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Results for “Differential Threshold”

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PubMed2005

Sex- and menstrual cycle-related differences in sweating and cutaneous blood flow in response to passive heat exposure.

To examine sex- and menstrual cycle-related differences in thermoregulatory responses to heat exposure, ten young women and six young men were heated passively by immersing their legs in water heated to 42 degrees C for 60 min (in ambient conditions of 30 degrees C and 45% relative humidity). The women underwent heat exposure during the mid-follicular (F) and mid-luteal (L) phases of the menstrual cycle, which were confirmed by assaying plasma female reproductive hormones. The rectal and mean body (T(b)) temperatures of women in the L phase were significantly greater than those of women in the F phase and of men during a pre-heating equilibration period (28 degrees C) and during heat exposure. During heat exposure, the local sweat rates (m(sw)) on the forehead, chest, back, and forearm of women in either phase were significantly lower than those of men, but the thigh (m(sw)) was similar to that of men. The m(sw) did not change at any site during the different phases of the menstrual cycle. The cutaneous blood flow (%LDF) was significantly greater on the thigh for women in either phase compared with men, but no difference was found at any other site (forehead, chest, back, and forearm). The %LDF on the back was significantly greater for women in the L phase than in the F phase, but those at other sites were similar in both phases. We conclude that, compared with men, heat loss from women depends more on cutaneous vasodilation (especially on the thigh) than on sweating, irrespective of the phase of the menstrual cycle. This phenomenon was due to peripheral mechanisms, as reflected in the greater slope of the relationship between %LDF and T(b) lower slope of the relationship between m(sw)) and frequency of sweat expulsion, and lower sweat output per gland. The menstrual cycle modified the T(b) threshold for vasodilation and sweat onset in women. Therefore, the sex difference in the T(b) threshold was more marked for women during the L phase than during the F phase. Moreover, the menstrual cycle modified the slope of the relationship between %LDF on the back and T(b).

Adult↗

PubMed1987

Relation of blood pressure during exercise to anaerobic metabolism.

To determine the effects of anaerobic metabolism on blood pressure (BP), 25 subjects were studied for BP response to graded and continuous leg-crank ergometry under both aerobic and anaerobic conditions. Measurements obtained during exercise also included ventilatory equivalents for oxygen and carbon dioxide to determine anaerobic threshold. Systolic and diastolic BP responses to exercise before anaerobic threshold were compared with responses after anaerobic threshold by linear regression analyses. The systolic BP response to graded exercise was significantly accelerated (p less than 0.01) after anaerobic threshold, demonstrating a nonlinear response to proportionately graded exercise demand. Comparison of the slopes or rates of change in diastolic BP before and after anaerobic threshold also indicated a significant difference (p less than 0.01) under these 2 different metabolic conditions. It is concluded that in contrast to the linear response of BP under conditions of aerobic metabolism, BP responds nonlinearly during anaerobic metabolism.

Anaerobiosis↗

PubMed1986

Behavioral evidence for a crossed ascending pathway for pain transmission in the anterolateral quadrant of the rat spinal cord.

Thresholds of two behavioral responses to noxious pressure of the hindpaws (withdrawal, vocalization) were analyzed before and 3 weeks after a lesion of various quadrants of the spinal cord at the cervical level. The threshold of the spinal reflex could not be modified. Threshold of vocalization elicited by the pressure of one hindpaw was significantly increased when, and only when, the opposite ventrolateral quadrant was cut. These results emphasize the role of the lateral spinothalamic tract in the transmission of noxious messages.

Afferent Pathways↗

PubMed1994

Shape discrimination for rectangles defined by disparity alone, by disparity plus luminance and by disparity plus motion.

