Strategic Pauses Relieve Listeners from the Effort of Listening to Fast Speech: Data Limited and Resource Limited Processes in Narrative Recall by Adult Users of Cochlear Implants

Abstract

Speech that has been artificially accelerated through time compression produces a notable deficit in recall of the speech content. This is especially so for adults with cochlear implants (CI). At the perceptual level, this deficit may be due to the sharply degraded CI signal, combined with the reduced richness of compressed speech. At the cognitive level, the rapidity of time-compressed speech can deprive the listener of the ordinarily available processing time present when speech is delivered at a normal speech rate. Two experiments are reported. Experiment 1 was conducted with 27 normal-hearing young adults as a proof-of-concept demonstration that restoring lost processing time by inserting silent pauses at linguistically salient points within a time-compressed narrative ("time-restoration") returns recall accuracy to a level approximating that for a normal speech rate. Noise vocoder conditions with 10 and 6 channels reduced the effectiveness of time-restoration. Pupil dilation indicated that additional effort was expended by participants while attempting to process the time-compressed narratives, with the effortful demand on resources reduced with time restoration. In Experiment 2, 15 adult CI users tested with the same (unvocoded) materials showed a similar pattern of behavioral and pupillary responses, but with the notable exception that meaningful recovery of recall accuracy with time-restoration was limited to a subgroup of CI users identified by better working memory spans, and better word and sentence recognition scores. Results are discussed in terms of sensory-cognitive interactions in data-limited and resource-limited processes among adult users of cochlear implants.

Keywords: cochlear implants; narrative recall; pupillometry; speech vocoding; time-compressed speech.

Figure 1.

Waveform of a sample sentence recorded at its original normal speech rate (top panel), the time-compressed version of the same sentence (middle panel), and the time-compressed version with silent periods inserted after the main clause and sentence ending to restore the lost processing time due to the time-compression (lower panel).

Figure 2.

Mean percentage of narrative content (propositions) recalled from narratives presented in clear speech (left panel), with 10-channel vocoding (middle panel) and with 6-channel vocoding (right panel) when presented at their original normal speech rate (N), when time-compressed to 60% of their original playing time (TC) and when the lost processing time was restored by inserting silent periods at clause and sentence boundaries (TR). Error bars are one standard error.

Figure 3.

Top two panels show the time course of mean pupil dilations, adjusted to baseline and to each individual's dynamic range while normal-hearing young adults listened to narrative passages. Data are shown for narratives that were time-compressed, presented at a normal speech rate, and in the time-restoration condition. Data are collapsed across vocoder conditions. (A) shows pupillary responses aligned to the end of the narrative passage, with time 0 indicating the end of the passage. (B) shows these data as a percentage of the duration of each passage. The three bottom panels show the mean adjusted pupil dilations separately for clear speech (C), 10-channel vocoding (D), and 6-channel vocoding (E) as a percentage of the passage duration. Error ribbons are one standard error.

Figure 4.

Generalized additive mixed model predicted difference curves of adjusted pupillary responses when young adults listened to narrative passages. The top panels show contrasts in predicted pupil size between conditions of time-compression versus normal rate (A), time-restored versus normal rate (B), and time-compressed versus time-restored (C). The bottom panels display contrasts in predicted pupil size between conditions of 6-channel vocoding versus clear speech (D), 10-channel vocoding versus clear speech (E), and 6-channel vocoding versus 10-channel vocoding (F). A higher estimated difference is an indication of the greater difference between compared conditions, with regions highlighted in red indicating time points where there is a statistically significant difference. Shaded ribbons are 95% confidence intervals.

Figure 5.

Mean percentage of narrative content (propositions) recalled from narratives by CI users when the narratives were presented at their original normal speech rate (N), when time-compressed to 60% of their original playing time (TC) and in the time-restoration (TR) condition in which the lost processing time was restored by silent periods inserted at clause and sentence boundaries. Error bars are one standard error.

Figure 6.

Time course of mean pupil dilations adjusted to baseline and to each individual’s dynamic range while CI users listened to narrative passages. Data are shown for narratives that were time-compressed (red), presented at a normal speech rate (blue), and heard with time-restoration (green). Data are transformed to represent changes in pupil size as a percentage of the passage duration. Error ribbons are one standard error.

Figure 7.

Generalized additive mixed model predicted difference curves of adjusted pupillary responses when CI users listened to narrative passages. The top panels show contrasts in predicted pupil size between conditions of time-compression versus normal rate (A), time-restored versus normal rate (B), and time-compressed versus time-restored (C). A higher estimated difference is an indication of the greater difference between compared conditions, with regions highlighted in red indicating time points where there is a statistically significant difference. Shaded ribbons are 95% confidence intervals.