Introduction to Brain-Computer Interfaces. Saim Rasheed Dipartimento Di Informatica e Comunicazione Università Degli Studi Di Milano - PDF

Introduction to Brain-Computer Interfaces Saim Rasheed Dipartimento Di Informatica e Comunicazione Università Degli Studi Di Milano On the Way What is BCI? Different Brain s Regions Motor Map and Motor

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Introduction to Brain-Computer Interfaces Saim Rasheed Dipartimento Di Informatica e Comunicazione Università Degli Studi Di Milano On the Way What is BCI? Different Brain s Regions Motor Map and Motor Imagery Neuronal Structure Why EEG? Brain Rhythms Electrode Placement System Different kinds of Artefacts BCI Example Generic Model of BCI Feedback types and their Effects on classification BCI System Setup Conclusion 2 Background and Motivation Imagine being alert and aware of your environment, but unable to move, speak or express yourself! This condition is called locked in syndrome. Patients are truly locked in to their bodies. 3 Background and Motivation The term locked-in refers to a state in which individuals are conscious and alert, but unable to use their muscles and therefore cannot communicate their needs, wishes, and emotions: the healthy brain is locked into a paralyzed body. This condition motivates us to restore their communication of those individuals who have lost the ability to communicate by speech or other muscular activities. 4 Introduction - What is BCI? The promise of Brain-Computer Interfaces (BCI) technology is to augment human capabilities by enabling people to interact with a computer through their brainwaves after a short training period. BCI translate your brain s electrical activity into messages or commands. A BCI is like a closed loop system where information is visually fed back to the user. Total Slides 89 5 Introduction - What is BCI? For example, Healthy participants may move forward within a virtual environment (VE) by the imagination of foot movement It is achieved by using a brain-computer interface (BCI) that transforms thought-modulated electroencephalogram (EEG) recordings into a control signal. A BCI establishes a communication channel between the human brain and the computer. 6 Introduction What is BCI? The basic principle of the BCI is the detection and classification of motor-imagery-related EEG pattern. Remember that BCI cannot: Write to the brain Read your thoughts... 7 Human Brain 8 Responsibilities of each part Frontal Lobe- associated with reasoning, planning, parts of speech, movement, emotions, and problem solving Parietal Lobe- associated with movement, orientation, recognition, perception of stimuli Occipital Lobe- associated with visual processing Temporal Lobe- associated with perception and recognition of auditory stimuli, memory, and speech 9 Activation sequence for motor areas 10 Activation sequence for motor areas The basic function of the brain is to produce behaviours (set of movements produced by the muscles). Planning for any given movement is done in the forward portion of the frontal lobe. This part of the cortex receives information about the individual's current position from several other parts. Then it issues its commands, to Area 6. It decides which set of muscles to contract to achieve the required movement. which issues the corresponding orders to the Primary motor cortex. This area in turn activates specific muscles or groups of muscles via the motor neurons in the spinal cord. 11 Motor Map 12 Motor Map A Penfield homunculus is a physical representation of the primary motor cortex, i.e., the portion of the human brain directly responsible for the movement and exchange of sense and motor information (namely touch: sensitivity, cold, heat, pain etc.) of the rest of the body. The most striking aspect of this map is that the areas assigned to various body parts on the cortex are proportional not to their size, but rather to the complexity of the movements that they can perform. 13 Motor Map Hence, the areas for the hand and face are especially large compared with those for the rest of the body This is no surprise, because the speed and dexterity of human hand and mouth movements are precisely what give us two of our most distinctly human faculties: the ability to use tools and the ability to speak. 14 Motor Imagery It is broadly accepted that mental imagination of movements involves similar brain regions that are used in programming and preparing such movements. Motor Imagery defined as mental simulation of a movement an efficient mental strategy to operate a BCI. 15 Motor Imagery The imagination of different types of movements, for example, right hand, left hand, foot, or tongue movement, results in a characteristic change of the brain signals over the sensorimotor cortex. 