We measured the just-noticeable difference in aspect ratio a/b for dotted rectangles defined entirely by disparity (DD rectangles), dotted rectangles defined by both disparity and luminance contrast (DD+LD rectangles) and rectangles defined by both disparity and relative motion (DD+MD rectangles) over a wide range of uncrossed (0-126 min arc) and crossed (0-126 min arc) disparities. The rectangle's height and width were, respectively, a and b. Vergence was monitored by nonious lines, fusion was monitored by a fusion line, and subjects were instructed to look at the rectangle. We conclude that for DD rectangles, aspect ratio discrimination for crossed and uncrossed disparities cannot be compared accurately unless (a) ocular vergence is monitored, (b) a wide range of crossed and uncrossed disparities are investigated and (c) trials are repeated until threshold has stabilized for the entire range of both crossed and uncrossed disparities. Intersubject variations in fixation disparity is the reason for (a). The reason for (b) is that the effect of disparity on aspect ratio discrimination threshold is different for crossed and uncrossed disparities. The reason for (c) is that, for some subjects, the lowest discrimination threshold continues to fall with practice for uncrossed disparities beyond the point at which the lowest threshold has effectively stabilized for crossed disparities. We report that aspect ratio discrimination threshold for a DD rectangle first decreased and then levelled out as its disparity was progressively increased from zero, while the perceived depth of the rectangle increased smoothly and approximately linearly. We found that the lowest value of aspect ratio discrimination threshold was the same for both crossed and uncrossed disparities. This lowest value occurred at or just before the disparity at which fusion was lost for the dots surrounding the rectangle. The lowest value of discrimination threshold was lower for DD+LD and for DD+MD rectangles than for DD rectangles. The lowest value of discrimination threshold for DD rectangles was 4.0, 3.4, 7.4 and 3.1% for our four subjects. If we assume that a and b are encoded directly, then a 3.1% discrimination threshold implies that the precision of encoding a and b is better than 1 min arc--considerably better than the 9 min arc mean dot separation, and considerably better than the 3-5 c/deg estimate for cyclopean grating acuity.

Adult↗

PubMed1999

Lowering the hemoglobin threshold for transfusion in coronary artery bypass procedures: effect on patient outcome.

BACKGROUND: There is controversy regarding the application of transfusion triggers in cardiac surgery. The goal of this study was to determine if lowering the hemoglobin threshold for red cell (RBC) transfusion to 8 g per dL after coronary artery bypass graft surgery would reduce blood use without adversely affecting patient outcome. STUDY DESIGN AND METHODS: Consecutive patients (n = 428) undergoing elective primary coronary artery bypass graft surgery were randomly assigned to two groups: study patients (n = 212) received RBC transfusions in the postoperative period if the Hb level was < 8 g per dL or if predetermined clinical conditions required RBC support, and control patients (n = 216) were treated according to individual physician's orders (hemoglobin levels < 9 g/dL as the institutional guideline). Multiple demographic, procedure-related, transfusion, laboratory, and outcome data were analyzed. Questionnaires were administered for patient self-assessment of fatigue and anemia. RESULTS: Preoperative and operative clinical characteristics, as well as the intraoperative transfusion rate, were similar for both groups. There was a significant difference between the postoperative RBC transfusion rates in study (0.9 +/- 1.5 RBC units) and control (1.4 +/- 1.8 RBC units) groups (p = 0.005). There was no difference in clinical outcome, including morbidity and mortality rates, in the two groups; group scores for self-assessment of fatigue and anemia were also similar. CONCLUSIONS: A lower Hb threshold of 8 g per dL does not adversely affect patient outcome. Moreover, RBC resources can be saved without increased risk to the patient.

Aged↗

PubMed2001

Pacing during ventricular fibrillation: factors influencing the ability to capture.

INTRODUCTION: Recent studies showed that pacing atrial and ventricular fibrillation (VF) is possible. The studies presented here determined which parameters influence the efficacy of a pacing train to capture fibrillating ventricular myocardium. Electrode type, current strength, order of pacing trains, polarity, and VF morphology preceding the pacing trains were investigated. METHODS AND RESULTS: A 504-electrode recording plaque sutured to the right ventricle of pig hearts was used to record the activations of VF and those resulting from the pacing stimulation. Capture of VF by pacing was determined by observing an animated display of the first temporal derivative of the electrograms. A series of electrodes in a line captured the heart more frequently during VF than did a point electrode. Increasing the current strength to 10 x diastolic pacing threshold increased the incidence of capture, but increasing this strength further did not. The second or third train of 40 stimuli had greater capture rates than did the first train during the same VF episode. Anodal and cathodal unipolar, and bipolar stimulation were equally efficacious in capturing VF. VF activation during the 1-second interval preceding pacing was more organized for pacing trains that captured than those that did not. The highest incidence of capture, 46% to 61% of pacing trains, occurred with a line of electrodes at 10 x diastolic pacing threshold delivered by the second or third train. CONCLUSION: The probability of a pacing train capturing fibrillating myocardium can be influenced by the pacing protocol parameters.

