81 - Special Senses II (Vision) Flashcards
Accommodation
The automatic adjustment by which the eye adapts itself to distinct vision at different distances
Presbyopia
Farsightedness due to ciliary muscle weakness and loss of elasticity in the crystalline lens
Blind spot
A small area on the retina that is insensitive to light due to the interruption, where the optice nerve joins the retina, of the normal pattern of light-sensitive rods & cones
Fovea
Area consisting of a small depression in the retina containing cones and where vision is most acute
Binocula disparity
The difference in images of an object seen by left and right eye resulting from the eyes’ horizontal separation
Prosopagnosia
An inability or difficulty in recognizing familiar faces; may be congenital or result from injury/disease of the brain.
Object agnosia
Loss of ability to recognize objects
Scotoma
Loss of vision in part of the visual field; blind spot
How is the image on the retina different from the image you see?
Image on Retina is Inverted and Reversed
Describe the difference between the location of rods and cones
Rods and cones are found in the retina, fovea and central fovea
Note that the fovea ONLY has cones
What is the function of rods?
Rods are responsible for scotopic vision (dim light vision)
Rods have more pigment & see in dim light
What is the function of cones?
Cones are responsible for phototopic vision (bright light vision)
Cones have three types of pigments (red, blue, green)
They need more light for acute vision
How can the location and function of rods and cones be used to explain night vision?
- If you look directly at something in the dark, you can’t see it well (b/c you are using cones)
- If you look at it from the side, you wil be ablt to see an outline (b/c you are using rods)
Which receptor mediates scotopic vision?
Rods
Which receptor mediates phototopic vision?
Cones
Describe the electrophysiological changes that occur when a photoreceptor is stimulated in darkness
In the dark, the rods are depolarized
- Na & Ca channels are open whenever there is enough cGMP
- Na & Ca keep coming in & K keeps going out
- Causes depolarization & release of neurtransmitter in the dark
Describe the electrophysiological changes that occur when a photoreceptor is stimulated in the light
When there is light, the intracellular cGMP is reduced & the channels close
- K+ keeps leaving the cell & allows for hyperpolarization of the cell
- Result = decrease in release of neurtransmitter
Describe the term receptive field for visual neurons
The receptive field of a neuron in the visual system can be defined as the area of retina from which the neuron can be influenced
Describe the receptive field of bipolar cells
Bipolar cells have receptive fields with a center - surround organization. This means that they will respond more to a properly positioned spot of light (or dark spot in light) than to even illumination
What is the overall function of photoreceptors?
Capture photons, release neurotransmitters onto bipolar cells
What is the overall function of bipolar cells?
Receive neurotransmitters from photoreceptors, release neurotransmitters onto ganglion cells
What is the overall function of ganglion cells?
The ONLY retina cells that produce action potentials (since their axons form the optic nerve) & are the output cells of the retina
Which four nuclei accept retinal projections?
1 - Suprachiasmatic nuclei
2 - Pretectal nuclei
3 - Lateral geniculate nucleus
4 - Superior colliculi
What is the function of retinal projections to the suprachiasmatic nuclei?
Circadian rhythms
What is the function of retinal projections to the pretectal nuclei?
Mediate papillary light reflexes
- Papillary muscle relaxes for flat lens, distance vision
- Papillary muscle contracts for rounded lens, near vision
What is the function of retinal projections to the lateral geniculate nucleus?
The LGN relays information to the visual cortex & receives input from visual cortex (90% of retinal axons terminate in LGN)
What is the function of retinal projections to the superior colliculi?
In response to visual (or auditory/somatic) stimuli, the superior colliculi can cause eyes, head and even body to turn towards stimuli
Also part of a major visual pathway (retina -> superior colliculus -> pulvinar -> cortex)
Describe the flow of information from the ganglia to the lateral geniculate to the striate cortex
There are several types of ganglion cells in the retina which are responsible for transmitting different kinds of information to the lateral geniculate. The lateral geniculate cells then carry this information to the striate cortex.
What are the two main visual pathways from the striate cortex to higher order visual areas?
- Dorsal (spatial vision) pathway
- Ventral (object recognition) pathway
Describe the dorsal (spatial recognition) pathway from the striate cortex to higher order visual areas
Dorsal stream responsible for the analysis of motion and relative positions of objects in visual scene. Neurons show selectivity for direction and speed of movement.
Describe the ventral (object recognition) pathway from the striate cortex to higher order visual areas
Ventral stream is responsible for high-level form vision and object recognition. Neurons show selectivity for shape, color and texture. The most selective highest order neurons can respond preferentially to faces and objects
How does this relate to the two separate hemispheres of the brain?
