Magnitude Of Reflex Research Articles

Impaired classical eyeblink conditioning in elderly human subjects: the role of unconditioned response magnitude

Classical conditioning is impaired in elderly subjects, and an experiment investigated the role of unconditioned response magnitude and autonomic arousal in this aging-related impairment. Two groups of young (n = 20, mean age 23.8 years) and elderly (n = 21, mean age 73.8 years) healthy subjects participated in the experiment. Single-cue classical eyeblink conditioning was in effect for 72 trials, with a tone as the conditioned stimulus (CS) and an airpuff to the eye as the unconditioned stimulus (US). The CS was presented alone in eight additional trials to allow recordings of skin conductance and heart rate responses. Startle eyeblink reflexes were elicited either alone or in the presence of the CS after the end of the conditioning procedure to investigate startle potentiation. More young (95%) than elderly (76%) subjects identified the CS - US contingency in a post-conditioning questionnaire, whereas only 38% of the elderly and 85% of the young subjects acquired the conditioned response (CR). Unconditioned blink reflex magnitudes were greater in the subjects who had acquired the CR. Heart rate deceleration was seen in both young and elderly subjects who had acquired the conditioned response. Startle reflexes and skin conductance responses were related to conditioning in young but not in elderly subjects. Unconditioned response magnitude was the best predictor for the acquisition of the CR. Together with the questionnaire data, this indicates that peripheral factors play a significant role in the impairment of classical eyeblink conditioning in elderly people. Autonomic arousal was not related to acquisition of the CR in elderly subjects.

Routine post-weaning handling of rats prevents isolation rearing-induced deficit in prepulse inhibition

Rats reared under isolation conditions from weaning present a number of behavioral changes compared to animals reared under social conditions (group housing). These changes include deficits in prepulse inhibition (PPI) of the startle reflex to a loud sound. PPI refers to the reduction of the magnitude of the startle reflex when a relatively weak stimulus (the prepulse) precedes by an appropriate time interval the intense startle-elicing stimulus (the pulse). PPI is useful for studying sensorimotor integration. The present study evaluated the effect of handling on the impairment of PPI induced by isolation-rearing. Male Wistar rats (N = 11-15/group) were housed in groups (5 per cage and handled three times a week) or isolated (housed individually) since weaning (21 days) for 10 weeks when they reach approximately 150 g. The isolated rats were divided into "minimally handled" animals (handled once a week for cleaning purposes only) or "handled" animals (handled three times a week). This handling consisted of grasping the rat by the tail and moving it to a clean cage (approximately 5 s). A statistically significant reduction (52%) in the PPI test was found only in the isolated group with minimal handling while no difference was seen between grouped animals and isolated handled animals. These results indicate that isolation rearing causes disruption in the PPI at adult age, which serves as an index of attention deficit. This change in the sensory processing of information induced by post-weaning isolation can be prevented by handling during the development of the animal.

Influence of individual differences in Behavioral Inhibition System on the magnitude and time course of the fear-potentiated startle

Previous research has shown that the startle reflex is potentiated during experimentally induced anxiety (fear-potentiated startle). In the present study, the magnitude and time course of the startle blink reflex were examined among 35 undergraduates scoring one standard deviation above ( n = 14) or below ( n = 21) the mean in a self-report measure of sensitivity to punishment (SP) in a paradigm involving the anticipation of electric shocks. Contrary to our predictions, SP did not relate to differences in the magnitude or in the time course of fear-potentiated startle. Our data do not support an association between this individual differences variable and the fear-potentiated startle.

Characteristics of propofol-evoked vascular pain in anaesthetized rats

In this study we have assessed vascular pain caused by the i.v. anaesthetic agent, propofol, using the flexor reflex response and compared this with that of capsaicin in anaesthetized intact rats. Experiments were performed on 133 male Sprague-Dawley rats weighing 280-340 g. The animals were anaesthetized with urethane (1.3 g kg(-1), i.p.), and an arterial cannula was inserted to the level of the bifurcation of the femoral artery. The magnitude of the flexor reflex was examined by recording the electromyogram from the posterior biceps femoris/semitendinosus muscles. Our data show that the flexor reflexes evoked by intra-arterial (i.a.) injection of propofol (1%, 25-100 microl) and capsaicin (0.05-0.2 microg) were dose dependent. An initial i.a. injection of procaine (2%, 200 microl) blocked both responses. Furthermore, the flexor reflex induced by these chemical stimuli were inhibited by morphine (5 mg kg(-1), s.c.) and restored with naloxone (1.5 mg kg(-1), s.c.). Pre-treatment with capsazepine (20 microg, i.a.), a selective VR1 antagonist, inhibited the capsaicin-evoked response, but not that of propofol. Indomethacin (10 mg kg(-1), i.p.), a non-selective cyclo-oxygenase inhibitor, inhibited only the propofol-evoked response and this recovered with arterial PGE2 (5 microg). Collectively our data suggest that propofol-evoked vascular pain is mainly initiated by prostanoids.

