# A strong direct link from the layer 3/4 border to layer 6 of cat primary visual cortex

## TL;DR

This study identifies a new neural pathway in the cat's visual cortex that connects layer 3/4 to layer 6, filling a gap in understanding how visual information is processed.

## Contribution

The discovery of a previously unidentified layer 3/4 border neuron population projecting strongly to layer 6 in the cat's visual cortex.

## Key findings

- Over 80% of neurons projecting to layer 6 are located within V1 itself.
- Layer 3/4 border neurons provide a significant portion of the missing input to layer 6.
- This connection may support the fast-conducting Y pathway and integrate distant visual field inputs.

## Abstract

The cat primary visual cortex (V1) is a cortical area for which we have one of the most detailed estimates of the connection ‘weights’ (expressed as number of synapses) between different neural populations in different layers (Binzegger et al in J Neurosci 24:8441–8453, 2004). Nevertheless, the majority of excitatory input sources to layer 6, the deepest layer in a local translaminar excitatory feedforward loop, was not accounted for by the known neuron types used to generate the quantitative Binzegger diagram. We aimed to fill this gap by using a retrograde tracer that would label neural cell bodies in and outside V1 that directly connect to layer 6 of V1. We found that more than 80% of labeled neurons projecting to layer 6 were within V1 itself. Our data indicate that a substantial fraction of the missing input is provided by a previously unidentified population of layer 3/4 border neurons, laterally distributed and connecting more strongly to layer 6 than the typical superficial layer pyramidal neurons considered by Binzegger et al. (Binzegger et al in J Neurosci 24:8441–8453, 2004). This layer 3/4 to layer 6 connection may be a parallel route to the layer 3 – layer 5 – layer 6 feedforward pathway, be associated with the fast-conducting, movement-related Y pathway and provide convergent input from distant (5–10 degrees) regions of the visual field.

The online version contains supplementary material available at 10.1007/s00429-024-02806-3.