title: Single-trial classification of motor imagery differing in task complexity: a functional near-infrared spectroscopy study.
abstract: BACKGROUND
  For brain computer interfaces (BCIs), which may be valuable in neurorehabilitation, brain signals derived from mental activation can be monitored by non-invasive methods, such as functional near-infrared spectroscopy (fNIRS). Single-trial classification is important for this purpose and this was the aim of the presented study. In particular, we aimed to investigate a combined approach: 1) offline single-trial classification of brain signals derived from a novel wireless fNIRS instrument; 2) to use motor imagery (MI) as mental task thereby discriminating between MI signals in response to different tasks complexities, i.e. simple and complex MI tasks.
  
  
  METHODS
  12 subjects were asked to imagine either a simple finger-tapping task using their right thumb or a complex sequential finger-tapping task using all fingers of their right hand. fNIRS was recorded over secondary motor areas of the contralateral hemisphere. Using Fisher's linear discriminant analysis (FLDA) and cross validation, we selected for each subject a best-performing feature combination consisting of 1) one out of three channel, 2) an analysis time interval ranging from 5-15 s after stimulation onset and 3) up to four Δ[O2Hb] signal features (Δ[O2Hb] mean signal amplitudes, variance, skewness and kurtosis).
  
  
  RESULTS
  The results of our single-trial classification showed that using the simple combination set of channels, time intervals and up to four Δ[O2Hb] signal features comprising Δ[O2Hb] mean signal amplitudes, variance, skewness and kurtosis, it was possible to discriminate single-trials of MI tasks differing in complexity, i.e. simple versus complex tasks (inter-task paired t-test p ≤ 0.001), over secondary motor areas with an average classification accuracy of 81%.
  
  
  CONCLUSIONS
  Although the classification accuracies look promising they are nevertheless subject of considerable subject-to-subject variability. In the discussion we address each of these aspects, their limitations for future approaches in single-trial classification and their relevance for neurorehabilitation.
authors.affiliation: Biomedical Optics Research Laboratory (BORL), Division of Neonatology, Department of Obstetrics and Gynecology, University Hospital Zurich, Zurich, Switzerland. holper@ini.phys.ethz.ch
authors.forename: Lisa
authors.initials: L
authors.lastname: Holper
authors.forename: Martin
authors.initials: M
authors.lastname: Wolf
mesh_terms: D000328:Adult; D001921:Brain / Q000502:physiology*; D003627:Data Interpretation, Statistical; D016002:Discriminant Analysis; D004576:Electromyography; D005260:Female; D005385:Fingers / Q000502:physiology; D007839:Functional Laterality; D006439:Hemodynamics / Q000502:physiology; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D007092:Imagination / Q000502:physiology*; D007699:Kinesthesis; D016014:Linear Models; D008297:Male; D009068:Movement / Q000502:physiology*; D009422:Nervous System Diseases / Q000534:rehabilitation; D011597:Psychomotor Performance / Q000502:physiology; D012046:Rehabilitation / Q000295:instrumentation*; D015203:Reproducibility of Results; D019265:Spectroscopy, Near-Infrared; D014584:User-Computer Interface
chemical_list: D006454:Hemoglobins