Biomedical subjects

G Schöner

Publications and source records attributed to G Schöner.

At least 19 recordsLinked to original sources

PubMed1998

Prior information preshapes the population representation of movement direction in motor cortex.

Single neuron activity was recorded in monkey motor cortex during the execution of pointing movements in six directions. The amount of prior information was manipulated by varying the range of precued directions. A distribution of neural population activation was constructed in the space of movement directions. This population representation of movement direction was preshaped by the precue. Peak location and width reflected the precued range of movement directions. From this preshaped form, the population representation evolved continuously in time and gradually in parameter space toward a more sharply peaked distribution centered on the parameter value specified by the response signal. A theoretical model of motor programming generated a similar temporal evolution of an activation field representing movement direction.

Animals↗

PubMed1998

Position and velocity coupling of postural sway to somatosensory drive.

Light touch contact of a fingertip to a stationary surface provides orientation information that enhances control of upright stance. Slight changes in contact force at the fingertip lead to sensory cues about the direction of body sway, allowing attenuation of sway. In the present study, the coupling of postural sway to a moving contact surface was investigated in detail. Head, center of mass, and center of pressure displacement were measured as the contact surface moved rhythmically at 0.1, 0.2, 0.4, 0.6, and 0.8 Hz. Stimulus amplitude decreased with frequency to maintain peak velocity constant across frequency. Head and body sway were highly coherent with contact surface motion at all frequencies except 0.8 Hz, where a drop-off in coherence was observed. Mean frequency of head and body sway matched the driving frequency </=0.4 Hz. At higher frequencies, non-1:1 coupling was evident. The phase of body sway relative to the touch plate averaged 20-30 degrees at 0.1-Hz drive and decreased approximately linearly to -130 degrees at 0.8-Hz drive. System gain was approximately 1 across frequency. The large phase lags observed cannot be accounted for with velocity coupling alone but indicate that body sway also was coupled to the position of the touch plate. Fitting of a linear second-order model to the data suggests that postural control parameters are not fixed but adapt to the moving frame of reference. Moreover, coupling to both position and velocity suggest that a spatial reference frame is defined by the somatosensory system.

Adaptation, Physiological↗

PubMed1997

Coupling of fingertip somatosensory information to head and body sway.

Light touch contact of a fingertip with a stationary surface can provide orientation information that enhances control of upright stance. Slight changes in contact force at the fingertip provide sensory cues about the direction of body sway, allowing attenuation of sway. In the present study, we asked to which extent somatosensory cues are part of the postural control system, that is, which sensory signal supports this coupling? We investigated postural control not only when the contact surface was stationary, but also when it was moving rhythmically (from 0.1 to 0.5 Hz). In doing so, we brought somatosensory cues from the hand into conflict with other parts of the postural control system. Our focus was the temporal relationship between body sway and the contact surface. Postural sway was highly coherent with contact surface motion. Head and body sway assumed the frequency of the moving contact surface at all test frequencies. To account for these results, a simple model was formulated by approximating the postural control system as a second-order linear dynamical system. The influence of the touch stimulus was captured as the difference between the velocity of the contact surface and the velocity of body sway, multiplied by a coupling constant. Comparison of empirical results (relative phase, coherence, and gain) with model predictions supports the hypothesis of coupling between body sway and touch cues through the velocity of the somatosensory stimulus at the fingertip. One subject, who perceived movement of the touch surface, demonstrated weaker coupling than other subjects, suggesting that cognitive mechanisms introduce flexibility into the postural control scheme.

Adult↗

PubMed1997

Low-frequency oscillations of visual, auditory and somatosensory cortical neurons evoked by sensory stimulation.

Low-frequency oscillations-LFOs-below 20 Hz in the activity of cortical neurons are a commonly observed property across all sensory modalities. However, the functional significance and potential role of these intrinsic oscillations are not well understood. Here, we attempt to provide a general framework for the interpretation of this phenomenon by considering its properties across several sensory modalities. In the first part, we provide a survey and a general description of low-frequency oscillations (LFOs) at a cellular level observed following adequate [Basar, and Schürmann, 1994]. Sensory stimulation of neurons recorded in three sensory modalities of neocortices in higher mammals. The second part will address some functional aspects of low-frequency oscillations (LFOs) such as stimulus selectivity and so-called 'interference' phenomena, specifically with findings related to 'resetting' and 'gating' of sensory processing streams. Finally, a hypotheses is outlined in which the low-frequency oscillations are regarded as an organizational principle by which continuity of sensory and motor states over time could be accomplished.

Acoustic Stimulation↗

PubMed1997

The influence of adaptation and stochastic fluctuations on spontaneous perceptual changes for bistable stimuli.

