properties required for their clinical utility. An experimental and theoretical framework for predicting and controlling the outcome of HSC stimulation by exogenous cytokines would thus be useful. An emerging theme from recent HSC expansion studies is that a net gain in HSC numbers requires the maintenance of critical signaling ligand(s) above a threshold level. These ligand-receptor complex thresholds can be maintained, for example, by high concentrations of soluble cytokines or by extracellular matrix- or cell-bound cytokine presentation. According to such a model, when the relevant ligand-receptor interaction falls below a critical level, the probability of a differentiation response is increased; otherwise, self-renewal is favored. Thus, in addition to the identity of a particular receptor-ligand interaction being important to the regulation of stem cell responses, the quantitative nature of this interaction, as well as the dynamics of receptor expression, internalization, and signaling, may have a significant influence on stem cell fate decisions. This review uses examples from hematopoiesis and other tissue systems to examine existing evidence for a role of receptor activation thresholds in regulating hematopoietic stem cell self-renewal versus differentiation events. (Blood. 2000;96:1215-1222)

Animals↗
PubMed2004

MRI investigation of the threshold for thermally induced blood-brain barrier disruption and brain tissue damage in the rabbit brain.

The ability of MRI-derived thermometry to predict thermally induced tissue changes in the brain was tested, and the thermal thresholds for blood-brain barrier (BBB) disruption and brain tissue damage were estimated. In addition, the ability of standard MRI to detect threshold-level effects was confirmed. These safety thresholds are being investigated to provide guidelines for clinical thermal ablation studies in the brain. MRI-monitored focused ultrasound heating was delivered to 63 locations in 26 rabbits. Tissue changes were detected in T(2)-weighted imaging and T(1)-weighted imaging (with and without contrast) and with light microscopy. The probability for tissue damage as a function of the accumulated thermal dose, the peak temperature achieved, the applied acoustic energy, and the peak acoustic power was estimated with probit regression. The discriminative abilities of these parameters were compared using the areas under the receiver operator characteristic (ROC) curves. In MRI, BBB disruption was observed in contrast-enhanced T(1)-weighted imaging shortly after the ultrasound exposures, sometimes accompanied by changes in T(2)-weighted imaging. Two days later, changes in T(2)-weighted imaging were observed, sometimes accompanied by changes in T(1)-weighted imaging. In histology, tissue damage was seen at every location where MRI changes were observed, ranging from small (diameter <1.0 mm) areas of tissue necrosis to severe vascular damage and associated hemorrhagic infarct. In one location, small (diameter: 0.8 mm) damage was not detected in MRI. The thermal dose and peak temperature thresholds were between 12.3-40.1 equivalent min at 43 degrees C and 48.0-50.8 degrees C, respectively, and values of 17.5 equivalent min at 43 degrees C and 48.4 degrees C were estimated to result in tissue damage with 50% probability. Thermal dose and peak temperature were significantly better predictors than the applied acoustic energy and peak acoustic power (P < 0.01). BBB disruption was always accompanied by tissue damage. The temperature information was better than the applied acoustic power or energy for predicting the damage than the ultrasound parameters. MRI was sensitive in detecting threshold-level damage.

Animals↗
PubMed1996

Cyclopean discrimination thresholds for the direction and speed of motion in depth.

We measured just-noticeable differences in the direction and speed of motion in depth of cyclopean and monocularly visible targets. Our stimulus set comprised different combinations of (d phi/dt)/(d delta/ dt), d delta/dt, d phi/dt and delta delta, where d phi/dt was the angular frontal plane speed of the binocularly-fused target, d delta/dt was its rate of change of disparity and delta delta was its disparity displacement. Our three subjects based their direction discriminations entirely on the task-relevant variable (d phi/dt)/(d delta/dt), and based their speed discriminations entirely on the task-relevant variable d delta/dt. They ignored all task-irrelevant variables in both tasks. Performance on both tasks was the same for motion within the horizontal and vertical meridians. Direction discrimination threshold rose significantly as the reference direction grew more oblique with respect to a line passing midway between the eyes and perpendicular to the frontal plane. Performance on the direction discrimination task was significantly better for the noncyclopean than for the cyclopean target, but the difference was not great. For the cyclopean target, the lowest value of the direction discrimination threshold was 0.70 deg (mean of three observers and two meridians). The Weber fraction for discriminating speed was not significantly different for the cyclopean and monocularly visible targets, and did not depend on the direction of motion in depth. The lowest values (mean of three observers and two meridians) were 0.12 (cyclopean) and 0.10 (noncyclopean). Findings did not scale for viewing distance. We propose that the human visual pathway contains: (a) a cyclopean mechanism sensitive to variations in the ratio (d phi/dt)/(d delta/dt) that is comparatively insensitive to both d phi/dt and d delta/dt; and (b) a speed-sensitive cyclopean mechanism that responds to variations in the value of d delta/dt, but is comparatively insensitive to d phi/dt. We also propose that a single speed-sensitive mechanism determines speed discrimination thresholds for both cyclopean and monocularly visible targets.

