Neighbourhood density effects in auditory nonword processing in aphasia

The question how prior knowledge of words affects processing of nonwords is important with respect to the study of lexical access and lexical competition. According to the Neighbourhood Activation model (Luce & Pisoni, 1998), the auditory presentation of a word or nonword yields automatic activation of all words in the mental lexicon that sound similar. Experimental results with unimpaired listeners on a lexical decision task showed that nonwords with dense lexical neighbourhoods led to slower responses and lower accuracy rates than nonwords with sparse neighbourhoods: the nonword must be compared against existing entries, which takes more time when there are more neighbours. The present study examined these effects of neighbourhood density on auditory lexical decision performance for nonwords with three groups of listeners: Broca’s aphasic patients, Wernicke’s aphasic patients, and age-matched nonbrain-damaged participants. Broca’s and Wernicke’s aphasic patients have been shown to be differentially impaired in lexical access: whereas a number of auditory lexical access studies have shown that lexical activation levels are reduced in Broca’s aphasic patients (Aydelott Utman, Blumstein, & Sullivan, 2001; Milberg, Blumstein, & Dworetzky, 1988), these levels are increased in Wernicke’s aphasic patients (Milberg et al., 1988) who may thus be impaired in selecting one winning word candidate among the activated candidates. In unimpaired speech processing, words are activated relative to the overall goodness of fit with the auditory input. In Wernicke’s aphasia, less appropriate acoustic forms seem to be able to activate lexical items almost to the same extent as exact-match items. Broca’s aphasics, on the other hand, did not activate words such as coat when presented with an acoustically distorted form (VOT-manipulated c*oat) if there was a lexical competitor (such as goat: cf. Aydelott Utman et al., 2001). If overall lexical activation is reduced for Broca’s aphasic patients, the difference between nonwords from dense neighbourhoods and from sparse neighbourhoods may be reduced as well. For Wernicke’s aphasic patients with increased lexical activation levels, however, neighbourhood density may exert a bigger influence. Thus, based on the assumption that the magnitude of the neighbourhood density effect is influenced by the overall level of lexical activation, smaller neighbourhood density effects on lexical decision performance were expected for Broca’s aphasic patients, whereas enlarged density effects were expected for Wernicke’s aphasic patients, compared to age-matched control participants.