Flashcards in Visual System Deck (20)
Describe Rods vs cones.
there are NO action potentials in either rods or cones. (no AP in photoreceptors which are rods/cones) or bipolar cells (horizontal)
first AP is in the axon of the ganglion cells.
(see slide 5)
these are neuromodulators -interneurons in retina begin processing information right away.
-info from cones and fovea goes straight through to ganglion cell from peripheral part - info is modulated (light modulated)
they are contrast detects not light detectors. (visual system processes information so relative output of ganglion you can see things brighter or darker than they are)
Describe the pathway from retina. (Memorize!)
Optic chiasm (Axons from nasal retina cross here)
Optic tract to Lateral geniculate nucleus (thalamus)
Optic radiations (geniculocalcarine tract area 17) to
MEMORIZE THIS ! !
T: Temporal portions of retina (which passes uncrossed)
N: Nasal portion of retina (which crosses)
So there is a nasal and temporal component for each eye, all light from right field comes to nasal retina of right eye but temporal retina of left eye
(brain wants to take visual information from right to left cortex so nasal part of retina crosses over to join temporal in the other eye and go back to lateral geniculus)
Describe the 2 major classes of ganglion cells corresponding to the size of ganglion cells receptive fields.
(nice to know, not need to know)
Corresponding to the size of the ganglion cells receptive fields, there are 2 major classes of ganglion cells:
M-type: Large cell bodies and dense arborizations; large receptive fields; transient response to continuous light (rapidly adapting); respond best to movement and large objects; RODS
P-type: Small cell bodies, with 1:1 relation to cones; small receptive fields; more numerous near fovea; wavelength selective (color); respond best to color and fine detail (high acuity); CONES
Describe the fovea.
The fovea occupies only 5% of the retina, but transmits to 67% of the lateral geniculate (koniocellular layers). This is another reason the fovea has the highest area of visual acuity.
there are CONES only at fovea (no rods)
contributes to vidual acuity...light is directed to photoreceptors, axial stimulation so best visual acuity is here
Describe the direct connection to the brainstem.
perception begins in the primary visual cortex (aka. area 17, striate cortex, V1), but that some information enters the brainstem via the pretectal area (for pupillary reflexes... will go to midbrain and synapse in Edinger Westphal nucleus then to synapse in ciliary ganglion of CN III to control size of pupil) and the superior colliculus (for head and eye movements, MLF orienting reflex).
if relative amount of light into eye is really bright there there is a lots of stimulation of Edinger Westphal so pupil will constrict.
Describe visual pathways: optic radiations
Upper fibers via parietal lobe
Lower fibers via temporal lobe (Meyer’s loop)
Describe lesions of the visual pathway.
Lesions you should know (refers to figure on SLIDE 14, end of powerpoint)
1 – Blind right eye
3 – Bitemporal hemianopia
4 – Left homonymous hemianopia
5 – Left homonymous upper quadrantanopia (“Pie in the Sky” lesion)
6 – Left homonymous hemianopia
7 – Left homonymous lower quadrantanopia (macular sparing often present)
8 – Left homonymous hemianopia with macular sparing
How do you test the optic nerve?
What are you testing?
To test the optic nerve, a common test is the pupillary light reflex. A light is shone through one pupil, which causes BOTH pupils to contract equally. The response of the illuminated eye is the direct reflex, the response of the unilluminated eye is the consensual response.
Both pupils constrict due to the bilateral connections to the Edinger-Westphal nucleus by the optic fibers traveling through the brachium of the superior colliculus to the pretectal area.
Preganglionic fibers travel in the Oculomotor nerve (CN III) to the sphincter pupillae muscle, causing it to contract. Testing both eyes confirms that the rostral midbrain is functional.
What does optic nerve damage produce?
Optic nerve damage produces equal pupils, neither of which responds to light shone in the eye ipsilateral to the lesion, but both eyes respond normally to light shone in the contralateral eye.
What does occulomotor nerve damage produce?
Oculomotor nerve damage causes a dilated ipsilateral pupil that does not respond to light shone into either eye.
What does Horner's syndrome produce?
Horner’s Syndrome (loss of sympathetic output to the head, superior cervical ganglion or its axons damaged) will result in a meiotic (constricted) pupil, ptosis of the eyelid, and anhydrosis (loss of sweating) ipsilateral to the lesion.
Describe the accomodation reflex.
Changing gaze to focus on a nearby object will elicit the accomodation reflex, which consists of vergence of the eyes (contraction of both medial recti muscles), ciliary muscle constriction (which causes the lens to thicken by releasing tension on the zonular fibers), and constriction of both pupils which improves optical performance and reduces light entering the eye due to greater reflectance from the nearer object.
Because this is a conscious act to change your gaze from far to near, it also involves the visual cortex.
Describe the corneal eye blink reflex.
How is the afferent information from each cornea distributed?
The corneal eye blink reflex is initiated by the free nerve endings in the cornea and involves the trigeminal nerve and ganglion, the chief sensory (light touch) and/or the spinal trigeminal tract and nucleus (pain), interneurons in the reticular formation, motor neurons in the facial nucleus and nerve, and the orbicularis oculi.
The afferent information from each cornea is distributed bilaterally to facial motor neurons by the reticular formation interneurons, therefore the eye blink response is consensual (both eye lids will close to stimulation of the cornea of either eye).
How is information in primary visual cortex processed?
Information in the primary visual cortex is processed in 3 specialized columns called hypercolumns.
1. Orientation - spatial representations (edges)
2. Blobs - color specificity
3. Occular Dominance - left vs. right
Describe cortical processing.
Each layer of the cortex has projections to other sites in the CNS via the large pyramidal cells. Integration and convergence continue as information "ascends" - thus allowing the formation of complex and abstract perceptions and associations.
higher up you go is more complex... nose 3,4 face 5, mother -6,7
Describe the 2 basic ways to detect motion of an object.
There are two basic ways to detect motion of an object:
1. The image moves temporally across the retina while the eye remains stationary - “temporal association.”
The head and eyes move to fix the image of the object on the fovea.
2. Movements are represented best in the Middle Temporal area of V5 and the Medial Superior Temporal area (V5a)
Motion perpendicular to the orientation axis can be detected by the primary visual cortex (V1), but V5 is essential for the analysis of complex movements.
Describe the cones in the retina.
Color is a property of an object, and is determined by the object’s reflectance and the light source.
The human retina has 3 separate cone systems (trichromatic: Red, Green & Blue), each with different but overlapping preferences.
Describe cones and how they process color.
What is needed to percieve color?
Cones do NOT transmit specific wavelength information, but rather they increase their response to varying intensities of light from a stimulus. For example: a "red" cone will absorb more energy from a 550nm photon than a "blue" cone, by about a factor of 2; but both will respond to this particular wavelength of light. All photoreceptors respond to a wide range of wavelengths, but respond "best" (highest output) to a particular wavelength.
We need at least 2 different photoreceptor types to perceive color; the cortex compares the differences in brightness and output to assign a "color" to that particular combination.
Due to chromatic aberration, there are no "blue" cones in the fovea.
Describe the Ishihara test.
These test charts are probably the most widely used in the testing of color blindness. They consist of a collection of varied colored dots, arranged in each plate so that the person with normal color vision reads one number; the color blind person perceives a different number or figure.