Dynamical changes in corticospinal excitability during imagery of unimanual and bimanual wrist movements in humans: a transcranial magnetic stimulation study.

This study explored the dynamical changes in corticospinal excitability during the imagination of cyclical unimanual and bimanual wrist flexion-extension movements. Transcranial magnetic stimulation was applied over the left motor cortex to evoke motor evoked potentials in the right wrist flexor and extensor muscles. Findings provided evidence for increased reciprocal excitability changes during imagery of symmetrical in-phase movements as compared to asymmetrical (anti-phase) or unimanual movements. This suggests that in-phase movements may reinforce whereas anti-phase movements may reduce the temporal representation of the task in the corticospinal motor networks of the brain.

Frequency-dependent effects of muscle tendon vibration on corticospinal excitability: a TMS study.

The aim of the present study was to investigate the effects of muscle tendon vibration at different frequencies on corticospinal excitability by means of transcranial magnetic stimulation (TMS). A second objective was to describe whether the observed modulations in motor evoked potentials (MEPs), as a function of vibration frequency, reflect the behavior of Ia afferents during and after vibration. In ten subjects, muscle tendon vibration (duration 30 s) was applied to the flexor carpi radialis (FCR) muscle at three different frequencies (20, 75 and 120 Hz). MEPs following single-pulse TMS were recorded from the targeted muscle during a previbration, vibration, and postvibration period. Muscle tendon vibration at 75 Hz increased the MEP amplitude significantly during vibration, whereas a smaller but still significant effect was observed during 120 Hz vibration. No significant MEP changes could be observed during 20 Hz vibration and during the postvibration period for each frequency. Our findings indicate that muscle tendon vibration exerts a frequency-dependent effect on corticospinal excitability. Furthermore, evidence is provided for the notion that the excitatory effect of muscle tendon vibration on the primary motor cortex is mediated by Ia afferent input.

Proprioceptive control of cyclical bimanual forearm movements across different movement frequencies as revealed by means of tendon vibration.

The effect of unilateral tendon vibration on the performance of cyclical bimanual forearm movements was investigated across different cycling frequencies (from 0.67 to 2.53 Hz). The spatiotemporal features of the individual limb motions as well as their coordination were studied. Tendon vibration was found to result in a substantial reduction in the amplitude of the vibrated arm, leaving the nonvibrated arm unaffected. The vibration-induced amplitude reduction decreased from 26% to 11% as cycling frequency increased even though significant reductions were still observed at the highest cycling frequencies. Tendon vibration was also found to result in an increase of the phase lead of the dominant arm with respect to the nondominant arm, but this effect was not modulated by cycling frequency. The data argue in favor of a closed-loop mode of movement control during cyclical high-speed movements. It is suggested that kinesthetic afferent information is processed and used to guide action up to near-maximal movement speeds, reinforcing recent claims with respect to visual information processing.

Proprioceptive regulation of interlimb behavior: interference between passive movement and active coordination dynamics.

The coordination of homolateral effectors (right arm/right leg) according to the in-phase or anti-phase mode was perturbed through passive movement of a third segment (left arm or left leg) imposed by the experimenter. The manipulated parameters of the passive segment were frequency and amplitude along with their degree of scaling. Results showed that passive movement degraded anti-phase patterns more than in-phase patterns. Furthermore, the anti-phase mode deteriorated profoundly during frequency manipulation, but scaling did not induce additional effects, whereas a linear association was observed between anti-phase deterioration and amplitude manipulation. Together, these data indicate that passive movement disturbed the coordination dynamics of an actively performed task. The fact that interference depended on the manipulated parameter suggests a distinction in the degree of intrusiveness of the irrelevant afferent information induced by the passive limb. It is concluded that sensory discrimination between irrelevant and relevant input is critical in performing a coordinated task adequately under perturbed conditions.

Directional tuning effects during cyclical two-joint arm movements in the horizontal plane.

The present study explored the effect of different movement orientations on the arm end-effector kinematic features, levels of muscle activity and intermuscular coordination between shoulder and elbow muscles during cyclical movement. Subjects were instructed to trace cyclical lines with their dominant arm along vertical, horizontal, and right (low inertia) or left diagonal (high inertia) orientations. EMG activity from the biceps, triceps and anterior and posterior deltoids were monitored along with the displacements of the end-effector of the arm. The results suggested a differential role for the shoulder versus elbow muscles in the manipulation of the hand end-effector trajectory. The activity in the shoulder flexors was predominantly in anti-phase with that of the shoulder extensors and was therefore presumed to manipulate the global features of the trajectory. Biceps and triceps tended to show less orchestrated activity and were therefore assumed to be responsible for making the fine adjustments and to compensate for intersegmental interactions. The most pronounced differences in kinematics and EMG features among the four principal movement orientations were observed between the two diagonal orientations, which differed profoundly in arm inertial resistance. The findings converged upon the principle of 'inertial anisotropy,' as previously identified for discrete movement, suggesting that the central nervous system did not fully preplan the actual kinematic requirements of cyclical task performance. Moreover, inertial anisotropy was evident in spite of the fact that movement was performed under temporal constraints (metronome pacing) and with availability of a visual template of the task, suggesting that enhancement of the feedback loop did not fully eliminate these effects.

