Fig. 6: Experimental paradigm for mVEP speller.

a Example of “on-target” visual stimuli. The lines flash across the row or column of the letter the subject is looking at. b Example of “off-target” visual stimuli. The lines flash on a row or column not associated with the letter of interest. c The online BCI decoder pipeline. The raw EEG is down-sampled to 100 Hz, bandpass filtered between 1 and 20 Hz, common average referenced (CAR), and z-scored across channels. The processed data is fed to a Support Vector Machine (SVM) classifier, predicting the letter of intent based on neural activity for on and off-target stimuli. d (left) The 128-element 700 kHz fundamental frequency (f₀) tFUS pressure wave profile measured through a skull fragment in a water tank. (right) A visualization of the profiled 2 mm focal point scaled to a 93 mm brain and centered on V5 (red circle), as well as at an example tFUS-GP area 1.41 cm away (green x). e A typical trial epoch consists of 12 row/column 200 ms line flashes and four 500-ms tFUS sonications. The first sonication begins 100 ms prior to the first BCI stimulation. The ultrasound parameters used for this experiment were as follows: a customized 128-element 700 kHz f₀, pulsed for 140 cycles (cycles-per-pulse, CPP). Each sonication consists of 1500 pulses (pulse number, PN), repeated at a pulse-repetition frequency (PRF) of 3 kHz.