Affective modulation of nociception at spinal and supraspinal levels

This study was designed to examine the effect of emotion on the nociceptive flexion reflex and pain ratings. To do so, 28 participants viewed pictures varying in emotional valence (unpleasant, neutral, pleasant) and electric stimulations were delivered during and in between pictures. Biceps femoris EMG resulting from the stimulations was used to quantify the nociceptive flexion reflex (spinal nociception), and pain ratings to the stimulations were used as an evaluative measure of supraspinal nociception. Manipulation checks suggested that pictures effectively manipulated emotion. Moreover, nociceptive flexion reflex magnitudes and pain ratings were modulated in a parallel manner. Specifically, viewing unpleasant pictures enhanced the nociceptive flexion reflex and pain, whereas viewing pleasant pictures inhibited the reflex and pain. Analyses suggested that emotional valence, but not arousal, mediated the effects of pictures.

Development of a New Model to Investigate the Fetal Nociceptive Pathways

Objective: The aim of the study was to develop an experimental model to investigate the fetal nociceptive pathways and fetal analgesia. Methods: We tested the electromyographic (EMG) response from the biceps femoris to electrical stimulation of the sural nerve in chronically-prepared fetal lambs with and without sufentanil. Results: An EMG response could be recorded 140 ms after the electrical stimulation above a threshold of current’s intensity. The response presents the characteristics of a nociceptive flexion reflex. The reflex magnitude increased with the stimulus intensity. Sufentanil decreased the response. Bradycardia was noted 10 s after the stimulation and was not observed after sufentanilinfusion. Catecholamine concentrations were not altered by the stimulation. Conclusion: Our study shows that a nociceptive flexion reflex can be recorded in the ovine fetus. We suggest that this reflex can be used as a new tool to study the ontogenesis of the nociceptive pathways and the effects of analgesic drugs during fetal life.

Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 +/- 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials (P < 0.01) and by 38% during the Recovery trials (P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials (P < 0.05) and by 28% during the Recovery trials (P < 0.01). The discharge rate of motor units increased from 10.3 +/- 2.7 pulses/s (pps) before vibration to 12.2 +/- 3.1 pps (P < 0.01) during the After trials and to 11.9 +/- 2.6 pps during the Recovery trials (P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.

Effects of electrically induced muscle contraction on flexion reflex in human spinal cord injury

Flexion reflex study in motor complete human spinal cord injury (SCI). To examine changes in the magnitude of the flexion reflex following functional electrical stimulation (FES) of the rectus femoris (RF) muscle. Bioengineering Unit, University of Strathclyde, Glasgow, Scotland, UK. The flexion reflex was evoked by electrical stimulation of the sural nerve, and was recorded in the tibialis anterior (TA) muscle. RF muscle conditioning stimulation was performed at 0.7, 1, and 2 times motor threshold ( x MT) over a range of conditioning test intervals. The incidence of the early component of the flexion reflex (<100 ms) was low, suggesting that this reflex component might be suppressed in SCI. The long latency flexion reflex component (>120 ms) was observed in all subjects during control conditions and following sensorimotor conditioning. FES applied to the RF muscle (above and below MT) in the main induced a significant early and long lasting depression of the long latency flexion reflex. The depression of the flexion reflex was a result of multisensory actions on flexion reflex pathways resulting from the direct and indirect (mechanical) consequences of electrically induced muscle contraction on cutaneous and muscle afferents. Our findings emphasize the importance of sensory feedback mechanisms in modulating flexion reflex excitability, and highlight the need for rehabilitation professionals to consider the central actions of FES-induced afferent feedback when incorporating FES into a rehabilitation program. State Scholarships Foundation (IKY) of Hellas.

