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Results for “Differential Threshold”

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At least 505 records · Page 28Linked to original sources
PubMed1999

Notch-1 inhibits apoptosis in murine erythroleukemia cells and is necessary for differentiation induced by hybrid polar compounds.

Strikingly increased expression of notch-1 has been demonstrated in several human malignancies and pre-neoplastic lesions. However, the functional consequences of notch-1 overexpression in transformed cells remain unclear. We investigated whether endogenously expressed notch-1 controls cell fate determination in mouse erythroleukemia (MEL) cells during pharmacologically induced differentiation. We found that notch-1 expression is modulated during MEL cell differentiation. Premature downregulation of notch-1 during differentiation, by antisense S-oligonucleotides or by enforced expression of antisense notch-1 mRNA, causes MEL cells to abort the differentiation program and undergo apoptosis. Downregulation of notch-1 expression in the absence of differentiation inducer increases the likelihood of spontaneous apoptosis. We conclude that in MEL cells, endogenous notch-1 expression controls the apoptotic threshold during differentiation and growth. In these cells, notch-1 allows differentiation by preventing apoptosis of pre-committed cells. This novel function of notch-1 may play a role in regulating apoptosis susceptibility in notch-1 expressing tumor cells.

Acetamides↗
PubMed2002

Fat is sexy for females but not males: the influence of body reserves on reproduction in snakes (Vipera aspis).

Reproduction is energetically expensive for both sexes, but the magnitude of expenditure and its relationship to reproductive success differ fundamentally between males and females. Males allocate relatively little to gamete production and, thus, can reproduce successfully with only minor energy investment. In contrast, females of many species experience high fecundity-independent costs of reproduction (such as migration to nesting sites), so they need to amass substantial energy reserves before initiating reproductive activity. Thus, we expect that the relationship between energy reserves and the intensity of reproductive behavior involves a threshold effect in females, but a gradual (or no) effect in males. We tested this prediction using captive vipers (Vipera aspis), dividing both males and females into groups of high versus low body condition. Snakes from each group were placed together and observed for reproductive behavior; sex-steroid levels were also measured. As predicted, females in below-average body condition had very low estradiol levels and did not show sexual receptivity, whereas males of all body condition indices had significant testosterone levels and displayed active courtship. Testosterone levels and courtship intensity increased gradually (i.e., no step function) with body condition in males, but high estradiol levels and sexual receptivity were seen only in females with body reserves above a critical threshold.

Adipose Tissue↗
PubMed1990

A method for changing the avian endocochlear potential by current injection.

The endocochlear potential (EP) in the pigeon was found to be +10 +/- 3 mV. A method for changing the EP by current injection into the scala media is described. The EP change brought about was 1.6-2.9 mV/microA and was dependent on the distance from the tip of the current electrode. Single fiber activity in the cochlear ganglion could be recorded simultaneously with the current injection. These experiments have shown that decreases in the EP reduce the sound-evoked activity of primary afferent nerves and elevate their threshold.

Acoustic Stimulation↗
PubMed2006

The dependence of force enhancement on activation in human adductor pollicis.

It has been well recognized that the steady-state isometric force after active muscle/fiber stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions (MVC). However, recent evidence obtained for sub-MVC suggests that force enhancement properties are different from those observed for electrically induced and MVC. Specifically, it appears that force enhancement is activation-dependent and that there is a subject-specific threshold for force enhancement in sub-MVC. To address these suggestions, the relationship between force enhancement and voluntary activation during stretch was investigated in 11 healthy subjects. Human adductor pollicis muscles were studied and force enhancement was measured while muscle activation during the steady-state isometric phase was controlled at a level of 30% of MVC. In order to study the effects of activation on force enhancement, subjects performed stretch contractions at 0, 10, 30, 60, and 100% of maximal voluntary effort while the steady-state isometric force after stretch, obtained at 30% of activation in all cases, was compared to the corresponding values measured in the isometric reference contractions. There was no force enhancement if muscle stretching occurred passively but all subjects showed force enhancement when muscle stretching occurred at maximal voluntary effort. When increasing the level of activation during the stretch phase, force enhancement increased, and the number of subjects who showed force enhancement increased as well. We conclude from these results that force enhancement during voluntary contractions is activation-dependent with a threshold that is subject-specific.

Adaptation, Physiological↗
PubMed2001

Infant light adaptation shows Weber's law at photopic illuminances.

In the present study we investigate the dependence of photopic contrast thresholds on retinal illuminance in infants and adults. Contrast thresholds were measured at five retinal illuminances between about 6 and about 20,000 Td in subjects in both age groups. The forced-choice preferential looking technique was used in 3-month-old infants, and standard forced-choice techniques were used in adults. The stimulus was a 0.25 cy/deg squarewave grating phase alternated at 6 Hz. Infants' contrast thresholds were more than two log units higher than those of adults at all retinal illuminances. Contrast thresholds had a similar dependence on retinal illuminance in both infants and adults. For both age groups, contrast thresholds initially decreased with increasing retinal illuminance. However, at both ages, above a critical illuminance of about 200 Td, contrast thresholds remained constant, following Weber's law. Thus a vertical shift was sufficient to bring the two data sets into correspondence. In the context of a two-site model of light adaptation, our results imply that infants' elevated contrast thresholds cannot be explained solely on the basis of photoreceptoral immaturities. Later physiological immaturities must also limit infants' photopic contrast thresholds.

