Examination of the neighborhood activation theory in normal and hearing-impaired listeners.

OBJECTIVE: Experiments were conducted to examine the effects of lexical information on word recognition among normal hearing listeners and individuals with sensorineural hearing loss. The lexical factors of interest were incorporated in the Neighborhood Activation Model (NAM). Central to this model is the concept that words are recognized relationally in the context of other phonemically similar words. NAM suggests that words in the mental lexicon are organized into similarity neighborhoods and the listener is required to select the target word from competing lexical items. Two structural characteristics of similarity neighborhoods that influence word recognition have been identified; "neighborhood density" or the number of phonemically similar words (neighbors) for a particular target item and "neighborhood frequency" or the average frequency of occurrence of all the items within a neighborhood. A third lexical factor, "word frequency" or the frequency of occurrence of a target word in the language, is assumed to optimize the word recognition process by biasing the system toward choosing a high frequency over a low frequency word. DESIGN: Three experiments were performed. In the initial experiments, word recognition for consonant-vowel-consonant (CVC) monosyllables was assessed in young normal hearing listeners by systematically partitioning the items into the eight possible lexical conditions that could be created by two levels of the three lexical factors, word frequency (high and low), neighborhood density (high and low), and average neighborhood frequency (high and low). Neighborhood structure and word frequency were estimated computationally using a large, on-line lexicon-based Webster's Pocket Dictionary. From this program 400 highly familiar, monosyllables were selected and partitioned into eight orthogonal lexical groups (50 words/group). The 400 words were presented randomly to normal hearing listeners in speech-shaped noise (Experiment 1) and "in quiet" (Experiment 2) as well as to an elderly group of listeners with sensorineural hearing loss in the speech-shaped noise (Experiment 3). RESULTS: The results of three experiments verified predictions of NAM in both normal hearing and hearing-impaired listeners. In each experiment, words from low density neighborhoods were recognized more accurately than those from high density neighborhoods. The presence of high frequency neighbors (average neighborhood frequency) produced poorer recognition performance than comparable conditions with low frequency neighbors. Word frequency was found to have a highly significant effect on word recognition. Lexical conditions with high word frequencies produced higher performance scores than conditions with low frequency words. CONCLUSION: The results supported the basic tenets of NAM theory and identified both neighborhood structural properties and word frequency as significant lexical factors affecting word recognition when listening in noise and "in quiet." The results of the third experiment permit extension of NAM theory to individuals with sensorineural hearing loss. Future development of speech recognition tests should allow for the effects of higher level cognitive (lexical) factors on lower level phonemic processing.

Preferred frequency response for two- and three-channel amplification systems.

The purpose of these investigations was to compare the preferred frequency-gain responses obtained from two- and three-channel amplification systems. The current experiments were limited to a linear system in which the crossover frequency dividing the channels was systematically varied. The subjects for the experiment were nine individuals with mild to moderately severe sensorineural hearing loss with various audiometric configurations. The subjects listened to continuous discourse, in noise, via a computer-controlled digital master hearing aid containing two real-time data acquisition processors. Initially, a modified simplex procedure was used to obtain preferred frequency-gain responses using several different crossover frequencies. A round-robin procedure was then conducted in which each preferred response from the simplex was compared with every other preferred response. The frequency-gain responses chosen most often for the two- and three-channel systems were compared. The results showed no significant differences between the preferred frequency-gain response for the two- versus the three-channel system. In addition, the preferred response chosen most often was not consistently observed at the same crossover frequency for all subjects, with the exception of those with steeply sloping hearing loss who chose 1,120 Hz as the first or second preference for the two-channel system. The round-robin results were rank-ordered according to the number of times each frequency-gain response was chosen. In general, subjects chose several frequency-gain responses at various crossover frequencies, which were not significantly different from each other statistically. The results of a final experiment suggested that physical similarities in the preferred responses chosen at the various crossover frequencies played a role in the rank-ordering of the preference judgments obtained in the original investigation.