Windows into the Living Brain
For most of history the working brain was a black box. Modern imaging techniques opened it, and they divide into two families: those that reveal structure (anatomy) and those that reveal function (activity).
| Technique | Measures | Reveals |
|---|---|---|
| EEG | Electrical activity via scalp electrodes | Function; excellent time resolution |
| CT | X-rays combined into a cross-section | Structure |
| MRI | Magnetic fields and radio waves | Structure; high detail |
| fMRI | Blood-oxygen changes over time | Function; good spatial resolution |
| PET | A radioactive tracer's uptake | Function (e.g. glucose use) |
The key trade-off is between temporal and spatial resolution. EEG records electrical activity through scalp electrodes with millisecond timing — it tells you when neurons fire with great precision, but is poor at pinpointing where. fMRI tracks blood-oxygen changes (the BOLD signal) and locates activity to within millimeters — it tells you where activity is with great precision, but lags by a second or more because it measures blood flow, not the firing itself. CT and MRI show structure (MRI in far finer detail than CT), while PET uses a radioactive tracer to reveal metabolic activity such as glucose consumption.
Common pitfall: treating fMRI as a direct measure of neural firing. It measures blood-oxygen changes that follow neural activity by a second or more — a proxy, not the electrical signal itself. That lag is exactly why fMRI has excellent spatial but poor temporal resolution.