Browsing by Author "Hoffmann, Ulrich"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Affective brain-computer interfaces: Psychophysiological markers of emotion in healthy persons and in persons with amyotrophic lateral sclerosis(2009) Nijboer, Femke; Carmien, Stefan P.; Leon, Enrique; Morin, Fabrice O.; Koene, Randal A.; Hoffmann, Ulrich; Tecnalia Research & InnovationAffective Brain-Computer Interfaces (BCI) are systems that measure signals from the peripheral and central nervous system, extract features related to affective states of the user, and use these features to adapt human-computer interaction (HCI). Affective BCIs provide new perspectives on the applicability of BCIs. Affective BCIs may serve as assessment tools and adaptive systems for HCI for the general population and may prove to be especially interesting for people with severe motor impairment. In this context, affective BCIs will enable simultaneous expression of affect and content, thus providing more quality of life for the patient and the caregiver. In the present paper, we will present psychophysiological markers for affective BCIs, and discuss their usability in the day to day life of patients with amyotrophic lateral sclerosis (ALS).Item Automatic determination of parameters for multipad functional electrical stimulation: Application to hand opening and closing(2012) Hoffmann, Ulrich; Deinhofer, Martin; Keller, Thierry; Tecnalia Research & InnovationTranscutaneous functional electrical stimulation (FES) is a method used for rehabilitation of patients having suffered a stroke or spinal cord injury. When applying FES a common problem is that stimulation electrodes have to be placed with great care in order to avoid activation of muscles close to the target muscles. A promising approach to circumvent this problem is to employ multipad FES, i.e. to employ electrode arrays containing many small electrodes allowing selective activation of muscles. In this work an algorithm is presented which automatically determines subsets of active electrodes and stimulation currents such that movements with user-specified amplitudes are induced. Using a recently developed portable multipad FES system and a virtual reality dataglove, the algorithm was tested with seven able-bodied subjects. Stimulation with parameters determined by the algorithm led to movements with a median deviation of between 0° and 5° from the specified wrist angle and between 0% and 12% from the specified degree of finger flexion.Item Brain-computer interface based on high frequency steady-state visual evoked potentials: A feasibility study(2009) Hoffmann, Ulrich; Fimbel, Eric J.; Keller, Thierry; Tecnalia Research & InnovationBrain-computer interfaces (BCIs) based on steady-state visual evoked potentials (SSVEPs) are systems in which virtual or physical objects are tagged with flicker of different frequencies. When a user focuses on one of the objects its flicker frequency becomes visible in the electroencephalogram (EEG) and so the object on which the user focuses can be determined from brain activity alone. A significant problem inherent to such systems is that typically flicker with frequencies in the range 5 - 30 Hz is used. Flicker in this frequency range is known to elicit easily detectable SSVEPs but is very tiring and annoying for users and can possibly trigger epileptic seizures. In this paper we study the feasibility of using higher frequencies for which the perceived flicker is less intensive. We compare the classification accuracy that can be achieved for stimuli flickering with low frequencies (15 - 20 Hz), medium frequencies (30 - 45 Hz), and high frequencies (50 - 85 Hz). The classification of the data is done with a Bayesian algorithm that learns classification rules and selects optimal electrode pairs. The results show that the medium frequency range can be used to build a high-performance BCI for which the flicker is hardly visible. We also found that for some subjects even high frequency flicker evokes reliably detectable SSVEPs.Item Detection and removal of stimulation artifacts in electroencephalogram recordings(2011) Hoffmann, Ulrich; Cho, Woosang; Ramos-Murguialday, Ander; Keller, Thierry; Tecnalia Research & InnovationStimulation artifacts are short-duration, high-amplitude spikes which can be observed in electroencephalogram (EEG) recordings whenever surface functional electrical stimulation (FES) is applied during recordings. Stimulation artifacts are of non-physiologic origin and hence have to be removed before analysis of the EEG can take place. In this paper, algorithms for the detection and removal of stimulation artifacts are presented. The algorithms require only little computational resources and can be applied online, while signals are recorded. Therefore, the algorithms are suitable for applications such as online