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PubMed2002

Excitability changes in human corticospinal projections to forearm muscles during voluntary movement of ipsilateral foot.

Excitability of the H-reflex in the relaxed flexor carpi radialis (FCR) muscle was tested during voluntary oscillations of the ipsilateral foot at five evenly spaced delays during a 600 ms cycle. In some experiments the H-reflex was conditioned by transcranial magnetic stimulation (TMS). With the hand prone, the amplitude of the FCR H-reflex was modulated sinusoidally with the same period as the foot oscillation, the modulation peak occurring in coincidence with contraction of the foot plantar-flexor soleus and the trough during contraction of the extensor tibialis anterior. When the H-reflex was facilitated by TMS at short latency (conditioning-test interval: -2 to -3.5 ms), the modulation was larger than that occurring with an unconditioned reflex of comparable size. This suggests that both the peripheral and the corticospinal components of the facilitated response were modulated in parallel. When the H-reflex was tested 40-60 ms after conditioning, i.e. during the cortical "silent period" induced by TMS, no direct effect was produced on the reflex size but the foot-associated modulation was deeply depressed. These results suggest that the reflex modulation may depend on activity fluctuations in the cortical motor area innervating the forearm motoneurones. It is proposed that when the foot is rhythmically oscillated, along with the full activation of the foot cortical area a simultaneous lesser co-activation of the forearm area produces a subliminal cyclic modulation of cervical motoneurones excitability. Should the two limbs be moved together, the time course of this modulation would favour isodirectional movements of the prone hand and foot, indeed the preferential coupling observed when hand and foot are voluntarily oscillated.

Action Potentials↗
PubMed1980

Response patterns of human lumbo-sacral motoneurone pools to distant somatosensory stimuli.

The response of human lumbo-sacral motoneurones to electrical stimuli applied to the trunk, the face and the arms was evaluated by means of H reflex testing in extensors and averaging the rectified tonic EMG recorded from extensors as well as flexors. In extensor and flexor muscles, the reflex pattern consisted of a non-reciprocal inhibition-facilitation, occurring with latencies of about 60 and 80 msec, respectively. With reflex testing, only facilitation was observed in extensors, beginning after a conditioning-test interval of about 40 msec and peaking at about 80 msec. This discrepancy was interpreted to indicate excitation supraliminal in a facilitatory and subliminal in an inhibitory pathway, the latter requiring an additional input to impinge on motoneurones. Afferents responsible for the described reflex actions were identified as belonging mainly to low-threshold skin nerve fibres, conducting with a maximum velocity of about 45--50 m/sec. However, medium threshold skin afferents as well as muscle and joint afferents may also play a role. Stimulation of C5 and T10 skin areas close to the neuraxis revealed similar effects, the latencies being shorter or equal with T10 compared with C5 stimulation. The possibility of a directly descending propriospinal pathway is discussed, mediating the inhibitory as well as the excitatory reflex actions described.

Adolescent↗
PubMed1991

Poly(A)-poly(U) induces circulating colony-stimulating activity resulting from interactions between endogeneous interleukin 6 and serum components.

The immunostimulant poly(A)-poly(U) induces a rapid enhancement of circulating colony-stimulating activity (CSA) in normal mice, culminating 2 h after i.v. injection. A dose of 200 micrograms per mouse is sufficient for a maximal effect. The colonies formed in response to sera from poly(A)-poly(U)-injected mice are mainly granulocytic with few macrophages. These sera are devoid of detectable interleukin 3 (IL-3) or granulocyte-macrophage colony-stimulating factor (GM-CSF), but contain large amounts of interleukin 6 (IL-6) that are perfectly correlated with circulating CSA levels. Although, in our hands, IL-6 alone induces no colony formation in the standard methylcellulose colony assay, it is nevertheless requisite for this biological activity because 1) monoclonal antibodies against IL-6 strongly diminish colony formation in response to sera from poly(A)-poly(U)-injected mice, and 2) recombinant (r)IL-6 induces colonies when tested in combination with low amounts of normal murine serum. At the concentrations used (0.3%-2.5%), the latter has no or a very slight effect alone. Low amounts of hematopoietic growth factors, that is, macrophage colony-stimulating factor (M-CSF), granulocyte colony-stimulating factor (G-CSF), GM-CSF, or IL-3 that are almost ineffective in the absence of IL-6 can replace normal serum. Taken together, these data suggest that circulating IL-6, induced by i.v. injection of poly(A)-poly(U), promotes colony formation by interacting with serum components that might be identical with hematopoietic growth factors present in normal serum at subliminal concentrations. Finally, the involvement of lipopolysaccharide (LPS) in this phenomenon has been ruled out by the use of the low responder strain of mice (C3H/HeJ) that leads to similar results.

