lecture 2 Flashcards
what is the distal stimulus for vision
light
physiological representation of wavelength
colour
physiological representation of intensity
brightness
physical properties of light
wavelength, intensity
what is light
form of electromagnetic energy
visible spectrum wavelength range
~400-700nm
where are receptors located in the eye
in the retina
function of the eye
to focus image onto the retina
proximal stimulus representing light
retinal image
role of the iris an pupil
How does it achieve this?
limit the amount of light passing through
has an adjustable aperture
allows us to deal with a great range of light intensities
what is the diameter range for the pupil
2-9mm
role of the cornea and lens
to focus light onto the retina
how much focusing power does the cornea have?
80%
how much focusing power does the lens have?
20%
how can the lens change shape?
due to the actions of the ciliary muscles
what shape is the lens when focusing a close object
it becomes fatter and shorter
what shape is the lens when focusing far objects
thinner and longer
myopia
near sightedness
light focused in front of the retina
hyperopia
far-sightedness
light focussed behind the retina
how to correct myopia
concave lens
how to correct hyperopia
convex lens
retina
photosensitive layer at the back of the eye
role of retina
carry out transduction, transform light into electrical impulses
cells in the eye from front to back
photoreceptors (horizontal cells) bipolar cells (amacrine cells) retinal ganglion cells optic nerve
what must light pass through to reach photoreceptors?
blood vessels, cells and axons
where are receptors?
layer furthest from incoming light
how does transduction occur in the eye?
visual photopigments reacting to light and triggering electrical signals
what is the action of a change in the shape of the lens called?
accommodation
how many rods are there?
120 million
when are rods most useful?
most useful at night, useless during the day
what type of vision do rods produce?
monochromatic
how sensitive are rods?
very sensitive
what light do rods respond well to?
dim light
how many types of rods are there?
one
what wavelength do rods respond best to?
medium wavelengths (green light)
what is the purkinje shift
at night red looms darker than green
what causes the purkinje shift?
rods respond to green light
how many cones in the human eye?
6 million
are cones more or less sensitive than rods?
less sensitive
when do cones ework best?
work best during the day, useless at night
what are cones responsible for?
colour vision
how many types of cones are there? what are they?
three types: red, green, blue
what wavelength do red cones respond best to?
long wavelengths
what wavelength do green cones respond best to?
medium wavelengths
what wavelength do blue cones respond best to?
short wavelengths
what receptors are active during scotopic vison?
only rods
what receptors are active during photopic vision
only cones
what receptors are active during mesopic vision?
both rods and cones are active?
what light intensities/ times of day does mesopic vision cover?
full moon/ bright moonlight to early twilight
what effect doe bright light have on photoreceptors?
bleaches photopigments causing them to stop responding
what is dark adaption?
increase in eyes sensitivity in the dark
how many times greater is eyes sensitivity after 20-30 minutes in the dark?
approximately 100,000 times greater than sensitivity in the light
what photoreceptors are concentrated on the fovea?
cones
convergence
one neuron receives signals from many other neurons
what receptors does the peripheral contain?
a large amount of rods
a small amount of cones
what is the blind spot
an area on the retina that contains no rods and no cones
convergence of rod cells
120 rod cells to 1 ganglion cell
convergence of cone cells
6 cone cells to 1 ganglion cell
acuity
the ability to detect fine details of a stimulus
high acuity
can detect fine details
low acuity
can detect only course details
do rods or cones have a higher acuity? why?
cones have a higher acuity due to less convergence
where is the highest acuity in the retina?
the fovea
where does acuity decrease?
toward the peripheral/ away from the fovea
in low lighting conditions (scotopic)
what ensures fovea is focussed on the fovea/ high acuity?
eye movements
