PSY280 - 2. Signal detection theory Flashcards
Response criteria
what people do when they’re unsure.
subjective magnitude: when they’re willing to say yes
response criteria can be variable even within the same person
signal detection theory
half there is a stimulus, half there isn’t
hit: present + yes, miss: no + present
false alarm: absent + yes, correct rejection: no + absent
signal detection theory
you want pavel to say no more: manipulate payoffs - make correct rejections really attractive
make false alarms repulsive - take money from you
vivian more liberal: hits attractive + miss unattractive
test them in 3 conditions: neutral, encouraged to say yes + then no
receiver operating characteristic
ROC
curve plots a person’s proportion of hits vs. false
alarms for different 50 response criteria.
only need these 2 because they’re proportions of miss + correct rejection
essentially right on top of one another: sensitivity is identical
Response criteria
decision- making under conditions of uncertainty.
The ability to detect a faint stimulus is about perceptual sensitivity.
highly sensitive = yes more without increasing false alarms
Signal Detection Theory
recognize that sensitivity can be evaluated using hits & false alarms
interpret an ROC curve
recognize that response criteria is orthogonal (unrelated) to perceptual sensitivity
sensory systems
near/far with respect to detecting external stimuli
near: gustation, somatisation - touch
middle: olfaction, audition
far: vision
Vision
vision is more informative
dominant sensory system in humans, and takes up about 1/2 of the brain for processing.
complex: sheer magnitude of info requires considerable computative power
Vision
knowledge - brain
light - receptors - afferent neurons - brain
afferent: from receptors into the brain
Light
electromagnetic radiation, a moving energy field that is produced by vibrations of electrically charged material
exists everywhere in nature
Light
Visible light: energy within electro magnetic spectrum that humans can perceive has wavelengths ranging from about 400 to 700 nm
Light consists of small packets of energy called photons with one photon being smallest possible packet of light energy
Light
wave that travels through a medium without permanently displacing the medium
stream of photons, tiny particles that consist of one quantum of energy
light as wave when moving around
light as photon when being absorbed
electromagnetic spectrum
can vary in length from kilometers to nm
wavelengths are not coloured, the colour is how our visual system has interpreted the physical properties of wavelength
continuum of electromagnetic energy that is produced by electric charges + is radiated as waves
Energy described by its wavelength: distance between the peaks of the electromagnetic waves
Light
travels through empty space at about 186,000 miles/sec
atmosphere some gets absorbed, some gets scattered
when a light from a star hits the atmosphere some of it is absorbed turned into heat
some will penetrate
The remaining light will eventually hit a surface, where it might be
reflected, absorbed, transmitted
colour represents wavelength of light it most reflects
white reflects all wavelengths equivalently + black does same but absorbed
if light isn’t absorbed/reflected it is transparent
transparent objects tends to refract the light
water bends the light a lot
eyes
sclera - tough outer membrane fiber - white part - protein is randomly - light is reflected
opaque - reflecting light
cornea - transparent window
outer membrane - transparent - allows light to pass through
eyes
same protein but it’s neatly stacked in parallel that allows light to pass through
aqueous humor - fluid, provides oxygen & nutrients
does the same job as blood, derivative but it transmits light
iris - colored muscular diaphragm - controls how much light is allowed to enter the eye
eyes
pupil - light enters
crystalline lens - changes focus - actively bends light to focus it on the back of the retina
vitreous humor - holds retina in place
retina - light-sensitive membrane
optic nerve - bundle of nerve fibres: collects electrical signals created by the retina + transmits them to the brain
eyes
behind the lend is a chamber filled with vitreous humour - gelatinous - hold shape of the eye
make sure retina stays attached to the back of the eye
vitreous humour is not replenished - floaters are waste from proteins
lens
actively refracts light by changing it’s shape to help focus light on the retina.
cornea, aqueous humour + lens
cornea does most of it - but passive
light naturally focuses on the back of the retina
The Eye
Rods + cones contain light-sensitive chemicals called visual pigments that react to light + sugar electrical signals
Signals flow through the network of neurons that make up the retina + emerge in the optic nerve which conducts signals toward the brain
Light is focused by the eye
Cornea: 80% of the eyes focusing power, it is fixed in place
lens can change its shape to focus for objects located at different distances
Lens controls accommodation
Light is focused by the eye
Accommodation: change in lenses shape that occurs when the ciliary muscles at the front of that eye tighten + increased the curvature of the line so that it gets thicker
Increased curvature increases the bending of the light rays so the focus point is pulled back to A to create a sharp image on the retina
Near point: distance at which your lens can no longer accommodate to bring close objects into focus
accommodation
lens fatter when gaze is directed toward nearer objects
behind eye when near object is seen with relaxed eye
Presbyopia: distance of the Nearpoint increases as a person gets older
Loss of ability to accommodate occurs because the lens hardens with age + ciliary muscles become weaker
myopia
object is near, light focussed farther back, so near objects are not a problem for people
emmetropia: perfectly converges light to the retina
nearsightedness - eyeball is too long, for distant objects, it converges at the front
need concave lenses to bend light out a little bit
Myopia
Brings parallel rays of light into focus at a point in front of the retina to the image that reaches the retina is blurred
Refractive myopia: cornea/lens Bends the light too much
Axial myopia: eyeball is too long
Farpoint: distance at which light becomes focused on the retina
Glasses bend incoming light focused as if it were at the far point
Laser assisted… (lasik): sculpting cornea
Hyperopia
Farsighted: can see distant objects clearly but has trouble seeing nearby objects
Focus point for parallel rays of light is located behind retina usually because eyeball is too short
short eyeball
convex lenses to follow shape
Astigmatism
due to an irregularly shaped cornea
cornea is not spherical - football shaped
intersections look blurry
horizontal + vertical lines intersect at diff depths
neurons
dendrites listen
axons talk
cell body integrates all the signals
There is some variation in structure, but neurons tend to have a basic “recipe”
Sensory receptors
pick up information from environment, rather than from other neurons.