Animals↗

PubMed2006

Learning to discriminate interaural time differences: an exploratory study with amplitude-modulated stimuli.

The advent of bilateral cochlear implants (CIs) has increased interest in learning on binaural tasks, and studies in normal-hearing listeners provide important background information. However, few studies have considered learning with discrimination of interaural time difference (ITD). Here, learning with ITD was explored using stimuli that are more relevant to bilateral CIs than used previously. Inexperienced listeners were trained with envelope-based ITD using high-frequency amplitude-modulated tones with or without an interaural carrier frequency difference (IFD), the former to simulate asymmetrical bilateral CI insertions. All were tested with and without IFD before and after training. In most listeners, ITD thresholds improved substantially with training, not necessarily reaching asymptote after 3,000 trials. In these, the magnitude and time-course of learning was larger than anticipated from a previous study with low-frequency ITD. Learning generalized across IFD and the effect of IFD on ITD thresholds at post-test was smaller than reported previously. These results have implications for studies of bilateral CIs, such as the need to provide extensive training to avoid over-estimating any apparent 'impairment'.

Acoustic Stimulation↗

PubMedDate not supplied

Electrical stimulation of the human brain in epilepsy.

The concordance between the epileptogenic area as determined by intracranially recorded spontaneous seizure onset and that determined by electrically induced auras or seizures in 126 patients with medically intractable epilepsy was studied. Four to 11 multicontact depth electrodes were chronically implanted in patients as part of a preoperative investigation procedure. Localization of the epileptic area was obtained from the recording of three spontaneous seizures and from stimulation data including self-reported auras, seizures, and intracranial EEG afterdischarges recorded during an incremental stimulation sequence at all cortical pairs of contacts. Positive responses to stimulation were obtained in 63% of patients. Concordance between spontaneous and induced auras or seizures was greater than 90% with single unilateral foci and less with multiple foci. Stimulation data reliably predicted the resection area in unilateral and bilateral temporal foci but not in other locations. Afterdischarge thresholds were not reliable predictors of the spontaneous seizure focus. The intracranial stimulation procedure used appears to provide useful confirmatory and complementary localizing information in relation to that obtained from spontaneous seizures.

Brain↗

PubMed2002

Distinct sensitizing effects of the cAMP-PKA second messenger cascade on rat dural mechanonociceptors.

Activation of the cAMP/protein kinase A (PKA) second messenger cascade has been implicated in the induction of mechanical hyperalgesia by inflammatory mediators. We examined the role of this cascade in mechanical sensitization of nociceptive neurons that innervate the meninges, a process thought to be involved in the pathophysiology of headache syndromes such as migraine. Single unit activity was recorded in the trigeminal ganglion from 40 mechanosensitive dural afferents (conduction velocitity: 0.3-6.6 m s(-1)) and nine mechanically insensitive dural afferents (MIAs) (conduction velocitity: 0.3-2.8 m s(-1)) while stimulating the dura with a servo force-controlled stimulator or von Frey monofilaments, respectively. Local application to the dura of dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP, 100 microM), a stable membrane-permeant cAMP analogue, produced mechanical sensitization in the majority of mechanosensitive units (19/29, 66 %). Two distinct patterns of mechanical sensitization were observed. Thirty-eight per cent of the units exhibited only a decrease in threshold (TH group), while 28 % showed only an increase in suprathreshold responses (STH group). dbcAMP also induced mechanosensitivity in the majority of MIA units (6/9, 67 %). dbcAMP-induced sensitization was blocked by the PKA inhibitors, Rp-cAMP (1 mM) and H-89 (100 microM). A mixture of inflammatory mediators induced both components of sensitization in the majority of mechanosensitive units tested. However, in each unit, PKA inhibitors blocked only one of the two effects (either TH or STH). Units that were classified as TH or STH also differed in their baseline stimulus-response slopes, thresholds and conduction velocities. These findings implicate the cAMP-PKA cascade in sensitization of dural mechanonociceptors and suggest that this cascade may produce sensitization through at least two different mechanisms operating in separate neuronal populations.

