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PubMed2006

Mastering the molecular dynamics of a bistable molecule by single atom manipulation.

At low temperature (5 K), a single biphenyl molecule adsorbed on a Si(100) surface behaves as a bistable device which can be reversibly switched by electronic excitation with the scanning tunneling microscope tip. Density functional theory suggests that the biphenyl molecule is adsorbed with one dissociated hydrogen atom bonded to a neighbor surface silicon atom. By desorbing this hydrogen atom with the STM tip, the interaction of the molecule with the surface is modified such that it becomes transformed into a multistable device with four stable states having switching yields increased by almost 2 orders of magnitude.

Journal Article↗

PubMed2000

Multiplicative effects of intention on the perception of bistable apparent motion.

When viewing ambiguous displays, observers can, via intentional efforts, affect which perceptual interpretation they perceive. Specifically, observers can increase the probability of seeing the desired percept. Little is known, however, about how intentional efforts interact with sensory inputs in exerting their effects on perception. In two experiments, the current study explored the possibility that intentional efforts might operate by multiplicatively enhancing the stimulus-based activation of the desired perceptual representation. Such a possibility is suggested by recent neurophysiological research on attention. In support of this idea, when we presented bistable apparent motion displays under stimulus conditions differentially favoring one motion percept over the other, observers' intentional efforts to see a particular motion were generally more effective under conditions in which stimulus factors favored the intended motion percept.

Adult↗

PubMed2007

Bistability with hysteresis in the activity of vasopressin cells.

Magnocellular vasopressin neurones generate distinctive 'phasic' patterns of electrical activity during which periods of spiking activity (bursts) alternate with periods of no spikes or occasional spikes. The mechanisms of burst termination in vivo are still not clearly understood. We recorded from single phasic vasopressin cells in vivo and here we show that burst terminations in some phasic cells is preceded by transient increases in activity, consistent with bursts ending as a result of activity-dependent inhibition. We show that extrinsically imposed increases in activity, evoked by brief stimulation of the organum vasculosum of the lamina terminalis, can either trigger bursts if given when a cell is silent, or stop bursts if given when a cell is active. Thus, the magnocellular vasopressin system is a population of independent bistable oscillators. The population as a whole is insensitive to transient changes in input level, whether these are excitatory or inhibitory. The vasopressin cell population thus acts like a 'low-pass filter'; although brief large changes in input rate have little overall effect, the population responds very effectively to small, sustained changes in input rate by evolving a pattern of discharge activity that efficiently maintains secretion. We note that these filtering characteristics are the opposite of the filtering characteristics that are typically associated with neurones.

Action Potentials↗

PubMed2005

Stripping Bacillus: ComK auto-stimulation is responsible for the bistable response in competence development.

In Bacillus subtilis competence for genetic transformation develops only in a subpopulation of cells in an isogenic culture. The molecular mechanisms underlying this phenotypic heterogeneity are unknown. In this study, we stepwise simplify the signal transduction cascade leading to competence, yielding a strain devoid of all regulatory inputs for this process that have been identified so far. We demonstrate that auto-stimulation of ComK, the master regulator for competence development, is essential and in itself can be sufficient to generate a bistable expression pattern. We argue that transcriptional regulation determines the threshold of ComK to initiate the auto-stimulatory response, and that the basal level of ComK (in a wild-type strain governed by MecA-mediated proteolytic control) determines the fraction of cells that reach this threshold, and thus develop competence.

Adaptation, Physiological↗

PubMed2005

A neural signature of colour and luminance correspondence in bistable apparent motion.

The 'correspondence problem' refers to the ambiguity of apparent motion (AM) paths if several similar objects are displaced across successive displays. We investigated the effect of intrinsic object properties such as colour and luminance on AM paths, and used functional magnetic resonance imaging to localize neural correlates of correspondence matching in visual cortical regions. Human subjects looked at an AM display where two dots in diagonally opposite corners of an implicit rectangle were flashed in alternation with two dots in the other two corners, yielding spontaneous alternations between horizontal and vertical AM. The dots differed in colour or luminance, or were identical. Neural activity was analysed as a function of whether the perceived AM path matched the dots' colour or luminance, and was also compared to activity during bistable AM displays without correspondence cues. When AM paths matched colour and luminance cues, activity in early visual cortex was the same as during perception of uncued displays, whereas it was suppressed when perceived AM paths violated colour or luminance cues. Colour-sensitive extrastriate cortex (V4 complex) transiently activated whenever AM perception switched from a pattern violating colour correspondence to one consistent with colour. We propose that the neural correlate of correspondence in early visual cortex reflects regulatory mechanisms that flexibly gate early visual feature processing in accord with an overriding perceptual decision. Conversely, activation of feature-selective extrastriate regions depends on the type of cue used for correspondence matching and may reflect the salience of percepts that match in colour and motion.

