As with oversensitivity, undersensitivity can also sound like a good deal. The ability to screen out excess input is one we value, and being especially adept at it should, theoretically, allow a child to focus only on things that are vitally important. You may admire the kind of hyperfocus that inspires inventors to invent, actors to put on stirring displays of make-believe, writers to create worlds out of words, all the while shutting out whatever deflects them from that goal.

For the children with sensory integration disorder whom we've called under-reactive, undersensitive, hyporeactive, or hyposensitive, however, even things that are or should be of compelling interest get screened out. That makes paying focused attention harder, not easier.

Think again of that radio, the one that was turned up so loud you couldn't think of anything else. But this time, imagine that the radio is faint and clouded with static, and to get any music at all you have to spend every second fiddling with the controls, turning the antenna every which way, and straining to get any useful content. It's a frustrating and unproductive process, and it takes all the attention and ingenuity you have.

Maybe sometimes you just give up. Other times, when there's news you need to hear or a program you really want to catch, you stand on your head with that antenna to try to make that signal come in. This is the experience your child has with any sensory input to which he is hyporeactive.

It's a cruel irony that much of the behavior that bothers you in your hyporeactive child is in fact his desperate attempt to pay attention. He is fiddling with every control in his body, moving around looking for better “reception,” making heroic attempts to turn the static and faint input into information that his brain and his body can use. All that running, jumping, bumping, pushing, rocking, swaying, and singing may be his equivalent of dancing around with an antenna, trying to find that one spot that will make the reception clear.