control of FES based neuroprostheses by a brain-computer interface. Tests are performed with datasets recorded from two subjects for artifact durations ranging from 0.5 ms to 10 ms. After application of the artifact removal algorithms the signal-to-noise ratio of the reconstructed signals ranges from 15 dB to 45 dB, depending on the duration of artifacts and the type of algorithm.Item The effect of visual cues on the number and duration of freezing episodes in Parkinson's patients(2012) Velik, Rosemarie; Hoffmann, Ulrich; Zabaleta, Haritz; Marti Masso, Jose Felix; Keller, Thierry; Tecnalia Research & InnovationFreezing of gait is a phenomenon common in Parkinson's patients and significantly affects quality of life. Sensory cues have been known to improve walking performance and reduce freezing of gait. Visual cues are reported to be particularly effective for this purpose. So far, sensory cues have generally been provided continuously, even when currently not needed. However, a recent approach suggests the provision of cues just in the case that freezing actually occurs. The arguments in favor of this on-demand cueing are reduced intrusiveness and reduced habituation to cues. Here, we analyzed the effect of visual cues on the number and duration of freezing episodes when activated either just on-demand or continuously and compare it to the baseline condition where no cue is provided. For this purpose, 7 Parkinson's patients regularly suffering from freezing of gait repeatedly walked a pre-defined course and their reaction to parallel laser lines projected in front of them on the floor was analyzed. The results show that, in comparison to the baseline condition, the mean duration of freezing was reduced by 51% in continuous cueing and by 69% in on-demand cueing. Concerning the number of freezing episodes, 43% fewer episodes were observed for continuous cueing and 9% less episodes for on-demand cueing.Item Neurophysiology of robot-mediated training and therapy: A perspective for future use in clinical populations(2013) Turner, Duncan L.; Ramos-Murguialday, Ander; Birbaumer, Niels; Hoffmann, Ulrich; Luft, Andreas; Medical TechnologiesThe recovery of functional movements following injury to the central nervous system (CNS) is multifaceted and is accompanied by processes occurring in the injured and non-injured hemispheres of the brain or above/below a spinal cord lesion. The changes in the CNS are the consequence of functional and structural processes collectively termed neuroplasticity and these may occur spontaneously and/or be induced by movement practice. The neurophysiological mechanisms underlying such brain plasticity may take different forms in different types of injury, for example stroke vs. spinal cord injury (SCI). Recovery of movement can be enhanced by intensive, repetitive, variable, and rewarding motor practice. To this end, robots that enable or facilitate repetitive movements have been developed to assist recovery and rehabilitation. Here, we suggest that some elements of robot-mediated training such as assistance and perturbation may have the potential to enhance neuroplasticity. Together the elemental components for developing integrated robot-mediated training protocols may form part of a neurorehabilitation framework alongside those methods already employed by therapists. Robots could thus open up a wider choice of options for delivering movement rehabilitation grounded on the principles underpinning neuroplasticity in the human CNS.Item Stimulation Discomfort Comparison of Asynchronous and Synchronous Methods with Multi-Field Surface Electrodes(2013-06) Imatz-Ojanguren, Eukene; Hoffmann, Ulrich; Veneman, Jan; Malešević, Nebojša; Keller, Thierry; Tecnalia Research & InnovationFunctional Electrical Stimulation (FES) is a technique that artificially stimulates motor nerves in order to restore motor/sensory functions for assistive and therapeutic applications. Recently, multi-field surface electrodes for transcutaneous electrical stimulation have been suggested to overcome problems of single channel surface stimulation. This study compares sensation perceived by 15 healthy subjects on upper limb when two different stimulation methods are applied by means of multi-field electrodes. Asynchronous and synchronous stimulation methods are compared for four different cases: activation of two neighbor fields, three neighbor fields, two distant fields and three distant fields. Two descriptors rated from 1 to 5 are used to describe discomfort: superficial discomfort and deep discomfort. Results expressed no differences in superficial discomfort for any case, but showed significant differences in deep discomfort for distant field activations. In these cases, synchronous stimulation resulted in higher perceived deep discomfort than asynchronous stimulation and affected its efficacy.