Spontaneous perceptual change was studied by measuring the probabilities of the first two spontaneous pattern switches as a function of time following the onset of a bistable apparent quartet for which either horizontal or vertical motion is perceived. Contrary to the classical satiation hypothesis (Köhler & Wallach, 1944), differential time-dependent adaptation of the perceived compared with the unperceived motion directions was not necessary to account for the first spontaneous switch. In addition, adaptation of the perceived motion accompanied by recovery from adaptation of the unperceived motion was not necessary to account for the increased probability of the second spontaneous switch. It was concluded that regardless of possible adaptation effects, stochastic fluctuations are necessary for the actual reversal of activation levels that produces the spontaneous switch. When the difference in detector activation is reduced by differential adaptation of competing motion detectors (or by the occurrence of a prior spontaneous pattern change), smaller stochastic fluctuations are sufficient to reverse the relative activation of competing detectors. Thus, adaptation can increase the probability of spontaneous switches without directly causing them.

Adaptation, Ocular↗

PubMed1996

Identification of the nonlinear state-space dynamics of the action-perception cycle for visually induced postural sway.

Human subjects standing in a sinusoidally moving visual environment display postural sway with characteristic dynamical properties. We analyzed the spatiotemporal properties of this sway in an experiment in which the frequency of the visual motion was varied. We found a constant gain near 1, which implies that the sway motion matches the spatial parameters of the visual motion for a large range of frequencies. A linear dynamical model with constant parameters was compared quantitatively with the data. Its failure to describe correctly the spatiotemporal properties of the system led us to consider adaptive and nonlinear models. To differentiate between possible alternative structures we directly fitted nonlinear differential equations to the sway and visual motion trajectories on a trial-by-trial basis. We found that the eigenfrequency of the fitted model adapts strongly to the visual motion frequency. The damping coefficient decreases with increasing frequency. This indicates that the system destabilizes its postural state in the inertial frame. This leads to a faster internal dynamics which is capable of synchronizing posture with fast-moving visual environments. Using an algorithm which allows the identification of essentially nonlinear terms of the dynamics we found small nonlinear contributions. These nonlinearities are not consistent with a limit-cycle dynamics, accounting for the robustness of the amplitude of postural sway against frequency variations. We interpret out results in terms of active generation of postural sway specified by sensory information. We derive also a number of conclusions for a behavior-oriented analysis of the postural system.

Humans↗

PubMed1996

Perceptual stability and the selective adaptation of perceived and unperceived motion directions.

Adaptation was studied in a paradigm in which the adapting stimulus was a variably biased version of a bistable apparent motion stimulus, a motion quartet, and the post-adaptation test stimulus was a "neutral" motion quartet. Either horizontal or vertical motion was perceived, never both at the same time. When only one of these was perceived during the entire adaptation phase of a trial, and the perceived motion was highly stable, adaptation effects were greater for the perceived than the unperceived motion directions (i.e., adaptation was selective to the perceived motion). However, when the perceived motion during adaptation was relatively unstable (i.e., when the perceived motion was more likely to spontaneously change directions), similar levels of adaptation were obtained for perceived as well as unperceived, but possible motion directions. Thus, adaptation occurs prior to the determination of which of the competing motion directions will be perceived. The relationship between the stability of the adapting percept and the selectivity of adaptation is explained in terms of differences in the activation of mutually inhibitory horizontal and vertical motion detectors.

Adaptation, Ocular↗

PubMed1995

Saccadic motor planning by integrating visual information and pre-information on neural dynamic fields.

A functional model of target selection in the saccadic system is presented, incorporating elements of visual processing, motor planning, and motor control. We address the integration of visual information with pre-information, which is provided by manipulating the probability that a target appears at a certain location. This integration is achieved within a dynamic representation of planned eye movement which is modeled through distributions of activation on a topographic field. Visual input evokes activation, which is also constrained by lateral interaction within the field and by preshaping input representing pre-information. The model describes target selection observable in paradigms in which visual goals are presented at more than one location. Specifically, we model the transition from averaging, where endpoints of first saccades fall between two visual target locations, to decision making, where endpoints of first saccades fall accurately onto one of two simultaneously presented visual targets. We make predictions about how metrical biases of first saccades are induced by pre-information about target locations acquired by learning. When coupled to a motor control stage, activation dynamics on the planning level contribute to stabilizing gaze under fixation conditions. The neurophysiological relevance of our functional model is discussed.

Computer Simulation↗

PubMed1994

Frequency dependence of the action-perception cycle for postural control in a moving visual environment: relative phase dynamics.