Bimanual coordination and limb-specific parameterization in patients with Parkinson's disease.

Bimanual coordination and the capability to parameterize the individual limb movements were examined in patients with Parkinson's disease (PD) as compared to healthy control subjects. In-phase and anti-phase patterns were performed while the individual limb movements were subjected to amplitude and loading manipulations. Findings showed that PD patients produced the bimanual configurations with lower degrees of phasing accuracy and consistency than control subjects, indicating an impairment at the global (coordinative) level of simultaneously produced movements. At the local (limb-specific) level, the imposed distances with and without loading were unaffected in PD patients as compared to control subjects, whereas cycle times were prolonged and depended on the task requirements. This illustrates a disturbance at the limb-specific level in complying with the execution of the submovements. The finding that movement slowness only became evident in the more complex conditions, suggests that it did not mainly represent a deficit in the execution of coordinated movements, but rather an inability to accommodate the motor output during stringent spatiotemporal task constraints.

Motor learning and Parkinson's disease: refinement of within-limb and between-limb coordination as a result of practice.

Even though the basal ganglia have been assigned a role in motor learning, few studies have addressed motor learning capabilities in Parkinson patients. In the present experiment, improvement of bimanual figure drawing across practice was compared between Parkinson patients and normal age-matched controls. At regular intervals during acquisition, performance was assessed under normal vision and blindfolded conditions. At initiation of practice, the typical signs associated with Parkinson's disease became evident, such as bradykinesia and hypometria. Moreover, reduced synchronization between the force-time specifications of both limbs was observed. When vision was withdrawn, Parkinson patients showed a larger drift of drawing performance across the workspace, indicative of a decline in proprioception. In spite of the aforementioned deficits, Parkinson patients made marked improvements in the speed of execution, the consistency of the spatial trajectories, and the synchronization between the limbs across practice, even though they never reached the performance levels obtained in elderly controls. The findings demonstrate that Parkinson patients do benefit from practice to refine their upper limb control and to alleviate their most basic motor deficits.

Morphing techniques for manipulating face images.

This paper describes morphing techniques to manipulate two-dimensional human face images and three-dimensional models of the human head. Applications of these techniques show how to generate composite faces based on any number of component faces, how to change only local aspects of a face, and how to generate caricatures and anticaricatures of faces. These techniques are potentially useful for many psychological studies because they permit realistic images to be generated with precise control.

Stability and Intermittency in Large-Scale Coupled Oscillator Models for Perceptual Segmentation

The coupled map lattice, a system of locally coupled nonlinear maps, is proposed as a model for perceptual segmentation. Patterns of synchronized activity are obtained in the model from high-dimensional, deterministic chaos. These patterns correspond to segmented topographical mappings of the visual field. The chaotic dynamic has a dual role of contributing to pattern creation in unsynchronized states and of noise revolting against stabilization in synchronized states. The dynamic allows rapid transitions between unsynchronized and synchronized states. Their stability characteristics are explored using analytical tools and numerical simulations. Stability or instability are shown to be determined by network coupling strength, in proportion to the rate of chaotic divergence. The introduction of adaptive connections, in combination with stimulus-controlled oscillation, enables stable or meta-stable patterns of synchronized activity to occur, depending on the perceptual structure in the visual field. For a perceptually ambiguous pattern, the system switches between alternative meta-stable segmentations. The switching-time distribution obtained from the model was found in agreement with those observed in the experimental literature. Copyright 1997 Academic Press. Copyright 1997 Academic Press

A model for recognition memory: REM-retrieving effectively from memory.

A new model of recognition memory is reported. This model is placed within, and introduces, a more elaborate theory that is being developed to predict the phenomena of explicit and implicit, and episodic and generic, memory. The recognition model is applied to basic findings, including phenomena that pose problems for extant models: the list-strength effect (e.g., Ratcliff, Clark, & Shiffrin, 1990), the mirror effect (e.g., Glanzer & Adams, 1990), and the normal-ROC slope effect (e.g., Ratcliff, McKoon, & Tindall, 1994). The model assumes storage of separate episodic images for different words, each image consisting of a vector of feature values. Each image is an incomplete and error prone copy of the studied vector. For the simplest case, it is possible to calculate the probability that a test item is "old," and it is assumed that a default "old" response is given if this probability is greater than .5. It is demonstrated that this model and its more complete and realistic versions produce excellent qualitative predictions.