Chapter 2: Light and the Eyes Flashcards
Light
visible illumination type of electromagnetic radiation
Light has properties of both a wave and a stream of particles
- Wave: wavelength is a property if electromagnetic radiation
- Stream of particles: receptors absorb individual particles photons of light
Visible Light
370-730 nm
Optic Array
spatial patterns of light rays, entering eyes from different locations in a scene
- limited by where you are in space and what can reach your eyes
Brightness
perceived intensity of light reflected/ emitted
Field of View
portion of surrounding space visible when your eyes are in their socket without moving eyes
- specific eye position is species variant
- eyes in front of head in humans
Acuity
measure of how clearly fine details are seen
The eye is encased in […] layers of membrane
The eye is encased in three layers of membrane
- Sclera: tough outer membrane; white of eyes; keeps junk out of eyes
- Choroid: middle; contains blood vessels that supply O2 and nutrients to eyes
- Retina: inner membrane that contains neurons, including photoreceptors
Extraocular Muscles
Three pairs of muscles move the eyes (keeps eyes pointed in the same direction)
- Superior/ Inferior Rectus: moves eyes up and down
- Medial/ Lateral Rectus: side to side
- Superior/ Inferior Oblique: rotation of eyes and helps with spatial navigation
Shape and Size of the Eye
roughly spherical- 24 mm diameter
Cornea
transparent membrane at front of eye that refracts light
Iris and Pupil
Iris- colored part of eye; involved in inclusion of light
Pupil- opening in iris
Pupillary Reflex
automatic process where iris contracts and relaxes to control size of pupil in response to light
Chambers of Eye
Anterior Chamber:
- Between cornea and iris
- Filled with aqueous humor
Posterior Chamber:
- Smaller space between iris and lens
- Filled with aqueous humor
Vitreous Chamber:
- Large; main interior portion of eye
- Filled with vitreous humor
Intraocular Pressure
pressure of fluids in chamber of eyes
- important in glaucoma- too much fluid present
Focal Length
distance from a lens at which image of object is in focus when object is far away
Diopters
units used to express lens power (1/ focal length)
Lens
power of any lens to refract light determines its focal length
- Weak lens doesn't refract light much (info doesn't make it to retina) - Strong lens refracts light sharply
Zonule Fibers
connect choroid and lens
- changes lens and shape
Ciliary Muscle
- muscles attached to choroid
- control tension of zonule fibers
Accomodation
adjustment of shape of lens so light of object from different distances focuses correctly on retina
- ciliary muscles are relaxed= choroid pull on zonule fibers= lens become thin and flat (weak lens with long focal length)
- ciliary muscles are contract= choroid pull on zonule fibers are opposed (stronger lens with shorter focal length)
Retinal Image
clear image of optic array on retina
- image is flipped 180, but is fixed in brain
Optic Disk
axons of RGC exit the eye
- no photoreceptors
- blind spot
- no blood vessels
Types of photoreceptor in retina
Rods- black and white vision in dim light
Cones- high acuity vision in bright light
Cells in visual system
Bipolar Cells
- receive from photoreceptors - send to amacrine cells and RGC
Horizontal Cells
- receives and sends from photoreceptors and other horizontal cells
Amacrine Cells
- receive and send to bipolar cells and other cells - sends to RGC
Ganglion Cell Layer
- layer of retina that contains RGCs
Retinal Ganglion Cells (RGCs)
- receives from bipolar and amacrine cells
- sends AP to brain via optic nerve
Fovea
has no rods but lots of cones
- receives what you’ve paying most attention to
Pigment Epithelium
has embedded photoreceptors
Anatomy of Retina
Nuclear Layer
- Ganglion Cell Layer- RGCs - Inner Nuclear Layer- bipolar, horizontal, and amacrine - Outer Nuclear Layer- photoreceptors
Synaptic Layer- where retinal neurons make synapses with each other
- Inner Synaptic Layer- between ganglion and inner nuclear layer
- Outer Synaptic Layer- between inner nuclear and outer nuclear layer
Optic Nerve
nerve formed by bundling together axons of RGCs
- exits eye through optic disk
Luminance Contrast
difference in intensity of illumination at adjacent retinal locations
- due to lateral pathway
Photopigments
molecule with ability to absorb light and initiate transduction of light with neural signals
- vary in ability to process light
Order: S-cones Rods M- cones L- cones
Spectral Sensitivity
degree to which photopigment molecules absorb lights of different wavelengths
Photoisomerization
change in shape of photopigment molecules between isomers when molecule absorbs a photon
- initiates transduction of light to neural signal
*Photopigment regeneration- occurs after photoisomerization
