L21-L23 (Vision cont. & Olfactory) Flashcards
Selective adaptation
“the psychologist’s electrode”
- prolonged exposure to a specific stimulus (orientation or spatial frequency) produces a change in perception or a reduction in sensitivity (tilt aftereffect or contrast threshold elevation) for a selective stimulus range
- suggests humans have neurons tuned to orientation and spatial frequency (pattern analyzers)
What happens to firing rate when neurons selectively adapt to a 20° grating?
selective adaptation of visual pattern analyzers to orientation
- prior to adaptation, peak activation of neurons in response to a vertical grating is at 0°
- adaptation to a 20° grating fatigues neurons tuned to orientations close to 20°
- peak activation by vertical grating shifts to -10° and vertical grating appears to be tilted 10° counterclockwise
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When is it hardest to see the test stimulus during selective adaptation?
when the test stimulus has the same orientation and spatial frequency as the adapting stimulus
no effect on visibility when its orientation is different!
What happens when neurons selectively adapt to a 7 cycles/degree grating?
selective adaptation of visual pattern analyzers to spatial frequency
- adaptation fatigues set of neurons (i.e. pattern analyzers) tuned to 7 cpd, which results in a loss of sensitivity (i.e. threshold elevation) at spatial frequencies close to 7 cpd
- spatial frequencies farther away from the adapting stimulus are processed by different pattern analyzers
Interocular transfer during selective adaptation of pattern analyzers
adapting with the left eye and testing with the right eye produces the same (or slightly reduced) tilt aftereffect or contrast threshold elevation in the right eye as adapting with the right eye
conclusion: interocular transfer shows that pattern analyzers are binocular and located in the visual cortex (not the retina or LGN)
What determines a contrast sensitivity function?
pattern analyzers (or spatial frequency channels): sets of cortical neurons tuned to a selective range of spatial frequencies and orientations
specific channels are compromised by certain diseases/disorders (e.g. multiple sclerosis, glaucoma or Alzheimer’s, double vision, amblyopia)
Amblyopia
i.e. lazy eye
- poor visual acuity in an otherwise healthy eye that can’t be corrected with lenses
- model system for studying cortical plasticity (ocular dominance columns may be affected)
- treated by patching the eye with good visual acuity
results in a general loss of sensitivity in pattern analyzers at all spatial frequencies (greater loss with more severe amblyopia)
What causes amblyopia?
abnormal visual experience during a critical period in development (until around age 8)
* congenital cataract (or lens opacity)
* strabismus (or misaligned eyes)
* anisometropia (unequal refractive errors in the 2 eyes)
Fourier’s theorem and Fourier’s analysis
- theorem: a sinewave is a basic unit from which all more complex patterns are made
- analysis: mathematical procedure for separating a complex pattern into component sinewaves that vary over space (vision) and time (hearing)
a complex stripe pattern may be broken down into component sinewave gratings
Fourier analysis of a complex scene
different spatial frequencies emphasize different types of information
- B: fine scale (high sfs or high contrast) carries information about fine details, which allows us to identify facial expressions (e.g. frowning)
- C: coarse scale (low sfs or low contrast) emphasizes the broad outlines of the face
complex scenes can be described as a set of component sinewaves because each neuron is sensitive to a narrow range of sfs
How do pattern analyzers interact at high contrasts?
or high sfs
- masking technique: high sfs introduced by these blocks masks low sfs that could reveal identity
- blurring the blocks (e.g. taking off your glasses) makes image identification of faces possible
Odor
a sensation associated with stimulation of the olfactory system
Odorant
- odor-inducing chemical
- molecule that is small, volatile (floats through air), hydrophobic (repellant to water), and fat soluble
not all molecules with these properties are odorants! (e.g. methane and carbon monoxide don’t produce odor)
2 routes in the olfactory system
- retronasal olfaction (near sense): odorants reach receptors through the throat; related to taste and flavor
- orthonasal olfaction (distance sense): odorants reach receptors through the nostrils
Olfactory sensory neurons
- project cilia into overlying mucus (5-10 million in each nasal cavity)
- axons form the olfactory nerve (cranial I), which synapses with mitral and tufted cells in glomeruli in olfactory bulb
- 5-10 million OSNs in humans and 100x more in dogs
- thousands of OSNs synapse with only a few mitral and tufted cells in each glomerulus
- each OSN lives for around 28 days
When does transduction occur?
