Using Knowledge: Top-Down Processing • 57Using Knowledge: Top-Down Processing
We will now consider some further examples of how perception depends on more
than just stimulation of the receptors. In this section we consider the role of top-down
processing, processing that depends on a person’s prior knowledge or expectations. We
have already described two examples of top-down processing: the naming of objects
created by different arrangements of geons, and the blob with the multiple personalities.
Another example of top-down processing is illustrated by something that happens
when, as I channel-surf on TV, I stop at Telemundo, a channel that often has dramatic
programs in which the action seems extremely interesting. My problem, however, is
that Telemundo is a Spanish-language station and I don’t understand Spanish. So while
the people on the program understand each other, to me the dialogue often sounds like
an unbroken string of sound, except occasionally when a familiar word like gracias
pops out. My perception refl ects the fact that the sound signal for speech is generally
continuous, and when there are breaks in the sound, they do not necessarily occur
between words. You can see this in ● Figure 3.13 by comparing the place where each
word in the sentence begins with the pattern of the sound signal.
But when my Spanish-speaking acquaintances watch Telemundo, they perceive this
unbroken string of sound as individual, meaningful words. Because of their knowledge
of the language, they are able to tell when one word ends and the next one begins, a
phenomenon called speech segmentation. The fact that a listener familiar only with
English and another listener familiar with Spanish can receive identical sound stimuli
but experience different perceptions means that each listener’s experience with lan-
guage (or lack of it!) is infl uencing his or her perception.
This example illustrates how knowledge that a person brings to the situation can
infl uence perception. In our example, this knowledge is prior knowledge of Spanish,
which makes it possible to perceive the individual words and therefore identify where
one word ends and the other begins. The idea that perception depends on knowledge
is not a new one. The 19th-century physicist and physiologist Hermann von Helmholtz
(1866/1911) proposed a theory based on this idea.HELMHOLTZ’S THEORY OF UNCONSCIOUS INFERENCE
Helmholtz proposed a principle called the theory of unconscious inference, which
states that some of our perceptions are the result of unconscious assumptions that we● (^) FIGURE 3.13 Sound energy for the sentence “Mice eat oats and does eat oats and little
lambs eat ivy.” The italicized words just below the sound record indicate how this sentence
was pronounced by the speaker. The vertical lines next to the words indicate where each word
begins. Note that it is diffi cult or impossible to tell from the sound record where one word
ends and the other begins. (Source: Speech signal courtesy of Peter Howell.)
meiz it oaz n doaz eet oaz n litl laamz eet ievee
mice
0 sec 0.5 1.0 1.5 2.0 2.5
eat oats and does eat oats and little lambs eat ivy
Time
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