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PubMed1999

Age-related changes in the brainstem auditory evoked potentials of the marmoset.

Changes in brain-stem auditory evoked potentials (BAEPs) with age were recorded in common marmosets (Callithrix jacchus) at the age of 1-2, 6-8 and 10-12 years. The auditory function was assessed by thresholds, latencies and amplitudes of BAEPs evoked by use of tone burst stimulations with audible frequencies ranging from 1 to 99 kHz. Prolongation of the latencies of later waves was observed in the animals at the age of 6-8 and 10-12 years at high frequencies, suggesting that aging in marmosets, as reported previously in humans and other animals, may cause earlier hearing loss at high frequency than at low frequency within the hearing range. At 10-12 years of age, the elevations of BAEP thresholds and the declines of BAEP amplitudes in older animals were also observed. As the differences in the parameters are small, it was suggested that only a moderate hearing loss occurred with onset late in life in common marmoset similar to that in CBA/Ca mice. Based on the results obtained in this study, BAEP latencies appear to be more sensitive indicators than BAEP thresholds and amplitudes for the early detection of hearing impairment.

Aging↗
PubMed1977

Threshold conditions for binocular rivalry.

A series of three experiments was performed to determine the minimum contrast necessary for a grating pattern to engage in binocular rivalry with a dissimilar pattern presented to the other eye. It was found that a pattern at its own contrast threshold can suppress temporarily a contralateral pattern, regardless of the contrast of the latter. In a fourth experiment it was found that a near-threshold pattern is as effective, in terms of the duration of visibility, as one that is four times greater in contrast; only the average duration of suppression varied with pattern contrast. This last set of data extends the findings of others to near-threshold conditions and poses difficulties for models of binocular rivalry based on the notion of reciprocal inhibition.

Differential Threshold↗
PubMed2002

Short-interval paired-pulse inhibition and facilitation of human motor cortex: the dimension of stimulus intensity.

Paired transcranial magnetic stimulation has greatly advanced our understanding of the mechanisms which control excitability in human motor cortex. While it is clear that paired-pulse excitability depends on the exact interstimulus interval (ISI) between the first (S1) and second stimulus (S2), relatively little is known about the effects of the intensities of S1 and S2, and the effects of manipulating neurotransmission through the GABA(A) receptor. When recording the motor evoked potential (MEP) from the resting abductor digiti minimi (ADM) muscle, using a fixed ISI of 1.5 ms, and expressing the interaction between S1 and S2 as MEP(S1+S2)/(MEP(S1) + MEP(S2)), then a systematic variation of the intensities of S1 and S2 revealed short-interval intracortical facilitation (SICF) if S1 and S2 were approximately equal to MEP threshold (RMT), or if S1 > RMT and S2 < RMT. In contrast, short-interval intracortical inhibition (SICI) occurred if S1 < RMT and S2 > RMT. Contraction of the ADM left SICI unchanged but reduced SICF. The GABA(A) receptor agonist diazepam increased SICI and reduced SICF in the resting ADM while diazepam had no effect during ADM contraction. Surface EMG and single motor unit recordings revealed that during ADM contraction SICI onset was at the I3-wave latency of S2, whereas SICF typically "jumped up" by one I-wave and started with the I2-wave latency of S2. Findings suggest that SICI is mediated through a low-threshold GABA(A) receptor-dependent inhibitory pathway and summation of IPSP from S1 and EPSP from S2 at the corticospinal neurone. In contrast, SICF originates through non-synaptic facilitation at the initial axon segment of interneurones along a high-threshold excitatory pathway.

Adult↗
PubMed2005

Shared decision signal explains performance and timing of pursuit and saccadic eye movements.

Each voluntary eye movement provides physical evidence of a visuomotor choice about where and when to look. Primates choose visual targets with two types of voluntary eye movements, pursuit and saccades, although the exact mechanism underlying their coordination remains unknown. Are pursuit and saccades guided by the same decision signal? The present study compares pursuit and saccadic choices using techniques borrowed from psychophysics and models of response time. Human observers performed a luminance discrimination task and indicated their choices with eye movements. Because the stimuli moved horizontally and were offset vertically, subjects' tracking responses consisted of combinations of both pursuit and saccadic eye movements. For each of two signal strengths, we constructed speed-accuracy curves for pursuit and saccades. We found that speed-accuracy curves for pursuit and saccades have the same shape, but are time-shifted with respect to one another. We argue that this pattern occurs because pursuit and saccades share a decision signal, but utilize different response thresholds and are subject to different motor processing delays.

Adult↗
PubMed1983

Spatial-frequency adaptation affects spatial-probability summation.