16 Brain s Unit 17 Brain s Unit Cell body - This main part has all of the necessary components of the cell, such as the nucleus (contains DNA), endoplasmic reticulum and ribosomes (for building proteins) and mitochondria (for making energy). If the cell body dies, the neuron dies. Axon - This long, cable-like projection of the cell carries the electrochemical message (nerve impulse or action potential) along the length of the cell. Depending upon the type of neuron, axons can be covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed transmission of a nerve impulse down a long axon. Myelinated neurons are typically found in the peripheral nerves (sensory and motor neurons), while nonmyelinated neurons are found in the brain and spinal cord. Dendrites or nerve endings - These small, branch-like projections of the cell make connections to other cells as well as to other dendrite and axons. 18 Brain s Unit Synapses 19 Brain s Unit Neurons also vary with respect to their functions: Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system. Motor neurons (motorneurons) carry signals from the central nervous system to the outer parts (muscles, skin, glands) of your body. Receptors sense the environment (chemicals, light, sound, touch) and encode this information into electrochemical messages that are transmitted by sensory neurons. Interneurons connect various neurons within the brain and spinal cord. 20 Knee Experiment Reflex Action When the doctor taps the right spot on your knee with a rubber hammer, receptors send a signal into the spinal cord through a sensory neuron. The sensory neuron passes the message to a motor neuron that controls your leg muscles. Nerve impulses travel down the motor neuron and stimulate the appropriate leg muscle to contract 21 Along an Axon-Action Potential 22 Action Potential A stimulus is received by the dendrites of a nerve cell. This causes the Na+ channels to open. If the opening is sufficient to drive the interior potential from -70 mv up to -55 mv, the process continues. Having reached the action threshold, more Na+ channels (sometimes called voltage-gated channels) open. The Na+ influx drives the interior of the cell membrane up to about +30 mv. The process to this point is called depolarization. The Na+ channels close and the K+ channels open. Since the K+ channels are much slower to open, the depolarization has time to be completed. Having both Na+ and K+ channels open at the same time would drive the system toward neutrality and prevent the creation of the action potential. 23 Action Potential 24 Action Potential With the K+ channels open, the membrance begins to repolarize back toward its rest potential. The repolarization typically overshoots the rest potential to about -90 mv. This is called hyperpolarization and would seem to be counterproductive, but it is actually important in the transmission of information. Hyperpolarization prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new stimulus. Part of the importance of hyperpolarization is in preventing any stimulus already sent up an axon from triggering another action potential in the opposite direction. In other words, hyperpolarization assures that the signal is proceeding in one direction. After hyperpolarization, the Na+/K+ pump eventually brings the membrane back to its resting state of -70 mv. 25 Neuro Transmitters - Acetylcholine The chemical compound acetylcholine is one of the neurotransmitters in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans. 26 Average Thickness and Resistivities average skull thickness. men : 6.5 millimeters, women : 7.1 mm average front-to-back 176 mm for men 171 mm for women average width 145 mm for men 140 mm for women 27 Electroencephalogram Electroencephalogram (EEG) are the electrical signals that can be recorded from the brain, either directly or through the scalp by hooking up the electrodes. On the scalp the amplitudes commonly lie within μv A useful way of observing human brain activity A new communication channel. An Austrian psychiatrist named Hans Berger was the first to record this activity in humans, in the late 1920s. 28 Why do all BCIs use EEGs? Technology Electro-corticography (ECoG) Magneto-encephalography (MEG) Computed Tomography (CT) Single Photon Emission Computerized Tomography (SPECT) Positron Emission Tomography (PET) Magnetic Resonance Imaging (MRI) Functional Magnetic Resonance Imaging (fmri) Electroencephalogram (EEG), Electromyogram (EMG), Electroocculogram (EOG) Remarks Highly invasive, surgery Very expensive Only anatomical data Radiation Exposure Radiation Exposure Only anatomical data Extremely Expensive Non-invasive, relatively cheaper, No exposure to radiation, non-anatomical data, easily portable 29 Why do all BCIs use EEGs? Other approaches are not portable. They also require millions of Euro s of equipment and highly trained technician and are less universal. 