Visual perception Flashcards
what is the retina?
the light-sensitive surface on 75% of the inner eye
what is the peripheral retina?
all the retina outside of the fovea. it contains rods & cones
what is the fovea?
what the image in our central vision falls on. high acuity (detailed). contains only cones
what is the optic disk?
point of exit for ganglion cell axons. it corresponds to the blind spot (no rods/cones)
what is macular degeneration?
deterioration of the retina in the macula (around the fovea). it’s a progressive illness that causes irreversible blindness & loss of central vision
short wavelengths
blue/green & high electromagnetic energy
long wavelengths
red/orange/yellow & low electromagnetic energy
process of accomodating to a far object
ciliary muscles relax & lens slims (light only bent a little)
process of accomodating to a near object
ciliary muscles tighten & lens thickens (light bent a lot)
what is accommodation?
the process by which the eye changes optical power to focus on an object as it’s the distance varies
what is myopia?
(i.e. near-sightedness) far objects are out of focus because the lens is too thick or the eyeball is too long. requires concave correction
What is hyperopia?
(i.e. far-sightedness) near objects are out of focus because the eyeball is too short. requires convex correction.
what is transduction?
the image on the retina is transformed into electrical energy
how many rods and cones?
120 million rods & 6 million cones
where are cones and rods?
only cones in the fovea, both cones and rods in the peripheral retina (except for in optic nerve)
how do cones adapt to darkness?
fast adaptation and low sensitivity (tested using only fovea)
how do rods adapt to darkness?
slow adaptation and high sensitivity (adjust more)
what is visual acuity?
can see detail (higher in cones)
what is neural convergence?
allows a neuron to receive input from many neurons in a network.
the difference between neural convergence in rods & cones
more convergence of rods than cones. 1 ganglion cell receives input from 120 rods & 6 cones (on average)
what is the spectral sensitivity of rods and cones?
rods are sensitive to shorter wavelengths (max of 500nm) & cones to longer wavelengths (max 560nm)
what is the Purkinje shift?
there’s more sensitivity to shorter wavelengths in the dark-adapted eye
what are the 3 types of cones
S cones (respond to short wavelengths, blue), M cones (respond to medium wavelengths, green), & L cones (respond to long wavelengths, red)
what is photopic vision?
cone-dominated, fovea & peripheral vision. light-adapted, high visual acuity, most sensitive to long wavelengths, the basis of colour vision, & few neural conversions
what is scotopic vision?
rod-dominated, peripheral vision, dark-adapted, low visual acuity, sensitive to short wavelengths, & no colour sensation
what is mesopic vision?
rod & cone vision together
what are ganglion cells?
receive input from photo receptors. input excites or inhibits ganglion cell.
they have on/off centre/surround receptive fields (on = excitatory when stimulated by light)
how many photo receptors do ganglion cells receive input from?
it varies, but approx 126
what’s uniform illumination?
light on on & off area of ganglion cell. doesn’t have an excitatory or inhibitory effect.
what is luminance discontinuity?
edge of light is touching the centre of a ganglion cell. this causes activation based on the surround
what are the 3 types of ganglion cells?
Magnocellular (M or parasol),
parvocellular (P or midget),
Koniocellular (K)
Magnocellular ganglion cells
get most of their input from rods & aren’t colour specific
Parvocellular ganglion cells
get input from single M or L cones. they’re colour specific (green or red on/off)
Koniocellular ganglion cells
excitatory input from S cones & inhibitory input from M & L comes (blue on)
eye anatomy
axon of ganglion cells -> retino-geniculo-striate pathway -> temporal/nasal representation -> optic nerve -> optic chasm -> optic tract -> LGN -> visual cortex
which side of the brain does visual information go to?
things in left visual field go to temporal representation in right eye or nasal representation in left eye (visa versa for right visual field).
at optic chasm, the nasal axons cross over to the other side of the brain & temporal axons stay on the same side. the visual fields are represented in the contralateral hemisphere.
what is the structure of the lateral geniculate nucleus?
