In this study, we use an auditory picture–word interference task to examine an anomic individual, NP. NP produced semantic errors in picture naming, but his comprehension was relatively well preserved. In the task, pictures to be named were accompanied by semantically, phonologically or unrelated distractors, presented at onsets ranging from −200 ms (before target) to +400 ms (after target). Naming latencies were measured. A group of 12 older controls showed semantic interference (slower latencies with semantic than with unrelated distractors), which was significant at −200 ms, and steadily diminished across later onsets. In contrast, at 0 ms, NP showed powerful semantic facilitation. There were no significant semantic effects at other onsets, but the trends, particularly at later onsets, were towards interference. Phonological effects for NP were in the same direction as for controls (facilitation) but were of greater magnitude. Indeed, NP showed a reliable facilitatory effect at 0 ms (and trends at −200 ms and +200 ms), but a similar trend in controls failed to reach significance. Within recent models of this task, in which semantic facilitation effects are attributed to an early, pre-lexical semantic processing stage, NP's pattern indicates that semantic processing is abnormally prolonged. The phonological facilitation effects are also consistent with this interpretation. We discuss their implications and future applications of the task to aphasia.
According to most current theories, word production involves at least two major processes or stages—lexical selection and phonological encoding (e.g., Caramazza, 1997, Dell, 1986, Dell, 1988 and Levelt, 1989; Levelt, Roelofs, & Meyer, 1999; MacKay, 1987, