Visual disorders

Functional specialisation in the 'visual' brain

• Scientists have identified multiple specialised visual processing areas in the human occipital cortex
• Brain imaging studies

Brain imaging evidence is correlational

What is wrong with these statements?

• Presence of bear patrol + absence of bears = bear patrol working like a charm
• Presence of rock + absence of tigers = rock repels tigers
• Processing of colour stimulus + increased blood flow area V4 =V4 responsible for colour perception

Neurological evidence

• Brain imaging studies indicate that the processing of specific features of visual information (e.g. colour, motion, faces) is associated with activity in specific areas of the brain (e.g. V4, V5, FFA)
• BUT they do not show clearly that these areas are necessary for perception
• Additional evidence that these areas are of major importance for specific types of perception comes from neurological studies on patients with various visual disorders

Achromatopsia

• There are rare neuropsychological cases where patients have lost their colour vision, with other visual attributes remaining intact
• Heywood and Kentridge (2003) concluded that damage to area V4 is often involved 
• Achromatopsia patients lose the ability to disinguish differences in hue, but retain some ability to discriminate brightness/reflectance

Akinetopsia

• Extremely rare cases exist where patients suffer motion blindess, such as patient LM. Perception was akin to viewing the world as snapshots. Colour and form perception were intact, but the ability to judge the direction and speed of moving objects was severely impaired
• Caused by damage to area V5 (referred to as MT in monkeys) in the visual cortex - located in occipital lobe of the brain
• Supports brain imaging evidence that this area is specialised for the processing of motion

Prosopagnosia

• The inability to identify people on the basis of their face
• Cause by damage to the left or right fusiform gyrus, located in lateral occipital and temporal lobes of the brain
• Suggests that this area is specialised for the processing of faces
• Individuals use other information to identify people: body, clothes, voice, context, etc.

Visual Agnosia

• The inability to percieve and/or recognise an object by sign (in the absence of any visual or intellectual impairment)
• In apperceptive Agnosia, individuals cannot bind together lines and basic shapes to form an integrated percept of an object
• In associative Agnosia, an integrated percept is formed, but has no meaning

Dorsal = how stream

Ventral = what stream

• Visual agnosia caused by damage to the lateral occipital and/or temporal lobes of the brain
• We can infer that these areas of the brain are involved in:
• The generation of a visual object as a discrete percept
• The connection between the perception of an object and associated contextual information such as object's name and function

Blindsight

• Patients with damage to the primary visual cortex following a stroke are unable to detect objects within portions of space 
• areas of blindness are called a Scotoma
• Sometimes the patient may be able to fixate of look straight at, or even point to, a visual object in the affected visual field, even though they do not consciously "see" the object. 

Patient D.B.

• A light beam was projected to one of 7 locations within his scotoma
• Upon hearing a tone, DB was required to move his eyes to that location. In control conditions, no target was illuminated
• DB reported seeing nothing within his scotoma and saw both conditions as identical, but still managed to move his eyes to the appropriate location

Cortical vs. Subcortical Pathways

• 90% of neural connections from the eyes run to area V1 in the visual cortex via the lateral geniculate nucleus (LGN)
• Most of the remaining 10% connect directly to a sub-cortical structure called the superior colliculus - this pathway was intact in DB's brain

A primitive Visual system

• This neual pathway to the superior colliculus might represent a primitive visual system that supplies information about the basic form and location of objects, but is not capable of generating conscious visual experience
• On this view, perhaps over the course of evolution this system was superceded by the much larger and more sophisticated visual system that supports conscious visual perception

Summary

Evidence from brain imaging studies for the functional specialisation of visual areas is supported by evidence from studies investigating visual disorders

1. Achromatopsia - loss of colour vision linked with damaeg to area V4
2. Akinetopsia - motion blindness linked with damage to area V5
3. Prosopagnosia - disrupted face perception linked with damage to FFA
4. Visual agnosia - difficulty recognising objects linked with damage to the ventral 'what' stream
5. Blindsight - perception without awareness linked to damage to area V1 but intact sub-cortical pathway