LANGUAGE DISCRIMINATION BY HUMAN NEWBORNS AND BY COTTON-TOP TAMARIN MONKEYS
change is significant (p<0.05, chi-square test). This re-
sult suggests that the tamarins discriminated Dutch from
Japanese, irrespective of speaker variation. Surprisingly,
such a pattern is not observed when the sentences are played
backwards: only 5 out of 13 tamarins dishabituated to the
backwards language change (p=0.87), and this pattern is
not significantly different from the speaker change condition
(p>0.2). These results parallel those obtained with newborns
on the synthetic stimuli.
forward
backward
CONDITION
Figure 2. Number of tamarins responding positively (white bars)
and negatively (hatched bars) to the test sentence depending on the
condition: language or speaker change, sentences played forward or
backwards. 2A: natural sentences. 2B: synthesized sentences. 2C:
data from experiments 2A and 2B pooled together. * p<0.05. **
p<0.01.
In Experiment 2B, the same tamarins were tested on both
the speaker and language conditions, but with synthesized
sentences. Figure 2B shows that 10 out of 13 tamarins disha-
bituated to the forward language change (p<0.05). Although
the number of subjects dishabituating to the speaker change
failed to reach statistical significance (p=0.29), the increased
numbers in this condition led to a non-significant difference
between language and speaker change for the synthesized
sentences (p>0.3). For backward sentences, subjects failed
to show a statistically significant level of dishabituation to
either the language or speaker change (p=0.29 and p=0.13).
Experiment 2B suggests that tamarins’ ability to discriminate
Dutch and Japanese is diminished when only prosodic cues
are available.
When the data from Experiments 2A and 2B are pooled
(Figure 2C), the overall result is clear: when sentences are
played forward, tamarins significantly dishabituate to the
language change (p=0.005) but not to the speaker change
(p=0.58), and the difference between language and speaker
change is significant (p<0.05). When sentences are played
backwards, no such effect is observed. This overall result
parallels that obtained with human newborns: both species
discriminate sentences of Dutch and Japanese played for-
ward, but not backwards.
The pattern of our results suggests both striking similari-
ties and differences between the monkey and the human au-
ditory systems. First, we have shown that tamarins, like hu-
man newborns, are able to process not just isolated syllables,
but also whole strings of continuous speech, and to extract
enough information thereof to discriminate between Dutch
and Japanese. Second, their ability to do so above and be-
yond speaker variability suggests that they are able to ex-
tract auditory equivalence classes, that is to extract abstract
linguistic invariants despite highly variable acoustic shapes
(17, 32). Third, the fact that, like newborns, tamarins fail
to discriminate when speech is played backwards, suggests
that their language discrimination capacity does not rely on
trivial low-level cues, but rather on quite specific proper-
ties of speech. Since tamarins have not evolved to pro-
cess speech, we in turn infer that at least some aspects of
human speech perception may have built upon pre-existing
sensitivities of the primate auditory system. Finally, unlike
newborns, tamarins fail to discriminate the language change
more than the speaker change when speech is resynthesized.
This leaves open the possibility that human newborns and
tamarins may not be responding to exactly the same cues in
the sentences : tamarins might be more sensitive to phonetic
than to prosodic contrasts.
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