Gravity's Effect On Perception

You may have, at some point in your life, noticed gravity.  Good!  It's super-important.

Gravity has been with us for our entire biological evolution.  Thus it only stands to reason that our biology would, when possible, take advantage of its influence.  For instance, since gravity on Earth is always pointing downward (technically inward, as a central force), it makes for an extremely reliable vertical compass.

FUN FACT:  the strength (and indeed, even direction) of gravity on the Earth's surface actually varies from location to location, dependent on factors like altitude, the local topography (say you had some mountains nearby) and geology (distribution of density).

Sauce

Local gravity can even change over time, due to differences in water levels and such.  Here's the gravity in South America shifting as the floods and recedes (holy snickerdoodles we're smart enough to watch gravity change from space):

Oh right I was going somewhere with this.

So, important and unavoidable as it is, gravity has been a perpetual influence in our evolution.  How else would you walk and balance yourself, how else would you know the orientation of your head, if not for an ever-present acceleration?

It also makes sense that gravity's influence would have seeped into other aspects of our brain.  Here we get to the whole point of this post: scientists conducted some visual illusion experiments on astronauts before, during and after trips into space, and found an interesting trend concerning vertical perception of these illusions.

Here's the Inverted-T illusion (wherein each line is actually the same length), with a graph detailing the perceived size difference by participants at various times:

They also asked participants to draw squares and crosses, measuring the horizontal-to-vertical ratio.

The extremely wide error bars are due to the small sample size, but the trend exists nonetheless.  As they spend time in space, a seemingly gravity-induced perception of vertical length dissipates.

One of the primary causes of speciation (if not the primary cause) in biology is some sort of geographic or geologic separation.  I can think of no geographic separation more significant than departing the planet altogether.  As people take longer and longer trips (longest cumulative time belonging to Sergei Krikalev with 803 days), it will be interesting to see the effects on human physiology and psychology, and how that will confer benefits and differences.  Sure, bone and muscle density decreases, but who needs bones in space?

h/t Neuroskeptic (amidoingitrite?)