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"Sixth sense" a fishy perception

By janra in Science
Wed Mar 12, 2003 at 07:58:55 PM EST
Tags: Science (all tags)

In the darkest depths of the ocean or the siltiest, murkiest river bottom, what's an inquisitive robot to do? Bright lights don't shine far even when the water's clear; navigational sonar may disturb the very thing you're seeking. Both take lots of equipment and power to operate, severely limiting your range.

Somehow, fish can navigate in to and out of the darkest, murkiest locations, and swim farther and faster than our robots and submarines can on the same amount of energy. The obvious place to look, then, is at the fish.

It turns out that a fish's lateral line is a sixth sense, related to the sense of touch in the same way the senses of smell and taste are related. Along the lateral line, which stretches from gill to tail and is visible in most fish species, is a row of specialized cells that sprout microscopic hairs, each attached directly to the end of a nerve cell. The hairs grow in clusters; the size of the clusters and of the hairs themselves vary between species. In some species, the hairs are even hidden in a protective cavity under the scales. In all fish species, however, the hairs send signals to the brain when the slightest current touches them. Using this row of extraordinarily sensitive sensors, fish can tell not only when something is approaching them, they can distinguish predator from prey from falling rock, and even detect unmoving obstructions by the eddies they produce in the ambient currents.

An artificial sensor with the same capabilities would first need to be on the same scale - a hair-like structure just under a millimetre tall. It would also have to stand up from the surface it's mounted on so fluid flow could move and bend it, and it would have to trigger a signal that can be sent to a computer for processing.

A group of researchers at the University of Illinois developed a cantilever design with a piezoresistive strain gauge at the base, and manufactured it using micromachined silicon.

The strain gauge is formed by diffusion doping part of the silicon substrate with boron, to provide a semiconductive material with the desired properties. A 300nm sacrificial layer of copper is deposited then etched into the desired design using photolithography, then a 600nm layer of gold is added and etched. The back of the silicon wafer is selectively removed to create the cantilever base with the right properties. The copper is dissolved away in dilute HCl, leaving a free - but not yet vertical - artificial hair attached at one end to the cantilever base opposite the strain gauge. The whole thing is then slowly lowered on top of a permanent magnet, and the hair is deformed by the magnetic field to stand vertically. To protect and strengthen the assembly, and to insulate the electrical connections so they can operate in conductive or reactive fluids, the entire thing can be coated with a thin Parylene film of a thickness dictated by a balance between insulation and sensor stiffening.

Using the process briefly described above, arrays of these artificial hairs can be created, packed as densely as 100 per square millimetre.

The micromachining turned out to be the easy part, however. Under simple laminar (layered, so that the centre moves fastest and the fluid velocity decreases smoothly as it nears the stationary surface of an object) or plug (extremely turbulent, so that the fluid seems to move in a solid "plug" past the stationary surface) flow, an equation for the relationship between the strain measured by the sensor and the average fluid velocity could be determined. However, pure laminar and pure turbulent flow aren't that common; the most common type of flow is a mixture of laminar and turbulent flow. The laminar flow, in such a mixed flow situation, is also often called the "boundary layer" because it exists along the surface, or boundary, of an object. Past the boundary layer, the fluid abruptly becomes more turbulent but not quite "plug", with eddies and vortices making the fluid flow harder to predict on a small scale. The boundary layer can be very thin, on the order of 1mm. The hair sensors - in fish and in the lab - are thus long enough to reach past the laminar boundary layer and into the turbulent flow regime, but short enough that the boundary layer can't be ignored. Because of the different flow regimes that affect the sensor, the finite-element numerical method, where the effect of the current on very short fractions of the hair is calculated then all of the fractions are combined to provide an overall picture of the whole hair, will often have to be used to calculate the flow velocity.

And this is just one hair.

This is also assuming unidirectional flow impinging on the hair directly in line with the cantilever. Outside of laboratory test equipment, water often flows crossways and opposite the main current, creating eddies that would push the sensor around and sometimes even counteract the effect of the main current. The fish (or robot) could also be oriented in any direction relative to the fluid flow.

To properly characterize the environment - and to have a hope of identifying, or even merely detecting, underwater objects - the hairs, like the hairs in fish, have to be made in clusters of hairs of different sizes and orientations, and some serious computing power will have to be used to figure out how the different sensor readings describe even a single current's direction and speed.

