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PubMed1987

The effects of contrast on visual orientation and spatial frequency discrimination: a comparison of single cells and behavior.

We have compared the effects of contrast on human psychophysical orientation and spatial frequency discrimination thresholds and on the responses of individual neurons in the cat's striate cortex. Contrast has similar effects on orientation and spatial frequency discrimination: as contrast is increased above detection threshold, orientation and spatial frequency discrimination performance improves but reaches maximum levels at quite low contrasts. Further increases in contrast produce no further improvements in discrimination. We measured the effects of contrast on response amplitude, orientation and spatial frequency selectivity, and response variance of neurons in the cat's striate cortex. Orientation and spatial frequency selectivity vary little with contrast. Also, the ratio of response variance to response mean is unaffected by contrast. Although, in many cells, response amplitude increases approximately linearly with log contrast over most of the visible range, some cells show complete or partial saturation of response amplitude at medium contrasts. Therefore, some cells show a clear increase in slope of the orientation and spatial frequency tuning functions with increasing contrast, whereas in others the slopes reach maximum values at medium contrasts. Using receiver operating characteristic analysis, we estimated the minimum orientation and spatial frequency differences that can be signaled reliably as a response change by an individual cell. This analysis shows that, on average, the discrimination of orientation or spatial frequency improves with contrast at low contrasts more than at higher contrasts. Using the optimal stimulus for each cell, we estimated the contrast threshold of 48 neurons. Most cells had contrast thresholds below 5%. Thresholds were only slightly higher for nonoptimal stimuli. Therefore, increasing the contrast of sinusoidal gratings above approximately 10% will not produce large increases in the number of responding cells. The observed effects of contrast on the response characteristics of nonsaturating cortical cells do not appear consistent with the psychophysical results. Cells that reach their maximum response at low-to-medium contrasts may account for the contrast independence of psychophysical orientation and spatial frequency discrimination thresholds at medium and high contrasts.

Animals↗
PubMed2004

Registering the response of tissues exposed to an interferential electric current stimulation.

Electrotherapy is a treatment method in which an electric stimulus is used to achieve physiological changes in the body. When increasing the level of the electric stimulus, three sensory, three motor and a pain response can be found. In this paper the three sensory threshold levels (1st just noticeable weak tingling sensation, 2nd a very fine but definite tingling and 3rd moderate strong tingling) were studied with an interferential electric current stimulus. The interferential electric current can be achieved by summing two alternating currents with little different frequencies with each other. As a current source, the study used a value intended for interferential electric current therapy. The threshold value measurements were performed on 5 days to 6 volunteers. The method of limits is a frequently used technique for determining sensory thresholds where ascending or descending stimuli is used and the observer reports of the appearance or disappearance of the sensation, respectively. A Bekesy's algorithm is a threshold tracking method with a variable stimulus. They were used for controlling the interferential electric current stimulus, and hence, for studying the,thresholds. By using the method of limits, the measurements were done at three sensory threshold levels at interferential frequencies deltaf of 10 Hz, 30 Hz, 50 Hz, and 100 Hz caused by two sine waves, one with a frequency between 2490 Hz and 2400 Hz, and the other constant 2500 Hz. In the method based on Bekésy's algorithm the interferential frequency deltaf was increased from 1 Hz to 100 Hz by changing the first frequency from 2499 Hz to 2400 Hz and the measurement was repeated for all three sensory thresholds. The sensory thresholds for interferential current stimuli seem to be quite unaffected by the interferential electric current frequency among the healthy test persons, and no distinct interferential electric current frequencies at which the threshold values should be measured were found in this study.

Adult↗
PubMed1984

Rocket-ship psychophysics. Assessing visual functioning in young children.

A set of psychophysical procedures is described for rapidly measuring visual functions such as spatial contrast sensitivity, flicker fusion, and rod and cone two-color increment thresholds in young children. The psychophysical procedures, disguised as electronic space games, use a forced-choice paradigm combined with the method of descending limits. Because children enjoy the tasks and are highly motivated, the tests yield good sensory thresholds in a short period of time. However, when thresholds from 6-8-year-old children are compared with those of adults, the children have slightly higher thresholds for all of the functions tested. The differences are ascribed to the tendency of children to adopt a guessing strategy when stimuli are close to threshold.

