Neural characteristics of successful and less successful speech and word learning in adults

Abstract

A remarkable characteristic of the human nervous system is its ability to learn to integrate novel (foreign) complex sounds into words. However, the neural changes involved in how adults learn to integrate novel sounds into words and the associated individual differences are largely unknown. Unlike English, most languages of the world use pitch patterns to mark individual word meaning. We report a study assessing the neural correlates of learning to use these pitch patterns in words by English-speaking adults who had no previous exposure to such usage. Before and after training, subjects discriminated pitch patterns of the words they learned while blood oxygenation levels were measured using fMRI. Subjects who mastered the learning program showed increased activation in the left posterior superior temporal region after training, while subjects who plateaued at lower levels showed increased activation in the right superior temporal region and right inferior frontal gyrus, which are associated with nonlinguistic pitch processing, and prefrontal and medial frontal areas, which are associated with increased working memory and attentional efforts. Furthermore, we found brain activation differences even before training between the two subject groups, including the superior temporal region. These results demonstrate an association between range of neural changes and degrees of language learning, specifically implicating the physiologic contribution of the left dorsal auditory cortex in learning success.

Figure 1

Imaging sequence. Subjects were imaged at the first 2 s of a TR followed by 12 s (6 stimulus pairs) of stimulus presentations when no scanning took place.

Figure 2

Subjects' pitch discrimination performance (in the scanner). Error bars show standard errors of the mean.

Figure 3

Brain activation revealed by the Post‐Training Speech vs. Sinewave (All Subjects) contrast. For this and subsequent figures, activation is superimposed on an averaged T1‐weighted volume (in Talairach space). Color scale represents normalized t value and applies to this and subsequent figures.

Figure 4

Brain activation revealed by the Successful vs. Less Successful Learners (Post‐Training Speech) contrast. Stronger activation for the successful and less successful learners are indicated by red and blue clusters, respectively.

Figure 5

Brain activation revealed by the Successful vs. Less Successful Learners (Pre‐Training Speech) contrast.

Figure 6

Left STG activation (in the strongest activating 5 mm³ cluster) in Speech (Sp) and Sinewave (Sw) conditions before (top) and after (bottom) training. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]