Fast and slow components of cerebral blood flow response to step decreases in end-tidal PCO2 in humans

This study examined the dynamics of the middle cerebral artery (MCA) blood flow response to hypocapnia in humans (n = 6) by using transcranial Doppler ultrasound. In a control protocol, end-tidal PCO2 (PETCO2) was held near eucapnia (1.5 Torr above resting) for 40 min. In a hypocapnic protocol, PETCO2 was held near eucapnia for 10 min, then at 15 Torr below eucapnia for 20 min, and then near eucapnia for 10 min. During both protocols, subjects hyperventilated throughout and PETCO2 and end-tidal PO2 were controlled by using the dynamic end-tidal forcing technique. Beat-by-beat values were calculated for the intensity-weighted mean velocity (VIWM), signal power (P), and their instantaneous product (P.VIWM). A simple model consisting of a delay, gain terms, time constants (tauf,on, tauf, off) and baseline levels of flow for the on- and off-transients, and a gain term (gs) and time constant (taus) for a second slower component was fitted to the hypocapnic protocol. The cerebral blood flow response to hypocapnia was characterized by a significant (P < 0.001) slow progressive adaptation in P.VIWM, with gs = 1.26 %/Torr and taus = 427 s, that persisted throughout the hypocapnic period. Finally, the responses at the onset and relief of hypocapnia were asymmetric (P < 0.001), with tauf,on (6.8 s) faster than tauf,off (14.3 s).     

IL-10-mediated suppression of TNF-alpha production is independent of its ability to inhibit NF kappa B activity

It is hypothesized that carotid body chemosensory activity is coupled to neurosecretion. The purpose of this study was to examine whether there was a correspondence between carotid body tissue dopamine (DA) levels and neuronal discharge (ND) measured from the carotid sinus nerve of perfused cat carotid bodies and to characterize interaction between CO2 and O2 in these responses. ND and tissue DA were measured after changing from normoxic, normocapnic control bicarbonate buffer (PO2 >120 Torr, PCO2 25-30 Torr, pH approximately 7.4) to normoxic hypercapnia (PCO2 55-57 Torr, pH 7.1-7.2) or to hypoxic solutions (PO2 30-35 Torr) with normocapnia (PCO2 25-30 Torr, pH approximately 7.4) or hypocapnia (PCO2 10-15 Torr, pH 7.6-7.8). Similar temporal changes for ND and tissue DA were found for all of the stimuli, although there was a much different proportional relationship for normoxic hypercapnia. Both ND and DA increased above baseline values during flow interruption and normocapnic hypoxia, and both decreased below baseline values during hypoxic hypocapnia. In contrast, normoxic hypercapnia caused an initial increase in ND, from a baseline of 175 +/- 12 (SE) to a peak of 593 +/- 20 impulses/s within 4.6 +/- 0.9 s, followed by adaptation, whereas ND declined to 423 +/- 20 impulses/s after 1 min. Tissue DA initially increased from a baseline of 17.9 +/- 1.2 microM to a peak of 23.2 +/- 1.2 microM within 3.0 +/- 0.7 s, then declined to 2.6 +/- 1.0 microM. The substantial decrease in tissue DA during normoxic hypercapnia was not consistent with the parallel changes in DA with ND that were observed for hypoxic stimuli.     

Dyspnea sensation and chemical control of breathing]

In order to estimate the role of peripheral chemosensitivity in dyspnea sensation, we performed BH experiment under the acute or chronic hypoxic condition. The former was simulated by a given rate (0-3.2 mg/kg/hr) of doxapram infusion. The latter experiment was carried out during sojourn in Lhasa (3700 m), China. Subjects conducted BH by inhaling 7% CO2 in O2 and assessed dyspnea sensation by visual analog scale (VAS) while repeatedly measuring PCO2 at breaking point (BP). Lowering of resting PETCO2 by augmented ventilation was derived by doxapram infusion and during acclimatization at high altitude. The effect of PCO2 on VAS was enhanced by doxapram. However, altitude acclimatization resulted in attenuated effect of PCO2 on VAS despite of further development of hypocapnia. The rate of PCO2 elevation during doxapram infusion was reduced and it might be attributed to decreased body storage of CO2. On the other hand, its rate was tended to recover to sea level value after acclimatization at high altitude and it may have cancelled the mitigated dyspnea sensation. Thus, BHT almost comparable period in both acute hypoxia and during altitude acclimatization. These results suggest that CO2 storage in the body contributes to modify dyspnea sensation as well as augmented peripheral chemosensitivity.     

