Thursday, 12 July 2012

Thursday, 17 May 2012

Following a tutorial Prof. Giovannoni gave in his clinic last week I would like to share an excellent link about eye movements. Tutis Vilis of the Department of Physiology, University of Western Ontario has developed a flash tutorial explaining many normal and abnormal eye movements, which can be found here:

There are a few points to develop around the link, once you've worked through it:

1) The accommodation reflex or near response comprises three movements - the first two are mentioned in the eye movement tutorial:
  • both eyes rotate medially towards the target  - vergence
  • the ciliary muscle contracts, causing the lens to be come more spherical/convex - accommodation
  • the pupils constrict
Why do the pupils need to constrict? The quality of the lens results in spherical aberration - light passing through the edge of the lens will be focussed more poorly onto the retina, reducing the quality of the image. Pupillary constriction prevents light from passing through the edges of the lens and ensures the eye can produce a sharp image on the retina. In a given light level, a near object will reflect more light onto the retina than one further away, so the pupillary constriction does not typically impair vision by inadequate illumination.

2) Humans have 6 extraocular muscles which are responsible for positioning the eye and one which elevates the eyelid (levator palpebrae superioris). Cows (for example) manage with just four muscles to rotate the globe. Why does Man need two more?

This image of the right orbit (from above) shows the origins and insertions of the orbital muscles.The orbit is cone shaped and this cone is externally rotated by about 20 degrees. The rectus (straight) muscles all originate from a common point at the back of that cone - the common tendinous ring (also known as the Annulus of Zin). Due to the cone's shape and position, the rectus muscles act not only in the direction they are named, but also cause rotation in other axes.

For example superior rectus (in the middle of the image above) when contracting rotates the globe upwards (elevation) but also has some internal rotation (intorsion) and medial rotation (adduction). The inferior rectus (not seen on the illustration) causes not only depression of the eye but extorsion and adduction. With only four muscles, there would be no control over in- and ex-torsion of the globe. Our bovine friends do not encounter this problem due to the shape of their orbits - presumably the tendinous ring lies deep to the centre of the globe, so their rectus muscles pull straight, as the name "rectus" would normally suggest.

The addition of two oblique (slanted) muscles allows for control over intorsion and extorsion. Superior oblique acts to intort, depress and abduct the eye. Some use the acronym SO-LID as a reminder - Superior Oblique Lateral rotation, Intorsion, Depression. The muscle is of interest as it originates in the common tendinous ring, then passes through a sling or pulley attached to the superio-medial orbit before attaching at the superior posteriolateral segment of the globe (see below):

Inferior oblique acts to extort, elevate and abduct the eye; this muscle does not arise from the common tendinous ring, but instead from the maxillary component of the anterior orbit and attaches to the inferior posteriolateral globe, see below, deep to the inferior rectus:

3) Why do we examine eye movements in an "H" shape?

Although above I have talked about eye movements from the neutral position, in clinical practice it is not possible to ask a patient to contract, for example, only their superior oblique muscle. If we examined simply elevation/depression and adduction/abduction, we would get a mixed picture of which muscle was acting. The picture below is of the right eye movements in a patient, as observed by the examiner, i.e. lateral rectus abducts the eye:

By examining in an "H" pattern, we can isolate the action of individual muscles. This image is also of right eye movements in a patient, as observed by an examiner. For the muscles in the left eye, simply consider a mirror image.

You'll note that even though the action of the superior rectus is elevation, intorsion and adduction, once the eye is already abducted by lateral rectus, superior rectus acts solely to elevate the eye. This is because once the eye is abducted, the other eye elevator - inferior oblique - becomes slightly slack and is unable to contribute to elevation.

Similarly even though the action of inferior oblique is extortion, elevation and abduction, once the eye is adducted by medial rectus, the superior rectus is unable to act as it now is slack, leaving inferior oblique as the only muscle able to elevate the eye.