Since different aspects of vision are handled by different higher visual cortices, and each hemisphere only sees the opposite visual hemi-field, it is difficult to understand how we perceive a unified visual world. The corpus callosum helps by linking the cortical areas that represent the two hemifields
What is the clinical relevance of parallel processing of different aspects of visual information? i.e. processing the spatial and object recognition parts of vision separately at the same time
Selective visual deficits are possible due to specific lesions
- Patients have been described who have problems visually identifying faces (prosopagnosia) or objects (object agnosia).
- Patients can lose the ability to see motion (cerebral akinetopsia).
What would happen if there is a unilateral lesion in the region below the calcarine fissure (primary visual cortex)?
Selective loss of color vision in just one half of the visual field can be caused by unilateral lesions in a region below the calcarine fissure.
Describe red-green color blindness
- Called red - green color blind if missing either red or green cone
- Such people have trouble distinguishing among green, yellow, orange, and red.
What if a patient is missing both red and green cones?
No real color vision
Describe monochromatopsia
Monochromatopsia (achromatic) people can have either one cone or no cones
How do you test for color blindness?
Ishihara charts
What is the inheritance pattern of red-green color blindness?
Genes for red and green cone pigments are on the X-chromosome. This is why red - green color blindness and color weakness is more common in males.
Describe the inheritance of blue cone pigment
gene for blue cone pigment is on an autosomal chromosome (chromosome 7)
Describe the inheritance of rhodopsin
Rhodopsin gene is on autosomal chromosome (chromosome 3)
Rhodopsin = pigment in rods
What are three mechanisms of depth perception?
- Size of image of know objects on retina
- Motion parallax
- Stereoscopic vision
Describe the size of image mechanism
Size of the image of known objects on the retina; since you know how big an object is you can judge how far away it must be.
Only requires ONE eye (monocular depth cue)
Describe the motion parallax mechanism
Motion (or monocular movement) parallax. If you move your head or body nearby objects seem to move quickly, but distant objects move more slowly.
Only requires ONE eye (monocular depth cue)
Describe the stereoscopic vision mechanism
You can use binocular vision for nearby objects
Requires BOTH eyes (binocular depth cues)
Describe strabismus
a condition that interferes with binocular vision because it prevents a person from directing both eyes simultaneously towards the same fixation point; the eyes do not properly align with each other.
Cross-eyed
Describe amblyopia
Also called lazy eye, is a disorder of sight. It involves decreased vision in an eye that otherwise appears normal, or out of proportion to associated structural problems of the eye.
What causes strabismus?
- Can be caused by failure of the eyes to fixate on the same point
- This would cause diplopia or double vision
- Some people with problem alternate which eye fixates objects
How can strabismus affect vision?
In young children, tendency is to use one eye for fixation and eventually suppress vision from other eye. The brain learns to ignore the suppressed eye which develops amblyopia.
Why is it clinically important to detect strabismus early in life?
Early treatment (before age 6) can prevent loss of vision from the suppressed eye. Treatment can include intermittent patching of the “good” eye to force the brain to pay attention to the suppressed eye.
What can occur if strabismus is not treated?
Untreated children can develop amblyopia. Studies are underway to determine if older children (or even adults) with amblyopia can be helped by the treatments which are used with young children.
What are three things that you need to remember regarding the relationship of the retinal image to the visual field and receptive field?
- Remember, image on retina inverted and reversed.
- Ganglion cell axons from nasal retina cross at optic chiasm
- Ganglion cell axons from temporal retina uncrossed.
What is the result of these three relationships?
The left visual field projected to nasal retina of left eye and temporal retina of right eye, both of which project to the right lateral geniculate which in turn projects to right hemisphere striate cortex
What are the four clinically important effects of lesions?
- Monocular blindness
- Bitemporal hemianopsia
- Homonymous hemianopsia
- Superior quadrant anopsia
What causes monocular blindness?
A lesion after the optic chiasm directly before the eye
What causes bitemporal hemianopsia?
Damage to central part of optic chiasm
What causes homonymous hemianopsia?
Homonymous hemianopsia occurs contralateral to optic tract damage
What causes superior quadrant anopsia?
Superior quadrant anopsia contralateral to lesion of
Meyer’s loop
Which type of vitamin deficiency can cause night blindness?
Vitamin A
Why is vitamin A important for vision?
Vitamin A is involved in the intermediate step of recycling photopigments after capture of a photon of light
Needed from diet (can’t synthesize it)