3D visualization library for multimodal medical images

To clarify the control mechanism of ventilation during posture change, ventilatory parameters, PETCO2, and ventilatory response to CO2 were examined in 11 healthy male subjects at supine (0 degrees) and 75 degrees head-up tilt positions. Minute expiratory ventilation (V.E), tidal volume (VT), respiratory frequency (f), end-tidal and transcutaneous PCO2 and CO2 output (V.CO2), and ventilatory response to CO2 were measured during a steady state condition. V.E (V.A) and VT increased significantly at 75 degrees tilt with significant decrease in PETCO2 from 40.1 mmHg (0 degrees) to about 36.1 mmHg (75 degrees). Transcutaneous PCO2 also decreased during tilt, by 3.3 mmHg. Physiological dead space (VD/VT) and V.CO2, however, remained unchanged, and ventilatory equivalent (V.E/V.CO2, V.A/V.CO2) increased significantly. The CO2-ventilatory response curve shifted upward (or leftward) without significant change in the response slope. At 75 degrees tilt, EMG activity of gastro-cnemius muscle increased. These findings suggested that PETCO2 decreased because of increased V.E (V.A) with a leftward shift of CO2-ventilatory response curve. Various signals such as afferents from lower extremities might have net stimulatory effects on a CO2-ventilation control system to reset the controlled level of PETCO2 to a lower range, but without significant change in CO2-ventilatory response during upright position.     

Neural respiratory responses to cortically induced seizures in cats

Seizure activity can lead to profound respiratory stimulation in spontaneously breathing animals with intact respiratory feedback mechanisms (Paydarfar et al., Am. J. Physiol. 260, R934, 1991). The present study was designed to test the hypothesis that peripheral respiratory feedback mechanisms are not important for the genesis of seizure-induced hyperpnea. Studies were performed in 16 anesthetized, vagotomized, glomectomized cats whose end-tidal PCO2 (PETCO2) was kept constant. Integrated phrenic nerve activity was used to represent respiration. Seizures were induced by injection of penicillin into the parietal cortex and electrocorticographic (ECoG) and biceps femoris nerve activities, arterial pressure, airway PCO2 and brain temperature were recorded continuously. Progressive seizure activity was associated with progressive increases of respiratory frequency and peak phrenic activity, despite constancy of PETCO2 and brain temperature. Patterns of entrainment were identified among ECoG spikes, biceps femoris nerve and phrenic nerve activities. Phrenic nerve activity became highly irregular during generalized ictal seizures and ceased to respond to changes of PETCO2. Acute intercollicular decerebration in all experiments resulted in normalization of respiratory rhythm even while ictal ECoG activity continued. We conclude that stimulation of breathing during seizures occurs in the absence of respiratory feedback mechanisms. The findings suggest that an important cause of the respiratory response is a feedforward mechanism, whereby activation of subcortical structures above medulla and pons results in stimulation of breathing.     

