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At least 145 records · Page 8Linked to original sources
PubMed2001

Assessment of the risk of fall, related to visual stimulation, in patients with central vestibular disorders.

In order to assess the influence of visual stimulation in the triggering of imbalance and falls in the elderly population, the postural responses of 18 elderly patients with central vestibular disorders and clinical evidence of instability and falls were studied while receiving different types of visual stimuli. The stimulation conditions were: (i) no specific stimuli; (ii) smooth pursuit with pure sinusoids of 0.2 Hz as foveal stimulation; and (iii) optokinetic stimulation (OK) as retinal stimuli. Using a platform AMTI Accusway platform, the 95% confidence ellipse (CE) and sway velocity (SV) were evaluated with a scalogram using wavelets in order to assess the relationship between time and frequency in postural control. Velocity histograms were also constructed in order to observe the distribution of velocity values during the recording. A non-homogeneous postural behavior after visual stimulation was found among this population. In five of the patients the OK stimulation generated: (i) significantly higher average values of CE ( > 3.4+/-0.69 cm2); (ii) a significant increase in the average values of the SV ( > 3.89+/-1.15 cm/s) and a velocity histogram with a homogeneous distribution between 0 and 18 cm/s; and (iii) a scalogram with sway frequencies of up to 4 Hz distributed in both the X and Y directions (backwards and forwards and lateral) during visual stimulation with arbitrary units of energy density > 5. These three qualitative and quantitative aspects could be "markers" of visual dependence in the triggering of the mechanism of lack of equilibrium and hence falls in some elderly patients and should be considered in order to prevent falls and also to assist in the rehabilitation program of these patients.

Accidental Falls↗
PubMed2003

Grammar processing outside the focus of attention: an MEG study.

To address the cerebral processing of grammar, we used whole-head high-density magnetoencephalography to record the brain's magnetic fields elicited by grammatically correct and incorrect auditory stimuli in the absence of directed attention to the stimulation. The stimuli were minimal short phrases of the Finnish language differing only in one single phoneme (word-final inflectional affix), which rendered them as either grammatical or ungrammatical. Acoustic and lexical differences were controlled for by using an orthogonal design in which the phoneme's effect on grammaticality was inverted. We found that occasional syntactically incorrect stimuli elicited larger mismatch negativity (MMN) responses than correct phrases. The MMN was earlier proposed as an index of preattentive automatic speech processing. Therefore, its modulation by grammaticality under nonattend conditions suggests that early syntax processing in the human brain may take place outside the focus of attention. Source analysis (single-dipole models and minimum-norm current estimates) indicated grammaticality dependent differential activation of the left superior temporal cortex suggesting that this brain structure may play an important role in such automatic grammar processing.

Acoustic Stimulation↗
PubMed2002

Differential fatigue of paralyzed thenar muscles by stimuli of different intensities.

Muscles paralyzed by injury or disease fatigue excessively when stimulated. This study examined whether the first few paralyzed thenar motor units recruited by electrical stimulation of the median nerve were more fatigue resistant than the total thenar motor unit population. The paralyzed thenar muscles of four subjects with chronic cervical spinal cord injury were fatigued by a 2-min intermittent 40-HZ protocol on 2 days. One experiment involved submaximal stimulation, the other supramaximal stimulation. These stimuli resulted in activation of part and all of the thenar muscles, respectively. Relative force loss, force-time integral decline, and slowing of half-relaxation time were always significantly less when only part rather than all of the muscles was fatigued. The part of the paralyzed muscles that was activated was also relatively fatigue resistant compared with control single thenar motor units. Thus, a reversal of recruitment order from fatigable to fatigue-resistant units cannot explain the extreme fatigability of paralyzed muscles. Use of submaximal stimulation during functional electrical stimulation may therefore help to reduce muscle fatigue because it recruits the more fatigue-resistant units.

Adult↗
PubMed1995

Overdrive suppression of spontaneously beating chick heart cell aggregates: experiment and theory.

