W3: Chemical Senses Flashcards
Chemical Senses
olfaction and gustation
Olfaction detection & JND
Chemical Senses: Olfaction
Humans are good at detecting odor although considered a “minor” sense - survival purposes
Odor’s JND is similar to others , 5-7%
Olfactory stimuli: Odourants vs volatiles
Chemical Senses: Olfaction: Physiology: stimuli
Olfactory stimuli: molecules/chemical compounds
Volatiles: undetectable smells in form of gas (why propane is laced w/ strong detectable odorant to prevent gas leak deaths)
Odourants: detectable scents, form = organic compounds/carbon chain (no. of carbon in chain indicates type of odorant)
^same olfactory receptor types connect to single mitral cell - receptors are selective twrds types of compounds and odourants (i.e. certain lengths of carbon chains)
Odours vs odourants
Chemical Senses: Olfaction: Physiology: Stimuli
Odours: whole odour percept = combo of odourants.
Odourants: detectable scents, most in the form of organic compounds/carbon chain (amount of carbon in chain indicates type of odorant)
Olfactory Receptors
Chemical Senses: Olfaction: Physiology: transduction
-chemoreceptor found on olfactory epthelium .
-Forever regenerating - lasts 60 days.
-500-1000 receptor types. aprox 200 cells/type
-Different neurons activated by different molecules to encode the compound - one receptor can detect multiple odorants that share the same molecule, therefore one odor can activate multiple receptors as odors contain a combination of molecules.
Olfactory Epithelium & Cilia
Chemical Senses: Olfaction: Physiology: transduction
Olfactory Epthelium: located on roof of nasal cavity, houses 6 mil olfactory receptors
Cilia: “hair like” structures in which connect to nasal tissue and olfactory receptors (5-40 cilia/receptor). In order for chemicals to be detected by olfactory receptors, they must dissolve in olfactory mucus
Olfactory mucosa
Chemical Senses: Olfaction: Physiology: transduction
tissue lining of the nose which cilia connect to
Olfactory Free Nerve Endings
Chemical Senses: Olfaction: Physiology: transduction
in the olfactory mucosa. Are nerve cells that “mediate the sensations of coolness, tingling, and burning that arise from high concentrations of chemicals.”
Olfactory Pathway Process
Chemical Senses: Olfaction: Physiology: Pathways
Receptors Axon breaks through skull to reach Mitral Cell in Olfactory Bulb -> synapses connect receptor cells axons to Mitral Cell dendrites to create 2000 Olfactory Glomeruli (5-25MC / thousands of receptors) -> Mitral Cell axons travel along olfactory tract to Primary Olfactory Cortex (to be sent to the thalamus and Orbitofrontal Cortex to mediate flavor) and Amygdala (which causes the emotional impact of memories and sends info to hippocampus in order to consolidate information as an emotional memory)
Mitral Cell
Chemical Senses: Olfaction: Physiology: Pathways
“A neuron in the olfactory bulb that receives signals from [the axon of] olfactory receptor neurons and relays them to the brain; there are 50,000 mitral cells in the human olfactory bulb.”
Olfactory Bulb
Chemical Senses: Olfaction: Physiology: Pathways
is the piece of brain that is exposed in the orthonasal passage.
Orthonasal passage/olfaction
Chemical Senses: Olfaction: Physiology: Pathways
refers to the passage of nostrils which is the main pathway for orthonasal olfaction.
Orthonasal Olfaction: smelling through the nose
Retronasal passage/olfaction
Chemical Senses: Olfaction: Physiology: Pathways
detecting scents that travel to the epithelium from the mouth through the retronasal passage (airways at top/back of mouth connecting to nose -why you cant fully taste food when holding nose as both passages lack air flow).
Odorant Receptors
Chemical Senses: Olfaction: Perception
-receptor cells which detect different odor compounds - each type picks up a different odor (i.e. carbon chain length) and responds less to similar odors
-calculate smell by combo of odors = levels of activity across different types of odorant receptors.
