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
