Dynamic, minimally invasive electrical brain stimulation improves speech perception

Electrical brain stimulation (EBS) has been studied as a tool to improve speech perception. Available EBS methods compromise between precision and power and rarely consider temporal dynamics. Noninvasive EBS delivers currents to wide areas but may lack regional specificity and is susceptible to current shunting through the skin. We introduce a novel minimally-invasive electrical brain stimulation approach using clinically implanted epicranial electrodes and cranial bolts, circumventing the current shunting limitations of traditional transcranial stimulation. Two types of contacts were used in five epilepsy patients (one female) over six experimental sessions undergoing intracranial monitoring: epicranial electrodes (served as reference electrodes for clinical recordings) and cranial bolts (holding fixtures for depth electrodes). Participants monaurally listened to pseudo-randomly presented sentences from matrix sentence speech-in-noise task in 3 conditions: no-stimulation, 50 ms, or 200 ms stimulation lag relative to sentence onset. EBS parameters were 100 Hz, biphasic, amplitude-balanced, square-wave pulses where the amplitude (1-6 mA) was modulated by the ongoing speech envelope. Accuracy in the task improved in 5 of 6 sessions while not reaching significance across sessions possibly limited due to the limited number of sessions and trials (p = 0.087 and 0.068 for 50 ms and 200 ms delay, linear mixed-effects model). In 5/6 sessions, there was increased accuracy with 50 ms delay, and in 3 of those, there was further improvement with 200 ms delay. We successfully demonstrated the dynamic EBS patterns delivered through minimally-invasive electrodes during speech perception with 83% responder rate for improvement in speech perception with short latency (50 ms) and 50% with longer latency (200 ms) stimulation.