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Results for “Differential Threshold”

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PubMed1999

Safety of single and repetitive focal transcranial magnetic stimuli as assessed by intracranial EEG recordings in patients with partial epilepsy.

The safety of single and repetitive (paired and quadruple) focal transcranial magnetic stimuli as possible inducers of epileptic discharges or clinically manifest seizures was investigated in 21 patients with intractable epilepsy during invasive presurgical monitoring. Subdural and/or intracerebral depth electrodes had been implanted in close proximity to the suspected epileptogenic zone, and the anticonvulsant medication had been reduced. Focal transcranial magnetic stimuli were applied by a Magstim QuadroPulse magnetic stimulator over the hand area of the motor cortex ipsilateral to the epileptogenic focus at intensities of 120% and 150% of motor threshold and additionally as close as possible to the suspected epileptogenic zone at 40-100% of maximal stimulator output. Stimulation did not induce any complex partial or secondary generalized tonic-clonic seizures. One patient with hippocampal sclerosis experienced an aura associated with rhythmic electroencephalographic discharges restricted to the ipsilateral intrahippocampal depth electrode after stimulation over his left temporal lobe. This patient, however, also had frequent spontaneously occurring auras with focal ictal discharges originating from this hippocampus. Interictal discharges were not influenced significantly by single or repetitive magnetic stimuli. In conclusion, from this study there is no evidence that single or serial focal transcranial magnetic stimuli activate epileptogenic foci. At least four high-frequency repetitive stimuli of high intensity may thus be applied with a low risk of seizure induction even in patients with low seizure threshold.

Adolescent↗
PubMed1990

Illusory contour orientation discrimination in the cat.

We present the first evidence that a non-human species (the cat) is able to discriminate the orientation of illusory contours. Following Vogels and Orban45, we used two types of illusory contours. In one type, the illusory contour was defined by a number of contour-inducing semicircles, of which the endpoints were separated by a gap. In the other pattern, the inducing semicircles were shifted in phase along their diameter and their endpoints were aligned along the contour. Just noticeable differences in orientation were measured (at the 73.5% correct level), using a Wetherill and Levitt49 staircase procedure. Values in the order of 11 degrees were obtained when using the first type of illusory contour. Just noticeable differences with the second type were in the order of 17 degrees. Reducing the salience of the illusory contour, whether by scrambling the contour, or by decreasing the number or the contrast of inducing semicircles, systematically increased discrimination thresholds.

Animals↗
PubMed1990

Noise and hypoxia induced temporary threshold shifts in rats studied by ABR.

Rats were exposed for 2 h either to 115 dB SPL noise, to 5% oxygen in nitrogen (hypoxia) or to both hypoxia and noise. Auditory nerve-brainstem evoked responses (ABR) to 80 dB HL clicks and threshold were recorded prior to exposure, immediately (5-10 min) after the exposure, 2 h after and 2 weeks after the exposure. The findings in each experimental animal were compared to those in the control (non-exposed) group and to those in the other groups. Thresholds were elevated in each of the experimental groups, but these were temporary threshold shifts, since 2 weeks following the exposure, threshold had returned to normal. Latencies (wave I and the IV-I interpeak latency difference- (IPLD] were prolonged in the groups exposed to hypoxia (hypoxia alone and hypoxia + noise). These results are discussed in view of possible mechanisms of these noise and hypoxia induced temporary threshold shifts.

Animals↗
PubMed2004

A simple and reliable method for clinical assessment of odor thresholds.

We investigated whether presenting of dilutions of phenyl ethyl alcohol at random succession according to the method of constant stimuli can replace the standard procedure of presenting a various number of dilutions in a staircase paradigm. Forty-six men and 44 women, aged 19-76 years, participated in this study. Phenyl ethyl alcohol was diluted in a ratio of 1:2, starting from 4%. Presentation of the odorant followed a three-alternative, temporal forced-choice paradigm with two blanks in addition to the odorant. Twenty dilutions were administered in a randomized order. Odor threshold was obtained by logistic regression of the correct and incorrect identifications of the probe containing the odorant. Thresholds were also calculated on the basis of the first 16 dilution steps only. Results from these procedures were compared with 'gold-standard' threshold assessment employing a three-alternative, temporal forced-choice staircase paradigm with seven reversals using 16 dilutions of phenyl ethyl alcohol. The method of constant stimuli took a shorter and less variable testing time than the staircase technique. The use of 20 dilution steps provided no better results than the use of 16 steps. The method of constant stimuli exhibited a good test-retest reliability (r = 0.7; P < 0.001) comparable to that of the staircase method and provided unbiased results highly correlated (r = 0.8; P < 0.001) with those of the staircase technique with similar inter-test variability. Applying 16 dilutions (1:2 steps) of phenyl ethyl alcohol at random succession in a three-alternative, temporal forced-choice paradigm is thus a simple and reliable procedure for the reproducible assessment of odor thresholds that may be contemplated as an alternative to the 'gold-standard' staircase method of clinical odor threshold assessment.

