Friday, 27 May 2022

Playing it by Ears (Hearing 3)

This post builds upon the previous two about the sense of hearing, here and here. It is probably best to read at least the previous one, because some understanding is required about the 'cone of confusion'. That is a key concept to understand why combining information from two ears is not enough to determine the direction of a sound source accurately.    

To summarise, sound from a given source will take longer to arrive at the ear farthest from the source  than at the nearest ear; it will also arrive less loud at the farthest ear. The brain measures the differences in arrival time and in loudness between the ears and computes an angle between two lines; the first runs from the nearest ear to the source of the sound, and the second is the axis connecting the two ears.

If you know something about how to indicate a direction in 3D space, you will realise that you cannot do that with just one angle. If you wish to indicate the position of the sun in the sky from a point on the Earth's surface, you need two angles: one tells you the compass direction, and the other indicates elevation above the ground. With hearing with tow ears, the one angle you get is relative to the axis between the ears. The sound can come from all directions obtained by sweeping that angle around that axis. The result is the 'cone of confusion'. Do not worry, an image may help.

Click to enlarge; copyright Gert van Dijk

Here is a new animal, one whose head is elongated enough to accommodate two pairs of ears: the front and hind pairs. Let's explore whether having more than two ears helps. The small golden globe represents a sound source. To start with, consider just the front pair of ears. I have drawn the line from the source to the nearest ear, the axis between the ears, and the angle between them. Rotating the around the axis results in the specific 'cone of confusion' for this particular sound source for the front ears. Let's call it the 'front CoC'. By the way, the cone is drawn as if it stops at the sound source; in reality it extends into space.

Click to enlarge; copyright Gert van Dijk

The sound will also be picked up by the hind ears, and we will assume that the animal’s brain performs a similar analysis for the hind ears. There is therefore a new angle, now between the source and the axis through the hind ears, and also a new cone: the ‘hind CoC’.

Click to enlarge; copyright Gert van Dijk

There would be no point in having four ears if the two localisation systems could not be combined. The source of the sound must lie on both cones, so we need to find the parts that the two cones have in common: their intersection. That intersection is, in this case, a nice parabola. It is shown as stopping at the end of the cones but should again extend into space along with the cones. Does this improve localisation? Yes: the possible source of the sound is reduced from the surface of a cone to a line in 3D. 

Click to enlarge; copyright Gert van Dijk

But the brain can do more with the information already available. We can also combine left ears and right ears. In the drawings, the sound source was placed at the right side of the animal, so let’s use the right ears, with the sound nearer the front than the hind ear. The principles are exactly the same: we draw the axis running through the ears, determine the angle between the source and the axis and rotate it around the axis: the ‘right CoC’. In this example the cone is a bit more difficult to visualise because the source lies between the two ears involved, but the principle remains the same. If we combine the new cone with the parabola we already had, we find that the only solution now consists of two points.

I stopped here, even though the ‘left CoC’ could still be added. You could also form two diagonal combinations, meaning the left front with right hind ear, and the right front with the left hind ear. With four ears, there are therefore six possible CoCs, and together that should be enough to solve the problem. However, I doubt that the animal’s brain would perform all the calculations after one another; there is probably a smarter way to get the correct answer.

The position of the ears could be more creative too. In the example above, the four ears all lied on a plane, but that is not necessary. Suppose that the front ears lie on a horizontal line, so there would be a front left and a front right ear, like in the example. But the hind ears might lie on a vertical line, yielding an upper hind ear and a lower hind ear. The four ears would lie on the corners of a tetrahedron. There may also be other ways to improve the location of sounds, so perhaps this will not be the last post of sound localisation.    

Sunday, 24 April 2022

14 Years on (and one million views later)

The first post of this blog was published on 22 April 2008. It was an experiment, and I had no idea how long I would continue to keep writing posts. I still don't,  but the fact that the blog is still here after 14 years, was not something I foresaw at the time and for now I shall continue. Let's start this post  update with an overview of past results, as per 23 April 2022:

Number of posts: 278 (not counting this one)
Number of comments: 2485
Number of followers at present: 220
Number of views: 1,001,012

This means that each posts generates an average of 9.8 comments. I like the idea that the number of views surpassed 1 million but have no idea how many of these views represent people taking an active interest, and how many 'views' in fact represent bots.

Which posts attracted the most interest?

