Nonhuman primate vocalizations support categorization in very young human infants

Abstract

Language is a signature of our species and our primary conduit for conveying the contents of our minds. The power of language derives not only from the exquisite detail of the signal itself but also from its intricate link to human cognition. To acquire a language, infants must identify which signals are part of their language and discover how these signals are linked to meaning. At birth, infants prefer listening to vocalizations of human and nonhuman primates; within 3 mo, this initially broad listening preference is tuned specifically to human vocalizations. Moreover, even at this early developmental point, human vocalizations evoke more than listening preferences alone: they engender in infants a heightened focus on the objects in their visual environment and promote the formation of object categories, a fundamental cognitive capacity. Here, we illuminate the developmental origin of this early link between human vocalizations and cognition. We document that this link emerges from a broad biological template that initially encompasses vocalizations of human and nonhuman primates (but not backward speech) and that within 6 mo this link to cognition is tuned specifically to human vocalizations. At 3 and 4 mo, nonhuman primate vocalizations promote object categorization, mirroring precisely the advantages conferred by human vocalizations, but by 6 mo, nonhuman primate vocalizations no longer exert this advantageous effect. This striking developmental shift illuminates a path of specialization that supports infants as they forge the foundational links between human language and the core cognitive processes that will serve as the foundations of meaning.

Keywords: conceptual development; developmental tuning; infancy; language acquisition; language and thought.

Fig. 1.

Experimental design. During the familiarization phase, each infant viewed eight different exemplars, presented sequentially, in conjunction with either the lemur vocalization (experiment 1) or backward speech (experiment 2). During the test phase, each infant viewed images from the familiar and novel categories, presented simultaneously in silence.

Fig. 2.

Infants’ responses to lemur vocalizations (experiment 1) and backward speech (experiment 2). At 3 and 4 mo, infants hearing lemur vocalizations were not only more likely than chance, but also more likely than those hearing backward speech, to discriminate between the novel and familiar test objects; at 6 mo, neither lemur vocalizations nor backward speech supported object categorization. For the purpose of comparison, we also show infants’ responses to human vocalizations (e.g., “Look at the modi!”) and to pure sine-wave tone sequences [e.g., 400- or 800-Hz tones; matched to the human vocalization stimuli for mean frequency, duration, and pause length (17, 23)]. Error bars represent ±1 SEM. Significant differences between preference score and chance performance (0.50) are marked by an asterisk (P < 0.05).

Fig. 3.

Correlations between age (in days) and preference score for infants hearing (A) lemur vocalizations and (B) backward speech. For the purpose of comparison, we also show the correlations for the human vocalization and sine-wave tone conditions (17). Significant correlations are marked by an asterisk (P < 0.05).