Adapting to Changes in Lighting
- Highest level of light commonly experiences is at least a million times as great as the lowest level
- Light adaptation- opposite of dark adaptation (dark –> bright environment)
- Dark adaptation- process of adjusting retinal sensitivity when going from bright–> dark environment
Operating Range
visual systems sensitivity to range of light intensities in current scene
- visual system adjusts operating range based on current conditions
Dark Adaptation
picture on phone
Convergence
property of retinal circuits in which multiple photoreceptors send signals to one RBG
Convergence in Retinal Circuits
Numerous rods (reasons why rods are more light-sensitive) have to converge ganglion cells much more than do cone (more likely to have 1:1 ratio)
- Circuits with convergence have a property called spatial summation
- Firing rate of the RGC increases as the number of photoreceptors that are activated by light increases - Degree of convergence affects acuity and firing rate
Spatial Summation
signals from photoreceptors in same small space on retina add up to affect response of RGC in circuit
Receptive Fields
Area that, when stimulated, causes a change in the firing rate of a single cell
- Receptive field vary in size; areas in the fovea are much smaller than areas in the periphery
- Midget (smaller) and parasol ganglion (larger) cell have different- sized receptive fields
- Enhance complexity of circuits
Preferred Stimulus
type of stimulus that produces neuron’s maximum firing rate
Center- Surround Receptive Fields
- Can be on- and off- center, with different patterns of response from RGCs
- On- center: RGCs increase firing rate when light in center is brighter than surrounding
- Off- center: RGCs increase firing rate when light in center decreases - Little change in firing rate when light uniformly covers the center and surround (canceled out)
- RGCs respond to luminance contrast, not so much to uniform illumination
Lateral Inhibition
refers to the inhibitory signals sent by horizontal cells, which modify the responses of photoreceptors and provide a way in which neural activity in one part of the circuit can influence neural activity in adjacent parts of the circuit
Edge Enhancement
process where visual system makes edges visible, helping perception of objects and surfaces
- detecting boundaries is crucial vision, especially in dim light where differences in brightness at the edges may be very small
- lateral inhibition works to sharpen boundaries to enhance the process
- RGCs with center- surround receptive fields that exhibit lateral inhibition (spot detectors) make edge enhancement happen
Mach bands
changes in intensity near each edge
Single-Cell Recording
how we are able to see activation
- edges –> orientation –> form –> motion –> etc
Strabismus
disorder of extraocular muscles where two eyes aren’t aligned with each other
- double image; impairs binocular depth perception
Amblyopia
misalignment of eyes leads to suppression of signals from one eye so fine vision doesn’t develop in that eye
- eyes develop normally, but signals from one eye aren’t processed properly
Myopia
- nearsightedness
- optic axis- too long
- can’t make lens thin enough
- focus in front of retina
Hyperopia
- farsightedness
- optic axis- too short
- can’t make lens thick enough
- focus behind of retina
Presbyopia
lens become less elastic with age
- can’t make lens thick enough
Near Point begins to increase
Near Point- closest distance at which person can bring object into focus
Astigmatism
curvature of cornea or lens is slightly irregular or asymmetrical, making it impossible for lens to fully accommodate
LASIK- surgery to reshape the cornea in order to correct disorders of accommodation
Cataracts
progressive “clouding” of lens that can lead to blindness
Causes:
- exposure to UV radiation
- diabetes and other diseases
- aging
Treatment:
- surgery to remove and replace lens (of fixed focal length)
Glaucoma
intraocular pressure is too high for person’s eye
Cause:
- most commonly caused by blockage of aqueous humor drain from the anterior chamber
Treatment:
- drugs, laser, or surgical therapies
Floaters
shadows on retina thrown by debris within vitreous humor
- perceived as small, semitransparent spots/ threads that float in front of eye
Phosphenes
brief, tiny bright flashes person’s field of view not caused by light
Macular Degeneration
damage to photoreceptors in region at center of retina
- Dry- no treatment
- Wet - prescription treatment
Retinitis Pigmentosa
inherited condition is where there is gradual degeneration of photoreceptors over many years
- night blindness and tunnel vision
Night-Vision Devices
Thermal Imaging- uses IR radiation
Image Enhancement- converts photons to electrons then produces patterns of intensities