Adolescent↗
PubMedDate not supplied

[Transformation of the structuro-functional organization of the motor neuron pool of the spinal cord during the postnatal ontogeny of dogs].

Data are presented concerning input-output characteristics of motoneuronal pool in puppies at various age periods (beginning from the first days of life) and distribution histograms of thresholds in motoneuronal populations. It was established that in early postnatal life, before realization of anti-gravitational reactions, low amplitude action potentials in the ventral root are recorded; threshold variation is rather insignificant accounting for the immediate maximum reflex reaction of motoneuronal pool. This is manifested in a rapid (relay) onset of plateau of input-output parameters. At this period, motoneurones of the pool are homogenous with respect to both the size and functional properties. During further postnatal life, relative amount of low amplitude elements of the pool decreases, modal value of threshold distribution being shifted to higher thresholds, indicating differentiation of motoneurones in the pool. After realization of antigravitational reactions, two plateaus in input-output parameters of the system are established together with corresponding two modal values in threshold distribution, indicating the onset of two main groups of motoneurones which differ in the size and functional properties.

Action Potentials↗
PubMed1996

Alcohol and workload as factors affecting the detection of angular acceleration.

INTRODUCTION: Previous findings have demonstrated that a low blood alcohol concentration (BAC) decreases subjects' ability to detect changes in angular motion and that this effect persists after the BAC returns to zero. METHODS: Angular motion thresholds of six alcohol and six placebo subjects holding pilots' certificates were obtained under both low and high task load conditions. The alcohol subjects were tested under double blind conditions in an enclosed simulator apparatus prior to drinking, after drinking (mean BAC = 0.038%), and at a time when the alcohol subjects' BACs had reached zero. RESULTS: Alcohol subjects' mean threshold values for the three threshold test sessions were 0.361, 0.512, and 0.465 degree.s-2 and 0.292, 0.422, and 0.379 degree.s-2 for the high and low task conditions, respectively. Corresponding values for placebo subjects were 0.364, 0.37, and 0.356 degree.s-2 for the high task load condition and 0.311, 0.317, and 0.312 degree.s-2 for the low task load condition. Statistical analyses showed significant task load and sessions effects (p < 0.025). Significant alcohol by test sessions interactions were found for both high and low task load conditions (p < 0.01) with subsequent t-tests showing no alcohol-placebo differences for the pre-alcohol test session, but significant alcohol-placebo differences for the alcohol test session and the test session conducted after the subjects' BACs returned to zero. CONCLUSIONS: The results indicate that the increased threshold for detecting changes in angular motion found after alcohol ingestion is relatively insensitive to task workload and, thus, it is unlikely that attentional changes underlie the threshold increase. These data together with the finding that an elevated threshold can persist after BAC reaches zero suggest that the threshold increase after alcohol ingestion reflects changes in vestibular functioning.

Acceleration↗
PubMed2005

Testing threshold and hormesis in a random effects dose-response model applied to developmental toxicity data.

Here we describe a random effects threshold dose-response model for clustered binary-response data from developmental toxicity studies. For our model we assume that a hormetic effect occurs in addition to a threshold effect. Therefore, the dose-response curve is based on two components: relationships below the threshold (hormetic u-shaped model) and those above the threshold (logistic model). In the absence of hormesis and threshold effects, the estimation procedure is straightforward. We introduce score tests that are derived from a random effects hormetic-threshold dose-response model. The model and tests are applied to clustered binary data from developmental toxicity studies of animals to test for hormesis and threshold effects. We also compare the score test and likelihood ratio test to test for hormesis and threshold effects in a simulated study.

Animals↗
PubMedDate not supplied

Changes in excitability in motor cortex (representation of foreleg) of the dog depending on signal signification of flexor reaction during elaboration of transswitching.

Excitability in a cortical representation of a foreleg was determined during elaboration of transswitching in dogs in three variations depending upon the stimulus used as a Conditioned stimulus (CS). If the flexor reaction to direct stimulation of the motor cortex was used as a CS, excitability changes of opposite signs appeared in the cortical representation of the foreleg, the threshold of flexion decreased in the alimentary situation and increased in the defense situation. Such changes in excitability were observed during instrumental conditioned transswitching. If subthreshold stimulation of the motor cortex was used as the CS, the threshold of flexion increased in both situations in approximately equal degree.

