We critically review the cognitive literature on olfactory memory and identify the similarities and differences between odor memory and visual-verbal memory. We then analyze this literature using criteria from a multiple memory systems approach to determine whether olfactory memory can be considered to be a separate memory system. We conclude that olfactory memory has a variety of important distinguishing characteristics, but that more data are needed to confer this distinction. We suggest methods for the study of olfactory memory that should make a resolution on the separate memory system hypothesis possible while simultaneously advancing a synthetic understanding of olfaction and cognition.

Three families of North American passerines – chickadees, nuthatches and jays – store food. Previous research has shown that memory for the spatial locations of caches is the principal mechanism of cache recovery. It has also been previously shown that the hippocampal complex (hippocampus and area parahippocampalis) plays an important role in memory for cache sites. The present study determined the volume of the hippocampal complex and the telencephalon in 3 food-storing families and in 10 non-food-storing families and subfamilies of passerines. The hippocampal complex is larger in food-storing birds than in non-food-storing birds. This difference is greater than expected from allometric relations among the hippocampal complex, telencephalon and body weight. Food-storing families are not more closely related to each other than they are to non-food-storing families and subfamilies, and the greater size of the hippocampal complex in food-storing birds is therefore the result of evolutionary convergence. Natural selection has led to a larger hippocampal complex in birds that rely on memory to recover spatially dispersed food caches.

A systematic review of scientific experimentation addressing olfactory effects on mood, physiology and behavior was undertaken. From this review, 18 studies meeting stringent empirical criteria were then analyzed in detail and it was found that credible evidence that odors can affect mood, physiology and behavior exists. To explain these effects, pharmacological and psychological mechanisms were explored and a psychological interpretation of the data was found to be more comprehensive. Methodological problems regarding dependent measures and stimuli, which led to inconsistencies in the data were discussed, as were the mediating variables of culture, experience, sex differences, and personality.

Using the definition that an illusion is observed when a stimulus is invariant but context alters its perception, we examined whether verbal context could produce olfactory illusions. To test this effect, we chose five odors with minimally fixed sources and that could be interpreted with various hedonic connotations. The odors were violet leaf, patchouli, pine oil, menthol, and a 1:1 mixture of isovaleric and butyric acids. Subjects individually sniffed each odor at two different sessions separated by one week. At each session an odor was given a different verbal label (either positive or negative) and subjects rated the odors on several hedonic scales and provided perceptual and interpretative responses to them. Results showed that the perception of an odor could be significantly influenced by the label provided for it. We propose that the cases where verbal labels inverted odor perception are the first empirical demonstrations of olfactory illusions.