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
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
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
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!
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
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