The previous two paragraphs probably do not make much sense unless you've understood the attachments and direction of rotation caused by each muscle earlier in this post. Please feel free to ask questions or for clarification below!

Next time from me: what happens when these movements go wrong...

Spurious Elevations of Vitamin B12 with Pernicious Anemia

Yang DT, Cook RJ. Spurious elevations of vitamin B12 with pernicious anemia. N Engl J Med. 2012 May 3;366(18):1742-3.

Level 2

Have you ever seen a patient with pernicious anemia with spuriously elevated levels of vB12? 

This report of 2 cases, of pernicious anaemia, highlights an important the problem of false normal results for vitamin B12 levels generated by automated analysers when the serum of patients with pernicious anaemia are evaluated. The results have been attributed to high levels of intrinsic factor–blocking antibodies that interfere with the assay.

Subacute combined degeneration of the spinal cord due to vB12 deficiency.
Teaching point: If you have a high-index of suspicion that a patient has vB12 deficiency please check plasma levels of homocysteine and methylmalonic acid and screen the serum for anti-intrinsic factor–blocking antibodies and antiparietal-cell antibodies. Or you can do the methionine loading test. 

Smooth red tongue due to vB12 deficiency 

Sunday, 6 May 2012

TED Talk: Oliver Sacks

If you are not a convert already can I suggest watching TED talks; I am an addict! 

Oliver Sacks is a British trained neurologist and a gifted writer and storyteller. He is well known as a writer and you will a learn a lot of neurology from reading his books:
  1. Migraine (1970) London: Faber and Faber; Berkeley: University of California Press
  2. Awakenings (1973) London: Duckworth; New York: Doubleday
  3. A Leg to Stand On (1984) London: Duckworth; New York: Summit Books
  4. The Man who Mistook his Wife for A Hat (1985) London: Duckworth; New York: Summit Books
  5. Seeing Voices: A Journey into the World of the Deaf (1990) Berkeley: University of California Press; London: Picador
  6. An Anthropologist on Mars (1995) New York: Alfred A. Knopf; London: Picador
  7. The Island of the Colorblind (1996) New York: Alfred A. Knopf; London: Picador
  8. Uncle Tungsten: Memories of a Chemical Boyhood (2001) New York: Alfred A. Knopf; London: Picador
  9. Oaxaca Journal (2002) Washington DC: National Geographic Directions
  10. Musicophilia: Tales of Music and the Brain (2007) New York: Alfred A. Knopf; London: Picador
  11. The Mind’s Eye (2010) New York: Alfred A. Knopf; London: Picador
I would recommend starting with "The man who mistook his Wife for a Hat"

Saturday, 5 May 2012

Case study: ascending motor weakness

Ms SE a 26-year old single woman presents with an acute ascending motor weakness. On examination she has a lower motor neurone pattern of weakness and areflexia. She had recent diarrhoeal illness 2 weeks ago. At present she is unable to walk due to lower limb weakness and has mild distal weakness in her hands that is limiting her ability to perform fine motor tasks. Her vital capacity is normal, she has full control of her bladder and bowel and there is no evidence of autonomic dysfunction. General examination is unremarkable with no evidence of systemic disease.

Teaching points:

Level one

Areflexia: What is a reflex? What does areflexia mean?
What is a lower motor neurone weakness?
Differential diagnosis of acute ascending weakness?
How do you differentiate a motor neuropathy from acute spinal cord pathology?

Level two

What do you expect the nerve conduction study results to show?
How do you monitor respiratory function in patients with neurological disease? At what level do you admit a patient to ITU for monitoring?

Level three

What findings on the nerve conduction studies support a diagnosis of demyelination?
Apart from nerve conduction studies what other investigations would you do an why?
The consultant recommending treating this patient with IVIG. How do you use IVIG? What are the complications of using IVIG?