Animals↗
PubMed1986

In vitro ovarian responses to pulsatile and continuous gonadotrophin administration on steroid secretion and oocyte maturation in the frogs, Rana pipiens and Rana catesbeiana.

An in vitro superfusion system was used to study the relative effects of pulsatile and continuous gonadotrophin administration on steroid secretion and oocyte maturation in Rana pipiens ovaries. Pulsatile (10 min pulse/hr) delivery of pituitary extract (PE) resulted in a slightly (insignificantly) lower level of testosterone (T) secretion over a 12-hr period. Administration of a fivefold lower, subliminal amount of PE instead of hormone-free media between pulses did not change the pattern of T secretion. When the interpulse frequency was increased to 2 or 3 hr, there were notable oscillations in T secretion which corresponded to the peaks in luteinizing hormone associated with the PE. In four experiments ovarian fragments underwent oocyte maturation, but this occurred only in fragments that received continuous PE stimulation. Progesterone (P) secretion was measurable only when oocyte maturation was observed in the ovarian fragment. Rana catesbeiana ovarian fragments exposed to continuous superfusion with homologous PE produced more T than those receiving hourly pulses of PE over a 6-hr period. Oocyte maturation accompanied by P secretion was observed in one experiment under continuous, but not pulsatile administration of PE. These results suggest that the frog ovary may be more resistant than the mammalian ovary to "down-regulation" under continuous gonadotrophic stimulation. The implications of these results on the frog pituitary--gonadal axis are discussed.

Animals↗
PubMed2002

Mechanisms of intracortical I-wave facilitation elicited with paired-pulse magnetic stimulation in humans.

In order to elucidate the mechanisms underlying intracortical I-wave facilitation elicited by paired-pulse magnetic stimulation, we compared intracortical facilitation of I1-waves with that of I3-waves using single motor unit and surface electromyographic (EMG) recordings from the first dorsal interosseous muscle (FDI). We used a suprathreshold first stimulus (S1) and a subthreshold second stimulus (S2). In most experiments, both stimuli induced currents in the same direction. In others, S1 induced posteriorly directed currents and S2 induced anteriorly directed currents. When both stimuli induced anteriorly directed currents (I1-wave effects), an interstimulus interval (ISI) of 1.5 ms resulted in extra facilitation of the responses to S1 alone. The latency of this effect was equivalent to that of the I2-wave from S1. When S1 evoked posteriorly directed currents (I3-wave recruitment), facilitation occurred at a latency corresponding to the I3-wave from S1. This facilitation occurred at an ISI of 1.5 ms when both S1 and S2 flowed posteriorly, and at an ISI of approximately 3.5 ms when S1 was posteriorly and S2 was anteriorly directed. Based on these findings, we propose the following mechanisms for intracortical I-wave facilitation. When S1 and S2 induce currents in the same direction, facilitation is produced by summation between excitatory postsynaptic potentials (EPSPs) elicited by S1 and subliminal depolarization of interneurones elicited by S2 directly. When S1 and S2 induce currents in the opposite direction, facilitation is produced by the same mechanism as above or by temporal and spatial summation of EPSPs elicited by two successive stimuli at interneurones or corticospinal neurones of the motor cortex.

Adult↗
PubMed2005

The neurophysiology of response competition: motor cortex activation and inhibition following subliminal response priming.

Some widely used tasks in cognitive neuroscience depend on the induction of a response conflict between choice alternatives, involving partial activation of the incorrect response before the correct response is emitted. Although such ''conflict tasks'' are often used to investigate frontal-lobe-based conflict-monitoring processes, it is not known how response competition evolves in the motor cortex. To investigate the dynamics of motor cortex activation during response competition, we used a subliminal priming task that induced response competition while bypassing pre-response stage processing conflict. Analyses of movement-related EEG potentials supported an interaction between competing responses characterized by reciprocal inhibition. Inhibitory interactions between response channels contribute to the resolution of response conflict. However, the reciprocal inhibition at motor cortex level seemed to operate independent of higher level conflict-monitoring processes, which were relatively insensitive to response conflict induced by subliminal priming. These results elucidate how response conflict causes interference as well as the conditions under which frontal-lobe-based interference control processes are engaged.

Adult↗
PubMed1972

Factors forming the edge of a receptive field: the presence of relatively ineffective afferent terminals.

