The Cortical Structural Model Extends to Thalamocortical Connections

The structural model predicts the laminar patterns and strength of corticocortical connections. Here, we addressed whether the structural model extends to connections between the thalamus and prefrontal cortices, which are connected with the mediodorsal (MD) nucleus and with other thalamic nuclei. The prefrontal cortex is composed of a series of areas ranging from caudal orbital and medial limbic areas that have the simplest trilaminar architecture through successive areas that show increasing elaboration into six delineated layers. Here, we compiled detailed, quantitative tract-tracing data from connectivity studies of the thalamus and cortex in macaques, which revealed that the structural model extends to thalamocortical connections. The phylogenetically ancient limbic areas were more diffusely connected with thalamic nuclei, projected to the thalamus from canonical Layer VI, and also substantially from Layer V, and were innervated more broadly by thalamic pathways that terminated in the middle and other layers. The pattern of thalamocortical connections became increasingly sharper for prefrontal areas with progressive laminar differentiation, with decreasing contribution of thalamic nuclei besides MD, sharpening of thalamic terminations to the middle cortical layers, gradual decreasing contribution by Layer V, and increased projection from canonical Layer VI to the thalamus. These findings support the hypothesis that the structural model can be extended to the broad thalamic connections and laminar-specific interactions with the thalamus, tested in a series of prefrontal cortices with a gradual increase in laminar complexity.