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PubMed1998

Individual differences in brain dynamics: important implications for the calculation of event-related band power.

Measures of event-related band power such as event-related desynchronization (ERD) are conventionally analyzed within fixed frequency bands, although it is known that EEG frequency varies as a function of a variety of factors. The question of how to determine these frequency bands for ERD analyses is discussed and a new method is proposed. The rationale of this new method is to adjust the frequency bands to the individual alpha frequency (IAF) for each subject and to determine the bandwidth for the alpha and theta bands as a percentage of IAF. As an example, if IAF equals 12 Hz, the widths of the alpha and theta bands are larger as compared to a subject with an IAF of, e.g., only 8 Hz. The results of an oddball paradigm show that the proposed method is superior to methods that are based on fixed frequencies and fixed bandwidths.

Adult↗
PubMed1998

Role of temperature in quanta mechanisms of facilitation in the frog neuromuscular junction.

The results of computer simulations on the Double Barrier Synapse (DBS) model are presented which quantify the relationship between the synapse parameters and the quanta transfer process. The DBS model is applicable to a variety of states of synaptic activity, and by changing the synapse parameters it is possible to simulate various conditions of quanta transmission. The influence of the bathing solution temperature change on the synaptic parameters under different conditions of transmitter release in the frog neuromuscular junction is investigated. Simulations demonstrate that several synaptic parameters, including the parameters of the presynaptic membrane, are not affected by the temperature change. It is shown that a stimulation frequency exists at which the steady-state level of facilitation during a long train of stimuli is the same for a wide range of temperatures.

Animals↗
PubMed1998

A single pacemaker cell model based on the phase response curve.

A single pacemaker cell model and its response to repetitive external depolarization stimulations is described in this paper. This model is a simple model based on the two most important functional properties of the cardiac pacemaker cells. The first property is the intrinsic pacemaker cycle length, which is an 'internal' parameter of the cell, describing the most important feature of a pacemaker cell. The second functional property is the phase response curve (PRC), which is an 'overall collective' function: it contains all the 'information' about the possible interactions of the pacemaker cell with the outside world (external stimulus, interaction with surrounding cells, etc.). This study demonstrates that by representing the pacemaker cell only by two fundamental features, and by applying a simple physical-mathematical model, a global picture of the system can be achieved, allowing us to explore qualitatively various physiological phenomena related to the pacemaker function. For example, we demonstrated that the PRC is a crucial parameter in the prediction of the entrainment phenomena of a single pacemaker cell in response to a periodic train of depolarization pulses. Specifically, the PRC permits a quantitative determination of the 1:1 synchronization range for a single pacemaker cell and an external depolarization pulse. Moreover, we show that the PRC can be used to represent the type of external stimulus applied to the pacemaker (e.g. depolarization pulse) and its intensity. Therefore, the PRC emerges as an important determinant and a useful 'tool' for the understanding of the dynamic interaction of pacemaker cells with the outside world. As a result of our simulations, we unveil a new important parameter: the 'degree of influence', which determines the range of 1:1 synchronization between an external depolarization pulse and a pacemaker cell. This interaction parameter is a direct function of the PRC parameters. It appears to be a helpful 'tool' for the understanding of synchronization and mutual entrainment mechanisms between the pacemaker cell and an external stimulus, and therefore it supports the basic importance of the PRC in the description and determination of these mechanisms.

Animals↗
PubMed1998

A pacemaker cell pair model based on the phase response curve.

