Dissociating Cortical Activity during Processing of Native and Non-Native Audiovisual Speech from Early to Late Infancy

Abstract

Initially, infants are capable of discriminating phonetic contrasts across the world's languages. Starting between seven and ten months of age, they gradually lose this ability through a process of perceptual narrowing. Although traditionally investigated with isolated speech sounds, such narrowing occurs in a variety of perceptual domains (e.g., faces, visual speech). Thus far, tracking the developmental trajectory of this tuning process has been focused primarily on auditory speech alone, and generally using isolated sounds. But infants learn from speech produced by people talking to them, meaning they learn from a complex audiovisual signal. Here, we use near-infrared spectroscopy to measure blood concentration changes in the bilateral temporal cortices of infants in three different age groups: 3-to-6 months, 7-to-10 months, and 11-to-14-months. Critically, all three groups of infants were tested with continuous audiovisual speech in both their native and another, unfamiliar language. We found that at each age range, infants showed different patterns of cortical activity in response to the native and non-native stimuli. Infants in the youngest group showed bilateral cortical activity that was greater overall in response to non-native relative to native speech; the oldest group showed left lateralized activity in response to native relative to non-native speech. These results highlight perceptual tuning as a dynamic process that happens across modalities and at different levels of stimulus complexity.

Figure 1

Methods. (A) Infants were exposed to speech stimuli while seated on a caregiver’s lap. (B) Infants wearing the NIRS headband, localized using the 10-20 coordinates T3 and T4. (C) Infants were tested using a block-design that consisted of 20 s long stretches of non-native and native infant-directed audiovisual speech. Each trial was preceded by a 10 s silent baseline. The two types of audiovisual speech block were further separated by two 20 s long trials of animated shapes with no accompanying audio.

Figure 2

Hemodynamic Response Functions. Native speech is plotted in black, Non-Native speech is plotted in dashed gray. Each channel location is shown: Right Anterior (R ant), Right Posterior (R pos), Left Anterior (L ant) and Left Posterior (L pos). Error bars represent standard error of the estimate. Data from 3-to-6-month-olds are in the left panel; those from 7-to-10-month-olds are in the middle panel, and those from 11-to-14-month-olds are in the right panel.