In spontaneously beating chick heart cell aggregates, sustained periodic stimulation at a rate faster than the intrinsic frequency is generally followed by a transient slowing of the automatic rhythm called "overdrive suppression." We characterize the qualitative aspects of overdrive suppression using three sets of experimental protocols: 1) stimulation at a fixed frequency with various numbers of stimuli, 2) stimulation at different frequencies, 3) stimulation with different intensities. We develop a mathematical model based on a system of nonlinear ordinary differential equations to account for the experimental observations. The main idea of the model is that overdrive suppression arises as a result of a hyperpolarizing current that is induced by action potentials. This work shows that the frequency of action potentials is the major determinant of overdrive suppression. Consequently, during periodic pacing of spontaneous oscillators at different rates, the fastest frequency where 1:1 entrainment can be maintained is associated with maximal overdrive suppression. This type of model is complementary to the development of a rigorous ionic model and can help provide insight into the physiological mechanisms of overdrive suppression.

Animals↗
PubMed2006

Medullary raphe neurons facilitate brown adipose tissue activation.

Recent evidence suggests that neurons in the medullary raphe are critical to the activation of brown adipose tissue (BAT), the major source of nonshivering heat production in the rat. Yet it is unclear which medullary raphe cells participate in cold defense and how participating cells contribute to BAT activation. Therefore, we recorded extracellularly from raphe cells during three thermoregulatory challenges that evoked an increase in BAT temperature in anesthetized rats: central cold, ambient cold, or intracerebroventricular prostaglandin E2 (PGE2) injection. Physiologically identified serotonergic (p5HT) cell discharge increased in response to cold or PGE2 administration and was positively correlated with BAT temperature. However, none of the 147 physiologically identified non-serotonergic (non-p5HT) cells recorded responded to thermoregulatory challenges that evoked an increase in BAT temperature. To test for modulation of BAT activation by non-p5HT cells that are either excited (ON cells) or inhibited (OFF cells) by noxious cutaneous stimulation, noxious stimuli were applied during evoked BAT temperature increases. Noxious stimulation suppressed BAT activation, suggesting that cells inhibited by noxious stimulation facilitate spinal circuits controlling BAT. To test whether medullary OFF cells modulate BAT activity, the mu-opiate receptor agonist (d-Ala2, N-Me-Phe4, Gly-ol5)-enkephalin (DAMGO) was microinjected into the raphe magnus, a manipulation that selectively activates OFF cells. DAMGO microinjection blocked noxious stimulation-evoked suppression of PGE2-induced BAT temperature increases. Thus, both p5HT and non-p5HT OFF cells in the medullary raphe facilitate BAT activation in response to cold challenge or pyrogen.

Adipose Tissue, Brown↗
PubMed2006

A coherence-based technique for separating phase-locked from non-phase-locked power spectrum estimates during intermittent stimulation.

In addition to evoked responses, which are phase-locked to the stimuli, the stimulation may also change the ongoing EEG in a time-locked manner. This change has been investigated in event-related synchronization/desynchronization (ERS/ERD) studies by comparing the spectra before and during stimulation or alternatively by using the intertrial variance method (IVM). In the present work, a technique based on the coherence estimate (kappa(y)(2)(f)) between the stimulation signal and the EEG is proposed for separating the ongoing EEG activity spectrum from that of the evoked responses. Furthermore, a statistical criterion is applied to reduce spurious spectral peaks. The performance of this procedure was assessed through simulation and illustrated with EEG during photic stimulation. For simulated data (signal-to-noise-ratio of 0.995 within 10-12.5 Hz) kappa(y)(2)(f) led to a non-phase-locked spectrum estimate with an average normalized error of 12.4%, which is reduced to only 0.2% after applying the statistical criterion. The methodology proposed is asymptotically equivalent to the IVM but it does not require previous filtering the EEG data. Kappa(y)(2)(f) together with the statistical correction criterion allows investigating the entrainment within a narrow-band range, particularly in frequencies close to that of the alpha peak. Hence it is useful in ERS/ERD studies. Moreover, it can be also used for characterizing frequencies within the gamma band.

