The cortex encodes speech timing as departure from expectation, not as physical measurement

Abstract

Every syllable and every phoneme in natural speech has a different duration. This variability is conventionally treated as jitter, noise the brain must overcome to recover an underlying regularity. Here we show that the cortex encodes it as information. We recorded magnetoencephalography from 25 native listeners of Spanish during twenty minutes of spontaneous narrative speech in Spanish, a syllable-timed language in which durational variability is low, making it a conservative test case. We modelled cortical activity with three timing regressors defined at successively finer grains: deviation from the expected syllabic beat, from the expected phoneme onset, and from the expected duration of the phoneme just completed. Fitted jointly, and against acoustic and linguistic-surprisal baselines, each regressor explained unique activity in bilateral superior temporal cortex, with distinct response latencies spanning 70 to 250 ms. The duration regressor explained cortical activity beyond raw phoneme duration, whereas raw duration explained no additional variance once it was included. At the onset grain, by contrast, predicted and physical timing contributed jointly. The onset departure was encoded with its sign, distinguishing events that arrived early from those that arrived late. In a separate sample, removing this durational variability was costly: comprehension in noise degraded under imposed isochrony relative to natural timing. Durational variability is therefore not noise the cortex discards but information it encodes, across a hierarchy of timescales - and at the finest grain, what is encoded is not how long a speech event lasted but how far its duration departed from expectation.