The mismatch negativity to frequency deviant stimuli during natural sleep

Eight subjects spent a single night in the sleep laboratory. Event-related potentials (ERPs) were recorded during the presentation of two auditory "oddball' stimulus conditions in which tonal frequency was manipulated. In the first condition, 1000 Hz "standard' and 2000 Hz "deviant' tones were presented. In the second condition, the deviant tone was reduced to 1050 Hz. In both conditions, deviant probability was 0.2. Stimuli were presented every 600 ms during wakefulness and stages 2, 4, and REM of sleep. A distinctive N1 wave was visible in both stimulus conditions when the subject was awake. The deviant stimuli elicited a "mismatch negativity' (MMN) that inverted in polarity at the mastoid. In REM sleep, an N1 and a MMN were also elicited in both conditions. In the large deviance condition, the MMN had a slightly attenuated amplitude and was shorter in duration while in the small deviant condition, its peak latency was unusually early. Neither the N1 nor the MMN could be recorded in non-REM sleep.     

Diagnostic possibilities for the nuchal evoked potential

In each of ten healthy young subjects breathing different concentrations of CO2 in O2, four alveolar CO2-tension levels were obtained, ranging from about 20 mmHg when hyperventilating in O2 to 50 mmHg. Maximum expiratory flows at 60% total lung capacity were measured at each level and corrected for the influence of the expired gas on the flow. The corrected maximum flow decreased significantly when the alveolar CO2 tension was below 30-35 mmHg, while there was only slight or no influence of CO2 on the maximal flow when the tension was above 35 mmHg. The decrease is taken as evidence of a constrictor effect on peripheral bronchi of hypocapnia.     

What is a migraine, anyway, and when is it gone?

PURPOSE: To assess the effect of pneumoperitoneum on P(a-ET)CO2 gradient in children. METHODS: Sixty one ASA I and II children (10.7 +/- 3.0 yr, 38.4 +/- 14.2 kg, mean +/- SD), scheduled for visceral or urological laparoscopic procedures, were studied. They were anaesthetized, intubated, paralysed and their lungs ventilated with constant ventilator settings to obtain PETCO2 values between 4.3 and 4.8 kPa. Intra-abdominal pressure was maintained between 8 and 14 mmHg. The following measurements were performed at steady state, before the pneumoperitoneum (T1) and 15 min later (T2): heart rate, systolic and diastolic arterial pressure; peak airway and intra-abdominal pressure; PaCO2 corrected for the patient's temperature; PETCO2 drawn between the micropore filter and the ventilator tubes, corrected for BTPS conditions; P(a-ET)CO2. Values between -1.0 and +1.0 mmHg were considered nil; patient position (horizontal or head-down tilt): all patients were horizontal at T1. RESULTS: Arterial pressure, heart rate and peak airway pressure increased at T2: PaCO2 and PETCO2 increased by 14%. The incidence of negative gradients increased from 54 to 67% although mean P(a-ET)CO2 remained clinically unchanged. No difference was found in P(a-ET)CO2 gradient, whatever the position and intra-abdominal pressure. The 95% confidence intervals for P(a-ET)CO2 were [-5.6; +3.2] at T1 and [-8.8; +4.8] at T2. CONCLUSION: PETCO2 often overestimates PaCO2 during laparoscopy in children, by up to 8.8 mmHg. Arterial blood gas analysis should be performed during long procedures to avoid hyperventilation.     

