Teaching model — not a real scan
How signals are captured
Follow the physical chain from neural activity to EEG, MEG, MRI and other sensors.
EEG · Electroencephalography
What it measures
A voltage difference at the scalp, usually tens of microvolts (µV).
- 01Aligned cortical pyramidal cells fire synchronous postsynaptic currents
- 02These currents act as tiny electrical dipoles
- 03Current spreads (volume conduction) through CSF, skull and scalp
- 04An electrode measures the resulting voltage vs. a reference
1–10 ms
Temporal resolution
~5–9 cm (19-ch); ~cm with high-density source imaging
Spatial resolution
Non-invasive (scalp contacts)
Invasiveness
high
Portability
$100–$2,000 consumer; $5k–$50k+ research (32–128 ch)
Rough cost
Limitations
- The skull smears the signal; source localization is non-unique (the inverse problem)
- Strong eye, muscle, movement, sweat and 50/60 Hz mains artifacts
Modality comparison
| Modality | Temporal resolution | Spatial resolution | Invasiveness | Portability | Signal path | Rough cost |
|---|---|---|---|---|---|---|
| EEG | 1–10 ms | ~5–9 cm (19-ch); ~cm with high-density source imaging | ○○ | high | Direct neural | $100–$2,000 consumer; $5k–$50k+ research (32–128 ch) |
| MEG | ~1 ms | ~3–10 mm (superficial); ~1–1.5 cm (deep) | ○○ | none | Direct neural | ~$3–5 million with shielded room; ~$1.5k–$5k per scan |
| fMRI | 1–3 s sampling; BOLD peaks ~5–6 s after activity | ~2–3 mm at 3T; sub-mm at high field | ○○ | none | Indirect / hemodynamic | $1.6–3M scanner; $500–$2,000+ per session |
| fNIRS | 0.1–1 s useful; responses evolve over seconds | ~1–3 cm laterally; only ~10–15 mm past the skull | ○○ | high | Indirect / hemodynamic | $15k–$150k research; hundreds–few $k for low-channel |
| ECoG | <1–5 ms | ~4–10 mm clinical; sub-mm with micro-ECoG | ●● | medium | Direct neural | $20k–$100k grid/amp; procedure many tens of $k+ |
| MEA | ~0.05–1 ms (spike timing) | ~50–150 µm around a tip | ●● | medium | Direct neural | $50k–$250k+ rig/arrays, excluding neurosurgery and long-term care |
| EMG | ~1 ms; sampling 1–5 kHz | Surface EMG: cm-scale detection volume | ○○ | high | Direct neural | $100–$2k education/wearable; $3k–$30k+ multichannel research |