interact with environ to translate stimuli into electrical signals
receptors respond to diff energies
touch: mechano energy
vision: electromagnetic energy
Ions
Ion channels + ion transporters regulate # of ions inside & outside of the cell.
ion transporter: selective + require energy to transport ions
Resting Membrane
sodium more concentrated outside + K inside
At rest, there are more positive ions outside than inside.
membrane potential is all relative
negative membrane potential
action potential
sodium (Na+) ion channels open + Na+ rushes into axon, resulting in fewer positive ions outside.
more positive inside + flip open allowing sodium to enter + makes membrane more positive which triggers opening of next one
shoots up to +40 membrane potential causes Na+ channels to close + potassium (K+) channels to open, letting K+ out.
sodium-potassium pump
trade 3 Na+ for 2 K+, so that ions end up in their original concentrations.
Na + K+ pumps actively put back ions in original locations
action potential
15 mv difference for threshold, diff neurons have diff resting membrane
refractory period can’t do anything for this period
whole process happens within 2 milliseconds
all or none principle
release of neurotransmitters
Neurons usually produce & release only one kind of neurotransmitter:
•excitatory: (Na+) sodium channels open
•inhibitory: (K+) potassium channels open
excitatory: easier to get to threshold
inhibitory: even more negative - threshold farther
reuptake: take up neurotransmitter + stop releasing
release of neurotransmitters
Neurons usually have receptors for multiple neurotransmitters
can receive both excitatory + inhibitory signals
rate of firing dictated by summated signals
Neurons
Neurons fire in absence of stimuli (spontaneous activity)
signals can involve either an increase/decrease in firing rate
no change means signals are cancelling each other out = no perception
Transforming light energy into electrical energy
Transduction occurs in rods + cones
Millions of molecules of light-sensitive visual pigment that are contained in the outer segments of the receptors
Visual pigments contain a long protein called opsin + a much smaller light-sensitive component called retinal
Transforming light energy into electrical energy
Retinal is the crucial part of the visual pigment molecule because when combined the resulting molecule absorbency visible light
changes its shape from being bent to straight – Isomerization creates a chemical chain reaction that activates thousands of charged molecules to create electrical signals in receptors
Chain reaction amplifies effect of isomerization
Visual pigments shape specific aspects of our perceptions
retinal info processing
light needs to interact with all of these cells before photoreceptors can translate these signals
layer of the retinal that is further away from the retina
these cells need to be transparent in order to reach the photoreceptors
the electrical signals comes forward to the ganglion cells
retinal info processing
outer segments are packed with photopigments like rhodopsin, composed of opsin and a retinal molecule.
diff receptors have diff photopigments
opsin is always a huge chain protein molecule that weaves in + out of the membrane + retinal molecule at the end, but it’s the thing that responds to light
isomerized
When a photon hits the retinal
when light hits it is isomerizes + detaches, it becomes activated
unstimulated, on in the dark
dark current
current running through photoreceptors in the dark
change in membrane potential because ions keep moving across membrane
Dark adaptation: increasing sensitivity in dark
Rod and cone receptors adapt to the dark at different rates
Macular degeneration: destroys Cone rich fovea and a small area that surrounds it
Creates a blind region in the central vision
Adapting to the dark
Retinis pigmentosa: degeneration of retina that is passed from one generation to the next
Attacks peripheral rod receptors and results in poor vision in the peripheral visual field
Blind spot: absence of receptors
Blind spots are hard to detect
Some mechanism in the brain fills in the place where the image disappears
In the dark:
Na+ ions are flowing inward via cGMP-gated sodiums channels in the outer segment
K+ is flowing outward via potassium channels in the inner segment
The sodium-potassium pump is working to restore the balance
In the dark:
outer membrane to be more positive
in inner segment, K+ channels open + leaving the cell - makes cell more negative
pump tries to restore balance
-40mv resting potential
In the light
Activated rhodopsin activates a G-protein, which activates an enzyme - converts cGMP to GMP, closing the sodium channels
no more cgmp, sodium channels slam shut
In the dark:
photoreceptors releasing excitatory neurotransmitter (glutamate)
action potentials don’t start until the ganglion cells, photoreceptor responses are graded
in dark -40mv, releasing a lot of glutamate
with isomarization - becomes more negative
in the light
with isomarization - becomes more negative
photoreceptors are hyperpolarized & release less glutamate
just about how much neurotransmitter is being released
Rods + Cones
Rods work in the low lighting conditions, but not sensitive to detail.
