I started considering the connection between sex and perception because of an article on the James Randi Educational Foundation's website called "The Lurking Pornographer: Why Your Brain Turns Bubbles into Nude Bodies." In it, we find photos that lead to the rather startling conclusion that when a swimsuit-clad individual has the swimsuit covered up by strategically placed blank space, our brain makes the executive decision that the person in the photograph is naked.
Don't believe me? Take a look:
And lest you think it's just because men are sex-obsessed, it works for photographs of guys, too:
That you have a white triangle overlaid on top of a triangular outline and three black circles is a simpler explanation than having three V-shaped bits and three black Pac-Man shapes all laid out just so as to appear to make two triangles. But amodal completion, like any inference based on incomplete information, can also get it wrong. Consider the horse(s) and cat(s) in the following drawing:
Two horses (in the left-hand drawing) and two cats (in the right-hand one) are merged into one by the brain "forcing" a wrong interpretation -- amodally completing the two animals into a single, extra-long horse and cat because of trying to fill in the missing pieces in the simplest possible way.
Likewise, when we have no other information about a person -- all we see is skin -- inferring a swimsuit seems like a jump. The easier solution is that they're running around naked. And of course, the fact that this stimulates our brains in a different way makes us stick with that solution once we've arrived at it.
It's like our neural network is hardwired with a perceptual form of Ockham's Razor, especially when the simpler solution is one that's kind of fun to look at.
Still, there's no doubt that most of our brains are obsessed with sex. The three chemicals that mediate the majority of the sexual response -- dopamine, oxytocin, and endorphin -- are a mighty powerful cocktail, and none of them have much to do with thinking. Interestingly, dopamine is the same neurotransmitter that is involved in addiction -- which may explain why the "sex drive" is called a "drive."
Oh, and about all of the claims that men think about sex twice as much as women do; there may be something to it. According to recent research by Dr. Ananya Mandal, the preoptic nucleus of the hypothalamus, which is one area of the brain involved in sexual response and mating behavior, is 2.2 times larger in men than in women. Size apparently matters -- in that respect, at least.
Anyhow, I thought all of this was pretty cool. It's always interesting to find out why we do what we do. It's why I found Desmond Morris' classic book The Naked Ape so fascinating when I first ran into it, at age 17 -- I'd never before considered human behavior from the standpoint of looking at humanity as if we were just another animal species. And far from being demeaning, that perceptual shift leaves me feeling interconnected to the rest of the natural world in a far more intricate way. We have reasons for doing what we do, just like every other living thing on Earth -- including the birds and the bees.