Acoustic reflexes to Schroeder-phase harmonic complexes in normal-hearing and hearing-impaired individuals

Harmonic complexes generated with positive or negative Schroeder-phases may result in differences in cochlear excitation, even though their long-term spectra and amplitudes are equal. As a measure of possible differences in cochlear excitation resulting from these harmonic complexes, thresholds and growth of the acoustic reflex were assessed in normal-hearing and hearing-impaired subjects. Harmonic complexes with fundamental frequencies of 50, 100, and 200 Hz were constructed with positive and negative-Schroeder phases. In normal-hearing subjects, acoustic reflex thresholds for the 50- and 100-Hz fundamental waveforms were typically lower for negative Schroeder-phase complexes than for positive Schroeder phase stimuli. At the highest fundamental frequency of 200 Hz, there were no significant threshold differences due to phase. Hearing-impaired subjects showed a similar pattern for thresholds between the two phase selections, but with smaller differences than those observed in normal-hearing subjects. At levels above reflex threshold, the magnitude of the acoustic reflex was greater for the negative-phase than the positive-phase stimuli for the lowest fundamental frequency, but no significant differences were observed at fundamental frequencies of 100 and 200 Hz. These results are consistent with generally greater cochlear excitation in response to negative than to positive Schroeder-phase stimuli when the fundamental frequency is sufficiently low. Increased excitation may reflect a synchronization of response across a wide band of frequencies in the cochlea when the rate of frequency sweep within periods of these harmonic complexes is appropriately matched to timing characteristics of the traveling wave.

Vagal response varies with valsalva maneuver technique: A repeated-measures clinical trial in healthy subjects

Study objective Variable success rates of the Valsalva maneuver in treatment of paroxysmal supraventricular tachycardia may be due to variations in performance technique. This study aimed to compare the magnitude of the vagal reflexes initiated by 5 variations of the Valsalva maneuver technique (supine, supine with epigastric pressure, supine with leg raise, semirecumbent position, and sitting position). Methods This was a single-blinded, repeated-measures, clinical trial of 65 subjects in sinus rhythm. Subjects performed each Valsalva maneuver technique 5 times in random order. The means of the longest ECG R-R intervals during the relaxation phase (postmaneuver R-R interval) and the postmaneuver pulse rates for each technique were compared. The mean differences between the pre- and postmaneuver R-R intervals for each technique were also compared. Results The supine with epigastric pressure and supine techniques resulted in longer mean postmaneuver R-R intervals (1.082 seconds [95% confidence interval (CI) 1.045 to 1.119 seconds] and 1.075 seconds [95% CI 1.035 to 1.114 seconds], respectively) than the leg raise, semirecumbent, and sitting position techniques (1.053 seconds [95% CI 1.019 to 1.086 seconds], 1.044 seconds [95% CI 1.006 to 1.081 seconds], and 1.024 seconds [95% CI 0.990 to 1.059 seconds], respectively), which equates to slower mean postmaneuver pulse rates for the supine with epigastric pressure and supine techniques (55.5 and 55.8 beats/min, respectively) than the leg raise, semirecumbent, and sitting position techniques (57.0, 57.5, and 58.6 beats/min, respectively). The supine with epigastric pressure and supine techniques also resulted in the largest premaneuver versus postmaneuver differences. Conclusion For healthy subjects in sinus rhythm, the supine with epigastric pressure and supine techniques generated stronger vagal responses, as measured by R-R intervals and pulse rates, than the other techniques examined. However, the vagal responses of these 2 techniques were similar, and the addition of epigastric pressure may confer little advantage.

Variation of magnitude and timing of wrist flexor stretch reflex across the full range of voluntary activation.

