The brain is key to our existence, but there’s a long way to go before neuroscience can truly capture its staggering capacity. For now though, our Brain Control series explores what we do know about the brain’s command of six central functions: language, mood, memory, vision, personality and motor skills – and what happens when things go wrong.
Mr B presented to the emergency department because of frequent falls and an inability to grasp anything in front of him. He could recognise his family by their voices but not by sight. When a pen was put in front of him, he claimed nothing was there, and he was unable to see the neurologist’s hands waving at him.
He was diagnosed with total blindness. However, Mr B was unaware of and unperturbed by his blindness. He vividly described surroundings that did not actually exist. Brain imaging revealed strokes that had damaged his visual cortex.
Mr B had a rare condition called Anton’s syndrome, characterised by denial of blindness. This is intriguing as it combines the most basic failure of visual perception (a complete loss of the ability to see) with a complete unawareness of this failure.
In this sense, it is one of the most dramatic illustrations of the consequences of damage to the brain’s visual system.
The visual system
The pathway from eye to brain begins in the retina, where light is converted into neuronal signals. Signals from the eye are transmitted, through a part of the thalamus (which sits near the centre of the brain) called the lateral geniculate body, to the primary visual cortex at the back of the brain.
The primary visual cortex (shown in blue in figure ii below) is the first point at which visual signals reach the cortex – the sheet of wrinkly neural tissue that makes up the outer surface of the brain. The primary visual cortex can be thought of as a bottleneck through which all visual information must pass before it is distributed to other visual areas of the cortex.
The above figure shows how light arriving from the right visual field – that part of the world to the right of wherever we happen to be looking – lands on the left half of our retinas, the light sensitive part of the eye.
From there it is then transmitted to, and processed in, the left hemisphere of the brain. Conversely, light arriving from the left visual field is ultimately transmitted to and processed in the right hemisphere of the brain. This means damage to, say, the left primary visual cortex will result in blindness in the right visual field.
Visual processing areas
The visual areas beyond the primary visual cortex have different relative visual specialisations, such as for colour, motion, or face processing. As a rough rule of thumb, the further across the cortical surface a visual area is from primary visual cortex, the more complex the processing carried out by that area.
For instance, there are neurons in the medial temporal lobe – an area near the top of the visual pathway, at the interface between vision and memory – that appear to respond exclusively to particular faces.
As an example, one study reported encountering neurons that responded exclusively to pictures of Jennifer Aniston’s face, but not to images of other people’s faces, of animals, or of prominent landmarks such as the Eiffel Tower.
Conversely, neurons in the primary visual cortex respond to much simpler attributes of visual images. They are sensitive to certain visual features, such as a small vertical edge at a particular location in the visual field, regardless of the object or scene giving rise to that edge – be it the edge of Jennifer Aniston’s face or the edge of a tree trunk or building.
A useful way of thinking about the organisation of visual areas is provided by the idea of two visual processing streams: the dorsal and ventral visual pathways.
‘Where’ and ‘what’ we see
The dorsal pathway extends from the primary visual cortex up into parietal areas, where bodily sensations such as touch are represented. This pathway is assumed to support the analysis of “where” things are located in space.
Lesions along the dorsal pathway can give rise to a range of deficits, such as optic ataxia, where people cannot use visual information to accurately reach for and grasp objects; and oculomotor apraxia, where people have reduced ability to voluntarily shift their gaze away from wherever they happen to be looking (sometimes called “sticky fixation”).
Other deficits resulting from dorsal pathway lesions are simultanagnosia, which is the inability to perceive more than one object at a time; and a complete lack of awareness of one half of the visual field, usually the left, known as visual neglect.
The ventral pathway extends from the primary visual cortex down into the temporal lobe. This pathway supports analysis of “what” is seen, such as distinguishing whether we are looking at Donald Trump or a monkey.
Lesions occurring relatively early in the ventral pathway, in areas close to the primary visual cortex, can leave visual sensation intact but disrupt accurate processing of those sensations.
An example is apperceptive agnosia, in which patients have near normal acuity and sensitivity to light but are unable to accurately perceive visually presented stimuli. Such patients would, for instance, be unable to name an elephant that was shown to them, or produce a copy drawing of one.
Lesions occurring further along the ventral pathway can result in associative visual agnosia, in which patients have normal perceptual abilities (for instance, producing good copy drawings) yet are unable to demonstrate knowledge of objects they perceive accurately, such as what they are used for.
An interesting case of associative agnosia is provided by the report of a farmer who was no longer able to recognise previously familiar cows following ventral pathway damage.
Alexia, an acquired disorder of reading in which people can accurately perceive written text but no longer extract meaning from it, can also be thought of as a form of visual associative agnosia.
While our knowledge of some of the basic mechanisms of visual processing has dramatically increased in recent times, we have a poorer understanding of how these processes are integrated into a larger whole. Many fascinating questions remain, such as how is it that individuals can exhibit complete unawareness of blindness as in Anton’s syndrome?