The visual system.

By Alex Antoniades and Robert Joaquim.

Overview

The visual system mechanism. Light. The eye. Structure and visual fields. The receptors role. The connection to Visual cortex. Visual cortex and other areas of occipital lobe. Experiment Prenatal visual system – Development What where pathways. How the visual system develops. Improvement of vision. Cortical development in newborns. Externality. Color vision. Sex differences.

Our visual world.

Radiant electromagnetic energy – Light.

Light is similar to sound waves (rise & fall).

The wavelength of light (frequency) denotes different color.

Intensity of frequency refers to brightness.

The eye

Each cell in visual cortex receives its input via the retina, from a particular part of the visual field.

This is the known as the receptive field of the cell.

‘Retinotopy‘ is the term referred to stimulated cells in the retina and the visual cortex that are linked together.

Pictures of retina.

Structure and visual fields.

Two visual fields in each eye. Right and left.

Lining of eyes transforms light into neuronal signals.

Information from right half of the visual field goes to the left primary visual cortex, and vice versa.

Structure and visual fields, cont.. Information from the

center of our visual fields goes to the most posterior part of the visual cortex to be represented.

Information from peripheral parts are mapped anteriorly in the primary visual cortex.

Less area is devoted in visual cortex for peripheral view receptors.

The receptors role.

Rod cells : 1000 times more sensitive to light (night vision), 20 times more than cone cells.

Cone cells : 3 different types, used for color vision, are not used in dark.

Rod cells have more membranous disks than cone cells, they are used for containing photopigment as a part of the outer segment of the cell.

The receptors role, cont..

The fovea contains only cones (colored vision,not used in dark) and the surrounding contains mainly rods (used for night vision).

When light energy occurs on the cells, they are excited throughout a chemical process known as phototransduction.

The receptors role, cont..

Absorption of light activates the cell. The information passes to the bipolar cell which synapses with the photoreceptor that is excited. The bipolar cell after modifying the signal, it passes the information to the ganglion cell. Amacrine and horizontal cells are changing the signal when still in the bipolar cell.

The connection to Visual cortex.

Signals from right retinas of both eyes travel through the optic nerve, optic tract, and optic radiations to the primary visual cortex of the right hemisphere, whereas signals from the left retinas travel to the left hemisphere.

When ganglion cell axons pass along the optic nerve they combine to form the optic chiasm (centre of picture).

The connection to Visual cortex, cont..

Some axons from optic tract form connections with hypothalamus (learning).

Most of the axons after the optic chiasm pass to Lateral Geniculate Nucleus of dorsal thalamus and then to the primary visual cortex also know as area 17, and area V1 of the occipital lobe.

Lateral Geniculate Nucleus.

Has 6 layers, starting with ventral (bottom).

Layers 1 & 2 contain bigger neurons -Magnocellular.

Layers 3 to 6 contain smaller neurons – Parvocellular.

80% of the input in LGN comes only from the visual cortex.

It is projected to the primary visual cortex.

Primary Visual Cortex.

9 different layers. Cell arrangement highly

complex. 3 separate processing

channels along Calcarine sulcus (line of Gennari).

M-channel – Motion. P-IB-channel – Shape. Blob-channel – Color. Striate cortex has around

1000 modules

Experiment.

Pick up one of the slips of paper and hold it with your right hand. Close your left eye with your left hand.

Focus on the left side of the slip on the color face, and watch the right hand side face disappearing as you move it closer to you eye.

Blind spot.

Color blind spot.Pick up one of the slips of paper and hold it with your right hand. Close your left eye with your left hand.

Focus on the right side of the slip on the grey face, and watch the left hand side changing color as you move it closer to you eye.

What & Where pathways

Important for parallel processing Where stream – deals with visual space,

detects speed, direction, and location in 3-D space. Directs eye movements to follow targets

What stream – identifies objects or familiar features, detects colour, shape, and fine detail

How the visual system develops

Visual development begins 4th week of embryonic life 1st = neurons & synapses in retina, 2nd = subcortical

visual areas, 3rd = primary visual cortex, 4th = higher visual centers

Takes months for whole system to be up & running Years before pathways are stabilised First optic tissue = 22 days after fertilisation By 5 weeks eye cups have developed

How the visual system develops cont…

8 weeks = eyelids form & fuse shut Retina neurons divide into layers for specific functions 1st layer to form, ganglion cells between 6 & 20 weeks

of gestation. Foveal cells formed by 14 weeks of gestation

Some rods & bipolar cells in peripheral parts of retina still developing many months after birth

All of 100 million neurons in primary visual cortex formed between 14 & 28 weeks of gestation

How the visual system develops cont…

Synapses in primary visual cortex form in 5th month. Process continues for another year at 10 billion synapses per day

Evidence suggests ‘where’ stream develops earlier than ‘what’ stream. By 4 months of age ‘where’ stream reaches max synaptic density while ‘what’ stream takes another 4 months

8 months after birth, primary visual cortex reaches max synaptic density

The improvement of vision

Fetuses 24 weeks postconception have been seen to react in response to strong light pointed at mother’s abdomen

At 2 months old, baby’s cerebral cortex takes over most visual tasks from subcortical circuits

By 1 year old, baby’s vision nearly as good as adult’s

Baby Audience

That’s how the presentation would look like if you were 4 weeks old

Newborn Vision

Vision similar to looking out of frosted glass Can only focus between 7 & 30 inches, can

see some colour. Better vision at edge of visual field

See motion better than other visual features due to ‘where’ stream

Innate preference for faces or facelike objects

Newborn Vision cont…

Track slowly moving objects Unsteady eye movements – saccades Smooth by 2 months 3 to 6 months – baby can anticipate movements & focus

ahead Vision 30 times worse than adults Acuity improves a lot due to changes in retina & cerebral

cortex

Improving vision

Newborn 4 Weeks 8 Weeks 3 Months 6 Months

Newborn Vision cont…

Fovea slowest part of retina to mature

Foveal cones short & fat After birth, cones grow

significantly longer & slimmer

Therefore, more cones & increased sensitivity to light

Cortical development in newborns

At birth, few action potentials in response to visual stimuli

No. of active neurons increase due to rapid growth of cortical synapses & myelin

Hyperacuity starts at 4 months

Externality

Emphasis on peripheral vision

Newborns rely on this much more than adults

Lasts up to 2 months until fovea is mature & cerebral cortex takes over fine vision

Colour vision

Colour vision improves as cones mature At 8 weeks colours can be differentiated as long as

large & bright e.g. apple & orange Blue cones aren’t so good at this age By 3 months, blue cones are as good as red & green

ones By 4 months, areas in primary visual cortex used for

processing colour are fully developed

Colour vision cont…

Habituation experiments Babies find it easier to

remember colours than shapes

Older babies prefer bright reds or blues

Maybe related to 4 types of colour opponent cells in LGN or cortex

Sex differences

Between 4 & 6 months of age, hyperacuity is significantly better in girls

Between 7 & 10 years of age, males perform better on visual spatial tasks

Questions.

1) In which part of the brain is the line of Gennari located?

2) How long does it take for fovea to mature in newborns.

3) What is acuity?