A specialized type of spinal cord cell has its cell body in lamina IV and has a small low threshold cutaneous receptive field which is remarkable for its abrupt edge. No signs could be found of a subliminal fringe to this field since its size remains fixed during wide excursions of the cell's excitability. Reversible blocking of peripheral nerves and dorsal roots showed that the afferents responsible for exciting these cells following natural stimuli, run in a restricted area of peripheral nerve and dorsal root. When the fibres necessary to sustain the natural stimulus receptive field were blocked, it was shown that other large myelinated fibres in neighbouring roots were still capable of firing the cell monosynaptically following electrical stimulation of the root or periphery although no natural stimuli were able to change the cell's excitability. It is necessary to divide the afferent synapses on such cells into a class which is highly effective in firing the cell on natural stimulation and a second class which has no effect yet detected following natural stimuli but which can fire the cell monosynaptically if synchronously activated by electrical stimulation. Suggestions are made for possible presynaptic and post-synaptic mechanisms which might divide the effect of arriving impulses into two such classes.

Animals↗
PubMed1970

Differences in recruitment order of motor units in phasic and tonic flexion reflex in "spinal man".

The recruitment order of motoneurones in muscle contractions has been held to be largely constant and determined by the size of the cell. However, as shown in a previous investigation using electromyographic techniques, the order in which different motor units are activated during voluntary muscle contractions changes in normal human subjects on shifts from phasic to tonic contraction. In order to investigate these two types of activity also in cases in which the cerebral influence on the motoneurone pool is blocked, an analysis was made of the recruitment order in phasic and tonic flexion reflexes in 10 patients with total interruption of the spinal cord. The following four principles were found to apply and presumed to be generally valid for the isolated human spinal cord: (1) in the phasic exteroceptive reflex, the order of recruitment varies despite application of a standardized stimulus; (2) in the tonic reflex, the first unit to be recruited is usually the same even with widely different types of stimuli; (3) a shift from phasic to tonic reflex activation may result in considerable changes in recruitment order; (4) after facilitation by a subliminal long-lasting stimulus, the first unit to be recruited in the phasic reflex is also the first to be recruited in the tonic reflex. It is suggested that a tonic influence on the motoneurone pool is required for the presupposed constancy of the recruitment order.

Action Potentials↗
PubMed1990

Modification of the visual response properties of cerebellar neurons by norepinephrine.

Extracellular recordings were conducted in the paraflocculus of anesthetized Long-Evans pigmented rats to determine how ionotophoresis of norepinephrine (NE) affects the responsiveness of individual Purkinje cells and interneurons to presentations of visual stimuli within their visual receptive fields. Presentations of moving or stationary visual stimuli during the control (pre-NE) period elicited simple spike excitations or inhibitory responses in slightly more than one-half (55%, n = 32) of the cells tested (20 of 38 Purkinje cells, 12 of 20 interneurons). The predominant effect of NE iontophoresis was to improve visually evoked responses in those neurons which showed modulations in their simple spike discharge to control presentations of visual stimuli. A clear enhancement of visual responses by NE (i.e., absolute increase over control) was observed in 18 of the units, and in 12 of the 14 remaining cells, reductions in stimulus-bound discharge during catecholamine iontophoresis were accompanied by much larger depressions in background activity, resulting in a net enhancement in the ratio of signal-to-noise. NE differentially affected responses to stimulus movement in the preferred and non-preferred direction in one-third of these neurons, such that directional selectivity was increased. However, the orientation bias of individual units was unchanged by NE. Iontophoretic application of the beta-adrenergic antagonist sotalol but not the alpha-adrenergic antagonist phentolamine blocked these facilitating noradrenergic effects. An additional feature of noradrenergic action was revealed in tests conducted in 26 cells which did not respond to control presentations of visual stimuli. Iontophoresis of NE resulted in the elicitation of visual responses in 11 of these units, suggesting the possibility that NE might act in some cases to gate the efficacy of subliminal synaptic input conveyed by classical afferent channels. It is proposed that an important aspect of noradrenergic action within local cerebellar circuits might be to refine the receptive field properties of individual neuronal elements and thereby improve information flow through the cerebellum.

Action Potentials↗
PubMed1976

Different excitability of type 1 and type 2 alpha-motoneurons. The recruitment curve of H- and M-responses in slow and fast muscles of rabbits.

