Our minds are like a tangle of wires, and those wires are the neural pathways we've developed over the course of our lives.  Since everyone's life is different and everyone's experiences unique, we all have different neural pathways and we're all wired differently.  Humans are social animals, and to get along in society, we need to have certain of those neurons wired up in a more-or-less compatible way with the other humans with whom we're in contact, and I believe that many so-called mental disorders are simply due to an individual having that tangle of wires plugged in a little (or a lot) differently that others.

If in the course of one's life and experience one somehow developed a neural association of, say, "food" with, oh, "aggression," and therefore started behaving very aggressively at mealtime, there's nothing fundamentally "wrong" with that person, it's just that their wiring is incompatible with that of those around them.  There are those who have developed wiring such that they associate "sexual arousal" with "self loathing" (see far left, above) or "anger" (see center, above).  If everyone else were wired the same way, these individuals wouldn't seem so freakish or strange - they only stand out because their wiring is different from the norm.     

Many times, this miswiring is far more benign and not incompatible with societal construct. For example, synesthesia is typically defined as a neurological condition in which responses to certain sensual sensations get "crossed."  It can occur when stimulation of one sensory or cognitive pathway (e.g., vision) leads to an automatic, involuntary experiences in a second sensory or cognitive pathway (e.g., hearing). About 5 percent of the population reportedly has some form or another of synesthesia, and over 60 types have been reported. The most common form of synesthesia is in people who perceive individual letters of the alphabet and numbers to be "shaded" or "tinged" with a color. Others commingle sounds with scents, sounds with shapes, or shapes with flavors.

I often associate perceptions of facial expressions with sounds.  A smile has a certain, very specific sound to me, as does a frown or a scowl.  What's interesting is that while I can "hear" those sounds very clearly even when I imagine the facial expressions, when I try to conjure up the sounds from my memory independent of the smile or the frown, I can find nothing.  I mean, when I try to describe (even to myself) what the sound of a smile is, I simply can't, yet I can hear it as soon as I imagine a smile, or see an actual one.  

The reason, I believe, is that since I'm experiencing a visual sensation (that is, a sight) using the faculty of hearing, but there is no real, actual sound heard by the ear, when my mind searches the "hearing" portions of my memory, there's nothing there.  But when it searches through visual memories and finds a smile, voila, I immediately "hear" that sound.  I can only experience that sound based on visual stimuli, or by remembering or imagining that stimuli, and not directly as a sound or the memory of a sound.   No smile, no "sound" of a smile.

I wonder if the term "synesthasia" could also be applied beyond the realm of the senses.  Could that odd feeling of deja vu be due to the perception of what's happening in current time getting crossed with, and therefore experienced as, the perception of memory?  Could deja vu be considered a form of synesthesia? What about getting signals crossed between pleasure and pain?  Between joy and sorrow?  Affection and anger?  Arousal and shame?