|Fig 1. Kim Peek could read a page in less than
eight seconds and had memorized 12,000 books.
He suffered congenital damage to the cerebellum and
missing corpus callosum.
Savant syndrome is a condition in which an individual with mental disabilities or CNS injuries displays profound and extraordinary skills, referred to as ‘islands of genius’. It most commonly co-exists with neurodevelopmental disorders like autism, but may also accompany other forms of mental disability or CNS injury/disease. Savant syndrome refers to observable behavioural characteristics, and is NOT considered a disorder itself. Savants demonstrate exceptional and prodigious abilities in specific areas, such as memorization, calculation, music, art, or language. Whatever the skill is, however, it is always connected to extraordinary memory that is exceedingly deep but narrow. Savant syndrome most commonly arises congenitally, but incidents of acquired cases have been reported, where savant skills suddenly appeared in adulthood following a brain damage. Understanding savants would give us deeper insight into human memory and cognition, as well as shed light on the limitless potential of the human brain.
Table of Contents
History & Epidemiology
|John Langdon Down|
|Fig 2. John Langdon Down
coined the term “idiot savant”.
The first accounts of savant syndrome occurred in the 1780s, when Karl Phillipp Moritz (1783) and Benjamin Rush (1789) each described the cases of Jedediah Buxton and Thomas Fuller, both nicknamed ‘lightning calculators’. The first scientific description, however, was made in 1887 by Dr. John Langdon Down, who is also known for his description of Down Syndrome. He coined the term “idiot savant” to describe someone who had “extraordinary skills, but with a great defect in reasoning power”. He noted that the skills were characterized by “verbal adhesion”, meaning that a savant’s extraordinary memory was without comprehension. The term idiot savant was later discarded because not all savants seemed to fit the description. The term “autistic savant” was then widely used due to the condition’s strong association with autism, but is now considered a misnomer because not all savants are autistic. About 50% of savants have autism, and about 10% of autistic individuals display savant syndrome. The other 50% of savants have other forms of neurodevelopmental disability or CNS injury/disease. Males outnumber females by a ratio of 6 to 1.
|Fig 3. Daniel Tammet is a prodigious savant.
He can perform complex arithmetic in seconds
without conscious effort. He has accurately recited
the value of pi from memory to 22,514 digits.
Tammet has stated that synesthetic experiences
aid him memorizing numbers.
Savants are classified into three categories based on the spectrum of their abilities.
- Splinter savants are individuals with some skills and interest in a specific area, that stand out in contrast to their overall level of functioning. For example, they may be obsessively preoccupied with memorizing maps, historical facts, and sports trivia.
- Talented savants are those with impressive skills that stand out among individuals with similar level of functioning. Talented savants show musical, artistic, or mathematical skills that seem very conspicuous compared to their overall disability.
- Prodigious savants are individuals that display extraordinary skills not only in contrast to their general level of functioning, but that are also remarkable compared to the general population. Prodigious savants are extremely rare but are most easily recognizable due to their conspicuity, while many savants of the other two types go unnoticed. There have been less than one hundred reported cases of prodigious savants in the past century.
|Fig 4. Derek Paravicini, a blind autistic, can play back
any piece of music he hears once, has the perfect pitch,
and can identify 10 simultaneous notes. Yet, he can't tell
left from right or count to ten.
Savants display specific skills within an intriguingly narrow range of mental functions. However, whatever the skill is, it is always connected to extraordinary memory. Most skills observed in savants tend to be right-hemisphere oriented – non-symbolic, concrete, artistic, directly perceived – rather than the left hemisphere – logical, sequential, symbolic. The skills tend to be rule based, rigid, and highly structured, and lack creativity and cognitive flexibility. Most savants have low IQ’s, but their skills seem to be IQ independent. Due to the lack of reasoning ability, savants have difficulty putting their skills to use. Many savants show inability to perform simple tasks such as tying shoe laces, counting to ten, counting money, or following instructions. Impaired social and communication skills are common in savants, and many savants display repetitive and stereotyped patterns of behaviour.
|Fig 5. Orlando Serrell is an example of an acquired
savant. At age 10 he was hit by a baseball to the head and
suffered braind injury. He can now instantly recall the day of
the week and the weather for any date since the incident.
- Mass memorization: some savants are able to memorize and regurgitate 12,000 books, entire census data such as population statistics of every city in the U.S., name of every street on a city map, every number in a phone book, and the value of π to 22,514 digits.
- Arithmetic: some savants are able to calculate complex arithmetic at a lightening speed. They can calculate square root of large numbers, multiply three-figure numbers in seconds, and find prime numbers instantly.
- Calendar calculation: some savants can instantly identify the day of the week that a particular date will occur or has occurred in any particular year. In one reported case, an individual showed a calendar calculating span of over 80,000 years and even remembered the weather for every day of his life.
