1 - Central Visual Processing

Question 2

Where do RGC (Retinal Ganglion Cells) axons synapse?

Main function?

Answer 2

Lateral Geniculate Nucleus (LGN)

Function: Relay–not processing

Question 3

Where does visual information processing begin?

Answer 3

In retina–retinal output (retinal ganglion cells) is processed

Question 4

Three Type of Retinal Ganglion Cells:

M-Type (5%)

P-Type (90%)

nonM-nonP Type (5%)

Answer 4

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

Question 5

Where are RGC axons (optic nerve) sorted?

Answer 5

Optic Chiasm

Optic Tract: Axons from BOTH eyes; carries half visual field

Optic Nerve: Axons from ONE eye

Question 6

What is the target for Lateral Geniculate Nucleus (LGN) neurons?

Answer 6

Primary Visual Cortex (aka striate cortex, V1 cortex, B.A. 17)

Question 7

How many layers does the primary visual cortex have; and which receives input from the thalamus?

Answer 7

6 Layers

Layer IV of neocortex receies input from the thalamus

Question 8

What is unique about layer IV of primary visual cortex?

Answer 8

Receives large volume of visual input; it is especially thick

Line of Gennari (stria)

Question 9

How is Visual Input organized to the striate cortex?

Answer 9

Retinotopic (topographic) organization

Question 10

Ocular Dominance Columns:

Monoculus Neurons?

Binocular Neurons?

Answer 10

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)

Question 11

Blobs

Answer 11

Defined histologically by high concentration of cytochrome oxidase (mitochondrial enzyme)

Wavelength sensitive responses to visual stimular and important in color perception

Question 12

What does the primary visual cortext contain for functionally distinct processing?

What defines these?

Answer 12

Three Channels

1. Motion (magnocellular or M channel)
2. Shape/Form (parvocellular-interblob or P-IB channel)
3. 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)

Question 13

M Channels:

Main Function?

Input?

Response Properties?

Answer 13

Function: Analysis of Motion

Input: Magnocellular LGN Neurons (M)

Properties: Circular, Center-Surround Receptive fields; monocular; wavelength insensitive

Like M-type retinal ganglion cells

Question 14

Cortical Simple Cells

Type?

Shape?

Orientation Selectivity?

Directional Selectivity

Answer 14

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

Question 15

How are Orientation-selective neurons arranged in columns?

Answer 15

Neurons in SAME columns share preferred orientation

Question 16

Directional Selectivity?

Answer 16

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***

Question 17

P-IB Channels (Parvocellular Interblob)

Function?

Input?

Response Properties?

Answer 17

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

Question 18

Cortical Complex Cells

Type?

Properties?

Answer 18

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)***

Question 19

Blob Channels

Function?

Input?

Response Properties?

Answer 19

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***

Question 20

Hypercolumn

How is information processed?

Answer 20

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

Question 21

Extrastriate Visual Cortex

Two major “streams”?

Answer 21

~ 2 dozen visual cortical areas exists; each with a complete retinotopic map

1. Analysis of Motion - Dorsal

2. Analysis of Form/Color - Ventral

Question 22

Dorsal Stream Extrastriate Cortex

Function of area MST?

Clinical: Lesion in Dorsal Stream?

Answer 22

Function: Linear, Radial, Circular Movement

Clinical: Lesion

1. Deficit in perception of motion (can’t see coffee being poured until it’s full)

Question 23

Ventral Stream Extrastriate Cortex

Function of area V4?

Clinical: Lesion in Ventral Stream?

Answer 23

Function: Large receptive field, orientation and color selective

Clinical: Lesion

1. Deficit in shape and color discrimination; Achromatopsia - partial or complete loss of color vision; “shades of gray”

Question 24

Ventral Stream Extrastriate Cortex

Function of area IT (Inferior Temporal)?

Clinical: Lesion in Ventral Stream (IT)?

Answer 24

Function: Colors/highly specific shapes; Fusiform Gyrus = Faces

Clinical: Lesion

1. Prosopangnosia - Facial recognition deficit resulting from lesion in face area

Question 25

Clinical important of early visual experience?

Clinical: Stabismus

Answer 25

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)

Question 26

What underlying structures are hindered if early visual deficits are not corrected?

Answer 26

LGN axons from deprived eye fail to synapse properly in Layer IV

Question 27

Clinical: Repair of Strabismus

Critical Period?

Answer 27

Surgical Corrections, or eye patching allows stimulation of weak eye

For best outcome, correction should be done before critical period ends

Question 28

Depth Perception

Near vs Far?

Answer 28

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

Question 29

Mechanisms for Far Field Depth Perception?

Answer 29

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