Topic 8 - The Visual System Flashcards
Cornea
Absolute level of refraction (can’t change shape of cornea)
Causes Bending of light energy in order to focus it on certain areas of the retina
Lens
Change the amount of refraction of light
(Shape changes to allow for more refraction when you bring object closer)
Shape of lens determines the amount of refraction that occurs
Ciliary muscle
Determines shape of the lens
Iris
Series of muscles that determines size of pupil
Pupil
Window that allows light energy into the eye
Pupil gets bigger in darker room to let more light into eye
Retina
Area of eye that converts light energy into APs
Important Functional area of light
Fovea
Specialized area of retina
Cornea and lens refract light onto fovea
Fovea is where highest level of visual processing takes place
- highest special acuity/resolution
2 layers (ganglion and bipolar cell layers) are pulled off to the side so that light energy can influence photoreceptors with less distortion (remember distortion of light entering)
Highest concentration of CONES
Lowest ratio of recptors-ganglion cells = smallest receptive field = highest acuity in bright light
Optic disk
Output Cells from retina exit here
Refraction
Bending of light energy
Photoreceptors of the eyes
Rods & cones
Convert light energy into APs
3 layers of the retina
- Photoreceptors (Ross & comes)
- Bipolar cells
- Ganglion cells
If there is more light energy is there more or less neurotransmitter released?
More light energy = less neurotransmitter release
Less light energy = more neurotransmitter release
When is the ON cell depolarizer (excited)?
When there is MORE Light (less neurotransmitter)
When is the OFF cell depolarized (excited)?
When there is LESS light (more neurotransmitter)
Differences between rods and cones pathway
- Rod pathway can only activate ON bipolar cells (they then use interneuron called amacrine cell to activate the bipolar cells of the cones pathway)
- Rods have more convergence to bipolar cells while cones have less convergence to bipolar cells
- Rods respond better in less light, cones respond better in bring light
What does the amacrine cell (interneuron) do?
Excites (depolarizers) the ON cell of the cones pathway
Inhibits the OFF cell of the cones pathway
How do you get ⬆️ acuity?
⬇️ ratio of receptors:ganglion cells = ⬇️ receptive field = ⬆️ visual acuity
Centre-surround mechanism
“The centre of a visual field responds differently to light than the surrounding (annulus) of a visual field”
Receptive field
Area of retina that can be influenced from the firing of a ganglion cell
What do horizontal cells do
Enhance the centre-surround mechanism
Magnocellular pathway of LGN
provides motion info (important for visual feedback during movement)
More rods (higher sensitivity, don’t like light)
Larger receptive fields bc rods are mainly part of the peripheral retina and have higher ratio of convergence
Parvocellular pathway of LGN
Applies meaning to what we’re seeing like identifying objects
More cones (less sensitive bc cones like light)
Smaller receptive fields bs made mainly of cones which is was the fovea is made of and cones have less convergence meaning smaller receptive fields
What part of brain does dorsal stream go to
Posterior parietal cortex
What part of brain does ventral stream go to
Occipital-temporal cortex
How does the dorsal stream represent space
Egocentrically
Object location/size are coded in reference to the body
Absolute coordinates used
How does the ventral stream represent space
Allocentrically
Object location/size are coded relative to other objects in the environment (info therefore does not need to be updated as you move closer or further from objects)
Uses relative coordinates for size and location
When are tasks immune to size-contrast illusions?
- When they are performed in ‘real time’ (immediate)
2. When they use absolute coordinates