Odor Detection
Chemical Senses: Olfaction: Perception
“Some chemicals are detectable at concentrations thousands of times weaker than others (sensitive to musk and require thousands higher concentration of methyl salicylate) ”
Odor Recogniton
Chemical Senses: Olfaction: Perception
odor memories similar to other modalities. “Humans are quite poor at identifying odors - detect 50% of daily household items by odor, women outperform men, ppl easily detect different odors” struggle to describe odors
Theories of Odor Recogniton: Different neurons activated by different molecules to encode the compound - one receptor can detect multiple odorants w/ same molecule, therefore one odor can activate multiple receptors (due to combo of molecules).
olfaction population coding
Chemical Senses: Olfaction: Perception: Recognition
“combinational receptor code for odor”: combo of responses across cell types reflects stimulus features ^each odor compound creates a pattern of activity across receptors - how they encode odors.
^ mitral cells connect to receptors = reflect same activity
^This theory explains odourant receptors selectivity towards types of odourant compounds (certain length of carbon atoms within a chain - respond at lower rates to similar length carbon chains (forms the tuning curve))
olfaction as a synthetic process
Chemical Senses: Olfaction: Perception: Recogniton
Synthetic processes: are processes where a combo of components creates a new perceptual whole that cannot be broken down into individual parts and cannot sense the individual parts within the whole. Olfaction is a synthetic process as combinations of odourants create a whole new odour and we cannot smell the different odourants within the odour. Also applies to colours - you cannot detect the hints of red and blue in purple.
Odor Adaptation
Chemical Senses: Olfaction: Perception
‘continuous exposure = odor intensity decreases by 30+%’ - studies: drops past 50% after 12 mins - recovery rates proportional to time (e.g. in 12 mins case, 12 mins to recover to pre-adaptation rates). Detection decreases
*adaptation is constantly happening as humans adapt to smells on their body (e.g. why smokers cant smell their smoke)
Adaptation is selective towards odors
Similar odorants can only raise the threshold of a particular odor
Anosmia
Chemical Senses: Olfaction: Perception
Odor Blindness. Partial means they cant detect specific odors - may reflect deficiency in an olfactory receptor
Smell Descriptors
Chemical Senses: Olfaction: Perception
Functional role of smell: cant predict smells by molecules but can by descriptors as they tend to be consistent across people
Two dominant dimensions of smell descriptors
Pleasentness-unpleasentness/hedonic tone: e.g. fruit = pleasant, animal = unpleasant
Edible-Unedible: spicy smells = edible; cleaning product smells = inedible
*smell is simple, primitive but reliable classification system which classifies all odors by avoidability/edibility.
Olfaction & Memory
Chemical Senses: Olfaction: Perception
olfactory perception requires memory (emotional impact of olfactory memories evidence = olf bulb -> amygdala -> hippocampus = emotional memory).
Reduced discriminability: Repeated exposure to a pair of separate odours can condition us to perceive an odourant when we detect its paired odourant
Training can allow people to better detect different odours (e.g. wine tasters can discriminate between 100,000 odours) or the odorant components (more than two - naturally we can maybe discriminate between two)
HM can only discriminate between odors by their intensity rather than the smell itself due to no memory due to severance of piriform/primary olfactory cortex
Gustation
Chemical Senses
Gustation establishes food preferences in humans
Minor sense but crucial to detecting nutrition, poisons and pathogens
taste receptors
Chemical Senses: Gustation: Physiology: Transduction
Chemoreceptor cells located on the end of microvilli (hairs) which project out of taste buds throughout the mouth, tongue & throat.
gustation transduction process
Chemical Senses: Gustation: Physiology: Transduction
chemical substance dissolves in saliva -> detected by receptors when molecules bind to them -> membrane allows molecules through which alters receptors potential
^process varies for different receptor neurons
taste buds
Chemical Senses: Gustation: Physiology: Transduction
10000 in mouth, located on Gustatory Papillae,
50-150 receptor cells are clumped = taste bud.
