3040 quiz one

do we perceive the world in the same way? (yes)

- common language for things in the world

- no immediately obvious differences in low-level perception

- shared biology and genetics

do we perceive the world in the same way? (counterarguments)

- in language common words can have different meanings

- we do not talk about low-level perception

- biology and genetics cause us to have differences on other levels so why not perception

aphantasia

- inability to form mental imagery

- imagine an apple in your head but you can't see it

- prevalence is around 4% (high uncertainty)

- ignored until recently

hyperphantasia

- having extremely vivid imagery

- prevalence is around 3%

synesthesia

- the coming together of different senses

- seeing colors when hearing sounds

- numbers/letters evoking specific colors

- all three show that perception is not universal

conclusions on perceptual idiosyncrasy

- we all share experiences but our perception of these experiences varies more than we expect

- no individual differences besides synesthesia and aphantasia is widely studied

sensation

activation of sensory receptors

- stimulation of receptors (raw input)

perception

conscious awareness of sensations

- interpretation/conscious awareness of those sensations (shaped by context, knowledge, expectations)

perceptual process

- distal stimulus = stimulus in the environment (object in the world)

- proximal stimulus = light is reflected and focused (the retinal image/direct energy input)

- sensation = receptors activated/processes

- neural processing + perception = brain interprets the signal

- recognition + action

-- knowledge and expectation feedback into perception (top-down processing) --

absolute threshold

the minimum stimulus intensity detectable 50% of the time

method of limits

experimenter increases or decreases stimulus intensity until it becomes or stops being perceptible

- until participant can or cannot detect

method of adjustment

subject adjusts the stimulus to the threshold themselves

method of constant stimuli

present a number of intensities in random order

problems with the concept of a threshold

- thresholds vary each time you measure them

- thresholds depend on the method used to measure them

- thresholds are strongly influenced by context

- subjects sometimes report seeing a stimulus even when no stimulus is presented at all

classical/universal laws of human perception

- weber's law

- fechner's law

- steven's power law

weber's law

- the minimum amount by which stimulus intensity must be changed in order to produce a noticeable variation in sensory experience

- JND (just noticeable difference) = proportional to the intensity of the stimulus

problem: JND increases for higher intensities but relationship may not be as orderly as weber's law

magnitude estimation

assigning subjective intensity values to different stimuli

problem: subjective, varies across individuals, difficult to argue that it is universal, may reflect bias

steven's power law

- ψ(I) = k*Ia

- ψ(I) is the subjective magnitude of the sensation evoked by the stimulus

- I is the magnitude of the physical stimulus

- k is a proportionality constant that depends on the units used

- a is an exponent that depends on the type of stimulation

- brightness has a smaller exponent (<1) = compresses sensation

- electric shock has a larger exponent (>1) = amplifies sensation

problems: exponent seems to change with every detail, the relationship usually breaks down for extreme value

are there universal laws of sensation?

short answer: no subjective judgements simply do not neatly follow the objective stimulus (though they are clearly related)

long answer: there are perceptual principles that stem from how our sensors work (e.g., we are more sensitive to low intensities)

- the final percept is formed by a nonlinear inference process in the brain

- neat laws (e.g., weber's, stevens', etc.) are useful oversimplifications

perception is systematic but messy

electromagnetic spectrum

the range of wavelengths or frequencies over which electromagnetic radiation extends

- humans see ~400-700 nm (visible light)

- this range is adaptive; it is the most intense part of sunlight and also penetrates water well

focusing

you can not focus on near and far objects simultaneously; accommodation adjusts lens shape but only within a range

cornea

transparent tissue covering the front of the eye (iris and pupil)

~ 75% of focusing power of the eye

what happens when the cornea gets damaged?

astigmatism

astigmatism

a visual defect caused by unevenness in the curvature of one of the refractive surfaces of the eye (usually the cornea)

lens

flexible tissue used to focus light onto the retina

what happens when the lens gets damaged?

cataracts

cataracts

progressive clouding of the lens that usually worsens with age

- only form of treatment for advanced cataracts is cataract surgery (lens replacement)

pupil

opening in the iris that controls light entry

what does the pupil do?

- regulates the amount of light going into the eye

- in bright light the pupil closes to let in less light

- in dim light the pupil opens to let in more light

- in the dark the enlarged pupil lets in up to 16x more light than in the light

retina

sheet of neural tissue at back of eye

- light foes through all of the tissue before hitting the photosensitive receptors

- cephalopods have a different structure -- photoreceptors are at the surface (no blind spot)

- brain filters out internal "noise"

what signals does the brain filter out?

- eye vasculature shadows

- illusions like motion-induced blindness

optic nerve

bundle of axons leaving the eye

- optic nerve leaves a gap in photoreceptors = blind spot

- brain fills in missing info from context

cones

- function best in moderate and bright light

- three types of photopigment (S, M, L) = blue, green, red

- chromatic (color) vision

- 5-6 million cones (per eye)

- concentrated in fovea

- are often 1:1 ganglion cells = high detail, low sensitivity

rods

- function in very low light conditions ("night vision")

- achromatic (without color) vision

- 120 million (per eye)

- more convergence (many rods --> one ganglion cell) = high sensitivity, low detail

- concentrated in periphery

neural convergence in the retina

- 126 million photoreceptors --> ~1 million ganglion cells

- rods converge more which explains higher sensitivity but poorer detail

- cones converge less so they support sharp vision and color

- average of 120 rods to one ganglion cell

- average of 6 cones to one ganglion cell

-- cones in fovea have 1-to-1 relation to ganglion cells --

corollary

neural firing is minimized whenever possible

implications of neural convergence

- cone vision is less sensitive than rod vision

- cone vision supplies more detail than rod vision

infant vision

- at birth rods are fairly developed

- cones are underdeveloped (fewer, cover less of fovea) so --> babies have poor color vision and visual activity

- improves with development as cones mature

how do we study vision in pre-verbal infants?

- exploit the fact that infants prefer to look at new objects

- record from their brains