Responses of cutaneous A-fiber nociceptors to noxious cold

Responses of cutaneous nociceptors to natural stimuli, particularly mechanical and heat stimuli, have been well documented. Although nociceptors are excited by noxious cold stimuli, there have been few studies of their stimulus-response functions for cold stimuli over a wide range of stimulus temperatures. Furthermore, the proportion of nociceptors excited by noxious cold is not clear. In the present study, we examined responses of mechanosensitive A delta-nociceptors and low-threshold mechanoreceptors to a wide range of cold stimuli that included stimulus temperatures <0 degrees C. Electrophysiological recordings were made from single primary afferent fibers in the saphenous nerves of anesthetized rats. Cutaneous sensory receptors were classed according to their conduction velocity and subgrouped functionally according to their responses evoked by mechanical, heat, and cold stimuli (0 degrees C). Responses evoked by a wide range of cold stimulus intensities that included stimuli considered innocuous and noxious (painful) were then assessed. Stimuli of 20 to -20 degrees C were delivered to the receptive field via a 1-cm2 contact thermode from a base temperature of 32 degrees C. Stimuli were applied in descending order of 2 degrees C decrements. Stimulus ramp rate was 5 degrees C/s, and stimulus temperatures were applied for a duration of 10 s. A total of 90 A fibers was studied, of which 61 were nociceptors and had conduction velocity in the A delta-range (2-30 m/s). Nociceptors were classed initially as mechanical, mechanoheat, and mechanocold nociceptors. The remaining 29 fibers were low-threshold mechanoreceptors with conduction velocity in the A delta- or A beta-range (>30 m/s). These were subgrouped according to their adaptive properties as slowly or rapidly adapting, and according to whether they were excited by hair movement (hair follicle afferent fibers). All nociceptors were excited by noxious cold. Only 30% of nociceptors were considered sensitive to cold on initial classification with the use of a cold stimulus of 0 degrees C. However, all nociceptors were excited by stimulus intensities <0 degreesC. Response thresholds for cold ranged from 14 to -18 degrees C (-4.6 +/- 1.07 degrees C, mean +/- SE). The total number of impulses, discharge rate, and peak discharge increased monotonically as intensity of cold stimuli increased. Power functions were used to determine the rate at which the number of impulses increased as stimulus intensity increased. The slopes of power funcions ranged from 0.12 to 2.28 (mean 1.07 +/- 0.13). Most mechanoreceptors were not excited by cold stimuli. The only types of mechanoreceptors that responded reliably to cold stimuli were the slowly adapting mechanoreceptors. Responses usually occurred during the temperature ramp when the skin temperature was decreasing. There was no evidence that mechanoreceptors encoded the intensity of cold stimuli at intensities above or below 0 degrees C, because evoked responses did not increase with intensity of cold stimuli. It is concluded that the proportion of cutaneous A delta-nociceptors excited by noxious cold stimuli has been underestimated in previous studies. All nociceptors were excited by stimulus temperatures <0 degrees C and encoded the intensity of cold stimuli. It is therefore likely that cutaneous A delta-nociceptors contribute to the sensation of cold pain, particularly pain produced by stimulus temperatures <0 degrees C.     

Carotid body chemoreception: the importance of CO2-HCO3- and carbonic anhydrase. (review)

The current hypotheses of carotid body (CB) chemoreception regard the glomus cells as the initial site of stimulus transduction. The consensus is that the transduction of chemical stimulus is coupled with the release of transmitter(s) from the glomus cells, which in turn generates action potentials in the afferent nerve terminals. Carbonic anhydrase (CA) is present in the glomus cells of the CB. Inhibition of CA activity in the CB in situ reduces the carotid chemosensory responses to CO2 and to O2, suggesting a common mechanism of chemosensing for both stimuli. However, CA inhibitors also block the red blood cell enzyme. Thus, the CO2 hydration reaction does not come to completion within the transit time of the blood from the lung to the CB. A steady-state reaction is not reached until later and so the PCO2 and pH levels in arterial blood samples are not the same as those sensed by the CB. Experiments in vitro using cat CB perfused and superfused with cell-free solutions, which had been pre-equilibrated with respiratory gases, strongly support the proposition that the CA activity in CB cells is essential for the speed and amplitude of the initial response to CO2 and for its subsequent adaptation. The immediate response to hypoxia also is delayed, but the late steady-state was less dependent on CA activity. In the nominal absence of CO2-HCO3- from the perfusate, hypoxic chemoreception persisted and its magnitude is not affected by CA inhibition, except for a delay which may be due to the initial alkaline pH of the glomus cells. Recent experiments performed in isolated glomus cells and in the whole CB show that hypoxia does not modify significantly the intracellular pH. By its simple catalytic function, CA can speed up the approach of the CO2 hydration reaction to equilibrium. However, CA may also contribute in the steady-state to the regulation of pHi by providing a continuous supply of H+ and HCO3-. Furthermore, CA performs a facilitatory role in the physiological chemosensory responses to CO2 and O2 in the presence of extracellular CO2-HCO3-. This role is likely to be related to the ion exchanger function and then to pHi regulation in the chemoreceptor cells.     