Hard to understand cells function as they last 10 days
papillae
Chemical Senses: Gustation: Physiology: Transduction
houses free nerve endings which detect spiciness (cool and tingling sensations)
taste receptor selectivity
Chemical Senses: Gustation: Physiology: Transduction
“each nerve responds best to one kind of substance it also responds to some extent to other substances. This may reflect the fact that a single sensory neuron connects to more than one type of receptor.”
Gustatory Pathway Process
Chemical Senses: Gustation: Physiology: Pathways
chemical substance dissolves in saliva -> molecules bind to receptor =detection -> membrane allows molecules through = alters receptors potential -> gustatory nerve axons are sent along 7th, 9th and 10th cranial nerves to medulla in brainstem and end here -> medullas neurons send axons to amygdala and thalamus -> thalamus sends projections to primary gustatory cortex and frontal operculum -> insula sends projections to Orbitofrontal Cortex to mediate 5 modes into flavor
Primary Gustatory Cortex
Chemical Senses: Gustation: Physiology: Pathways
Primary Gustatory Cortex is also referred to as the Insula Cortex
Gustatory cortical processing
Chemical Senses: Gustation: Physiology: Cortical Processing
AI/FO’s primary role in brain is processing taste (using FMRI and single-unit recordings)
^region involves in somatosensory and taste motor functions such as mouth feel
Most cortical cells respond to range of stimuli while still holding a preference (38% favor sweet, 34% salts, 22% bitter, 5% acidic)
Gustation: topographical map
Chemical Senses: Gustation: Physiology: Cortical Processing
“gustotopic” map with different clusters of cells responding to different taste qualities.” OUTDATED
Taste Adaptation & cross-adaptation
Chemical Senses: Gustation: Perception
“Prior exposure to one taste can affect perception of a later taste, either by diminishing that taste or by enhancing it” (one sour taste reduces intensity of later sour and bitter tastes)
Cross-adaptation: ‘adapting to sourness makes water taste sweet’ and vice versa
Conditioned taste aversion
Chemical Senses: Gustation: Perception: Adaptation
aversion to food arises after sickness follows (doesnt need to cause it - just needs to appear as association) on only one occasion - strong conditioning (can happen with cancer patients who eat food within a couple of hours of chemo, or after excessive alcohol consumption)
taste selectivity
Chemical Senses: Gustation: Perception: Encoding/selectivity
taste neurons/nerve fibers respond to all taste qualities but respond most to a particular taste quality (therefore assume that taste cells mostly include one receptor type).
taste qualities
Chemical Senses: Gustation: Perception: Encoding/selectivity
Taste Qualities: signal info to the taste system about edibility and nutrition
Sweetness: caused by sugar/simple carbs. Broken down quickly to provide energy. Moderate consumption. Universally preferred - very palatable
Saltiness: caused by sodium chloride. Maintains water distribution among membranes (sweating counteracts this). Palatable - Moderate consumption. Sodium deprivation= preference for salty foods
Sourness: caused by acidic substance. Not palatable - food is usually unripe or contaminated - adaptive to avoid
Bitterness: caused my many compounds (e.g. toxic alkaloids in plants). ‘Unpalatability protects us from these plant-based poisons’
Saviourness: unami. Caused by amino acids (e.g. MSG). MSG suggests protein. Increases palatability. No scientific basis for allergy to MSG
taste as an analytic process
Chemical Senses: Gustation: Perception: Encoding/selectivity
Analytic Process: are senses where their percept can be dissected into different parts to better understand the percept as a whole. Hearing is analytic as chords can be broken down into individual notes and these notes can be identified separately when played as a whole.