A pacemaker cell pair model and the dynamic interaction between the two pacemaker cells is described in this paper. It is an extension of our single pacemaker cell model, in which we studied its response to repetitive external depolarization stimulations. This model is a simple model based on the two most important functional properties of the cardiac pacemaker cells: its intrinsic pacemaker cycle length, which is an 'internal' parameter of the cell, and the phase response curve (PRC), which is an 'overall collective' function. The PRC contains all the 'information' about the possible interactions of the pacemaker cell with the outside world (interaction with surrounding cells, external stimulus, etc.). First, we examined the properties and solutions of 1:1 synchronization between two pacemaker cells. We found that in order to achieve synchronization between two pacemaker cells, there should be limitations on the PRC parameters, which depend on the cells intrinsic cycle lengths. Next, we investigated the 2:1 entrainment state between two interacting pacemaker cells. We found that there is not necessarily a unique solution for this state as there was for the 1:1 state. Finally, we ran our computer model to investigate the properties of more complex patterns of entrainment between two pacemaker cells. As a result of our analytical study, we unveil two new important parameters, which are fully defined as a function of the PRC parameters: (1) the 'accelerator factor' which describes the tendency of a pair of interacting pacemaker cells to synchronize at a common cycle length, which is closer to the faster cycle of the pair; (2) the 'degree of coupling', which describes the range of the 1:1 synchronization and the 'strength' of the interaction between a pair of interacting pacemaker cells. Those two interaction parameters arise as helpful 'tools' for the understanding of synchronization and mutual entrainment mechanisms between pacemaker cells. Therefore, this study establishes the PRC as an important determinant and a useful approach for the understanding of the dynamic interaction of pacemaker cells among themselves and with the outside world.

Animals↗
PubMed1998

A neural network solution to the transverse patterning problem depends on repetition of the input code.

Using computer simulations, this paper investigates how input codes affect a minimal computational model of the hippocampal region CA3. Because encoding context seems to be a function of the hippocampus, we have studied problems that require learning context for their solution. Here we study a hippocampally dependent, configural learning problem called transverse patterning. Previously, we showed that the network does not produce long local context codings when the sequential input patterns are orthogonal, and it fails to solve many context-dependent problems in such situations. Here we show that this need not be the case if we assume that the input changes more slowly than a processing interval. Stuttering, i.e., repeating inputs, allows the network to create long local context firings even for orthogonal inputs. With these long local context firings, the network is able to solve the transverse patterning problem. Without stuttering, transverse patterning is not learned. Because stuttering is so useful, we investigate the relationship between the stuttering repetition length and relative context length in a simple, idealized sequence prediction problem. The relative context length, defined as the average length of the local context codes divided by the stuttering length, interacts with activity levels and has an optimal stuttering repetition length. Moreover, the increase in average context length can reach this maximum without loss of relative capacity. Finally, we note that stuttering is an example of maintained or introduced redundancy that can improve neural computations.

Computer Simulation↗
PubMed1998

Neural network simulations of the primate oculomotor system. III. An one-dimensional, one-directional model of the superior colliculus.

This report evaluates the performance of a biologically motivated neural network model of the primate superior colliculus (SC). Consistent with known anatomy and physiology, its major features include excitatory connections between its output elements, nigral gating mechanisms, and an eye displacement feedback of reticular origin to recalculate the metrics of saccades to memorized targets in retinotopic coordinates. Despite the fact that it makes no use of eye position or eye velocity information, the model can account for the accuracy of saccades in double step stimulation experiments. Further, the model accounts for the effects of focal SC lesions. Finally, it accounts for the properties of saccades evoked in response to the electrical stimulation of the SC. These include the approximate size constancy of evoked saccades despite increases of stimulus intensity, the fact that the size of evoked saccades depends on the time that has elapsed from a previous saccade, the fact that staircases of saccades are evoked in response to prolonged stimuli, and the fact that the size of saccades evoked in response to the simultaneous stimulation of two SC sites is the average of the saccades that are evoked when the two sites are separately stimulated.

Animals↗
PubMed1998

Geometry-induced features of current transfer in neuronal dendrites with tonically activated conductances.