When standing human subjects are exposed to a moving visual environment, the induced postural sway displays varying degrees of coherence with the visual information. In our experiment we varied the frequency of an oscillatory visual display and analysed the temporal relationship between visual motion and sway. We found that subjects maintain sizeable sway amplitudes even as temporal coherence with the display is lost. Postural sway tended to phase lead (for frequencies below 0.2 Hz) or phase lag (above 0.3 Hz). However, we also observed at a fixed frequency, highly variable phase relationships in which a preferred range of phase lags is prevalent, but phase jumps occur that return the system into the preferred range after phase has begun drifting out of the preferred regime. By comparing the results quantitatively with a dynamical model (the sine-circle map), we show that this effect can be understood as a form of relative coordination and arises through an instability of the dynamics of the action-perception cycle. Because such instabilities cannot arise in passively driven systems, we conclude that postural sway in this situation is actively generated as rhythmic movement which is coupled dynamically to the visual motion.

Humans↗

PubMed1994

Temporal stability of the action-perception cycle for postural control in a moving visual environment.

When standing human subjects are exposed to a moving visual environment, the induced postural sway forms a stable temporal relationship with the visual information. We have investigated this relationship experimentally with a new set-up in which a computer generates video images which correspond to the motion of a 3D environment. The suggested mean distance to a sinusoidally moving wall is varied and the temporal relationship to induced sway is analysed (1) in terms of the fluctuations of relative phase between visual and sway motion and (2) in terms of the relaxation time of relative phase as determined from the rate of recovery of the stable relative phase pattern following abrupt changes in the visual motion pattern. The two measures are found to converge to a well-defined temporal stability of the action-perception cycle. Furthermore, we show that this temporal stability is a sensitive measure of the strength of the action-perception coupling. It decreases as the distance of the visual scene from the observer increases. This fact and the increase of mean relative phase are consistent with predictions of a linear second-order system driven by the visual expansion rate. However, the amplitude of visual sway decreases little as visual distance increases, in contradiction to the predictions, and is suggestive of a process that actively generates sway. The visual expansion rate on the optic array is found to decrease strongly with visual distance. This leads to the conclusion that postural control in a moving visual environment cannot be understood simply in terms of minimization of retinal slip, and that dynamic coupling of vision into the postural control system must be taken into account.

Algorithms↗

PubMed1993

Bistability and hysteresis in the organization of apparent motion patterns.

In a paradigm for which 2 distinct patterns are perceived for the same stimulus, perceptual hysteresis (persistence of a percept despite parameter change to values favoring the alternative pattern) and temporal stability (persistence despite intrinsic propensities toward spontaneous change) are interdependent. Greater persistence during parameter change reduces temporal stability, slowing the rate of parameter change reduces hysteresis by increasing opportunity for spontaneous change, and increasing temporal stability (by enlarging the stimulus) increases hysteresis. Hysteresis results in the perception of parametrically disfavored patterns; a parameter can influence a percept without specifying it. The visual system thus exhibits time-dependent behavior analogous to dynamical behavior observed in other systems, both physical and biological, for which there is competition among alternative patterns that vary in relative stability.

Adult↗

PubMed1993

Dynamic properties of cortical evoked (10 Hz) oscillations: theory and experiment.

Experiments probed the dynamic properties of stimulus-evoked (approximately 10 Hz) oscillations in somatosensory cortex of anesthetized rats. Experimental paradigms and statistical time series analysis were based on theoretical ideas from a dynamic approach to temporal patterns of neuronal activity. From the results of a double-stimulus paradigm we conclude that the neuronal response contains two components with different dynamics and different coupling to the stimulus. Based on this result a quantitative dynamic model is derived, making use of normal form theory for bifurcating vector fields. The variables used are abstract, but measurable, dynamic components. The model parameters capture the dynamic properties of neuronal response and are related to experimental results. A structural interpretation of the model can be given in terms of the collective dynamics of neuronal groups, their mutual interaction, and their coupling to peripheral stimuli. The model predicts the stimulus-dependent lifetime of the oscillations as observed in experiment. We show that this prediction relies on the basic concept of dynamic bistability and does not depend on the modeling details.

Animals↗

PubMed1992

Evoked oscillatory cortical responses are dynamically coupled to peripheral stimuli.

We report that the response of neurons in rat somatosensory cortex to tactile stimulation consists of two components, a short-latency response and an oscillatory response, observable as up to 8 peaks in the post-stimulus-time-histogram with interpeak intervals in the order of 100 ms (10 Hz). While the first component is always stimulus locked, the second component is strictly stimulus-locked only when elicited from the resting state: once started, the oscillations are only weakly affected by further stimulation. This implies generally that the question of stimulus locking of oscillatory response components is not a yes/no question. Instead, the concept of dynamic coupling is shown to adequately capture the different limit cases. We present a simple dynamic model that exemplifies this point.

Animals↗

PubMed1991

Dynamic theory of action-perception patterns: the "moving room" paradigm.

