Parallel-distributed processing in olfactory cortex: New insights from morphological and physiological analysis of neuronal circuitry

A working hypothesis is proposed for piriform cortex (PC) and other olfacto ry cortical areas that redefines the traditional functional roles as follow s: the olfactory bulb serves as the primary olfactory cortex by virtue of e ncoding 'molecular features' (structural components common to many odorant molecules) as a patchy mosaic reminiscent of the representation of simple f eatures in primary visual cortex. The anterior olfactory cortex (that has b een inappropriately termed the anterior olfactory nucleus) detects and stor es correlations between olfactory features, creating representations (gesta lts) for particular odorants and odorant mixtures. This function places ant erior olfactory cortex at the level of secondary visual cortex. PC carries out functions that have traditionally defined association cortex-it detects and learns correlations between olfactory gestalts formed in anterior olfa ctory cortex and a large repertoire of behavioral, cognitive and contextual information to which it has access through reciprocal connections with pre frontal, entorhinal, perirhinal and amygdaloid areas. Using principles deri ved from artificial networks with biologically plausible parallel-distribut ed architectures and Hebbian synaptic plasticity (i.e. adjustments in synap tic strength based on locally convergent activity), functional proposals ar e made for PC and related cortical areas. Architectural features incorporat ed include extensive recurrent connectivity in anterior PC, predominantly f eedforward connectivity in posterior PC and backprojections that connect di stal to proximal structures in the cascade of olfactory cortical areas. Cap abilities of the 'reciprocal feedforward correlation' architecture that cha racterizes PC and adjoining higher-order areas are discussed in some detail . The working hypothesis is preceded by a review of relevant anatomy and ph ysiology, and a non-quantitative account of parallel-distributed principles . To increase the accessibility of findings for PC and to advertise its sub stantial potential as a model for experimental and modeling analysis of ass ociative processes, parallels are described between PC and the hippocampal formation, inferotemporal visual cortex and prefrontal cortex.