If You Had Half a Brain..

A great story made its way onto the interwebz lately. The Daily Mail reports:

"A 10-year-old girl born with half a brain has both fields of vision in one eye, scientists said today. The youngster, from Germany, has the power of both a right and left eye in the single organ in the only known case of its kind in the world.

"University of Glasgow researchers used Functional Magnetic Resonance Imaging (fMRI) to reveal how the girl’s brain had rewired itself in order to process information from the right and left visual fields in spite of her not having a whole brain."

BBC News goes further with:

"In the case of the German girl, her left and right field vision is almost perfect in one eye. Scans on the girl showed that the retinal nerve fibres carrying visual information from the back of the eye which should have gone to the right hemisphere of the brain diverted to the left ... 'Despite lacking one hemisphere, the girl has normal psychological function and is perfectly capable of living a normal and fulfilling life. She is witty, charming and intelligent.'"

Get that? The only known case in the world where brain plasticity (the ability of the brain to reorganise itself after injury) is displayed for all to see. Plasticity doesn't always work this way, there are many cases where plasticity effects haven't achieved the mark of restoring all or most of the impaired brain function. Epilepsy patients, for example, who undergo a hemispherectomy (removal of a half of a brain) in order to prevent the onset of severe seizures, among other things tend to lose an entire field of vision in both eyes; they only see people and objects in one half of their visual field, as in the illustration below:



Neither was this a case of brain injury; the anonymous girl (known only as 'AH') failed to adequately develop her cerebral right hemisphere in the womb. As a result, she is without a right-brain and also without the use of her right eye. She also has a slight left-hemiparesis (weakness affecting half of the body) but close to normal vision in both hemifields of her normal left eye.

In a study published by the Proceedings of the National Academy of Sciences (PNAS), a team led by Lars Muckli of the University of Glasgow used fMRI to investigate how the visual cortex had remapped itself. In a healthy individual, the cerebral cortex contains "maps" for vision, sound, motion and touch, which develop and modify over time dependent on several factors including genetic cues and neural activity. In the mammalian brain (that is, human brain) the visual cortex is made up of distinct sections dealing with vision, the main one being an area known simply as 'V1', the primary visual cortex. 'V2' deals with quarterfield representations in the area of vision, effectively dealing with the 'up' and 'down' areas of both the right and left hemispheres of vision, while 'V3' is a structure in front of V2 that, among other things, performs a supporting role for V2. There is also the question of retinotopic maps, a direct mapping of the spatial arrangement of the retina, located in visual structures including the cortex and thalamus.

As per materials provided by the University of Glasgow, "visual information is gathered by the retina at the back of the eye and images are inverted when they pass through the lens of the pupil so that images in your left field of vision are received on the right side of the retina, and images from the right are received on the left." The part of the retina close to the nose is known as the nasal retina whereas the other part is referred to as the temporal retina, being in proximity to the temples. Both halves transmit received information through separate nerve fibres. In a normal situation, the nerve fibres of the nasal retina cross over in the optic chiasm, a brain structure located at the bottom of the brain near the hypothalamus, and are processed by the hemisphere on the opposite side. The nerve fibres of the temporal retina remain in the same hemisphere (ipsilateral), meaning that the left and right visual fields described earlier are processed by opposite sides of the brain.



[DIGRESSION]Vision is not the only modality to be processed in this strange way. It actually reflects the larger processing activities of the intact brain which tends to process all other modalities in opposite sides of the brain. To wit, touch and hearing for example that is "entered" into the right side of the body (right body, right ear) are processed by the left-brain, and touch/hearing entered into the left body/ear is processed by the right-brain. This is generally referred to as contralateral processing, when input is processed by the 'opposite' half of the brain. Those inputs processed by the 'same' side of the brain is known as ipsilateral processing. For more information, please read about Basic Visual Pathways.[/DIGRESSION]

The MRI scan displays the complete lack of a right-hemisphere: The optic chiasm is shown here (top l-r) in the transverse and enlarged transverse planes, and (bottom l-r) in the coronal and saggital planes. A rudimentary optic nerve is pointed out in the enlargement by the green arrow but with no discernible optic tract, and it can also be seen how the left-hemisphere is spilling over into the right-domain. The vacant right-hemisphere is filled with cerebrospinal fluid (CSF).

In AH's fascinating case, it was found that the nasal retinal nerve had connected to her left-brain. A possible interpretation for AH's condition is suggested by the authors: The lack of a right-brain prevented an opposite connection from being made, which led the optic nerve fibers to "connect" with ipsilateral structures instead.

Remembering that normal cases require a crossing in the optic chiasm, and AH's connections were essentially ipsilateral, how exactly does AH see both visual fields with only one eye? After all, if the entire right hemisphere is missing, AH should see only the left hemifield. The answer lies with the Lateral Geniculate Nucleus (LGN), a structure that is embedded deep in the thalamus and which processes visual information from the retina. In AH, both the nasal and temporal retina would need to be mapped onto the LGN to allow for the processing of both hemifields. Again a similar suggestion of ipsilateral projections were presented as being the solution, instead of the usual contralateral connections, and that a mirror-symmetric representation of the hemifields would be received and processed by the thalamus. Similar cases have been seen in achiasmatic dogs where optic nerve fibres terminated in the ipsilateral LGN.

'Islands' were also found to have formed in the left-hemisphere to deal especially with processing of the left hemifield, to compensate for the missing right-brain activity.

The loss of AH's right-hemisphere was discovered at age 3 when she was treated for brief seizures and twitching taking place on her left side. It is speculated that the right-brain failed to develop between Day 28 and Day 49 of embryonic development. Despite the situation, she is able to engage quite capably in activities that require a fair amount of balance, such as riding a bicycle or roller-skating. Truly an extraordinary case in more ways than one.

For a professional view, please see Dr. Steven Novella's entry on this case.
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Muckli, L., Naumer, M., & Singer, W. (2009). Bilateral visual field maps in a patient with only one hemisphere Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0809688106