4.2 The visual system Flashcards
eyes rely on light energy
eyes take in light information and convert it into neural signals that the brain can interpret
eyes
tool for collecting light energy
light
travels through space in the forms of waves
wavelength
the distance b/w two waves
amplitude
range
distance b/w the lowest and highest points
high amplitude is brighter than low amplitude
the visual light spectrum is just a small part
gamma rays (too short to see ex, used for tanning)
x-rays
ultraviolet rays
visible light
infrared rays
radar
radio waves (too long for us to see)
ac circuits
shortest waves lengths we can see are
blue or purple
longest visible wave lengths is the colour
red
intermediate visible wave lengths are the colours
yellow and green
saturation
*light that consists of a mixture of different wavelengths have low saturation (faint colour)
# of wavelengths they receive from a single region in space
complex mixtures of wavelengths are perceived as white
*light waves that consists of mostly one wavelength have high saturation
what we perceive as colour represents the …
wavelengths that bounce off of objects
structure of the eye
designed to let light enter but also to direct light to the right spot of the inside of the eye
- sclera
- cornea
sclera
white surface of the eyes
cornea
clear part in the front of the eye
-shape of cornea helps direct light to right area of the eye by suspending the light waves toward the point that it needs to strike at the back of the eye
pupil
the dark opening part
- after traveling through the cornea, light then travels through the pupil
- size of pupil varies depending on how much light is available
- in bright context pupil is smaller to reduce the amount of light that can enter
- in dim context pupil is larger to allow more light to enter
iris
difference in pupil size depends on the iris
- the part around the pupil that has a colour
- it is a muscle that tightens around the pupil constricting it
- when the muscle relaxes the pupil dilates
the lens
behind the pupil
- the clear part that changes shape
- bends light more or less
- accommodation reflex
accommodation
refers to the lens changing shape to bend light that enters the eye, so that it strikes the right spot at the back of the eye
- close objects need the lens to be more spherical because the light from those objects must be bent more significantly to bring them into focus
- bringing far objects into focus requires the lens to be flat since light energy does not need need to be bent as much to direct them to the proper spot at the back of the eye
the retina
after passing through the lens the light will travel through the liquid contents of the eye and strike a structure at the back called the retina
- photoreceptors
- ganglion cells
- optic nerve
photoreceptors
pair of light receptors designed to absorb light
- transduce light energy into neural signals
- rods and cones are two types of photoreceptors
ganglion cells
take signals from photoreceptors and relay that info to the brain
optic nerve
where the axons of ganglion cells get bundled together
fibres going to the brain
blind spot
rods
are more sensitive to light and they do not provide as fine-grained an image as the cones
- they can detect light at very low intensities so we rely on them more in dim lighting
- grey, black, white
- yellow dots dominate outside of the fovea which represents the periphery of the visual field (not as defined as cones )
cones
are less sensitive to light so we don’t rely on the as much in the dark
-different cones respond to different wavelengths, so we rely on them for colour vision
(the blue green and red dots dominate in the fovea which represents the centre of the visual field
dark adaptation
the process of rods and cones adjusting their sensitivity to dark lighting conditions
this is why dark environments feel less dark as we spend more time there
tri-chromatic (or young-Helmholtz) theory
colour vision depends on three types of cones that are each sensitive to different wavelengths of light that correspond to the colours blue, green and red.
colour blindness
people missing one or more of the cones
negative afterimage
starting at one colour and the afterimage is a different colour
opponent-process theory
colour vision depends on patterns of neural signals that put pairs of colour in opposition
red vs green
blue vs yellow
black vs white
individual ganglion cells respond most vigorously to one colour (red) but their activity is inhibited when presented with the opposing colour (green),
this explains negative afterimages, cells that responds to (green) get exhausted over time when presented with something green and viewing green inhibits cells that respond to red
tired green ganglion cells cant compete with a rebound effect from the previously inhibited red ganglion cells, once we stop viewing something green. instead of a return to a neutral state, we end up experiencing an illusion of red after staring at green
nearsightedness (or myopia)
prevents bringing far scenes into focus
this occurs when the eye is slightly too long
the light needs to bent less
farsightedness (or hyperopia)
prevents bringing close objects into focus
- when the eye is just a bit short that the light is not sufficiently bent by the time it strikes the retina at the proper spot
- the light needs to bent more
optic chiasm
the crossover of optic nerves at the brains midpoint
cross over for the optic nerve fibres from the right side of the brain for the left eye and the optic nerve fibres from the left side for the right eye
left visual field is processed by the
right hemisphere
right visual field is processed by the
left hemisphere
on the way to the occipital cortex, visual inputs get routed through the …
lateral geniculate nucleus of the thalamus
feature detection cells
cells in the occipital cortex of humans and other animals preferentially respond to edges presented at a specific orientation and at a specific location in the visual field
ventral stream of visual processing
deals with processing visual info according to its identity (its a pencil!)
perceptual constancies
info received by our eyes vary a great deal according to :
-viewing angle
-lighting conditions
-distance
we are able to unconsciously correct for these variations and perceive the properties of objects as constant or unchanging
shape constancy
different viewing angles do not lead us to conclude objects are changing shape
colour constancy
light and shadow change the wavelengths that bounce off of objects to strike our retinas
(amount of wavelengths in dim or bright light reflecting off the objects does not change colour)
size constancy
the size of image an object projects varies with distance, but we are able to correct for this variability
closer objects project larger images than further objects
dorsal stream of visual processing
deals with processing visual info for the purpose of guiding motor actions (enabling use of the pencil for writing)
depth perception
despite the 2D image projected to our retinas, depth perception enables us to perceive the world in 3D
binocular depth cues
-combined influence and info from both eyes
convergence
retinal disparity
convergence
(extreme convergence is fully crossing our eyes)
-closer objects need more convergence (most objects we see already are close though)
retinal disparity
since both of our eyes view the world from different angles we can use the difference in viewing angles to determine how far an object is away from us
(3D movies )
separate but overlapping images to each eye
monocular depth cues
-each eye separately accommodation motion parallax interposition object brightness linear perspective texture gradient height in plane relative size
accmmodation
lens changing shape in response to brining close vs far objects into focus
-source we can use to determine the distance of objects
motion parallax
helps with depth perception when we are on the move
in those situations, visual inputs that are closer to us will appear to move faster and in the opposite direction we are moving, where the ones that are farther away appear to move slowly and is moving in the same direction as we are
interposition
these cues conclude interposition in which one can infer that an object is closer than another one because it obscures the view of the other object
object brightness
since the light received by our retinas from objects that are further away will be at lower intensity than light from closer objects
linear perspective
occurs when parallel lines precede into the distance, when they do the lines appear to get closer together
texture gradient
far objects will tend to be less defined and blurry then close objects
height in plane
aspects of the environment that are higher in our visual field will tend to be further away
relative size
an object will project a smaller image onto our retinas if it is further away from us
the visual system
helps guide and helps our interactions