Friday, 4 May 2012

Clinical skills: Hoffman's reflex

Level 1
The test involves tapping the nail or flicking the terminal phalanx of the middle or ring finger. A positive response is seen when terminal phalanx of the thumb flexes. A positive Hoffman's reflex indicates an upper motor neuron lesion or a pyramidal sign. 

In my experience Hoffmann's reflex are often symmetrically positive in normal people, particularly in subjects with brisk reflexes, which is why I seldom use this test myself. An asymmetrically positive Hoffmann's sign is more worrying and is likely to be pathological.

Hoffman's reflex is similar to a deep tendon reflex and is mediated by spindle fibres and is therefore a monosynaptic reflex pathway. In other words it should be interpreted as you do tendon reflexes. In comparison, the plantar or Babinski's reflex is not a deep tendon reflex and hence is always considered pathological in adults.

Other relevant posts of interest: Babinski's sign, tendon reflexes

History of neurology: Johann Hoffmann (March 28, 1857 – November 1, 1919, Heidelberg) was a German neurologist remembered for describing the reflex above and Werdnig-Hoffmann disease (infantile spinal muscular atrophy or SMA).

Thursday, 3 May 2012

Lecture Halls without Lectures — A Proposal for Medical Education

If you are a medical educationalist or are interested in medical education you should read this article:

Prober & Heath. Lecture Halls without Lectures — A Proposal for Medical Education. | N Engl J Med 2012;366:1657-1659.

Some extracts:

"It’s time to change the way we educate doctors. Since the hours available in a day have not increased to accommodate the expanded medical canon, we have only one realistic alternative: make better use of our students’ time."

"As Sir William Osler aptly said, “He who studies medicine without books sails an uncharted sea, but he who studies medicine without patients does not go to sea at all.” Yet conversations with medical students about the first-year medical curriculum reveal that about half of lectures proceed without even the briefest example involving patients."

".... rather than a standard lecture-based format, the instructors provided short online presentations. Class time was used for interactive discussions of clinical vignettes highlighting the biochemical bases of various diseases. The proportion of student course reviews that were positive increased substantially from the previous year. And the percentage of students who attended class shot up from about 30% to 80% — even though class attendance was optional."

"I am an old convert! Lectures are the worst platform for teaching; they encourage passive learning and turn medical students into zombies. When I was a 3rd year medical student I got so despondent with my pathology lecturer, who essentially was rehashing what he had written in his published book of pathology lecture notes, that I stopped going to pathology lectures. I spent the time learning pathology myself in the library using a text book and other sources. This made me an active learner. When something interested me, for example the pathology of the thymus, I asked the pathology tutors to show me thymic sections in our pathology practicals; pathology of the thymus was not in our syllabus. I won the pathology prize that year, with a mark that was nearly 10% higher than the second best student. After my pathology viva the head of pathology summoned me to a meeting to find out why I had done so well in my pathology exams and to encourage me to do an intercalated BSc in pathology, with a further option of a follow-on intercalated PhD. Needless to say I did not take up his offer, but I did stop going to medical school lectures. Not attending lectures turned me into an active learner, which was the main reason I did well at medical school.  I only attended lectures when the lecturer had an established reputation for giving inspirational talks; these were few and far between." 

"In short  this paper suggests doing away with classic lectures and using the time instead to encourage active, rather than passive, learning, around clinical cases. Becoming an active learner and putting patients at the center of your learning experience will change your life. It will also make you a neurophile. Try it!"

Monday, 2 April 2012

Froment's sign

Level 2

Froment's sign tests for the action of adductor pollicis, which is weak with an ulnar nerve palsy.

A patient is asked to hold an object, usually a flat object such as a piece of paper, between their thumb and index finger (pinch grip). The examiner then attempts to pull the object out of the subject's hands. A normal individual will be able to maintain a hold on the object without difficulty. With ulnar nerve palsy, the patient will experience difficulty maintaining a hold and will compensate by flexing the FPL (flexor pollicis longus) of the thumb to maintain grip pressure causing a pinching effect.

