Exam 13: Speech Perception

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Based on their studies of brain-damaged patients, 19th-century researchers Paul Broca (1824-1880) and Carl Wernicke (1848-1905) showed that damage to specific areas of the brain causes language problems, called aphasias. When Broca tested patients who had suffered strokes that damaged their frontal lobe, in an area that came to be called Broca's area, he found that their speech was slow and labored and often had jumbled sentence structure. Patients with this problem-slow, labored, ungrammatical speech caused by damage to Broca's area, are diagnosed as having Broca's aphasia. Later research showed that patients with Broca's aphasia not only have difficulty forming complete sentences, they also have difficulty understanding sentences in which word order is critical for understanding the meaning of those sentences, as in "The boy was pushed by the girl,"
The patients studied by Wernicke, who had damage to an area in their temporal lobe that came to be called Wernicke's area, produced speech that was fluent and grammatically correct but tended to be incoherent. Patients such as this not only produce meaningless speech but are unable to understand speech and writing. While patients with Broca's aphasia have trouble understanding sentences in which meaning depends on word order, Wernicke's patients have more widespread difficulties in understanding and would be unable to understand "The apple was eaten by the girl" as well. In the most extreme form of Wernicke's aphasia, the person has a condition called word deafness, in which he or she cannot recognize words, even though the ability to hear pure tones remains intact (Kolb & Whishaw, 2003).

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Noise-vocoded speech is created by dividing the speech signal up into different frequency bands and then adding noise to each band. This process transforms the spectrogram of the original speech stimulus into a noisy spectrogram. The loss of detail in the frequency representation of the noise-vocoded signal transforms clear speech into a harsh noisy whisper.
Subjects in Matthew Davis's (2005) experiment listened to a vocoded sentence and then wrote down as much of the sentence as they could. This was repeated for a total of 30 sentences. Performance was near zero for the first three sentences, and then increases, until by the 30th sentence, subjects are reporting half or more of the words. The increase in performance is important because all the subjects were doing is listening to one sentence after the other. 
What information do listeners use to achieve this clearer understanding
One hint comes from experiments in which subjects first listened to a degraded sentence and wrote down what they heard, as before, and then heard a clear undistorted version of the sentence, followed by the distorted sentence again (hear degraded sentence ? hear clear sentence ? hear degraded sentence again). Subjects reported that when they listened to the second presentation of the degraded sentence, they heard some words they hadn't heard the first time. Davis calls this ability to hear previously unintelligible words the "pop-out" effect. 
The pop-out effect shows that higher-level information such as listeners' knowledge can improve speech perception. But this result becomes even more interesting when we consider that after experiencing the pop-out effect subjects became better at understanding other degraded sentences that they were hearing for the first time. Even more interesting, the pop-out effect and later improvement in performance also occurred in a group of subjects who read the sentence after hearing the degraded version (hear degraded sentence ? read written sentence ? hear degraded sentence again). What this means is that it wasn't listening to the clear sound that was important, but knowing the content (the speech sounds and words) of what they were hearing that helped with learning.