• The spatial distribution of directed attention is coordinated by a large-scale neural network. The three principal cortical components of this network are located in the region of the frontal eye fields, posterior parietal cortex, and the cingulate cortex. We injected a retrogradely transported fluorescent dye into the frontal eye fields and another into the posterior parietal cortex of the monkey brain. Large numbers of neurons in the cingulate cortex were retrogradely labeled with each of the two fluorescent dyes. The two types of retrogradely labeled neurons were extensively intermingled, but neurons labeled with both tracers constituted less than 1% of retrogradely labeled cingulate neurons. Other cortical areas that contained retrograde neuronal labeling included the premotor, lateral prefrontal, orbitofrontal, opercular, posterior parietal, lateral temporal, inferior temporal, parahippocampal, and insular regions. These areas contained neurons labeled with each of the two dyes but virtually no neurons labeled with both. In the thalamus, retrogradely labeled nuclei failed to display evidence of double labeling. The overlap between the two populations of retrogradely labeled neurons was far more extensive at the cortical than at the thalamic level. These observations show that cortical and thalamic projections to the frontal eye fields and posterior parietal cortex do not represent axonal collaterals of single neurons but originate from two distinct and partially overlapping populations of neurons.
Morecraft RJ, Geula C, Mesulam M. Architecture of Connectivity Within a Cingulo-Fronto-Parietal Neurocognitive Network for Directed Attention. Arch Neurol. 1993;50(3):279-284. doi:10.1001/archneur.1993.00540030045013