Poster A76, Tuesday, August 20, 2019, 10:15 am – 12:00 pm, Restaurant Hall
Noninvasive neurostimulation reveals a causal role for left superior temporal lobe in speech adaptation
Ja Young Choi1,2, Tyler Perrachione2;1Harvard University, 2Boston University
Speech perception requires listeners to map acoustic speech signals onto abstract phonemic representations despite the variability in acoustic-to-phonemic mapping across talkers. To facilitate this mapping, listeners use preceding speech to adapt to talker-specific phonetic idiosyncrasies. Previous studies have reported increased neural activity in superior temporal lobe in the presence of talker variability. However, it is unknown how this region is causally involved in processing variability in speech. In this study, we used high-definition transcranial direct current stimulation (HD-tDCS) to investigate the causal involvement of left superior temporal lobe in talker adaptation. Participants (N = 60 right-handed native English-speaking adults) performed a speech processing task in which they identified the target word they heard (“boot” / “boat”) as quickly and as accurately as possible. The task factorially manipulated talker variability (single vs. mixed talkers) and speech context (isolated words vs. connected speech). Throughout the experiment (~13 min), participants received anodal (n = 20), cathodal (n = 20), or sham (n = 20) stimulation in a between-subjects design. For the active stimulation groups (anodal and cathodal), current intensity was linearly ramped up to 2 mA over 30 seconds before the experiment started, and the intensity was ramped back down to 0 over 30 seconds upon completing all trials. For the sham stimulation group, current intensity was linearly ramped up to 2 mA over 30 seconds before the experiment started, immediately followed by a linear ramp-down to 0 over 30 seconds, which generated the initial dermal sensation of active stimulation but did not actually stimulate the neurons underneath the scalp during the task. Stimulation sites were chosen so that maximum current intensity was localized to left superior temporal lobe. Our results consistently showed that listeners were slower to identify target words in mixed-talker condition than in single-talker condition overall (p ≪ 0.0001), reflecting the additional processing cost associated with talker variability. We also observed a significant talker variability × speech context interaction (p ≪ 0.0001), demonstrating that magnitude of additional processing cost incurred by talker variability was reduced by connected speech – i.e., speech preceding a target word helped listeners resolve cross-talker variability. While overall response time did not differ across the three stimulation groups (p = 0.36), the effect of connected speech was significantly attenuated under both anodal and cathodal stimulation compared to sham (anodal vs. sham p < 0.01; cathodal vs. sham p < 0.01), showing that active stimulation disrupts the brain’s ability to use connected speech to rapidly adapt to talkers. However, active stimulation did not have a modulatory effect on talker adaptation in the isolated-word condition (anodal vs. sham p = 0.45; cathodal vs. sham p = 0.90), showing a dissociation in the effect of tDCS on talker adaptation on two different timescales. This set of results suggests that stimulation of left superior temporal lobe disrupts the brain’s ability to use speech context to rapidly adapt to a talker, revealing this region’s causal role in talker adaptation.
Themes: Perception: Speech Perception and Audiovisual Integration, Phonology and Phonological Working Memory
Method: Neurostimulation