Animals↗

PubMed2005

Laminar variation in threshold for detection of electrical excitation of striate cortex by macaques.

Macaques were trained to signal their detection of electrical stimulation applied by a movable microelectrode to perifoveal striate cortex. Trains of < or =100 cathodal, 0.2-ms, constant current pulses were delivered at 50 or 100 Hz. The minimum current that could be reliably detected was measured at successive depths along radial electrode penetrations through the cortex. The lowest detection thresholds were routinely encountered when the stimulation was applied to layer 3, particularly just at the juncture between layers 3 and 4A. On the average, there was a twofold variation in threshold along the penetrations, with the highest intracortical thresholds being in layers 4C and 6. Variations as high as 20-fold were obtained in some individual penetrations, whereas relatively little change was observed in others. The minimum detectable current was 1 muA at a site in layer 3, i.e., 10-100 times lower than that for surface stimulation. Because macaques, as do human subjects, find electrical stimulation of striate cortex to be highly similar at all loci (a phosphene in the human case), it is puzzling as to how such uniformity of effect evolves from the exceedingly intricate circuitry available to the effective stimuli. It is hypothesized that the stimulus captures the most excitable elements, which then suppress other functional moieties, producing only the luminance of the phosphene. Lowest thresholds presumably are encountered when the electrode lies among these excitable elements that can, with higher currents, be stimulated directly from some distance or indirectly by the horizontal bands of myelinated axons, the stria of Baillarger.

Animals↗

PubMed1973

Behavioral assessment of pain detection and tolerance in monkeys.

Cebus albifrons monkeys received electrical stimulation of the hindlimbs over a wide range of intensities. On trials signalled by a blue light, the animals were permitted to escape shock by pressing a disc, or shock was terminated after 8 sec (free escape). Escape force (disc pressure) was found to increase as stimulation intensity increased well beyond escape threshold, while shock duration curves reached plateau at the mid-range of intensities. The shock duration curves generated by free escape responses should be comparable to pain detection functions obtained by similar operations in humans, and the curves were stable over months of testing, as is generally found in pain-detection studies. On trials signalled by a red light, the animals received intense tail shock immediately after escape responses (punished escape), or, if they endured leg shock for 8 sec without escaping, then they could avoid tail shock with a panel press. The shock duration curves generated by punished escape responses should be comparable to pain tolerance functions as defined for human subjects, and the escape thresholds were considerably higher on red-light trials. As in human studies, the tolerance curves were not stable over repeated testing sessions, and some feature of the paradigm forced a progression toward extremely high levels of tolerance.

Animals↗

PubMed1976

Temporal summation effects on lingual vibrotactile thresholds.

Ascending lingual vibrotactile thresholds at 250 Hz were obtained for four groups of normal-speaking adult subjects. Stimulus durations were varied for each respective group. Threshold values plotted as a function of stimulus duration indicated a temporal summation function occurring up to 200 msec. Findings are discussed in terms of underlying peripheral receptors and some implications for using summation measures as a basis for identifying oral sensory disorders.

Adult↗

PubMed1994

Neuronal and psychophysical sensitivity to motion signals in extrastriate area MST of the macaque monkey.

