Examination of the Neighborhood Activation Theory in Normal and Hearing-Impaired Listeners

Objective E-periments were conducted to e-amine the effects of le-ical information on word recognition among normal hearing listeners and individuals with sensorineural hearing loss. The le-ical 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 conte-t of other phonemically similar words. NAM suggests that words in the mental le-icon are organized into similarity neighborhoods and the listener is required to select the target word from competing le-ical 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 le-ical 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 e-periments were performed. In the initial e-periments, word recognition for consonant-vowel-consonant (CVC) monosyllables was assessed in young normal hearing listeners by systematically partitioning the items into the eight possible le-ical conditions that could be created by two levels of the three le-ical 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 le-icon-based Webster’s Pocket Dictionary. From this program 400 highly familiar, monosyllables were selected and partitioned into eight orthogonal le-ical groups (50 words/group). The 400 words were presented randomly to normal hearing listeners in speech-shaped noise (E-periment 1) and “in quiet” (E-periment 2) as well as to an elderly group of listeners with sensorineural hearing loss in the speech-shaped noise (E-periment 3). Results The results of three e-periments verified predictions of NAM in both normal hearing and hearing-impaired listeners. In each e-periment, 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. Le-ical 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 le-ical factors affecting word recognition when listening in noise and “in quiet. ” The results of the third e-periment permit e-tension of NAM theory to individuals with sensorineural hearing loss. Future development of speech recognition tests should allow for the effects of higher level cognitive (le-ical) factors on lower level phonemic processing.