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Let me start off with a couple of metaphors.

In the third Lord of the Rings film, a signal for help is sent from Minas Tirith to Gondor, using a chain of beacons. When the keepers of one of these beacons sees that the one next to his lit, his orders are to light his own thus passing on the chain reaction. When the last one within sight of Gondor is lit, the message that Minas Tirith wants help is received. There is nothing conveyed in the fire itself, no information within the flames, the beacons are either on or off. The only reason the beacons convey a message is that the citizens of Gondor know why that beacon would be lit.

Imagine a simple pressure sensor inside a building that is connected by a wire to an electric light on a control panel in a security room. When someone steps on the sensor it completes the circuit which sends electricity straight to the light turning it on. A label on the light says what it's connected to and where it is. So that when the light comes on, the person at the control panel knows someone is in that sector of the building. But to be more precise, he knows someone has activated that particular pressure sensor, if it wasn't labelled he would have no idea what the message meant.

My understanding of the human nervous system is that it works in the same way. Based on information I learned from children's educational programs as a child and a few bits I picked up since.

Our ears have sensors designed to respond to sound. Each and every one has a direct connection to the brain via a chain of nerve cells. When one of the sensors is sufficiently stimulated, it zaps the neuron it's connected to, this in turn causes the neuron to zap the next one and so on all the way up to the brain.

The impulses/spikes themselves contain no actual information, they are just pure electricity, every spike contains the same amount of voltage. The only reason the brain is able to interpret them is because it knows exactly where each and every chain is connected to. When the impulse reaches the brain it knows it comes from a sensor in the ear that is responsible for a particular level of sound, the brain takes these signals and translates them into the sounds we hear. Nerve cells work something like binary, they are either on or off with nothing in between. The frequency of the spikes going along the chain indicates how intense the sound is. If the brain didn't know where that chain was connected to then the signal could mean absolutely anything.

This is supported by the condition known as synaesthesia where, because "wires are corssed" in the brain there are people who hear colour or taste music.

My questions are, is my reasoning correct? Is this exactly how nerves and perception work? Has this been scientifically proven beyond a doubt? Is there something within the impulses that contains additional information? Are the workings of the nervous system debated in scientific circles? I've been told we've never been able to plug an eyeball or any other kind of human sensory device into a computer so we've never been able to examine it as it's working, is this true?

But most of all, does this mean there are things our senses cannot detect? I already know there are things beyond our range of vision and hearing and the brain blurrs then together. What I mean is this...

I know that we have different senses in our ears that are designed to respond to higher or lower sounds. Say we have a sensor that responds to a level 1 sound (I don't know the unit of measurement for sound) and next to it is a sensor that responds to a level 2 sound. What happens if we try to hear a 1.5 sound? Is it not detected by the ear because we don't have sensors in that range?

Can we only sense something if we have a sensor for it and are those sensors only built to sense one ultra specific thing?

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I'll attempt an answer, but please know you don't have a clear nor defining question to address. This is more suited for conversation or open discussion, and I recommend starting a chat on the topic. Please be wary the question may be put on hold. I'll give it a go and respond from the top down. I absolutely don't think this is going to be fruitful for other people. Here goes:

Our ears have sensors designed to respond to sound. Each and every one has a direct connection to the brain via a chain of nerve cells. When one of the sensors is sufficiently stimulated, it zaps the neuron it's connected to, this in turn causes the neuron to zap the next one and so on all the way up to the brain.

Take the example of the eye; the retina is composed of photosensitive neurons, a type of sensory neuron. The inner nose is innervated with olfactory sensory neurons. The cochlea is similarly innervated by neurons. Firstly, you haven't made it clear that the neurons are the sensors. But indeed, the sensory neuron is the first order neuron. The processing of sensory information proceeds through first, second, third and higher order neurons until the information is integrated functionally somewhere in the brain. There exist neurons that modulate the signal along the way (e.g. lateral inhibitory interneurons) that are not on the information highway but rather beside it, regulating the input and output of neurons. It's quite complex, but you have the general gist of the hierarchy of neurons from sensory input (e.g. odor) to output (behavior, response, perception).

