1000 ms? lol
You have to be more specific about what you mean by 'respond'. There are several different levels of response.
become aware of change?
Our brain receives and processes information via neurons. Neurons rely on ion exchanges occuring across their membrane to transmit impulses.
The process of actually sending an impulse and resetting the neuron is quite involved... the Na+ ions have to bump their way through the ion channels into the cell, the K+ bumps it's way out, then the K+ is pumped back in and the Na+ out... lots of steps basically. The cell can do this remarkably quick considering it's complexity but the bandwidth of a neuron is only ~0.5 - 1kHz, so it can send a maximum of 500 to 1000 signals per second.
Therefore, the maximum temporal resolution of a neuron is about one or two milliseconds, ignoring the time it takes for the impulse to actually travel down the neuron's length - which isn't long but it's still a delay.
It depends a lot on what you're sensing and how many neurons you have in place. Humans usually take a few tens of milliseconds to respond to something because their brain has to actually do something with the information.
If it's a pin prick in the finger, the response is to pull away regardless of the cup of tea behind your hand. But if it's a more complex problem, it'll take longer to come up with a suitable output.
This is where it's important to use very specific questions. For example... when you're not trying to work out any kind of complex solution, the response time is shorter (because you don't need to come up with a solution and then do it). I think this is what you're talking about, roughly how long does it take for something to enter our consciousness from it happening.
Music is a good example. Delays are used a lot in the music industry to put all kinds of effects on the artists' voices. A delay of hundreds of milliseconds if easy to hear as separate echos and it makes the person sound like they're in a big room, like a church. If the echos are musically in time, it helps join the song together by providing an echo of the last note to reference to the new one being played or sung - important for something like a guitar solo where you can have lots of notes being played one at a time.
But if you shorten the delay time down towards tens of milliseconds, it gets quite hard to hear the gap between the original note and the echo. Instead, you start getting a chorus sound, that sounds like there's two people or instruments producing a sound at the same time, just note quite in sync with each other. This effect is used a lot to 'fatten up' the voices of people on practically every tune you hear now. Either they get a processor to do it or they just get the artist to sing the track a few times over themselves to make it sound like they have a really thick and strong voice. A lot of Slim Shady's tracks have tonnes of it on. Listen carefully and you'll hear it. The trick to these effects is to make them subtle enough that most people will never be able to tell what's happening.
Below ten or twenty milliseconds, the extra voice disappears and instead you start hearing all kinds of funny phase interactions that make a 'swooshing' noise like you'd get if you were washing out a really wide pipe with a jet washer.
Even in this example, your brain is processing the sound to try and work out what it's hearing. So it's not a 100% pure example of the time it takes just to get to your consciousness.
Judging by the impulse time of a neuron's membrane and the kinds of effects I've looked at to do with things like music... I would estimate it probably takes a few ms for a signal to get into your brain, probably a bit more.
Your ears are able to hear up to 20kHz not because the neurons are carrying a 20kHz signal but because they connect to a hair that can vibrate that fast, and when it does vibrate it signals a nerve that represents a 20kHz vibration has been detected - it doesn't carry the actual 20kHz signal down it.
You may be interested to know, as we're on hearing, that your brain hears things as 16db louder if there's any kind of intelligence in the sound. You'd hear my voice as 16db louder than the background noise around us, even if I was talking at the sound volume as the noise. Your brain 'turns up the gain' for intelligent signals. This is actually a trick exploited to supress noise in electronics... white noise that doesn't make any sense is purposefully injected into the signal sometimes to mask over any intelligent noise related to the original intelligent signal.
It's also interesting to note that while our neurons have a max of ~0.5 - 1kHz, the best transistor last time I checked was working at 604GHz - 604 million times faster.
Gimmee a minute. I'm still thinking.
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