Clinically, this compensation manifests as flexion of the IP joint of the thumb (rather than extension, as would occur with correct use of the adductor pollicis). The compensation of the affected hand results in a weak pinch grip with the tips of the thumb and index finger, therefore, with the thumb in obvious flexion. The FPL is innervated by the anterior interosseous branch of the median nerve, which comes off more proximally than the wrist. 

Simultaneous hyperextension of the thumb MCP joint is indicative of ulnar nerve compromise. This is also known as Jeanne's Sign.

Extra reading: Jules Froment

Lord Brain Memorial Lecture

"A date for your diary; please try and attend! Professor Blakemore is an excellent speaker and the topic is very relevant to anyone studying neurology."

RSVP via Mr Surinder Pal:

Thursday, 29 March 2012

c-MAF transcription factor and touch

Level: 3+

Wende et al. The transcription factor c-Maf controls touch receptor development and function. Science. 2012 Mar 16;335(6074):1373-6.

The sense of touch relies on detection of mechanical stimuli by specialized mechanosensory neurons.  This study shows that the transcription factor c-Maf/c-MAF is crucial for mechanosensory function in mice and humans. The development and function of several rapidly adapting mechanoreceptor types are disrupted in c-Maf mutant mice. In particular, Pacinian corpuscles, a type of mechanoreceptor specialized to detect high-frequency vibrations, are severely atrophied. In line with this, sensitivity to high-frequency vibration is reduced in humans carrying a dominant mutation in the c-MAF gene. This work identifies a key transcription factor specifying development and function of mechanoreceptors and their end organs.

"In humans, dominant c-MAF mutations are associated with ocular developmental abnormalities and cataracts, but their effects on mechanosensation have not been examined. Encouraged by our results in mice, we tested touch sensitivity in a family comprising four carriers of the dominant Arg288→Pro288 (R288P) c-MAF mutation. This mutation in the auxiliary DNA binding domain interferes with c-MAF–dependent transcriptional activation but does not eliminate c-MAF function and represents one of three known cataract-causing point mutations in c-MAF. To assess the function of Meissner and Pacinian corpuscles, we tested vibrotactile acuity over a wide range of frequencies (5 to 240 Hz). Pacinian corpuscles are essential for the detection of small-amplitude high-frequency vibrations. We observed a large increase in the vibration amplitude required to elicit responses in c-MAF mutant carriers at high but not low frequencies. Tactile spatial acuity, that is, the ability to distinguish grids of different spacing, is thought to depend on hair follicle afferents/Meissner corpuscles/Merkel cell-neurite complexes and was not significantly changed. Thus, the R288P c-MAF mutation interferes with normal vibration detection in humans."

Additional reading:  Pacinian corpuscles

Integrative Approaches to Developing Therapeutics for Neurodegenerative Diseases

Young Life Scientists' Symposium
24th August 2012. Cardiff, UK

Neurodegenerative Diseases, such as Alzheimer’s Disease, Parkinson’s Disease and Huntington’s Disease, are becoming increasingly common in our ageing population. Young and early-career researchers all over the country are dedicating their work to understanding and combating these devastating diseases .Integrating our knowledge from every approach is the next step towards fully understanding and treating these diseases.

The focus of this unique symposium is to integrate ALL approaches to researching neurodegenerative diseases, with an overall aim to informing the development of therapeutics. All PhD students and early-career Post-Docs from every school working on neurodegenerative disease are welcome to attend. Researchers based in Psychology, Pharmacy, Physiology, Neuroscience, Biology, Biochemistry, Genetics, Behavioural Neuroscience, Medicine, Physiotherapy, Imaging and the Social Sciences are all encouraged to attend and share their work.

For more information about this symposium and to register, please visit our website: 

Wednesday, 28 March 2012

Evolutionary medicine: prehistoric proteins

Level: 3+

For those interested in the molecular cogs that underpin evolution this article is a must:

This work shows that evolution also relies on random events; I would be very interested to see this type of work done on ion channels. 

