  Simultaneously measuring the activities of all neurons in a mammalian brain
at millisecond resolution is a challenge beyond the limits of existing
techniques in neuroscience. Entirely new approaches may be required, motivating
an analysis of the fundamental physical constraints on the problem. We outline
the physical principles governing brain activity mapping using optical,
electrical,magnetic resonance, and molecular modalities of neural recording.
Focusing on the mouse brain, we analyze the scalability of each method,
concentrating on the limitations imposed by spatiotemporal resolution, energy
dissipation, and volume displacement. We also study the physics of powering and
communicating with microscale devices embedded in brain tissue.
