Cortical Layers

The neocortex is organized into six horizontal layers (I-VI), each with distinct cell types, connectivity patterns, and functional roles. Understanding cortical laminar structure is essential for interpreting multi-site recordings.

Layer characteristics:

• Layer I: Mostly dendrites and axons, few cell bodies
• Layers II/III: Small pyramidal cells, local and intracortical connections
• Layer IV: Granule cells, main target of thalamic input (sensory cortex)
• Layer V: Large pyramidal cells, main output to subcortical targets
• Layer VI: Corticothalamic feedback, modulatory functions

Recording considerations: Different layers show distinct activity patterns and serve different computational roles. Laminar probes (sites spanning multiple layers) reveal layer-specific processing and information flow.

Depth structure: In rodents, full cortical depth is ~1-2 mm. In primates, it can exceed 3 mm. Site spacing of 50-100 µm provides good layer resolution.

Why it matters for probe selection: If you're interested in layer-specific processing, input-output transformations, or cortical microcircuits, you need sufficient recording span and site density to sample multiple layers simultaneously.