Visual System Flashcards
What is light?
Waves of electromagnetic energy
Wavelengths that are visible to humans
What are the two properties of light?
Wavelength (colour)
Intensity (brightness)
How does light work with your eye?
Light enters the eye through the pupil and reaches the retina
How is the amount of light that reaches the retina regulated?
Pupil size
- regulated by iris - regulates the size of the pupil to regulate the amount of light that comes in (which gives your eye its colour)
Pupil size is compromise between sensitivity (ability to see objects) and acuity (ability to see details)
What is involved within the lens of the pupil?
Focuses incoming light on the retina
What is focus called within the lens
Accommodation
What does it mean when the lens is cylindrical?
When focused on something near
What does it mean when the lens is flattened?
When focused on something far away
Why is eye position important?
Most mammals have two eyes on the front of their heads - compared to some animals with eyes on the side of their heads
Most of what is seen is seen through both eyes
Eyes see things from a slightly different perspective
What is binocular disparity?
The difference in the two retinal images
-Greater for closer things
- Helps create depth perception (3D perception) - takes two-dimension perspectives to turn it into one 3 dimensions perspective
Whats the difference between predators and prey eye placement
Predators - eyes in front; helps predictors build dept-preception to hunt prey (Owls)
Prey - eyes on side; Better view to escape a predator by running away (Mice)
What are the five layers of the retina (the structure of the back of the eye) in order
-Retinal ganglion cell layer (near the front of the retina)
-Amacrine cell layer
-Bipolar layer
-Horizontal cell layer
-Receptor layer (farthest from the light) - the back of the retina
What are two important components of the fovea?
Location
High-acuity vision - think of the times trying to thread a needle
What two important components of the optic disk or optic nerve?
Blind spot
Completion (filling in)
- The axons on the retinal ganglion cells must leave a gap, in the receptor layer, which creates a blind spot
What is duplexity theory?
Rods and cones mediate different types of vision
What are the red coloured and blue coloured cells respectfully?
Red coloured cells are cones
Blue coloured cells are rods
What is photopic? - one of two visual systems
Cone-mediated, lighted conditions
What is scotopic? - one of the two systems
Rod-mediated, dim light
What is the difference between low and high degree convergence?
Low degree of convergence in cone-fed pathways (one receptor)
High degree of convergence in rod-fed pathways (multiple receptors)
What is spectral sensitivity?
More intense lights appear brighter
Wavelength can impact the perception of brightness
What does differential sensitivity to wavelengths mean?
Lights the same intensity, but of different wavelengths, can differ in brightness
What is spectral sensitivity curves?
Relative brightness of lights at different wavelengths
What does it mean when fixtion occurs during eye movement?
Continous movement usualy occurs, if stopped, then the visual image will disappear around the one fixations
What is saccades during eye movement?
Rapid movements between fixations - fixations are connected by these rapid movement
What is a temporal integration in eye movement?
Sum of the inputs
Explains why images are detailed , coloured, and wide-angled
Explains why things don’t disappear when we blink
Eyes are always moving, keeping up to date on the visual environments and adds this information together
Why do images start to disappear due to eye movement?
Most visual neurons respond to change so if you block the change (artificially stabilized) then images start to disappear
What is visual transduction?
Transduction is the conversion of energy
Visual transduction is conversion of light to neural signals by the visual receptors
How does transduction occur by rods?
- Rhodopsin is bleached
- Separates into retinal and opsin
- Hyperpolarizes the rods
- At rest, rods are slightly depolarized
- Rods transmit signals through inhibition
How does the inhibitory response to rods to light work?
When light bleaches rhodopsin molecules, the rods’ sodium channels close; as a result, the rods become hyperpolarized and release less glutamate
Rods transmit signals through the neural system via inhibition
Where does the retina’s information travel from?
From rods and cones
Where does the geniculate’s information travel from?
Located from the thalamus
What are the two different pathway from the retina-geniculate-striate system that visual information can take
Nasal hemiretinas decussate (crosses) at optic chiasm
Temporal hemiretinas stay ipsilateral (remains on the same side of the body)
What is the parvocellular layer in the M and P channels
Top four layers of lateral geniculate nucleus
Composed of small-body (parvo) neurons
Responsive to colour, fine detail, slow/stationary objects
What is the magnocellular layers in the M and P channels
Bottom two layers of lateral geniculate nucleus
Composed of large (mango) body neurons
Responsive to rods and movements
What is the Hubel and Wiesal methodology in order?
