Feedforward and Feedback Connections

Abstract The mammalian sensory neocortex consists of hierarchically organized areas reciprocally connected via inter-areal feedforward (FF) and feedback (FB) pathways. As information ascends through this cortical hierarchy, via FF connections, neuronal receptive fields (RFs) become tuned to increasingly complex stimulus features, and an increasingly abstract representation of the sensory inputs is achieved. FF connections are reciprocated by FB connections from higher-order areas to lower-order areas. Reciprocal FF-FB connections between cortical areas are found throughout the neocortex of all mammalian species, suggesting that they carry out fundamental computations. This chapter reviews data primarily from primate, cat, and mouse on the anatomical organization, functional connectivity, synaptic physiology, and functional impact of FF and FB pathways, with a particular focus on the visual system, where these connections have been mostly studied. The picture that emerges from these studies is one in which both FF and FB connections show a high degree of anatomical specificity with respect to areas, layers, and cell types they interconnect. This specificity serves to parse FF and FB connections into specialized processing streams. Within each stream, FF connections drive sensory responses in higher-order areas and mediate the complexification of RFs along the cortical hierarchy; instead, FB connections provide stream-specific top-down modulations of lower-order areas reflecting sensory context, predictions, and the internal states of the organism. These complementary functions of FF and FB connections result from differences in their visuotopic extent, targeted cell types, synaptic physiology, and location of their synaptic contacts on the postsynaptic neurons.