What do A and B stand for?

Context:

PSYCHOLOGY
Science Starts Early
Frank C. Keil

......
Each of these abilities has early origins.
Consider, for example, how children respond
to the challenge of noticing correlations as
they encounter them in the fl ow of experi-
ence. For instance, an infant learning lan-
guage, upon hearing streams of syllables, not
only has to notice how often certain syllables
occur but also needs to infer higher-order
patterns arising from those syllables. One
study ( 2) showed that 5-month-old infants
can handle this challenge by rapidly tracking
not only the sounds of the syllables but also
visual patterns associated with each syllable.
In the experiment, infants looking at a com-
puter screen were repeatedly presented with
abstract patterns of syllables and shapes. An
“ABB” pattern, for instance, could be repre-
sented by certain shapes corresponding to
the syllables “di ga ga.” When presented with
a new pattern (ABA) with new syllables—
such as “le ko le”—the infants looked longer
at the shapes on the screen than if the new
syllables were in the old ABB pattern. This
suggests that they recognized it as a new,
unfamiliar correlation.