To evaluate the different excitability of Type 1 and Type 2 alpha-motoneurones, we analyzed comparatively the threshold and amplitude of the H-reflex and the Hmax/Mmax ratio in a slow muscle, soleus, and in a fast muscle, lateral gastrocnemius, of the rabbit. The H-reflex had almost always the same threshold in the muscles examined, but in soleus its amplitude increased much more than in lateral gastrocnemius when the stimulus intensity was increased. A clear difference in amplitude of the H-reflexes was already evident with stimuli subliminal for direct responses (M). The maximal H-reflex was always much higher in soleus than in lateral gastrocnemius. The calculation of the mean Hmax/Mmax ratio showed that it was 3 times higher in the slow than in the fast muscle. On the basis of these results, showing a clear-cut difference in the reflex excitability of slow and fast muscles and in the light of other experimental data available in the literature, we tend to conclude that Type 1 alpha-motoneurones are exclusively, or at least predominatly, depolarized by stimulation of afferent Ia fibres.

Animals↗
PubMed2004

Human sensorimotor tracking of continuous subliminal deviations from isochrony.

We show that people continuously react to time perturbations in the range 3-96 ms in otherwise isochronous sound sequences. Musically trained and untrained participants were asked to synchronize with a sequence of sounds, and these two groups performed almost equally below the threshold for conscious detection of the perturbations. Above this threshold the motor reactions accounted for a larger proportion of the stimulus deviations in musically trained participants.

Acoustic Stimulation↗
PubMed1992

Interindividual variability of central delay changes in the soleus H reflex pathway.

Central delay (CD) changes in the soleus H reflex pathway, as demonstrated by variations in the time interval between afferent (P1) and efferent (P2) neurographic volleys underlying the reflex response, were assessed in a group of normal subjects, both during the steady state and after homosynaptic spatial summation of afferent impulses. The maximal range of CD changes, regardless of whether "spontaneous" or provoked, showed significant interindividual differences whose size was positively related to the Hmax/Mmax ratio, provided that the extension of the subliminal fringe was suitably normalized. Comparatively similar variations in amplitude of the reflex motoneuronal discharge under different experimental conditions can be associated with different CD changes. Indeed, "spontaneous" CD fluctuations occurring during the steady state were consistently greater than CD reductions provoked by spatial summation, the gap size being negatively related to the Hmax/Mmax ratio.

Adolescent↗
PubMed1979

Effects of H-reflex conditioning upon the contralateral alpha motoneuron pool.

H-reflex recovery curves have been elicited by subliminal conditioning stimuli applied to the ipsilateral and contralateral posterior tibial nerves in 10 healthy female subjects. In both types of recovery curve there was clear evidence of a period of facilitation in the ipsilateral soleus motoneuron pool 75-250 msec after the conditioning stimulus. These results indicate the bilateral nature of the facilitation and show it to be most probably produced by stimulus-evoked inputs as opposed to twitch-evoked inputs. If the facilitation is produced by descending long-loop reflex influences, then complementary evidence is provided for previous electromyographic data showing the bilaterality of long-loop reflexes evoked by percutaneous electrical stimulation. It is impossible, however, at the present time, to rule out the possible involvement of cutaneous afferent discharges or other stimulus evoked inputs in the late facilitation.

Adult↗
PubMed1995

Adenosine antagonists have differential effects on induction of long-term potentiation in hippocampal slices.

How adenosine leakage and tetanic release might affect long-term potentiation (LTP) was investigated by applying adenosine antagonists 8(p-sulfophenyl)theophylline (8SPT) or 8-cyclopentyl-3,7-dihydro-1,3-dipropyl-1H-purine-2,6-dione (DPCPX) to slices, while recording CA1 field EPSPs and population spikes. In the first series of experiments, we applied weak double tetani (at 100 Hz, for 1 s) that were subliminal for evoking LTP in initial control runs. In the presence of 8SPT--at concentrations (10-50 microM) which block both A1 and A2 receptors--the same tetani consistently evoked LTP of population spikes but not of excitatory postsynaptic potentials (EPSPs), whereas DPCPX (50 nM), which blocks only A1 receptors, facilitated LTP of both EPSPs and population spikes. These results are consistent with previous evidence that tetanic adenosine release on the one hand depresses LTP via A1 receptors but on the other facilitates LTP via A2 receptors. In a second set of experiments, 8SPT (50-100 microM) did not prevent the induction of LTP of both EPSPs and population spikes by stronger tetanic stimulation. Therefore A2 receptor activation is not essential for the induction of LTP when stronger tetani are applied. Overall, the main effect of endogenous adenosine release is to oppose LTP induction.

Adenosine↗
PubMed1964

BINOCULAR DEPTH PERCEPTION WITHOUT FAMILIARITY CUES.