Somatosensory evoked potentials during hypoxia and hypocapnia in conscious humans

PURPOSE: The objective of the study was to evaluate the effects of moderate hypoxia and hypocapnia on the latency and amplitude of cortical somatosensory evoked potentials (SSEPs) in conscious human subjects. METHODS: In ten volunteers the amplitude and latency of the cortical somatosensory evoked potentials were recorded during stimulation of the left posterior tibial nerve. Measurements of SSEPs and respiratory variables were made breathing ambient air, air containing a reduced oxygen percentage (17% O2, 14% O2 (n = 6) or 11% O2 (n = 10)), and again during voluntary hyperventilation breathing ambient air (PETCO2 = 20 mmHg, n = 10). RESULTS: Hypoxia (11% O2) caused mild stimulation of ventilation (P < 0.05) but had no effects on the latency or amplitude of the SSEP. Lesser degrees of hypoxia had no effects. Hyperventilation caused a small (2-4%) decrease) in the latency of the SSEP and an increase in the amplitude of the SSEP (P < 0.05). CONCLUSIONS: These findings in conscious subjects were consistent with previous observations in anaesthetized humans and anaesthetized dogs and show that the decrease in latency of the SSEP associated with hypocapnia is not due to changes in the depth of anaesthesia. These effects of hypocapnia may contribute to small variations in the latency of the SSEP when monitoring is performed during surgery, but are unlikely to be large enough to be of clinical concern.     

Haptic localization and the internal representation of the hand in space

As the hand actively explores the environment, contact with an object leads to neuronal activity in the topographic maps of somatosensory cortex. However, the brain must combine this somatotopically encoded tactile information with an internal representation of the hand's location in space if it is to determine the position of the object in three-dimensional space (3-D haptic localization). To investigate the fidelity of this internal representation in human subjects, a small tactual stimulator, light enough to be worn on the subject's hand, was used to present a brief mechanical pulse (6-ms duration) to the right index finger before, during, or after a fast, visually evoked movement of the right hand. In experiment 1, subjects responded by pointing to the perceived location of the mechanical stimulus in 3-D space. Stimuli presented shortly before or during the visually evoked movement were systematically mislocalized, with the reported location of the stimulus approximately equal to the location occupied by the hand 90 ms after stimulus onset. This pattern of errors indicates a representation of the movement that fails to account for the change in the hand's location during somatosensory delays and, in some subjects, inaccurately depicts the velocity of the actual movement. In experiment 2, subjects were instructed to verbally indicate the perceived temporal relationship of the stimulus and the visually evoked movement (i.e., by reporting whether the stimulus was presented "before," "during," or "after" the movement). On average, stimuli presented in the 38-ms period before movement onset were more likely to be perceived as having occurred during rather than before the movement. Similarly, stimuli in the 145-ms period before movement termination were more likely to be perceived as having occurred after rather than during the movement. The analogous findings of experiments 1 and 2 indicate that the same inaccurate representation of dynamic hand position is used to both localize tactual stimuli in 3-D space and construct the perception of arm movement.     

Differences between estimates and measured PaCO2 during rest and exercise in older subjects