Adult↗
PubMed1987

Anterior pretectal stimulation alters the responses of spinal dorsal horn neurones to cutaneous stimulation in the rat.

1. The behavioural effects of stimulating sites in the anterior pretectal nucleus (a.p.t.n.) were studied in unanaesthetized rats; 1-2 weeks later these rats were anaesthetized with Fluothane and the effects of similar electrical stimulation determined on the responses of spinal neurones to cutaneous stimuli. 2. Stimulation of the a.p.t.n. for 15 s with 35 microA r.m.s. sine-wave current inhibited the tail-flick response to noxious heat of unanaesthetized animals for up to 1 h. 3. Stimulation of the same sites in anaesthetized rats inhibited the responses to noxious heat of forty-two multireceptive and two high-threshold neurones located deep in the spinal dorsal horn. 4. The high-threshold responses of seven cells were unaffected or slightly potentiated by pretectal stimulation. These seven cells were all recorded from the dorsal margin of the dorsal horn, were not multireceptive neurones and could be made to discharge only by water above 50 degrees C. 5. The responses of twelve multireceptive cells to low-threshold stimulation were not affected by pretectal stimulation. All these cells were recorded from deep within the dorsal horn. 6. On ten occasions, cells deep in the dorsal horn were identified as projection neurones which were driven antidromically by high-frequency (300 Hz) stimulation of the contralateral anterolateral tract at cervical levels. The high-threshold responses of all these cells were reduced by pretectal stimulation. No cells were driven antidromically by pretectal stimulation. 7. Ipsilateral lesions of the dorsolateral funiculus abolished the inhibitory effects of prectectal stimulation. Lesions of the dorsal columns were without effect. 8. It is concluded that stimulation of the a.p.t.n. inhibits the tail-flick reflex of unanaesthetized rats and inhibits the high-threshold discharge of deep dorsal horn cells to cutaneous stimuli in anaesthetized rats. Cells recorded from the dorsal margin of the dorsal horn are not affected. The inhibition is mediated via the dorsolateral funiculus and affects cells which project rostrally in the contralateral anterolateral funiculus.

Action Potentials↗
PubMed1972

The importance of timing on the respiratory effects of intermittent carotid sinus nerve stimulation.

1. The respiratory response, measured directly as tidal volume or indirectly by using integrated peak phrenic activity, to intermittent electrical stimulation of the carotid sinus nerve was determined in anaesthetized cats.2. Stimulation at rates of 20-25 Hz for 0.5 sec had a rapid effect, increasing inspiratory airflow and phrenic discharge, but only if applied during inspiration. An increase in tidal volume or peak level of integrated phrenic discharge occurred only if the stimulus was exhibited during the second half of inspiration. Continuous stimulation had no greater effect on size or frequency of breathing than did intermittent inspiratory stimuli alone. Stimulation during expiration had no effect on the form or magnitude of subsequent breaths.3. Stimuli in expiration led to a prolongation of expiration. Stimuli in late inspiration caused a prolongation of both inspiration and expiration. Because of these effects, the respiratory rate could be changed by stimulation; in some instances entrainment of respiration by the intermittent carotid sinus nerve stimuli occurred.4. The findings are attributable to modulation of incoming carotid sinus nerve information by the central respiratory neurones, which use primarily that which arrives during inspiration. They show a possible mechanism by which oscillating signals may have a different effect than their mean level would indicate.

Action Potentials↗
PubMed1979

Responses of single units in the monkey superior colliculus to moving stimuli.

