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Final

PSYCH 1XX3 Study Guide - Final Guide: Retinal Ganglion Cell, Optic Chiasm, Visual Acuity


Department
Psychology
Course Code
PSYCH 1XX3
Professor
Joe Kim
Study Guide
Final

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Vision
Introduction Our Visual Sense
Humans are highly visually reliant animals as 1/3 of our brain’s function is to process visual info
If visual info is in conflict with info from another sense, we bias our trust towards vision
o Ex. In a movie theatre, we will hear audio from speakers but perceive it as coming from
the actors. Seeing visual cues takes more importance than the actual source of the sound.
We see with our brain and the eye is used to collect, focus, and sense light stimuli
Our visual system captures, processes, and interprets visual info to give us a rich interaction with
the environment
Case Study: John was blind by an occipital lobe stroke at 30. He cannot see but processing
pathways are intact allowing him to recognize object locations, and coordinate with the motor
cortex to avoid them while moving
The Light Stimulus
Light travels as a wave w/ 3 physical characteristics: height (amp), wavelength (distance bw wave
peaks), purity
Variations in amplitude affect the perception of brightness
o Greater amplitude = more light being reflected/ emitted by that object = appears brighter
and intense
Variations in wavelength affect the perception of colour (measured in nm)
o Smaller wavelengths = waves w/ high frequency as there is less distance bw peaks
o Larger wavelengths = waves w/ low frequency
Purity affects the perception of the saturation or richness of colours
o Light of 1 wavelength = pure light that is completely saturated
o Natural light is a combination of wavelengths = desaturated and not as intense as pure
colours
The visible spectrum is species dependent
o Humans can see wavelengths in the visible spectrum within the total electromagnetic
radiation spectrum
Shortest wavelength we can see 360 nm (violet)
Longest wavelength we can see 750 nm (red)
o Bees see wavelength shorter than 360 nm in the UV spectrum
They see flowers as being different colours
o Snakes see wavelength longer than 750 n in the infrared spectrum
This helps them see the body heat emitted by their prey in the dark
The Eye
Cornea the curved structure through which light first passes through
o transparent window at the front of the eye
Pupil black dot through which light passes through second
Iris coloured part of the eye that controls the size of the pupil
o Has muscles that are controlled by the brain
o If less light is reaching the retina, muscles dilate the pupil into a larger opening
o If too much light enters, muscles cause the pupil to constrict into a tiny opening
Lens transparent structure that does the final focusing of light onto the retina
o Lens curvature causes pic to land on retina upside-down & reversed left to right

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o The brain corrects the image so we see a properly oriented image
o Flexible tissue whose shape can be altered by muscles to focus on close or far objects
(accommodation)
If the object is close, the lens gets fatter or rounder
If the object is far, the lens elongates
Vitreous Humour light passes through this clear jelly like substance inside the eyeball and lands
on the retina after passing through the lens
Sclera tough membrane that covers the white part of the eye
Retina - Begins the translation of light into neural impulses
The Retina Retinal Layer 1 (Photoreceptors)
Retina paper thin sheet that covers the back of the eye
o Made of neural cells arranged in 3 layers
o The layer at the back of the eyes, farthest away from the light is where photoreceptors are
located
Photoreceptors cells that translate the physical stimulus of light into a neural signal that the brain
can understand
o To reach the photoreceptors, light must pass though 2 transparent layers of retinal tissue
The retina has an inside out arrangement because the photoreceptors get their nutrients from the
retinal pigment epithelium (RPE) at the back of the eye
Photoreceptors would die without access to RPE cells
If photoreceptors were located at the front of the retina, facing the light, then they would not have
access to RPE and would die
The Retina Retinal Layer 1 (Photoreceptors Rods and Cones)
2 types of photoreceptors rods and cones
Humans have more rods than cones
Cones operate at high light intensities and are used for day vision
o Give us the sense of colour, visual acuity, sharpness, detail
o Become more concentrated at the fovea
Fovea tiny spot in the middle of the retina that contains cones
If we want to see something in detail, we move our eyes so the image falls on the
fovea
Rods operate at low light intensities and are used for night vision
o Give no colour vision and offer poor visual acuity
o There are no rods in the fovea but there is an increasing concentration of rods in the area
around the fovea
This is why rods are useful for peripheral vision
When we look at an object in a dimly lit environment (ex. stars), we’re better off
looking slightly to 1 side of the object than staring right at it
Photoreceptors send info to the next layer of bipolar cells that send info to the next layer of
ganglion cells
The Retina Retinal Layer 2 and 3
Ganglion cells collect info from a larger segment of the retina
Axons of these cells converge on 1 point called the optic disc
o The optic disc is an exit hole for ganglion axons
o This area has no photoreceptors so it makes up the blind spot

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It then leaves the eye to join the optic nerve which travels to the brain
The Retina Summary of the Passage of the Neural Signal
Light entering the eye passes through the ganglion cells, bipolar cells, strikes the photoreceptors
on the retina at the back of the eye
At that point light is converted into a neural signal that is sent from the photoreceptors to the
bipolar cells, and to the ganglion cells, whose axons make up the optic nerve
The Retina Processing in the Retina
Amacrine and horizontal cells are cells in the retina that let areas in a retinal layer communicate
with each other
o They allow info from adjacent photoreceptors to combine their info
o Info in rods & cones converge to travel along axons in the optic nerve
o A little visual processing is done in the retina before the signal is sent to the brain
The Retina Receptive Field in the Retina
The photoreceptors in the retina are divided into specific groups
o Info from each group gets assimilated into 1 signal that eventually affects the ganglion
cell
In the fovea, the photoreceptor group for a ganglion cell may have 1 cone
o This means the ganglion cell is representing a small area of the image
o Each fovea cone has a direct link w/ the brain, thus detail is preserved, and more visual
acuity occurs here
Sometimes, input from many cones and rods is combined into 1 neural signal for 1 retinal
ganglion cell
Groups get larger closer to the periphery of the eye
o This is why visual acuity is lower for peripheral vision
Photoreceptors form a receptive field and converge their neural signal
The collection of rods and cones in the retina when stimulated affects the firing of a particular
ganglion cells is called the receptive field of that retinal ganglion cell
Visual Pathways Visual Fields and Hemispheres
Visual perception is extensively processed in the brain
Right & left halves of our visual field are processed by the contralateral brain side
Input from the right visual field travels along the optic nerve to left hemisphere
Input from the left visual field travels along the optic nerve to right hemisphere
Visual fields send info to both eyes so each hemisphere gets info from both eyes
Before reaching the hemispheres, the axons from the inner part of the retina (region of the retina
close to the nose) have to cross over to the opposite hemisphere
o The point at which the optic nerves from the inside half of each eye cross over to the
opposite hemisphere is called the optic chiasm
Visual Pathways Main Pathway (Occipital Lobe)
After the optic chiasm, the info from each visual field arrives in the opposite hemisphere
Here, the optic nerve fibres split and travel along 2 pathways
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