Swimming in sand 1: the sandworms of Dune                                             10,100  
The anatomy of giants in 'Game of Thrones'; did they get it right?                9,500
A future book on future evolution from France                                             6,470
Avatar's "Walking with hexapods" or "Don't walk this way"                        6,380
Warren Fahy's "Fragment"                                                                             4,340
More future evolution in Japan                                                                      4,250
Alternate future evolution in Japan                                                                3,890
Future evolution from France: "Demain, les animaux du futur" Review I   3,340
A century of thoats                                                                                        3,070
The future is wild... and it is Manga!                                                            3,030

Comparing the list with earlier ones show that the top 10 hasn't really changed that much. The lesson is the same as before: if I would want to maximise the number of views, I should write about popular films, TV series and books. I largely stopped doing that, which explains why the top 10 is stable.

   

Click to enlarge

It is interesting that the attraction of posts can differ much over time. For instance, the sandworm post attracted a great deal of interest in the beginning, followed by a steady trickle of about 20 to 100 views per day. In contrast, the two posts with Japan in the title showed a comeback in 2020 and 2021.  

Just another dragon

A post without any new material or thoughts is at best mildly interesting, so here is a minor illustration showing a schamtic view of a 'Megadraco' taking off. This is a species of the clade Dialata, discussed here.

Click to enlarge; copyright Gert van Dijk

This animal is much larger than Earth's birds, so how does it fly? Well, readers will realise that pterosaurs grew much larger than birds. pterosaurs definitely flew, so how did they outsmart birds in this respect? Well, you may remember that lift is proportional not only to wing area but also to the square of air velocity. In other words, doubling velocity has the same effect on lift as a fourfold increase in wing area. Speed pays to remain airborne. The problem is how to get to such a speed from a start on the ground (jumping off a branch or a cliff is a much easier way to achieve speed) . The problem is more difficult for larger animals; if you have seen a swan or heavy goose take off, you may realise just how difficult it can be for a heavy bird to achieve that all-important speed.

The idea is that pterosaurs achieved high starting speed in a radically different way: they jumped into the air, powered not just by their hind legs, but also by their much more powerful front legs: the wings! That quadrupedal launch should be enough to get them high enough in the air for a first powerful downwards wing stroke, and from then on, they were in business. 

I like that idea and thought that evolution might well do its familiar 'parallel' trick again. If your basic body plan involves six limbs, of which the middle pair are wings, you have four legs left to propel yourself up into the air. That should help! Of course, once in the air, those four limbs weigh something but do not contribute to flight, so perhaps the resulting animals do not grow as large as the biggest pterosaurs. But even so, with a quadrupedal launch and a clap-and-fling first wing beat, these Furahan dragons get up fast enough to fly, large as they are.          

The future


The 'Great Hexapod Revolution' means that the anatomy of various animals in various already finished paintings was no longer correct. Sadly, these paintings were therefore instantly no longer 'finished'. I am working my way through them, changing legs and heads left and right. I have only about 5 more of such reviews to do, and after that I will make only two or three completely new paintings. Meanwhile, I will work on some additional material such as a Glossary, and then the manuscript of The Book is all done. I expect to achieve that goal this year, but will also move from one house to another, which always takes more time than you think, even if you take that into consideration.

And then there is the matter of finding a publisher. That is an open question. I wonder what the post '15 years on' will have to say on that subject...   

Sunday, 6 March 2022

Lend me your ears! (Hearing 2)

In nature, localising the source of a sound has obvious survival value, to localise prey, predators, mates, competitors, etc. The previous post started the subject, but this one deals with how mammals, humans in particular, try to solve that problem. As we shall see, localising sounds is not all that easy. Of course, some animals are much better at this than others, but there are some fundamental problems. Humans make use of no less than three different mechanisms. This post might be a bit technical, but I left the mathematics to a nice free good review, here.   

The main problem is that sound can bend around obstacles, and so change direction. A sound reaching your ear can therefore come from just about anywhere. You can therefore hear things you cannot see, which is good news in the dark or in a jungle. Of course, you do not know where it is coming from. Sound provides a good alerting but a poor localising system, whereas a well-developed eye is a localising organ (see here for a comparison of echolocation with vision). 

Click to enlarge; copyright Gert van Dijk

If we would have just one simple ear of the hole-in-the head variety, but with an eardrum and vibration sensors, we would be stuck at that level. But bilateral symmetry provides us with two ears, and evolution made clever use of that. The image above shows that the waves from a sound source travel directly to the nearest ear but must travel further to reach the farthest ear. The sound arrives at the ears with a time difference: the 'interaural time difference (ITD)'. The difference in arrival time depends on the extra distance the sound has to travel, shown in red. That difference depends on where the sound comes from, relative to the axis between the two ears. The maximum arrival distance occurs with sources placed on that axis, because the sound must travel farthest around the head. The minimum is no difference at all: this occurs when the source is placed on the plane of symmetry. You could make a table telling you which difference corresponds to which angle, and that is basically what the nervous system does for you. 

