PMC3246120_01
A) The 6x6 matrix used in the current study. A row or column flashes for 100 ms every 175 ms. The letter in parentheses at the top of the window is the current target character "D." A P300 should be elicited when the fourth column or first row is flashed. After 15 flash sequences, the collected brain responses are processed, classified, and online feedback is provided directly below the character to be copied. The process is then repeated for the subsequent target characters. B) Post-implant brain MRI. T2-weighted coronal sections through anterior, middle and posterior temporal lobe (from left to right) demonstrating the intraventricular location of stereotactic depth electrode in the left temporal lobe and the intraparenchymal location of the stereotactic depth electrode in the right temporal lobe (encircled in yellow).
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F1
The distal contacts lie in contact with the ventricular surface of the left hippocampal formation (Figure 1B). All data were collected in the copy speller mode: words were presented on the top left of the video monitor and the character currently specified for selection was listed in parentheses at the end of the letter string as shown in Figure 1A.
The ERPs and the respective r2 correlations with the task are plotted in the periphery, ordered corresponding to the provided axial viewpoint representing the approximate relative electrode positions (note that for the purposes of the illustration the left hemisphere electrodes are not shown in the ventricle). These waveforms represent the average responses to the target (blue) and non-target (red) stimuli, with 0 ms indicating the stimulus onset. Note that the four most posterior responses in the right hemisphere are plotted on different scales for emphasis. The classification accuracy after 15 flash sequences for subsets of electrodes in the left and right hemispheres is provided in the bar graphs. The blue bars indicate the accuracy using individual common referenced electrodes ordered from anterior to posterior. The intermediate red bars indicate the accuracy using adjacent electrode pairs ordered in the same fashion. The green dashed line indicates the accuracy using all 8 electrodes within a given hemisphere (left: 84.4%, right: 93.8%), and the solid black line indicates the accuracy using all 16 electrodes (96.9%), which also represents the accuracy achieved during the online experiments. Chance accuracy for the task is 2.8%.
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F2
This amplitude discrepancy was also observed during the BCI sessions as shown by the averaged responses provided in Figure 2. The ERPs from all electrodes and their r2 correlations (i.e. the proportion of the variance of the instantaneous signal amplitude accounted for by the stimulus type, i.e., target or non-target) with the task are presented in Figure 2.