The threshold of intelligibility for a word in a wide-spectrum noise is shown to be a decreasing function of the frequency with which the word occurs in general linguistic usage (word frequency). The drop in threshold is about 4.5 db per logarithmic unit of word frequency. This rate is independent of the length of the word, although the thresholds for words of given frequency of occurrence are lower for long words.
The effect of restricting the listener's alternatives in an intelligibility test to a specified number of words is calculated from this relationship. These calculations come within 1 db of published experimental data. Theoretical functions relating intelligibility threshold to word length are also calculated from the word-frequency effect, on the assumption that listeners can discriminate the length of a word at levels too low for it to be identified. These functions are in general agreement with the experimental results.
Implications for intelligibility testing procedures are discussed.
Simple reaction time is significantly increased in patients with unilateral lesions of either cerebral hemisphere responding with the hand ipsilateral to the lesion, but the effect is much greater when the lesion is in the non-dominant hemisphere. This difference cannot be attributed to asymmetries in the size or type of lesion. It applies over the complete distribution of reaction times as well as to the means, and is not diminished by practice. Neither the classical conduction model of reaction time nor the mass-action hypothesis proposed by De Renzi and Faglioni (1965) can account for the findings. Although the increase of about 100 ms observed with lesions of the dominant hemisphere probably represents a non-specific effect of brain disease, the much larger increases resulting from lesions of the non-dominant hemisphere appear to be of focal origin. The critical areas cannot yet be specified with certainty, but maps of the brain scans for patients with the highest reaction times suggest that structures in or near the basal ganglia and the posterior parietal region of the non-dominant hemisphere are involved, and other studies show that the focal effect is not found when patients with contralateral motor involvement are excluded.
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