Visual system (lecture 3)

The retina is an extension of the brain as it develops from a portion of the neural ectoderm that gives rise to the rest of the brain.

3The retina is a sheet of photoreceptors and connecting neurons 

The organisation of the retina  - learn diagram

The photoreceptors (the first link in visual processing) are situated behind the neurons that connect them to the rest of the brain 

Rods:

Rods mediate nocturnal vision

Convergent pathway to ganglion cells

High sensitivity but poor spatial resolution(acuity) 

Canes:

Cones mediate colour vision

Direct pathway to ganglion cells

Low sensitivity but good spatial reso5lution 

Different distribution of cones and rods in the retina:

The fovea contains only cones

In the rest of the retina rods are more frequent than cones

Light - to

rhodopsin and iodopsin (light- absorbing molecules) split apart - to

change in membrane permeability of the outer segment of rods and cones (decrease of sodium NA+) - to

gradual electrical potential in the photoreceptors - to

bipolar cells - to

ganglion cells  - to
Three types of cones: sensitive to red, green or blue.

7

Ganglion cells differ in type cells (large, medium and small) and concentration (greater in the fovea)

These differences account for:
1) processing different visual information and relay it to different part of the brain
2) the representation of particular points in the retina

The axon of ganglion cells stream towards the optic disc exit the retina become myelinated form the optic nerve 

Blind spot demo (see graphic) - Macula cieca

Close your right eye and look at the cross, at about 50 cm. Move forward and backward until the circle will “disappear”

(i.e. it falls in your macula cieca) 

Receptive fields of retinal ganglion cells

The group of photoreceptors that converge onto a single ganglion cell represent its receptive field.

Receptive fields:

- Are circular & size varies from small (in the fovea) to large (in the periphery)

- Contain two zones: OFF and ON, which produce different neural activity in response to retinal stimulation.

-Two parallel pathways  ON and OFF cells (crucial for contrast) 

Ganglion cells – receptive field

ON-CENTER  central excitatory zone and an inhibitory surround.

OFF-CENTER central inhibitory zone and an excitatory surround. 

[see slides demoing reflective surfaces]

Simultaneous brightness contrast

Firing rate of neurons whose receptive fields intersect a contrast boundary will differ from those of neurons whose receptive fields fall entirely on either side of the boundary.

Neurons whose receptive field centres lie just within the target on the dark background centres  will fire at a higher rate than neurons whose receptive field centres lie just within the target on the light background.

The former are less inhibited by their oppositely disposed receptive field surrounds than the latter. 

learn diagrams of visual pathways

Form of visual stimuli

M cell (magnocellular):
large cell body, rich dendritic ramification

P cell (parvocellular):
small cell body, and limited dendritic ramification

M and P cells consist of ON-centre and OFF-centre cells

M cell  large receptive fields  large objects, movement

P cell  small receptive fields  small objects, details, colour 

Architecture of lateral geniculate nuclei (LGN) is highly organized.

Six layers:

1. a)  Three layers (2, 3 and 5) receive input from ipsilateral retina, other three (1, 4 and 6) from the contralateral retina

2. b)  Each layer has a retinotopic representation. A stimulus in a certain position in the space will activate cells within each layer that fall along a line perpendicular to the LGN’s surface.

3. c)  Each layer has different cytoarchitecture. Axons in the lower two layers (magnocellular – M) are larger than in the other four layers (parvocellular – P). 

4. Receptive fields in lateral geniculate nucleus is quite similar to those found in the retina.

   There are on-center and off-center cells 

Colour of visual stimuli

LGN receptive fields for colour.

When a portion of the receptive field is illuminated with the colour shown, the cell’s rate of firing increases. When a portion is illuminated with the complementary colour, the cell’s rate of firing decreases.

Concentric single opponent cells one type of cone activates the centre and the other type of cone has the opposite effect on the surround 

LGN - to

Optical radiation - to

Primary visual area (BA 17 o V1 o striate cortex) - to

The segregation of M and P pathways, started in LGN, is maintained in the cortex.

M pathway - movement
P pathway  - colour and details 

Receptive fields in the cortex are more complex than those found in retina and lateral geniculate nucleus

Input from lateral geniculate nucleus reaches the IV layer of visual cortex.

In this layer, cells show an increase of activity ONLY when the stimulus has linear properties, i.e. when has clear edge such as a bar or a line.

They do not respond to a simple spot of light 

“WHAT” and “WHERE”

The output from V1 follow to pathways:

The superior longitudinal fasciculus (dorsal route) and the inferior longitudinal fasciculus (ventral route)

Dorsal pathway  location of objects (WHERE)

Ventral pathways  object recognition (WHAT) 

Simple cells

Simple cells have discrete excitatory and inhibitory zones, which are larger than those found in retina and in LGN, and their size is rettangular rather than circular

Receptive fields of simple cells respond to different orientation (vertical, horizontal, oblique) 

Complex cells

Simple cells and, at some extent, also geniculate cells form the input to Complex cells.

These are larger than simple cells and they have NOT discrete excitatory and inhibitory zones.

Stimulus orientation is crucial, but position within the receptive filed is less critical. 

Visual cortex is organised in columns.

This implies that complex cells have direct connections with simple cells within the same column.
In turn, this facilitates exchange of local information. 

Visual cortex and colours Blobs: clusters of cells processing colour information

Similar receptive field than those observed in the LGN

Parvocellular cells of LGN processing colour terminate mainly on the blobs of the visual cortex, but also in the interblobs.

Also some magnetocellular cells of LGN terminate on the blobs to provide information about brightness or contrast. 

Visual deficits following damage of visual central pathways 

Macular vision spare lesion in occipital cortex 40

macular vision not spare  lesion in optic radiation 

Foveal vision spare lesion in occipital cortex 44

Fovealvisionnotspare lesioninopticradiation 

Brain lesion encompassing V5 have been associated to Akinetopsia (e.g. patient MP described by Planck in 1983)

Brain lesion encompassing V4 and the area anterior to V4 have been associated to Achromatopsia

Bilateral lesions of parieto-occipital area

BALINT’S SYNDROME (optic ataxia, ocular ataxia, possible simultagnosia)

Lesions in the occipital-temporal areas

AGNOSIA (inability to recognise objects through visual input)