Back in time: Reversed speech as a window into cognitive language processing-A systematic review.

The speech signal unfolds in time. This has repercussions for speech production and processing. For production, processing units are assembled in a language-specific order, while in perception, the acoustic signal is segmented into specific, temporally ordered processing units. Specific insights into the forward processing of speech and its assumed mechanisms can be gained from reversed speech. Reversed speech can be either the production of speech in reverse order by skilled speakers (subjectively reversed speech), or speech played back in reverse order for investigating the mechanisms underlying speech segmentation and perception (objectively reversed speech). Direct research on reversed speech has hitherto been very sparse, leaving out opportunities to further the understanding of underlying segmentation and perception mechanisms together with temporal constraints on forward processing. To this end, a systematic literature review on reversed speech is presented here. Three insights can be gained from this review. First, findings from subjectively and objectively reversed speech can be integrated in coupled oscillator models, capitalizing on neural oscillations that align with individual speech sounds, syllable and phrases. Second, the temporal order of information in time windows < 40 ms is relatively irrelevant for perception, while the relative order of processing units is important for both perception and production. Third, objectively reversed speech is predominantly used as a control condition in speech processing studies but should be more carefully implemented, depending on the exact research questions. In general, cross-frequency coupling can be considered a domain-general neural mechanism underlying cognitive flexibility of which reversed speech is a prime candidate.