Human piriform cortex as a potential substrate for encoding higher-order odor representations
Episodic memory is often imbued with multisensory richness. Thus, recall of a complex event may be endowed with the sights, sounds, and smells of its prior occurrence. While hippocampus and other medial temporal structures are implicated in episodic memory retrieval, the participation of sensory-specific cortex is less well established. In this study subjects underwent olfactory fMRI scanning as they took part in a cross-modal contextual memory paradigm (Gottfried et al., Neuron 2004). During an initial study phase, subjects were given combinations of smells and pictures and asked to imagine a link between each pair of cross-modal stimuli. The aim of this session was to encourage episodic memory encoding of odor-object pairs. In a subsequent recognition memory test, subjects had to decide whether they were viewing a study (old) or novel (new) picture, in the absence of any odor cues. Comparison of correctly remembered items to correct rejections ("old/new" effect) revealed significant memory-related activity in piriform cortex. Our findings indicate that sensory features of the original engram are preserved in unimodal sensory cortex and suggest that reactivation of traces distributed across modality-specific brain areas underpins the sensory vividness of episodic memories. The ability of piriform cortex to sustain sensory-specific memory traces implies that this region may have access to higher-order representations of odor quality by providing a substrate for synthetic, experience-based odor coding.