PSYCH 1XX3 Lecture Notes - Lecture 5: Visual Acuity, Vitreous Body, Optic Disc

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11 Feb 2016
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Vision Module
-Nearly 1/3 of the brain devoted to processing visual information
-Visual cues taken on more importance than the actual source of sounds ex: Movies
Light
-Travels as a wave. Can vary in 2 Respects:
1. The height of each wave (amplitude)
2. Distance between the peaks of successive waves (Wavelength)
Amplitude
-Variations in amplitude affect the perception of brightness
-Greater the amplitude, the more light that is being reflected/emitted by that object so appears
more bright
Wavelength
-Variations affect the perception of colour
-Measured in nanometers/ millionths of a millimetre
-Small wavelengths= light waves with higher frequency because less distance between each
peak
-Large wavelengths have light waves with lower frequency
-Humans are only sensitive to tiny portion of wavelengths and this is called the VISIBLE
SPECTRUM
-Shortest wavelength we can see is 360 nanometers (violet) and longest wavelength is 750
nanometers (red)
-BUT, there are other species that can see outside the humans “visible spectrum” ex: Bees
can see wavelengths shorter than 360 nm in ultraviolet spectrum and see differences in
colours of flowers that look the same to humans
-Ex 2: Snakes can see wavelengths longer than 750 nm in the infrared spectrum which allow
them to see prey in the dark from the body heat emitted by the prey
Purity
-Affects the perception of saturation/richness of colours
-Light that is made up of a single wavelength
-The perceived colour would be described as saturated
-Natural light will give a combo, so described as desaturated
-Most of colours we see in everyday life are NOT pure, but a mixture of wavelengths so less
intense than pure colours
The Eye
-Light first passes through the curved cornea; begins focusing process (does 80% of work)
-Cornea= Transparent window at front of the eye
-Rest of eye covered by white part called sclera (tougher membrane)
-Next, light passes through the pupil (the round window black dot in the middle of your eye)
Light Property
Our Perception
Amplitude
Brightness
Wavelength
Colour
Purity
Saturation
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-The iris consists of a band of muscles controlled by the brain; if not enough light is reaching
the retina, muscles causes the pupils to dilate into larger opening, and if too much light is
entering the eye, than it constricts into a tiny opening
-After passing pupil, light passes through the lens; transparent structure that does the final
focusing of light onto the retina at the back of the eye
The Lens
-The curvature of the lens causes images to land on the retina upside down and reversed from
left to right
-Final perceived image is a product of brain activity… A correction takes place
-Lens is flexible piece of tissue, and the shape can be altered by surrounding muscles
allowing it focus on close or far objects
-Object close: Lens gets fatter/rounder to produce clear image
-Object far: Lens gets elongated to focus on image on the back of your eye
-Change in shape of the lens to focus is called ACCOMMODATION
The Retina
-After travelling through the lens, light passes through the vitreous humor (clear jelly-like
substance that comprises the main chamber inside the eyeball)
-Light lands on the retina, which is the neural tissue that lines the back of the eye
Sequence that light passes through the eye:
1. Cornea
2. Pupil
3. Lens
4. Vitreous Humor
5. Retina
The Retina
-Where the physical stimulus of light is first transmitted into neural impulses
-Paper thin sheet that covers the back of the eye
Retinal Layer 1: Photoreceptors
-Layer at very back of the eye, farthest away from light
-Cells in the retina that are responsible for translating the physical stimulus of light into a
neural signal that the brain can understand
-To reach the photoreceptors, light must pass through the other two layers of retinal tissue
which are transparent
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-Reason for inside out arrangement: The photoreceptors get their nutrients from layers of cells
at the very back of the eye called the retinal pigment epithelium (RPE)
-Photoreceptors would die without RPE so has to be in the back
Photoreceptors: Rods and Cones
-Named for their shape
CONES
-6 million
-Day vision; high light intensities
-Colour
-Good visual acuity/ sharpness of detail
-Concentrated in fovea (a tiny spot in middle of the retina that exclusively has cones)
** consider when want to see more detail, we move our eyes so image falls
directly onto fovea**
RODS
-125 million
-Night Vision; low light intensities
-No colour
-No rods in Fovea, but increasing concentration surrounding the Fovea.. Makes rods useful for
peripheral vision
-ex: When trying to see an object somewhere that is dimly lit, better off looking slightly to one
side of the object instead of staring right at it because the cone rich fovea does not work well
in that environment.
-Poor visual acuity
-Concentrated in periphery
Bipolar and Ganglion Cells
-Photoreceptors send information to next layer of cells in the retina called the bipolar cells
-Bipolar cells then send information on to next layer of cells in the retina called the ganglion
cells
-Ganglion Cells: Collect information from larger segment of the retina. Axons of these cells all
converge at one point in the eye called the optic disc and then leave the eye to join the optic
nerve which travels to the brain
-Optic Disc: Exit hole in the eye for ganglion axons, small area contains no photoreceptors so
it constitutes our blind spot
OVERVIEW OF LIGHT THROUGH RETINA
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