Page 238 - Foundations of Cognitive Psychology : Core Readings

The Auditory Scene 243

the shape of objects. The second was the fact that perceptual organization could
be demonstrated with very young animals.
To the arguments offered by the Gestaltists can be added the following one:
From an engineering point of view, it is generally easier to design a machine
that candosometaskdirectlythantodesignone that can learn to do it. We can
design machines that can parse or generate fairly complex sentences, but there
has been limited success in designing one that could learn grammatical rules
from examples without any designed-in knowledge of the formal structure of
those rules. By analogy, if you think of the physical world as having a ‘‘gram-
mar’’ (the physical laws that are responsible for the sensory impressions that
we receive), then each human must be equipped either with mechanisms capa-
ble of learning about many of these laws from examples or with a mechanism
whose genetic program has been developed once and for all by the species as a
result of billions of parallel experiments over the course of history, where the
lives of the members of the species and its ancestors represent the successes and
the lives of countless extinct families the failures. To me, evolution seems more
plausible than learning as a mechanism for acquiring at least a general capa-
bility to segregate sounds. Additional learning-based mechanisms could then
refine the ability of the perceiver in more specific environments.
The innate influences on segregation should not be seen as being in opposi-
tion to principles of learning. The two must collaborate, the innate influences
acting to ‘‘bootstrap’’ the learning process. In language, meaning is carried by
words. Therefore if a child is to come to respond appropriately to utterances, it
is necessary that the string be responded to in terms of the individual words
that compose it. This is sometimes called the segmentation problem. Until you
look at a spectrogram of continuous speech occurring in natural utterances, the
task seems easy. However, on seeing the spectrogram, it becomes clear that the
spaces that we insert into writing to mark the boundaries of words simply do
not occur in speech. Even if sentences were written without spaces, adults
could take advantage of prior knowledge to find the word boundaries. Because
they already know the sequences of letters that make meaningful words, they
could detect each such sequence and place tentative word boundaries on either
side of it. But when infants respond to speech they have no such prior learning
to fall back on. They would be able to make use only of innate constraints. I
suspect a main factor used by infants to segment their first words is acoustic
discontinuity. The baby may hear a word as a unit only when it is presented in
isolation, that is, with silence (or much softer sound) both before and after it.
This would be the result of an innate principle of boundary formation. If it
were presented differently, for example, as part of a constant phrase, then the
phrase and not the word would be treated as the unit. The acoustic continuity
within a sample of speech and the discontinuities at its onset and termination
would be available, even at the earliest stage of language acquisition, to label it
as a single whole when it was heard in isolation. Once perceived as a whole,
however, its properties could be learned. Then, after a few words were learned,
recognition mechanisms could begin to help the segmentation process. The in-
fant would now be able to use the beginnings and ends of these familiar pat-
terns to establish boundaries for other words that might lie between them. We