Arterial PCO2 (PaCO2) has been estimated during exercise with good accuracy in younger individuals by using the Jones equation (PJCO2) (J. Appl. Physiol. 47: 954-960, 1979). The purpose of this project was to determine the utility of estimating PaCO2 from end-tidal PCO2 (PETCO2) or PJCO2 at rest, ventilatory threshold (VTh), and maximal exercise (Max) in older subjects. PETCO2 was determined from respired gases simultaneously (MGA 1100) with arterial blood gases (radial arterial catheter) in 12 older and 11 younger subjects at rest and during exercise. Mean differences were analyzed with paired t-tests, and relationships between the estimated PaCO2 values and the actual values of PaCO2 were determined with correlation coefficients. In the older subjects, PETCO2 was not significantly different from PaCO2 at rest (-1.2 +/- 4.3 Torr), VTh (0.4 +/- 2.5), or Max (-0.8 +/- 2.7), and the two were significantly (P < 0.05) correlated at Vth (r = 0.84) and Max (r = 0.87) but not at rest (r = 0.47). PJCO2 was similar to PaCO2 at rest (-1.0 +/- 3.9) and Vth (-1. 3 +/- 2.3) but significantly lower at Max (-3.0 +/- 2.6), and the two were significantly correlated at Vth (r = 0.86) and Max (r = 0. 80) but not at rest (r = 0.54). PETCO2 was significantly higher than PaCO2 during exercise in the younger subjects but similar to PaCO2 at rest. PJCO2 was similar to PaCO2 at rest and Vth but significantly lower at Max in younger subjects. In conclusion, our data demonstrate that PaCO2 during exercise is better estimated by PETCO2 than by PJCO2 in older subjects, contrary to what is observed in younger subjects. This appears to be related to the finding that PETCO2 does not exceed PaCO2 during exercise in older subjects, as occurs in the younger subjects. However, PaCO2 at rest is best estimated by PJCO2 in both younger and older subjects.     

Effect of different levels of hyperoxia on breathing in healthy subjects

We have recently shown that breathing 50% O2 markedly stimulates ventilation in healthy subjects if end-tidal PCO2 (PETCO2) is maintained. The aim of this study was to investigate a possible dose-dependent stimulation of ventilation by O2 and to examine possible mechanisms of hyperoxic hyperventilation. In eight normal subjects ventilation was measured while they were breathing 30 and 75% O2 for 30 min, with PETCO2 being held constant. Acute hypercapnic ventilatory responses were also tested in these subjects. The 75% O2 experiment was repeated without controlling PETCO2 in 14 subjects, and in 6 subjects arterial blood gases were taken at baseline and at the end of the hyperoxia period. Minute ventilation (VI) increased by 21 and 115% with 30 and 75% isocapnic hyperoxia, respectively. The 75% O2 without any control on PETCO2 led to 16% increase in VI, but PETCO2 decreased by 3.6 Torr (9%). There was a linear correlation (r = 0.83) between the hypercapnic and the hyperoxic ventilatory response. In conclusion, isocapnic hyperoxia stimulates ventilation in a dose-dependent way, with VI more than doubling after 30 min of 75% O2. If isocapnia is not maintained, hyperventilation is attenuated by a decrease in arterial PCO2. There is a correlation between hyperoxic and hypercapnic ventilatory responses. On the basis of data from the literature, we concluded that the Haldane effect seems to be the major cause of hyperventilation during both isocapnic and poikilocapnic hyperoxia.     

Averaging of temporal memories by rats.

Rats were trained on a mixed fixed-interval schedule in which stimulus A (tone or light) indicated food availability after 10 s and stimulus B (the other stimulus) indicated food availability after 20 s. Testing consisted of nonreinforced probe trials in which the stimulus was A, B, or the compound AB. On single-stimulus trials, rats responded with a peak of activity around the programmed reinforced time. On compound-stimulus trials, rats showed a single scalar peak of responding at a time midway between those for stimulus A and B. These results suggest that when provided with discrepant information regarding the temporal predictability of reinforcement, rats compute an average of the scheduled reinforcement times for the A and B stimuli and use this average to generate an expectation of reward for the compound stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of familiar and complex stimuli on infant attention.

