An excellent question! Sensory Processing Disorder (SPD; also known as Sensory Integration Dysfunction, or SID) basically means that one's sensory systems aren't working smoothly; somewhere between receiving sensory input, interpreting the input, and/or responding to the input, there are breakdowns in the system. In some cases, the system is oversensitive; in others, it is undersensitive. There are lots of ways this might "look" in a child's (or adult's) behavior and it can have far-reaching impacts physically, socially, and emotionally.
JoNo's SPD problems mostly have to do with oversensitvity to sensory input and difficult regulating the input and responses. In other words, his brain doesn't accurately interpret sensory input or effectively moderate sensory input/response. For example, light touches (especially unexpected ones) seem more aggravating than they really are; noises sound louder and more piercing; movements (like falling) seem farther (and therefore more frightening) than they really are. He also has a hard time regulating or modulating his sensory input, so he doesn't recognize when he's nearing sensory overload, and he has trouble regulating his arousal levels: he goes from happy to tantrumy-angry in half a second; he has trouble staying asleep at night, knowing when he needs to use the bathroom before it's actually happening, even regulating his body temperature (waking up at night drenched in sweat from his body over-responding to temperature changes).
The good news is that he's really smart and seems to compensate fairly well, so even though it's a lot harder for him to do normal things, he tends to figure out how to do many (though not all) things his own way. The bad news is that he's really smart and compensates fairly well in a lot of areas so people can't readily tell that things that seem effortless are actually a great struggle for him.
While it looks like he's been effortlessly enjoying the play area, his neurological system has actually been on high alert the entire time, trying to make sense of every unexpected and seemingly-too-large noise, touch or movement.
While it looks like he's just being stubborn about wanting to be fed, he really has exceeded his patience and motor planning abilities spooning the first six bites of soup from the bowl, so even though he's still hungry, the task of getting it to his mouth is daunting enough to ask for help.
While he almost never falls down the stairs, you'll notice, if you pay attention, that he is still very deliberate about each step, using both feet on each step and regaining his equilibrium every time, unless his vestibular system has the safety net of mom's hand to hold, when he is able to take the steps with only one foot.
While it looks like he's just being a baby about the fall he's taken, his brain thought he fell 10 feet rather than 2, and even though he knows he's not hurt, he's really shaken up.
While it looks like he's just greedy and territorial, he's really just protecting his personal space to ward off as much unexpected sensation as possible.
While it looks like he can't possibly be oversensitive to touch when he can't seem to keep his hands from touching everything he sees, or to sound when he seems to enjoy making lots of it with his voice or "instruments" around the house, he's really self-medicating; his brain knows that the only way to blaze those neurological trails is to use them over and over and over, and without a good sensory trail in his brain, every touch really does feel new, like he's never touched it before.
While it looks like he's just a picky spoiled eater, he's really having trouble getting past new textures, strong (or bland) tastes and smells, and even coordinating the feel of chewing and swallowing without panicing.
While it may seem that he wakes up several times at night because he's spoiled and knows we'll come to him, it's really a problem with his brain keeping his arousal levels low enough to stay asleep. He'd keep sleeping if he could, but his brain won't let him.
While it looks like he's got a temper that is off-the-charts and out-of-control, he's really amped up about all the extra sensory input he's receiving, and he doesn't even notice that he's getting overwhelmed until the moment it rushes out. He's having to manage bigger-than-age-appropriate emotions with less-than-age-appropriate sensory skills to regulate his body's response to stress.
While it's validating to know that my mommy instincts were right that something wasn't right and I'm able to get some help with figuring it all out now, it's also really tough learning to accept and understand how to cope with having a child with special needs, especially one that others won't automatically recognize as having special needs. More on coming to grips with that in future posts . . .