Adult↗

PubMed2005

Electrical coupling induces bistability of rhythms in networks of inhibitory spiking neurons.

Information processing in higher brain structures is thought to rely on the synchronization of spiking neurons. Increasing evidence indicates that, within these structures, inhibitory neurons are linked by both chemical and electrical synapses. However, how synchronized states may emerge from such circuits is not fully understood. Using snail neurons interconnected through a dynamic-clamp system, we show that networks of spiking neurons linked by both reciprocal inhibition and electrical coupling can express two coexisting coordination patterns of different rhythms. One of these patterns consists of antiphase firing of the network partners whereas, in the other, neurons fire synchronously. Switching between patterns may be evoked immediately by transient stimuli, demonstrating bistability of the network. Thus electrical coupling can provide a potent way for instantaneous reconfiguration of activity patterns in inhibitory spiking networks without alteration of intrinsic network properties by modulatory processes.

Algorithms↗

PubMed1989

Serotonin-induced bistability of turtle motoneurones caused by a nifedipine-sensitive calcium plateau potential.

1. The effect of serotonin on the firing properties of motoneurones was studied in transverse sections of the adult turtle spinal cord in vitro with intracellular recording techniques. 2. In normal medium, turtle motoneurones adapt from an initial high frequency to a low steady firing during a depolarizing current pulse. In the presence of serotonin (4-100 microM) motoneurones responded with accelerated firing and a frequency jump during a depolarizing current pulse followed by an after-depolarization outlasting the stimulus. From a depolarized holding potential motoneuronal activity was shifted between two stable states by brief depolarizing and hyperpolarizing current pulses. As an expression of this bistable firing behaviour, the frequency-current relation in response to a triangular current injection was counter-clockwise in serotonin while clockwise in normal medium. 3. The delay to onset of the frequency jump was shortened as the amplitude of the activation pulse was increased. From a positive holding potential the after-depolarization exceeded spike threshold and its duration increased with an increase in steady bias current. The effect of serotonin on turtle motoneurones could be blocked by methysergide (10 microM). 4. When action potentials were depressed by tetrodotoxin, a voltage-dependent, non-inactivating plateau potential, intrinsic to the motoneurone, was revealed. Activation of this voltage plateau provides the motoneurones with two stable states of firing. The apparent input resistance was 2-4-fold lower during the plateau than at rest. 5. The serotonin-induced plateau potential was Ca2+-dependent and was blocked when Ca2+ was replaced by either Co2+ (3 mM) or Mn2+ (3 mM). 6. The Ca2+ plateau was blocked by nifedipine (1-15 microM). 7. Serotonin reduced the slow after-hyperpolarization following action potentials. The change in balance between inward and outward currents seems to be sufficient to reveal the plateau response. 8. Although a small plateau response was induced by Bay K 8644 (1-15 microM), this L-channel agonist could not reproduce the pronounced effect of serotonin. 9. It is concluded that serotonin induces a Ca2+-dependent and nifedipine-sensitive plateau potential in turtle motoneurones primarily by reducing a K+-current responsible for the slow after-hyperpolarization.

Animals↗

PubMed2002

Magneto-opto-electronic bistability in a phenalenyl-based neutral radical.

A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic. In the paramagnetic state, the unpaired electrons are located in the exterior phenalenyl units of the dimer, whereas in the diamagnetic state the electrons migrate to the interior phenalenyl units and spin pair as a pi-dimer. Against all expectations, the conductivity increases by two orders of magnitude in the diamagnetic state, and the band gap decreases. This type of multifunctional material has the potential to be used as the basis for new types of electronic devices, where multiple physical channels are used for writing, reading, and transferring information.

Journal Article↗

PubMed1991

Bistability and the ordered bimolecular mechanism.

An equation describing the instantaneous velocity of an ordered bimolecular enzymatic reaction that exhibits inhibition by substrate and product was derived. Using kinetic constant values for horse liver alcohol dehydrogenase, the velocity expression was applied to an open-reaction system. The calculated steady-state surfaces displayed regions of bistability, which further substantiates the link between substrate inhibition and multiple steady states. This general computational approach may be applied to any system that can be described by an instantaneous velocity equation.