The reported phenomena were obtained through the use of special techniques. (i) All monocular depth and familiarity cues were removed from the stimuli (through the use of randomdot stereo patterns). (ii) The statistical and topological properties of the stimuli were precisely known (since they were generated according to a specific computer program). (iii) Convergence motions of the eye and proprioceptive cues were eliminated (through the use of tachistoscopic illumination). (iv) The time of presentation was under control (through erasure of the persistent afterimages). Under these conditions stereopsis could be studied in its purest form. It was shown that depth can be perceived in the absence of monocular depth and familiarity cues and of all binocular depth cues except for disparity. These findings have important implications for some existing theories of stereopsis and open up areas for further research. Some phenomena based on stereo erasure are reported here for the first time. It has been demonstrated that the perception of ambiguous depth organizations can be influenced, even subliminally, by a preceding unambiguous stimulus. Perhaps the most interesting result is the finding that the correspondence of objects and patterns in the two retinal projections can be established without actual recognition of the objects and patterns. This pattern matching is based on some relatively simple processes of finding connected clusters formed by adjacent points of similar brightness, and the processes seem to be amenable to rigorous analysis.

Biomedical Research↗
PubMed2002

De Vries-Weber gain control and dark adaptation in human vision.

Thresholds for seeing light from a stimulus are determined by a mechanism that pairs subliminal excitations from both halves of a twin unit. Such excitations stem from a package of k > or = 1 receptor responses. A half-unit contains one red or one green cone and P rods. The receptor's "Weber machine" controls the receptor's gain. Each half of a twin unit contains a "de Vries machine," which controls the half's k number. In the dark the receptor's dark noise events reset its Weber machine and the receptor's relation to its de Vries machine. A pairing product for light perception also represents a direction event. The local time signs of the two subliminal excitations are crucial for the polarity, size, and pace of the direction event. In relation to the time when and the area in which the stimulus is presented, these signs have average latency periods that depend on intensity and average locations that depend on movement. Polarity depends on which of the two subliminal excitations happens to arrive first at the twin's pairing facility. The intra- and inter-twin pairings in a persepton for the perceptions of light, edge and movement and the probability summation of the pairing products of the mutually independent three sets of twins of the retrinet improve intensity discrimination. Cross-pairings of intra-receptor pairings in red and green cones of a trion for yellow improve visual discrimination further. Discrimination of stimuli that exploit the model's entire summation mechanisms and pairing facilities represents "what the perfect human eye sees best." For the model this threshold of modulation in quantum absorption is the ideal limit that is prescribed by statistical physics. The lateral and meta interaction in a twin unit enhance the contrast of an edge and of a temporal transient. The precision of the local time sign of a half's stimulation determines the spatiotemporal hyperfunctions for location and speed. The model's design for the perfect retinal mosaic consists of red twins situated along clockwise and counterclockwise spirals and green twins along circles that are concentric with the fovea. The model's descriptions of discrimination, adaptation, and hyperfunctions agree with experimental data.

Dark Adaptation↗
PubMed2006

Amygdala-prefrontal dissociation of subliminal and supraliminal fear.

Facial expressions of fear are universally recognized signals of potential threat. Humans may have evolved specialized neural systems for responding to fear in the absence of conscious stimulus detection. We used functional neuroimaging to establish whether the amygdala and the medial prefrontal regions to which it projects are engaged by subliminal fearful faces and whether responses to subliminal fear are distinguished from those to supraliminal fear. We also examined the time course of amygdala-medial prefrontal responses to supraliminal and subliminal fear. Stimuli were fearful and neutral baseline faces, presented under subliminal (16.7 ms and masked) or supraliminal (500 ms) conditions. Skin conductance responses (SCRs) were recorded simultaneously as an objective index of fear perception. SPM2 was used to undertake search region-of-interest (ROI) analyses for the amygdala and medial prefrontal (including anterior cingulate) cortex, and complementary whole-brain analyses. Time series data were extracted from ROIs to examine activity across early versus late phases of the experiment. SCRs and amygdala activity were enhanced in response to both subliminal and supraliminal fear perception. Time series analysis showed a trend toward greater right amygdala responses to subliminal fear, but left-sided responses to supraliminal fear. Cortically, subliminal fear was distinguished by right ventral anterior cingulate activity and supraliminal fear by dorsal anterior cingulate and medial prefrontal activity. Although subcortical amygdala activity was relatively persistent for subliminal fear, supraliminal fear showed more sustained cortical activity. The findings suggest that preverbal processing of fear may occur via a direct rostral-ventral amygdala pathway without the need for conscious surveillance, whereas elaboration of consciously attended signals of fear may rely on higher-order processing within a dorsal cortico-amygdala pathway.

Adult↗