Click to enlarge; copyright Gert van Dijk
 

Unfortunately, this is not a perfect solution. The arrival difference tells you what that angle is, but not whether the source is up, down, to the front, the back, or anywhere in between. The image above tries to explain that. The source could be anywhere on the brilliantly named ‘cone of confusion’; that is the surface you get if you rotate the angle around the ‘hearing axis’. One way to get around this problem  is to rotate your head and listen again, because then you get a different cone of confusion, giving you an additional clue where the sound comes from.  

Mammalian ears make use of a second trick. When sound bends around an object, its volume decreases. Your head provides a ‘sound shadow’: The volume of sound in the nearest ear is louder than that in the farthest ear, in the sound shadow, so there is an 'interaural level difference (ILD)'. The difference in volume depends on the angle, and again the brain constructs a table telling you which difference in sound level corresponds to which angle. But that irritating cone of confusion is still there...

Mammal evolution came up with a third trick: the external ear! The complex shape of the external ear alters the spectrum of the sound, and how it is altered depends on the location of the source of the sound: the 'head-related transfer function ('HRTF)'. This works to an extent with just one ear. When I first read this, I wondered how that could work, because people have such differently shaped ears. Well, the solution is brain learns to live with the filtering characteristics of the ears you happen to have. This has been put to the test by altering people's ears by placing a mould on the ear, and that indeed that fooled the brain to make mistakes in sound location. 

Click to enlarge; source and rights here

You may wonder why you would have three systems for the same purpose. One part of the answer is that the efficacy of each system depends on sound frequency. The figure above shows the frequency: ITD ('arrival time') works best at low frequencies, and ILD ('loudness') and HRTF ('external ear') work best at high frequencies. Together, they do a nice job for all frequencies. But there are strong clues that the situation still is not optimal, and that is behaviour. Many animals can move their ears, and people can tilt and turn their head to locate the source of a sound: that shrinks the cone of confusion. But we still start a visual search of the general area of the sound, hoping that vision will provide the ultimate localisation. Of course, that is in part because we are diurnal mammals with very good vision; but part of the explanation is that localising sounds is inherently difficult.

Could animals on other planets do better? I think so, but that speculation will have to wait for another post.



Saturday, 26 February 2022

Tabulae Mortuae V (Archives XV): Digital paintings die too...

 Every now and again I show an image from the Creature Vaults, those hidden domains where old sketches, failed paintings and discarded designs find their final resting place. 'Final', unless they are dragged out to be presented to the world, usually for the first time.

Click to enlarge; copyright Gert van Dijk

This image is one such, and it is the first to come from a vault without physical form. Other vaults consist of large cardboard folders, or of stacks of oil paintings carelessly stacked against the back wall of a closet. This vault is digital.

I started the conversion of the Furaha project from oil paintings to digital art some 11 years ago. The project, now nearly done, changes as time passes. The Great Hexapod Revolution had as a result that legs, heads and jaws or earlier hexapods no longer followed my self-imposed rules. The changes were too large to be solved with moderate cosmetic changes (I tried), so many paintings are now seeing a 'Mark II". In fact, some started as oil paintings (MkI), were later redone as digital paintings (MkII), and are now revisited to become MkIII. Mind you, most paintings these days are entirely new.

Click to enlarge; copyright Gert van Dijk

Here is some more detail of the head of this now defunct animal. It is a pity that I had to discard it, as I rather like the painting. But I kept the overall design and colour scheme for the MkIII version, which is nearly finished, and looks just as well or better, I think.

The animal is a 'thresher', with the Latin name 'Ira tarda'. That means 'slow anger', a name that was inspired by memories of an old teacher of mine. Threshers are solitary, grumpy and are best left to their own devices. They do have to meet from time to time, in view of the perpetuation of the species, but their behaviour at such times gives little indication of a mood upswing. Best not talk about it, really.



Saturday, 29 January 2022

Hear, hear! (The sense of hearing 1)

There have been many attempts to design interesting sensory systems for alien animals. Some tried to equip animals with radio or radar, which poses the difficulty that such radiation passes easily through biological tissues, making them hard to detect. There have been attempts to do away with vision altogether and replace it with something else, such as echolocation. The most famous example is probably Barlowe's Expedition. Although his artistic prowess made the result look spectacular, life without vision on a planet with light seems very unlikely. The echolocation was explored in four posts: one, two, three and four

I cannot remember a fictional world in which the total absence of hearing is presented as an interesting twist. That absence suggests that hearing is accepted as a simple given. Are there environments in which hearing cannot work? Yes, there are. If we define hearing as the ability to pick up mechanical vibrations in a surrounding medium, close to the Wikipedia definition, then the absence of that medium will abolish hearing. The resulting environment coins the nice phrase 'In space no-one can hear you scream'. True, but you cannot breathe there either, and that is definitely a bigger problem than not being able to scream. I intend to write two posts about hearing in a terrestrial environment with an Earth-like atmospheres. The present one will have a look at evolution of hearing on Earth, to see if that helps designing alien hearing. 

