Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOlfactionPSY 310Greg FrancisLecture 34Why can’t you tickle yourself?Purdue UniversityOlfaction Sensation of molecules in the air around you Every animal (and some plants) have something like olfaction Olfaction is less important for humans than for other animals Often disparaged by philosophers as an ignoble sense Compared to vision and hearing Indeed, olfaction has been largely misunderstood throughouthistory Garlic to ward off vampires Don’t breath in a dying person’s last breath Judged in the 1800s used antimephitic (anti-poison) smells to ward offtyphus when visiting a prison Needed chemistry and medicine to reveal how smell is related tomolecules in the airProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOlfaction Not every molecule produces a smell The molecule must be able to exist in the air For example, the molecules of a piece of glass do not evaporate in tothe air, so it produces no smell If it cannot get in to the air, it cannot enter our nose Inside the nose is an organ that is involved in olfactionPurdue UniversityNose The top of your nasal cavity is defined by bone There are tiny holes in this bone in an area called the cribriformplate Olfactory receptors stick through these holes to get informationabout moleculesProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityNose Here is anotherviewPurdue UniversityNose One more view All thereceptors arestuck in abunch ofmucus The moleculesmust getthrough themucus to bedetectedProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityClose up A close up looks likethis Olfactory mucosa The region with a theolfactory receptors You take a whiff inorder to bring moleculesin the air up to thesereceptors You hold your nose toprevent molecules fromreaching thesereceptorsPurdue UniversityClose up Here is another view The olfactory bulb of the brain sits right above the receptors Different from almost every other sensory system (thalamus)Prof. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHow does it work? Molecules arecombinations of basicelements (periodic table) Molecules havecharacteristic molecularshapes There is no simplecorrespondence toshape and smellPurdue UniversityHow does it work? The olfactory receptors are strings of long molecules that alsotake particular shapes The right odorant will fit into the receptor and set off a processthat ends up in a neural response The receptors are very sensitive, a single molecule can triggersome of themProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOlfactory receptors There are roughly1000 olfactoryreceptors (Nobel prizewinning work) They are regeneratedevery few weeks Why might this benecessary?Purdue UniversityOlfactory receptors We can distinguish at least 10,000 different odors Experts may be able to distinguish 100,000 odors A single odorant might be bound to several different receptors So a stimulus generates a pattern of responses across thereceptors Similar to color perception, thethree cone types can lead tomany different patterns Think also of IT cells anddistributed codesProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOdor discrimination People are not particularlygood at identifying odors ifjust given the odor alone There are sex differences Women tend to be betterthan men high end of white bars Men are better at someodors High end of dark barsPurdue UniversityOdor discrimination People are not particularly goodat identifying odors if just giventhe odor alone There are sex differences Women to be better than men high end of white bars Men are better at some odor High end of dark bars Many of the problems seem tobe less involved in perceptionand more to do with memory ofthe labels for odors Training helps a lotProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTesting odors It is rather difficult to produce agood odor stimulus You have to go to great lengths toclear the air of various molecules The physiology also adapts tosmells Receptors Neural responsesPurdue UniversityPerception It is unclear exactly how to describe odor perception Much seems to be tied to the sources For vision, we have the following kind of terms to describeaspects of perceptual experience Brightness Color Texture Intensity Depth Shiny Movement Many others…. What are the differences between these two colors?Prof. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversityPerception In contrast, there are few words to describeolfactory perception In English Aromatic: strong scented Fragrant: pleasant smelling Pungent: sharp, biting Redolent: spreading sweet scent Stinky: bad We do use other words Floral, fruity: reference to the sources of the smell, not thesmell itself Sweet: reference to a taste that accompanies the smell Green: reference to a visual association with a smellPurdue UniversityPerception It’s fairly easy to demonstrate the lack of words fordescribing odors Try to identify the difference in odor between thesetwo sourcesProf. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySmell and language Language and smell seem to generally be disconnected Different hemispheres of the brain Smells can influence behavior without conscious awarenessPurdue UniversityPrimary smells Researchers have tried to identify primary smells In the style of primary colors But there is not a lotof agreement Nor do they seem toeasily match upwith receptors There is currently no theorythat predicts what a moleculewill smell like But there is a lot ofinterest!Prof. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityAnosmia Some people lose their sense of smell Usually due to brain injury The textbook makes it out to be quite serious That has not been consistent with my own observations ofmy wife, who has anosmiaPurdue UniversityOlfaction There are more questions than answers There are no overall theories of how the brainrepresents smells Candidates include: Vibration of molecules (largely