olfactory system
when odorants bind to odorant receptors (protein molecules) on cilia
all olfactory sensory neurons with the same type of odorant receptor send their axons to the same glomerulus pair (1 medial and 1 lateral)
4 steps in olfactory transduction
- odorant binds to G-protein-coupled odorant receptor
- receptor-odorant complex activates G protein, which combines with GTP molecule, displacing GDP
- G protein α subunit dissociates and activates adenyl cyclase, which produces cyclic GMP (cAMP)
- cAMP binds to and opens Na+ channel, allowing Na+ ions to enter and depolarizing the neuron
neuron fires action potentials until the odorant is swept away and the neuron returns to its unbound state
Olfactory tract
after transduction and signals pass olfactory nerve
axons of mitral and tufted cells send signals directly to the primary olfactory cortex (or piriform)
How many odorant receptor genes are in the mammalian genome?
1000+ different odorant receptor genes, each coding for a single type of odorant receptor
How many odorant receptor genes are in humans?
2 kinds!
- ~350-400 functional odorant receptor genes
- ~450 pseudogenes (present in chromosomes but non-functional so don’t produce proteins)
variation in functional odorant receptor genes across humans affect odor perception and odor liking
Specific anosmia
inability to smell a specific odorant usually due to the absence of a specific odorant receptor protein
Anosmia
inability to smell caused by:
* infection (e.g. SARS-CoV-2)
* disease or drugs (e.g. damaged epithelium)
* head trauma (e.g. fractured cribiform plate, injured olfactory nerve)
not genetic!
Link between anosmia and COVID-19
- anosmia occurs in up to 50% of people infected with SARS-CoV-2 (less likely with Omicron variant)
- typically goes away in a few weeks but 7% persist for months or years
- can involve period of parosmia (abnormally unpleasant odors) during recovery
recovery may be fast due to turnover in OSNs every ~28 days but unclear why recovery is slower in some people
What causes the link between anosmia and COVID-19?
mechanism not clear
- support (or sustentacular) cells in the olfactory epithelium are infected
- inflammatory response from support cells block odorant receptor expression in OSNs
OSNs are not infected by COVID-19!
Current most successful treatment for anosmia
olfactory training: actively sniffing the same small number of odorants every day
3 theories of neural coding
- Amoore’s lock and key theory
- shape-pattern theory
- vibration theory
Amoore’s lock and key theory
theory of neural coding
- there are 7 primary odors, each with a corresponding molecule shape
- olfactory transduction is initiated when the correct molecule shape (key) bind with a particular odorant receptor (lock)
Shape-pattern theory
theory of neural coding
- odors have distinct receptor activation patterns
- each odorant activates several different types of odorant receptors
- different odorants activate different receptor arrays in olfactory epithelium
- produces specific pattern of glomerular activity in olfactory bulb
Evidence for shape-pattern theory
theory of neural coding
- different odorants (that produce different odors) have different spatial pattern of responses in OSNs
- may be how we can identify thousands of odors with only 350 different olfactory receptors
but some odorant molecules with similar shapes smell different!
