1 - Central Visual Processing Flashcards
Where do RGC (Retinal Ganglion Cells) axons synapse?
Main function?
Lateral Geniculate Nucleus (LGN)
Function: Relay–not processing
Where does visual information processing begin?
In retina–retinal output (retinal ganglion cells) is processed
Three Type of Retinal Ganglion Cells:
M-Type (5%)
P-Type (90%)
nonM-nonP Type (5%)
M-Type: Motion; large receptive fields, transient responses
P-Type: Shape/Fine Detail, Color; small receptive fields, sustained responsesn; some can be wavelength sensitive
nonM-nonP Type: Color cells; wavelength sensitive
Where are RGC axons (optic nerve) sorted?
Optic Chiasm
Optic Tract: Axons from BOTH eyes; carries half visual field
Optic Nerve: Axons from ONE eye
What is the target for Lateral Geniculate Nucleus (LGN) neurons?
Primary Visual Cortex (aka striate cortex, V1 cortex, B.A. 17)
How many layers does the primary visual cortex have; and which receives input from the thalamus?
6 Layers
Layer IV of neocortex receies input from the thalamus
What is unique about layer IV of primary visual cortex?
Receives large volume of visual input; it is especially thick
Line of Gennari (stria)
How is Visual Input organized to the striate cortex?
Retinotopic (topographic) organization
Ocular Dominance Columns:
Monoculus Neurons?
Binocular Neurons?
Layer IV is organized into ocular dominance columns or stripes
LGN neurons relay infromation from either left or right eye to cortical neurons in one column
Monocular: Receive input from only ONE eye
Binocular: Recive input from BOTH eyes (role in depth perceptions)
Blobs
Defined histologically by high concentration of cytochrome oxidase (mitochondrial enzyme)
Wavelength sensitive responses to visual stimular and important in color perception
What does the primary visual cortext contain for functionally distinct processing?
What defines these?
Three Channels
- Motion (magnocellular or M channel)
- Shape/Form (parvocellular-interblob or P-IB channel)
- Color (Blob channel)
- - -
Channels are deined by the specific visual respisne properties of the neurons within the channel
- Nature of stimulus (shape, motion, color)
- location in visual field (receptive field)
M Channels:
Main Function?
Input?
Response Properties?
Function: Analysis of Motion
Input: Magnocellular LGN Neurons (M)
Properties: Circular, Center-Surround Receptive fields; monocular; wavelength insensitive
Like M-type retinal ganglion cells
Cortical Simple Cells
Type?
Shape?
Orientation Selectivity?
Directional Selectivity
Type: M Channel
Elongated receptive field with anatagonistic on- and off-zones
Shape: Formed by converging inputs from multiple LGN neurons with circular receptive fields
Orientation Selectivity: Cells have preferred orientation for firing rate; stimulus that aligns with receptive field and falls only onto on-zone causes highest firing rate
Directional Selectivity: Responds to preferred movement; allows analysis of motion
How are Orientation-selective neurons arranged in columns?
Neurons in SAME columns share preferred orientation
Directional Selectivity?
Property of many orientation selective cells
Highest firing rate when object with preferred orientation moves in one direction
***Directional Selectivity of neurons within M-channel allow for analysis of OBJECT MOTION***
P-IB Channels (Parvocellular Interblob)
Function?
Input?
Response Properties?
Function: Analysis of shape
Input: Non-wavelength sensitive parvocellular LGN neurons
Properties: Small, circular center-surround receptive field, sustained firing, similar to P-type retinal ganglion cells
Cortical Complex Cells
Type?
Properties?
P-IB Channel cells are Cortical Complex Cells
Highly orientation selective (more than simple)
Fire in response to edges in receptove field, as long as stimuli have preferred orientation (no “on” “off” zones)
Small receptive field; mostly binocular; wavelength insensitive
***High degree of orientation selectivity and small size of receptive field allows complex cells to analysis details of shape (form)***
Blob Channels
Function?
Input?
Response Properties?
Function: Analysis of Color
Input: Wavelength Sensitive LGN neurons (intralaminar/parvocellular)
Response Properties: Wavelength sensitive, circular, NOT orientation or direction selective
***Wavelength Sensitivity and circular receptive fields allow Blob cells to detect small areas of color***
Hypercolumn
How is information processed?
Small cube of cortex; spans all cortical layers from I to VI
Each point in visual field is represented by a hypercolumn in primary visual cortex (retinotopic mapping)
Information is processed in parallel by the hypercolum for motion, form, color
Extrastriate Visual Cortex
Two major “streams”?
~ 2 dozen visual cortical areas exists; each with a complete retinotopic map
1. Analysis of Motion - Dorsal
2. Analysis of Form/Color - Ventral
Dorsal Stream Extrastriate Cortex
Function of area MST?
Clinical: Lesion in Dorsal Stream?
Function: Linear, Radial, Circular Movement
Clinical: Lesion
- Deficit in perception of motion (can’t see coffee being poured until it’s full)
Ventral Stream Extrastriate Cortex
Function of area V4?
Clinical: Lesion in Ventral Stream?
Function: Large receptive field, orientation and color selective
Clinical: Lesion
- Deficit in shape and color discrimination; Achromatopsia - partial or complete loss of color vision; “shades of gray”
Ventral Stream Extrastriate Cortex
Function of area IT (Inferior Temporal)?
Clinical: Lesion in Ventral Stream (IT)?
Function: Colors/highly specific shapes; Fusiform Gyrus = Faces
Clinical: Lesion
- Prosopangnosia - Facial recognition deficit resulting from lesion in face area
Clinical important of early visual experience?
Clinical: Stabismus
Early experience permanently alters visual system
Strabismus - Misalignment of the eyes; produces diplopia; compensates by suppressing input from one eye; IF untreated–permanent visual impairment (low acuity, poor depth perception)
What underlying structures are hindered if early visual deficits are not corrected?
LGN axons from deprived eye fail to synapse properly in Layer IV
Clinical: Repair of Strabismus
Critical Period?
Surgical Corrections, or eye patching allows stimulation of weak eye
For best outcome, correction should be done before critical period ends
Depth Perception
Near vs Far?
Different mechanisms exist for near-field and far-field depth perception
Near Field: (about 30 m) - Eyes separated horizontally, so 3D objects produce slighlt different images one ach retina; binocular areas help detech differences
Far Field: Very little retinal disparity; depth cues are monocular
Mechanisms for Far Field Depth Perception?
Size: Looks small, far away
Interposition: Object in front must be closer
Linear Perspective: Parallel lines converge
Light and Shadow: Distant objects look hazy
Motion Parallax: During movement, near objects appear to change more quickly