30 Brain Rhythms EEG of human brain contains different types of oscillatory activities. The oscillations in the alpha and beta band are particularly important to use in discriminating between different brain states. DELTA HZ Deep dreamless sleep, in infants THETA 4 8 Hz Deep relaxation and meditation, mental imagery, amplitude usually greater than 20 μ V. ALPHA 8 13 Hz Usually found over occipital region of brain, indicate relaxed awareness without any attention or concentration, not thinking Amplitude is normally less then µv. 31 Brain Rhythms BETA Hz Appear in frontal and parietal region, Associated with active thinking, active attention, focus on the outside world or solving concrete problems or simply normal alert consciousness. GAMMA Hz Having very low amplitude and rarely occurs, detection of gamma waves can be used for confirmation of certain brain diseases. Mu-Rhytm associated with motor activities and maximally recorded over motor cortex. Mu wave is in the same frequency band as in the alpha wave 32 EEG Recording and Measurement The first electrical neural activities were registered using simple galvanometers. More recently EEG systems consist of a number of delicate electrodes. the effective bandwidth for EEG signals is limited to approximately 100 Hz. a minimum frequency of 200 samples/s is often enough for sampling the EEG signals In some applications where a higher resolution is required for representation of brain activities in the frequency domain, sampling frequencies of up to 2000 sample/s may be used. 33 EEG Recording and Measurement Representation of each signal sample with up to 16 bits is very popular for the EEG recording systems. A simple calculation shows that for a one hour recording from 128-electrode EEG signals sampled at 500 samples/s a memory size of Gbits 0.45 Gbyte is required. To enable a satisfactory recording the electrode impedances should read less than 5 k Ω and be balanced to within 1 kω of each other. 34 Types of electrodes disposable (gel-less, and pre-gelled types); reusable disc electrodes (gold, silver, stainless steel, or tin); headbands and electrode caps; saline-based electrodes; needle electrodes. 35 EEG Electrode Placement 36 EEG Electrode Placement Relationship between location of an electrode and the underlying area of cerebral cortex. the actual distances between adjacent electrodes are either 10% or 20% of the total front-back or right-left distance of the skull. The letters F, T, C, P and O stand for Frontal, Temporal, Central, Parietal and Occipital respectively. A z (zero) refers to an electrode placed on the midline. Even numbers refers to right hemisphere and odd numbers refers to left hemisphere. Nasion: point between the forehead and nose. Inion: Bump at back of skull. 37 21 Electrode Placement 38 75 Electrode Placement 39 Maudsley electrode positioning system In Maudsley electrode positioning system the outer electrodes are slightly lowered to enable better capturing of the required signals from epileptic foci in epileptic seizure recordings. 40 Artefacts Physiological Artefacts patient-related or internal artefacts are body movementrelated, EMG, ECG (and pulsation), EOG, ballistocardiogram, and sweating System Artefacts The system artefacts are 50/60 Hz power supply interference, impedance fluctuation, cable defects, electrical noise from the electronic components, and unbalanced impedances of the electrodes. 41 Artefact Waveforms 42 Event-related (de)synchronization ERD/ERS is the task-related or event-related change in the amplitude of the oscillatory behavior of specific cortical areas within various frequency bands. An amplitude (or power) increase is defined as event-related synchronization while an amplitude (or power) decrease is defined as event-related desynchronization. 43 Visual evoked potential The visual evoked potential (VEP) is the electrical response of the brain's primary visual cortex to a visual stimulus. 44 Steady state Visual evoked potentials SSVEPs occur when sensory stimuli are repetitively delivered at high enough rates so that the relevant neuronal structures are prevented to return to their resting states 45 P300 Potential The P300 is a positive evoked potential that appears 300 ms after presentation of an attended stimulus embedded in a sequence of irrelevant stimuli. Event-related potential (ERP) P300 auditory evoked potential measure reflects the speed of neural events related to attention and short term memory. 46 BCI Example A user views a matrix containing letters and other characters. This is an example of a matrix: 47 BCI Example One row or column is briefly flashed, then another, and so on until all rows and columns are flashed. The user counts each time a target (such as the letter K) flashes while ignoring other flashes. 48 BCI Example Your brain produced different EEGs in response to ignored vs. counted flashes. EEGs from a P300 BCI 6 voltage (uv) Nontargets Targets K K -4 time after flash begins (ms) 49 Earlier VR-BCI Research Based on two types of visually evoked responses. steady-state visual evoked potential event-related P300 potential In contrast, different BCI paradigm for VR control based on motor imagery. 