(4 components)
- Visual stimuli presented on a screen
- Subject is curarized - blocked eye movements by freezing it
- Extracellular electrode placed in one neuron
- Neuron’s receptive field is mapped
What is the receptive field of visual neurons?
The area of the visual field where appropriate visual stimuli can influence neuron firing
What are the receptive fields of a sensory cell?
The stimulus region and the features that excite or inhibit the cell
What are 3 very similar receptive fields in the retina-geniculate-striate system
- Retinal ganglion cells
- Lateral geniculate nucleus neurons
- Lower layer IV of striate cortex
What are the similar characteristics of the receptive fields
-Smaller in the foveal area
- Circular
- Monocular
-Excitatory and inhibitory area separated by a circular boundary
What are the two patterns of responding with respect to firing?
On firing: burst of firing when light was turned on
Off-firing: inhibition of firing when the light was turned on, burst of firing when light was turned off
Whats the difference of on-center and off-center cell responses
On-center firing: When a spot of light is shone anywhere in the center of the field
There is an “off” response when a spot of light is shone anywhere in the periphery of the field
Off-center firing: When a spot of light is shone anywhere in the center of the field
There is an “on” response when a spot of light is shone anywhere in the periphery of the field
What are some characteristics of firing?
Responds to brightness contrast between centers and peripheries of their visual fields
Responds. best to fully illuminated “on” or “off” area
Responds poorly to diffuse light
What are some simple striate cells?
Responds best to bars or edges in a particular location and orientation
Monocular
What are some complex straite cells?
Responds best to straight lines of particular orientation
Many complex cells are binocular
Retinal ganglion cells with receptive fields are selective too?
- Uniform illumination
- Orientation
- Motion
- Direction of motion
Lateral geniculate cells have receptive fields are sensitive to
- Orientation
- Motion
- Direction of motion
How do contextual influences shape properties of the receptive fields?
Timing, location, amount of light but also particular actions or emotional states
What is the primary visual cortex responsible for
Located in occipital lobe
Recieves most inputs from visual relay nuclei of thalamus
What is the secondary visual cortex responsible for?
Located in the prestriate cortex (surrounds primary visual cortex)
Recieves input from primary visual cortex
What is the visual association cortex responsible for?
Areas
- Inferotemporal cortex
-Posterior parietal cortex
Recieves input from secondary visual cortex
What results from damage to the primary visual cortex
Areas of blindness in corresponding areas of visual field
Scotomas are plotted by perimetry tests
What is completion?
Your visual system “completes” an image when your unable to see - it fills in the blank of an image
Many patients with large scotomas are unaware of them bc of this
What is blindsight?
The ability to respond to visual stimuli in a scotoma with no conscious awareness
Thought to be mediated by visual pathways that are not part of the retina-geniculate-straite system
What are the portions of secondary and association cortex do
Areas specilized for particular type of visual analysis
Connections between areas have been identified using anterograde/retrograde tracing
What responsibilities does the dorsal stream have?
Information flows from primary visual cortex
Travels through dorsal prestriate secondary visual cortex
Ends in associations cortex of posterior parietal region
Originally proposed to be “where” pathway
More recently proposed to be behavioural control path
What reponsibilities does the ventral stream have?
Information flows from primary visual cortex
Travels through the ventral prestraite secondary visual cortex
Ends in association cortex of posterior parietal region
Originally proposed to be “what” pathway
More recently proposed to be conscious perception pathway
What is agnosia?
Failure to recognize
What is prosopagnosia?
Inability to recognize faces
May not be specific to faces
- Difficulty distinguishing between visually similar members of stimuli
Associated with damage to fusiform face area
- area between the occipital and temporal lobes
- Confirmed prosopagnosia suffers could recognize faces unconsciously (with a change in skin conductance)
What is akintospia?
Deficiency in the ability to see smooth movement
Can be triggered by high doses of antidepressants
Results of damage to the medial temporal area (MT)
What are the four lines of research that support MT as the area being affected by akintospia?
Patients with damage tend to have unilateral or bilateral damage to MT
Activity in the MT increases when humans view movements (fMRI)
Blocking activity of the MT with TMS produces motion blindness
Electrical stimulation of the MT induces visual perception of motion
