# Extracellular stimulation in neocortex lacks specificity

## TL;DR

Extracellular stimulation in the neocortex can unintentionally activate multiple pathways, making it less precise than intended.

## Contribution

The study demonstrates antidromic activation of pyramidal cells during extracellular stimulation, revealing a lack of specificity.

## Key findings

- L2/3 pyramidal cells responded to L4 stimulation even after synaptic blockade, indicating antidromic activation.
- Extracellular stimulation in L4 activates both ascending and descending pathways, mixing different types of neural responses.

## Abstract

By activating axons, extracellular stimulation elicits synaptic responses in patch-clamped cells. However, because axonal signaling is both orthodromic and antidromic, extracellular stimulation may lack specificity. For example, stimulating neocortical layer (L) 4 to recruit ascending inputs to L2/3 pyramidal cells (PCs) may also antidromically activate presynaptic L2/3 PCs, as their descending axons traverse L4. This contaminates L4→L2/3 activation with L2/3→L2/3 responses, which is problematic since these pathways have different properties. Using 2-photon calcium imaging, we found L2/3 PCs that responded to L4 stimulation even after synaptic blockade, demonstrating antidromic activation. Our findings highlight limitations of extracellular stimulation specificity in neocortex.