This paper reports an investigation of the magnitude and timing of the stretch reflex over the full range of activation of flexor carpi radialis. While it is well established that the magnitude of the reflex increases with the level of muscle activation, there have been few studies of reflex magnitude above 50% of maximum voluntary contraction (MVC) and virtually no study of the timing of the response in relation to activation level. Continuous small amplitude (approximately 2 degrees) perturbations were applied to the wrist of 12 normal subjects while they maintained contraction levels between 2.5-95% MVC, monitored via surface electromyography (EMG). Both narrow band (4-5 Hz) and broad band (0-10 Hz) stretch perturbations were employed. The gain (EMG output/stretch input) and phase advance of the reflex varied with the level of muscle activation in a similar manner for both types of stretch, but there were significant differences in the patterns of change due to stretch bandwidth. Consistent with previous studies, the group average reflex gain initially increased with muscle activation level and then saturated. Inspection of individual data, however, revealed that the gain reached a peak at about 60% MVC and then decreased at higher contraction levels, the pattern across the full range of activation being well described by quadratic functions (mean r2=0.82). This quadratic pattern has not been reported previously for the neural reflex response in any muscle but is consistent with the pattern that has been reliably observed in studies of the mechanical reflex response in lower limb muscles. In contrast to the pattern for reflex gain, the phase advance of the reflex (at a stretch frequency of 4.5 Hz) decreased linearly from approximately 130 degrees at the lowest contraction levels to approximately 50 degrees as maximum voluntary contraction was reached (mean r2=0.69). This decrease corresponds to a delay of 49 ms introduced centrally in reflex pathways. All subjects showed clearly defined quadratic functions relating reflex gain and linear functions relating reflex phase to activation level, but there were considerable individual differences in the slopes of these functions which point to systematic differences in synaptic behaviour of the motoneuron pool. Thus, there was wide inter-subject variation in both the contraction level at which the reflex gain reached a peak (31-69% MVC) and the highest target contraction level that could be sustained during reflex measurement (47-95% MVC). A high correlation between these variables (r2=0.78) suggests a linear relation between afferent support of contraction and muscle fatigability. The decline in reflex gain at high levels of muscle activation signals a failure of muscle afferent input and subjects in whom the gain reached a peak and declined early were unable to sustain higher target contraction levels. The results of the study show that both the timing and magnitude of the stretch reflex vary markedly over the full range of voluntary muscle activation. The pattern of variation may account for why the stretch reflex contributes most effectively to muscle mechanics over the lower half of the range of activation, while progressive reductions in both gain and phase advance at higher levels render the reflex mechanically less effective and make tremor more likely.

Isoflurane facilitates hiccup-like reflex through gamma aminobutyric acid (GABA)A- and suppresses through GABAB-receptors in pentobarbital-anesthetized cats.

The mechanism by which volatile anesthetics exert inconsistent effects on hiccups is unknown. We elicited a hiccup-like reflex by mechanical stimulation of the dorsal epipharynx in mechanically ventilated cats. The magnitude of the hiccup-like reflex was measured as the peak negative esophageal pressure (nPes) generated against an occluded airway. First, we examined the effects of different end-expiratory concentrations of isoflurane on nPes. Second, we determined the effects of 1.0 minimum alveolar anesthetic concentration of isoflurane on nPes after a peripherally restricted gamma aminobutyric acid (GABA)(A)-receptor antagonist, bicuculline methiodide (BM), a GABA(B)-receptor antagonist, CGP 35348, a peripherally restricted GABA(B)-receptor antagonist, CGP 54626, or saline had been administered IV. Third, BM, CGP 35348, or artificial cerebrospinal fluid was administered intracisternally before 1.0 minimum alveolar anesthetic concentration of isoflurane exposure. During isoflurane anesthesia, nPes was inversely proportional to the end-expiratory isoflurane concentration. The rank order of nPes values obtained after IV drug pretreatment and isoflurane exposure was BM < saline < CGP54626 < CGP35348. After intracisternal drug pretreatment and isoflurane administration, the order of nPes was BM < artificial cerebrospinal fluid < CGP35348. Isoflurane modulates the hiccup-like reflex in opposite directions through both central and peripheral GABA(A) and GABA(B) receptors, with the net effect being a dose-dependent suppression. Isoflurane facilitated the hiccup-like reflex through activation of central and peripheral gamma aminobutyric acid (GABA)(A) receptors but suppressed it via activation of central and peripheral GABA(B) receptors. The net result was that the hiccup-like reflex was inhibited in proportion to the alveolar isoflurane concentration.

Affective style and in vivo immune response: neurobehavioral mechanisms.