Despite the fact that fish are not particularly intelligent, they can still beat our best computers when it comes to real-time analysis of fluid dynamics using an array of sensors mounted on a moving surface, much like our brains can beat any computer out there when it comes to things like analyzing light impinging on an unsteady-state detector to instantly recognize and identify complex patterns.

It looks like our underwater robots will remain clumsy and "deaf" to the subtle movement of the surrounding water for a while yet. The information carried past their skin will remain unheard until computers become powerful enough to perform the extensive analysis needed to make this as valuable a sensor system as it is for a simple fish.

References: "Design and fabrication of artificial lateral line flow sensors", Z. Fan et al, Journal of Micromechanics and Microengineering, June 2002


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"Sixth sense" a fishy perception | 49 comments (33 topical, 16 editorial, 0 hidden)
Numbering senses (a bit OT) (4.00 / 3) (#10)
by tang gnat on Wed Mar 12, 2003 at 05:14:20 PM EST

Somehow there is this popular idea that humans have five senses. Besides the usual, we have a sense of balance, a sense of self-orientation (knowing where my hand is without looking at it), and many more

one more that you forgot (more OT) (5.00 / 1) (#27)
by fishling on Wed Mar 12, 2003 at 08:46:26 PM EST

a sense of outrage.  :-)

[ Parent ]
and more important and most often lacking... (none / 0) (#40)
by Wah on Thu Mar 13, 2003 at 01:06:04 PM EST

...a sense of humor.
Parent ]
Holographic sense. (none / 0) (#36)
by porkchop_d_clown on Thu Mar 13, 2003 at 09:44:32 AM EST

If I remember correctly, the sense of where your body parts are is called the "holographic sense". It should also be noted that it really isn't an independent sense - it's a combination of remembering how you last moved your arm (for example) and the feeling of stress in your muscles. It is easy to demonstrate this, simply by noting that person waking from deep sleep has no idea about where his arms and legs are for the first few seconds after waking - which is why I've been known to accidentally hurl a pillow across the room if I'm awakened suddenly. (I tend to put my arms under my pillow when I sleep...)

You can lead a horse to water, but you can't make him go off the high dive.

[ Parent ]
(OT) Why throw the pillow? (none / 0) (#37)
by Cheetah on Thu Mar 13, 2003 at 09:53:07 AM EST

I'm curious why you often throw the pillow across the room.  I almost always sleep with my arms underneath my pillow (propping it and thus my head up a bit), and I don't think I've ever knocked the pillow out of bed, let alone thrown it across the room.  I'm having a hard time even imagining how the muscle sequence goes that ends with the pillow airborne.

[ Parent ]
I haven't quite figured it out either... (none / 0) (#38)
by porkchop_d_clown on Thu Mar 13, 2003 at 10:27:56 AM EST

As near as I can tell, if something happens that makes me sit up as I wake up, and as I bring my arms around, the pillow comes with them, then keeps going...

My wife thinks that I've managed to stick my arm *in* the pillow case, which is possible, but I've never noticed that I've done that...

You can lead a horse to water, but you can't make him go off the high dive.

[ Parent ]
Proprioception (5.00 / 3) (#41)
by bob6 on Thu Mar 13, 2003 at 01:48:52 PM EST

The word you're looking for is proprioception, the sense of position and movement. It is a complex cognitive function comparable to vision which is not yet fully understood. As you pointed out, it uses different information sources.
The proprioception uses mainly the somesthetic pathway which is a general term for the touch senses including texture, heat and pain. We have organs all inside our body, especially around skeletal parts (bones & muscles), which are sensitive to pressure variations. The whole thing is summed up by our brain in order to build an accurate representation of the body position.
The fun fact is that those receptors are stimulated by high-amplitude and low-frequency sounds, hence the fat bass sensation experienced by your bowels when listening to loud music.

[ Parent ]
Making sense... (none / 0) (#45)
by gidds on Thu Mar 13, 2003 at 10:13:29 PM EST

It really depends how you count the senses. My list has thirteen, though few will come as a surprise.

Non-contact senses:

  • Sight (rods and cones).
  • Hearing (direct and bone conduction).
Chemical senses:
  • Smell.
  • Taste (sweet, sour, salty, bitter, and maybe a fifth one, `umami', for monosodium glutamate). What we think of as taste is really a combination, involving smell and the touch senses on the tongue.)
Skin senses:
  • Touch.
  • Pressure.
  • Heat.
  • Cold.
  • Pain.
(Of course, these also operate inside us give information about the state of our digestive system, &c.)