The impact of dendritic geometry on somatopetal transfer of the current generated by steady uniform activation of excitatory synaptic conductance distributed over passive, or active (Hodgkin-Huxley type), dendrites was studied in simulated neurons. Such tonic activation was delivered to the uniform dendrite and to the dendrites with symmetric or asymmetric branching with various ratios of branch diameters. Transfer effectiveness of the dendrites with distributed sources was estimated by the core current increment directly related to the total membrane current per unit path length. The effectiveness decreased with increasing path distance from the soma along uniform branches. The primary reason for this was the asymmetry of somatopetal vs somatofugal input core conductance met by synaptic current due to a greater leak conductance at the proximal end of the dendrite. Under these conditions, an increasing somatopetal core current and a corresponding drop of the depolarization membrane potential occurred. The voltage-dependent extrasynaptic conductances, if present, followed this depolarization. Consequently, the driving potential and membrane current densities decreased with increasing path distance from the soma. All path profiles were perturbed at bifurcations, being identical in symmetrical branches and diverging in asymmetrical ones. These perturbations were caused by voltage gradient breaks (abrupt change in the profile slope) occurring at the branching node due to coincident inhomogeneity of the dendritic core cross-section area and its conductance. The gradient was greater on the side of the smaller effective cross-section. Correspondingly, the path profiles of the somatopetal current transfer effectiveness were broken and/or diverged. The dendrites, their paths, and sites which were more effective in the current transfer from distributed sources were also more effective in the transfer from single-site inputs. The effectiveness of the active dendrite depended on the activation-inactivation kinetics of its voltage-gated conductances. In particular, dendrites with the same geometry were less effective with the Hodgkin-Huxley membrane than with the passive membrane, because of the effect of the noninactivating K(+)-conductance associated with the hyperpolarization equilibrium potential. Such electrogeometrical coupling may form a basis for path-dependent input-output conversion in the dendritic neurons, as the output discharge rate is defined by the net current delivered to the soma.

Animals↗
PubMed1998

Multiple synchronization strategies in rhythmic sensorimotor tasks: phase vs period correction.

To characterize synchronisation strategies in the tracking of auditory rhythm with rhythmic finger tapping, the adaptation process after unexpected step changes of an interstimulus interval (ISI) of 500 ms was investigated. Step changes of 2% (10 ms), 4% (20 ms), and 10% (50 ms) of ISI were applied to the stimulus sequence. Synchronisation patterns of 5 subjects were analyzed based on synchronisation error (SE) and interresponse intervals (IRI). A strategy shift contigent upon the size of the introduced step change was detected. After small ISI changes, rapid IRI matching to the new ISI was accompanied by temporarily enlarged SE values, which slowly returned to preferred SE values before the step change. Large ISI changes showed quick SE adaptations accompanied by a temporary overcorrection of IRI. Response asymmetry between ISI decreases and increases emerged, showing a stronger adaptation during ISI increases. A two-dimensional difference equation was formulated to simulate the time series of intertap intervals and explain the control process during IRI and SE adjustments. The system constants were optimized to minimalize the deviations between the computed and the observed response trajectories, consisting of the time series of SE and IRI. It was shown that a successful model fit using a linear two-dimensional difference equation was based on the size and direction of the ISI changes. MANOVA procedures showed that differences in equation parameters during small and large step changes were statistically significant (P < 0.05). It is therefore suggested that a uniform model accounting for synchronization responses to all step changes would require the introduction of nonlinear system properties.

Acoustic Stimulation↗
PubMed1998

Bifurcations in the decremental propagation of a spike train in the Hodgkin-Huxley model of low excitability.

Response of a nerve fiber of low excitability to periodic stimulus pulses is studied with computer simulation of the Hodgkin-Huxley model. The excitability of the Hodgkin-Huxley model is reduced by decreasing the equilibrium potential for the sodium ion and by increasing the temperature, so that the decremental propagation of spikes occurs in the refractory period. It is shown that, as the period of stimulus pulses is decreased, the propagation length of the spikes is continuously changed, and period-doubling bifurcations occur. The response of a nerve fiber of low excitability is then qualitatively different from that of a normal fiber.

Animals↗
PubMed1998

Forming independent components via temporal locking of reconstruction architectures: a functional model of the hippocampus.

The assumption is made that the formulation of relations as independent components (IC) is a main feature of computations accomplished by the brain. Further, it is assumed that memory traces made of non-orthonormal ICs make use of feedback architectures to form internal representations. Feedback then leads to delays, and delays in cortical processing form an obstacle to this relational processing. The problem of delay compensation is formulated as a speed-field tracking task and is solved by a novel control architecture. It is shown that in addition to delay compensation the control architecture can also shape long-term memories to hold independent components if a two-phase operation mode is assumed. Features such as a trisynaptic loop and a recurrent collateral structure at the second stage of that loop emerge in a natural fashion. Based on these properties a functional model of the hippocampal loop is constructed.