50 Why Virtual Reality? If it is possible to show that people are able to control their movements through space within a VE, it would justify the much bigger expense of building physical devices like a robot arm controlled by a BCI. 51 Why Virtual Reality? It is a safe environment. It can be used to control the experience and reduce distractions. It can be highly motivating. VR as a feedback enhances the classification accuracy and reduces the time needed for BCI training sessions. 52 BCI Goals/Research Problems Analysis of the influence of different feedback types on performing the same task. Demonstration of how to move through the Virtual Environment, using imagination of foot/hand movement. No muscular activity should be involved. 53 Two Learning Systems What Machine should learn? discrimination between different patterns of brain activity as accurate as possible What user of BCI should learn? performing different mental tasks in order to produce distinct brain signals. If the subjects are well trained (say, classification accuracies above 90%) they can produce the same/distinct EEG patterns over a long/short period for the same/distinct tasks. 54 Basic Components of Brain-Computer Interface A BCI system is, in general, composed of the following components: signal acquisition, preprocessing, feature extraction, classification (detection), application interface 55 Generic Model for BCI 56 Signal acquisition The signal acquisition component is responsible for recording the electrophysiological signals and providing the input to the BCI. We have planned to use g.tec s MOBIlab+ for this purpose through non-invasive approach. 57 Pre-Processing Preprocessing includes artifact reduction (electrooculogram, EOG, and electromyogram, EMG), application of signal processing methods, that is, low-pass or highpass filter, methods to remove the influence of the line frequency and in the case of multichannel data the use of spatial filters (bipolar, Laplacian, common average reference) 58 Pre-Processing Eye blink Removal An eye blink or an eye movement is generally detected by a voltage increase above 100µV. Rejection method consist of discarding contaminated EEG, based on either automatic or visual detection. Their success crucially depends on the quality of the detection, and its use depends also on the specific application for which it is used 59 Pre-Processing Artefact Removal Discarding segments of EEG data with artifact can greatly decrease the amount of data available for analysis. 60 Pre-Processing Eye blink Removal Subtraction methods are based on the assumption that the measured EEG is a linear combination of an original EEG and a signal caused by eye movement, called EOG (electrooculogram). The EOG is a potential produced by movement of the eye or eyelid. The original EEG is recovered by subtracting separately recorded EOG from the measured EEG 61 Pre-Processing Artefact Removal Other approaches include: Principal component analysis, Independent component analysis, signal fraction analysis canonical correlation analysis. Common Spatial patterns 62 Feature Extraction The goal of this component is to find a suitable representation (signal features) of the electrophysiological data that simplifies the subsequent classification or detection of specific brain patterns. There are a variety of feature extraction methods used in BCI systems. A less than exhaustive list of these methods includes amplitude measures, band power, autoregressive parameters, and wavelets. 63 Classification The task of the classifier component is to use the signal features provided by the feature extractor to assign the recorded samples of the signal to a category of brain patterns. The classifier output, which can be a simple on-off signal or a signal that encodes a number of different classes, is transformed into an appropriate signal that can then be used to control a VR system. 64 BCI-Two Adapting Systems Without Feedback To acquire subject specific data for the used imaginations With Feedback Wrong feedback can elicit frustration, a response likely to be associated with a widespread EEG desynchronization. Correct feedback reinforces the desired EEG patterns. 65 Possibilities of Feedback Types Through conventional monitor e.g. simple moving bar graph. Through VE experiments with HMD. Through highly immersive projection environment e.g. CAVE. 66 What is BCI2000? BCI2000 is a general-purpose system for BCI research and development. It can also be used for data acquisition, stimulus presentation, or brain observation applications. BCI2000 is available free of charge for research purposes to academic and educational institutions (see for further information). g.tec is offering complete BCI2000 systems with optimally configured components. 67 Using BCI2000 with g.mobilab+ 68 Using BCI2000 with g.mobilab+ The portable device g.mobilab+ supports 8 EEG channels and sends the data via a serial cable or via Bluetooth to the computer system. The wireless transmission allows the subject to move freely in a range of about 30 meters. This enables the easy usage of the system over 36 hours with one battery set. BCI2000 allows to select the channels that should
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