Looking for the roots of cortical sensory computation in three-layered cortices

Abstract

Despite considerable effort over a century and the benefit of remarkable technical advances in the past few decades, we are still far from understanding mammalian cerebral neocortex. With its six layers, modular architecture, canonical circuits, innumerable cell types, and computational complexity, isocortex remains a challenging mystery. In this review, we argue that identifying the structural and functional similarities between mammalian piriform cortex and reptilian dorsal cortex could help reveal common organizational and computational principles and by extension, some of the most primordial computations carried out in cortical networks.

Figure. Connectivity in mammalian piriform cortex (PCx) and turtle dorsal cortex (DCx)

(a). Transverse view (see inset) of the basic microcircuits. Sensory afferents from the Lateral Olfactory Tract (in PCx) or Lateral Forebrain Bundle (in DCx) make en-passant synapses in superficial layer 1 on distal segments of layer-2 pyramidal cell dendrites and on superficial inhibitory interneurons. Layer-2 pyramidal neurons receive recurrent excitation from other pyramidal cells (associational connectivity), feed-forward inhibition from superficial interneurons (FF), and feed-back inhibition from layer-2/3 interneurons (FB). (b) Top view (see inset) of PCx and DCx connectivity. Afferents from the olfactory bulb (OB) project to PCx without apparent topographical order. In DCx, there may be a coarse topography of lateral geniculate nucleus (LGN) projections that preserves visual isoazimuth neighborhoods [10,40]. In both cases, recurrent excitation through local (grey) and long-range (not shown) associational connections contributes to broadening the stimulus selectivity of pyramidal cells and may mask any local anisotropy in the spatial distribution of the primary sensory afferents (see color tiles).