Wonky Brains and Crooked Quirks


Your brain is not symmetrical: a horizontal MRI scan will show you that it’s a bit wonky – as if it’s been twisted anticlockwise slightly inside the skull (the technical term for this is Yakovlevian anticlockwise torque).  The right frontal lobe, as a result, is bigger and wider than the left and it often protrudes forwards beyond the left frontal lobe; while the left occipital lobe is wider than the right and protrudes further backwards.

Yakovlevian torque

Yakovlevian torque

The curious thing about these neuroanatomical asymmetries is that they are not random variations between individuals: these are distinct patterns in anatomical design that appear to have some advantage.  While most parts of your body – your eyes, your lungs, your arms – are more or less symmetrical with a few idiosyncratic exceptions, something in the development of your brain causes these systematic adjustments.  Indeed, patterns in hemispheric symmetry and asymmetry that deviate from this norm are associated with schizophrenia, dyslexia, and a number of other disorders.  Why?  What causes wonky brains?

Vocalisation is lateralised to the left hemisphere – from monkeys to marmosets to mice

Many of these asymmetries are closely related to one of the most uniquely human abilities: language.  In keeping with this twisted Yakovlevian appearance, the Sylvian fissure (which lies, roughly speaking, underneath the main auditory and language processing areas) is longer and less steep in the left hemisphere, making way for a larger planum temporale.  Language (or other kinds of vocalisation) is lateralised to the left hemisphere – not just in humans, but a variety of other animals from monkeys to marmosets to mice (as well as song birds and frogs).  In humans, the planum temporale is heavily implicated in auditory processing and the other structural differences between the left and right frontal lobes match the areas that are associated with language and speech.  Indeed, in most autistic children it is the right hemisphere that dominates speech processing (unlike most other people, who show left lateralisation), and this is reversed as they improve their language skills and the left hemisphere gradually comes to dominate.

Asymmetry affords flexibility

It’s reasonable to assume that these asymmetrical quirks have some reason for being – nature doesn’t deviate from a pattern without good reason.   While language lateralisation is the most studied, visuospatial ability, attention, music perception, and mathematical ability have all been found to be dominated by one or another hemisphere.

The best reason is, simply, that asymmetry affords flexibility.  Having one part of the brain specialised for a particular task or function means that information can be processed more efficiently, freeing up other parts for other things – much the same as how most modern computers have dual-core processors for efficiency.  It also reduces the amount that the hemispheres interfere or conflict with each other by granting dominance to one over the other.

Bisazza and Dadda (2006) demonstrated this by breeding two strains of the same species of fish, one strain with asymmetrical brain structure, and the other with symmetrical brain structure.  When they were required to carry out one task (catching a shrimp), the two strains of fish performed equally well.  But when they were required to do two tasks simultaneously (catch a shrimp while avoiding a predator), those with symmetrical brains took twice as long to catch the shrimp.  Those with asymmetrical brains were barely affected by the distraction of the predator.

Translate this into humans, and lateralisation for speech means that your left hemisphere can perform the function of holding a conversation while your right hemisphere can get on with other cognitive tasks.  As for some of the other asymmetrical quirks – the fact that Alzheimer’s disease progresses faster in the left hemisphere, for example; or that the left also dominates the perception and expression of positive emotions; or the observation that  many of these asymmetries are most pronounced in right-handers – the answer is not as obvious.

 

References

Dadda, M., & Bisazza, A., (2006).  Does brain asymmetry allow efficient performance of simultaneous tasks?  Animal Behaviour, 72, 523-529.

Further reading

Ehr, G. (2006).  Hemisphere dominance of brain function – which functions are lateralized and why?  In 23 Problems in Systems Neuroscience, van Hemmen, J. L., & Sejnowski, T. J. (Eds.).  Oxford University Press: New York.

Petty, R. G. (1999).  Structural asymmetries of the human brain and their disturbance in schizophrenia.  Schizophrenia Bulletin, 25, 121-139.

Toga, A. W., & Thompson, P. M. (2003).  Mapping brain asymmetry.  Nature Reviews Neuroscience, 4, 37-48.

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Bird-brain: Why do pigeons bob their heads?


