The Functional Neuroanatomy of the Human Visual System

Sensory Processing 

• The visual system is a sensory mechanism which gathers information about the environment and sends it to the brain via neural impulses.
• This is achieved by means of transduction.
• Transduction represents transformation of one type of energy into another.

How does it work?

• The visual system responds  to a particular type of energy: Light
• Light energy is transduced into neural impulses via specialised receptor cells in the eyes.
• These receptor cells send electro-chemical messages to the neurons in the cerebral cortex.
• Light energy => chemical energy => electrical energy

What is light?

• A narrow band in the spectrum of electromagnetic radiation capable of stimulating receptors in the eye.

Three psychological dimensions of light

• Hue - determined by wavelength and roughly corresponds to colour names
• Saturation - relates to the purity of light (intense vs dull)
• If all radiation is of one wavelength, the colour is pure or saturated.
• If radiation contains all wavelengths, it has no hue- it is white.
• Brightness - relates to the amplitude/intensity of the radiation (light or dark)

Anatomy of the eye

• Visual information is contained in the light relfected from objects. To perceive objects, we need sensory detectors that respond to the reflected light 
• As light passes through the lens of the eye, the image is inverted and focused to project on the back surface of the eye, the retina. 

The retina and photoreceptors

• Retina contains several layers of cells including photoreceptors
• Photoreceptors respond to light energy by firing neural impulses
• There are two types of photoreceptor:
• CONES: provide most visual information and are responsible for colour vision
• RODS: more responsive in dim light. They are sensitive to black/white and shades of grey.

The Fovea

• Region at the centre of the retina that is specialised for high acuity
• Receptive field at the centre at the eye's visual field
• Depression at the fovea where photoreceptors are packed most densely and where vision is clearest

Acuity across the visual field

• Vision is better in the centre of the visual field than at the margins or periphery 
• When we look at the centre, letters at the periphery must be much larger than those in the centre for us to see them as well.

The optic disc

• Region of the retina where axons forming the optic nerve leave the eye and where blood vessels enter and leave
• Has no photoreceptors and thus is a 'blind spot'

From eye to brain

• Information from two retinal hemifields cross at the optic chiasm and travel to the opposite side of the brain.
• After this, most axons of the optic nerves project to the lateral geniculate nucleus  in the thalamus.
• Some project to the superior colliculus (sub-cortical pathway).

Lateral geniculate nucleus (LGN)

• It is the main relay centre from the retina to the visual cortex
• It receives the information from the retinal ganglion cells but also, importantly, receives feedback from the cortex
• It performs a range of computations (spatiotemporal correlations) to achieve a 3D representation of visual space

The primary visual cortex (V1)

• Also known as the Striate cortex because sections appear striped when stained
• Receives input from the LGN
• Basic processing of visual information
• Participates in the processing of form, colour and motion
• Hubel and Wiesel (1959) - single-cell recordings of electrical activity
• Most cells respond to best to lines or bars in a particular orientation in the visual field; some also respond to movement to any/specific directions
• The visual cortex is retinotopic
• Portions of the visual field are lawfully mapped onto areas of V1.
• Adjacent neurons process adjacent portions of of the visual field.

Functional specialisation 

• The occipital cortex is composed of multiple regions
• Each region processes specific features of visual information
• V1 and V2 - basic visual processing
• V3 and V3a - form perception (especially of moving stimuli)
• V4 - colour perception
• V5 - motion perception

Two visual subsystems

It has been argued that visual processing takes place within two partially independent visual sub-systems (e.g. Milner & Goodale, 2006).

1. The ventral stream - Perception for identification
2. The dorsal stream - Perception for action

Evidence for two distinct visual streams

Perenin and Vighetto (1988)

• Patients with damage to the parietal region (dorsal stream)
• Difficulty using vision to guide reaching and grasping movements but able to recognise what objects are 

Goodale, Milner, Jakobsen and Carey (1991)

• Patient with damage to lateral occipital cortex (ventral stream)
• Could not recognise objects visually, but could accurately guide actions by sight

 

• Different parts of the visual system analyse different features of the visual scene.

• Processing is hierarchical – initial analysis of basic features takes place in ‘lower-order’ areas and more complex processing occurs in ‘higher-order’ areas.

• How is it that these features are integrated into a meaningful, unified representation of the world around us?

• This describes the “binding” problem.