Examination of the neighbor-hood activation theory in normal and hearing-impaired listeners

Objective: Experiments were conducted to examine the effects of lexical inf ormation on word recognition among normal hearing listeners and individuals with sensorineural hearing loss. The lexical factors of interest were inco rporated in the Neighborhood Activation Model (NAM). Central to this model is the concept that words are recognized relationally in the context of oth er phonemically similar words. NAM: suggests that words in the mental lexic on are organized into similarity neighborhoods and the listener is required to select the target word from competing lexical items. Two structural cha racteristics of similarity neighborhoods that influence word recognition ha ve been identified; "neighborhood density" or the number of phonemically si milar words (neighbors) for a particular target item and "neighborhood freq uency" or the average frequency of occurrence of all the items within a nei ghborhood. A third lexical factor, "word frequency" or the frequency of occ urrence of a target word in the language, is assumed to optimize the word r ecognition process by biasing the system toward choosing a high frequency o ver 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 lev els of the three lexical factors, word frequency (high and low), neighborho od density thigh and low), and average neighborhood frequency thigh and low ). Neighborhood structure and word frequency were estimated computationally using a large, on-line lexicon-based Webster's Pocket Dictionary. From thi s program 400 highly familiar, monosyllables were selected and partitioned into eight orthogonal lexical groups (50 words/group). The 400 words were p resented randomly to normal hearing listeners in speech-shaped noise (Exper iment 1) and "in quiet" (Experiment 2) as well as to an elderly group of li steners with sensorineural hearing loss in the speech-shaped noise (Experim ent 3). Results: The results of three experiments verified predictions of NAM in bo th 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 tha n comparable conditions with low frequency neighbors. Word frequency was fo und to have a highly significant effect on word recognition. Lexical condit ions with high word frequencies produced higher performance scores than con ditions with low frequency words. Conclusion: The results supported the basic tenets of NAM theory and identi fied both neighborhood structural properties and word frequency as signific ant lexical factors affecting word recognition when listening in noise and "in quiet." The results of the third experiment permit extension of NAM: th eory to individuals with sensorineural hearing loss. Future development of speech recognition tests should allow for the effects of higher level cogni tive (lexical) factors on lower level phonemic processing.