Mapping the world onto the brain

January 10, 2008

—Nina Pierpont, MD, PhD

Some years ago a man’s sight was restored who had never had sight.  There was a blockage in his eyes which kept the light from getting in, which was fixed by surgery.  The rest of the structures were there—the retina for receiving the light, the optic nerves for transmitting retina signals, and the parts of the brain that receive the signals.  So we imagine that within days of the surgery, when the bandages were taken off, the man saw for the first time faces—his surgeon, his wife—and the things in his room.  Later, when he got up, we imagine he looked out the window and saw trees and sky, people walking, and cars driving by.

Not so.  The man saw light but not things.  He experienced a confusing, disturbing set of sensations of light which made no sense and did not congeal into anything in particular.  Time passed, and things did not change.  He found these sensations so distracting that he had to keep his eyes covered.

Sight is more than light falling on the retina, making electrical signals that travel to the brain.  Sight is a process by which the brain interprets the patterns of signals it receives.  The brain starts to build this process at birth.  The baby’s brain is an open book waiting for specific kinds of signals.  Newborns mostly stare at nothing in particular when they are not sleeping.  They love to stare at faces, though, and will focus on a face or gaze into eyes a foot or two away from their own—just about the distance to mom’s face when a baby is cradled in arms.  As the months go by the baby notices and focuses on things farther and farther away.  A three or four-month child watches people coming in across the room.  A nine-month-old crawling baby has the sharpest eyes imaginable, finding (and eating) every bit of lint on the carpet.

The brain at this age, during its critical period for development of sight, has a neurological plan in place waiting to receive the right signals.  The development of sight is a dance:  the brain is ready to learn to blend signals from the eyes into visual images, but without the signals won’t organize itself to see.

If something is wrong with an eye early in life, sight does not develop at brain level for that eye.  The problem can be anything that limits the clarity of images coming from the eye, or the eyes pointing in different directions.  The young brain does not support a double image.  If the signals from the two eyes don’t agree, the brain just blocks out one of them.

Language learning is similar.  Babies’ minds have templates for language.  They are pre-programmed, “hard-wired,” to make sense of the babble around them, whether they are born in America, France, or China.  Any normal baby can learn any real language.  A baby can even learn two at the same time and not mix them up, though this takes a little longer.  Babies can learn to pronounce any type of word sound, but older people lose the sounds which don’t occur in their own language.  A young child can also figure out the grammar in the words that surround him.  If a kindergartener is moved to another country, he is speaking like a native before the end of the year, way ahead of his parents in both accent and word use.  To him, it just sounds right.  The adults have to think about it more.

Grammatical structure is so hard-wired into young children’s brains that they are able to create complex language structures where they don’t exist, to satisfy their own needs for true language.  When people of two languages intermingle, they create a semi-language called a pidgin, which is a mixture of words from two languages stuck together with a lot of hand-waving to take the place of grammar.  Pidgins are rough, and can’t express fine gradations of meaning.  The babies born where pidgin is spoken, though, transform the pidgin as they grow up into what is called a creole, which is still a blend of the two languages but now with the grammatical structures of a true language.  Children’s minds impose this structure, not adult minds.

Learning, from the earliest days of a child’s life, is a reciprocal dance between what the brain is ready and adapted to do and the signals that come from the child’s senses.  This is the dance of development.

In future columns I’ll talk about other senses, the problems some children have figuring out the signals from their senses, and the more complex ways in which children’s nervous systems are waiting for certain things to happen in their lives.