Effects of healthy aging and left hemisphere stroke on statistical language learning

ABSTRACT

Spoken sentences are continuous streams of sound, without reliable acoustic cues to word boundaries. We have previously proposed that language learners identify words via an implicit statistical learning mechanism that computes transitional probabilities between syllables. Neuroimaging studies in healthy young adults associate this learning with left inferior frontal gyrus, left arcuate fasciculus, and bilateral striatum. Here, we test the effects of healthy aging and left hemisphere (LH) injury on statistical learning. Following 10-minute exposure to an artificial language, participants rated familiarity of Words, Part-words (sequences spanning word boundaries), and Non-words (unfamiliar sequences). Young controls (N = 14) showed robust learning, rating Words > Part-words > Non-words. Older controls (N = 28) showed this pattern to a weaker degree. Stroke survivors (N = 24) as a group showed no learning. A lesion comparison examining individual differences revealed that “non-learners” are more likely to have anterior lesions. Together, these findings demonstrate that word segmentation is sensitive to healthy aging and LH injury.

KEYWORDS:

• Word segmentation
• statistical learning
• language
• aging
• stroke

Acknowledgements

We are grateful to the participants who contributed their time and effort to this study. This work was supported by the NIH under grants F31DC014875 (to author MEF), R01DC014960 (to author PET), and R01DC016902 and R01HD037082 (to author ELN). Author MEF received additional training support through the ASHFoundation New Century Scholars Doctoral Scholarship.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 Note that we selected only two non-words to minimise the imbalance in the test between “legal” sequences in the language (the four words) and “illegal” sequences (four part-words and two non-words).

2 In one experiment, Toro and colleagues (2005) did find reduced statistical learning when participants were asked to attend to pitch changes in the speech stream itself. However, exposure was only 7 minutes, participants were told their goal was to detect the pitch changes (not to learn anything about the language), and pitch changes occurred often, approximately every 10 syllables.

3 A reviewer points out that there could be circumstances in which lesions to frontal regions may actually convey an implicit learning advantage. Frontal regions are known to be involved in cognitive control, which has been shown to interfere with implicit learning under some circumstances (see Friederici et al., 2013 for a discussion of this issue). Therefore, one could generate the hypothesis that impairments to cognitive control mechanisms might lead to improvements in implicit learning abilities. While we have not tested this proposal directly here, we agree that more work is needed to understand precisely what these brain regions contribute to the statistical learning mechanism.

Additional information

Funding

This work was supported by National Institute of Child Health and Human Development: [grant number R01HD037082]; the National Institute on Deafness and Other Communication Disorders: [grant number F31DC014875, R01DC014960, R01DC016902].