6 layers of axons. 4 from P neurons, 2 from M neurons, & K neurons are interlayered.
visual cortex structure
6 areas - V1 (primary visual/striate cortex), extrastriate areas (V2-V5), & IT (inferotemporal cortex)
describe the V1 area
where optic radiation terminates. has M & P cell layers.
some V1 neurons are orientation-selective, motion-direction selective, or selective for colour/brightness.
what are orientation-selective neurons?
elongated receptive fields that are side by side. stimulated by a stimulus with the preferred orientation
who studied orientation-selective neurons in cats in the V1 area?
Hubel & Wiesel (1959)
what are motion-direction selective neurons?
respond when a stimulus in their receptive field matches their preferred motion direction.
describe the V2 area
receptive fields 2x as large as in V1
respond to basic stimulus features (orientation, motion-direction, brightness etc.). integrate more info than in V1, so respond to more complex features, too (e.g. arcs, lengths, angle, shapes, texture)
who found evidence for the dorsal and temporal stream?
Ungerleider & Mishkin (1982) - studied monkeys with lesions.
whats the difference between the parietal and temporal stream?
parietal (dorsal) - processes object location (where) - V3 & V5
temporal (ventral)- processes object identity (what) - V4
describe the V4 area
5x larger receptive fields than in V1
neurons respond to object-defining features, e.g. colour, orientation, complex shapes, & texture
what neurons converge onto V4?
mainly P & K
what neurons converge onto V3?
mainly M
what is sensation?
the uninterpreted sensory impressions created by detecting a stimulus
what is perception?
the cognitive processes of making sense of sensations
what is structuralism?
studies the elements of consciousness. based on the idea that conscious experience can be broken down into sensory elements
who came up with structuralism?
Wundt (1879)
what is introspection?
someone describes a description of the sensory elements they experience (Titchener, 1901)
what did Fechner do?
1860-aims to measure the elements of perception.
how did Fechner measure the absolute perception threshold?
Method of adjustment
method of limits
method of constant stimuli
what is the absolute perception threshold?
how much stimulus intensity we need to perceive a stimulus.
what is the method of adjustment?
participants adjust intensity of a test light until they’re just able to perceive it (min threshold).
the more time in the dark, the less stimulus light needed for perception
what is the method of limits?
participants presented with trails of increasing/decreasing light intensity & asked if they can see the stimuli.
the trials were done with ascending/descending light intensity
mean absolute perception threshold = 2.25 candela
what is the method of constant stimuli?
same task as method of limits. more trials per light intensity & light intensity’s randomised for each trial.
80% of stimuli detected at 4 candela, & 25% at 2 candela
50% detected at intensity 3 (change level)
What did Weber do?
1834- aimed to measure the difference threshold (minimum intensity to discriminate between 2 stimuli)
what’s Weber’s law?
the change in a stimulus needed to discriminate it from another stimulus is a constant ratio of the original stimulus (difference threshold = 8%)
brighter stimulus 1 is, the greater required brightness for stimulus 2 to be perceived as different
what did Stevens do?
1957- aimed to come up with a subjective magnitude estimation to measure the relationship between stimulus intensity & perceived intensity.
an increase in the perceived stimulus intensity can be larger (response expansion), or smaller (response compression), than the increase in the measured stimulus intensity
what did Gibson do?
1966- ecological theory of perception. Assumed perception’s direct (perception= sensation). & real-world provides sufficient context for visual systems to directly perceive what’s there.
perception’s based on invariant info extracted by observer’s movement. when observer moves, optic array becomes ambient, but some info remains invariant
what are some examples of invariant visual info?
optic flow pattern
texture gradient
horizon ratio
gravity, straight lines, & reflectance on diff areas
what is optic flow pattern?
focus point remains motionless, whilst rest of visual field moves away from this point