Chapter 14: Smell Smell and taste (gustation and olfaction) are chemical senses in that the signal for the sense is a molecular chemical or ion which stimulates receptors in the nose or on the tongue. Studies show that people can identify poisonous substances, gender of others, and – for some traditional h-gs – scent can be used to track prey Smell Many diseases produce particular odors that can be used in diagnosis. Trimethylaminuria: liver disorder that leads to strong smell of rotting fish in people’s breath and sweat. Pheromones: special scents used usually in mating; a certain pheromone would be produced by fertile females exciting males. Classifying smells Henning's Smell prism: In 1916, Hans Henning attempted to classifying smells using a 6 general categories: Flowery, Putrid, Fruity, Spicy, Burnt, Resinous. Noncorner smells are combinations. However, many have criticized this as being ad-hoc and too restrictive. Classifying smells 2) Use of MDS: Schiffman (1974) has done extensive work using MDS in an attempt to determine the psychological dimensions of smell. Her research has generated two dimensions of smell. Pleasant unpleasant, and another dimension which has yet to be characterized. Molecular shape and odor One complicating factor in understanding smell perception is that there is no reliable relationship between odor perception the physical aspects of the signal. For example, molecular shape has little to do with what smell is perceived. Olfactory anatomy Odorous molecules are introduced into the nasal cavity via the nostrils or the mouth. Some of the molecules eventually attach themselves to the Olfactory epithelium. A section of tissue lining the top of the nasal cavity. 1) Olfactory binding protein: a substance found in the nose which helps to trap and transport odorous molecules to the olfactory epithelium. This helps increase olfactory sensitivity. 2) Olfactory receptor cells: neurons in the olfactory epithelium which begin the process of smelling. Unlike the other sensory systems, these receptor cells are actual brain cells, whose axons connect directly to brain center. There are about 10 mil. receptor cells in humans, compared to 200 mil in dogs. These are also the only brain cells which are capable of regeneration, which occurs about every 5 to 8 weeks. Olfactory anatomy Cilia at the end of each OSN have receptor proteins that act as lock and key sites for ordorous molecules. Deficiencies in receptor proteins can lead to anosmias – deficits in smell. Can be specific to certain kinds of smells Olfactory pathways Olfactory nerve projects to both subcortical (medial and lateral hypothalamus) and cortical regions (orbito frontal cortex). Subcortical more emotional aspects of smells; cortical emotional plus memory and interpretation. Common chemical sense Non odorous receptors in the nose that inform about the presence of certain potentially irritating chemicals (such as carbon dioxide). When stimulated in the presence of another odorant can affect the perception of smell of that odorant – often making it seem more pungent and unpleasant. Odor Perception 1) Humans are remarkably sensitive to odors, concentrations as small as 1 part per 50 billion are detectable, however, humans are remarkably poor at odor identification. Most likely because humans only rarely have to relay on smell exclusively to identify a substance. 2) Bistability: some odorous substances are ambiguous. Their interpretation affected by context. Ex: An odorous substance that can be perceived equally as either woody or citrusy will be interpreted as woody in the context of another unambiguously citrusy odor and vice-versa. 2) Most research tends to confirm that females are more sensitive to odors and are more able to indentify odors. 3) Adaptation to odors, both long-term and short-term is possible. Short-term is likely due to fatiguing receptor activity, while longterm may be due to structural changes in regenerated nerve cells. 4) Odor sensitivity declines with age and with smoking.