Chapter 10 Flashcards
Belief about vision - Ancient Greece
light beams from our eyes illuminated objectively
Belief about our vision - 16th Century Switzerland
Felix Platter proposed that the eye receives light
How we see
light from external source enters the eye, is refracted, and an image is sent to the back of the eye
Visible Light
-the adequate stimulus for vision
-part of the electromagnetic spectrum
Electromagnetic Spectrum
-includes a variety of energy forms
-the visible part accounts for 1/70 of the range
Light is described by its …
wavelength
Wavelength
the distance the oscillating energy travels before it reverses direction
Visible Light Range
38nm to 750nm
Sclera
opaque outer covering of the eye
Cornea
refracts light that enters the eye
Aqueous Humor
-sits behind cornea
-removes waster and supplies nutrients to the cornea and lens
Iris
-color of the eye
-circular muscle
Pupil
a hole in the iris
Lens
-receives light that passes through the pupil
-transparent
-controlled by ciliary muscles to achieve accommodation
Accommodation
-ciliary muscles stretch the lens flatter to focus the image of a distant object
-relax to focus the image of a near object
Refraction through the lens…
-causes image to be inverted when it hits the retina
-discovered by Rene Descartes
Vitreous Humor
-80% of eyes internal volume
-“jelly-like”
-behind lens
Retina
-detects light and tells the brain about aspects of light related to objects in the world
-where seeing begins
Two types of photoreceptors
-rods and cones
-turn light into neural signal
Photopigments
break down in the presence of light
Duplex retina
bc we have two types of receptors
Bipolar Cells
connect photoreceptors and ganglion cells
Ganglion Cells
form optic nerve
How the retina works
-At rest : photoreceptors are most active in darkness, Na+ and Ca+ channels open, glutamate is released, inhibiting bipolar cells (partially depolarized)
-When light strikes the photopigment : Na+ and Ca+ channels close, reducing glutamate release
- Bipolar cells then increase their firing rate, which increases firing in the ganglion cells, signal travels from optic nerve into the brain
Rods
-contain rhodopsin
-function well in dim light and poorly in bright light (scotopic vision)
-rods distinguish only different levels of light
-located in periphery of the retina
Fovea
-central point of focus
-no rods here
-contains most cones
Cones
-contain iodopsin
-function well in bright light; poorly in dim light (photopic vision)
-allow for color vision
-located mostly in fovea
3 Varieties of Iodopsin
- S-cones: blue hues (420nm)
- M-cones: green hues (534nm)
- L-cones: red hues (564nm)
Receptive fields
-circular regions on the retina in which certain light stimuli hitting a collection of cones or rods can influence a ganglion cells firing rate
- influence can be excitatory
- helps make a map of visual space
Visual Acuity
-the ability to distinguish details
-greatest in the fovea
Pathway of Visual Info to the Brain
- info about each visual field enters the eye and is registered on the receptive field in the retina
- this info from the receptive fields is taken by ganglion cell axons into the brain
- optic nerves join for a short distance at the optic chiasm and then separate again ; neurons from the outside half of the brain do not cross over, but go to the same side of the brain
- Once separated, the neurons of the optic nerve travel to their first synapse in the lateral geniculate nuclei (LGN) of the thalamus
- info is routed from the LGN to each visual cortex
