Basic Anatomy and Physiology of the Visual System Flashcards
laminar organization of the retina
- photo receptors
- rods and cones
- horizontal
- bipolar
- amacrine
- ganglion cells
layers of the retina
Retinal output to __________
CNS
how do photo receptors in the retina work?
- Hyperpolarize when stimulated by light
- Depolarize when exposed to dark or shadow
- Release neurotransmitter (glutamate) when DEPOLARIZED (i.e., exposed to dark such as shadow)
- COUNTER INTUITIVE: Photo Receptors release more glutamate by dark than by light
regional differences in retinal structure
Periphery of Retina:
- Higher ratio of rods to cones
- Higher ratio of photoreceptors to ganglion cells
- More sensitive to light
When center receptors are depolarized (dark) the Bipolar Cell is __________ via horizontal cell connections.
hyperpolarized
Photo Receptors, Horizontal Cells and Bipolar Cells:
The Big Picture
what are ganglion cells?
- Ganglion Cells Receive input from Bipolar cells
- Connected laterally by Amacrine cells (similar to horizontal cells)
- Ganglion Cells have a similar Center-Surround organization as Bipolar Cells
- On-Center Off-surround or Off-Center On-surround
- Uniform illumination: surround cancels out signal from center and averages out signal
- Ganglion Cells are Responsive to differences in illumination at EDGES
__________ is only electromagnetic radiation that we can see.
- Wavelength/frequency, amplitude
- Hot colors: Orange, red
- Cool colors: blue, violet
Color is constructed in the __________.
mind
Do rods or cones see color?
cones
- Color is perceived through mixing of signals from 3 cone types: blue, green, and red cones
- Each type has a special spectral sensitivity
- Young-Helmholtz trichromacy theory of color vision
how do we see all the colors?
Opponent Process Theory of Color Vision was developed to account for G-R B-Y oppositions
how do color opponent ganglion cells work?
[AFTER EFFECT: When one color is saturated the surround opposition color is activated and leads to perception of the opposite color]
three types of LGN cells
- P-type X ganglion cells (parvocellular [small])
- M-type Y ganglion cells (magnocellular [big])
- W ganglion cells (koniocellular [tiny])
P-type ganglion cells
- Parvocellular = “small”
- Form & color
- Narrow band
- Selective response to wavelength and fine detail
- Sustained response
retinofugal projection
Visual Pathways - Lateral Geniculate Nucleus
(parts of the brain)
- Primary projections to the Lateral Geniculate Nucleus (LGN) in the Thalamus prior to visual cortex
- geniculate = “elbow”
Visual Pathways - Lateral Geniculate Nucleus
(visual field)
non-thalamic targets of the optic tract
- Hypothalamus: Biological rhythms, including sleep and wakefulness
- Pretectum: Determines pupil size; certain types of eye movement
- Superior Colliculus: Orients the eyes in response to new stimuli
retinotopic and columnar organization of
V1 (striate cortex) BA 17
- Retinotopic organization
- Cellular layers: Wide layer IV, receptive layer
- Columnar organization
- Layer IV - simple cells (location & orientation)
- Complex cells (orientation & movement)
- Hypercomplex (orientation & movement plus other features e.g. width)
- Ocular dominance columns
- Blobs (color) & interblobs (orientation)
form perception in the visual cortex
- How does the visual system take an array of varying luminence levels of light impinging on the retina and create representations of objects in space?
- The Gestalt problem of figure-ground
- Hubel & Weisel discovered simple and complex cells in visual cortex for detecting lines, orientations and motion
Orientation Sensitivity of Simple Cells in V1
Movement / Directional Sensitivity of Complex Cells in V1
Visual Cortext Cytochrome Oxidase Blobs
- Blobs cut across ocular dominance columns
- Believed to carry color information
- Recently discovered
secondary visual areas of cortex:
dorsal vs. ventral streams
- Ventral stream (“what system”): Color, objects, & faces
- Dorsal stream (“where system”): Spatial orientation, location, object motion, exproprioception (navigation)
Secondary visual areas of cortex:
Unimodal Association Areas
Note the Middle Temporal Cortex (MT):
- Involved in motion processing (dorsal stream) yet shows up in ventral stream in some diagrams
- Problem is that MT is not well defined anatomically and differs between species
- Considered part of Dorsal Stream, but not so dorsal!
Tertiary visual areas of cortex:
Multimodal/Crossmodal Association Areas
- Angular Gyrus (reading)
- Supramaginal Gyrus (integration of somatosensory auditory and visual info); project into language areas
damage of Primary Projection Area:
cortical blindness
- Anton’s syndrome (denial of blindness)
- Blindsight (unconscious perception)
- Anopia Hemianopia (visual field deficit)
- Scotomas