Adaptation, Ocular↗
PubMed1978

Analysis of the detection of a moving line.

A model is proposed to explain the effects of line motion on visual sensitivity. The model incorporates recent evidence for parallel "pattern" and "flicker" systems. An element of either system consists of a spatial filter followed by a temporal filter; the characteristics of these filters were derived from previous psychophysical measurements. The model may be used to demonstrate why visual sensitivity to optimum triangular-wave motion is greater than for optimum square-wave motion, which, in turn, is greater than for on-off presentation. The predictions of the model are in reasonable agreement with threshold-amplitude data for triangular-wave and square-wave motion; the model is also consistent with threshold-frequency data for small-amplitude sinusoidal motion.

Differential Threshold↗
PubMed1990

Orientation discrimination in the cat: a distributed function.

Cats were trained to make fine orientation discriminations with stimuli similar to those used in physiological experiments--narrow, light bars 12 degrees long--before and after various combinations of lesions of areas 17 and 18. Discrimination thresholds were measured at different contrast levels and different bar widths, both pre- and postoperatively, for up to 1.5 years after the lesion. For high contrast stimuli, lesions restricted to area 17 or area 18 had little effect, but those lesions involving area 17 and a substantial part of area 18 raised thresholds. In the latter case there was a relationship between the amount of area 18 spared and the bar width at which discrimination was impaired. At low contrast deficits were seen only for narrow widths. These results lead to the following conclusions. (i) Orientation discrimination is a function distributed within and across areas 17 and 18. (ii) How this function is distributed in this cortex depends on stimulus width. (iii) The X system does not carry the signal necessary for orientation discrimination. (iv) Cells most narrowly tuned for orientation, which reside in the part of area 17 subserving central vision, do not determine the orientation discrimination threshold.

Animals↗
PubMed1999

Effect of altered body CO2 stores on pulmonary gas exchange dynamics during incremental exercise in humans.

The lactate threshold is a widely used and, at times, controversial construct in exercise physiology and pathophysiology. Its non-invasive estimation during incremental exercise depends upon CO2 output increasing as a function of O2 uptake, i.e. 'V-slope', as a result of bicarbonate buffering during the lactic acidosis. However, we hypothesised that the V-slope deflection could also occur as a consequence of metabolic CO2 being diverted proportionally more into the CO2 stores in the early phase of exercise. Eight healthy males performed two incremental exercise tests on a cycle ergometer, with and without controlled prior hyperventilation; the hyperventilation caused end-tidal PCO2 to decline by 10 mmHg, with the clearance of a CO2 volume averaging 2547 ml. This corresponded to an 'effective' CO2 capacitance of some 3.12 ml mmHg-1 kg-1. Gas exchange was determined breath-by-breath, and blood was sampled from the dorsum of the heated hand. Our results demonstrate that the early dynamics of CO2 wash-in to the previously depleted body stores can result in a 'pseudo-threshold', i.e. significantly before the onset of the actual lactic acidosis. Precautions should therefore be taken to avoid hyperventilation prior to non-invasive estimation of the lactate threshold.

Adolescent↗
PubMed1984

Scotopic threshold of the human retina during dynamic physical effort.

The scotopic threshold of 7 subjects was measured with a 1 degree circular white light located 15 degrees below fixation during 45 min at rest followed by 10 min of running on an exercise treadmill. The slope of the treadmill was adjusted individually so that the effort extended by each subject required 60-75% of his maximum oxygen uptake. Thresholds showed a sudden elevation of about 0.55 nLa (logarithmic scale) at the onset of the exercise period, and remained at this level throughout the following 10 min. It was concluded that the head movements and consequent eye instability imposed by the run were the main cause for the increase in scotopic threshold.

Adolescent↗
PubMed1985

Temporal (phase and time) disparities in lateralization.

The topic of this paper concerns the judgement of just noticeable differences (JND) and of complete lateral position for dichotic 1 000-Hz stimuli with phase or onset time or delay differences and for a click. The 1 000-Hz sinusoids had 30 ms steady state with 10 or 50 ms rise/decay time and were presented at 50 dB HL in listeners with normal hearing. For every value of delta phi, delta onset or delay, the presentation consisted of blocks of 20 trials and the JND was determined as the phase or onset time or delay differences producing 80% of correct responses (forced choice). The intracranial 1 000-Hz tone image with long disparities was evaluated for 90, 180 and 270 degrees delta phi, for 250, 500 and 750 microseconds delta onset and for 250, 500 and 750 microseconds delay. Our results demonstrate that the interaural time differences with suitable headphone onset cues have a negligible effect on JND in pure-tone lateralization tests; the JND for a 1 000-Hz tone is based on phase cues only. With 180 and 270 degrees phase shifts, the interaural onset time differences, with 10 ms rise-fall time, become critical and override phase information. The JND for clicks on the contrary is based on time cue only. The intracranial click image with long disparities and duality threshold was evaluated.

Adult↗