Cortical processing of odor objects

Abstract

Natural odors, generally composed of many monomolecular components, are analyzed by peripheral receptors into component features and translated into spatiotemporal patterns of neural activity in the olfactory bulb. Here, we will discuss the role of the olfactory cortex in the recognition, separation and completion of those odor-evoked patterns, and how these processes contribute to odor perception. Recent findings regarding the neural architecture, physiology, and plasticity of the olfactory cortex, principally the piriform cortex, will be described in the context of how this paleocortical structure creates odor objects.

Figure 1

Major local circuit components of the piriform cortex. See text for circuit description. Abbreviations: FF = feedforward inhibition mediated by interneurons in Layer I; FB = feedback inhibition mediated by interneurons in Layers II and III; MP = multipolar interneuron; SL = semilunar pyramidal cell; SP = superficial pyramidal cell; DP = deep pyramidal cell; ASSN = association fibers; AFF = afferent fibers; EndoP = endopiriform nucleus. Excitatory neurons depicted in red, inhibitory in blue.

Figure 2

Piriform cortical ensemble processing of odorants. (A) Odorant features, encoded as spatiotemporal patterns in olfactory bulb glomeruli and their outputs, converge in the piriform cortex through broadly dispersed, non-topographic projections. Information is also distributed through broad intracortical association fibers projections. Neurons activated by a given odor (black cells) are broadly distributed. Spiking in individual neurons can driven directly by afferent input (cell a), directly by association fiber input (cell b), or the combination (cell c). (B) Given the distributed afferent and association fiber projections, different odorants (X and Y) can activate widely distributed, overlapping ensembles of neurons. As described in the text, plasticity of association fiber synapses allows for pattern completion in the event of degraded familiar afferent inputs.

Figure 3

(A) Pattern separation allows a decorrelation between two scents with highly overlapping component features into two distinct odor objects. Pattern completion can reduce the distinction between two overlapping scents by recapitulating one familiar odor object from the partial input driven by the other. (B) Piriform cortical neural ensembles perform pattern separation and completion relative to olfactory bulb input – a function common to auto-associative arrays and similar to processes in the hippocampal system (adapted from Barnes et al., 2008). See text for description.