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Summary of 89 pages for the course Advances In Experimental Medicine And Biology at 123 University (Note)
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Processing the EnvironmentSensory Perception
Visual Cues
− Depth, Form, Motion, Constancy
− Binocular Cues -
● Retinal disparity (eyes are 2.5 inches apart)
● Convergence – things far away, eyes are relaxed. Things close to us, eyes contract.
− Monocular Cues
● relative size, interposition (overlap), relative height (things higher are farther away),
shading and contour, motion parallax (things farther away move slower)
● Constancy – our perception of object doesn’t change even if it looks different on retina.
♦ Ex. size constancy, shape constancy, color constancy.
Sensory Adaptation
− Hearing - inner ear muscle: higher noise = contract.
− Touch - temperature receptors desensitized
− Smell – desensitized to molecules
− Proprioception – mice raised upside down would accommodate over time, and flip it over.
− Sight – down (ex. Light adaptation, pupils constrict, rods and cones become desensitized to
light) and upregulation (dark adaptation, pupils dilate)
Weber’s Law
− 2 vs. 2.05 lb weight feel the same.
− 2 vs. 2.2 lb weight difference would be noticeable.
− The threshold at which you’re able to notice a change in any sensation is the just noticeable
difference (JND)
− So now take 5 lb weight, in this case if you replace by 5.2 weight, might not be noticeable. But if
you take a 5.5 lb it is noticeable.
− I = intensity of stimulus (2 or 5 lb), delta I = JND (0.2 or 0.5).
− Weber’s Law is delta I to intensity is constant, ex. .2/2 = .5/5 = .1.
● Delta I/I = k (Weber’s Law)
− If we take Weber’s Law and rearrange it, we can see that it predicts a linear relationship bet
− ween incremental threshold and background intensity.
● Delta I = Ik.
● If you plot I against delta I it’s constant
Absolute threshold of sensation
− The minimum intensity of stimulus needed to detect a particular stimulus 50% of the time
− At low levels of stimulus, some subjects can detect and some can’t. Also differences in an
individual.
− Not the same as the difference threshold (JND) – that’s the smallest difference that can be
detected 50% of the time.
− Absolute threshold can be influenced by a # of factors, ex. Psychological states.
● Expectations
● Experience (how familiar you are with it)
● Motivation
● Alertness
− Subliminal stimuli – stimuli below the absolute threshold.
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,The Vestibular System
− Balance and spatial orientation
− Focus on inner ear - in particular the semicircular
canals (posterior, lateral, and anterior)
− Canal is filled with endolymph, and causes it to shift
– allows us to detect what direction our head is
moving in, and the strength of rotation.
− Otolithic organs (utricle and saccule) help us to
detect linear acceleration and head positioning. In
these are Ca crystals attached to hair cells in viscous
gel. If we go from lying down to standing up, they
move, and pull on hair cells which triggers AP.
− Also contribute to dizziness and vertigo
● Endolymph doesn’t stop spinning the same time as we do, so it continues moving and
indicates to brain we’re still moving even when we’ve stopped – results in feeling of
dizziness.
Signal Detection Theory
− Looks at how we make decision under conditions of uncertainty –
discerning between important stimuli and unimportant “noise”
− At what point can we detect a signal
● Origins in radar – is signal a small fish vs. large whale.
● Its role in psychology – which words on second list were
present on first list.
● Real world example – traffic lights. Signal is present or absent
(red).
Strength of a signal is variable d’, and c is strategy
− d’: hit > miss (strong signal), miss <hit (weak signal)
− c: 2 strategies – conservative (always say no unless 100% sure signal is present. Bad thing is
might get some misses). Or liberal (always say yes, even if get false alarms).
For any signal, have noise distribution. And get a second
graph – the signal distribution.
− The difference between means of the two is d’. So if
signal shifted to right, d’ would be big and easy to
detect. If left, d’ very small and more difficult to
detect.
− X-axis have intensity.
− The strategy C can be expressed via choice of
threshold – what threshold individual deems as
necessary for them to say Y vs. N. Ex. B, D, C, beta,
just dif variables.
− If we were to use B, let’s say choose this threshold – 2. So anything greater than 2 will say Y to,
anything less say N. So probability of hit is shaded yellow, and false alarm is pink.
− D = d’-B, so let’s say d’ in this example is 1, so 2-1=1. So if we use D strategy, anything above 1 =
Y.
− C strategy is an ideal observer. Minimizes miss and false alarm. C = B – d’/2. So in our example,
it’s 2- ½ = 1.5. So anthing above a 1.5
● When C = 0, participant is ideal observer. If <1, liberal. If >1, conservative.
