Omar Feix do Nascimento

Optimization of wavelets for classification of movement-related cortical potentials generated by variation of force-related parameters

The paper presents a novel pattern recognition approach for the classification of single-trial movement-related cortical potentials (MRCPs) generated by variations of force-related parameters during voluntary tasks. The feature space was built from the coefficients of a discrete dyadic wavelet transformation. Mother wavelet parameterization allowed the tuning of basis functions to project the signals. The mother wavelet was optimized to minimize the classification error estimated from the training set. Classification was performed with a support vector machine (SVM) approach with optimization of the width of a Gaussian kernel and of the regularization parameter. The efficacy of the optimization procedures was representatively shown on electroencephalographic recordings from two subjects who performed unilateral isometric plantar flexions at two target torques and two rates of torque development. The proposed classification method was tested on four pairs of classes corresponding to the change in only one of the two parameters of the task. Misclassification rate (test set) in the classification of 1-s EEG activity immediately before the onset of the tasks was reduced from 50.8+/-2.9% with worst wavelet and nearest representative classifier, to 40.2+/-7.3% with optimal wavelet and nearest representative classifier, and to 15.8+/-3.4% with optimal wavelet and SVM with optimization of the kernel and regularization parameter. The proposed pattern recognition method is promising for classification of MRCPs modulated by variations of force-related parameters.

Movement-related parameters modulate cortical activity during imaginary isometric plantar-flexions

A multitude of studies have demonstrated a clear activation of the motor cortex during imagination of various motor tasks; however, it is still unclear if movement-related parameters (movement direction, range of motion, speed, force level and rate of force development) specifically modulate cortical activation as they do during the execution of actual motor tasks. Accordingly, this study examined whether the rate of torque development (RTD) and/or the torque amplitude modulates cortical potentials generated during imaginary motor tasks. Fifteen subjects imagined four different left-sided isometric plantar-flexion tasks, while EEG and EMG recordings were being performed. The averaged EEG activity was analyzed in terms of movement-related potentials (MRPs), consisting of readiness potential (RP), motor potential (MP) and movement-monitoring potential (MMP). It was demonstrated that RTD and torque amplitude indeed modulate cortical activity during imaginary motor tasks. Information concerning movement-related parameters for imaginary plantar-flexion tasks seems to be encoded in the supplementary motor area (SMA) and the primary motor cortex (M1). A comparison between MRPs of imaginary and actual motor tasks revealed that early MRPs were morphologically similar, but differed significantly in amplitude. One of the possible suggestions to explain such a difference may be an “abortion” of ongoing motor programs.

Relationship between plantar-flexor torque generation and the magnitude of the movement-related potentials.

This study investigates whether rate of torque development (RTD) and/or torque amplitude are reflected in the movement-related potentials (MRPs) preceding and accompanying isometric activation of plantar flexor muscles. Subjects were asked to perform six different tasks involving the left ankle joint. The tasks consisted of voluntary isometric plantar flexions at three different RTDs (two fixed rates and a ‘ballistic’ task) ending at two different torque amplitudes. The main observations from the analysis of the MRPs were: 1) the readiness potentials (RP) demonstrated a statistically significant discrimination between low and high torque amplitudes; 2) the RP, the motor potentials (MP) and the movement-monitoring potentials (MMP) could be statistically differentiated among the different RTDs; and 3) in general the MRPs demonstrated an ipsilateral tendency in relation to the involved limb. The results indicate that RP is a suitable parameter for differentiation between levels of isometric plantar flexion torque and MP and MMP are sensitive to a differentiation between RTDs. The correlation between MRPs and motor tasks involving different rates of torque development and levels of torque suggests that MRPs may comprise a potential solution for programming of intended movements to be executed by systems based on neural rehabilitation technology.

Afferent-mediated modulation of the soleus muscle activity during the stance phase of human walking

The aim of this study was to investigate the contribution of proprioceptive feedback to the amplitude modulation of the soleus muscle activity during human walking. We have previously shown that slow-velocity, small-amplitude ankle dorsiflexion enhancements and reductions applied during the stance phase of the step cycle generate, respectively, increments and decrements on the ongoing soleus activity. We have also shown that the increments in soleus activity are at least partially mediated by feedback from group Ia fibres. In the present study, we further investigated the afferent-mediated contribution from muscle group II afferents, cutaneous and proprioceptive afferents from the foot, and load-sensitive afferents to the soleus EMG. Slow-velocity, small-amplitude ankle trajectory modifications were combined with the pharmaceutical depression of group II polysynaptic pathways with tizanidine hydrochloride, anaesthetic blocking of sensory information from the foot with injections of lidocaine hydrochloride, and modulation of load feedback by increasing and decreasing the body load. The depression of the group II afferents significantly reduced the soleus response to the ankle trajectory modifications. Blocking sensory feedback from the foot did not have an effect on the soleus muscle activity. Changes in body load affected the ongoing soleus activity level; however, it did not affect the amplitude of the soleus EMG responses to the ankle trajectory modifications. These results suggest that the feedback from group II afferents, and possibly from load-sensitive afferents, contribute to the amplitude modulation of the soleus muscle activity during the stance phase of the step cycle. However, feedback from cutaneous afferents and instrinsic proprioceptive afferents from the foot does not seem to contribute to this muscle activation.

EEG based BCI-towards a better control. Brain-computer interface research at aalborg university

This paper summarizes the brain-computer interface (BCI)-related research being conducted at Aalborg University. Namely, an online synchronized BCI system using steady-state visual evoked potentials, and investigations on cortical modulation of movement-related parameters are presented.

Influence of directional orientations during gait initiation and stepping on movement-related cortical potentials

The influence of directional orientation on movement-related potentials (MRPs) during gait initiation and stepping has been investigated in the present study, as well as possible effects caused by the distinction between gait initiation and stepping. Accordingly, electroencephalographic (EEG), electromyographic (EMG) and kinetic recordings were conducted while eight subjects initiated gait and were stepping in three different directions (namely, forward, backward and lateral). Five different movement-related potentials were extracted from the EEG recordings and statistically analyzed. Movement parameters were extracted from kinetic recordings and statistically analyzed as well. Results indicated that variations in directional orientation of gait and stepping were associated to changes in MRPs, but the associations between movement parameters and MRPs were conditional to the kind of task performed. Gait tasks were mainly differentiated in early MRPs while stepping tasks were more differentiated in late MRPs, indicating that differences between gait initiation and stepping might be associated with different levels of preparation and execution. Apparently the changes found in the movement-related potentials were not simply caused by changes in the sensorial input due to perception of the spatial environment, but rather because of variations in the movement kinematics and kinetics.

Identification of movement-related cortical potentials with optimized spatial filtering and principal component analysis

Abstract We present a method for detecting movement intention from multichannel electroencephalographic (EEG) recordings. Movement intention is expressed as a slow negative deflection in amplitude of the EEG signal recorded above the motor cortex. This deflection is known as a movement-related cortical potential (MRCP). Detection of movement intention implies discrimination between MRCPs and noise. The signal-to-noise ratio of MRCPs was improved by an optimized spatial filter. Features were extracted with principal component analysis or locality preserving projections from the spatially filtered signals and classification between MRCPs and noise was performed with a k-nearest neighbors algorithm, modified by adjusting the decision rule to improve specificity, and a support vector machine approach. In one representative subject the sensitivity and specificity in detection were in the range 80–90% and 98–99.5%, respectively. The method seems promising for the development of asynchronous brain–computer interfaces (BCIs) based on MRCPs.