THE FUNCTIONAL ROLE OF ANATOMICAL FEEDBACK CONNECTIONS IN VISUAL ATTENTION
Entrenched in a dense, highly connected network, cortical neurons receive heterogeneous inputs from diverse connections, including local feedforward and feedback signals. Over the years, neuroscientists have focused on feedforward and local connections, amassing a wealth of information on the purpose of these signals. However, the functional role of feedback connections remains a mystery, despite their sheer abundance and prevalence when compared to feedforward connections. The field has long hypothesized that feedback acts as a conduit for attentional signals, carrying them throughout the brain. Previous work on this topic has been variable. Though cognitively demanding neural processes, like attention, have traditionally been studied in nonhuman primate models, the available methodologies have made it difficult to examine. On the other hand, work with mouse models has utilized modern methods, and studies have suggested that other top-down signals may be transported by anatomical feedback connections. In spite of this promising evidence, mouse findings have not always translated well to humans, presumably due to the vastly different anatomy between the two model systems. Recent technological advancements now allow nonhuman primate researchers to leverage the optogenetic methods commonly used in mouse models, providing an experimental design to specifically test the functional role of feedback in visual attention in an appropriate model system. Here, we show that feedback connections carry attentional signals through the brain, supporting a long-asserted view of the field. Although feedback has anatomically distinct laminar targets, the limited number of studies investigating any layer-specific changes have not reported any differences across the laminae. Here, we find that feedback-related effects predominantly occur in the supragranular layers of the feedback recipient area, the preferential target of feedback synapses. Finally, the current literature has neglected to determine if any feedback-specific changes propagate to feedforward areas. Here, we show that these feedback-mediated attentional signals can be identified in hierarchically higher areas. Together, these results support a decades old theory presumed by the field, and provide novel evidence about how feedback effects the laminae and feedforward areas of the brain.