Balance (kinaesthetic) senses:

  • Linear acceleration and orientation (in the otoliths).
  • Angular acceleration (in the semicircular canals).
Proprioceptive senses (how our body is arranged):
  • Position (muscles, tendons, joints).
  • Tension (").
(These are the senses most of us have - you'll note I haven't included senses of discipline, fair play, decency, irony, or restraint...)

I find it interesting just how much preprocessing we do on most of these. We're used to thinking about how our eyes detect distance, shape, texture, &c from two flat images, but we do a good deal of interpretation on all our senses - `wet', for example, being a combination of coldness and touch, and also pressure and muscle tension when judging the surface friction.)

[ Parent ]

Pretty good article (4.00 / 1) (#11)
by epepke on Wed Mar 12, 2003 at 05:22:13 PM EST

But back when I was a kid, people thought the lateral line generated a slight current and reacted to changes in the conductivity of the water. The electric eel's electrification mechanism was said to be an adaptation of this. Has popular wisdom changed?

The truth may be out there, but lies are inside your head.--Terry Pratchett

must have (3.00 / 1) (#12)
by janra on Wed Mar 12, 2003 at 05:25:56 PM EST

Just checked my reference's references, and the paper on fish biology they were working from to get the natural structure of the sensors is dated 1996.

Discuss the art and craft of writing
That's the problem with world domination... Nobody is willing to wait for it anymore, work slowly towards it, drink more and enjoy the ride more.
[ Parent ]
Thats a different system (4.00 / 2) (#14)
by Edgy Loner on Wed Mar 12, 2003 at 05:53:52 PM EST

Electric eels as well as other fish species, notably sharks, have specialized sensors for detecting low amplitude electric fields. In some cases these sensor systems are passive, in other cases they are active and produce a field and sense disturbances in that field. In all cases these sensory systems are seperate from the lateral line system. I'm not sure, but I think that only the more modern bony fish species have a lateral line system.

A few random links:
Electric eels
More eel stuff
Electroreception in sharks
Electrorecption and magnetic sensing

This is not my beautiful house.
This is not my beautiful knife.
[ Parent ]

That truly is a separate sense (none / 0) (#20)
by fluffy grue on Wed Mar 12, 2003 at 06:44:22 PM EST

Mmm. Being able to intrinsically sense electric fields (rather than relying on side-effects like feeling the slight pressure of a static charge or feeling your muscles convulse when you touch a live wire) would be a pretty useful ability, especially if you're a network engineer or electrician.
"Is a hyperlink" is a hyperlink.
"Is not a quine" is not a quine.

Cats: Nature's entropy generators

[ [ Parent ]

There is research going on at UIUC on that.... (none / 0) (#46)
by n3uropil on Fri Mar 14, 2003 at 02:34:02 AM EST

I know because I'm in that lab. There are several teleost species (called 'weakly electric' as the fields that they emit are <1mV, and can't hurt or be felt by most organisms) that have active electroception capabilites. They emit a very low amplitude electric field (some species emit 'pulse', others emit continuous sinusoidal 'waves'). The current flows around the fish and back into it's skin via electroreceptors distributed across the body surface. <p>Objects in the immediate environment (other electric fish, food animals, twigs) change the current flow (conductive objects increase the flux, resistive objects decrese the flux) this changes the voltage on the skin, forming an electric "image" that is dectected by the electroreceptors.

The info is carried back to the hindbrain along with purely passive electrosensory information (discussed in another reply here), and first processed in the ELL - electrosensory lateral line lobe.

The sensitivity is amazing - the fish can sucessfully detect and find small crustacean prey whos signal approaches the natural noise and fluctuations experienced by the receptors (~1 uV at about a cm or two).

Our lab studies behavior, physiology, and computation in the brains of South American wave-type fish - the Black Ghost Knife fish. Take a look at our lab website here.


[ Parent ]

Thanks! [n/t] (none / 0) (#47)
by epepke on Fri Mar 14, 2003 at 05:42:32 PM EST

The truth may be out there, but lies are inside your head.--Terry Pratchett

[ Parent ]
Electric fish at home (none / 0) (#49)
by Simon Quellen Field on Wed Mar 26, 2003 at 07:27:32 PM EST

Several common aquarium fish produce electric signals you can easily detect with simple equipment costing less than $20.

