Speech Segmentation and Lexicon Building
How do children find words in a stream of English?

Babies do not begin life already knowing the meanings of words. Even so, when they hear large amounts of speech, they start to notice that similar sound chunks appear again and again.

For example, even before a child knows what dog means, repeated exposure to sentences such as Look at the dogs. Dogs are running. There is a dog. helps the child notice the recurring sound chunk “dog” within them.

At this stage, the first step is to detect recurring sound chunks in continuous speech and treat them as word-like units. That is speech segmentation.

Children who do not yet have enough meaning or vocabulary use multiple cues to figure out where one word begins and ends in the stream of speech. One of those cues is stress.

English words contain strong and weak syllables, and many everyday words follow a trochaic pattern, beginning with a strong syllable followed by a weak one. As a result, children who grow up hearing English begin to use strongly heard syllables as cues and predict, “Maybe a new word starts here.”

This tendency also appears in young children’s speech. In banana, the first syllable ba is weak, while the middle syllable na carries the stress. Around age two, some children pronounce banana more like “nana.” Rather than simply forgetting the first sound, they are often perceiving the word around its stressed syllable. The strongly heard NA functions as the center of the word, and the chunk nana is easier for them to grasp than the full form beginning with a weak syllable.

A sound chunk heard once does not immediately become a stable word. But after repeated encounters with similar sound forms, children retain a memory that says, “I’ve heard this sound before.” If they hear the sound form dog again and again, the sequence of sounds itself stays in memory. At this stage, it does not yet need to be tied to the clear meaning “dog.”

The system that stores sound information recognized as language is called the lexicon.

The important point here is to separate storage in the lexicon from the acquisition of meaning. First, the child creates an index for “this recurring sound chunk,” and only afterward does meaning begin to develop.

In a paper dictionary, each headword comes with a definition. But when we think about a child’s early lexicon, it is easier to imagine the headword being created first, with the meaning added later.

For example, a child may hear the sound giraffe many times without ever having seen a giraffe or knowing what the word means. Even so, if the same sound form is repeated often enough, the child can still build the memory, “I hear this sound chunk again and again.” In other words, a word can exist in the mind before its meaning is understood.

Adults experience the same thing. When we repeatedly hear an unfamiliar foreign word or technical term, we often feel, “I don’t know what it means, but I’ve heard that word before.” That is an example of the sound form being remembered before the meaning.

As sound forms build up in the lexicon, speech segmentation itself becomes easier. If the sound form dog appears in a stream of speech and it is already stored, the child can recognize, “This part is dog.” That, in turn, makes it easier to detect the boundaries of the unknown sounds around it.

At first, language may sound like nothing more than an unbroken flow of sound. Gradually, however, children begin to hear it as a mix of “sound chunks I know” and “sound chunks I don’t know yet.” What is developing at this stage is a growing stock of word forms that children can recognize as the same word when they hear it, even before they know the meaning. That stored knowledge becomes the foundation for later vocabulary understanding.

When a sound form has already been stored in the lexicon, it becomes easier to connect that form to meaning when the word later appears in a concrete situation. Imagine a child who has already heard the sound form dog many times and has it stored in the lexicon, then hears Look at the dog. while someone points to a real dog.

That may be the first moment when the sound form dog becomes linked to the animal in front of the child. Each new encounter with another dog, a dog in a picture book, or a running dog enriches that meaning even more.

Language development begins with finding sound chunks in continuous speech, storing those chunks in the lexicon, and then gradually building meaning on top of them.