Alcohol Dehydrogenase↗

PubMed1994

Gelfand-Tsetlin principle of minimal afferentation and bistability of dendrites.

Neurons with bistable dendrites may fire persistently without persistent synaptic excitation while transient afferentation shifts the frequency of the persistent firing. These neurons change the firing frequency until they minimize their afferential input in a properly designed network. The afferentation minimum in this network is a stable state. The concept of the stable afferentation minimum generalizes Feldman's spring model of muscle control. An oversimplified network imitating the gamma-loop demonstrates the above statements.

Dendrites↗

PubMed1996

Bistability and its regulation by serotonin in the endogenously bursting neuron R15 in Aplysia.

1. Previous computational studies of models of neuron R15 in Aplysia have indicated that several distinct modes of electrical activity may coexist at a given set of parameters, that this multistability can be modulated by transmitters such as serotonin (5-HT) and that brief perturbations of the membrane potential can induce persistent changes in the mode of electrical activity. To test these predictions, the responses of R15 neurons to injections of brief (1.5 s) current pulses were recorded intracellularly in the absence and presence of 5-HT. 2. In the absence of 5-HT, brief perturbations induced abrupt transitions in the electrical activity from bursting to beating. Such transitions were observed in approximately 20% of the cases. The duration of beating activity varied from several seconds to tens of minutes. In the presence of low concentrations (1 microM) of 5-HT, both the probability of mode transitions and the duration of induced beating activity increased significantly. 3. These results indicate that at least two stable modes of electrical activity can coexist in R15 neurons and that this bistability can be regulated by 5-HT. In general, these conclusions agree with the results from analyses of mathematical models of R15. Although the function of these dynamic properties in R15 is speculative, our results, interpreted on the background of the model, support the notion that nonlinear dynamical properties of individual neurons can endow them with richer forms of information processing than has generally been appreciated.

Amino Acid Sequence↗

PubMed2006

Role of the prefrontal cortex in attentional control over bistable vision.

The primary source of top-down attentional control in object perception is the prefrontal cortex. This region is involved in the maintenance of goal-related information as well as in attentional selection and set shifting. Recent approaches have emphasized the role of top-down processes during elementary visual processes as exemplified in bistable vision where perception oscillates automatically between two mutually exclusive states. The prefrontal cortex might influence this process either by maintaining the dominant pattern while protecting it against the competing representation, or by facilitating perceptual switches between the two competing representations. To address this issue, we investigated reported perceptual reversals in patients with circumscribed lesions of the prefrontal cortex and healthy control participants in three experimental conditions: hold (maintaining the dominant view), speed (inducing as many perceptual switches as possible), and neutral (no intervention). Results indicated that although the patients showed normal switching rates in the neutral condition and were able to control perceptual switches in the hold condition as much as control subjects were, they were less able to facilitate reversals specifically in the speed condition. These results suggest that the prefrontal cortex is necessary to bias the selection of visual representations in accord with current goals, but is less essential for maintaining selected information active that is continuously available in the environment. As for attentional selection, the present results suggest that the prefrontal cortex initiates perceptual reversals by withdrawing top-down support from the dominant representation without (or prior to) boosting the suppressed view.

Attention↗

PubMed2005

Temporal dynamics in bistable perception.

Bistable perception is fundamentally a dynamic process: Our perceptual experience continuously alternates when an ambiguous or rivalrous stimulus is observed. Here we present a method to analyze instantaneous measures of dominance and transition between percepts. The analysis extracts three time-varying probabilities. First, the transient preference represents the probability of perceiving one interpretation at one instant. Second, the reversal probability is the probability that the current percept will change at the next evaluation. Finally, the survival probabilities are the probability that at one instant the current percept will not switch to the alternative interpretation. We derive the relationships between these probabilities and offer a test of independence between consecutive percepts. We also introduce a simple technique to sample the observer's perception at regular intervals. The analyzing method is illustrated with the example of binocular rivalry. We demonstrate Levelt's second proposition with the survival probability measure and show that the consecutive rivalrous percepts are not independent.

Dominance, Ocular↗

PubMed2000

Bistable membrane potential of the ciliate Coleps hirtus.

