Visual System Flashcards
Visual Agnosia
problems in perception (not sensation)
Visual Perception: Light
range of electromagnetic radiation (particles and waves)
- particles are photons
- waves of various lengths
(we don’t see all waves)
the eye
the light has to go through a lot of tissue before reaching the retina all the way back where it get perceived
Retina
photoreceptions absorb the light
rods and cones
rods
responsible for low levels of stimulation (e.g. night vision or dim lighting)
- 120 million
CLOSER TO PERIPHERY
cones
responsible for intense light (day vision)
- 6 million
- red, gree, and blue
-gives visual resolution/acuity
CLOSER TO FOVEA
optic nerve
axons of the cells that have collected information in the retina and sends it to the brain
optic chiasm
where the brain separates information to get the LGN (lateral geniculate nucleus)
ipsilateral
same side
contralateral
opposite side
blind spot
- at 17 degrees
- the other eye is functioning for you
- we mentally reconstruct the blindspot
- blindspot is created because the axons of the retinal ganglion cells are coming out
first synapse in the LGN
3 layers receive information from one eye
3 layers receive information from the other eye
LGN to V1
- many LGN cells synapse onto one V1 Cell
- receptive field of the LGN combine to make a larger receptive field in the V1 cells
V1
- organized into cortical modules (like little slices of cubes)
- cortical module holds the color and orientation
- helps make up the map of the “pixels” you can see
orientation
shapes, angles, size, edges → how something is composed
blind sight
never synapses onto the LGN, it goes to the superior colliculus and goes directly to the parietal cortex (bypasses LGN and goes to parietal)
LGN
- retinotopically organized –> spatial map
Top 4 Layers - Parvo (slow)
-larger cells
- color orientation
- ventral pathway
- had the fovea which has cones, which do color
Bottom 2 - Magno (fast)
- smaller cells
-input from the periphery
gives motion
10%
of information goes directly to the parietal cortex
left visual field
nasal (inner left eye)
temporal (outer right eye)
right visual field
nasal (right inner eye)
temporal (left outer eye)
preferences of photoreceptors
short, medium, long waves
high intensity lights (cones)
low intensity (rodes
Visual Cortex (V1)
- organizes color and orientation into cortical modules
- if damage to entire V1 in left hemisphere –> then you cannot see anything in right eye
= damage = completely bling
V2
detects corners (not just edges)
V4
- detects curvature and actual computation of color occurs here
- if someone has damage –> color blind
- color processing
- blindness = grayscale
cortical visual areas
- optimal stimulus
- more complex along route
- bidirectional connectivity
Retinotopic mapping
- put someone in MRI and show them a rotating checkerboard on a circle
- can identify the regions in the cortex and ask questions about what the groups of cells are doing
Hierarchy
Retina → points of light, very small
LGN → On/off cells, bit larger
V1 → larger RFs, orientation selectivity
V2-4 → larger Rfs, edges, T-junctions, curvature (early stages of visual processing; the cells have preferences)
Rods → LGN M cells → V1
Movement and contrast sensitive
Color and location insensitive
Cones → LGN P cells → V1
Color and orientation selective
Contrast insensitive
