Lecture 9 - Vision Flashcards
What is visible light?
The small portion of the electromagnetic spectrum that we can see
Scotopic
Function of eye in very low light
Mesopic
At threshold of cone vision, low light
Photopic
Eye function under well lit conditions, colour vision (mainly cones)
What is the purpose of a third eyelid?
-Keeps eye moist
-Removes irritants
-Shield eyes
What is the function of the cornea?
-Refracts light towards the retina
-Protects against dirt/debris
-Sensitive (blink reflex)
-Protects against infection
-Filters UV light
Where does most light refraction occur?
Cornea
Function of the lens?
-Filters some UV light
-Accommodation for far and near vision
-Finely focuses light
Function of the retina?
-Light detection
-Signal processing through 10 cell layers
-Signal transmission
-Adaptation
What kind of pupils do predators usually have and why?
Vertical pupils - good for depth perception and distance without moving head
What kind of pupils do prey usually have and why?
Horizontal pupils - Enhances light from front and behind and enhances ground view
What is cyclovergence?
The ability to keep eyes level to the ground as an animal bends its head to monitor environment while grazing
Where does the dorsal oblique muscle pull the eye?
Diagonal downwards
Where does the ventral oblique muscle pull the eye?
Diagonal upwards
Where does the medial rectus pull the eye?
Towards the midline
Where does the lateral rectus pull the eye?
Towards the lateral side
What is accommodation?
The process of bending (refracting) light rays to focus them on the retina
When is accommodation required and when is it not required?
Required - Viewing close objects
Not required - viewing far objects
Do horses use accommodation?
No, they may have a unique retinal structure
What does activation of the parasympathetic nervous system do to pupils?
It constricts pupils and promotes accommodation (near vision)
What does activation of the sympathetic nervous system do to pupils?
It dilates pupils and flattens the lens (far vision)
What happens when the ciliary muscles contract?
The suspensory ligaments become slackened, causing the lens to become more rounded.
How many layers is the retina organized into?
10 layers
What is the 1st layer of the retina?
Pigment epithelium
What is the 2nd layer of the retina?
Photoreceptor layer
What is the 3rd layer of the retina?
Outer limiting membrane
What is the 4th layer of the retina?
Outer nuclear layer (nuclei of photoreceptors)
What is the 5th layer of the retina?
Outer plexiform layer
What is the 6th layer of the retina?
Inner nuclear layer
What is the 7th layer of the retina?
Inner plexiform layer
What is the 8th layer of the retina?
Ganglion cell layer
What is the 9th layer of the retina?
Nerve fiber layer
What is the retina like?
An ogre (it has layers)
What 5 cell types accomplish visual processing?
-Photoreceptors
-Horizontal cells
-Bipolar cells
-Amacrine cells
-Ganglion cells
What do photoreceptors do?
Transduce light (photons) into electrical signals (action potentials)
What do Horizontal Cells do?
Enhance contrast and edge detection (detects visual changes)
What do Bipolar Cells do?
Intermediate cells that project to Ganglion Cells
Which cells do photoreceptors synapse with?
Bipolar Cells
What do Amacrine cells do?
Interconnect bipolar, ganglion cells and other amacrine cells to refine the visual signal
What do Ganglion Cells do?
Produce final signal that is propagated to the brain
What is convergence?
Where lots of rods converge on fewer bipolar cells, which converge on even fewer ganglion cells for processing
If there is decreased convergence, we can detect _________ detail
Greater
What is the fovea?
An area of high visual acuity
Do most veterinary species have a fovea?
No; they have an area of high visual acuity around the central retina, but with no distinct fovea
What is the tapetum lucidum?
A reflective layer of cells behind the retina to reflect light back into the photoreceptors when in low light conditions
What is the drawback of having a tapetum lucidum?
Reduced visual acuity (because the extra reflection bends the light)
What protein is involved in the activation of rods?
Rhodopsin (vitamin A derivative)
How are rods activated?
Light causes a conformational change in rod cells, which leads to hyperpolarization of the rod cells by closure of Na+ channels
Where are rods and cones usually located on the retina?
Rods - Peripheral retina
Cones - Central retina
Which cones do dogs have?
Green-yellow
Blue
Which cones do horses have?
Red
Blue
What is Occipital and Association Cortices responsible for?
Perception of objects
How are objects represented on the retina?
Inverted. Brain turns them right side up
What occurs through the lateral geniculate - visual cortex pathway?
Conscious perception
What does the retino-tectal pathway control?
-Pupillary reflexes
-Reflex orientation of the eye
What does the retino-hypothalamic pathway control?
Circadian rhythms and seasonal changes
Where are images from the left visual field processed?
Right occipital cortex
Where are images from the right visual field processed?
Left occipital cortex
What is the pathway of an action potential for simple processing?
Optic nerve –> lateral geniculate –> optic radiations –> occipital cortex in area VI
What is the pathway of an action potential for complex processing?
Optic nerve –> lateral geniculate –> optic radiations –> occipital cortex in area V2, 3
What does binocular vision provide?
-Stereopsis (depth perception)
-Binocular summation
What type of animal (role in ecosystem) has greater binocular vision?
Predators. Eyes are placed forward
Why do prey animals tend to have eyes placed on the side of their head?
To better detect predators with a maximum field of view
What do simple cells in the visual cortex respond to?
Lines placed in certain areas of the visual field at certain orientations
What do complex cells in the visual cortex respond to?
Edges or lines in a particular orientation no matter where they are in the visual field
What is the visual stream for WHERE something is?
Occipital lobe –> parietal lobe
What is the visual stream for WHAT something is?
Occipital lobe –> temporal lobe
What occurs in cataracts?
-Cloudiness of the lens
-Scatters light so that it cannot be focused on the retina
What occurs in glaucoma?
Increased intraocular pressure
What occurs in conjunctivitis?
Inflammation of the thin mucous membrane of the eyelid
What is the Menace Response Pathway
Reflex blinking that occurs following a fast movement of an object
-Tests integrity of cranial nerve pathways (2-optic and 7-facial)
Which cranial nerves are involved in the palpebral reflex?
Trigeminal (5) and facial (7)
What is the pupillary reflex?
Reduction in pupil size in response to light
–> Response is consensual
–> Controlled by parasympathetic pathway
What is the Dazzle reflex?
Strong light causes immediate closing of the eyelids.
–> Controlled from subcortical areas of the brain
–> Useful when pupillary reflex is difficult to evoke
–> Can also cause 3rd eyelid protrusion, blinking, and head movements
