Controlling, Mapping, and Modeling Neural Circuits

## Measuring activity **Electrophysiology** records the electrical signals of neurons directly. Electrodes detect **spikes** (action potentials) with very high **temporal resolution** — individual spikes on a millisecond scale. **Optical methods** use light. **Calcium indicators** are engineered proteins that fluoresce more brightly when calcium rises inside an active neuron. With a microscope, many identified cells can be imaged at once. **Fiber photometry** uses an implanted optical fiber to collect the summed fluorescence of a genetically defined population — a **population signal** rather than single cells. ## Trade-offs - **Single-cell versus population resolution:** imaging and some electrodes resolve individual neurons; photometry reports a population average. - **Temporal resolution:** electrophysiology tracks individual spikes; calcium signals are slower and smooth over rapid firing. - **Spatial resolution:** imaging shows where each cell is; electrodes sample neurons near their tips, with less certainty about cell identity unless combined with tagging. No method is best for everything. Researchers choose — or combine — methods based on the question. ## Aligning activity with behavior To ask what a signal means, researchers align neural activity with behavior on a shared timeline: cue onset, movement, choice, reward. A neuron that rises before a lever press looks like it is involved in the decision. But alignment is still correlation. A signal tied to behavior might reflect: - **sensory input** (the cue itself), - **movement** (the press), - **internal state** (arousal, hunger), - **decision-making**, or - the **consequence** (reward or its absence). Careful task design separates these — for example, by varying cues and movements independently — and perturbation experiments test which signals matter for the behavior. ## The course framework GENES → CELL TYPES → CIRCUITS → NETWORK DYNAMICS → BEHAVIOR, studied by four complementary approaches: **map**, **record**, **perturb**, and **model**. Understanding a neural circuit requires several of these ways of observing and testing the same biological system. ## Further reading - [Deisseroth Lab (Stanford)](https://web.stanford.edu/group/dlab/) - Developmental xenocortication using human-derived organoids in mice. *Nature* (2026) — collaborative work by multiple Stanford groups and investigators.