For a quick introduction, see this page.

Free book: Science Toys You Can Make.
[ Parent ]

Somewhat OT, but... (4.00 / 3) (#15)
by subversion on Wed Mar 12, 2003 at 06:25:31 PM EST

This is pretty similar to how the human hearing mechanism works - small hairs, disturbed by motion in the inner ear fluid.

Any thoughts on this as either an aid to artifical hearing research or a method of surgical replacement of the hairs within the inner ear, which tend to grow damaged as time goes on?

If nothing else, maybe it could assist in research into how the inner ear works, maybe leading to better hearing aids and hearing aids that could work in circumstances where the current techniques don't.

If you disagree, reply, don't moderate.

The State of the Art (4.00 / 1) (#39)
by MilTan on Thu Mar 13, 2003 at 12:36:21 PM EST

The current approach, as explained here is not to replace the hairs themselves, but instead to implant devices which trigger the same nerves that the hairs do. Given that, as mentioned elsewhere, we simply model the eardrum instead of the hairs in building a microphone, this seems to be a fairly intriguing approach. Thus, in this case, it might not be necessary to replace the hairs themselves if the same effect can be had by different means.

[ Parent ]
Nice link. (none / 0) (#44)
by subversion on Thu Mar 13, 2003 at 09:56:16 PM EST

Exactly what I was getting at, to some extent.  However, the problem that was outlined in the link was that we can't provide enough channels.  Instead of trying to miniaturize enough processing to channelize a single signal (like current cochlear implants do), why not implant hairs and tie them to the nerves thereby allowing us to provide the massively parallel capabilities of our own ears via an artificial means?

If you disagree, reply, don't moderate.
[ Parent ]
Wrong way to do it (4.60 / 5) (#16)
by trhurler on Wed Mar 12, 2003 at 06:32:55 PM EST

If you try to do finite element analysis, combine that with velocity information based on other sensors and water velocity information based on propulsion settings and velocity and so on, combine all that into one big picture, and do it in realtime, then even the computers of a century down the road will probably be useless. This is not a mathematically reasonable problem. Fish don't do it this way, and neither should we.

The key is to empirically determine the domains and ranges of a set of functions that map from sensor states and other inputs and settings to given environmental conditions, then successively refine and add to this set of functions until you can do useful things with them. That's how fish do it, more or less(or rather, how the evolution of fish has brought them to do it.) Of course, this takes time and makes product development a slow process, but it also can yield results, which is more than I can say for trying to do hundreds of thousands of finite element anaylses simultaneously and nearly instantaneously, then combine that with other data, generate a picture, and do this often enough to provide a useful navigational ability.

'God dammit, your posts make me hard.' --LilDebbie

La, la, la, birds do it ... (4.33 / 3) (#21)
by pyramid termite on Wed Mar 12, 2003 at 06:45:38 PM EST

... bees do it,
goldfish in the privacy of bowls do it,
let's do it, let's empirically determine the domains and ranges of a set of functions that map from sensor states and other inputs and settings to given environmental conditions, then successively refine and add to this set of functions

mmm, doesn't scan.

Someone modslap me, I'm being silly.

On the Internet, anyone can accuse you of being a dog.
[ Parent ]
Lay off the crack (none / 0) (#25)
by trhurler on Wed Mar 12, 2003 at 07:25:05 PM EST

Seriously. A weed habit is sometimes ok, but crack gets you every time.

'God dammit, your posts make me hard.' --LilDebbie

[ Parent ]
Why immitate nature? (4.00 / 2) (#17)
by vadim on Wed Mar 12, 2003 at 06:37:52 PM EST

Really. Another poster pointed that our ears work in a similar way. But that's not how we record sound. Instead of messing with ears and trying to get sound from the movement of tiny hairs we made the microphone. And now we've got devices that can reproduce sound from the vibration of a glass.

Wouldn't it be possible to do this same thing in some other way?
<@chani> I *cannot* remember names. but I did memorize 214 digits of pi once.

actually, we *did* mimic ears (4.00 / 1) (#19)
by janra on Wed Mar 12, 2003 at 06:43:24 PM EST

Not the tiny hairs part of the ear, but the eardrum, the membrane that transmits the vibrations in the air to the fluid inside our ear. A microphone is also a membrane that transmits vibrations and generates an electrical signal.

