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Today me and my friend were coming on motor bike and I was sitting opposite direction because I was holding something in my hand (and it was fun :P). When he started bike I felt very high acceleration. I asked him to go slow however he said he is going with regular speed. Then out of curiosity, me and him tried different velocities and different accelerations while me sitting on either direction of acceleration and opposite to it. To my surprise I 'perceived' more acceleration when I was sitting opposite to direction of acceleration. While searching how we perceive acceleration, surprisingly it is not very well known. I found following explanations,

  • We overestimate arrival time ( ref: this )
  • Endolymph from Vestibular system ( ref: Wiki ) [Not exactly acceleration but rather balance]
  • Interpolated motion segments ( ref: this )

My question, Is it just visual perception or special mechanism for detection of acceleration? In any of these cases, why I was feeling more acceleration while siting opposite than in direction of acceleration? [ My initial guess is because I didn't have any support and visual cues were messed up ]

Update:

I found this post but I didn't get any satisfactory answer from book referred in answer.

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  • $\begingroup$ Interesting question. However, it is unclear to me whether the question is on the perception of acceleration (a), the perception of velocity (v), or both. In other words, did you perceive both a and v to be higher when sitting in reverse? Moreover, are you confident it has nothing to do with you being uncomfortable in reverse position on a bike? Personally, I would find it awkward and perhaps even scary to sit in reverse. Last, did you switch places with your friend? Any additional data, or is this an isolated anecdote? $\endgroup$
    – AliceD ♦
    Sep 22, 2015 at 11:43
  • $\begingroup$ I don't think we visually perceive acceleration. We just estimate it. However, we can perceive acceleration because of the vestibular system. Am I right @AliceD ? $\endgroup$
    – WYSIWYG
    Sep 22, 2015 at 11:43
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    $\begingroup$ @AliceD Yes, we switched positions. He also felt similar. Maybe you guys can also try :P. I did feel change in acceleration. It was hard to control precisely but we checked by changing bike speed from 0 to X in Y seconds. Change in velocity was little bit crude. It was just checking (rather 'feeling') velocity when bike has achieved constant speed. $\endgroup$
    – Dexter
    Sep 22, 2015 at 11:53
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    $\begingroup$ @Dexter What if you close your eyes? Did you try that too? To eliminate visual contribution. Moreover I guess we would be more cautious towards backward acceleration because it may give a feeling of falling. $\endgroup$
    – WYSIWYG
    Sep 22, 2015 at 11:58
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    $\begingroup$ The otolith detect linear acceleration (ncbi.nlm.nih.gov/books/NBK10792). The semicircular canals detect angular acceleration. $\endgroup$
    – kmm
    Sep 22, 2015 at 13:25

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My question, Is it just visual perception or special mechanism for detection of acceleration?

Acceleration is a synthesized conclusion from a multitude of systems.

Most prominent is the Endolymph system you already mentioned:

Wikimedia commons -- Otoliths

As you accelerate, the endolympth and the otoliths within (small, calcified deposits) pass over the hair cells and produce action potentials which travel to the brain. The higher the magnitude of acceleration, the more action potentials will be sent.

This is also where dizziness comes from, as the endolymph and otoliths do not come to rest at the same rate, momentarily giving the brain two different interpretations of what acceleration you're experiencing.

There is also the Doppler Effect:

Doppler Effect -- Wikipedia

Based in the cochlear system of the inner ear this time, instead of the vestibular system, the doppler effect is interpreted by the brain and gives a very rough interpretation of both velocity and acceleration (if the velocity of the object happens to be changing).

Then there is the entire visual side of things. Motion blur and the speed at which the perception of objects change size are large contributors to our sense of visual acceleration, but almost all of this is done via complex manipulation of visual data in the occipital and frontal lobes.

There are also countless minor contributors: skin and join tension (the sensation of weight due to acceleration), ease of breathing, sensory cells in your hair if your hair is exposed to the open air while acceleration, blood pressure sensors (and numbness when the body can't compensate), etc.

In any of these cases, why I was feeling more acceleration while siting opposite than in direction of acceleration?

My guess would be because your body is more visually attuned to facing the same direction as the acceleration, and when you were facing the opposing direction your mind overcompensated in an effort to protect you since you probably don't get a lot of opportunities to rapidly accelerate backwards over distances of more than a few feet.

Your brain might also have been panicking a bit because you couldn't see where you were going (usually very bad), and the heightened sensory state made the magnitude of the acceleration feel greater.

These are my best guesses, however. If someone has an academic source, please feel free to edit this answer!

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  • $\begingroup$ I didn't get Doppler effect part . $\endgroup$
    – Dexter
    Sep 22, 2015 at 14:19
  • $\begingroup$ @Dexter -- The doppler effect is the change in pitch of a sound due to the velocity of its source. If the source of a noise is moving, the sound waves in the direction of its velocity/acceleration are condensed (higher pitched), and those in the opposing direction are elongated (lower pitch). You can tell if something is heading towards or away based on that. A classic example are race cars or racing bikes, whose engine noises change pitch to an observer despite constant speeds: youtube.com/watch?v=qyxvPiOqEiE $\endgroup$
    – MCM
    Sep 22, 2015 at 19:32
  • $\begingroup$ Are I know what Doppler effect is. I was wondering how that is playing role in perception of acceleration ? $\endgroup$
    – Dexter
    Sep 23, 2015 at 3:31
  • $\begingroup$ The otoliths are not responsible for dizziness. The moving endolymph is. The endolympoh in turn does not recognize acceleration, but direction of movement afaik. $\endgroup$
    – AliceD ♦
    Sep 23, 2015 at 4:20
  • $\begingroup$ @AliceD - "the endolympth and otoliths..." I specified both. The otoliths are partially responsible. The vestibular system is designed to track 3-axial movement (XYZ), and the frequency of action potentials changes depending on how many hairs are open and how "open" they are. Unless I'm very, very incorrect, the vestibular system tracks acceleration in addition to vector. $\endgroup$
    – MCM
    Sep 23, 2015 at 14:39

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