Tuesday, 27 March 2012

Seminar: antisense approaches as a therapy for muscular dystrophy

Wednesday 28th March 2012, 13:15 – 14:15
Perrin Lecture Theatre, 4 Newark Street, Whitechapel
Lunch will be provided
University College London  

“Antisense approaches as a therapy for muscular dystrophy”

Professor Muntoni graduated in Medicine in Italy in 1984 and obtained his specialization in Child Neurology and Psychiatry in 1989. Since 1993 he has worked in London, initially as a Lecturer (1993); Senior Lecturer (1994); Reader (1996) and Professor of Paediatric Neurology at the Hammersmith Hospital, Imperial College. In 2007 he moved the clinical, pathology and academic group from Imperial College to UCL Institute of Child Health and Great Ormond Street Hospital for Children. He is director of the Dubowitz Neuromuscular Centre, leading national and international centre for the diagnosis, management, treatment and experimental research therapy development in childhood neuromuscular disorders. The Centre has very close clinical and academic links with the adult neuromuscular centre at the Institute of Neurology, also at UCL.


1) Cirak S et al. Exon skipping and dystrophin restoration in Duchenne Muscular Dystrophy patients after systemic phosphorodiamidate morpholino oligomer treatment. The Lancet, 2011;378:595-605.
2) Cirak S et al .Restoration of the Dystrophin-associated Glycoprotein Complex After Exon Skipping Therapy in Duchenne Muscular Dystrophy. Mol Ther. 2011 Nov 15
3) Anthony K et al. Dystrophin quantification and clinical correlations in Becker muscular dystrophy: implications for clinical trials. Brain. 2011 Dec;134(Pt 12):3544-56.
4) Kinali M et al. Local Restoration of Dystrophin Expression in Duchenne Muscular Dystrophy: A Single Blind, Placebo-controlled Dose Escalation Study Using Morpholino Antisense Oligomer AVI-4658. Lancet Neurol. 2009 8(10):918-28.
Coordinator: Professor Dean Nizetic

Making connections - the human connectome project

Level: 3+

You may be interested in the flowing news feature: 

Jon Bardin. Making Connections. Nature 2012;483:394-6.

"The network, dubbed the 'connectome', is a web of nerve-fibre bundles that criss-cross the brain in their thousands and form the bulk of the brain's white matter. It relays signals between specialized regions devoted to functions such as sight, hearing, motion and memory, and ties them together into a system that perceives, decides and acts as a unified whole...."

The Human Connectome Project is an a initiative to map this network!

Saturday, 24 March 2012

Case study: horizontal diplopia

A 32-year woman presents with horizontal double vision on looking to the left. It is noted that her right eye fails to adduct and that there is nystagmus in the left eye. The remainder of her neurological examination is normal except for bilaterally upgoing plantar responses, i.e. bilaterally positive Babinski responses.

Level 1
What is the differential diagnosis?
Describe the neuroanatomy of horizontal eye movements?
Why does she not have a 3rd nerve palsy?
What is the significance of the the upgoing plantar responses?

Level 2
What investigations would you do?

Level 3
Why does she have nystagmus in the abducting left eye?
What happens to convergence?

Monday, 19 March 2012

The split brain: left brain vs. right brain

Level 3

When I was in high school I read the first edition of "Left brain right brain" which is now in its 5th edition. This is one of the books that kindled my interest in neuroscience and was a fascinating read. Last week's Nature has a news feature on this topic, which I recommend you read:

David Wolman. The split brain: A tale of two halves. Nature 2012;483:260-263.

The article discusses the seminal observations made on studying patients who had their corpus callosums cut to control epilepsy. This procedure has now been abandoned and the patients who had this procedure done are now old; research on this cohort is coming to an end.  However, the insights from studying these patients have been seminal; for example blind-sight, responding to stimuli that are are not conciosuly perceived and a large number of the disconnection syndromes. 

