Neural Signals, Recording Modalities, and Biophysics
Biophysical Origins of Signals
Neural Sources
Recorded signals reflect transmembrane currents, synaptic potentials, action potentials, and field effects generated by populations of neurons. The observable voltage at a sensor is shaped by volume conduction, tissue conductivity, electrode geometry, and filtering by skull and meninges.
Spatial and Temporal Scales
Single-unit recordings emphasize millisecond-scale spiking from small neural populations, while EEG reflects broader synchronized activity filtered through the head volume conductor.
Common Modalities
| Modality | Strengths | Limitations |
|---|---|---|
| EEG | Low cost, good temporal resolution | Low spatial resolution, artifact-prone |
| MEG | Excellent temporal resolution, good source modeling | Expensive, constrained environments |
| ECoG | Higher SNR than scalp EEG, useful for decoding | Invasive, limited cortical coverage |
| Intracortical arrays | High information bandwidth | Invasive, tissue response, longevity challenges |
Which modality generally provides the highest recording bandwidth for fine motor decoding?
Intracortical microelectrodes can capture high-resolution spiking and local field activity, giving high bandwidth for decoding.
Correct answer: Intracortical microelectrodes
Why is scalp EEG more spatially blurred than intracortical recording?
The scalp sensor sees a spatially mixed version of neural activity after propagation through conductive layers.
Correct answer: Because the skull and tissues volume-conduct and spatially smooth the signals
Artifacts and Practical Constraints
Noise Sources
Advanced systems must address line noise, motion artifacts, EMG contamination, ocular artifacts, amplifier drift, stimulation artifacts, and nonstationary electrode impedance. Good experimental design often matters as much as the decoder itself.
Which is a common source of contamination in scalp EEG during movement tasks?
EMG from facial and neck muscles frequently contaminates EEG during movement.
Correct answer: Electromyographic artifact
Give one reason intracortical recordings may degrade over time.
Other acceptable answers include encapsulation, gliosis, micromotion, or electrode corrosion.
Correct answer: Foreign body response