Adult↗
PubMed2001

Introduction and evaluation of a gray-value voxel conversion technique.

In micro finite element analyses (microFEA) of cancellous bone, the 3D-imaging data that the FEA-models are based on, contain a range of gray-values. In the construction of the eventual FEA-model, these gray-values are commonly thresholded. Although thresholding is successful at small voxel sizes, at larger voxel sizes there is substantial loss of trabecular connectivity. We propose a new method: the gray-value method, where the microFEA-models use the information within the 3D-imaging data directly, without prior thresholding. Our question was twofold. First, how does the gray-value method compare to both plain and mass-compensated thresholding? Second, what is the effect of element size on the results obtained with the gray-value method? We used nine microCT-scans of human vertebral cancellous bone. These were degraded to represent different resolutions, and converted into microFEA-models using plain thresholding, mass-compensated thresholding, and the gray-value method. The apparent elastic moduli of the specimens were determined using microFEA. The different methods were compared on the basis of the apparent elastic moduli, compared to those calculated for a 28 microm reference model. The results showed that the gray-value method greatly improves the results relative to other methods. The gray-value method gives accurate predictions of the apparent elastic moduli, for voxel sizes up to one trabecular thickness (Tb.Th.). For voxel sizes greater than one Tb.Th. the accuracy, although still better than for both thresholding methods, becomes increasingly worse.

Biomechanical Phenomena↗
PubMed2001

Depth thresholds of motion parallax as a function of head movement velocity.

The lower parallactic depth threshold is determined by (a) the ratio of relative image velocity to head velocity when the head moves fast (>13 cm/s) and (b) the motion threshold when the head moves slow (<13 cm/s). These two results are explained by a single system that codes the ratio of relative image velocity to head velocity, using the same image velocity signal as that used for motion perception. In this explanation, ratios coded from low relative image velocities, which are slightly higher than the motion threshold, produce a perception of depth only when the head moves slowly.

Depth Perception↗
PubMed1985

Independent and interactive effects of digoxin and quinidine on the atrial fibrillation threshold in dogs.

To assess the effects of digoxin as single therapy and in combination with quinidine in the treatment of atrial fibrillation, the atrial fibrillation threshold was determined from the right atrial appendage and Bachmann's bundle in 11 open chest dogs. In group 1 (six dogs), the atrial fibrillation threshold was determined at baseline, post-quinidine (10 mg/kg intravenously) and then post-digoxin (50 micrograms/kg intravenously). In group 2 (five dogs), the order of drug administration was reversed. The results of this study were: 1) Digoxin had no significant effect on the atrial fibrillation threshold when given alone. 2) Quinidine significantly increased the atrial fibrillation threshold (p less than 0.002) and the addition of digoxin resulted in a further increase in threshold (p less than 0.002). 3) Quinidine produced greater suppression of atrial fibrillation induction at the right atrial site than at the Bachmann's bundle site, suggesting differential effects of quinidine on atrial fibers.

Animals↗
PubMed2001

Repetitive transcranial magnetic stimulation effects on language function depend on the stimulation parameters.

In previous studies it has been shown that picture-naming latencies can be facilitated with both suprathreshold single and repetitive transcranial magnetic stimulation (TMS/rTMS) over Wernicke's area. The aim of this study was to investigate whether low-frequency rTMS (1 Hz) or high-frequency rTMS (20 Hz) at subthreshold intensities is also capable of influencing picture naming. In 16 healthy right-hand male subjects, trains of 1 Hz or 20 Hz were applied over either Wernicke's area, Broca's area, or the primary visual cortex. The subjects had to name 20 black-and-white line drawings, which were shown immediately after rTMS and again 2 minutes later. Naming latency could be facilitated only immediately after Wernicke's area stimulation at a frequency of 20 Hz and at an intensity of 55% of the maximal stimulator output, which was more than the motor threshold. All other stimulation procedures failed to influence naming latencies. These results indicate that language functions can be facilitated in healthy subjects only by high-frequency rTMS with intensities at or above the motor threshold.

Adult↗