Whole-brain activity mapping
The aspiration to record every neuron in an entire brain at single-cell, single-spike resolution — the motivating vision behind initiatives such as the US BRAIN Initiative's original brain activity map proposal. It is genuinely achieved today only in small, transparent model organisms: light-sheet calcium imaging captures nearly the whole brain of a larval zebrafish (on the order of 10^5 neurons), with comparable coverage for C. elegans and Drosophila larvae.
For the mammalian brain the gap is many orders of magnitude — a mouse has on the order of 10^8 and a human 10^11 neurons — and no current or near-term technology approaches brain-wide single-neuron electrophysiology in a mammal. Honest discussion separates the demonstrated small-organism results from the mammalian aspiration, and notes that calcium imaging trades temporal resolution and depth for coverage.
Light-sheet microscopy captures calcium activity across nearly the entire brain of a larval zebrafish, on the order of 100,000 neurons, at cellular resolution — a genuine whole-brain recording only because the animal is tiny and transparent.
Whole-brain today means a millimetre-scale, see-through brain.
Recording activity is not the same as recording the connectome or the underlying causes; a brain-wide activity movie still needs models to become mechanistic understanding.