Discuss the art and craft of writing
That's the problem with world domination... Nobody is willing to wait for it anymore, work slowly towards it, drink more and enjoy the ride more.
[ Parent ]
Well, kind of, I suppose. (none / 0) (#23)
by vadim on Wed Mar 12, 2003 at 07:05:44 PM EST

I found this though. We can also capture sound by shining a laser at an object. Although I suppose this makes that object the membrane.

My point was that copying nature doesn't seem to always be the most efficent thing to do. After all we drive cars and not Mech-like things.
<@chani> I *cannot* remember names. but I did memorize 214 digits of pi once.
[ Parent ]

The wheel was quite an invention wasn't it? -nt- (none / 0) (#35)
by iasius on Thu Mar 13, 2003 at 03:27:33 AM EST

the internet troll is the pinnacle of human evolution - circletimessquare
[ Parent ]
kind of flawed analogy (4.50 / 2) (#29)
by suntzu on Wed Mar 12, 2003 at 09:10:45 PM EST

Recording data isn't nearly on the same level of difficulty as processing it in real time and making decisions based on it. In addition to capturing sound, the cillia in our ears helps us maintain balance and figure out our orientation relative to the direction of gravity. They also help determine locational aspects of sound. So, no, a microphone isn't nearly as robust as a set of ears.

This project isn't looking to record stuff using the mimicked-fish-sensor. It's looking to use that sensory apparatus to successfully navigate an uncharted environment, and so that other tasks (one of which may be recording) can be performed.

[ Parent ]
I agree (none / 0) (#48)
by werner on Wed Mar 26, 2003 at 12:47:44 PM EST

Nature has often found very cunning ways of doing things well beyond our engineering capabilities, but the fundamental differences in human and natural technologies often makes imitation pointless.

Imagine driving to work in a car with legs, or flying on holiday in a plane which flapped its wings.

Absurd, I know. What about artificial hearts? Making one which pulses is pretty tricky, but one which just pumps constantly is no problem, and I believe they work, too. Mother Nature, on the other hand would have a really hard time building something on wheels or any kind of "constant" mechanisms - wheels which roll and roll, pumps which produce a constant stream - with the building blocks she has.

Meat likes to go back and forth, metal likes to go round and round.

Hairs sound like a good idea, but I think thousands are over the top. Why not a few dozen of different lengths, upto 30cm in length? Many thousands sounds like overkill. Of course, I know nothing about hydrodynamics.

[ Parent ]
serious computing power (4.33 / 3) (#26)
by radish on Wed Mar 12, 2003 at 08:29:42 PM EST

serious computing power is quite the understatement.  algorithmic modeling of complex patterns is fine for simulation but pitiful for real time analysis.  it's like the difference between vector and bitmapped graphics - chaotic data is very inefficient to translate into algorithms and vice versa, but if you use a model that matches the data you can employ tricks to make the job a lot easier.

thus, at risk of borrowing further from nature, neural nets are the obvious way to go for something like this.  you have to train them, but you're trading a reasonable up-front development effort for huge gains at runtime.

very cool sensors though.  neural net integrated circuits are bound to gain some momentum pretty soon, and micro- and nano-engineering seems to be blossoming.  maybe proper artificial critters are pretty close after all...

neural net issues (4.00 / 2) (#28)
by suntzu on Wed Mar 12, 2003 at 09:06:38 PM EST

The unsatisfactory thing about that, is that while it will perform well in most situations, it can leave many things unaccounted for, which could cause very costly equipment failures.

But, of course, those are the conditions that fish operate under. And if a predator attacks it in a totally novel way, it's dead. It's just that, it's been naturally selected for long enough that it "knows" its environment pretty damn well.

So that means you'd have to do a hell of a lot of training on your neural net. Still, you're probably right. At least for the forseeable future, the trained neural net will outperform the algorithmic analyzer that tries to "figure out" exactly what any given flow means.

