While researches know that the hippocampus acts to associate sensory memories into a coherent whole, they know little about how the sensory-related brain regions contribute to that association.
Specifically, Gottfried and his colleagues sought to find out whether the brain's olfactory centers contribute to reconstructing multisensory memories. While other researchers have found that the visual and auditory brain regions were activated during memories of pictures and sounds, it wasn't known whether taste and smell brain regions similarly participated in memory association.
In their experiments, the researchers presented human volunteers with random combinations of an odor and the image of an object and asked them to imagine a link or story that associated the two. For example, a subject might smell the aroma of a rose and see a picture of a duck and be asked to invent a link.
Then, as their brains were imaged using functional magnetic resonance imaging, the subjects were presented with a series of both previously seen images and new images. They were not asked about the odors, but only to recall whether they were viewing new or old images.
Gottfried and his colleagues discovered that the subjects' odor-processing brain region, called the piriform cortex, was significantly activated when they saw objects previously associated with odors -- even though the odors were not presented during the image recognition. Also, the subjects' hippocampal areas were activated during the image recognition, as occurs in normal memory recall.
One caveat, said the researchers, is that the activation of the odor-processing centers might have been due to subjects' direct recall of odor "imagery," rather than participation of the olfactory center in the memory-associating process. However, when the scientists asked the subjects about the experience of recognizing the images, they said they had recalled the story associated with the image-odor pairs and had not tried to conjure the memory of the odor itself. Also, the subjects did not show breathing changes that would be associated with recalling odors.
Thus, concluded the researchers, these findings confirm models of memory recall in which the sensory-specific components of a memory are preserved in the sensory-related brain regions, and the hippocampus draws on those components to reconstruct the rich, ethereal experience that is a memory of things past.
Jay A. Gottfried, Adam P.R. Smith, Michael D. Rugg, and Raymond J. Dolan: "Remembrance of Odors Past: Human Olfactory Cortex in Cross-Modal Recognition Memory"
Published in Neuron, Volume 42, Number 4, May 27, 2004, pages 687-695.