Considerable evidence exists to support an association between psychological states and immune function. However, the mechanisms by which such states are instantiated in the brain and influence the immune system are poorly understood. The present study investigated relations among physiological measures of affective style, psychological well being, and immune function. Negative and positive affect were elicited by using an autobiographical writing task. Electroencephalography and affect-modulated eye-blink startle were used to measure trait and state negative affect. Participants were vaccinated for influenza, and antibody titers after the vaccine were assayed to provide an in vivo measure of immune function. Higher levels of right-prefrontal electroencephalographic activation and greater magnitude of the startle reflex reliably predicted poorer immune response. These data support the hypothesis that individuals characterized by a more negative affective style mount a weaker immune response and therefore may be at greater risk for illness than those with a more positive affective style.

Poster 48: Aging-related changes in Achilles’ tendon reflexes

Objective: To evaluate the aging-related changes of Achilles’ tendon reflex activities. Design: Case-control study. Setting: Institutional research center. Participants: 14 elderly subjects (8 women, 6 men; mean age, 69.4±7.1y) and 18 young subjects (9 women, 9 men; mean age, 30.0±6.6y). Intervention: Subjects were seated in a custom-designed chair with the trunk, thigh, and lower leg immobilized. The foot was fixed firmly on a footplate connected to a torque sensor. The Achilles’ tendon was tapped using a force transducer-mounted plastic hammer. Tendon-tapping force and the resulting reflex torque generated by the ankle joint were measured simultaneously at neutral ankle position in the isometric condition to minimize nonreflex viscoelastic contribution of muscle. Main Outcome Measures: Peak reflex-mediated torque, peak electromyography, and tendon reflex gain calculated from the relationship between the tapping force and joint torque were compared as parameters for reflex magnitude. Reflex loop delay and contraction time were analyzed as parameters for reflex timing. Results: The elderly subjects showed lower peak torque (2.33±1.29Nm) and tendon reflex gain (332.69±226.32cm) than the young subjects (3.75±1.92Nm, 657.62±401.37cm; P=.024, P=.009, respectively). Longer reflex loop delay was noticed in the elderly group (47.0±10.89ms) as compared with the young group (40.19±5ms) ( P=.028). The peak electromyographic activity was lower (.13±.12mV vs .17±.15mV, P=.43) and contraction time was longer (144.43±30.02ms vs 131.0±31.7ms, P=.24) in the elderly group, but neither was significant. Conclusions: Not only was the magnitude of Achilles’ tendon reflex decreasing but also the reflex loop latency was delayed along with the neuromuscular aging process. Relatively longer contraction time with lower peak reflex torque in elderly subjects may be considered as less synchronized reflex excitation with aging.

Static and dynamic changes in body orientation modulate spinal reflex excitability in humans.

In the present study, we investigated the modulation pattern of the soleus H reflex in healthy subjects in response to imposed static and dynamic changes in body angle, referenced to the vertical plane. Soleus H reflexes were recorded using conventional methods with subjects either supine or while they were erect. Changes in body angle were initiated with subjects lying supine on a tilt table. Table position was controlled via a motor and could move from the horizontal to the upright position and beyond. Elastic bands around the trunk (upper and lower part) and around the thigh and shank secured subjects' position. In the vertical position, the soleus H reflex exhibited a strong depression in all subjects tested, reaching amplitudes as low as 40+/-8.1% of the control reflex (Ho). With subjects supine, positioning the body at 10 degrees, 20 degrees, 40 degrees, 60 degrees, 90 degrees, -50 degrees and -20 degrees all resulted in a significant facilitation of the soleus H reflex. The reflex magnitude at these angles ranged from 140+/-17.2% to 180+/-10.9% of the Ho. Reflex facilitation was also observed following dynamic tilt of the body in the sagittal plane (at 1.8 degrees /s) with the H reflex reaching amplitudes as high as 300+/-18.3% of Ho. Our findings indicate that changes in body orientation induced a significant facilitation of the H reflex magnitude in soleus motoneurones that were essentially independent of angular change in body orientation or of movement direction. In addition, they highlight the potent modulatory effects that natural stimulation of the vestibular system can have on reflex excitability. The implications of our findings are discussed in relation to the maintenance of body posture.