Cones work in bright lighting conditions + process color & detail.
120 million rods to 6 million cones -most in the fovea, most sensitive
rods photoreceptors under dim light - hard to see stuff
cones need a lot of light to do their jobs
blind spot bundles ganglion nerves to take it to the back of the eye
method of adjustment
measure rate of dark adaptation
how long does it take to see effectively in the dark
constantly reevaluating threshold
2 stages of adaptation
photopigments of rods + cones regenerate at different rates
method of adjustment
Dark adaptation curve: junction relating sensitivity to light to time in the dark, beginning when lights are extinguished
Look at small fixation point while paying attention to a flashing test light that is off to the side
minimum amount of energy necessarily to just barely see the light is converted to sensitivity
method of adjustment
Sensitivity equals 1/threshold, high threshold corresponds to low sensitivity
Light adopted sensitivity: sensitivity measured in the light, measured while the eyes adapted to the light
Once in the dark observer continues adjusting the intensity of the flashing light so it can just barely be seen, tracking the increase in sensitivity that occurs in the dark
As observer becomes more sensitive to light she must decrease the light intensity to keep it just barely visible
method of adjustment
Dark adaptation curve shows that as adaptation proceeds, observer becomes more sensitive to the light
Dark adopted sensitivity: sensitivity at the end of dark adaptation, 100,000 times greater than the light adopted sensitivity measured before dark adaptation begin
Regenerate
time it takes to adapt to dark reflects how long it takes for visual pigment to regenerate.
regenerate: takes longer for rods than cones
Measuring Cone adaptation
Image of the test light falls only on the cones by having observer look directly at the test light so the image will fall on the all cone fovea
cones maxes out in the dark
rods continue to regenerate + become more sensitive overtime
rod cone break
Measuring rod adaptation
Rod monochromats: people who have no cones due to a rare genetic defect
Once dark adaptation begins, rods increase sensitivity + reach final dark adopted level in about 25 minutes
Rod – Cone break: place where the rod to begin to determine the dark adaptation curve
Spectral sensitivity
relative sensitivity to light as a function of it’s wavelength.
for short wavelengths it needs to be intense to detect it
how bright something appears at the the same intensity
yellowish green is brightest colour of the day
Visual pigment regeneration
Visual pigment bleaching: after retinal part of the visual pigment changes its shape retinal separate from the opsin part of the molecule causes the molecule to become lighter in color
retinal needs to return to its bent shape + become reattached to the opsin
Sensitivity to light depends on the concentration of chemical – Visual pigment
Visual pigment regeneration
speed at which our sensitivity increases in the dark depend on chemical reaction – regeneration of the visual pigment
Detached retina: bleach pigment separated right now and opsin can no longer be recombined in the person becomes blind in the area of the visual field serve by the separate area of the retina
Spectral sensitivity
Measuring spectral sensitivity curve
We present one wavelength at a time + measured observers sensitivity
Life of a single wavelength – mono chromatic light – created using special filters called a spectrometer
Ability to see what wavelengths is plotted in terms of sensitivity versus wavelength
Spectral sensitivity
stimulates only the cones
Measure rod spectral sensitivity curve by measuring sensitivity after the eye is dark adopted
Rods are more sensitive to short wavelength of light then cones
Purkinje shift: shift from cone vision to rod vision occurs at dusk because you’re eye begins dark adopting in low light levels so the rods begin to influence vision
Rod and cone absorption Spectra
Absorption spectrum: plot of amount of light absorbed versus the wavelength of the light
Rod and cone absorption Spectra
Short wavelength pigment absorbs light best at about 419 nano meters, medium wavelength pigment absorbs light at about 530 190 m and long wave length pigment absorbs light at 558 nm
fewer short wavelength receptors + much less of the short wavelength pigment
Sensitivity in the dark and sensitivity to different wavelengths are determined by the properties of the rod and cone visual pigment
Electrical signals in neurons
Signals are transmitted out of back of the eye in optic nerve to a group of neurons called the lateral geniculate nucleus and then to the visual receiving area of the cortex
Recording electrical signals in neurons
Electrodes are connected to a meter that records the difference in charge between the tips of the two electrodes
Difference in potential between the tips when an axon is at rest is -70 mV
Resting potential: inside the neuron is 70 mV more negative than the outside
Action potential last about one millisecond
Basic properties of action potential
Propagated response: once the response is triggered it travels all the way down axon without decreasing in size
Changing stimulus intensity affect the rate of firing
Refractory period: interval between the time A nerve impulse occurs in the next one can be generated by the axon – 1ms
Spontaneous activity: establish as a baseline level of firing for the neuron