- Musical ability: musical savants have the perfect pitch, exceptional sense of hearing, and can play any complex and long piece of music flawlessly after only hearing it once.
- Artistic ability: artistic savants show exceptional talent in painting, drawing, and sculpturing. The ability seems to involve detail focused processing.
- Language ability: some savants show exceptional ability at speed reading, learning a new language within a few days, and being able to translate up to 20 languages.
Although extremely rare, some savants have been reported to have exceptional mechanical ability, such as building or repairing intricate machines, as well as spatial ability, such as being able to memorize complex map routes or estimate distances by heart with precise accuracy.
|MRI of Kim Peek's brain|
|Fig 6. Normal brain (top) vs Kim Peek's brain (bottom).
Peek's brain shows smaller cerebellum, as well as missing
corpus callosum and the anterior and posterior comissures.
There are only few published studies that examine the neuroanatomy of the savant brain, and there is currently no general agreement in the findings. An MRI study of the brain of Kim Peek showed substantial damages to various structures. The brain showed multiple congenital abnormalities including macroencephaly and a shrunk cerebellum. More importantly, his brain lacked corpus callosum and the anterior and posterior commissures, the major connections that allow the two cerebral hemispheres to communicate. A study by Joy Hirsch on George Widener, a calendar calculator, showed that all of his brain structures were normal, but unusual areas of his brain were activated during various tasks. The brain showed activation in the motor areas while he performed mathematical calculations, and showed no activation at the language centres (Broca’s area and Wernicke’s area) while he performed language tasks. Corrigan et al. used multimodal neuroimaging techniques such as high resolution MRI, J-resolved NMR spectroscopy, and diffusion tensor imaging to investigate the neuroanatomy of a 63-old autistic savant, but none of the imaging techniques revealed any structural abnormalities. However, they found that the structures on the right side of the brain were larger than the left: the right cerebral hemisphere was 1.9% larger, the right amygdala and caudate nuclei were 24% and 9.9% larger, respectively, and fiber tract bundle volumes were larger on the right side for the amygdala, hippocampus, frontal lobe, and occipital lobe.
|Savant brain neuroimaging study|
|Fig 7. A multimodal neuroimaging study by Corrigan et al. revealed enlarged structures on the right cerebral
hemisphere of the savant brain, including the amygdala, caudate nuclei, and fiber tract bundle volumes.
Current knowledge & Theories
Savant syndrome is poorly understood and the causes and mechanism are currently unknown. Many researchers have presented theories to attempt to account for various aspects of the condition, but no one theory is sufficient.
|Fig 8. Stephen Wiltshire has a photographic
memory of aerial views. He can reproduce buildings
and structures from memory to an extraordinary degree
of detail after only seeing them once.
Rote memory & Inability to abstract reasoning
The memory of savants seems to be very deep but narrow, concrete, non-symbolic, rigid, repetitive, and unconscious. It relies on the more primitive and lower-level brain circuitry, and does not involve critical thinking or associative learning. They lack creativity and cognitive flexibility. Mishkin et al. referred to it as “habit memory”, as opposed to “cognitive memory”. In most reported cases, a musical savant’s talent shines not in their ability to compose, but in their ability to play back, note for note, pieces of music that they have heard. Similarly, an artistic savant’s talent tends to shine in their ability to reproduce with great realistic detail from memory. Schipperheijn et al. suggested that savant skills may be due to a “failure to forget” rather than an enhanced ability to remember.
Innate ability to access less-processed information
A hypothesis suggests that savants have an innate ability to directly access the less-processed, lower level information that exists in all human brains, but is normally unavailable to conscious awareness. Most savants have a failure of top-down inhibition, and therefore may be able to access the information that is normally blocked by perception and logic.
|Fig 9. An MRI of the brain of a patient with FTD shows damage
(atrophy) to the left anterior temporal lobe (bottom, right).
Left-right brain compensation theory
Left-right brain compensation is currently the most plausible theory to explain the causes of savant syndrome. The skills most frequently observed in savants are associated with the right hemisphere, and the skills that they lack are associated with the [http://en.wikipedia.org/wiki/Lateralization_of_brain_function left. The theory suggests that damage to the left hemisphere might cause the right hemisphere to dominate. A number of studies have examined the savant brain using CT or MRI scans and showed damage or abnormality in the left hemisphere. In addition, incidents of acquired savant syndrome in individuals with fronto-temporal dementia (FTD) further support the theory, because FTD involves defects in the left anterior temporal lobe. In contrast, individuals who develop neurological disorders that affect the entire brain, such as Alzheimer's, do not generally develop savant-like abilities. The Geschwind-Galaburba hypothesis states that the left hemisphere is more prone to damage during the fetal development, because it completes its development later than its right counterpart and therefore is exposed to environmental insults like neurotoxins or cerebral hypoxia for a longer period of time. Damage to the left hippocampus and amygdala may play an important role, as both are involved in the regulation of memory.
|Fig 10. A sketch by a 3-year-old artistic savant, Nadia.|
High-low brain compensation theory
In addition to left-right compensation, it has been hypothesized that damage to the higher-level cognitive semantic memory may lead to the compensation by the lower-level, primitive, procedural memory. Together with the left-right compensation, this would result in skills associated with the right brain, coupled with automatic “habit” memory.