The effects of spatial-frequency adaptation on spatial-probability summation were investigated by two techniques: the measurement of the dependence of cosine grating sensitivity on the number of cycles presented and the determination of the frequency-of-seeing curve for a cosine grating. Both measurements indicate that the contribution of spatial-probability summation to adapted grating thresholds is reduced in comparison with unadapted conditions. This reduction in the effects of spatial-probability summation, together with the decrease in local sensitivity, which depends on the spatial inhomogeneity of the visual system, is sufficient to account for the threshold elevation aftereffect measured when adapting and testing with cosine gratings.

Adaptation, Ocular↗
PubMedDate not supplied

Relationship between pacemaker fibrillation thresholds and electrode area.

Energy thresholds for electrical pacing of the heart are lower with small electrodes than with large. Because pacing of the heart during the vulnerable period may produce ventricular fibrillation, it is also pertinent to know if fibrillation threshold is affected by the electrode size. Electrodes of different surface area but of the same material were implanted in 40 dogs and the pacing thresholds were recorded. Ventricular fibrillation was electrically induced by discharging a 2-msec d.c. cathodal pulse of progressively increasing energy into the vulnerable period. It was found that small electrodes required more energy to produce ventricular fibrillation than large electrodes, and the ratio of fibrillation to stimulation threshold was higher for the small-surface-area electrode. The difference between the thresholds and ratios obtained with the various electrodes was statistically significant. A similar experiment was performed in animals with chronically implanted electrodes, producing comparable results. The results indicate that, in regard to pacing and fibrillation thresholds, small electrodes are preferable to large.

Animals↗
PubMed1977

The influence of age onthe sensitivity of the cornea,.

Corneal touch threshold (CTT) was measured in 205 healthy people of different ages. It was found that CTT increased gradually throughout life, although more significantly after the fifth decade of life. The results are found to be in good accord with those of Boberg-Ans, thus refuting an earlier report asserting that corneal sensitivity (threshold-1) increased up to the fifth decade and declined thereafter,

Adolescent↗
PubMed2003

Electrosensibility and electromagnetic hypersensitivity.

Electromagnetic sensibility, the ability to perceive electric and electromagnetic exposure, and electromagnetic hypersensitivity (EHS), developing health symptoms due to exposure to environmental electromagnetic fields, need to be distinguished. Increased electrosensibility is a necessary, however, not a sufficient condition for electromagnetic hypersensitivity. At an extended sample of the general population of 708 adults, including 349 men and 359 women aged between 17 and 60 years, electrosensibility was investigated and characterized by perception threshold and its standard deviation. By analyzing the probability distributions of the perception threshold of electric 50 Hz currents, evidence could be found for the existence of a subgroup of people with significantly increased electrosensibility (hypersensibility) who as a group could be differentiated from the general population. The presented data show that the variation of the electrosensibility among the general population is significantly larger than has yet been estimated by nonionizing radiation protection bodies, but much smaller than claimed by hypersensitivity self-aid groups. These quantitative results should contribute to a less emotional discussion of this problem. The investigation method presented, is capable of exclusion diagnostics for persons suffering from the hypersensitivity syndrome.

Adolescent↗
PubMed2001

Differential effect of near-threshold stimulus intensities on sound localization performance in azimuth and elevation in normal human subjects.

The ability of humans to localize sounds remains relatively constant across a range of intensities well above detection threshold, and increasing the spectral content of the stimulus results in an improvement in localization ability. For broadband stimuli, intensities near detection threshold result in fewer and weaker binaural cues used in azimuth localization because the stimulus energy at the high- and low-frequency ends of the audible spectrum fall below detection threshold. Thus, the ability to localize broadband sounds in azimuth is predicted to be degraded at audible but near threshold stimulus intensities. The spectral cues for elevation localization (spectral peaks and notches generated by the head-related transfer function) span a narrower frequency range than those for azimuth. As the stimulus intensity decreases, the ability to detect the stimulus frequencies corresponding to the spectral notches will be more strongly affected than the ability to detect frequencies outside the range where these spectral cues are useful. Consequently, decreasing the stimulus intensity should degrade localization in both azimuth and elevation and create a greater deficit in elevation localization due to the narrower band of audible frequencies containing elevation cues compared to azimuth cues. The present study measured the ability of 11 normal human subjects to localize broadband noise stimuli along the midsagittal plane and horizontal meridian at stimulus intensities of 14, 22, and 30 dB above the subject's detection threshold using a go/no-go behavioral paradigm. Localization ability decreased in both azimuth and elevation with decreasing stimulus intensity, and this effect was greater on localization in elevation than on localization in azimuth. The differential effects of stimulus intensity on sound localization in azimuth and elevation found in the present study may provide a valuable tool in investigating the neural correlates of sound location perception.

Adult↗
PubMed1994

Anticonvulsant activity of antagonists and partial agonists for the NMDA receptor-associated glycine site in the kindling model of epilepsy.

