By Melissa Wright
When the seeing brain goes blind
In the late 90’s, a blind 63-year old woman was admitted to a university hospital emergency room. After complaining to co-workers of light-headedness that morning, she had collapsed and become unresponsive. Within the 48 hours following her admission, after what was found to be a bilateral occipital stroke, she recovered with no apparent motor or neurological problems. It was only when she tried to read that an extraordinary impairment became apparent: despite the damage only occurring in the occipital lobe, which is typically devoted to vision, she had completely and specifically lost the ability to read Braille. Braille is a tactile substitution for written letters, consisting of raised dots that can be felt with the fingertips. Before this, she had been a proficient Braille reader with both hands, which she had used extensively during her university degree and career in radio (Hamilton, Keenan, Catala, & Pascual-Leone, 2000). So what happened?
The Visual Brain
It is estimated that around 50% of the primate cortex is devoted to visual functions (Van Essen, Anderson, & Felleman, 1992), with the primary visual areas located right at the back of the brain within the occipital lobe (also known as the visual cortex). Visual information from the retina first enters the cortex here, in an area named V1. Within V1, this information is organised to reflect the outside world, with neighbouring neurons responding to neighbouring parts of the visual field. This map (called a retinotopic map) is biased towards the central visual field (the most important part!) and is so accurate that researchers have even managed to understand which letters a participant is reading, simply by looking at their brain activity (Polimeni, Fischl, Greve, & Wald, 2010). These retinotopic maps are found in most visual areas in some form. As information is passed forward in the brain, the role of these visual areas becomes more complex, from motion processing, to face recognition, to visual attention. Even basic visual actions, like finding a friend in the crowd, requires a hugely complex chain of processes. With so much of the cortex devoted to processing visual information, what happens when visual input from the retina never occurs? Cases such as the one above, where a person is blind, suggest that the visual cortex is put to use in a whole new way.
In sighted individuals, lexical and phonological reading processes activate frontal and parietal-temporal areas (e.g. Rumsey et al., 1997), while touch involves the somatosensory cortex. It was thought that braille reading activated these areas, causing some reorganisation of the somatosensory cortex. However, as the case above suggests, this does not seem to be the whole story (Burton et al., 2002). Remember, in this instance, the unfortunate lady had damage to the occipital lobe, which is normally involved in vision, but as the lady was born blind it had never received any visual information. Although you might expect that damage to this area would not be a problem for someone who is blind, it turned out instead to impair abilities associated with language and touch! This seriously went against what scientists had understood about brains and their specialised areas, and had to be investigated.
Neuroimaging, such as functional Magnetic Resonance Imaging (fMRI), allows us to look inside the brain and see what area is activated when a person performs a certain task. Using this technique, researchers have found that in early blind individuals, large portions of the visual cortex are recruited when reading Braille (H. Burton et al., 2002). This activity was less apparent for those who became blind in their later years, though was still present, and it wasn’t there at all for sighted subjects. That the late-blind individuals had less activity in this region seems to show that as we get older and as brain regions become more experienced, they become less adaptable to change. A point to note however – fMRI works by correlating increases in blood oxygen (which suggests an increase in energy demand and therefore neural activity) with a task, such as Braille reading. As any good scientist will tell you, correlation doesn’t equal causation! Perhaps those who cannot see are still somehow ‘visualising’ the characters?
So is there any other evidence that the visual areas can change their primary function? Researchers have found that temporarily disrupting the neural activity at the back of the brain (using a nifty technique called Transcranial Magnetic Stimulation) can impair Braille reading, or even induce tactile sensations on the reading fingertips (e.g. Kupers et al., 2007; Ptito et al., 2008)!
Other fMRI studies have investigated the recruitment of the occipital lobe in non-visual tasks and found it also occurs in a variety of other domains, such as in hearing (e.g. Burton, 2003) and working memory (Harold Burton, Sinclair, & Dixit, 2010). This reorganisation seems to have a functional benefit, as researchers have found that the amount of reorganisation during a verbal working memory task is correlated with performance (Amedi, Raz, Pianka, Malach, & Zohary, 2003). As well, it has been reported that blind individuals can perform better on tasks such as sound localisation (though not quite as good as Marvel’s Daredevil!) (Nilsson & Schenkman, 2016).
But Is It Reorganisation?
This is an awesome example of the ability of the brain to change and adapt, and this seems true also for areas that are so devoted to one modality. How exactly this happens is still unknown, and could fill several reviews on its own! One possibility is that neuronal inputs from other areas grow and invade the occipital lobe, although this is difficult to test non-invasively in humans because we can’t look at individual neurons with an MRI scan. The fact that much more occipital lobe activity is seen in early-blind than late-blind individuals (e.g. H. Burton et al., 2002) suggests that whatever is changing is much more accessible to a developing brain. However, findings show that some reorganisation can still occur in late-blind, and even in sighted individuals who undergo prolonged blindfolding or sensory training (Merabet et al., 2008). This rapid adaptation suggests that the mechanism involved may be making use of some pre-existing multi-sensory connections that multiply and reinforce following sensory deprivation.
Cases of vision restoration in later life are rare, but one such example came from a humanitarian project in India, which found and helped a person called SK (Mandavilli, 2006). SK was born with Aphakia, a rare condition in which his eye developed without a lens. He grew up near blind, until the age of 29 when project workers gave him corrective lenses. 29 years with nearly no vision! Conventional wisdom said there was no way his visual cortex could have developed properly, having missed the often cited critical period that occurs during early development. Indeed, his acuity (ability to see detail, tested with those letter charts at the optometrists) showed initial improvement after correction, but this did not improve over time suggesting his visual cortex was not adapting to the new input. However, they also looked at other forms of vision, and there they found exciting improvements. For example, when shown a cow, he was unable to integrate the patches of black and white into a whole until it moved. After 18 months, he was able to recognise such objects even without movement. While SK had not been completely without visual input (he had still been able to detect light and movement), this suggests that perhaps some parts of the visual cortex are more susceptible to vision restoration. Or perhaps multi-sensory areas, that seem able to reorganise in vision deprivation, are more flexible to regaining vision?
So Much left to Find Out!
From this whistle-stop tour, the most obvious conclusion is that the brain is amazing and can show huge amounts of plasticity in the face of input deprivation (see the recent report of a boy missing the majority of his visual cortex who can still see well enough to play football and video games; https://tinyurl.com/yboqjzlx). The question of what exactly happens in the brain when it’s deprived of visual input is incredibly broad. Why do those blind in later life have visual hallucinations (see Charles Bonnet Syndrome)? Can we influence this plasticity? What of deaf or deaf-blind individuals? Within my PhD, I am currently investigating how the cortex reacts to another eye-related disease, glaucoma. If you want to read more on this fascinating and broad topic, check out these reviews by Merabet and Pascual (2010), Ricciardi et al. (2014) or Proulx (2013).
Edited by Chiara & Sam
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