taste is an analytic process as “its sensations can be decomposed perceptually into the contributions from each of the primary sensations/taste primaries”
Different taste coding theories
Chemical Senses: Gustation: Perception: Encoding/selectivity
Taste Primaries and Cross-Fibre Theory
Henning’s Taste Tetrahedron
Taste primaries/cross-fibre theory
Chemical Senses: Gustation: Perception: Encoding/selectivity
theory of analytic coding of taste - taste is a combination of the taste qualities signified by activity across the channels which represent each individual taste quality as well as which channel fired the most
OUTDATED as taste fibers respond to all qualities (therefore cant be channels?) & doesnt incl saviourness/unami
^Psychophysical linking hypotheses: labelled lines/four neurons detect activity patterns to encode the type of odor. Supporting evidence = topographical organisation HYP/EVIDENCE ARE BOTH OUTDATED ^ location of receptors have no function
Henning’s Taste Tetrahedron
Chemical Senses: Gustation: Perception: Encoding/selectivity
is another outdated encoding theory of taste - tetrahedron represents how tastes are diff combos of taste qualities.
OUTDATED: doest indicate diff flavor intensity and doesnt incl. unami
Flavor
Chemical Senses: Gustation: Perception: Encoding/selectivity
eating sensation created by combination of modes (i.e. taction, vision, gustation and olfaction) mediated by the Orbitofrontal Cortex. Taste and smell are only sensory modalities required to produce flavor - food travels up to nose through the retronasal passage (at top/back of mouth) in order to make contact with the olfactory epithelium. Change in associated color to food can change taste perception
What is the Orbitofrontal cortex’s relation to flavor?
Chemical Senses: Gustation: Perception: Encoding/selectivity
Explanation for combo of senses creating flavor: Orbitofrontal cortex is sensory pathway for taste, smell and vision - ‘cells responding to these modalities are in close proximity of eachother’ - a third of neurons are bimodal: respond to pairs of these senses in all combos possible (taste-smell, taste-vision, vision-smell)
Chemical senses: qualia and modality
Chemical Senses: Evaluation
taste and smell modalities create qualia such as the clean smell of limes or disgusting smell of faeces. distinct modalities together create flavour.
Chemical senses: Psychophysics
Chemical Senses: Evaluation
sensitivity towards particular odors.
Thresholds vary for odors.
Taste and smells detectability varies for the sake of adaptation.
Taste primates theory = psychophysical linking hypothesis to orbitofrontal cortex (taste/smell/vision).
Chemical senses: Cognitive Neuroscience
Chemical Senses: Evaluation
taste and smell chemoreceptors hold preferences bur respond to various stimuli. Distinct hierarchy of processing from sensory organs to cortex. Topographical maps for both systems.
Chemical senses: Computational Neuroscience
Chemical Senses: Evaluation
representation is present in coding of odor (combinational code of activity = specific odor) and taste (activity combo across receptors represents combo of taste qualities). Computations = approach/avoidance, nutrition and edibility
Olfaction
sense of smell
Odor’s prevent ppl from illness,
evokes memories w/ lrg emotional impact (amygdala)
Facts (not all proven):
-humans can determine sex by hands and breaths natural scent (most dont know this);
-olfaction may sync menstural cycles ;
-“pheromones play role in sexual attraction?”;
- snort drugs, receptors embedded in tissue = no barrier
-can detect odor from couple molecules
- discriminates btwn thousands of compounds
Primary Olfactory Cortex
also referred to as the piriform cortex
Olfactory Cortical Processing
Smell is unique = straight to olfactory bulb rather than thalamus (shows smell was an early part of evolution) -
Olfactory bulb send mitral cell axons to primary olfactory cortex & amygdala (=smells emotional impact)
^then sent to orbitofrontal cortex to perceive smell (ofc mediates flavor)
One nostril stimulates both hemispheres of brain (unlike other senses)
“breathing modulates Cortical olfactory activity”