Several previous studies have shown that competitive and uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists are ineffective as anticonvulsants in fully amygdala kindled rats, i.e. a widely-used model of complex-partial seizures (limbic epilepsy). In the present experiments, different categories of ligands for the NMDA receptor-associated glycine modulatory site were evaluated in the kindling model in rats. For this purpose, the 'silent' glycine antagonist 7-chlorokynurenic acid (7-CKA), the low efficacy glycine partial agonist (+)-HA-966 (R(+)-3-amino-1-hydroxypyrrolid- 2-one), and the high efficacy glycine partial agonist D-cycloserine (D-CS) were used. In view of the poor brain penetration of some of these compounds after systemic administration, all drugs were injected bilaterally into the lateral ventricles of fully kindled rat. Anticonvulsant activity was evaluated by determination of the threshold for induction of focal afterdischarges in the amygdala (ADT) and by monitoring seizure parameters (seizure severity, seizure duration, afterdischarge duration) at ADT current. In addition to anticonvulsant activity, the behavioural adverse effects of test drugs were determined. All three drugs increased the focal seizure threshold, although at different potencies. In terms of injected dosages and percentage ADT increase, the most potent compound was 7-CKA, followed by (+)-HA-966 and D-CS. None of the drugs induced behavioural adverse effects or motor impairment (measured in the rotarod test) at anticonvulsant doses. However, D-CS produced proconvulsant effects at higher doses. The data demonstrate that pharmacological manipulation of the glycine modulatory site of the NMDA receptor is an effective means of increasing seizure threshold in amygdala-kindled rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Non-astigmatic children's contrast sensitivities differ from anisotropic patterns of adults.

Monocular contrast thresholds were estimated for horizontal (H), vertical (V) and left and right diagonal 10 c/deg sinusoidal gratings for non-astigmatic children (4:10--11:2 yr) and adults using yes-no signal detection (YN) methods. Some children were also tested with two-interval forced-choice (2IFC). Younger children were less likely to have the typical adult pattern of anisotropic sensitivity, favoring H and V, while 10--11 yr olds had more adult-like anisotropy. Within all age groups patterns of anisotropy varied considerably. Deviation of adults from the typical pattern may be related to small amounts of astigmatism in their untested eyes. 4- and 5-yr olds were confused by 2IFC. For children 6 yr and older, 2IFC and YN threshold estimates were equivalent, but 2IFC required less testing time.

Adolescent↗
PubMed1990

Two mechanisms for localization? Evidence for separation-dependent and separation-independent processing of position information.

The Weber function for separation--i.e. delta s as a function of separation s--is typically measured using a pair of targets presented roughly symmetrically relative to the fovea. With this paradigm, as the separation increases, the eccentricity of the individual targets increases also. To disentangle the effects of separation and eccentricity on the Weber function for separation, we systematically examined each of these variables and also examined the effects of target size and exposure duration. Separation discrimination thresholds were measured for average separations from 3 to 6 deg across a wide range of eccentricities, and for eccentricities of 2.5-10 deg for a range of separations. The dependence of threshold on target size was measured by varying the length of the stimuli from 1 to 120 min arc; the dependence on exposure duration was measured using durations of 100 and 500 msec at 10 deg eccentricity for comparison with data collected previously at smaller eccentricities. We found that for separations less than the eccentricity of the targets, thresholds depend primarily on separation; for larger separations, thresholds depend solely on eccentricity. In general, unless the targets are very small or quite brief, the spatial and temporal characteristics of the targets are not major contributors to the slope of the Weber function. Two mechanisms are proposed to account for thresholds in the two regions, one separation-dependent and one separation-independent.

Differential Threshold↗
PubMed1985

Electrical stimulation of pulp nerves--comparison of monopolar and bipolar electrode coupling.

In the present study monopolar and bipolar electrode couplings were compared in electrical stimulation of intradental nerves. In experiments on cats the threshold current values of A- and C-fibres and the jaw-opening reflex were measured with both techniques using different durations of current pulses (0.2-50 msec). In human studies the perception and pain thresholds were determined in the same manner. In experiments on anaesthetized cats 170 intradental nerve units were recorded. Of these 118 were A-fibres (conduction velocity (CV) greater than 2 m/sec) and 52 C-fibres (CV less than or equal to 2 m/sec). The threshold current values (pulse duration 10 msec) of A-fibres were almost 2-fold (12.2 microA, S.D. = 7.9) and of C-fibres over 2-fold (91.3 microA, S.D. = 52.3) with bipolar stimulation compared to monopolar (6.9 microA, S.D. = 7.3 and 42.5 microA, S.D. = 21.5 respectively). The mean threshold of the jaw-opening reflex obtained with bipolar electrode coupling was about twice the value obtained with monopolar (8.2 microA, S.D. = 3.7 and 3.4 microA, S.D. = 0.7 respectively). In human experiments the mean perception threshold was 11.7 microA (S.D. = 6.7) with bipolar and 7.2 microA (S.D. = 3.2) with monopolar stimulation. The mean pain threshold was also higher with bipolar than with monopolar stimulation, 16.0 microA (S.D. = 8.8) and 10.3 microA (S.D. = 4.5) respectively. The subjective sensations were different with these two electrode couplings; the pain (at pain threshold level) induced by bipolar stimulation was experienced as sharp or cutting and with monopolar stimulation as dull or more indefinite. It is concluded that there is a marked difference in the electrical threshold values of pulpal nerves and perception threshold of individual human teeth between monopolar and bipolar tooth stimulation. The thresholds of the A-fibres measured from the cat canine tooth closely correspond to the perception thresholds of the human tooth, but the threshold values of the pulpal C-fibres were significantly higher.