1. Single unit responses of pan-directional cells to moving and stationary flashing stimuli were studied in the superficial layers of the superior colliculus in paralysed, anaesthetized rhesus monkeys. The aim of this study was to see how far cell responses to moving stimuli fit in with what would be expected from their responses to stationary flashing stimuli. 2. Both the leading and the trailing edge of a moving stimulus evoke a transient response. If the diameter of moving light spots is increased the strength of the leading edge response increases, reaches a maximum and decreases to a constant value which is similar to the behaviour of the on response when the diameter of flashing spots is increased. The strength of the trailing edge response increases and reaches the same strength as that of the leading edge response. If the width of a long moving slit is increased, the strength of the leading edge response is the same at all slit widths, while the strength of the trailing edge response shows a course similar to that of the trailing edge response if the spot diameter is increased. If the length of a wide moving slit is increased both the leading and the trailing edge responses decrease. These results indicate that the strength of both leading and trailing edge responses is dependent on the degree the inhibitory surround is activated. 3. The leading and the trailing edge of a stimulus evoke their responses at the same position in the receptive field independent of the direction of movement. 4. Increasing the velocity of a moving stimulus shows that in general the leading edge response is present up to higher velocities than the trailing edge response independent of the sign of contrast. The burst duration to moving stimuli decreases with increasing stimulus velocity and appears to be determined by the time a moving edge is present in the receptive field centre. When this time becomes shorter than 10--20 ms, the burst duration for moving stimuli is constant and about the same as for flashing stimuli. This indicates that, although spatial receptive field properties can vary considerably, temporal receptive field properties show a strong similarity among different units. 5. The response latencies to light and dark moving edges are the same, which in turn are about equal to the response latencies to stationary flashing stimuli. 6. Stimulation experiments show that the general response characteristics to moving stimuli can be predicted by using a set of receptive field parameters derived from responses to stationary flashing stimuli. The most important variable of moving stimuli appears to be the period of time a moving contour is present within the receptive field centre, besides the degree of activation of the inhibitory surround.

Animals↗
PubMed1976

Digital stimulator control system for introducing premature stimuli.

A control system to convert a general-purpose stimulator into one which will generate premature stimuli at variable but controlled intervals is described. The control system was developed to sense spontaneous cardiac depolarizations and inject premature stimuli at preset elapsed times after every 10th or every spontaneous depolarization. The delay may be set by an analog circuit or by digital thumb-wheel switches. The output of the stimulator also may be coupled to the input, permitting the use of the system as a standard asynchronous stimulator with digital control of the pulse interval. A block diagram is presented to permit duplication of the system using standard digital and analog integrated circuits.

Animals↗
PubMed1989

Disruption of the connections between the mediodorsal and sulcal prefrontal cortices alters the associability of rewarding medial cortical stimulation to place and taste stimuli in rats.

This study involved 2 tests of conditioned reward with self-stimulation (SS) of the prefrontal cortex. In Experiment 1, rats were tested for a conditioned taste preference (CTP) induced by pairing a novel flavor with SS of the medial prefrontal cortex (MC). Normal rats displayed a CTP. Rats with bilateral cuts of the connections between the MC and sulcal prefrontal cortex (SC) did not show a CTP. In Experiment 2, similar cuts had no effect on the ability of SC SS to promote a CTP, showing that the cuts spare the ability to learn a CTP. In Experiment 3, rats were tested for a conditioned place preference by pairing MC SS with environmental cues. Lesioned rats, but not intact rats, had a CPP. Results suggest the presence of prepotent relations, dependent on intrinsic prefrontal connections, between the rewarding effects of prefrontal stimulation and distinct sensorimotor domains.

Animals↗
PubMed2006

Interactions between luminance and colour channels in visual search and their relationship to parallel neural channels in vision.

We have compared visual search under conditions that tend to isolate the magnocellular, parvocellular and koniocellular channels of the human visual system. We used isoluminant red-green stimuli that do not modulate short-wavelength sensitive (SWS) cones to isolate the parvocellular pathway, isoluminant SWS-cone isolating stimuli to stimulate only the koniocellular system and addition of small luminance contrasts to selectively activate the magnocellular pathway. We found that in the case of conjunction search, where attentional resources were required, the red-green (parvocellular) system can use accompanying small luminance (magnocellular) signals to improve visual search. On the other hand, when using SWS-cone isolating stimuli to selectively stimulate the blue-yellow (koniocellular) system, addition of similar luminance signals did not increase the efficiency of the serial visual search. The results indicate that S-cone signals may be processed in a separate pathway that does not get converging inputs from the magnocellular pathway. This is unlike the case with the red-green opponent system, which functions more synergistically with the magnocellular system.

Adult↗