Noninvasive signal modality directly from the cortex are certainly possible (e.g. Ultrasound, fNIR). But they lack the spatial and temporal resolution of implan

Noninvasive signal modality directly from the cortex are certainly possible (e.g. Ultrasound, fNIR). But they lack the spatial and temporal resolution of implanted electrodes. Intracortical brain signals are the most well understood and mature in terms of instrumentation as well. So in the short term, implanted electrodes offer the highest capacity for a BCI.