We recorded the responses of single neurons in extrastriate area MST while rhesus monkeys discriminated the direction of motion in a set of stochastic visual displays. By varying systematically the strength of a coherent motion signal within the visual display, we were able to measure simultaneously the monkeys' psychophysical thresholds for direction discrimination and the responses of single neurons to the same motion signals. Neuronal thresholds for reliably signaling the direction of motion in the visual display were calculated from the measured responses using a method based in signal detection theory. Neurons in MST were exquisitely sensitive to motion signals in the display, having thresholds for discriminating the direction of coherent motion that were, on average, equal to the psychophysical thresholds of the monkeys. For many MST neurons, the intensity of the response was correlated with the monkey's psychophysical judgements for repeated presentations of a given near-threshold stimulus; the monkey tended to choose the preferred direction of the neuron under study when that neuron responded more strongly to the stimulus. In both of these respects, MST neurons were indistinguishable from neurons in extrastriate area MT, a major source of afferent input to MST. In a second set of experiments, we found that both of these results held true in the face of pronounced manipulations of the visual stimulus. Severe reductions in stimulus size and speed, for example, compromised neuronal and psychophysical sensitivities by similar amounts so that the average neuronal and psychophysical thresholds remained approximately equal. In addition, the trial-to-trial covariation of neuronal response and perceptual decision was unaffected by our stimulus manipulations. Thus, MST neurons carry signals appropriate for supporting psychophysical performance on our task over an impressively wide range of stimulus configurations.

Animals↗

PubMed1995

Threshold sodium excretory and renal blood flow effects of angiotensin converting enzyme inhibition.

OBJECTIVE: To probe the potential influence of a high renal tubular level of angiotensin II on sodium reabsorption using angiotensin converting enzyme inhibitors and a non-peptide angiotensin antagonist. METHODS: Systemic arterial blood pressure, renal blood flow (RBF) and electrolyte and urinary excretion were measured in anesthetized uninephrectomized Wistar rats. Captopril (n = 12), ramiprilat (n = 10) or EXP 3174 (n = 9) was infused into the renal artery in graded doses to examine whether the threshold dose that increased RBF was lower than that which increased sodium excretion rate (Na+ excretion rate). RESULTS: Whereas ramiprilat (0.5-4 micrograms/kg/min intra-arterially) and EXP 3174 (0.5-4 micrograms/kg/min intra-arterially) decreased blood pressure in all but the lowest dose of 0.5 micrograms/kg/min, captopril (1-8 micrograms/kg/min intra-arterially) did not change blood pressure except for a slight effect with the two higher doses. These three agents increased RBF to about the same degree, between 6% and 18%, which was relatively large compared with the maximal vasodilator response achievable in the kidney with acetylcholine (30-37%). Captopril given intra-arterially had a more consistent effect on Na+ excretion rate than either ramiprilat or EXP 3174. An increase in Na+ excretion rate occurred with captopril ranging from 44% to 78%, whereas no significant change was obtained with the other drugs. The dose of captopril that increased RBF was the same as that which increased Na+ excretion rate. In those experiments in which ramiprilat resulted in an increase in Na+ excretion rate (five of 10 experiments), the effective dose was the same as that which increased RBF. CONCLUSION: When administered by the intra-arterial route, captopril was more effective in increasing Na+ excretion rate than either ramiprilat or EXP 3174. Although the threshold dose of captopril and ramiprilat required to increase Na+ excretion rate and RBF was similar, suggesting blockade of a common angiotensin II pool, there was not a good correlation between these two effects.

Angiotensin II↗

PubMed1995

Assessment of heat pain perception in relation to vibration exposure.

The effect of vibration on thin unmyelinated sensory fibers was studied by perception threshold measurement of heat pain. The investigation was a cross-sectional study of 98 vibration-exposed and 53 non-vibration-exposed workers. Pain perception was determined by using the "Marstock" method. The perception threshold of contact heat induced pain was assessed by the method of limits. Quantified personal energy-equivalent vibration exposure was assessed for all subjects on a group basis. The cumulated lifetime equivalent frequency-weighted vibration exposure was estimated based on measurements according to ISO 5349. The mean heat perception threshold was 46.2 degrees C (SD 2.6 degrees C) for the right hand and 46.0 degrees C (SD 2.7 degrees C) for the left. The results showed no mean difference (left = 0.1 degree C, right = 0.6 degree C) in heat pain perception between vibration-exposed and non-exposed subjects. There was a high correlation between the right and left hand measurements (r = 0.81). The risk (rate ratio) for impairment was 1.02 (95% CI 0.81-1.30) for the right hand and 1.00 (95% CI 0.79-1.26) for the left. The results of the study did not indicate any impairment as assessed by an increased perception threshold for heat pain. The lack of an increased heat pain threshold among the vibration-exposed workers in our study cannot exclude a possible lesional somatosensory influence from the exposure, as the lesional effect may also be a lowering of the threshold. The multiple and unrelated percepts of thermal sensations imply that it is not legitimate to draw conclusions about the total status of the small calibre afferents when testing only the heat pain perception.