Animals↗
PubMed1990

Changes over time in thresholds for electrical stimulation of the cochlea.

The purpose of this paper is to better characterize changes over time that occurred in psychophysical detection thresholds for electrical stimulation of the cochlea. Threshold changes observed in nonhuman primates implanted with cochlear electrode arrays can be divided into at least three types based on the patterns of change over time. Short-term increases and subsequent decreases in threshold were commonly observed during the first months after implantation and were often followed by periods of long-term threshold stability. Long-term slow increases in thresholds and more rapid increases after a period of threshold stability have also been observed. The threshold changes may be divided into at least two classes based on their dependence on the waveforms used for the threshold measurements. Some changes occurred primarily in thresholds for long phase-duration signals while other changes were equal in magnitude (in decibels) for all tested stimuli. This suggests that at least two mechanisms underlay these threshold changes. The observed changes in thresholds have implications for experimental studies of electrical stimulation and for clinical application of auditory prostheses.

Animals↗
PubMed1999

Sodium channels (from rat, mouse, and man) in neuroblastoma cells and different expression systems have similar sensitivities to pentobarbital.

Current understanding of the molecular mechanisms of anaesthesia entails multiple molecular sites of anaesthetic action, each more or less important for a specific anaesthetic drug used. For some molecular anaesthetic targets such as the GABA(A), and acetylcholine receptors as well as potassium channels, large differences in anaesthetic effects and sensitivity between different receptor subtypes have been found. Voltage-gated sodium channels have been shown to be affected at clinical concentrations of pentobarbital. However, these results were obtained in an expression system, and anaesthetic sensitivity may be dependent on sodium channel subtype and subunit composition, too. Therefore, we compared the sensitivity of voltage-gated sodium channels to pentobarbital in human (SHSYSY) and mouse (N1 E-115) neuroblastoma cell lines as well as an HEK293-cell expression system with previously reported data with other sodium channel subtypes. Remarkably, for all sodium channel subtypes studied as well as all subtypes reported in the literature, pentobarbital had qualitatively identical effects with some quantitative differences, and at potentials near the action potential threshold sodium currents were significantly reduced by clinical concentrations of pentobarbital.

Action Potentials↗
PubMed2005

Haptic detection of sine-wave gratings.

We studied human haptic perception of sine-wave gratings. In the first experiment we measured the dependence of amplitude detection thresholds on the number of cycles and on the wavelength of the gratings. In haptic perception of sine-wave gratings, the results are in agreement with neural summation. The rate at which detection thresholds decrease with increasing number of cycles is much higher than can be accounted for by probability summation alone. Further, neural summation mechanisms describe the detection thresholds accurately over the whole spatial range probed in the experiment, that is wavelengths from 14 mm up to 225 mm. Earlier, we found a power-law dependence of thresholds on the spatial width of Gaussian profiles (Louw et al, 2000 Experimental Brain Research 132 369-374). The current results extend these findings; the power-law dependence holds not only for Gaussian profiles, but also for a broad range of sine-wave gratings with the number of cycles varying between 1 and 8. Haptic perception involves tactual scanning combined with an active, dynamic exploration of the environment. We measured characteristics of the velocity and force with which stimuli were scanned while performing a psychophysical task. One particularly surprising finding was that, without being instructed, participants maintained an almost constant scanning velocity during each 45-min session. A constant velocity in successive trials of the experiment might facilitate or even be necessary for discrimination. Further, a large systematic dependence of velocity on scanning length was found. An eightfold increase in scanning length resulted in about a fourfold increase in scanning velocity. A second experiment was conducted to study the influence of scanning velocity on psychophysical detection thresholds. This was done by systematically imposing specific scanning velocities to the participants while the thresholds were measured. The main result of the second experiment was that psychophysical detection thresholds are constant over a relatively broad range of scanning velocities.

Adult↗
PubMed2004

Unexpected epidemic thresholds in heterogeneous networks: the role of disease transmission.

We reformulate several recent analyses of infection processes on highly heterogeneous networks (e.g., scale-free networks) which conclude that diseases will spread and persist even for vanishingly small transmission probabilities. The results of these latter studies contrast with conventional epidemiological models where there are clear threshold effects, namely, should the transmission probability fall below a critical threshold level the disease is expected to die out. Here we show that epidemic propagation depends equally on the infection scheme as well as the network structure. Connectivity-dependent infection schemes can yield threshold effects even in scale-free networks where they would otherwise be unexpected.

