Efficient functional localization of language regions in the brain.

Important recent advances in the cognitive neuroscience of language have been made using functional localizers to demarcate language-selective regions in individual brains. Although single-subject localizers offer insights that are unavailable in classic group analyses, they require additional scan time that imposes costs on investigators and participants. In particular, the unique practical challenges of scanning children and other special populations has led to less adoption of localizers for neuroimaging research with these theoretically and clinically important groups. Here, we examined how measurements of the spatial extent and functional response profiles of language regions are affected by the duration of an auditory language localizer. We compared how parametrically smaller amounts of data collected from one scanning session affected (i) consistency of group-level whole-brain parcellations, (ii) functional selectivity of subject-level activation in individually defined functional regions of interest (fROIs), (iii) sensitivity and specificity of subject-level whole-brain and fROI activation, and (iv) test-retest reliability of subject-level whole-brain and fROI activation. For many of these metrics, the localizer duration could be reduced by 50-75% while preserving the stability and reliability of both the spatial extent and functional response profiles of language areas. These results indicate that, for most measures relevant to cognitive neuroimaging studies, the brain's language network can be localized just as effectively with 3.5 min of scan time as it can with 12 min. Minimizing the time required to reliably localize the brain's language network allows more effective localizer use in situations where each minute of scan time is particularly precious.

Foreign language talker identification does not generalize to new talkers.

Listeners identify talkers less accurately in a foreign language than in their native language, but it remains unclear whether this language-familiarity effect arises because listeners (1) simply lack experience identifying foreign-language talkers or (2) gain access to additional talker-specific information during concurrent linguistic processing of talkers' speech. Here, we tested whether sustained practice identifying talkers of an unfamiliar, foreign language could lead to generalizable improvement in learning to identify new talkers speaking that language, even if listeners remained unable to understand the talkers' speech. English-speaking adults with no prior experience with Mandarin practiced learning to identify Mandarin-speaking talkers over four consecutive days and were tested on their ability to generalize their Mandarin talker-identification abilities to new Mandarin-speaking talkers on the fourth day. In a "same-voices" training condition, listeners learned to identify the same talkers for the first 3 days and new talkers on the fourth day; in a "different-voices" condition, listeners learned to identify a different set of voices on each day including the fourth day. Listeners in the same-voices condition showed daily improvement in talker identification across the first 3 days but returned to baseline when trying to learn new talkers on the fourth day, whereas listeners in the different-voices condition showed no improvement across the 4 days. After 4 days, neither group demonstrated generalized improvement in learning new Mandarin-speaking talkers versus their baseline performance. These results suggest that, in the absence of specific linguistic knowledge, listeners are unable to develop generalizable foreign-language talker-identification abilities.

Implicit and explicit learning in talker identification.

In the real world, listeners seem to implicitly learn talkers' vocal identities during interactions that prioritize attending to the content of talkers' speech. In contrast, most laboratory experiments of talker identification employ training paradigms that require listeners to explicitly practice identifying voices. Here, we investigated whether listeners become familiar with talkers' vocal identities during initial exposures that do not involve explicit talker identification. Participants were assigned to one of three exposure tasks, in which they heard identical stimuli but were differentially required to attend to the talkers' vocal identity or to the verbal content of their speech: (1) matching the talker to a concurrent visual cue (talker-matching); (2) discriminating whether the talker was the same as the prior trial (talker 1-back); or (3) discriminating whether speech content matched the previous trial (verbal 1-back). All participants were then tested on their ability to learn to identify talkers from novel speech content. Critically, we manipulated whether the talkers during this post-test differed from those heard during training. Compared to learning to identify novel talkers, listeners were significantly more accurate learning to identify the talkers they had previously been exposed to in the talker-matching and verbal 1-back tasks, but not the talker 1-back task. The correlation between talker identification test performance and exposure task performance was also greater when the talkers were the same in both tasks. These results suggest that listeners learn talkers' vocal identity implicitly during speech perception, even if they are not explicitly attending to the talkers' identity.