Picking up mechanical vibrations from the air is not fundamentally different from doing so in another medium, such as water. The medium could be expanded to include touching an object, such as the ground. Picking up ground vibrations is so close to hearing that you could arguably include that in the 'hearing' concept. The ability to register vibrations lies at the heart of our kind of 'air hearing'. Where it is easy to evolve 'air hearing' for animals that once came out of water will depend on their starting point. Here is a very nice paper comparing vertebrate and insect hearing. The basic problem has to do with being surrounded by air. Bodies are mostly water, so vibrations in water will readily pass into watery bodies, where they can be picked up. But picking up air vibrations in a watery body requires a signal transformation. The main way to do that, in vertebrates and insects, is to use a taut membrane to pick up air vibrations. 

Click to enlarge; source and copyright: https://www.frontiersin.org/articles/10.3389/fevo.2021.667218/full

 Vertebrates 

Vertebrates apparently evolved their eardrum, or tympanic membrane, from skin. Making an eardrum work means the drum must be open to the air on one end, but there also must be air on the other side of the drum, inside the animal. Having bubbles of air in the body is not a standard part of animal anatomy though, so the middle ear had to evolve from scratch too. The illustration above shows that eardrums were present at the start of the Mesozoic, not earlier. As if that was not difficult enough, the system also needed pressure transducers, for which rearranged jaw bones were press-ganged. 

It seems that hearing was not really an easy process for Earth vertebrates, so perhaps 'air hearing' is not an automatic given. The paper suggests that vertebrates, before they evolved proper 'air hearing', may have picked up vibrations through the ground, perhaps to sense predators. That makes me wonder whether predators at the time would have been under evolutionary pressure to walk softly. 

Insects 

The paper also goes into insect hearing and makes the point that insects had it much easier: forming an eardrum was a matter of thinning the exoskeleton, and the insects' exoskeleton, right at the air-body interface, already carried lots of mechanoreceptors that could easily be given a new job. Insects have air-filled tracheae (breathing tubes) everywhere, so forming the equivalent of a middle ear was not difficult either. Apparently, hearing evolved in insects independently at least 19 times, resulting in a great variety of ear designs, found all over insect bodies. 

Click to enlarge; source here

Another paper provided details of such insect ears. A few examples are shown above the arrows indicate ears. The authors write that about the only place where you will not find insect ears is on the sides of their heads. Instead, insect ears are most often found on the body itself, but they can also be situated on the legs, mouth parts and even on wings. As for the number of legs, many insects have just two ears, but some grasshoppers have no less than 6 paired 'functional auditory organs in their abdomen', giving them 12 ears. Some insects have two ears per leg, giving them four ears. Mantids are perhaps the oddest, with just one ear in a groove on the midline of the thorax. Although there is a tympanic membrane on each side of a narrow grove, these are so close together that they function as one ear. Mantids are 'the only known terrestrial animal with a single ear'. The figure above shows an exception: some mantids do have two ears, but both are in the midline, and they are sensitive to different frequencies. 

click to enlarge; source here
 Spiders 

There are no tympanic membranes to be found in spiders at all, so for a long time it was thought that they were deaf. However, they are apparently able to pick up sounds with hairs on the tip of their legs. This was only published in 2016, so there will be probably more discoveries in the field of spider hearing. 

 


The investigators made a very nice video of their somewhat accidental findings, that you can find on their website (they allow downloading, and the quality there is better than shown here; I just enclosed it for ease of use). If you need more, the authors went on to study hearing in 'ogre-faced spiders' later, also with a nice video). 

What does it all mean for speculative biology? 

As usual, there are various solutions to the problem (and we haven't even touched upon frequency ranges and other intricacies!). People who design vertebrate analogues might do well to think about how Earth vertebrates and insects developed eardrum-based hearing: you need some suitable tissue with easy access to outside air, so that tissue can be thinned to form a membrane. You also need a way to have air on the inside of the membrane too. That alone would suggest a place near the mouth. It will also help if that place is already equipped with the ability to sense movement, of jaws, hairs, scales, etc. But the spider example shows, as usual, that there are other ways too, and hairs seem to excellent starting material to evolve hearing. 

The next post will be about sound localisation. Sound does not travel in neat straight lines, which is just one of the things that complicate that particular problem.