Vibration theory
theory of neural coding
- every odorant has a molecular vibrational frequency
- odorants with the same frequency smell the same (e.g. citrus odors)
- vibrational frequencies of odors, not shape, activate odorant receptors through electron tunneling
odorants with the same shape can smell different
* e.g. methyl alcohol (364 Hz) and methyl sulfide (265 Hz) have the same shape but smell different (sweet vs. rotten eggs)
* tetraborane (265 Hz) also smells like rotten eggs
What odorant did Turin synthesize?
based on vibrational theory of neural coding
Tonkene, which smells like coumarin (sweet, herbaceous-warm, hay-like, tobacco), but without the carcinogenic properties
Specific anosmia for androstenone odorant
due to differences in the expression of OR7D4 (odorant receptor pseudogene in chromosome 19):
* 30% people can’t smell it due to nonfunctioning OR7D4
* 25% perceive it as “sweet musky-floral” due to reduced OR7D4 expression
* 45% perceive it as “urinous” due to functioning OR7D4
consistent with shape-pattern theory but inconsistent with vibration theory (vibrational patterns for androstenone are constant)
Stereoisomers
mirror-image rotated odorant molecules
* same vibrational frequencies but different odor (inconsistent with vibration theory)
* activate different odorant receptors (consistent with shape-pattern theory)
odor may depend on the amount of time vibrating odorant stays in a receptor
Which of the theories of neural coding is the most supported?
- shape-pattern theory is supported by research on the variability in perception of androstenone and stereoisomers
- vibration theory is controversial but supported by research on perfumes and fruit flies
Conclusion: odor perception may involve an interaction between spatial and temporal patterns (including temporal order and speed of activation) based on molecular shape and vibration
5 factors that affect olfactory detection thresholds
- sex
- age
- experience
- attention
- odorant
Effect of sex on olfactory detection
females generally have lower thresholds (i.e. higher sensitivity) than males, especially during the ovulation, but their sensitivity isn’t heightened during pregnancy
Effect of age on olfactory detection
- OSNs aren’t replaced as quickly as they die
- half of population becomes anosmic after age 85
Effect of experience, attention, and odorant on olfactory detection
- experience: can develop the ability to detect androstenone with repeated exposure
- attention: thresholds increase (i.e. sensitivity decreases) during demanding visual tasks
- odorant: easier to detect (i.e. lower threshold) ones with longer carbon chains
Odor discrimination
the ability to judge a change in the intensity of an odorant
JND is 18-25% of standard concentration in uncontrolled settings
Olfactometer
equipment used for proper control of stimuli in olfactory experiments
JND improves to 7-11% of standard concentration
How does an olfactometer work?
- flow meters and valves precisely control the concentration of odorant, duration, temperature, and humidity
- flow meter in mask measures the timing, rate, and volume of sniffs
Odor recognition
i.e. knowing you’ve smelled it before
- requires 3x more odorant molecules than detection
- durable even after several days, months, or a year
memory for odors is even more durable if initial exposure is accompanied by emotion
Odor identification
attaching a verbal label to a smell
2 kinds of factors that contribute to sex differences in olfactory perception
involving detection, discrimination, and identification
sex differences are small but significant
1. hormonal factors
2. anatomical factors: more neurons in female olfactory bulbs
recent attempt to isolate hormonal contributions found no change in olfactory perception in transgender men and women with hormonal changes following gender-affirming hormone treatment
Development of odor identification ability with age
- peaks at around age 20 and declines after age 50
- rapid decline in older ages may partly be due to decline in verbal and semantic processing
Criteria: >35 normal; ≤ 18 anosmia; ≤ 10 pure chance
females performed slightly better than males!
Tip-of-the-nose phenomenon
difficulty with odor identification
the inability to name a familiar odor
- like the tip-of-the-tongue phenomenon, may know first letter, syllables, etc. but unable to come up with the name of something we know
- anthropologists found that there fewer adjectives for the experience of smell in many languages compared to other sensations (expect for Jehai and Semaq Beri hunter-gatherer groups in Malay Peninsula)
Why is the sense of smell disconnected from language?