Action-perception patterns are studied theoretically in terms of equations of motion that capture the coordination capacity of the nervous system. We consider intrinsic dynamics in the absence of visual information that contain a single posture state as a fixed point attractor. We couple these intrinsic dynamics to visual information that stabilizes posture in the visual world. This leads to a theory of postural sway induced by an optic flow field ("moving room" paradigm). The optic flow is parametrized in a simplest approximation by the expansion rate of a relevant perceptual target. We show how temporal stability as the key concept of this theory can lead to prediction and serve as a measure of perceptual coupling. Finally, we discuss the relation of the present theory to biological cybernetics.

Animals↗

PubMed1990

A synergetic theory of quadrupedal gaits and gait transitions.

We present a theoretical analysis of the patterns of interlimb co-ordination in the gaits of quadrupedal locomotion. Introducing as collective variables a set of relative phases that describe the co-ordination patterns, we classify gaits by their symmetry properties, which can be expressed as invariances under groups of transformations. We define dynamics of the collective variables, on which we impose symmetry restrictions. The stable observable gait patterns correspond to atractors of these dynamics. A non-trivial consequence of this theoretical viewpoint is that gait transitions can take the form of non-equilibrium phase transitions that are accompanied by loss of stability. We show how various types of such phase transitions involving hysteresis, slowing down and fluctuation enhancement can occur. Also the difference between smooth and abrupt transitions is given theoretical foundation. While existing experimental evidence is consistent with the theory developed here, we propose new experimental measures that can serve to test the present theoretical framework. Finally, the influence of underlying symmetries of the dynamics on the nature of the gait patterns and their stability is analyzed. For example, breaking of a front-hind symmetry can lead to a change from absolute to relative co-ordination in the sense of von Holst (1939, Ergebnisse der Physiologie 42, 228). Also, differential stability of straight and reverse gaits results from thus lowering the symmetry.

Gait↗

PubMed1990

A dynamic theory of coordination of discrete movement.

The concepts of pattern dynamics and their adaptation through behavioral information, developed in the context of rhythmic movement coordination, are generalized to describe discrete movements of single components and the coordination of multiple components in discrete movement. In a first step we consider only one spatial component and study the temporal order inherent in discrete movement in terms of stable, reproducible space-time relationships. The coordination of discrete movement is captured in terms of relative timing. Using an exactly solvable nonlinear oscillator as a mathematical model, we show how the timing properties of discrete movement can be described by these pattern dynamics and discuss the relation of the pattern variables to observable end-effector movement. By coupling several such component dynamics in a fashion analogous to models of rhythmic movement coordination we capture the coordination of discrete movements of two components. We find the tendency to synchronize the component movements as the discrete analogon of in-phase locking and study its breakdown when the components become too different in their dynamic properties. The concept of temporal stability leads to the prediction that remote compensatory responses occur such as the restore synchronization when one component is perturbed. This prediction can be used to test the theory. We find that the discrete analogon to antiphase locking in rhythmic movement is a tendency to move sequentially, a finding that can also be subjected to empirical test.

Cybernetics↗

PubMed1989

Learning and recall in a dynamic theory of coordination patterns.

A dynamic theory of learning and recall of coordination patterns is developed in the context of relative timing skills. Characterizing the coordination patterns in such skills by the collective variable, relative phase, we choose a model system in which the intrinsic pattern dynamics as well as the influence of environmental and memorized information are well understood from previous experimental and theoretical work. To describe learning we endow memorized information with dynamics which is determined by a phenomenological strategy. Similarly, additional degrees of freedom must be introduced to understand recall. As such recall variables we choose the relative strengths with which each memorized pattern acts on the pattern dynamics and model their dynamics phenomenologically. The resulting dynamical system that resembles models used in pattern recognition theory is shown to adequately describe the learning and recall processes. Moreover, due to the operational character of the theory, several predictions emerge that are open to experimental test. In particular, we show under which conditions phase transitions occur in the dynamics of the coordination patterns during learning and during recall. Considering different time scales and their relations we demonstrate how these phase transitions can be identified and observed. Other predictions include the influence of the intrinsic pattern dynamics on the recall process and the existence of history and hysteresis effects in recall. We discuss different forms of "forgetting" and differentiation of memorized information. The results show how a new theoretical view of learning and recall as change of behavioral dynamics can lead to a different understanding of these processes by providing testable predictions.

Extremities↗

PubMed1988

Dynamic pattern generation in behavioral and neural systems.

In the search for principles of pattern generation in complex biological systems, an operational approach is presented that embraces both theory and experiment. The central mathematical concepts of self-organization in nonequilibrium systems (including order parameter dynamics, stability, fluctuations, and time scales) are used to show how a large number of empirically observed features of temporal patterns can be mapped onto simple low-dimensional (stochastic, nonlinear) dynamical laws that are derivable from lower levels of description. The theoretical framework provides a language and a strategy, accompanied by new observables, that may afford an understanding of dynamic patterns at several scales of analysis (including behavioral patterns, neural networks, and individual neurons) and the linkage among them.

Animals↗