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Chapter 15: The Chemical Senses

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Western University
Psychology 2115A/B
Christine Tsang

Chapter 15 Review: The Chemical Senses By the end of this section, you should know:  Functions of smell and taste  Physiology of smell and taste  Flavour perception: sensory interaction of smell and taste The Chemical Senses  Smell and taste as “gatekeepers” o Identify things that need to be absorbed for survival o Detect bad things in our environment (smell of decay, etc.) o Smell and taste are the initial ways of our defense system to determine if something should be approached and later be absorbed into our system or if something is bad and should avoid it so it doesn‟t get absorbed into your system  Neurogenesis of receptors (rebirth) o In the other systems, if you damage or destroy the receptors, they are gone (neural conductive deafness, etc.) o This is not the case for taste and smell in the sense that these receptors have constant neurogenesis: reborn over and over again after a cycle of several weeks  5-7 week cycle for olfactory receptors  1-2 week cycle for taste receptors o Due to their direct one-to-one contact with a chemical substance – absorbing a chemical and using that chemical for the basic sensation o As a result, they become quite damaged over a short period of time The Chemical Senses  Taste (gustation)  Smell (olfaction)  Sensory interaction (flavour)  Taste and smell are phyogenetically old receptor systems in our bodies  The sensory systems that allow us to detect and discriminate tiny molecules in the environment and help us link external environmental cues and internal needs (thirst, hunger, etc.)  Unlike vision and auditory, taste and smell are emotionally vivid and perceptually vivid  Very much related to memory and cognition  Interactions with limbic activity  We have pretty good memories for smell and taste – can bring back memories of the past The Olfactory System  Macrosmatic vs. Microsmatic animals o Macrosmatic: certain animals have a very keen sense of smell that is critical to their survival (rats, dogs, etc.) o Microsmatic: humans have a relatively good sense of smell but is not as keen as other animals and not as crucial to survival  The role of pheromones: chemical signals released by an individual that affect the physiology and behaviour of other individuals o Olfactory cues that are not largely consciously detectable o Play a big role in animal mating practices o Not known whether we do the same in order to signal certain biological events Stern & McClintock (1998)  Underarm secretions and menstrual synchrony  Thought that maybe women are secreting pheromones in the environment that other women are picking up on, causing menstrual synchrony  Thought that this is a hold over our evolutionary past  Collected underarm secretions from 9 different women  Took the secretions and wiped them on the upper lip of recipient female subjects  Wanted to know whether the recipients would have synched menstrual cycles with the donor participants  Found that menstrual synchrony occurred because secretions taken from the beginning of the cycles shortened the length of the recipient‟s cycles  Suggests that the pheromones were part of the secretion even though the women did not report any sort of odor detection and yet the changes still occurred  This suggests that perhaps human women are in fact secreting pheromones that are altering the physiology of the women around them How do we Measure Odor Detection?  Yes/no procedure o Basic detection – present or not present o Given trials with odors and no odors o Say yes or no if they do or don‟t smell anything  Forced choice o Give participants two trials, one that has an odor and one that doesn‟t o The participant has to indicate whether trial one or tow contains the smell  Both these procedures can be used to measure the detection threshold OR the different threshold Detection Thresholds  Detection threshold: the minimum amount of a stimulus required to know that something is there – for the participant to indicate that they detect the presence of the stimulus  Dog odor sensitivity > rat odor sensitivity  Rat odor sensitivity > human odor sensitivity  Why? o Sensitivity receptors? – NO  Maybe the types of receptors we have for smell just aren‟t sensitive  However humans and animals have the same receptors with equal sensitivity o Number of receptors? – YES  The biggest difference seems to be the number of receptors they possess  Whereas the receptors themselves are the same, and therefore the sensitivity is the same, the number of receptors that dogs have in comparison to humans is enormous Difference Thresholds  Difference threshold: the smallest amount of difference that an individual can detect in the stimulus o In the case of odor, it is the smallest difference in chemical concentration that can be detected between two different samples  Olfactomer o Device used to precisely present specific concentrations of olfactory stimuli  Human difference threshold is approximately 11% o We can detect a difference at about 11% of chemical concentration – can detect differences that are 11% different o Tells us that we are not sensitive How do we Identify Odors?  