Vision Flashcards
Three steps from stimulus to perception
Reception, transduction, coding
Light
Adequate stimulus for vision. Described as a particle of energy wave of energy (e-magnetic energy)
2 properties of light
- Wavelength = color
2. Intensity= brightness
Wavelengths of light (violet-Red) based on rainbow
400-700
Pupil
Where Light enters the eye.
Size changed in response to changes in illumination
Sensitivity
Ability to see when light is dim
Acuity
Ability to see details
Lens
Focuses light on the retina
Ciliary muscles
Alter the shape of the lens as needed
Accommodation
Process of adjusting the lens to bring the images into focus
Blind spot
No receptors where info exits the eye
Completion
Visual system uses info from cells around the blind spot to fill in blind spot (completion)
Fovea
High acuity area at center of retina
Thinning of ganglion cell layer…
Reduces distortion due to cells between The pupil and the retina
Duplicity theory of vision
Cones and rods mediate different kinds of vision
Cones
Photopic (daytime vision)
High acuity color info in good lighting
Only cones found in fovea
Rods
Scotopic (nighttime vision)
High sensitivity, allowing low acuity vision in dim light. Lack detail and color info.
More convergence. Increasing sensitivity while decreasing acuity.
Visual transduction (in light) step 1
Light bleached rhodopsin molecules
Visual transduction (light) step 2
Sodium channels close
Visual transduction (light) step 3
Sodium ions cannot enter rods = rods hyperpolarized
Visual transduction (light) step 4
Glutamate release is reduced
Trichromatic theory of color vision
3 different kinds of cones
- Short wavelength: blue
- medium wl: green
- long wl: red and yellow
Discriminate among wavelengths by ratio of activity across 3 types
Intense light increases activity of all 3 but ratio of responses stays same
Opponent process theory
2 different classes of cells in the visual system for encoding color and another one for encoding brightness
Hering: each of 3 cells encoded 2 complenatry color perceptions
1class= signaled red by changing activity in one direction (ex:depolarization and hyperpolarization)
2class= signal blue and yellow in same opponent fashion.
Evidence to support opponent process
- If state at something yellow for 1 minute and look at white = see blue. (Same for is u stare at red, green = green, red) afterimage
- cells in lateral geniculate nucleus may be excited by green light and inhibited by red light.
Blue yellow color deficiency
Very rare
Red-green color deficiency
Most common
^ due to deficient cone that process that colors wavelength
Protanopia
Red weak
Brightness of red. Orange. And yellow is reduced. Shift to appearing green
Deuteranopia
Green weak
Same perceptual problem with red. Orange. Yellow. green brightness reduced. Shift to see red.
Receptive fields
Portion of retina in which light affects the activity of a given neuron
Light in retinal area leads to excitization or inhibition of cortical cell
Define receptive fields in 2 ways
An area of retina
An area of visual field
^ equivalent
Left visual field reaches…
Right primary visual cortex
Vice versa for Right visual field
Similarities seen at all three neurons (retina,geniculate,striate)
Receptive fields in fovea smaller than periphery
Neurons receptive fields are circular
Neurons Monocular
Many neurons with excitatory and inhibitory area.
Retinoptopic organization
(Striate system)
Info received at adjacent portions of retina remain adjacent in straite
More cortex is devoted to areas of high acuity
25% of primary visual cortex is dedicated to input from fovea
Magnocellular layers
Big cell bodies (bottom two layers of LGN)
Responsive to movement
Input from rods
Parvocellular layers
Small cell bodies, top 4 layers of LGN
Color, detail and still/slow objects
Input from cones
Visual cortex organization
Lateral geniculate area -> primary visual cortex (V1)-> secondary visual cortex (V2)-> association cortex
Parallel processing
Info flow through components over multiple pathways
Combined activity of many interconnected cortical and subcortical areas of each sensory system
V1+V2
Feature detectors: neurons respond to presence of a particular feature
Spatial frequencies
Inferior temporal cortex
Large receptive fields
Detailed info about stimulus shape
Posterior parietal cortex
Spatial attention: where in space stimulus allocated
Spatial neglect: ignore one power of body or extrapersonal space
Visual agnosia
Inability to recognize objects
Prosopagnosia
Inability to recognize faces. Able to read and write and reocgnize other objects.
Trouble recognizing different kinds of animals and cars.
Difficulty with complex visual discriminations
Evidence from brain imaging studies in normals: objects and faces processed by central stream.
Middle temporal cortex (MT) (V5)
Motion perception info is processed
Medial superior temporal cortex (MST) (V4)
Motion perception info is processed
Motion perception
Cells respond selectively to speed and direction of movement
Disorders of motion perception monkeys…
Monkeys with damage to MT or MST respond inaccurately to moving visual stimuli
MT damage in humans
Causes motion blindness
