Contrasting Patterns of Cortical Input to Architectural Subdivisions of the Area 8 Complex: A Retrograde Tracing Study in Marmoset Monkeys

Abstract

Contemporary studies recognize 3 distinct cytoarchitectural and functional areas within the Brodmann area 8 complex, in the caudal prefrontal cortex: 8b, 8aD, and 8aV. Here, we report on the quantitative characteristics of the cortical projections to these areas, using injections of fluorescent tracers in marmoset monkeys. Area 8b was distinct from both 8aD and 8aV due to its connections with medial prefrontal, anterior cingulate, superior temporal polysensory, and ventral midline/retrosplenial areas. In contrast, areas 8aD and 8aV received the bulk of the projections from posterior parietal cortex and dorsal midline areas. In the frontal lobe, area 8aV received projections primarily from ventrolateral areas, while both 8aD and 8b received dense inputs from areas on the dorsolateral surface. Whereas area 8aD received the most significant auditory projections, these were relatively sparse, in comparison with those previously reported in macaques. Finally, area 8aV was distinct from both 8aD and 8b by virtue of its widespread input from the extrastriate visual areas. These results are compatible with a homologous organization of the prefrontal cortex in New and Old World monkeys, and suggest significant parallels between the present pathways, revealed by tract-tracing, and networks revealed by functional connectivity analysis in Old World monkeys and humans.

Towards a comprehensive atlas of cortical connections in a primate brain: Mapping tracer injection studies of the common marmoset into a reference digital template

“…We chose these cases for the present analyses based on the quality of the injections and cell labeling (e.g. no involvement of white matter, and robust long-distance label including the thalamus), and because they had already been independently analyzed in detail by traditional neuroanatomical methods, thus providing a valid basis for comparison between the results of the expert-based and automated procedures Reser et al, 2013 ). Nisslstained images of sections from each case, together with the positions of injection sites and retrogradely labeled neurons, are publicly available through the Marmoset Brain Architecture web site (http://marmoset.braincircuits.org).…”

“…Fluorescence microscopy photomicrographs of the types of injection sites and quality of the cell filling obtained with these methods are shown in recent publications from our laboratory (e.g. Palmer and Rosa, 2006 ;Burman et al, 2011Burman et al, , 2015Reser et al, 2013 ). Estimates of injection extent for each case, drawn under microscopic examination, are available through the project's web site.…”

“…One in five sections was plotted under fluorescence microscope. Nissl staining was performed in sections adjacent to those used for fluorescence microscopy, as detailed in Burman et al (2011) and Reser et al (2013) . Although other stains (myelin, cytochrome oxidase and, in some animals, calbindin)…”

“…Additionally, normalized thickness was calculated so that 0 corresponds to the pial surface and 1 -to the border between the white matter and the gray matter. A surface defined by points of normalized cortical thickness equal to 0.5 was extracted (the mid-thickness surface) and converted to a flat map using the free, open-source, software package CARET (Computerized Anatomical Reconstruction and Editing Toolkit, Van Essen et al, 2001 ; http://brainvis.wustl.edu/wiki/index.php/Caret:About) and the approach described previously ( Reser et al, 2013 ;Burman et al, 2015 ).…”

“…The third method was to compare the percentage of cells per cortical area obtained by the automated mapping versus the expertbased cell count established by neuroanatomists, as reported previously in a tabular form (e.g. Burman et al, 2011 ;Reser et al, 2013 ). The numbers of cells per single cortical area were grouped to match the reference data; for example, in the present cases several small auditory areas were grouped as a supergroup "core and belt", due to low cell counts (e.g.…”