Thalamocortical network activity is necessary for conscious olfactory awareness
It is widely assumed that the thalamus is functionally irrelevant for the sense of smell, a feature that is thought to distinguish the olfactory system from all other sensory modalities. Animal studies suggest that odor information directly projects from piriform cortex (PC) to orbitofrontal cortex (OFC) ("direct" pathway), which is considered to be the dominant route for transmission of odor information to neocortical brain areas. A second ("indirect") pathway is a disynaptic projection from PC to OFC via the mediodorsal (MD) thalamus, but is generally regarded as inconsequential due to its sparse fiber density.
In a recent study we combined olfactory fMRI with novel effective connectivity techniques (Dynamic Causal Modeling, or DCM) to measure attention-dependent network coherence within direct (non-thalamic) and indirect (transthalamic) olfactory pathways (Plailly et al., Journal of Neuroscience 2008). Human subjects were presented with (or without) an odor, and with (or without) a tone, while selectively attending to either modality. Attention to odor significantly modulated neural coupling within the indirect pathway, strengthening mediodorsal thalamus-OFC connectivity. Critically, these effects were modality specific (odor > tone), directionally sensitive (forward > backward), and selective to route (indirect > direct). Our findings confirm that the human transthalamic pathway is an active modulatory target of olfactory attention. The results imply that olfaction, like all other sensory modalities, requires a thalamic relay, if only to consciously analyze smell.