Comprehension of natural and synthetic speech: effects of predictability on the verification of sentences controlled for intelligibility.

A sentence verification task (SVT) was used to study the effects of sentence predictability on comprehension of natural speech and synthetic speech that was controlled for intelligibility. Sentences generated using synthetic speech were matched on intelligibility with natural speech using results obtained from a separate sentence transcription task. In the main experiment, the sentence verification task included both true and false sentences that varied in predictability. Results showed differences in verification speed between natural and synthetic sentences, despite the fact that these materials were equated for intelligibility. This finding suggests that the differences in perception and comprehension between natural and synthetic speech go beyond segmental intelligibility as measured by transcription accuracy. The observed differences in response times appear to be related to the cognitive processes involved in understanding and verifying the truth value of short sentences. Reliable effects of predictability on error rates and response latencies were also observed. High-predictability sentences displayed lower error rates and faster response times than low-predictability sentences. However, predictability did not have differential effects on the processing of synthetic speech as expected. The results demonstrate the need to develop new measures of sentence comprehension that can be used to study speech communication at processing levels above and beyond those indexed through transcription tasks, or forced-choice intelligibility tests such as the Modified Rhyme Test (MRT) or the Diagnostic Rhyme Test (DRT).