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,− Beta, set value of threshold = to the ratio of height of signal distribution to height of noise
distribution. lnbeta = d’ x C = 1 x 1.5 = 1.5
Bottom-Up vs. Top-Down Processing
− Bottom up: stimulus influences our perception.
● Processing sensory information as it is coming in (built from smallest piece of sensory
information)
− Top-down: background knowledge influences perception. Ex. Where’s waldo
● Driven by cognition (brain applies what it knows and what it expects to perceive and fill in
blanks)
Gestalt Principles
− Similarity – items similar to one another grouped together
− Pragnanz – reality is often organized reduced to simplest
form possible (Ex. Olympic rings)
− Proximity – objects that are close are grouped together
− Continuity – lines are seen as following the smoothest path
− Closure – objects grouped together are seen as a whole
Sight (Vision)
Structure of the Eye
− Conjunctiva is first layer light hits
− Cornea – transparent thick sheet of tissue, anterior 1/6th.
− Anterior chamber – space filled with aqueous humour, which
provides pressure to maintain shape of eyeball.
− Pupil is hole made by iris, which determines eye color
− Lens bends the light so it goes to back of eyeball.
− Suspensory ligaments, attached to a ciliary muscle. These two things
together form the ciliary body, what secrets the aqueous humor.
− Posterior chamber Is area behind the ciliary muscle, also filled with
aqueous humor.
− Vitreous chamber – filled with vitreous humour, jelly-like substance
to provide pressure to eyeball.
− Retina is filled with photoreceptors.
● Macula – special part of retina rich in cones.
● Fovea – completely covered in cones, no rods.
− Choroid – pigmented black in humans, a network of blood vessels. Bc black all light is reflected.
− Sclera – whites of the eye, thick fibrous tissue that covers posterior 5/6th of eyeball. Attachment
point for muscles.
Visual Sensory Information
Sensation requires light -> neural impulse, by a photoreceptor
What is light?
− Electromagnetic wave part of a large spectrum
− EM spectrum contains everything from gamma rays to AM/FM waves. Visible light is in the
middle
● Violet (400nm) – Red (700nm)
− The Sun is one of most common sources of light
Light enters pupil and goes to retina, which contains rods and cones
− There are 120 million rods, for night vision
● Light comes in, goes through pupil, and hits rod. Normally rod is turned on, but when light
hits turns off.
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, ● When rod is off, it turns on a bipolar cell, which turns on a retinal ganglion cell, which goes
into the optic nerve and enters the brain.
− There are 6-7 million cones
● 3 types: red, green, blue
● Almost all cones are centered in fovea
Phototransduction Cascade – when light hits rods and cones
− Retina is made off a bunch of dif cells – rods and cones.
− As soon as light is presented to him, it takes light and converts it to neural impulse. Normally
turned on, but when light hits it’s turned off.
PTC is set of steps that turn it off.
− Inside rod are a lot of disks stacked on top of one another.
− A lot of proteins in the disks. One is rhodopsin, a multimeric protein with 7 discs, which contains
a small molecule called retinal (11-cis retinal). When light hits, it can hit the retinal, and causes it
to change conformation from bent to straight.
− When retinal changes shape, rhodopsin changes shape.
− That begins this cascade of events – there’s a molecule in green called transducin made of 3 dif
parts – alpha, beta, gamma
● Transducin breaks from rhodopsin, and alpha part comes to disk and binds to
phosphodiesterase (PDE).
● PDE takes cGMP and converts it to regular GMP. Na+ channels allow Na+ ions to come in,
but for this channel to open, need cGMP bound. As cGMP decreases, Na channels closes.
● As less Na+ enters the cell, rods hyperpolarize and turn off. Glutamate is no longer released,
and no longer inhibits ON bipolar cells (it’s excitatory to OFF bipolar cells).
● So bipolar cells turn on. This activates retinal ganglion cell which sends signal to optic nerve
to brain.
Photoreceptors (Rods and Cones)
− A photoreceptor is a specialized nerve that can take light and convert to neural impulse.
− Inside rod are optic discs, which are large membrane bound structures – thousands of them. In
membrane of each optic disc are proteins that fire APs to the brain.
− Cones are also specialized nerves with same internal structure as rod.
− Rods contain rhodopsin, cones have similar protein photopsin.
− If light hits a rhodopsin, will trigger the phototransduction cascade. Same process happens in a
cone.
− Differences:
● 120 M rods vs. 6 million cones.
● Cones are concentrated in the fovea.
● Rods are 1000x more sensitive to light than cones. Better at detecting light – telling us
whether light is present, ie. BW vision
● Cones are less sensitive but detect color (60% Red, 30% Green, 10% Blue)
● Rods have slow recovery time, cones have fast recovery time. Takes a while to adjust to dark
– rods need to be reactivated.
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