This is Bob.

This question occurred to me yesterday while I was waiting for a bus and spotted some pigeons nearby.  Their idiosyncratic, jerky, back-and-forth head movements sort of look like they’re continually pecking for invisible food suspended in the air – which is quite odd if you think about it, and must cause them to expend quite a bit of energy.

So why do they do it?

It turns out that the back-and-forth motion is an illusion, actually.  This was first described by Dunlap & Mowrer (1930) who found that birds (specifically chickens, in this case) don’t jerk their heads backwards; in fact, they hold their heads still relative to their surroundings and move their body forwards, before thrusting their head forwards for it to catch up with the body.  This is called the “thrust-hold cycle”.  (See picture: the head moves relative to the body, only because the body continues to move forwards while the head stays still.)

Thrust-hold cycle

It’s clear that this has something to do with their vision: pigeons don’t do it when they’re blindfolded and made to walk forwards, and they don’t do it when they are walking on a treadmill (such that they remain stationary relative to their surroundings) – which means the behaviour only occurs when the environment is unstable.

What’s happening is that they are keeping the image projected onto the retina still, to prevent the image blurring.  Humans have a similar optokinetic reflex: try slowly scanning the room from left to right in a straight line, while keeping your head still.  It’s extremely difficult to do smoothly because your eyes will keep making small, jerky saccadic movements.  But hold up a finger, slowly move it from left to right, and follow it with your gaze, and your eyes will fixate on the finger and move perfectly smoothly.  It’s because in the first instance the thing you’re looking at is the room, the retinal image of which is changing as you move your eyeballs.  Your vestibular system does its best to stabilise the image by keeping your eyes still for as long as possible and then quickly fixating on the next image of the room.  In the second instance, however, the visually important thing is your finger, and the best way to keep it in focus is by moving your eyes such that your the image of your finger is always projected onto the same part of your retina.

Rather than eye movements, pigeons make movements with their entire heads in a similar fashion to stabilise the image.  This explanation is further supported by the fact that if you put a pigeon on a treadmill such that it’s only moving passively (i.e. it’s not walking or moving its feet at all), it will still bob its head: the pigeon doesn’t even need to be walking in order to produce this behaviour, it just needs to have an unstable visual environment.

There are likely to be other functions of head-bobbing, however.  If nothing else, it’s a reasonable assumption to make simply because visual processing is such a complex task – the visual system always makes the most of what it’s got.

For instance, depth perception is more difficult for pigeons than for humans because the visual fields of each eye don’t overlap nearly as much as ours.  We can tell a lot about the depth or distance of an object by looking at it with both eyes and comparing the retinal image projected onto each.  Pigeons, with their eyes spaced more widely apart, don’t have that kind of information available to them.  One source of information they can use is motion parallax, which is based on the fact that objects closer to you will move faster than objects further away as your travel through your environment.

For instance, if you’re travelling in a car, you might notice the fence at the side of the road whizzing past so fast you can’t see it clearly; the trees behind it will move quite quickly, but not as quickly as the fence; the buildings in the background will move more slowly still; and the distant mountains you can make out in the distance will barely be moving at all.  You can get a pretty good idea of the relative distances between all these things in the foreground and background just from their relative motion.  The same idea is shown in this GIF.

Motion parallax

Pigeons create this motion parallax and maximise it in the thrust phase of their thrust-hold cycle: by moving their head a little bit, over and over again, they can get a rapidly updating idea of how far away things are, relative to the pigeon and to other things in the environment.  Even when the pigeon is moving too fast to be able to maintain the hold phase – e.g. when it’s running or landing – it will still thrust its head back and forth to sustain the motion parallax.

Incidentally, cat owners will have noticed a similar phenomenon in their pets: before making a big leap, cats will often move their heads back and forth a few times.  Although cats’ eyes overlap a great deal (even more than humans’) so they don’t face the same paucity of depth information that pigeons do, when making an ambitious jump it still helps to have all the information you can get about the distance between you and the landing spot you’re aiming for.

References

Dunlap, K. and Mowrer, O. H. (1930). Head movements and eye functions of birds. J. Comp. Psychol. 11, 99–113.