Chapter 2 Light and the Eyes Flashcards
Light
visible illumination; a type of electromagnetic radiation, corresponding to a small slice of wavelengths in the middle of the electromagnetic spectrum
Electromagnetic radiation
physical phenomenon that is simultaneously both a wave and a stream of particles
Wavelength
distance between 2 successive peaks of a wave, different types of electromagnetic radiation are defined by their differences in wavelength
Electromagnetic spectrum
entire range of wavelengths of electromagnetic radiation, from very short to very long
Photons
single particles of light; a photon is the smallest possible quantity of electromagnetic radiation
Optic array
spatial pattern of light rays, varying in brightness and color, entering your eyes from different locations in a scene
Field of view
Portion of the surrounding space you can see when your eyes are in a given position in their sockets
Acuity
a measure of how clearly fine detail is seen
Extraocular muscles
three pairs of muscles around each eye that enable us to move our eyes very rapidly and accurately and keep the eyes always pointed in the same direction
Optic axis
Imaginary diameter line from the front to the back of the eye, passing through the center of the lens
Sclera
Outer membrane of the eye; a tough protective covering whose visible portion is the white of the eye and the transparent cornea at the front of the eye
Choroid
Middle membrane of the eye, lining the interior of the sclera and containing most of the blood vessels that supply the inside of the eye with oxygen and nutrients
Retina
Inner membrane of the eye, made up of neurons, including the photoreceptors that convert the light entering the eye into neural signals
Cornea
Transparent membrane at the front of the eye; light enters the eye by first passing through the cornea, which sharply refracts the light
- Don’t adjust how much light passing through
- fixed, accounts for about 80% of focusing power
Lens
Transparent structure near the front of the eye that refracts the light passing through the pupil so that the light focuses properly on retina
- Fine adjustments necessary to bring light into sharp focus
- adjusts shape for object distance, accounts for the other 20%
Iris
colored part of the eye; small circular muscle with an opening in the middle (the pupil) through which light enters the eye
Pupil
Opening in the middle of iris, through which light enters the eye
- Bright light-> iris contract -> pupil constricts -> light decrease
- Dim light -> iris relaxes -> pupil dilates -> increase light
Anterior chamber
space between cornea and iris, filled with aqueous humor
Posterior chamber
space between iris and lens, filled with aqueous humor
Aqueous humor
Clear, thin fluid filling the anterior and posterior chambers of the eye
*Refract light but cannot be adjusted
Vitreous chamber
main interior portion of the eye filled with vitreous humor
Vitreous humor
Clear, somewhat gel-like fluid filling the vitreous chamber of the eye
*Refract light but cannot be adjusted
Intraocular pressure
pressure of the fluids in the three chambers of the eye
Focal length
Distance from lens at which the image of an object is in focus when the object is far away from the lens (at “optical infinity”)
- Weak lens doesn’t refract light much; thin and flat with large focal length
- Strong lens refract light sharply; thick and rounded with short focal length
Diopters
power of lens; diopters= 1/focal length
*increase diopters, increase power of lens, decrease focal length
Zonule fibers
fibers that connect the lens to choroid; they pull on the lens to change its shape
Ciliary muscles
Tiny muscles attached to the choroid; they relax and contract to control how the choroid pulls on the zonule fibers to change the shape of the lens
- When ciliary muscles are relaxed -> choroid pull zonule fibers -> stretches lens -> thin, flat shape -> weak lens with long focal length -> focusing light from distant objects
- When ciliary muscles contract -> oppose pull zonule fibers -> thick, round shape -> strong lengs with short focal length -> focusing light from nearer objects
Accommodation
Adjustment of shape of lens so light from objects at different distances focuses correctly on retina
Retina image
clear image on retina of optic array
Nuclear layers
three main layers of retina, including outer nuclear layer, inner nuclear layer, ganglion cell layer
Synaptic layers
in retina, two layers separating three nuclear layers- the outer synaptic layer and inner synaptic layer
Ganglion cell layer
layer of retina that contains retinal ganglion cells
Retinal ganglion cells (RGCs)
neurons in the ganglion cell layer of retina
Inner synaptic layer
layer of retina, contains synapses among bipolar cells, amacrine cells and RGCs
Inner nuclear layer
layer of retina that contains bipolar cells, horizontal cells, and amacrine cells
Outer synaptic layer
layer of retina contains synapses among photoreceptors, bipolar cells, and horizontal cells
Outer nuclear layer
layer of retina consisting of photoreceptors (but not including their inner and outer segments)
Photoreceptors
retinal neurons (rods and cones) that transduce light into neural signals
Rods
one of photoreceptors, named for distinctive shape
- Sensitive in black and white in dim light
- Cylindrical, enclosed disks
- Can respond to as little as a single photon of light
- High sensitivity, low acuity non color night vision
- Are very sensitive, respond to absorption of a single photon. High amplification of signal within rod photoreceptor. Detect lowest light levels
- Don’t exist in fovea but more in peripheral retina
- 100 million rods across retina
- Different absorption spectra of visual pigments opsins: absorbs best at 500nm
- Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells
- Trade-off is that rods cannot distinguish fine detail and cannot handle high light levels
Cones
one of photoreceptors, named for distinctive shape
- High acuity color in bright light
- Tapered, open folds
- High acuity daylight vision in color
- Less sensitive, require absorption of multiple photons per cone. Lower amplification of signal within cone photoreceptor. Operate only at higher light levels.
