@article {3142, title = {EEG-based communication and control: speed-accuracy relationships.}, journal = {Applied psychophysiology and biofeedback}, volume = {28}, year = {2003}, month = {09/2003}, pages = {217{\textendash}231}, abstract = {People can learn to control mu (8-12 Hz) or beta (18-25 Hz) rhythm amplitude in the EEG recorded over sensorimotor cortex and use it to move a cursor to a target on a video screen. In our current EEG-based brain-computer interface (BCI) system, cursor movement is a linear function of mu or beta rhythm amplitude. In order to maximize the participant{\textquoteright}s control over the direction of cursor movement, the intercept in this equation is kept equal to the mean amplitude of recent performance. Selection of the optimal slope, or gain, which determines the magnitude of the individual cursor movements, is a more difficult problem. This study examined the relationship between gain and accuracy in a 1-dimensional EEG-based cursor movement task in which individuals select among 2 or more choices by holding the cursor at the desired choice for a fixed period of time (i.e., the dwell time). With 4 targets arranged in a vertical column on the screen, large gains favored the end targets whereas smaller gains favored the central targets. In addition, manipulating gain and dwell time within participants produces results that are in agreement with simulations based on a simple theoretical model of performance. Optimal performance occurs when correct selection of targets is uniform across position. Thus, it is desirable to remove any trend in the function relating accuracy to target position. We evaluated a controller that is designed to minimize the linear and quadratic trends in the accuracy with which participants hit the 4 targets. These results indicate that gain should be adjusted to the individual participants, and suggest that continual online gain adaptation could increase the speed and accuracy of EEG-based cursor control.}, keywords = {Video Recording}, issn = {1090-0586}, doi = {10.1023/A:1024685214655}, url = {http://www.ncbi.nlm.nih.gov/pubmed/12964453}, author = {Dennis J. McFarland and Jonathan Wolpaw} } @article {2119, title = {Quantification of effects of global ischemia on dynamics of ventricular fibrillation in isolated rabbit heart.}, journal = {Circulation}, volume = {98}, year = {1998}, month = {10/1998}, pages = {1688-96}, abstract = {

BACKGROUND:\ 

Ventricular fibrillation (VF) leads to global ischemia of the heart. After 1 to 2 minutes of onset, the VF rate decreases and appears more organized. The objectives of this study were to determine the effects of no-flow global ischemia on nonlinear wave dynamics and establish the mechanism of ischemia-induced slowing of the VF rate.

METHODS AND RESULTS:\ 

Activation patterns of VF in the Langendorff-perfused rabbit heart were studied with the use of 2 protocols: (1) 15 minutes of no-flow global ischemia followed by reperfusion (n=7) and (2) decreased excitability induced by perfusion with 5 micromol/L of tetrodotoxin (TTX) followed by washout (n=3). Video imaging ( approximately 7500 pixels per frame; 240 frames per second) with a voltage-sensitive dye, ECG, and signal processing (fast Fourier transform) were used for analysis. The dominant frequency of VF decreased from 13.5+/-1.3 during control to 9.3+/-1.4 Hz at 5 minutes of global ischemia (P\<0.02). The dominant frequency decreased from 13.9+/-1.1 during control to 7.0+/-0.3 Hz at 2 minutes of TTX infusion (P\<0.001). The rotation period of rotors on the epicardial surface (n=27) strongly correlated with the inverse dominant frequency of the corresponding episode of VF (R2=0. 93). The core area, measured for 27 transiently appearing rotors, was 5.3+/-0.7 mm2 during control. A remarkable increase in core area was observed both during global ischemia (13.6+/-1.7 mm2; P\<0.001) and TTX perfusion (16.8+/-3.6 mm2; P\<0.001). Density of wave fronts decreased during both global ischemia (P\<0.002) and TTX perfusion (P\<0.002) compared with control.

CONCLUSIONS:\ 

This study suggests that rotating spiral waves are most likely the underlying mechanism of VF and contribute to its frequency content. Ischemia-induced decrease in the VF rate results from an increase in the rotation period of spiral waves that occurs secondary to an increase in their core area. Remarkably, similar findings in the TTX protocol suggest that reduced excitability during ischemia is an important underlying mechanism for the changes seen.

}, keywords = {Animals, Electrocardiography, Linear Models, Myocardial Ischemia, Rabbits, Rotation, Sodium Channel Blockers, Tetrodotoxin, Ventricular Fibrillation, Video Recording}, issn = {0009-7322}, doi = {10.1161/01.CIR.98.16.1688}, url = {http://www.ncbi.nlm.nih.gov/pubmed/9778336}, author = {Mandapati, R and Asano, Y and Baxter, Bill and Gray, R A and Davidenko, J M and Jalife, J} } @article {2118, title = {Technical features of a CCD video camera system to record cardiac fluorescence data.}, journal = {Ann Biomed Eng}, volume = {25}, year = {1997}, month = {07/1997}, pages = {713-25}, abstract = {

A charge-coupled device (CCD) camera was used to acquire movies of transmembrane activity from thin slices of sheep ventricular\ epicardial\ muscle stained with a voltage-sensitive dye. Compared with photodiodes, CCDs have high spatial resolution, but low temporal resolution. Spatial resolution in our system ranged from 0.04 to 0.14 mm/pixel; the acquisition rate was 60, 120, or 240 frames/sec. Propagating waves were readily visualized after subtraction of a background image. The optical signal had an amplitude of 1 to 6 gray levels, with signal-to-noise ratios between 1.5 and 4.4. Because CCD cameras integrate light over the frame interval, moving objects, including propagating waves, are blurred in the resulting movies. A computer model of such an integrating\ imaging\ system was developed to study the effects of blur, noise, filtering, and quantization on the ability to measure conduction velocity and action potential duration (APD). The model indicated that blurring, filtering, and quantization do not affect the ability to localize wave fronts in the optical data (i.e., no systematic error in determining spatial position), but noise does increase the uncertainty of the measurements. The model also showed that the low frame rates of the CCD camera introduced a systematic error in the calculation of APD: for cutoff levels \> 50\%, the APD was erroneously long. Both noise and quantization increased the uncertainty in the APD measurements. The optical measures of conduction velocity were not significantly different from those measured simultaneously with microelectrodes. Optical APDs, however, were longer than the electrically recorded APDs. This APD error could be reduced by using the 50\% cutoff level and the fastest frame rate possible.

}, keywords = {Action Potentials, Algorithms, Animals, Body Surface Potential Mapping, Calibration, Computer Simulation, Electric Conductivity, Fluorescent Dyes, Image Processing, Computer-Assisted, Models, Cardiovascular, Sheep, Ventricular Function, Video Recording}, issn = {0090-6964}, doi = {10.1007/BF02684848}, url = {http://www.ncbi.nlm.nih.gov/pubmed/9236983}, author = {Baxter, Bill and Davidenko, J M and Loew, L M and Wuskell, J P and Jalife, J} }