Cortical Activity Evoked by Odors

Excerpt

It has been hypothesized (Lynch 1986; Aboitiz et al. 2002; Montagnini and Treves 2003) that the mammalian cortex initially evolved as an associative structure, allowing features of the sensory world extracted by more peripheral circuits to be merged both within and between sensory modalities into objects capable of driving behavior. Associative cortical circuits generally have broadly distributed, overlapping inputs, allowing convergence of different pieces of information. This is in contrast to classic topographic, hierarchical cortical circuits, where information flow is more restricted to narrow, specialized channels, with much less cross-talk between disparate inputs.

The early mammalian cortex, like the modern reptilian cortex, was dominated by olfaction (lateral cortex) and hippocampus (medial cortex), with a multimodal interface (dorsal cortex) between the two. The olfactory cortex and hippocampus are characterized by nontopographic, associative networks capable of merging distributed, diverse, collections of inputs into everything from rich memories of specific life events, to maps of the visuospatial world, to single olfactory percepts derived from complex molecular mixtures. Only with continued evolutionary expansion of the cortex through the emergence of the neocortex did regional specialization and topographic, unimodal sensory processing come to be expressed, as seen, for example, in the mammalian primary visual or auditory cortex (Lynch 1986; Montagnini and Treves 2003).

Thus, the strongly associative nature of the primitive cortex—i.e., trilaminar cortices like the piriform cortex or hippocampus—promotes synthetic object processing, as opposed to analytical processing of features from complex mixtures. The processing of complex stimulus patterns as objects by associative circuits leads to robust stimulus recognition in the face of degraded inputs and enhanced discrimination of overlapping patterns (Whitfield 1979). It also leads to several testable predictions about cortical activity evoked by odors and the resulting sensory perceptions. Although there is great evolutionary conservation of peripheral features of odor processing across phyla (Hildebrand and Shepherd 1997), mammals have invested a substantial metabolic commitment to paleo- and neocortical olfactory circuits. This chapter will review the structure and function of the olfactory cortex, and describe data on the associative, multimodal, state- and expectation-dependent nature of cortical odor processing. This chapter will also attempt to outline issues that need to be addressed before we can answer how the olfactory cortex contributes to odor perception (Figure 14.1).