- Exists mostly in fovea
- 5 million cones across retina
- Different absorption spectra of visual pigments opsin: absorb best at 419, 532 and 558 nm
- One-to-one wiring leads to ability to discriminate fine details
- Trade-off is that cones need more light to drive later neurons than rods
Pigment epithelium
layer of cells attached to choroids; photoreceptors are embedded in it
Optic disk (blind spot)
location on retina where the axons of RGCs exit the eye; contains no photoreceptors
*The brain “fills in” the spot
Optic nerve
nerve formed by the bundling together of the axons of RGCs; exits eye through optic disk
Bipolar cells
neurons in inner nuclear layer of retina
Horizontal cells
neurons in inner nuclear layer of retina
Amacrine cells
neurons in inner nuclear layer of retina
Fovea
region in the center of retina where the light from objects at the center of our gaze strikes the retina; contains no rods and a very high density of cones
- Thinner cones so packed together in dense hexagonal grid
- Ganglion cell and inner nuclear layers are pushed off to side of fovea -> light reach without being scattered as much
“Through Pathway”
Cone -> Bipolar cells -> RGCs
- Bipolar cells cones only or rods only
- Rods and cones send signals out of retina via bipolar cells, ganglion cells, ganglion-cell axons
- Often display convergence and summation
- Contribute to centers of ganglion-cell receptive fields
“Lateral pathway”
Photoreceptors horizontal
- (Bipolar + Amacrine Amacrine + Bipolar) -> RGCs -> action potential to optic nerve
- Signals are sent across retina between receptors and bipolars by horizontal cells, between bipolar and ganglion cells by amacrine cells
- Often display lateral inhibition: make edges more visible
- Contribute to surrounds of bipolar and ganglion-cell receptive fields
Luminance contrast
difference in intensity of illumination at adjacent retinal locations
Photopigment
Molecule with ability to absorb light and initiate transduction
Spectral sensitivity
degree to which a photopigment molecule absorbs light of different wavelengths
Isomers
different possible shapes of molecules, such as the all-trans retinal and 11-cis-retinal shapes of photopigment molecules
Photoisomerization
change in shape by a photopigment molecule from one isomer (11-cis retinal) to another (all-trans retinal) when the molecule absorbs a photon; initiates transduction of light to a neural signal
- Decrease membrane potential -> change number of neurotransmitter molecules released by photoreceptor at synaptic terminals -> change in membrane potential of bipolar, horizontal, amacrine -> RGCs -> action potentials to optic nerve -> brain
- Trigger cascade of enzyme reactions in the receptor, providing high amplification of signal
- Cascade ultimately affects membrane potential and transmitter release at synapse at base of receptor
Operating range
visual system’s sensitivity to the range of light intensities within the current scene; the visual system adjusts its operating range according to current conditions
Dark adaptation
process of adjusting retinal sensitivity (changing the operating range) as the person moves from a bright environment to a darker one
- Results from an adjustment in the sensitivity of the photoreceptors so they can respond to lower levels of light
- Reverse process -> light adaptation
- At first, cone sensitivity increase , than rod
- Later, cone sensitivity decrease, and rod increase -> rod-cone break
Rod monochromats
individuals with a very rare genetic disorder in which the retina develops with rods but without cones; used in dark adapatation experiments to establish the curve for rods
Photopigment regeneration
process whereby photopigment molecules change back into the 11-cis shape after photoisomenzation
- Cone 5 minutes from dark to light
- Rod 20-30 minutes
Convergence
property of retinal circuits in which multiple photoreceptors send signals to one RGC
- Increase degree of convergence, increase circuit supports sensitivity to dim light because signals from all photoreceptors in circuit are combined by being funneled onto a single RGC
- Decrease degree of convergence, increase circuit supports visual acuity because different spatial patterns of light stimulate different photoreceptors, and the responses from different photoreceptors aren’t combined but are sent to separate RGCs
- Acuity is higher in fovea than periphery, sensitivity to dim light is higher in periphery than fovea because density of RGCs increase near fovea than periphery of retina -> fovea contains no or little convergence circuits and periphery contains mainly circuits with increase convergence
- Higher convergence of rods than cones (100 rods to one ganglion cell; 5 cones to one ganglion cell)
Spatial summation
Property of retinal circuits with convergence in which signals from photoreceptors in some small space on the retina summate (add up) to affect the response/firing rate of RGC in circuit
Receptive field
region of a sensory surface that, when stimulated, causes a change in the firing rate of a neuron that “monitors” that region of the surface; the receptive field of an RGC is the region of the retina occupied by the photoreceptors to which the RGC is connected
Center-surround receptive field
An RGC receptive field in which the center of the receptive field responds differently to stimulation than the surrounding portion of the field
On-center receptive fields
Receptive fields of RGCs with center-surround structure in which the RGCs increase their firing rate when the amount of light striking the center of receptive field increases relative to amount of light striking surround