Obsessive focus & Absence of distraction
Many savants, especially those with autism, tend to have underdevelopment in the brain areas that affect social and communication skills, and therefore spend most of their time alone. This isolation may cause them to find comfort in being obsessively preoccupied with memorizing or repeating certain tasks, in a desperate search for stimulation. In addition, a savant’s brain is entirely dedicated to the task at hand, which allows an extremely high level of focus without distraction. Normal individuals in extreme isolation have shown some degrees of mental calculation or rote memory as a way of adapting.
|Chromosome 15: 15q11-q13|
|Fig 11. Areas on chromosome 15 hypothesized to be
involved in savant syndrome: 15q11-q13.
Some researchers suggest a possible genetic basis of savant syndrome, where savant skills may be inherited in genes. Nurmi et al. reported that there seems to be a linkage between savant skills and chromosome 15q11-q13. In another study, Skuse et al. postulated that a cluster of genes on the X chromosome may be responsible for savant syndrome because the absence of those genes inhibited the normal development of the amygdala. In 1969, Horwitz et al. reported a case of identical, mentally disabled twins, both with calendar calculating abilities.
Explanation for the male bias
Savant syndrome occurs 4-6 times more in males than females, to a ratio of up to 6:1. The disproportionate ratio may be explained by the presence of testosterone in the male fetus. As mentioned above, in a developing human fetus, the left hemisphere is always exposed to an external damage or injury for a longer period of time. In the male fetus, high levels of circulating testosterone can act as neurotoxin during this period, causing damage to the left hemisphere. The same reason may also account for the high male:female ratio in other developmental disorders like autism and learning disabilities. Other studies showed that females have enhanced ability to eliminate environmental toxins during the early postnatal period, therefore reducing the risk for brain damage. Another hypothesis to explain the male bias has to do with the X chromosome. As mentioned above, the X chromosome may be involved in the proper development of the amygdala, and females are at a reduced risk because they have two X chromosomes, whereas males only have one.
Current research: Is there a savant inside all of us?
|Fig 12. Allan Snyder with his tDCS apparatus
which he calles "the thinking cap" (tDCS).
Many cases of acquired savant syndrome in individuals with brain injury or fronto-temporal dementia shed light on the fascinating possibility that the savant skills may be reproduced in normal individuals. Researchers hypothesize that all humans possess savant-like abilities, but they are normally suppressed by the brain's higher-level circuitry that thinks conceptually and applies preconceived thoughts to stimuli.
Unlocking the inner-savant: rTMS & tDCS
Allan Snyder and his team demonstrated the potential for artificially inducing savant skills in normal individuals by directing low-frequency repetitive transcranial magnetic stimulation (rTMS) to the left anterior temporal lobe, the area implicated with the fronto-temporal dementia. Magnetic pulse inhibits electric circuits on the left side of the brain and temporarily simulates the effects of FTD. When participants were asked to perform specific tasks before, during, and after the stimulation, some of them demonstrated superior recall, improved language ability, better sense of pitch, and an enhanced drawing ability. Four out of eleven participants noticeably improved their proofreading skills, ten out of twelve significantly improved the ability to guess random large numbers accurately. Their abilities, however, returned to normal within about an hour. In another study, Snyder used a technique called transcranial direct current stimulation (tDCS), which involves applying an electrical current to the scalp through electrodes. More than 40% of participants that received stimulation showed dramatic improvements in their problem solving skills, in contrast to the placebo group. While the results are striking and somewhat consistent with the hypothesis, no conclusions can be drawn with certainty from these findings. Snyder’s results are still preliminary with small data sets, and the experiment hasn’t been replicated with enough success and statistical significance. More research is expected to follow in the near future.
|Repetitive Transcranial Magnetic Stimulation (rTMS)|
|Fig 13. rTMS: a magnet connected to an electric current
is placed on the left side of the head. Magnetic pulses
disturb electric circuits on the left cerebral hemisphere.
|rTMS group vs. placebo group|
|Fig 14. Participants were asked to solve various problems before, during, and after the stimulation.
Mean problem solving ability was higher in the (a) rTMS group than in the (b) placebo group.