Animals↗
PubMed1999

Self-organization of the heartbeat as coordination among ventricular myocardial cells through mechano-electrical feedback.

To elucidate the mechanism of the self-organizing control of heartbeats, models that include the electrical and mechanical processes of heartbeat are proposed. As the mutual interaction between the two processes, mechano-electrical feedback (MEF), mediated by stretch-activated ionic channels, is considered. Analyses of the models show that coordination of beats among myocardial cells is established by MEF even in the absence of electrical coupling. The coordination of heartbeats is found to show in-phase synchronization under normal conditions, while it is out of phase or irregular under mechanically abnormal conditions. It is concluded that coupling by MEF is important for the self-organization of heartbeats.

Animals↗
PubMed1999

Bayesian inference in populations of cortical neurons: a model of motion integration and segmentation in area MT.

A major issue in cortical physiology and computational neuroscience is understanding the interaction between extrinsic signals from feedforward connections and intracortical signals from lateral connections. We propose here a computational model for motion perception based on the assumption that the local cortical circuits in the medio-temporal area (area MT) implement a Bayesian inference principle. This approach establishes a functional balance between feedforward and lateral, excitatory and inhibitory, inputs. The model reproduces most of the known properties of the neurons in area MT in response to moving stimuli. It accounts for important motion perception phenomena including motion transparency, spatial and temporal integration/segmentation. While integrating several properties of previously proposed models, it makes specific testable predictions concerning, in particular, temporal properties of neurons and the architecture of lateral connections in area MT. In addition, the proposed mechanism is consistent with the known properties of local cortical circuits in area V1. This suggests that Bayesian inference may be a general feature of information processing in cortical neuron populations.

Animals↗
PubMed1999

Activity-dependent enhancement in the reliability of correlated spike timings in cultured cortical neurons.

To study the use-dependent modification of activity in neural networks, we investigated the spike timing by simultaneously recording activity at multiple sites in a network of cultured cortical neurons. We used dynamical analysis to study the temporal structure of spike trains and the activity-dependent changes in the reliability and reproducibility of spike patterns evoked by a stimulus. We also used cross-correlation analysis to evaluate the interactions of neuron pairs. Our main conclusions are that even when no obvious change in spike numbers can be seen, use-dependent modification occurs, either enhancing or reducing in the reliability and reproducibility of spike trains evoked by a stimulus, and the fine temporal structure of stimulus-evoked spike trains and interactions between neurons are also modified by tetanic stimulation.

Action Potentials↗
PubMed1999

Investigation of the dynamics underlying periodic complexes in the EEG.

Periodic complexes (PC), occurring lateralised or diffuse, are relatively rare EEG phenomena which reflect acute severe brain disease. The pathophysiology is still incompletely understood. One hypothesis suggested by the alpha rhythm model of Lopes da Silva is that periodic complexes reflect limit cycle dynamics of cortical networks caused by excessive excitatory feedback. We examined this hypothesis by applying a recently developed technique to EEGs displaying periodic complexes and to periodic complexes generated by the model. The technique, non-linear cross prediction, characterises how well a time series can be predicted, and how much amplitude and time asymmetry is present. Amplitude and time asymmetry are indications of non-linearity. In accordance with the model, most EEG channels with PC showed clear evidence of amplitude and time asymmetry, pointing to non-linear dynamics. However, the non-linear predictability of true PC was substantially lower than that of PC generated by the model. Furthermore, no finite value for the correlation dimension could be obtained for the real EEG data, whereas the model time series had a dimension slighter higher than one, consistent with a limit cycle attractor. Thus we can conclude that PC reflect non-linear dynamics, but a limit cycle attractor is too simple an explanation. The possibility of more complex (high dimensional and spatio-temporal) non-linear dynamics should be investigated.

Acute Disease↗
PubMed1999

The role of cortical area MST in a model of combined smooth eye-head pursuit.