The impulses/spikes themselves contain no actual information [...] Nerve cells work something like binary, they are either on or off with nothing in between.

The exact temporal dynamics of this communication highway between neurons is very important. At some level, auditory volume may be coded by frequency, but this is restricted only to some level. Information is very quickly encoded and the 'raw' information sensory neurons provides becomes processed and transformed quite early in the chain. For instance, you may trigger circuits responsible for reflexes, which are (so-to-speak) dormant until a kind of neural activity activates them. Vision is well described in terms of encoding; at the first processing area of the brain for vision, things are simple, and are progressively more high-levels of encoding as the information is processed; there are regions responsible for motion, color, object recognition, even specialized regions for the recognition of faces, for instance.

If the brain didn't know where that chain was connected to then the signal could mean absolutely anything.

Correct. Remember though that the brain is very plastic, and can learn. There are many curious cases in neuroscience where this is apparent. One well-known widely described example: you can wear glasses that invert the world, and this would feel strange. After some time though, your perception would switch the world the "right way up". The retina has been largely unaffected, but the perception and the integration of the information can drastically vary quite independently.

Is there something within the impulses that contains additional information?

Remember that there are two kinds of connections between two neurons. Electrical and chemical synapses. A wave of depolarization does indeed travel through a neuronal axis, but in most cases, this does not occur across neurons. Instead, neurons (and related cells such as glia) release chemical messengers (neurotransmitters and neuromodulators) that act as inter-cell signals. These can inhibit or excite the post-synaptic neuron. Simply put, it's not a string of wires; it's a line of wires that interface with each other chemically; at each interface, chemicals must diffuse across the gaps and interact with the membrane-bound sensors found on the receiving neuron. This is not at all binary. And please know we are simplifying the system in order to describe it.

Is this exactly how nerves and perception work?

This is a simplification of how neural circuits work. How perception works is a completely different thing you have not even begun to address. This is a completely different topic of discussion. Bonus link!

But most of all, does this mean there are things our senses cannot detect?

A sensory system can only detect what it can detect. Your skin cannot detect radio waves. Your eyes cannot detect infra-red light. Your mobile phone, similarly equipped with sensors, cannot detect neutrinos.

I don't know the unit of measurement for sound

Decibel (volume) or hertz (pitch or frequency).

What happens if we try to hear a 1.5 sound? Is it not detected by the ear because we don't have sensors in that range?

You are describing the spectrum of volume as a graded, discontinuous thing. It's not! And more generally, yes. If your sensors cannot detect something, the organism is oblivious to it. Obvious.

Can we only sense something if we have a sensor for it [...]?

Yes. But you underestimate the power of a sensor such as a photosensitive cell. It can transmit information about the amplitude of the light, and responds to a range of light. Moreover, each photosensor has a response spectrum and does not respond equally to different (although similar) wavelengths of light. More so, if you have a patch of photosensitive cells, you can infer motion, direction, speed, size, distance, and all the merry things we associate with vision. The wealth of information is easy to underestimate.

... are those sensors only built to sense one ultra specific thing?

It depends on how you define specific. The equipment of a neuron in the retina is usually specific (e.g. expression of proteins responsible for light sensitivity) but it's possible to have a neuron that is responsive to several things. Have you ever tried to rub your eyes, and you see phosphenes? Your retina is responding to the increased pressure. You could say these cells are sensitive to light and pressure. That's not ultra specific. You can similarly activate these neurons at short range electrically, or magnetically. That's starting to sound like a pretty unspecific sensor, depending on how you define it for yourself.

I absolutely don't mean to aggravate, I very much suggest you do some readings on the topic. It's one thing to have a pub-level confidence in vaguely worded funfacts about sensory circuits, and another to understand and appreciate the nuances and complexity and the many knowns and unknowns of neuroscience. It's not too difficult to grasp. I would instead ask you to ask about fundamentals before you ask to correct your outlook on biological sensory systems. Books are cheap and the internet comes in handy sometimes, and you may even spare someone else's time!

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  • $\begingroup$ I think I was clumsy in trying to get to what I really wanted to ask and I think that's why I came across as a dimwit. I'm going to ask another question, this time specific to my issue. $\endgroup$ – Tailspin May 31 at 23:13

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