[ Parent ]
Boundary layer (3.00 / 3) (#31)
by phliar on Wed Mar 12, 2003 at 11:48:43 PM EST

The laminar flow, in such a mixed flow situation, is also often called the "boundary layer" because it exists along the surface, or boundary, of an object. Past the boundary layer, the fluid abruptly becomes more turbulent...
Hmmm... I'm not an aerodynamicist, so maybe I'm mistaken — but isn't the boundary layer the layer between the surface (where the local velocity is zero) and where the local velocity is the free stream velocity? The free stream is laminar, and the boundary layer may be either laminar or turbulent.

My interest in aerodynamics is from radio-controlled sailplanes. We obsess about where the boundary layer makes the transition from laminar to turbulent, and where the flow detaches from the surface. The Reynolds' number (Re) is right at the boundary (∼5·105) — some airfoil sections benefit from an early transition, and some from delaying the transition. Additionally, at these Reynolds' numbers the flow sometimes re-attaches after detaching, forming a "laminar separation bubble." One of the people working on low-speed aerodynamics is M. Selig, at the U. Illinois at Urbana-Champaign.

Faster, faster, until the thrill of...

aerodynamics or hydrodynamics? (none / 0) (#34)
by janra on Thu Mar 13, 2003 at 01:19:06 AM EST

You're quite right about the definition of the boundary layer - it is the layer between the surface and the bulk flow. However, in water, that layer is more often laminar than turbulent. Not sure about air; I haven't studied fluid dynamics with respect to air that much :-)

The bulk flow (I guess called the "free stream" in aerodynamics) is, at least to my knowledge, more often at least partially turbulent, except when the flow is laminar all the way through, as sometimes in a smooth pipe. I hated fluid dynamics, so even though it's only 2 or 3 years since my course, I can't properly back it up without finding my textbook, but that's what I remember. Or maybe that was just the only cases the text covered, but I'm pretty sure it was covered the most because it was the most common, at least from a chemical engineering point of view. Laminar boundary layer was basically always assumed, when we had to deal with edge effects.

Also, I remember that the boundary layer separation was generally most likely to happen at or past the widest part of an object, when the surface was "pulling away" from the flow and either the object or the flow was travelling quickly. Unless you did funny things to the surface to make it separate early, as in golf balls and their dimples.

Discuss the art and craft of writing
That's the problem with world domination... Nobody is willing to wait for it anymore, work slowly towards it, drink more and enjoy the ride more.
[ Parent ]
Incompressible flow so aero = hydro (1.00 / 1) (#42)
by phliar on Thu Mar 13, 2003 at 02:17:41 PM EST

I know nothing about handling compressibility effects... for ordinary subsonic speeds, hydrodynamics is the same as aerodynamics, eh?
The bulk flow (I guess called the "free stream" in aerodynamics) is, at least to my knowledge, more often at least partially turbulent, except when the flow is laminar all the way through, as sometimes in a smooth pipe.
Ah, pipes! So you consider wall effects? In aero, there is a well-defined free-stream velocity — it is the negative of the aircraft velocity. So since (assuming the atmosphere is at rest, i.e. ignoring turbulence from ground-based objects, wind-shear etc.) the atmosphere is still, in the aircraft's frame of reference the free stream flow is laminar.

One other difference: what kinds of Reynolds' numbers do you usually work with in fluid dynamics? My area of interest is about 0.5 million. In this region boundary layer separation seems mostly driven by the pressure distribution. The boundary layer itself can be laminar or turbulent; "tripping" the boundary layer with a small discontinuity to make it transition from laminar to turbulent adds energy to it and allows it to stay attached longer, just as with golf balls' dimples. Recently there has been some succesful work in designing airfoils to control the "separation bubble" and encouraging early re-attachment.

Full-scale sailplanes fly at much higher Re and try to maintain laminar flow as far back as possible. Glider pilots often wash and wax their wings before contests since surface imperfections can easily trip the flow.

Faster, faster, until the thrill of...
[ Parent ]

Biology vs. Technology (2.50 / 2) (#32)
by phliar on Wed Mar 12, 2003 at 11:53:44 PM EST

Despite the fact that fish are not particularly intelligent, they can still beat our best computers when it comes to real-time analysis of fluid dynamics...
Well, the state of the art in computer vision research is nowhere near as sophisticated as human vision, and we don't even have the best! However, evolution has been working at the problem a little longer than the engineers. Give them another couple of decades.

Faster, faster, until the thrill of...

"Sixth sense" a fishy perception | 49 comments (33 topical, 16 editorial, 0 hidden)
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