Why do human ears have a lower boundary for the lowest energy perceivable signal than eyes?

Question

I am currently hearing a lecture about human machine interaction. The lecturer is not a biologist (neither am I, we are both computer scientists), but he makes some statements about biology which I think are questionable.

One statement was that the energy of the lowest perceivable signal for eyes is and the energy of the lowest perceivable signal for ears is . He cites those values from the following (German) source:

Schmidtke, H. et al. Handbuch der Ergonomie mit ergonomischen Konstruktionsrichtlinien und Methoden. Koblenz: Bundesamt für Wehrtechnik und Beschaffung (BWB)

HdE (2002): 2.4.1, Abb. 2

[KIT-Bibliothek: 2007 E 1808-1(2) (Präsenzexemplar)]

It is clear that those values vary from person to person. Assuming they don't vary too much, it is interesting to know why the human ear can detect signals with lower energy than the human eye.

According to my lecturer, the reason is because we can hear in all directions, but see only in one direction. He argues that hearing is much more important for this reason to detect predators, even if they are silent.

However, this argumentation seems very weak to me. I would rather think that there is a range in which "interesting" signals occur. If the interesting signals have lower energy for sound than for visual signals the ear adapted to this range.

I guess if the hypothesis of my lecturer is correct, then animals which can see in all directions would (always / very often) have much worse ears. If my hypothesis is correct, then there would be no significant difference in animals with "panorama-vision" and animals without in terms of lowest perceivable energy for ears and eyes (assuming fair comparisions; unfair would be comparing fish with land animals as the medium might play a big role).

Is there any source (written by somebody from biology, not form computer science) which makes a claim about this question?

Answer 1

Preamble. There is a lot of misunderstood science here and you are more than right for questioning the lecturers interpretation of these energy values; something the other answers do not discuss. The problem arises from a dodgy reference and a lot of conjecture.

In summary.

• Light and sound cannot be compared energetically in a biological context.
• Our ears probably don't hear lower energies than our eyes see to avoid prey.

Let us ignore the reference...

The reference is from BWB which is as far as I can tell a non peer reviewed federal technical publisher and that this is a textbook about ergonomics. But given its obscurity, it having never undergone peer review, it being not biologically scientific, it is safe to ignore from the context of a scientific biological topic. The fact it is in German doesn't help either.

If anyone can provide an open link to this reference or correct me on this I would be grateful. Google scholar turned up no hits other than a citation.

Let us give your lecturer the benefit of the doubt. This answer assumes that there are derived energy values for sight and sound as your lecturer suggested and that eyes sense higher energy than ears.

Is hearing omnidirectional to detect predation?

I'm going to use the term sensory setup to describe that our senses use fixed ears that hear omnidirectionally and forward facing directional eyes.

... hearing is much more important for this reason [omnidirectional] to detect predators, even if they are silent.

Let's flesh out this hypothesis so that it makes sense in practical terms.

A small movement is made by the predator. This generates some sound and some change in light. In the case of forward sighted animals, the sound is much more difficult to detect than the light would be as it contains less energy. The ears must compensate by detecting lower energy signals than the eyes would need to in order to detect the predator.

Before we question the physics, this biological hypothesis is very tough to swallow. Without citation I would say it is unreasonable conjecture.

Examples that contradict this hypothesis:

• Gorillas have no natural predators or prey but have a very similar sensory setup to ours that helps social bonds.

• Meerkats on the other hand rely on sentries that broadcast a series of bleats after spotting prey by sight. Other meerkats immediately hide when hearing this signal (interestingly, this call is mimicked by Drongos who steal the meerkat food that is abandoned in the panic) before checking if a predator is present. Meerkats have a similar sensory setup to ours.

• There are other megafauna like deer that are prey for the big predators; predators that would have put ancestral humans on the menu. Deer-like prey animals rely heavily on near-omnidirectional eyesight and can angle their ears to focus on an area to hear stealthy predators. This is the "opposite" sensory setup that has the same conclusion of predator evasion.

Even those non-exhaustive examples makes the hypothesis questionable at best.

...there is a range in which "interesting" signals occur. If the interesting signals have lower energy for sound than for visual signals the ear adapted to this range.

This answer too is based on pure conjecture it seems, however it more aligns with how biology might work.

It is sensible to say evolution selected for animals that could detect noises and lights that, if reacted to, saved the organisms life to continue propagating. This hypothesis bypasses the idea that sound and light are comparable energetic signals too, which is good (discussed below).

This hypothesis also allows for various morphologies to have evolved depending on what an "interesting signal" is to that species.

Light vs Sound.

These aren't the same thing.

The elephant in the room of this question is that light and sound are non-comparable energies. This is due to source, speed, distance, medium, mechanisms for receiving the energy (eyes and ears) and most significantly in a biological context neural interpretation. These values are so abstract that even if an energy value is calculated, it is not in anyway quantitatively comparable in biological organisms. For example a really loud subsonic whale call is not the same as a small change of grass parted by a wolf as it sneaks up on you. One is absolutely essential for survival, the other is undetectable. Both could theoretically have the same "energy".

Note Frequencies are the biologically important part of the "energy" but must be considered as two different methods of acquiring sensory information. The difference between a chimp mating call and a territorial threat are very similar but for their frequency, and eating the delicious red berries compared to the toxic blue berries is also down to the frequency reflected from them!

There is little to no biological literature that uses energy values as sensory input. This would just obscure the actual important physical aspects of the sensory input values.

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I hope that helped clear things up and accurately discussed your dis-satisfaction with your lecturers' hypothesis despite being months too late!

Answer 2

I can't check the sources, but I think they must estimate the relative energy of a threshold-sensitivity signal. I don't think the energy of a sound wave can be assigned an arbitrary value the way a photon can...certainly only a photon can propagate in a vacuum...

Both light and sound waves scatter less, as they propagate, if they're higher frequency/energy. So all things being equal blue light scatters less than red light, and ultrasound scatters less than infrasound. So audible sound is less directional than visible light, and any sound receptor provides information that is less directional compared to any light receptor. So of course sound's a good thing to pick up for alert signals, that doesn't really mean it's been prioritized over vision through evolutionary processes.

Since most of vertebrate evolution occurs in the ocean, where sound and light propagation are quite different than in the terrestrial milieu, the lecturer's conjecture is likely irrelevant. Our sense organs are far more influenced by their evolutionary history than considerations of "best possible" (globally optimal) function.

Answer 3

It is my understanding that the dark-adapted eye can perceive single photons. Those have a well-defined range of energies in the visible spectrum. Also, as others have pointed out, light is very directional and localized: if you are not looking at the source, it will not be perceived. Sound comes to the ears from all directions at all times. The waves are so large that low frequencies cannot even "fit" inside a typical room, so direction is meaningless. Trying to define a particular energy for them when they can vary in both frequency and intensity (unlike photons, which only vary in frequency and number of them impinging) is not likely to be exact.

If I was told that two completely unlike forces differed by a factor of 4, I would say, "could be that one is actually more or less than the other, too close to tell." In electronics, component values can vary over a range of half to twice as much and be in specification.

Generally speaking, the senses evolved to the point where any further sensitivity would result in perceiving noise. Then, quite reasonably, they stopped. If you could hear the air molecules banging on your eardrums (if ears were 10 dB more sensitive) you would go mad. Light does not have this noise characteristic, so we can see the lowest possible value: one photon.