Connection from cortical area V2 to V3A in macaque monkey

Abstract

The V2 projection to V3A was labeled by pressure microinjecting biotinylated dextran amine (BDA) and Phaseolus vulgaris lectin (PHA‐L) into V2 just posterior to the lunate sulcus. Dense terminal labeling in clusters was found in layer 4, with a weaker terminal projection in layer 3. About 3.5–4.1% of the synapses in the densest bouton clusters in layer 4 were made by labeled boutons. All were asymmetric (Gray's type 1) synapses, made by spiny, excitatory neurons. The most frequently encountered synaptic targets were spines (76% in layer 4, 98% in layer 2/3). The remainder of the synaptic targets were dendritic shafts, of which just less than half (44%) had the characteristic ultrastructure of smooth (inhibitory) cells. Multisynaptic boutons were rare (mean synapses per bouton for layer 4 1.2, for layer 2/3 1.1). The mean size of the postsynaptic densities found on spines (0.11 μm²) was not significantly different from that for dendrites (0.09 μm²). In terms of their type, laminar location, number, and targets, the synapses that formed the V2 projection to V3A are typical of a major, excitatory, feedforward projection of macaque visual cortex. J. Comp. Neurol. 488:320–330, 2005.