Who said what? The effects of speech tempo on target detection and information extraction in a multi-talker situation: An ERP and functional connectivity study.

People with normal hearing can usually follow one of the several concurrent speakers. Speech tempo affects both the separation of concurrent speech streams and information extraction from them. The current study varied the tempo of two concurrent speech streams to investigate these processes in a multi-talker situation. Listeners performed a target-detection and a content-tracking task, while target-related ERPs and functional brain networks sensitive to speech tempo were extracted from the EEG signal. At slower than normal speech tempo, building the two streams required longer processing times, and possibly the utilization of higher-order, for example, syntactic and semantic cues. The observed longer reaction times and higher connectivity strength in a theta band network associated with frontal control over auditory/speech processing are compatible with this notion. With increasing tempo, target detection performance decreased and the N2b and the P3b amplitudes increased. These data suggest an increased need for strictly allocating target-detection-related resources at higher tempo. This was also reflected by the observed increase in the strength of gamma-band networks within and between frontal, temporal, and cingular areas. At the fastest tested speech tempo, there was a sharp drop in recognition memory performance, while target detection performance increased compared to the normal speech tempo. This was accompanied by a significant increase in the strength of a low alpha network associated with the suppression of task-irrelevant speech. These results suggest that participants prioritized the immediate target detection task over the continuous content tracking, likely due to some capacity limit reached the fastest speech tempo.