Chapter 5: Vison Flashcards
What some one sees depends on what?
How far one sees is dependent on how far
light travels before it strikes one’s eyes
where does the perception of what we see happen
it happens in our brain in fact the perception of all our sensory organs happens in our brain, not in our sensory organs
Law of specific nerve energies
this law states that activity by a particular nerve always
conveys the same type of information to
the brain
How light or visual stimuli go throughout the eye and to the brain
The light enters the eye through an opening called the pupil and goes to the retina which contains nerve cells: bipolar and ganglion and photoreceptors
bipolar cells are in the middle and connect between the photoreceptors and the ganglion cells so when the light comes in it goes through the ganglion cells and travels through the bipolar cells to the photoreceptors and that’s where transduction happens and info comes back to the ganglion cells and it sends it to the thalamus through the optic nerve which then will be sent to the occipital lobe which has to the primary visual cortex or V1
Bipolar Cells
Bipolar cells are one of the main retinal interneurons and provide the main pathways from photoreceptors to ganglion cells
ganglion cells
– The axons of ganglion cells join one another to form the optic nerve that travels to the brain
Amacrine Cells
Additional cells that receive information from bipolar cells and send it to other bipolar, ganglion, or amacrine cells
Control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli
The Optic Nerve
Consists of the axons of ganglion cells that band together and exit through the back of the eye and travel to the brain
blind spot
the point at which the optic nerve leaves the s back of the eye contains no receptors that are why its called a blind spot
this does not contain rodes or cones
Fovea
area in the center of the retina at which the vision is the best highly dense with cones and rods are absent
Each receptor in the fovea attaches to a single bipolar cell and a single ganglion cell is known as a midget ganglion cell
Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of the input
- Our vision is dominated by what we see in
the fovea
the adaptability of Visual Receptors
Highly adaptive: Example: predatory birds have a greater
the density of receptors on the top of the eye; rats
have a greater density on the bottom of the
eye
foveal vision
cones are the receptors responsible for it, Each ganglion cell excited by a single cone sensitive to bright light responds poorly to dim light Good detail vision because each cones own ganglion cell sends a message to the brain and there aremany cones in there
Peripheral vision
have many rodes, Each ganglion cell is excited by many
receptors Responds well to dim light, and dominates in darkness Poor detail vision because many receptors converge their input onto a given ganglion cell that has few cones.
this vision helps you sense motion, and view objects, and sceneries. It also helps you walk around without running into things.
types receptors in the retina
the retina consists of two kinds of receptors which are cones and rods
Visual Receptors: Rods
mostly serve in the periphery vision and respond to faint light (120 million per retina) they are very dominant in the darkness
Visual Receptors: Cones
Cones: most abundant in and around the fovea (6 million per retina) Essential for color vision and more useful in bright light mostly serves foveal vision and dominates in brightness thought there fewer cones compared to rods but they provide about 90% of the brain’s input
Rods and Cons ratio in other species that are more active in darkness
The ratio of rods to cones is higher in species that are more active in dim light
Photopigments
Chemicals contained by both rods and cones that release energy when struck by light
Light
is electromagnetic ration traveling in waves
Visible light
are a portion of the electromagnetic spectrum and “Visible” wavelengths are dependent upon the species’ receptors
Humans perceive wavelengths between 400 and 700 nanometers (nm)
How do we register or perceive different lights
light with higher wavelength and short frequency as bluish colors
and light with long wave length and low frequency as reddidh color
Colour Vision Theories
Trichromatic theory/Young-Helmholtz theory
Opponent-process theory
Trichromatic Theory
The trichromatic theory that states we have 3 types of cones of varying
lengths; red, green, and blue (sensitive to different wavelengths)
- Other colors are produced by a combination of these
The Opponent-Process Theory
Opponent process theory: color vision theory based on three
“systems”: red vs green, blue vs yellow, black vs white
- Exciting one color in a pair (red) blocks the excitation in the other member of the pair (green)
After Image Effect
Fatigue caused by one response will produce an afterimage of the opposite color
Limitations of Color Vision Theories
Both the opponent process and trichromatic theory has limitations
Color constancy, the ability to recognize color
despite changes in lighting, is not easily
explained by these theories
Retinex theory
suggests the cortex compares information from various parts
of the retina to determine the brightness and color for each area
Color Vision Deficiency
inability to perceive different colors, Caused by either the lack of a type of cone or a cone that has abnormal properties
the most common form is difficulty distinguishing between red and green
How the Brain Processes Visual Information?
optic chiasm
is the place where the two optic nerves leaving the eye meet
In humans, half of the axons from each eye cross to the other side of the brain the half closer to the temporal doesn’t cross to the other side the half that is closer to the nasal is the one that crosses to the other side.
Processing in the Retina
Lateral Inhibition in the Retina
Primate Receptive Fields: Ganglion cells of primates that are found in the rating pathways
Parvocellular neurons
Magnocellular neurons
Koniocellular neurons
Parvocellular Neurons> p = layer
Mostly located in or near the fovea
* Have smaller cell bodies and small
receptive fields
they respond to
color
fine details
still objects
slow moving object
Magnocellular Neurons> m layer
Distributed evenly throughout the retina
* Have larger cell bodies and visual fields
* respond to
objects in motion
Koniocellular Neurons
Have small cell bodies
* Found throughout the retina
* Have several functions, and their axons
terminate in many different places
blindsight
blindsight: an ability to respond to visual stimuli that they report not seeing Some people with damage to V1 showblindsight
The Primary Visual Cortex
The primary visual cortex (area V1) receives information from the lateral geniculate nucleus and is the area responsible for the first stage of visual processing
types of cells in the visual cortex
– Simple cells
– Complex cells
– End-stopped/hypercomplex cells
Types of cones
Long wave length> red cone
short wave length> blue cone
medium wavelength> green cone
visual fields
all the inputs from the left visual field are carried to the right primary visual cortex and inputs from the right visual field are carried to the left primary visual cortex
simple Cells
located in V1 they have the smallest receptive size and their receptive field is Bar- or edge-shaped, with the fixed excitatory and inhibitory zone.
Complex Cells
located in V1 andV2 and they have a medium receptive size and their receptive field is Bar-Bat- or edge-shaped, without fixed excitatory or
inhibitory zones
End-Stopped Cells
located in V1 andV2 and they have the largest receptive size their receptive field is the Same as the complex cell but with a strong inhibitory zone at one en
Columnar Organization of the Visual Cortex
In the visual cortex, cells are grouped
together in columns perpendicular to the
surface
- Cells within a given column process
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