Adult↗

PubMed1997

Critical levels of spinal cord blood flow and duration of ischemia for the acute recovery of segmental spinal cord responses in cats.

A broad range of residual lumbar spinal cord blood flows assessed by laser-Doppler flowmetry (rSCBF, 0-78%) was achieved by snare occlusion of the descending aorta and additional blood-volume reduction for 1-10 min in spinal cats (n = 30; 238 trials). The period of complete reflex suppression (delay until recovery) that revealed some correlation to duration (r = 0.72) and depth (r = -0.36) of ischemia showed comparable durations in mono- and polysynaptic reflexes, whereas it was significantly less for the cord dorsum potential (CDP). With rSCBF values > 50-60% reflexes and > 45%, the CDP was rarely abolished, irrespective of the duration of ischemia. The threshold of duration for a complete loss of reflex responses was found to be approximately 1 min of ischemia. The influence of rSCBF and duration of ischemia on the occurrence of incomplete recoveries of reflexes was assessed simultaneously in a logistic regression model. Compared with periods of ischemia of 3 min, all longer durations showed a steep risk gradient for incomplete recoveries; an increment of 10% in rSCBF led to a risk reduction for incomplete recoveries of nearly 25%. These findings were significant (p < 0.001) and indicated that blood-flow thresholds and limits for the development of neurologic deficits of the spinal cord are comparable to those of the brain, with the important difference that the blood-flow reserve of the spinal cord is smaller.

Animals↗

PubMed1998

Recovery cycles of neurons in the inferior colliculus, the pontine nuclei and the auditory cortex of the big brown bat, Eptesicus fuscus.

This study examines the recovery cycles of neurons in the inferior colliculus (IC), the pontine nuclei (PN) and the auditory cortex (AC) of the big brown bat, Eptesicus fuscus, using a pair of identical pulses at different interpulse intervals. Although the recovery cycle varies among neurons, on average, AC neurons have the longest recovery cycle and PN neurons have the shortest one. The recovery cycle of IC neurons is slightly longer than PN neurons. Neurons tend to have longer recovery cycles when determined with long pulse duration than short one. The minimum threshold of these neurons to the second pulse increases with decreasing interpulse interval. The difference in the recovery cycles of these neurons supports previous findings that IC and PN neurons can follow sound pulses at higher rates than AC neurons do.

Acoustic Stimulation↗

PubMed2002

Acquisition and expression of proximal and distal upper limb stimulus-response curves to transcranial magnetic stimulation.

Motor cortex stimulus-response (S/R) curves are an indication of cortical excitability and are of relevance to topographical mapping. The aims of this study were to compare two different methods of collecting data to construct a S/R curve for transcranial magnetic stimulation (TMS) in the upper limbs, to identify reliable summary statistics for the S/R curve, and to determine whether S/R curves predicted motor threshold. Motor evoked potentials (MEP) were obtained from biceps brachii (BB) and abductor pollicis brevis (APB) muscles at rest with a circular coil centered at the vertex. Motor threshold was determined using a validated protocol. MEPs were obtained with 1% increments in intensity or from the average of five trials at 5% increments. The S/R relationships were fitted to linear, S-shaped, and Boltzmann functions. A linear function determined from the average of five trials accurately summarized our data (r2 from 0.6 to 0.9, P < 0.05, n = 8, for right APB and from 0.6 to 0.9, P < 0.05, n = 8 for right BB). The X-axis intercept of the line determined using these methods fell between the upper and lower limits of motor threshold in all eight subjects. We propose that MEP values obtained at intervals of 5% averaged over five trials, fitted to a linear function provides a practical means of assessing the S/R characteristics of TMS for proximal and distal upper limb muscles.

Adolescent↗