The following is a figure that summarises the experimental paradigm that is used to study the split brain patients.

Similalry, you may want to catch the last weeks Horizon, "Out of control", on the BBC iPlayer. A lot of what is discussed in this episode is relevant to the split brain; for example blind-sight is discussed in detail. 

Wednesday, 14 March 2012

Evolutionary neurology: depth perception

Level 3+

Jumping spiders jump long distances to catch their prey. This require accurate depth perception. Nagata and colleagues show that jumping spiders use a process called image defocus, which allows depth perception through the comparison of a unfocused image to a focused image within the same eye. A single layer within the spider's eye that could not focus green light nevertheless contained a green sensitive pigment; therefore this layer always receives an unfocused image, while other layers receive images in focus. Spiders are accurate jumpers in green light, but jump short of their target in red light.  

Please see original article and perspective:

Nagata et al. Depth Perception from Image Defocus in a Jumping Spider. Science 27 January 2012: 469-471.

Herberstein & Kemp. A Clearer View from Fuzzy Images. Science 27 January 2012: Vol. 335 no. 6067 pp. 409-410 

"Why is this relevant to neurology? It is an example of convergent evolution; i.e. nature has invented the means of assessing depth several times; it is obviously an important trait. Man mainly uses binocular vision to judge depth; the physical phenomenon is parallax.

Some questions to make you think:
  1. How does someone with uniocular vision or an amblyopic eye judge depth? 
  2. What symptoms would someone with a lack of depth perception complain of?
  3. Is it an important sensory attribute? 
  4. What are the DVLA rules in relation to driving regulations concerning impaired depth perception? 
All these questions should be relevant to someone studying neurology."

Monday, 12 March 2012

Assessing vertigo

Level 1

Benign paroxysmal positional vertigo or BPPV is the most common vestibular problem in adulthood. Patients will present with vertigo - the sensation of either the room or their head spinning - typically worse on head movements. They (or a significant other) may have noticed nystagmus and this can provoke referral to many different specialities. See this YouTube video for common presentations and the underlying pathology:

We each have two sets of semi-circular canals. The diagram to the left shows a schematic diagram of one. The three canals are oriented at 90 degrees to each other, providing information about head movement in any axis. For more information on the anatomy, see the following YouTube video:

The Hallpike manoeuvre (or Dix-Hallpike Test) is a simple and useful addition the arsenal of clinical tests for many clinicians, including Neurologists, ENT surgeons, GPs, Geriatricians and A&E doctors. We would encourage medical students to learn about it. A Hallpike manoeuvre is positive when it provokes nystagmus (as witnessed by the examiner) and vertigo (as felt by the patient) - this is strongly suggestive that the underlying diagnosis is BPPV.

The manoeuvre was first described in M. R. Dix and C. S. Hallpike: The pathology, symptomatology and diagnosis of certain common disorders of the vestibular system. The Annals of Otology, Rhinology and Laryngology, December 1952, 61 (4): 987-1016. For more modern guidance on the details of the test, see The British Society of Audiology's guidance.

The importance of the test is all the more apparent as a positive test indicates the presence of posterior canal BPPV; negating the need for further cli tests and offering the potential for an easy treatment in the form of the Epley manoeuvre:

For one more video covering the Hallpike and Epley, click here

Tuesday, 28 February 2012

Modeling the human brain; can it be done?

Level 3

Please read the article of "Computer modelling: Brain in a box" by Mitchell Waldrop as a part of the Nature feature on Alan Turing

"The article describes what it will take to model the human brain and the sceptics view of the project. I have little doubt that modelling the human brain is possible. The fact that nature can encode the brain's development and function using a triplet coding system (DNA) I see no reason why we cannot do it using binary code in a computer. Judge for  yourselves."

