Does the brain always think of an appropriate response when reacting to a stimulus?

For example, when seeing things, does the brain have to think of an appropriate response to the eyes?

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    $\begingroup$ From the two answers seems this really unclear question. $\endgroup$
    – h22
    Jan 10, 2017 at 13:29
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    $\begingroup$ This question seems sort of oddly upvoted and answered - it is pretty low-quality and unclear what you are asking compared to the normal standards for Biology.SE. What does "appropriate" mean? $\endgroup$
    – Bryan Krause
    Jan 10, 2017 at 16:32
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    $\begingroup$ This is the most vague question I've ever come across. What are you on, exactly? And why was this upvoted? $\endgroup$
    – user28959
    Jan 10, 2017 at 19:22
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    $\begingroup$ @Servaes It was in Hot Network Questions $\endgroup$ Jan 11, 2017 at 10:35

3 Answers 3


Well, not always but most of the time; reacting to stimuli is what defines the nervous system and the part of the nervous system that includes the brain is called the 'central nervous system' or CNS. Reactions to stimuli (nerve impulses) don't always have to pass through the brain as the CNS consists of the brain and the spinal cord. This means that in the case of thinking, if what you see (or sense) is something harmful (a man with a knife, feel extreme heat) then you will react without the nerve impulse having to pass through your brain (instead it will simply pass through your spinal cord). This is because the body does not need nor want to waste time thinking about an obvious and protective response. This reaction that requires no conscious thinking is called a 'reflex reaction' or an 'automatic reaction'. If you want to know more about this you should probably look into the nervous system.

  • $\begingroup$ If you take the stimuli as light entering the eye, then this answer is a bit misleading, since even basic object recognition (e.g. to identify dangerous objects) is going to be performed by visual cortex in the brain. Yes other parts of the response may not be in the brain, and even if in the brain, it is not too clear what the OP means by "think". $\endgroup$ Jan 10, 2017 at 11:08
  • $\begingroup$ @NeilSlater: The contraction of the iris when the stimulus is bright light entering the eye is an interesting one. It does involve the midbrain, so it's not like the knee-jerk reflex that involves only the spinal cord. But it might serve as an example which the question doesn't consider under the heading "the brain thinking of a response", if "thinking" is intended to refer to activity of the cerebral cortex (including image-recognition). I do agree with you though: this answer says that the reaction to seeing a man with a knife involves only the spinal cord, not the brain, which is false. $\endgroup$ Jan 10, 2017 at 11:58
  • $\begingroup$ ... in fact I think arguably the retinae could be considered part of the brain. In any case they connect to the brain via the optic nerves, and the spinal cord isn't in that path at all. $\endgroup$ Jan 10, 2017 at 12:02
  • $\begingroup$ @Steve Jessop: Not only is it false, there's still the whole question of whether the response the brain comes up with is actually appropriate. Present the same "man with knife" stimulus to a random sample of people, and I think you're likely to get a wide range of responses. $\endgroup$
    – jamesqf
    Jan 11, 2017 at 8:26
  • $\begingroup$ @jamesqf of course it goes without saying that each individual's reaction will vary due to several factors: situation, environment raised in, personality, self-confidence, etc. However, it I no question that every single person WILL react and upon impulse too; since not a single person will do nothing at all (obviously) $\endgroup$
    – Alex P
    Jan 11, 2017 at 8:39


Optical illusions trick your brain and elicit an inappropriate response. Of course optical illusions don’t need to be engineered, they abound in reality (remember The Dress that went viral two years ago)?

In addition to optical illusions, other senses can be tricked. There are auditory illusions, an illusory sense of touch, etc.

  • $\begingroup$ I can barely look at that. What's actually going on in it? It's like it's moving. Wow! It's weird. Now it's sliding. It's worse if you focus your eyes, if you unfocus your eyes it's easier to look at. I still can't figure it out. What's it doing? $\endgroup$ Jan 10, 2017 at 18:06
  • $\begingroup$ @DanielCann It’s really just straight, equidistant, parallel lines with black/white coloured square boxes between them. The sequences of boxes in each row are offset at different distances from the left side. That’s all there is to it. $\endgroup$ Jan 10, 2017 at 18:11
  • $\begingroup$ How do you know? We're both human, and it's an optical illusion which is designed to deceive humans. $\endgroup$ Jan 10, 2017 at 18:20
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    $\begingroup$ @DanielCann Well I mean you can simply check with a straightedge and a set square/triangle/compass … $\endgroup$ Jan 10, 2017 at 18:43
  • $\begingroup$ @DanielCann: or assuming you trust your PC not to conspire with Konrad to trick you, zoom in on the image in any image-editing program. The illusion doesn't work on a small enough section. Then you'll see that nothing's moving and everything in any given small section of the image is straight and square. Pan around the image until you're convinced it's true everywhere. $\endgroup$ Jan 14, 2017 at 19:30

The answer to the question is if thinking is always a response to some external stimuli depends if we consider the memory. Thinking about the event in the past is surely possible many years after the stimuli is long gone. Continuous stimuli from the environment may not be required for the thinking process itself as spontaneous brain activity exists as well.


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