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Chapter 15

Sensation and Perception Psych 367 Chapter 15.docx

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Douglas Wylie

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Chapter 15 the chemical senses The olfactory system Functions of olfaction - Their primary window to the environment - One important contrast between humans and other species is that many animals are macrosmatic (having a keen sense of smell that is important to their survival), whereas humans are microsmatic (having a less keen sense of smell that is not crucial to their survival - Olfaction is also extremely important in sexual reproduction because it triggers mating behaviour in many species - Menstrual synchrony: women who live together often have menstrual periods about the same time - Stern and McClintock o Experiment that led them to conclude that menstrual synchrony is caused by human pheromones  Chemical signals released by an individual that affect the physiology and behaviour of other individuals o They found that underarm secretions taken from donor women in the initial phase of their cycle shortened the length of the recipients cycle o The did not detect any smell - Anosmia: the loss of the ability to smell as a result or injury or infection - People suffering from anosmia describe the great void created by their inability to taste many foods because of the close connection between smell and flavour Detecting odours - Our sense of smell enables us to detect extremely low concentrations of some odorants - The detection threshold for odours is the lowest concentration at which an odorant can be detected - Measuring the detection threshold o Present different concentrations of an odorant on different trials o The participant responds to yes I smell or something or no I cant smell anything o One problem with this procedure is that it is susceptible to bias o Some people will respond yes at the merest hint of a smell where as others will wait o The forced choice method avoids this problem by presenting the participant with blocks of two trials  One trial contains a weak smell and the other no smell o Threshold if 75% correct - There is a very large range of threshold - T butyl mercepatan, the odorant that is added to natural gas, can be detected in very small concentrations less that 1ppb - But even though human are unaware of odours that other animals can detect, humans individual olfactory receptors are as sensitive as any animals - Human olfactory receptors can be excited by the action of just 1 molecule of odorant - Humans just have fewer receptors then dogs - Another aspect of odour detection is the difference threshold – the smallest difference in the concentration of two odours that can be detected - Measurements of the difference threshold highlights one of the most important problems in olfactory research - the control of concentrations in stimulus presentation - Cain o Carefully measured the difference threshold by placing two odorants of different threshold by placing two odorants of different concentrations on absorbent cotton balls and asked participants to judge which was more intense, he found that difference threshold averaged 19% o He found that stimuli that were supposed to have the same concentration actually varied considerably o This variation was apparently caused by difference in the airflow pattern through the cotton in different samples o To deal with this problem, Cain re-measured the difference threshold using a device called olfactometer, which presents olfactory stimuli with much greater precision that cotton balls Identifying odours - When odorant concentrations are near threshold, so a person can just detect the presence of an odour, then person usually cannot sense the quality of the odour - The concentration of an odorant has to be increased by as much as a factor of 3 above the threshold concentration before the person can recognize an odours quality - Recognition threshold: The concentration at which quality can be recognized - Humans can discriminate between as many as 100 000 different odours - They often find it difficult to accurately identify specific odours - When you are asked to identify the substance associated with the odour, they are successful only half the time - When they presented participants with the names of the substances at the beginning of the experiment and then reminded them of the correct name when they failed to respond, they correctly got 98% - Knowing the correct label for the odour actually seems to transform out perception into that odour - Knowing a fishy goat smell actually came from leather, the smell was then transformed into that leather - When we have trouble identifying odours, this trouble results not form a deficiency in our olfactory system, but from an inability to retrieve the odours name from our memory - One reason for the difficulty is that we lack a specific language for odour quality - Another difficulty in relating odours to molecular properties is that some molecules that have similar structure can smell different The neural code for olfactory quality The olfactory mucosa - Olfactory mucosa: dime sized region located high in the nasal cavity that contains the receptors for olfaction - Odorant molecules are carried into the nose in the air stream which brings these molecules into contact with the mucosa - Olfactory receptor neurons are located in the mucosa Olfactory receptor neurons - Olfactory receptor neurons are dotted with molecules called olfactory receptors that are sensitive to chemical odorants - Other parallels between visual pigments and olfactory receipts are that they are both proteins that cross the membrane of the receptor neurons seven times - They are both sensitive to a specific range of stimuli - Each type of olfactory receptor is sensitive to a narrow range of odorants - An important difference between the visual system and the olfactory system is that while there are only four different types of visual pigments, there are 350 different types of olfactory receptors, each sensitive to a particular group of odorants - The large number of olfactory receptors is important because it is one reason we can identify 100 000 different odours - Just as a particular rod or cone receptor contains only one type of visual pigment, a particular olfactory receptor neuron contains only one type of olfactory receptor Activating olfactory receptor neurons - 350 different types of ORNs in the mucosa - There are about 10 000 of each type of ORN, so the mucosa contains millions of ORN - Calcium imaging o When Olfactory receptor responds, the concentration of calcium increases inside the ORN o One way of measuring this increase in Ca ions is by soaking olfactory neurons in a chemical that causes the ORN to fluoresce with a green glow when exposed to ultraviolent - Malnic o Determined the response to a large number of odorants using calcium imaging o 10 different ORNs are activated by each odorant o Recognition profile: the pattern of activation of odorants o Each odour causes different patterns of firing - This difference in perception is reflected in their different profiles - When two odorants smell different, they usually have different profiles - The idea that an odorants smell can be related to different response profiles is similar to the trichromatic code for colour vision Activating the olfactory bulb - Activation of receptors in the mucosa causes electrical signals in the ORNs that are distributed across the mucosa - These ORNs send signals to structures called glomerulus in the olfactory bulb - Each glomerulus therefore collects information about the firing of a particular type of ORN - Optical imaging o Can be used to measure the activity of large areas of the olfactory bulb by measuring how much red light is reflected from the olfactory bulb o The bulb must first be exposed by removing a patch of the skill o Red light is used because when neurons are activated, they consume oxygen from the blood o Areas that have been activate reflect less red light and look darker - As the length of the carbon chain increases, the area of activation moves to the left - The functional group associated with a particular type of compound determines the general area of the olfactory bulb that is activated, and the compounds chain length determines the position within each area - Molecules with similar structures can have very different odours - Linster o Looked at this phenomenon by studying pairs of molecules that have the same chemical formula, but in which a group within the molecule is rotated to a different position - 2 deoxyglucose technique o Injecting a radioactive 2 deoxyglucose molecule into an animal and exposing the animal to different chemical o The radioactive 2DG contains glucose is taken up by active neurons so by measuring amount of radioactivity in the various parts of a structure , we can determine which neurons are most activated by the different chemical s - The arrows indicate that the two forms of carvone have different patterns but the two forms of limonene has almost identical patterns - The result is that behavioural testing of the rats showed that they could tell the difference between the two forms of carvone but could not distinguish between the two forms of limonene - Therefore the pattern of activation on the OB is related not only to functional groups and structure, but also to the odour that is perceived - The olfactory system codes different odorants based on the response of ORNs and the olfactory bulb t
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