Monday, 6 February 2012

Anti-NMDA associated encephalitis

Level 2

Epub ahead of printGable et al. The Frequency of Autoimmune N-Methyl-D-Aspartate Receptor Encephalitis Surpasses That of Individual Viral Etiologies in Young Individuals Enrolled in the California Encephalitis Project. Clin Infect Dis. 2012 Jan 26.

Background:  In 2007, the California Encephalitis Project (CEP), which was established to study the epidemiology of encephalitis, began identifying cases of anti-N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis. Increasing numbers of anti-NMDAR encephalitis cases have been identified at the CEP, and this form rivals commonly known viral etiologies as a causal agent. We report here the relative frequency and differences among encephalitides caused by anti-NMDAR and viral etiologies within the CEP experience.

Methods: Demographic, frequency, and clinical data from patients with anti-NMDAR encephalitis are compared with those with viral encephalitic agents: enterovirus, herpes simplex virus type 1 (HSV-1), varicella-zoster virus (VZV), and West Nile virus (WNV). All examined cases presented to the CEP between September 2007 and February 2011 and are limited to individuals aged ≤30 years because of the predominance of anti-NMDAR encephalitis in this group. The diagnostic costs incurred in a single case are also included.

Results: Anti-NMDAR encephalitis was identified >4 times as frequently as HSV-1, WNV, or VZV and was the leading entity identified in our cohort. We found that 65% of anti-NMDAR encephalitis occurred in patients aged ≤18 years. This disorder demonstrated a predilection, which was not observed with viral etiologies, for females (P < .01). Seizures, language dysfunction, psychosis, and electroencephalographic abnormalities were significantly more frequent in patients with anti-NMDAR encephalitis (P < .05), and autonomic instability occurred exclusively in this group.

NMDA Receptor
Discussion: Anti-NMDAR encephalitis rivals viral etiologies as a cause of encephalitis within the CEP cohort. This entity deserves a prominent place on the encephalitic differential diagnosis to avoid unnecessary diagnostic and treatment costs, and to permit a more timely treatment.

"These results support my anecdotal experience with patients admitted to the Royal London Hospital in the last 5 years and is also supported by the findings of the recent surveillance study in England (see below)."

Granerod et al. Causes of encephalitis and differences in their clinical presentations in England: a multicentre, population-based prospective study. Lancet Infect Dis. 2010 Dec;10(12):835-44. 

Friday, 6 January 2012

Synesthesia: not as rare as you think

Level: 3

Brang & Ramachandran. Survival of the synesthesia gene: why do people hear colors and taste words? PLoS Biol. 2011 Nov;9(11):e1001205. Epub 2011 Nov 22. 

Synesthesia is a perceptual experience in which stimuli presented through one modality will spontaneously evoke sensations in an unrelated modality. The condition occurs from increased communication between sensory regions and is involuntary, automatic, and stable over time. While synesthesia can occur in response to drugs, sensory deprivation, or brain damage, research has largely focused on heritable variants comprising roughly 4% of the general population. Genetic research on synesthesia suggests the phenomenon is heterogeneous and polygenetic, yet it remains unclear whether synesthesia ever provided a selective advantage or is merely a byproduct of some other useful selected trait. Progress in uncovering the genetic basis of synesthesia will help us understand why synesthesia has been conserved in the population.

"Synesthesia may be a neurological oddity, but knowing about it and understanding the mechanisms that underpin it will help you appreciate the way the cerebral cortex functions."

Clinical significance of synesthesia:

Sensory deprivation and deafferentation (i.e., loss of sensory input through the destruction of sensory nerve fibers) can lead to synesthetic-like experiences. After early visual deprivation due to, for example, retinitis pigmentosa, touch stimuli can produce visual phosphenes, and after loss of tactile sensation from a thalamic lesion, sounds can elicit touch sensations. Arm amputees experience touch in the phantom limb merely by watching another person’s hand being touched. Hallucinogenic drugs can cause synesthesia-like experiences. 

"The article is a great read; enjoy!"