- lateralization: majority of olfactory processing occurs in the right side of brain while language processing occurs in left side
- competition between odor and language processing in the brain
e.g. word recognition is impaired when an odorant is present simulataneously (less activation because left and right hemisphere are competing)
Self-adaptation to odorant
prolonged exposure to a particular odorant reduces sensitivity to, or perceived intensity of, that odorant
e.g. delicious smell when you walk into a bakery disappears by the time you make a purchase
Cross-adaption to odorant
- sensitivity for particular odorant reduced after exposure to a similar but different odorant
- may occur between odorant molecules that fit the same odorant receptor (shape-pattern theory)
e.g. picking out a perfume in a department store
Mechanism for self- and cross-adaptation to odorants
- short-term biochemical phenomenon that occurs after continuous exposure to an odorant for ~15 mins
- receptor adaptation/recycling: odorant receptor is internalized into cell body of OSN then emerges again after several minutes
Cognitive habituation to odorant
inability to detect odorant or very diminished detection ability after long-term exposure to it
e.g. going out of town and coming back to the house with a funny smell; smelling a strong fragrance the person wearing it cannot smell
3 mechanisms involved in cognitive habituation
- odorant receptors internalized into cell bodies during receptor adaptation may be hindered after continuous exposure, taking longer to recycle
- odorant molecules may be absorbed into bloodstream and transported into OSNs via nasal capillaries, causing adaptation to continue
- cognitive-emotional factors (e.g. reduced habituation if odorant is believed to be harmful)
Olfactory hedonics
- the liking dimension of odor perception
- typically measured with scales pertaining to an odorant’s pleasantness, familiarity, and intensity
Relationships between pleasantness/familiarity, intensity, and liking
olfactory hedonics
linear releationship between pleasantness/familiarity and liking
* pleasant odors are perceived to be familiar
* we like pleasant/familiar odors more
complex relationship between odor intensity and liking
Developmental and cross-cultural evidence that hedonic responses are learned
nature vs. nurture
developmental (learning)
* infants and children often have different preferences from adults (e.g. don’t find feces unpleasant)
* some preferences develop through in utero exposure (e.g. garlic)
cross-cultural (associative learning)
* different cultural likes/dislikes (e.g. natto in Japan vs. cheese in America)
Evolutionary evidence that hedonic responses are learned
nature vs. nurture
- learned taste aversions: humans and rats avoid, based on smell, substances that have been paired with gastric illness
- innate odor responses are adaptive for specialist species (e.g. California ground squirrel) but can be disadvantageous for generalist species (e.g. humans, rats, cockroaches)
California ground squirrels have an instinctive defensive response to snakes (their predators) while generalist species have a more varied environment so they must learn to adapt
2 caveats before concluding that odor hedonics are learned
evidence for nature
- potential variability in functioning odorant receptor genes and pseudogenes expressed across individuals may affect perceived odor intensity and thus its pleasantness
- trigeminally-irritating odorants may elicit pain responses and all humans have innate drive to avoid pain
Pheromones
- chemicals for animal communication (don’t need to be odorants)
- secreted through urine and sweat glands
- triggers physiological or behavioral response in another member of the same species
2 kinds: releasers and primers!
Releaser pheromones
trigger an immediate, specific behavioral response
Examples
* swarming, attracting mates, recognition in insects
* mother recognition and sexual attraction in mammals (excluding humans)
Primer pheromones
trigger a slow physiological change, often hormonal
Examples
* new queen production and proportion of types of workers in insects
* accelerate puberty and control estrus cycles in mammal
* synchronization of human menstrual cycles (may occur through skin and difficult to replicate so may just be statistical)
Which organ in many animals detects pheromones?
vomeronasal organs, which project to accessory olfactory bulbs
- human embryos may have a VNO but it disappears shortly
- no AOB in humans
Chemosignals
chemicals emitted by humans that are detected by the olfactory system
Effect of chemosignals on other humans
may affect others’ mood, behavior, hormonal status or sexual arousal
- sniffing T-shirts worn by ovulating women increases testosterone levels in men
- sniffing women’s tears decreases testosterone levels and sexual desire in men
- EEG evidence that body odor carries information about potential partners in terms of gender and sexual orientation
Effect of trigeminal stimulation on sensation
- stimulation of free nerve endings (dendrites of trigeminal nerve or cranial V) in mouth and nose are responsible for the feeling that accompanies certain smells and tastes
- warning system for potentially harmful substances
e.g. burn when cutting onions, when eating chili peppers
