Vision Flashcards
The _____ and ____ focus light
cornea, lens
Refraction
The bending of light
Done by the cornea
Ciliary muscles
Adjust the focus of the eye by changing the shape of the lens
Accommodation
The process of focusing the lens
Pupil
Controls how much light enters the eye
Dilates or shrinks to adjust for light
Extraocular muscles
Control eye movements
Retina
Where visual processing begins
Contains several cell types
Photoreceptor cells
Light goes all the way to the back here first
Rods and cones
Bipolar cells
Light goes out to the bipolar cells; assisted by the horizontal cells
Receive input rom photoreceptors and synapse on ganglion cells
Ganglion cells
Light goes out to the ganglion cells; assisted by the amacrine cells
Axons from these cells form the optic nerve and exit the eye
Horizontal cells
In the retina
Contact photoreceptors and bipolar cells
Inside-out processing
Light goes to the back of the eye first and makes its way out
Photopic system
The one that uses cones
Good for color vision
Good for visual acuity and sharpness
Downside of photopic system
Require more light to be activated
Scotopic system
Uses rods
Works in dim light
Low visual acuity
Three general characteristics of the visual system
Sensitivity
Integration
Adaptation
Sensitivity
Weak stimuli are amplified to produce physiological effects
Integration
Takes time to integrate the stimuli, which makes vision relatively slow but increases sensitivity
Adaptation
The visual system is capable of adapting to a wide range of light intensities
Quanta
Unit of measurement for electromagnetic radiation that the visual system responds to
Each quantum has a _________
wavelength
Photons
Quanta of light energy with visible wavelengths
The visual system can deal with a wide range of light intensities by…
- adjusting pupil size
- range fractionation
- photoreceptor adaptation
Range fractionation
Different intensities handled by different receptors
Low threshold are rods
High threshold are cones
Photoreceptor adaptation
Ability of individual photoreceptors to adjust sensitivity to prevailing level of illumination
Threshold for perception of color
Light intensity has to be above 10^-5 to activate cones to see color
Visual acuity
Measure of how much detail we see and is sharpest in the center of the visual field, where the fovea is located
Vision is sharpest at the fovea because…
- there is a high density of tightly-packed cones
- this region receives direct light input that does not pass through other cells
Fovea
Indentation
Light reaches the cones without having to pass through blood vessels and other layers of cells
Rods
Mostly present at the periphery (outside of fovea) to capture the light that comes in when we widen our pupil
Optic disc
Where blood vessels and the optic nerve leave the eye, resulting in no photoreceptors
Blind spot
Due to lack of photoreceptors in the optic disc
Why don’t we see a hole in our vision?
Our eyes are constantly moving around and our brain fills in that picture for us
Rods are absent from the fovea
They are more numerous in the periphery and are more sensitive to dim light than cones are
Why does the pupil dilate under low illumination?
Helps to let in light to activate the rods
Saccades
Our eyes are constantly shifting a little bit to prevent adaptation of making the scene disappear
Lateral inhibition
A process where interconnected neurons inhibit their neighbors and produce contrast
Axons from retinal ganglion cells exit the eye and form the optic nerve; some cross at the _____________
optic chiasm
After passing the optic chiasm, the axons of the optic nerve are called the __________
optic tract
Left visual field…
hits the right side of both eyes
Sent to the right visual cortex for processing
Right visual field…
hits the left side of each eye
Sent back to the left visual cortex for processing
After optic tract…
Info goes to thalamus for processing
Then goes to primary visual cortex
Nasal hemiretina
Portion of retina closest to the nose
Projects its axons to contralateral side of the brain
Temporal hemiretina
Lateral portion of the retina
Projects its axons to the ipsilateral side (same side) of the brain
Most axons in the optic tract synapse on cells in the _____________ of the thalamus
lateral geniculate nucleus (LGN)
Optic radiations
Made by axons of postsynaptic cells in the LGN
Terminate in the primary visual cortex (also called the striate cortex and the occipital lobe)
Damage at the level of retina
Blindness
No transduction of light energy
Damage at the cortex
Individual can see something but cannot identify what it is
Where your eyes converge
Best acuity (when you’re looking forward)
Acuity falls off toward the periphery of the visual field
Visual field
The whole area you can see without moving your head or eyes
Which animal below has a wider visual field? Why?
Visual field for rabbit is very wide since eyes are on sides of head
Which animal has better visual acuity? What makes it so?
Owl, because there is more overlap between vision of both eyes
Evolutionarily, why might this be the case for the rabbit?
He can identify that something is coming and quickly get out of the way
Trichromatic hypothesis of color perception
Three types of cones
Each cone has a different type of opsin, or photopigment, that respond to the different part of the spectrum
Trichromatic theory– cone sensitivity
Short: peak sensitivity at 420 nm
Medium: 530 nm
Long: 560 nm
Color blindness
Due to absence of cones sensitive to medium-wavelength light (M cones)
Mostly in men (women can be carriers)
Amblyopia
Reduced visual acuity; one eye is stronger
Myopia
Nearsightedness; occurs if the eyeball is too long
Image hits in the middle of the eyeball instead of the fovea
Glasses help correct so that the image hits the back of the eye
