A conserved pulvinar projection to the amygdala revealed in macaque monkeys (Macaca mulatta)

Understanding the organization and function of thalamic pulvinar projections to the amygdala is of interest due to the proposal that this projection provides the amygdala with short-latency visual sensory input that eludes conscious awareness. However, most reports in primates have emphasized a projection from the multimodal medial pulvinar-a pulvinar division unique to primates-versus projections from visual pulvinar divisions (inferior or lateral). Further, reports in other closely related species such as tree shrews and rodents have yielded inconsistent results relative to primates when homology is considered. In these species, subdivisions of the lateral posterior/pulvinar complex, which are homologous to the visual inferior pulvinar of primates, project to the amygdala. Such a difference in pulvino-amygdala connections across these closely related species would be surprising. However, modern methods that reveal subdivisions of the pulvinar were lacking in previous anatomical studies of primate pulvino-amygdala connections. To better understand whether a major shift in pulvino-amygdala projections is truly present across species, we reevaluated the locations of pulvinar neurons projecting to the amygdala in rhesus monkeys (Macaca mulatta) using robust anatomical markers for delineating divisions of the pulvinar, and specifically highlighting the border between the medial and inferior pulvinar. Our findings show definitively that pulvino-amygdala projections in macaques share both conserved, via the visual inferior pulvinar, and novel, via the medial pulvinar, profiles. Further, our data provide a refinement in the available routes via which visual information could reach the amygdala, one that includes the inferior pulvinar nucleus of the thalamus.