Adult↗
PubMed1999

Direct HIV testing in blood donations: variation of the yield with detection threshold and pool size.

BACKGROUND: This study was designed to evaluate the potential yield of introducing nucleic acid amplification testing (NAT) in blood donation, according to the detection threshold and the pool size. STUDY DESIGN AND METHODS: A mathematical model of early HIV-1 population dynamics in blood has been developed and is used to predict the window period for NAT, according to the detection threshold and the pool size. The corresponding number of undetected, infected blood donations and the residual risk are estimated by using a previously published simulation model for the United States (9.96 million blood donations from regular donors, and an observed rate of HIV antibody-positive blood donations of 3.18/100,000) and for France (2.32 million blood donations, and a rate of antibody-positive donations of 0.9/100,000). RESULTS: The average window period from infection predicted by the mathematical model for NAT ranges from 8.4 to 15.6 days, according to the detection threshold and the pool size. The maximum yield of adding NAT to the current antibody tests is estimated at 14 donations for the United States and 2 for France. The maximum yield of adding NAT to the newly developed combined HIV antibody and p24 antigen tests is 7 donations for the United States and 1 for France. CONCLUSION: NAT at blood donation could reduce the HIV-1 window period to a minimal value of 8 days without pooling the blood samples, but the yield of NAT would be close to that of combined HIV antibody and p24 antigen tests for high values of the detection threshold and the pool size.

Blood Donors↗
PubMed1977

Neural interactions and subjective contours.

Increment thresholds were measured on either side of the locus of subjective contours and inequivalent loci for control figures. The data indicate a small change in increment threshold value in the direction predicted by theories postulating neural interactive components in the formation of subjective contours.

Differential Threshold↗
PubMed1990

Detection of disparity motion by the human observer.

A single-line stimulus in a context-free visual environment presented for 250 msec needs to move with a disparity velocity of 25 to 45 min of arc/s before the direction of depth motion can be correctly identified. Presence of a pair of flanking line stimuli improves this value severalfold, but even then the threshold is 18 to 28 times higher than for the detection of a static disparity difference.

Depth Perception↗
PubMed2006

Influence of drive cycle length on initiation of ventricular fibrillation during implantable cardioverter defibrillator threshold testing.

BACKGROUND: Programmed electrical stimulation of the heart as a method to induce tachyarrhythmias has been described since the 1960s. To date, no study has examined optimal drive cycle length in the induction of ventricular fibrillation (VF) during defibrillation threshold testing after implantable cardioverter-defibrillator placement. We hypothesized that longer drive cycle length, by means of the longer action potential duration, would promote intramyocardial phase 2 reentry and facilitate induction of VF. METHODS: Fifty consecutive implants were randomized in a prospective crossover format for this study. The group consisted of 40 men and 10 women, with each patient receiving either a 400 or 600 ms initial drive train prior to 1.2 J internal shock on the T wave with a goal to induce ventricular fibrillation. The timing of the T wave shock was determined by measuring the interval from the beginning of the QRS to the apex of the T wave in lead II. Successful inductions were defibrillated via the cardioverter defibrillator. Patients were then crossed over and the protocol repeated. RESULTS: Twenty of 23 (87%) patients were successfully induced into VF in the initial 400 ms drive train arm whereas 22 of 27 (81%) were successfully induced in the 600 ms arm. Thus, a total of 44 (88%) patients were successfully induced at 400 ms, 41 (82%) patients were successfully induced at 600 ms, and 2 (4%) patients were not inducible at either cycle length, but were inducible with 50 Hz ventricular stimulation. However, no significant difference was noted between the two groups. CONCLUSION: No investigation to date has questioned whether a relationship exists between drive cycle length and initiation of ventricular fibrillation. Our study addresses this question, though negative for difference between 400 and 600 ms drive trains. Further research into optimal strategies for inducing ventricular fibrillation will minimize patient sedation time and discomfort while undergoing defibrillator threshold testing.

Aged↗