Time-compression thresholds for Mandarin sentences in normal-hearing and cochlear implant listeners

Abstract

Faster speech may facilitate more efficient communication, but if speech is too fast it becomes unintelligible. The maximum speeds at which Mandarin words were intelligible in a sentence context were quantified for normal hearing (NH) and cochlear implant (CI) listeners by measuring time-compression thresholds (TCTs) in an adaptive staircase procedure. In Experiment 1, both original and CI-vocoded time-compressed speech from the MSP (Mandarin speech perception) and MHINT (Mandarin hearing in noise test) corpora was presented to 10 NH subjects over headphones. In Experiment 2, original time-compressed speech was presented to 10 CI subjects and another 10 NH subjects through a loudspeaker in a soundproof room. Sentences were time-compressed without changing their spectral profile, and were presented up to three times within a single trial. At the end of each trial, the number of correctly identified words in the sentence was scored. A 50%-word recognition threshold was tracked in the psychophysical procedure. The observed median TCTs were very similar for MSP and MHINT speech. For NH listeners, median TCTs were around 16.7 syllables/s for normal speech, and 11.8 and 8.6 syllables/s respectively for 8 and 4 channel tone-carrier vocoded speech. For CI listeners, TCTs were only around 6.8 syllables/s. The interquartile range of the TCTs within each cohort was smaller than 3.0 syllables/s. Speech reception thresholds in noise were also measured in Experiment 2, and were found to be strongly correlated with TCTs for CI listeners. In conclusion, the Mandarin sentence TCTs were around 16.7 syllables/s for most NH subjects, but rarely faster than 10.0 syllables/s for CI listeners, which quantitatively illustrated upper limits of fast speech information processing with CIs.