Preferred stimulus
type of stimulus that produces a neuron’s maximum firing rate; for RGCs with on-center receptive fields; the preferred stimulus is a spot of light that exactly fills the center of receptive field
Off-center receptive fields
receptive fields of RGCs with center-surround structure in which the RGCs decrease their firing rate when the amount of light striking the center of the receptive field decreases relative to the amount of light striking surround
*Important when we look at dark objects on bright background
Center-Surround Receptive Fields
After incoming light is transduced, neural signals are transmitted through circuit
- Cones in center and surround of receptive field send excitatory signals to bipolar and horizontal cells
- Center cones increase responses of bipolar cells and increase firing rate of RGC
- Surround cones-> horizontal-> center cone -> bipolar
- Horizontal send inhibitory signals to cones and decrease strength of signals sent by cones spread signals around
* Excitatory signals from center photoreceptors
* Inhibitory signals by horizontal -> RGC -> Brain
Lateral inhibition
Inhibitory neural signals transmitted by horizontal cells in retinal circuits
Edge enhancement
process by which the visual system makes edge as visible as possible, facilitating perception of where one object or surface ends in the retinal image and another begins
Strabismus
Disorder of extraocular muscles in which the two eyes are not aligned with one another, resulting in a double image, which impairs binocular depth perception
Amblyopia
condition in which both eyes develop normally but the neural signals from one eye aren’t processed properly, so that fine vision doesn’t develop in that eye
Myopia (nearsightedness)
condition in which the optic axis is too long and accommodation cannot make the lens thin enough to focus light from a distant object on the retina, so the light comes to a focus in front of the retina, so the light comes to a focus in front of the retina, and the image on the retina is blurry; the person can see nearby objects clearly but not distant objects
Hyperopia (farsightedness)
condition in which the optic axis is too short and accommodation cannot make the lens thick enough to focus light from a nearby object on the retina, so the light comes to a focus behind the retina, and the image on the retina is blurry; the person can see distant objects clearly but not nearby objects
Presbyopia
common condition in which the lens becomes less elastic with age, characterized by a progressive increase in the distance from the eye to the near point as the person ages; as in hyperopia, accommodation can’t make the lens thick enough to focus light from nearby objects
Near point
closest distance at which a person can bring an object into focus; prebyopia is characterized by a progressive increase in the distance from the eye to the near point as the person ages
Astigmatism
a condition in which the curvature of the cornea or lens is slightly irregular/asymmetrical, making it impossible for the lens to fully accommodate
LASIK (Laser-assisted in situ keratomileusis)
Surgery to reshape the cornea in order to correct disorders of accommodation
Cataract
a progressive “clouding” of the lens that can, if left untreated, lead to blindness
*Surgery -> replacement lens
Glaucoma
a condition in which the intraocular pressure is too high for the person’s eye, most commonly caused by blockage of the openings, that let aqueous humor drain from the anterior chamber
Floaters
Shadows on the retina thrown by debris within the vitreous humor; perceived as small, semitransparent spots or threads that appear to be floating before the person’s eyes and tend to move with the eys
Phosphenes
brief, tiny bright flashes in the person’s field of view not caused by light but by any of a variety of other causes
Macular degeneration
a condition characterized by damage to the photoreceptors in a region at the center of the retina; the leading cause of severe visual loss in the United States
- Dry: cells degenerate in pigment epithelium which causes loss of function of the photoreceptors overlaying those cells
- Wet: new blood vessels grow underneath the retina, leak fluid, bleed and ultimately scar, also leading to loss of function of the overlying photoreceptors
Retinitis pigmentosa (RP)
inherited condition in which there is gradual degeneration of the photoreceptors over many years, often leading to right blindness and “tunnel vision”
Night-vision devices (NVDs)
devices to enable vision in total or near-total darkness
Thermal imaging
a technology used in night-vision devices; infrared radiation emitted by objects and surfaces in a scene are converted into a visible electronic image
Image enhancement
technology used in night-vision devices; dim light is amplified by converting photons into electrons, amplifying the number of electrons and then using the electrons to produce a pattern of varying intensities on a phosphor-coated screen
Opsin
large protein; visual pigment molecules components
Retinal
light sensitive molecule; visual pigment molecules components
Why cones recover from light faster than rods?
After isomerization,
- Retinal and opsin split apart, are moved out of receptor, put back together (regenerated) with help of cells next to receptors (glia and pigment eithelium cells), and finally moved back into place in receptor membrane
- This process takes longer for rod receptors (20 mins) than cones (5 mins) because cone membranes are exposed to outside, in folds and whereas rod membranes are mostly in disks inside outer membrane, so retinal/opsin have to cross added membrane
