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Explain the pathway of the visual system.
Light enters the eye and focuses to the retina via the lens. Photoreceptors on the retina are activated and send an electrical signal, creating AP. Electrical signals (AP) are sent via pathways to the visual cortex.
An object in the visual field produces an ___ image on the retina. Light from the left side of the visual field enters the ___ halves of the retina of both eyes and vice versa. Light from the top strokes the ___ of the retina and vice versa.
The center of the eye is called the ___ and is specialized for detailed vision.
There are three main layers of the retina. These are:
Before reaching the photoreceptors, light must pass through the ___ and ___ cells.
Ganglion cell axons band together to form the ____.
receptor, bipolar, and ganglion cell layers
Light particles that enter the eye are called ___. The ___ is where the optic nerve exits and does not contain photoreceptors.
Photoreceptors are sensory receptors for ___. Photoreceptors are called modified neurons because they originate from neural tissue but do not have ___ or dendrites. The two main photoreceptors are __ and ___.
- Cone cells (cones)
- Rod cells (rods)
These photoreceptors are responsible for the presence/absence of light (not color, but __). They are very sensitive to light. They are found in the ___ of the retina and do not have a fovea. They are activated more in __ light.
- Rods; black and white
These photoreceptors need a lot more light to become activated. They are sensitive to color, specifically the colors __, __, or __, but not all 3. These are found in the ___ of the retina. They are activated more in __ light.
- (purple=both red and blue cones activated)
Phototransduction occurs in the ____.
*The outer segment contains photons that interact to cause electrical signals
Explain the process of phototransduction.
- Photopigments release energy when struck by light.
- 1. Light strikes the back of the retina and Na+ channel closes; receptor hyperpolarizes
- 2. Less NT is released as a result of hyperpolarization onto the postsynaptic neurons (bipolar cell)
- 3. Because of the reduction of NT release, the bipolar cell is not inhibited (always an inhibitory connection)
- 4. Increase in NT released (because of no inhibition)
- 5. EPSP in ganglion cell (axons from the optic nerve carry info into the brain)
- *Essentially, Decrease in NT=Decrease in inhibition/increase in excitation=more NT released
- *More light=greater hyperpolarization
Color vision deficiency, or color blindness, is the inability to perceive color differences. Some people lack two kinds of cones, whereas others have 3 types of cones, but have them in low numbers or unusual forms of one of them. The most common form is difficulty in distinguishing __ and __.
Red and green
The primary visual pathway:
Optic nerves in the thalamus meet at the optic chiasm, forming the ______. Most axons go to the ________, which in turn sends its axons to the visual cortex (occipital lobe).
Half of the fibers of each eye decussates at the ___, and the other half remains ipsilateral. Images presented to the left visual field are projected to the right hemisphere and vice versa.
- Optic tract
- lateral geniculate nucleus
- Optic chiasm
The left optic nerve carries information about the ___ visual field.
The left optic tract carries information about the ___ visual field.
The right optic nerve carries information about the ___ visual field.
The right optic tract carries information about the ___ visual field. Fibers decussate at the ___ portion, and remain ipsilateral at the temporal portion.
Damage to the right optic nerve will result in:
Damage to the optic tract in the center will affect fibers that cross midline, resulting in:
Damage the right optic tract will result in:
- Blindness in the right eye
- Blindness in the peripheral visual fields of both eyes (bitemporal hemianopsia)
- Blindness in left visual field in both eyes (left homonymous hemianopsia)
From the lateral geniculate nucleus, light travels to the _____ (V1), then to the _____ (V2). The ___ stream is for identifying and recognizing objects (the "what" pathway). The ___ stream is for finding objects and determining how to move toward and grasp them (the "where" and "how" pathway).
- Primary visual cortex
- Secondary visual cortex
- Ventral (in the temporal lobe)
- Dorsal (in the parietal lobe)
Damage to the ventral stream of the temporal lobe will result in _____. This is the inability to ______.
- Visual agnosia
- recognize objects
- *Vision is normal but person can describe or point to an object but cannot identify/name it. Visual acuity is fine, just can't identify.
Damage to the inferior temporal lobe will result in _____. This is the inability to recognize ____.
- *There is no problem in identifying other objects, but cannot identify faces; problem is not associated with vision or memory
Damage to the parietal cortex can result in the ability to ________.
- Convert vision into action
- *Person cannot reach out to grab object; can walk toward something they hear, but cannot walk toward something they see
Lesion of the (Right) optic nerve:
Lesion at (Right) lateral chiasm:
Lesion at optic chiasm:
Lesion at (Left) optic tract:
- Blindness in the entire right eye (right optic nerve carries info about entire visual field)
- Lose info from nasal portion of the right eye
- Lose peripherals of each eye
- Lose vision in right halves of each eye (Left tract carries info about right visual field)