the eye Flashcards
photoreceptor
cell specialised for light detection
photopigment
protein + light absorbing factor
light absorbing factor
photosensitive co-factor bound by a protein
xeopus: african clawed frog eyes
have optical apparatus that make photosensitive cells particularly sensitive to light coming from some directions
eyes make each photoreceptor cell respond to light from a restricted range of directions, which corresponds to light originating in a particular portion of the visual scene
xenopus skin
Photosensitive cells in their skin: respond to light, but not the direction it is coming from
detect ambient light levels
cup eyes
little pits lined with heavily pigmented cells (screening pigments), and photoreceptor cells sitting within them
Pigmented cells shield the photoreceptors from light coming in from some directions: they cast shadows across the pit
they can now determine which direction light is coming from
basic function cup eyes
render photoreception direction dependent
planaria use for cup eyes
negative phototaxis: to seek shade
the insect compound eye
Pigment cells form a tube surrounding a group of photoreceptors (ommatidia), there are thousands in each eye
light can only enter from one end; light can reach the photoreceptor from the direction the tube is pointed
many in different directions allows them to make up the whole visual scene
ommatidia
Pigment cells form a tube surrounding a group of photoreceptors
why do ommatidia have a cornea and a protective lens?
these act to maximise the efficiency of light transmission to the photoreceptors
do not play a major function in making light direction selective
what shape are ommatidia collecting area?
cone shaped, gets wider teh further from the ommatidia entrance (wider at the eye surface)
feature of neighbouring ommatidia and function
only have partially overlapping collecting cones
so by comparing activity across ommatidia it would be possible to determine the location of bright/ dim areas
examples of uses of the insect compound eye
Example 1: moth to a flame
* Light from blub falls withing the collecting cone of this ommatidium, but not its neighbouring ones
Example 2: finding flowers
* Light reflected from the flower will fall more within collection of cones of some ommatidia than others. These can tell the moth which direction to fly
acuity
capacity for seeing distinctly the details of an object
what do opticians use to measure acuity?
Snellen acuity charts measure acuity
the smaller the letters the harder to distinguish details, therefore to read smaller letters you need higher acuity
what does high acuity vision require?
an array of pixels each receiving light from a restricted range of points in visual space
what increases effective acuity?
The more independent representations of light (pixels) you can squeeze into an image, the higher the spatial resolution of that image and the higher the effective acuity of someone viewing it
each ommatidium represents how many pixels?
one
size of mirror eyes
1mm
design of mirror eyes
concave mirror that focus image onto an array of photoreceptors
- Eye has an aperture at the front to let light in
- Light enters aperture, passes through photoreceptors, and falls on a reflective surface
- Reflective surface is concave so when light bounces off it is reflected inwards to a focus point on the photoreceptors
photoreceptors relationship to pixels in mirror eye
- Each photoreceptor receives light preferentially from a different point in visual space and can be regarded as a separate pixel
this overcomes the fundamental limits to acuity of a compound eye
relationship between acuity and distance compound eye
inverse correlation
as distance increases, acuity worsens
at a longer the distance from the eye the rays appear…
more parallel
viewing long distances (mirror eye)
concave mirror bends the light to a single point on the photoreceptors
mirror focuses a clear image onto the photoreceptors
in a mirror eye at a shorter distance the rays appear..
more divergent
viewing shorter distances mirror eye
rays divergent when they hit the mirror therefore they are not reflected onto the photoreceptors
acuity is very poor
acuity and focus in mirror eye
object in focus at some viewing distances, but not at others
acuity is strong dependent upon viewing distance
what is invariant in the mirror eye
focal length of the mirror
what is acuity limited by in the compound eye?
the collecting area of teh ommatidium
how many times has the lens eye developed independently?
three times
animals that have the lens eye?
all vertebrates
invertebrates: spiders, squid, octopus
what shape are lens eyes?
sperical
what is the general passage of light onto the retina (lens eye)?
light to enters through an aperture, which passes through a lens, that focuses it onto an array of photoreceptors on the retina
what does the focus of the lens result in?
the projection of an image of the visual scene onto the photoreceptors
acuity of lens eye
very high acuity
lens eye photoreceptor pixel relationship
each photoreceptor receives light from a different point in visual space: becomes an independent pixel
how is light focused on to the retina (basic)
refraction
refraction
focusing divergent rays from a point in visual space into a single point on the retinal surface
what does the degree of refraction depend on?
distance from the object
refraction from the cornea is…
constant, invariant
what does refraction adjustment depend on
the lens, the lens is adjustable
what is the lens connected to?
a ring of muscle called ciliary body by suspensory ligaments
role of suspensory ligaments on the lens
hold the lens in place and pull it flat
the lens is elastic and wants to form a sphere, the suspensory ligaments stop it doing this
role of the ciliary muscles
pull the suspensory ligaments taught in order to maintain an appropriate lens shape
lens for distant objects
Light rays near parallel, require little refraction to focus
Lens thin and therefore has low refractive power
caused by the ciliary muscles relaxing and pulling the lens flat
lens for close objects
Lights rays diverging, require greater refraction to focus
Lens thick and has high refractive power
ciliary muscles contract, releasing tension in the suspensory ligaments, allowing the elastic lens to get rounder
accommodation
an involuntary response to changing distance of view
changing from distance to close vision
- Ciliary muscles contract and suspensory ligaments relax
- Lens thickens and refractive power increases
how can you externally see change from distance to close vision?
pupil constriction
refractive errors:
problems with corneal or lens refraction
what happens when a perfect image is not formed on the retina?
we observe a reduction in acuity
what is myopia
short sightedness, difficulty viewing distant objects
image is focussed in front of the retina
why can we still see near objects with myopia?
the accommodative change in the lens can compensate for the problem
what can cause myopia?
eye too deep (corneal:retinal distance too large)
effective refractive power too large
how can we correct myopia?
*Diverging (concave) lens corrects
makes parallel rays from distant objects more divergent as they hit the cornea
what is hyperopia/ hypermetropia?
long sightedness
difficulty viewing close objects
image not yet focussed by the time the light reaches the retina
what causes hyperopia?
eye too shallow (corneal:retina distance too small)
refractive power of the cornea too low
why can people with hyperopia still view distant objects?
accommodative changes in the lens compensate
how can we correct for hyperopia
- Converging (convex) lens corrects
make diverging rays from close objects more parallel as they hit the cornea
presbyopia
most widely experienced
loss of accommodation ability during aging
As people get older their lenses become less elastic
Even when ciliary muscles are full constricted and suspensory ligaments lose, the lens stays relatively thin (lacks elasticity to form full sphere when not under tension)
why older people have difficulty seeing things close up
what does speed of increase of myopia suggest?
speed of this increase suggests strong environmental influence
-time spent indoors with artificial light through childhood and adolescence, as time spent outdoors is negatively correlated with myopia incidence
what is genetically heritable in the lens eye? what can this cause?
eye depth is heritable to some extent
myopia is heritable
what function does metabotropic glutamate receptors perform in vision?
they allow retinal ganglion cells to be excited by increases in light