Recognition threshold: concentration needed to determine quality of an odorant o How much of that chemical is necessary for us to be able to identify a particular odor  Humans can discriminate among 100,000 odors, but they cannot label them accurately o Our ability to recognize smells is very good, but our ability to label an individual odor is terrible o Due to our relatively bad memory system in terms of labeling smells (cognition) How do we Organize/Categorize Odor?  What is the physical property of odor that is linked into our sensory system?  Very difficult to map perceptual experience onto physical attributes to a stimulus/chemicals  We don‟t have language to describe the quality of odors other than that it is good/you like it or it is bad/you don‟t like it  Some molecules that have a similar structure smell different and some molecules that have different structures smell the same The Olfactory System  Can be broken down into 3 parts  Olfactory mucosa (OM) o Located at the top of the nasal cavity o The chemicals comes into the system and are trapped in the mucosa  Olfactory receptor neurons (ORNs) o Once it gets to the mucosa, the chemicals flow over the mucosa and come in contact with the ORN o The signal is transformed into an electrical signal that is eventually transferred to the brain o Several types of ORNs – around 350 different types in humans  Olfactory bulb o Large amounts of convergence from ORN to olfactory bulb  Unlike vision or audition or even touch, the olfactory system is compromised of a whole bunch of receptors that is particularly specialized for a particular chemical type How do ORNs Respond to Different Smells?  Calcium imaging method o Increase of Ca+ inside ORN o Detect the amount of Ca+ inside ORN o When an ORN is active, Ca+ decreases inside the ORN o More Ca+ = less glow of ORN, therefore decrease in glow means higher strength of activation o Less glow, more activity  Smells are coded by recognition profiles o Has been used to determine that we encode smell neurally by sing a distributed code at the level of the ORN o Suggests that odorants are encoded by patterns of activation of ORNs depending on what the stimulus is  recognition profile Activating the Olfactory Bulb  ORN sends signals to the Glomeruli in the olfactory bulb  Olfactory bulb considered part of the brain by some but not others (mid/low brain)  ORNs sends their signal to one, maybe two, of the glomeruli How do Glomeruli Respond to Different Smells?  Two methods: o Optical imaging method  Uses the fact that cortical cells are using oxygen when active  Takes advantage of the fact that oxygen is being absorbed by using different kinds of light that detect if oxygen is present  Red light is less reflective when there is less oxygen present in the cell – the more red light you have, the more oxygen you have  Therefore, measuring the mount of light reflected from the cortex can tell us which regions are more or less active than others  Different parts of the olfactory bulb are active depending on the class of chemical that is being absorbed  Functional chemical group endings (alcohol, acid, etc.) indicated the general area of activation in the bulb itself whereas the chain length tells us exactly whether inside that sub region will be active o 2-deoxyglucose technique (2DG) d  inject 2DG into the species then expose the animal to different chemical odors  2DG will be taken up more by highly active areas vs. low active areas  perceived odor difference is related to differences in brain activity – not about just the pure perception of the chemical itself  molecule that smell similar have different recognition profiles  high order brain activity is more related to our perception of those odors that the perception or sensation of those chemicals themselves Are Some Odors Pre-Programmed?  Rats that are bred in laboratories and have not been exposed to cats for generations show a fear response to cat odor o Suggests hardwired-ness o Likely that some odors are pre-programmed and some are not  Female rabbits release pheromones that trigger nursing responses in newborn rabbits o Trigger responses in newborn rabbits, even those who are not pregnant/have babies Higher-Order Olfaction  Signals from the olfactory bulb being transferred to higher order places in cortex  Goes form bulb to piriform cortex  We have emotional responses to food – probably due to the fact that the amygdala is effected  Olfaction pathways ell us that because we have a back-and-forward movement (rather than one direction) suggests there is a lot of cognitive mediation that is going on that is modifying our perception of odor  In some way, what is going on in terms of backward responses in amygdala is that we can modify our perception of smells by our emotional context/memory  How is it that we are able to differentiate smells? o We can segregate all these sensations since they all come at us in a big jumble, causing the brain to have to separate out the different kinds of odor Olfactory Physiology  How do we separate odors form one another?  Piriform cortex is involved in odor discrimination  Wilson (2003) found that with experience, rat olfactory neurons were able to discriminate between different odors o Measured the response of neurons to perform cortex to two different types of odors o One was a mixture stimuli, one was a compound stimuli o If you give the rats enough exposure (50+ sniffs) the neurons in the rat‟s piriform cortex are able to differentiate between the compound and the mixture o Also found it is not happening at the level of olfactory bulb, but from the cortical level – purely a cortical response  Used people and exposed them to a mixture o
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