Animals↗
PubMed1985

Involvement of respiratory perception in the resistive load compensation.

Relation of threshold for perception of added inspiratory flow resistance to ventilatory and occlusion pressure response to hypercapnia during external flow-resistive loading was examined in 8 healthy volunteers and 14 patients with chronic obstructive pulmonary disease. Occlusion pressure response to hypercapnia without the load and the threshold for perception were similar between normal subjects and patients. Occlusion pressure at end-tidal PCO2 of 60 mm Hg increased to 134 +/- (SD) 23.4% (p less than 0.01) in normal subjects, but it did not change in patients (113.7 +/- 25.3%). Occlusion pressure response to hypercapnia increased to 155.4 +/- 60% (p less than 0.05) during ventilatory loading in normal subjects but it did not change in patients (105.9 +/- 33%). Inverse correlation between threshold values and changes after loading in occlusion pressure response to hypercapnia was seen in patients as well as in normal subjects. These results indicate in both normal subjects and patients with chronic obstructive pulmonary disease that occlusion pressure response to hypercapnia during the ventilatory loading includes signals from the higher central nervous system and that the lower the threshold, the higher the respiratory drive during the loaded ventilation.

Adult↗
PubMed1999

Relationship between EABR thresholds and levels used to program the CLARION speech processor.

The purpose of this study was 1) to describe the relationship between electrically evoked auditory brain stem response (EABR) thresholds and the behavioral measures needed to program the speech processor of the CLARION Multi-Strategy Cochlear Implant, and 2) to determine whether the relationship between EABR threshold and the behavioral levels used to program the Clarion device was correlated with the temporal integration abilities of these subjects. The EABR thresholds, psychophysical thresholds, and the Clarion speech processor programming levels were recorded from 29 postlingually deafened adults. The mean EABR thresholds approximated most comfortable levels (M levels) for the programming stimulus. However, intrasubject variability was substantial. Significant intrasubject variability in temporal integration also was observed. Correlations were found between the ability of a cochlear implant user to perform temporal integration and the relationship between his or her EABR threshold and the threshold for the programming stimulus.

Adult↗
PubMed1986

Exposure-duration effects in localization judgments.

The effects on localization accuracy of increasing exposure duration beyond 100 msec are explored for a wide range of object separations. Previous reports that localization accuracy for objects separated by a few minutes of arc increases for exposures up to at least 400 msec are confirmed. I report here that localization of larger objects at larger separations does not improve when the exposure duration is increased beyond 100 msec. This difference between the small- and large-scale results can be explained by the difference in the spatial-frequency content of the objects being localized: When high-frequency objects are substituted for spectrally broadband objects in the large-scale case, the exposure-duration effects for widely separated objects become similar to those obtained in the small-scale case. These results suggest that the exposure-duration effect previously reported in hyperacuity studies is not specific to the localization task per se but rather is a suprathreshold version of the familiar form of spatiotemporal interaction seen in contrast-threshold results. They also suggest that a single type of mechanism underlies small- and large-scale localization.

Differential Threshold↗
PubMed1995

Investigations of the suitability of pallesthesiometry in the diagnostics of functional disturbances of the peripheral nervous system.

This contribution presents the results of fundamental investigations of the suitability of measurements of vibration sensitivity threshold (pallesthesiometry) as a method for the assessment of vibration-induced nerve impairments. Sinusoidal vibration stimuli are applied at the fingertips of the subjects in this method. The sensitivity threshold of the subjects is determined similar like the hearing threshold in audiometry and is used as a measure for possibly existing nerve disorders. Laboratory and field experiments were carried out by means of a simple to serve and inexpensive pallesthesiometer. The test results proved the suitability of the pallesthesiometry for screening purposes in principle. Concentration and cooperation of the subjects as well as the suitable selection of the investigation frequencies according to the frequency content of the vibration exposure are of essential meaning for comparable and reliable measuring results. Furthermore the rest and vibration-free position of the hand and fingers during the measurement of the thresholds are important to avoid measurement falsifications. A most extensive standardization of the investigation methodology and of the measuring equipment as well as the establishment of normative values of the sensitivity thresholds for different age groups are necessary for a broad application of the pallesthesiometry in practice.

Adult↗