The study protocol outlines fundamental experiments to understand the neural dynamics underlying human memory and use direct brain stimulation as a tool (intervention) to study the relationship between those dynamics and memory performance. The investigators will collect recording and stimulation data from 250 patient volunteers as they perform carefully-matched verbal and spatial memory tasks. During non-stimulation sessions, the investigators will measure correlative neural biomarkers of memory encoding and retrieval using standard clinical depth electrodes and micro-wire recordings. To test the causal role of these biomarkers, the investigators will employ direct brain stimulation to briefly modulate neural activity, and measure ensuing changes in behavioral performance. With a set of causal biomarkers and predictive models in hand, the investigators will finally ask whether model-driven stimulation paradigms offer the investigators the ability to reliably modulate neural activity, and consequent behavior, in real-time.