In normal recording solution, the swimming pattern of the freshwater ciliate Coleps hirtus, belonging to the class Prostomatea, consists of alternating periods of nearly linear forward swimming and circular swimming within a small area. Current-clamp recordings were performed to elucidate the mechanism for this behaviour. No members of this class have previously been studied using electrophysiological techniques. The ciliates were maintained in culture and fed on the planctonic alga Rhodomonas minuta. The membrane potential showed spontaneous shifts between a more negative (deep) level of approximately -50 mV and a less negative (shallow) level of approximately -30 mV. The input resistance and capacitance at the more negative level were approximately 400 M capomega and 120 pF respectively. C. hirtus displayed a pronounced inward rectification, which was virtually insensitive to 1 mmol l(-1) Cs(+) and almost completely blocked by 1 mmol l(-1) Ba(2+). Depolarising current injections failed to evoke graded, regenerative Ca(2+) spikes. However, current-induced depolarisations from the more negative potential level (-50 mV) showed a pronounced shoulder during the repolarising phase. Increased current injections prolonged the shoulder, which occasionally stabilised at the shallow membrane potential (-30 mV). The membrane potential could be shifted to the deep level by brief hyperpolarising current injections. Similar biphasic membrane properties have not been reported previously in any ciliate. The bistability of the membrane potential was abolished in Ca(2+)-free solution containing Co(2+) or Mg(2+). In Ca(2+)-free solution containing 1 mmol l(-1) Ba(2+), brief depolarising current injections at the deep potential level evoked all-or-nothing action potentials with a prolonged plateau coinciding with the shallow potential. We conclude that the deep membrane potential in C. hirtus corresponds to the traditional resting potential, whereas the shallow level is a Ca(2+)-dependent plateau potential. In normal solution, the direction of the ciliary beat was backwards at the deep potential level and forwards at the shallow membrane potential, probably reflecting the two main phases of the swimming pattern.

Animals↗

PubMed2002

Low-intensity optical bistability in an active Fabry-Perot mirror induced by input-phase-insensitive parametric downconversion.

A Fabry-Perot resonator filled with second-order nonlinear optical material is investigated. Attention is devoted to making the resonator act as a nonlinear mirror for ultralow-intensity light signals that can be switched by a control beam at the second-harmonic frequency. The interaction process is an input-phase-independent parametric downconversion. The theoretical problem is solved through implementation of a dummy variable method optimized for a parametric process. Efficient amplification and bistability of low-intensity signals have been found.

Journal Article↗

PubMed2003

Nonlinear and bistable reflection minima at lambda = 10.6 microm in an attenuated total reflection configuration with a highly doped n-GaAs film.

n-GaAs can be so highly doped that its plasma frequency is larger than the associated frequency of a CO2 laser emission line, leading to a negative real part of the dielectric permittivity. We studied linear and nonlinear reflection properties of structures composed of such thin highly doped n-GaAs film in an attenuated total reflection (ATR) configuration. We show that deep reflection minima coincide with the excitation of Brewster-type modes of the multilayer structure. These minima feature strong nonlinear and bistable behavior and are sensitive to geometrical and material parameters. The proposed ATR configuration can be used to determine doping concentrations with an accuracy of better than 1 partper thousand and different deformation potentials of the higher-conduction bands in n-GaAs.

Journal Article↗

PubMed2003

Bayesian modeling of cue interaction: bistability in stereoscopic slant perception.

Our two eyes receive different views of a visual scene, and the resulting binocular disparities enable us to reconstruct its three-dimensional layout. However, the visual environment is also rich in monocular depth cues. We examined the resulting percept when observers view a scene in which there are large conflicts between the surface slant signaled by binocular disparities and the slant signaled by monocular perspective. For a range of disparity-perspective cue conflicts, many observers experience bistability: They are able to perceive two distinct slants and to flip between the two percepts in a controlled way. We present a Bayesian model that describes the quantitative aspects of perceived slant on the basis of the likelihoods of both perspective and disparity slant information combined with prior assumptions about the shape and orientation of objects in the scene. Our Bayesian approach can be regarded as an overarching framework that allows researchers to study all cue integration aspects-including perceptual decisions--in a unified manner.

Bayes Theorem↗

PubMed2003

Optical bistability in axially modulated OmniGuide fibers.

We demonstrate the feasibility of optical bistability in an axially modulated nonlinear OmniGuide fiber through analytical theory and detailed numerical experiments. At 1.55-microm carrier wavelength, the in-fiber devices that we propose can operate with only a few tens of milliwatts of power, can have a nearly instantaneous response and recovery time, and can be shorter than 100 microm.

Journal Article↗