The cortical medial superior temporal area (MST) is essential for the normal execution of smooth pursuit eye movements. Many pursuit-related neurons (visual-tracking neurons = VT neurons) in the lateral part of area MST (MSTl) are responsive to retinal image slip (r) as well as to eye (e) and head velocity (h) with similar preferred directions (isodirectionality). We show, by running a connectionist network with VT neuron-like elements, that an assembly of MSTl-VT neurons is able to reconstruct target motion in world-centered coordinates (t'). When t' is fed into a subsequent model stage, converting t' into gaze velocity (g') with varying contributions of e and h, the overall model is able to account for many of the salient properties of visually guided pursuit including the consequences of MSTl lesions. However, the analysis of the MSTl network also clearly indicates that isodirectionality is not a prerequisite for its performance. The investigation of a second model suggests that isodirectionality indeed does not result from functional but from developmental constraints. This second model is a connectionist network with hidden units, which similar to MSTl-VT neurons receive input from modality specific units encoding retinal slip, eye and head velocity. After training this network to offer t' as output, two subsets of hidden units emerged, one exhibiting isodirectionality, but not the other. Since only isodirectional hidden units contributed to the flow of information, the preponderance of isodirectional MSTl-VT neurons might be the result of developmental pruning, eliminating the second group.

Adult↗
PubMed1999

Kinematic construction of the trajectory of sequential arm movements.

A kinematic construction rule determining the trajectory of human sequential movements is formulated using minimum-jerk and minimum-angular-jerk trajectories. The kinematic construction rule states that the observed trajectory of sequential movements coincides with a weighted average of the minimum-jerk trajectory and the segmented minimum-angular-jerk trajectory. This rule covers not only point-to-point movements but also simple sequential movements. Five kinds of experiments that measure the trajectories in planar, multijoint sequential arm movements were conducted. The measured trajectories coincide with the predictions made on the basis of the kinematic construction rule presented here. Moreover, predictions of previous models such as the minimum-jerk, the equilibrium-trajectory, and the minimum-torque-change models are shown to be incompatible with our observations of sequential movements.

Arm↗
PubMed1999

An architectural hypothesis for direction selectivity in the visual cortex: the role of spatially asymmetric intracortical inhibition.

Within a linear field approach, an architectural model for simple cell direction selectivity in the visual cortex is proposed. The origin of direction selectivity is related to recurrent intracortical interactions with a spatially asymmetric character along the axis of stimulus motion. No explicit asymmetric temporal mechanisms are introduced or adopted. The analytical investigation of network behavior, carried out under the assumption of a linear superposition of geniculate and intracortical contributions, shows that motion sensitivity of the resulting receptive fields emerges as a dynamic property of the cortical network without any feed-forward direction selectivity bias. A detailed analysis of the effects of the architectural characteristics of the cortical network on directionality and velocity-response curves was conducted by systematically varying the model's parameters.

Animals↗
PubMed1999

Distortions of length perception.

A combined influence of stimulus orientation and structure on the judgment of length was tested in psychophysiological experiments. The subjects adjusted the test part of a stimulus to be equal in length to the reference part. The orientation of the parts of the stimulus varied in the experiments. The stimuli (three dots or the Oppel-Kundt figure, which had ten dots within the filled part) were generated on the monitor. In the Oppel-Kundt figure, the filled part was considered as a reference and the empty part as a test. In sessions of the experiments, values of errors were measured as functions of the size and orientation of the stimulus. The reference part length varied within 14-150 min are range, and the orientation was fixed in 0 degree, 90 degrees, 180 degrees or 270 degrees positions. The orientation of the test part varied from 0 degree to 360 degrees in 7 degrees steps. We assume, that the experiments with the three-dot stimuli yielded pure characteristics of visual field anisotropy, while those with the Oppel-Kundt figure showed the combined effect of both the components (anisotropy and spatial filtering). The data demonstrated independence of the two factors from each other in a simultaneous manifestation. The characteristics of a pure Oppel-Kundt illusion have been found to be in close correspondence with the predictions of the model of spatial filtering.

Cybernetics↗