Gradual increment/decrement of isometric force modulates soleus stretch reflex response in humans

We investigated how a gradual isometric force-increment and -decrement task modulates the behavior of a soleus stretch reflex. Six healthy subjects performed isometric plantar-flexion torque exertion tasks in which they adjusted the torque level to a target changing triangularly (3 s/cycle; the amplitude is 15% of maximal voluntary isometric contraction (MVC)) or to constant targets (3.75, 7.5, and 11.25% of MVC). The magnitude of the short latency stretch reflex evoked by dorsi-flexing mechanical perturbation was strongly modulated by the muscle contraction states; it was the largest and the smallest for the torque-increment and -decrement phases, respectively. On the other hand, within each muscle contraction, the magnitude was independent of the torque level. Similar results were observed for the medium latency stretch reflex. These results indicate that when the central nervous system increases or decreases the muscular force, it simultaneously regulates the sensitivity of the stretch reflex.

Flexor reflex excitability in mice lacking galanin receptor galanin-R1

This study was conducted to examine the excitability of the nociceptive flexor reflex and its sensitization by repetitive stimulation of C-fibers in anesthetized mice that lack the galanin-R1 receptor. Repetitive stimulation of C-fibers induced a gradual increase in reflex magnitude during the stimulation (wind-up), and a subsequent increase in spinal reflex excitability (central sensitization). This occurred in GAL-R1 −/−, GAL-R1 +/−, and +/+ wild-type controls, with no significant differences observed between genotypes. Intrathecal administration of galanin markedly blocked the sensitization following the repetitive stimulation in all three groups. No differences between wild-type or galanin-R1 receptor knockout mice were seen. These results confirm previous studies in rats, showing that intrathecal galanin reduces the central sensitization following wind-up. The present data indicate that this effect is probably mediated by receptors other than GAL-R1.

Peroneus longus stretch reflex amplitude increases after ankle brace application

Background: The use of external ankle support is widespread throughout sports medicine. However, the application of ankle bracing to a healthy ankle over a long period has been scrutinised because...

Galvanic and acoustic vestibular stimulation activate different populations of vestibular afferents.

To deduce whether similar or distinct populations of vestibular afferents are activated by acoustic and galvanic vestibular stimulation by comparing the effectiveness of 'matched' stimuli in eliciting vestibulospinal reflexes. Twelve subjects (5 men, 7 women) underwent individual 'matching' of 2 ms tone burst and galvanic stimuli, using vestibulocollic reflexes so that corrected reflex amplitudes to tone burst and galvanic stimuli were within 10% of each other. These same intensities were then administered using 20 ms durations to determine whether they were equally effective in evoking vestibulospinal responses. Corrected reflex amplitudes for vestibulocollic responses to tone burst and galvanic stimulation were not significantly different for the right (P=0.45) or left (P=0.68) sides. All subjects had vestibulospinal responses to galvanic stimulation (average intensity 4.0 mA for both sides). The short latency (SL) and medium latency (ML) components of the vestibulospinal reflexes were larger after galvanic compared to tone burst stimulation in 11 of 12 subjects (P<0.01). Despite evoking equal-sized vestibulocollic reflexes, there was a clear dissociation between the magnitude of tone burst and galvanic-induced vestibulospinal reflexes. Galvanic stimulation evoked SL and ML reflexes in all subjects. Tone burst stimuli evoked only small SL reflexes and, in most cases, no ML reflexes. Acoustically-evoked vestibulocollic reflexes are likely to be due to saccular excitation. The limited effectiveness of longer tone burst stimuli to evoke ML vestibulospinal reflexes suggests that saccular afferents have, at most, only a minor role in the production of these reflexes. We conclude that galvanic stimulation is more effective in eliciting vestibulospinal reflexes than tone burst stimulation, and that the two methods activate different populations of vestibular afferents.

Emotion among women with psychopathy during picture perception.

Emotional reactions in women with psychopathy were examined in a mixed-picture paradigm using psychophysiological measures. Startle probes were presented at 2.0 or 4.5 s following onset of a 6-s picture presentation. At 2.0 s, nonpsychopaths exhibited the typical pattern of eyeblink reflex magnitude: unpleasant > neutral > pleasant. Psychopaths with high general levels of anxiety also exhibited this pattern. Psychopaths with lower anxiety exhibited attenuated reflex magnitudes during unpleasant pictures. Similarly, when focusing on psychopathy components, only individuals expressing high antisocial behavior and high emotional detachment exhibited smaller reflex magnitudes during unpleasant pictures. At 4.5 s, all groups exhibited normal, potentiated reflex magnitudes during unpleasant pictures. Group differences were not observed for other measures.