Adult↗
PubMed1985

Ventilatory thresholds during a graded exercise test.

38 healthy male volunteers were administered a graded exercise test during which selected respiratory gas exchange variables were monitored. In each case, two distinct ventilatory thresholds were observed. The first threshold was detected by the power output at which the VE/VO2 ratio and FEO2 reached a minimum. The mathematical relationship between these two variables was such that at power outputs above this threshold, hyperventilation with respect to oxygen consumption had to occur. The second threshold was detected by the power output at which the VE/VCO2 ratio reached a minimum and the FECO2 reached a maximum. These two variables were related in a manner such that at power outputs above this threshold, hyperventilation with respect to carbon dioxide production had to occur.

Differential Threshold↗
PubMed1982

Role of threshold in afterimage visibility.

The negative afterimage from grating can be considered equivalent to a renal grating, the contrast of which decreases over time. The interval between the onset of the afterimage and the time at which the effective contrast of the afterimage falls below threshold defines afterimage duration. In a series of experiments with several predictions based on this formulation were confirmed. Square-wave gratings yielded longer afterimage durations than sinusoidal gratings, a difference that is attributable to the difference in threshold between these two types of grating. Also, grating adaptation before afterimage induction was found to abbreviate afterimage duration because of threshold elevation. Finally, it was found that, even after fading to invisibility, an afterimage could interact with a real grafting to influence threshold performance on a forced-choice detection task.

Afterimage↗
PubMed1972

The detection of visual intensity differences by pigeons.

Pigeons were trained in a conditional discrimination procedure to discriminate between visual stimuli that varied in intensity. The magnitude of the intensity difference ranged from 0.80 to 0.05 logarithmic units. Psychometric functions were calculated from the data and the mean difference threshold for the 11 subjects was approximately 0.12 logarithmic units. A signal-detection analysis of the data suggested that stimuli that were below the calculated threshold were detectible to the subjects.

Animals↗
PubMed1979

Effect of information feedback on temporal discrimination in concurrent and post-feedback measures.

College students estimated time intervals (5, 6, 7, 8, and 9 sec.) where one group (N = 20) received information feedback in terms of actual interval durations and a second group (N = 18) received no information. Theshold measures were then taken for all subjects using the method of constant stimuli (comparison stimuli of 5, 6, 7, 8, and 9 sec. and a 7-sec. standard). In addition to increased accuracy and consistency of judgments, the feedback group showed a decreasing Weber fraction during the estimation phase, while the Weber fraction for the no-feedback group increased. The feedback group retained only a slight advantage in subsequent threshold measures; group differences in difference thresholds and derived nonparametric estimates of d' were nonsignificant.

Differential Threshold↗
PubMed1991

[Visual scoring in the auditory brainstem response. Second report: determination of response threshold].

In the ABR audiometry we usually determine the threshold by visual scoring. The evaluation of visual scoring has been investigated in my laboratory for several years, using the traced samples of ABR obtained from 3 normal adults same as that described in my previous report. Stimuli were clicks of 0, 5, 10, 15, 20, 30dBSL and without stimulus. Thresholds were calculated by the two psychophysical methods, "method of limits" and "up and down method". Scorers were two groups: 3 experienced and 3 inexperienced. Results obtained were as follows: 1. In the method of limits, averages of thresholds measured by the experienced scorers were contained 6.0-9.8dB, their standard deviations being 3.82-6.40dB. 84-95% of all thresholds were contained 0-10dB or 5-10dB in the experienced group. 2. In the method of limits, the highest rate that all scorers of a group had the same threshold was 56% in the separate presentation in the experienced group. 3. In the up and down method averages of threshold were contained 4.6-9.1dB, which were smaller than the method of limits, and standard deviations were contained 1.77-3.18dB, which were about 1/2 times in the method of limits. 92-99% of all thresholds were contained 0-5dB or 5-10dB.

Audiometry, Evoked Response↗
PubMed1982

[Equal-sensation contours and a threshold curve for low-frequency vibrotactile stimuli on the glabrous skin of the human hand].

Using vibrotactile stimuli, equal-sensation contours and a threshold curve were determined on the glabrous skin of the human hand. Equal-sensation contours were measured between 1 Hz and 16 Hz. When plotted on log-log coordinates, they monotonically decreased as a function of frequency and were downward by concaved. The threshold curve was measured between 1 Hz and 50 Hz. Its values decreased linearly from 1 Hz to 16 Hz, became constant at 16-35 Hz, and decreased again at 35-50 Hz. Therefore, at the stimulus range of 1-16 Hz, equal-sensation contours were downward concave and the threshold curve was more or less linear. This discrepancy seems to be caused by the differences in mechanoreceptor types which take part in subjective responses: suprathreshold sensation depends on several types of mechanoreceptors, whereas vibrotactile thresholds depends mainly on RA receptors (Meissner corpuscles).

Adult↗