A total of 48 infants, divided into age groups of 2, 31/2, and 5 mo, were given habituation training during 2 experimental sessions taking place on consecutive days. Stimuli varying in familiarity and complexity were presented for 17 trials in each session. The S's attention to the stimulus was measured through the use of corneal reflections. 31/2- and 5-mo-old Ss showed habituation of looking to a familiarized stimulus within test sessions and between test sessions, demonstrating both short- and long-term memory effects. All 3 groups preferred to fixate complex stimuli over simple stimuli. (16 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Evaluation of a dynamical model of speech perception

Previous work (Tuller, Case, Ding, & Kelso, 1994) has revealed signature properties of nonlinear dynamical systems in how people categorize speech sounds. The data were modeled by using a two-well potential function that deformed with stimulus properties and was sensitive to context. Here we evaluate one prediction of the model--namely, that the rate of change of the potential's slope should increase when the category is repeatedly perceived. Judged goodness of category membership was used as an index of the slope of the potential. Stimuli from a "say"-"stay" continuum were presented with gap duration changing sequentially throughout the range from 0 to 76 to 0 msec, or from 76 to 0 to 76 msec. Subjects identified each token as either "say" or "stay" and rated how good an exemplar it was of the identified category. As predicted, the same physical stimulus presented at the end of a sequence was judged a better exemplar of the category than was the identical stimulus presented at the beginning of the sequence. In contrast, stimuli presented twice near the middle of a sequence with few (or no) stimuli between them, as well as stimuli presented with an intervening random set, showed no such differences. These results confirm the hypothesis of a context-sensitive dynamical representation underlying speech.     

The reduced responsiveness of neurones in nucleus reticularis gigantocellularis following their excitation by peripheral nerve stimulation

1. Post-stimulus histograms of neuronal activity, constructed from extracellular recordings in decerebrate, decerebellate cats, have been used to investigate the responsiveness of neurons in nucleus reticularis gigantocellularis following their excitation by a peripheral nerve stimulus. 2. The response to a testing stimulus applied to a peripheral nerve was depressed following the response to a conditioning stimulus applied to the same or a different peripheral nerve. This reduction in responsiveness was maximal within 50 msec of the peak of the response to the conditioning stimulus. Response latencies to the testing stimulus were increased during the period of reduced responsiveness. 3. Responsiveness to a peripheral nerve stimulus was also reduced following a spontaneous or an antidromically evoked spike, but this effect was weaker and much shorter-lasting than that following a nerve-evoked spike. Thus, the reduced responsiveness cannot be solely due to phenomena which are an inevitable consequence of an action potential in the neurone. 4. In spontaneously firing neurones, the duration of the reduced responsiveness to a testing stimulus generally outlasted the depression of spontaneous activity which often followed an excitation evoked by a peripheral nerve conditioning stimulus. 5. The reduction in responsiveness to a testing stimulus applied to the same nerve as the conditioning stimulus was greater and longer-lasting than that to a testing stimulus applied to a different nerve. 6. When stimuli were applied to one nerve at a relatively high rate, the neurone became much less responsive to that input, but simultaneously became more responsive to low rate stimulation of other nerves. 7. It is concluded that the greater part of the reduced responsiveness is due to events occurring on the input pathway to a reticular neurone, or possibly in the region of the afferent endings on its dendrites. These processes may allow selective changes in responsiveness to different inputs, and enable the units to act as novelty detectors.     

Directed attention prolongs the perceived duration of a brief stimulus

Stelmach, Herdman, and McNeil (1994) suggested recently that the perceived duration for attended stimuli is shorter than that for unattended ones. In contrast, the attenuation hypothesis (Thomas & Weaver, 1975) suggests the reverse relation between directed attention and perceived duration. We conducted six experiments to test the validity of the two contradictory hypotheses. In all the experiments, attention was directed to one of two possible stimulus sources. Experiments 1 and 2 employed stimulus durations from 70 to 270 msec. A stimulus appeared in either the visual or the auditory modality. Stimuli in the attended modality were rated as longer than stimuli in the unattended modality. Experiment 3 replicated this finding using a different psychophysical procedure. Experiments 4-6 showed that the finding applies not only to stimuli from different sensory modalities but also to stimuli appearing at different locations within the visual field. The results of all six experiments support the assumption that directed attention prolongs the perceived duration of a stimulus.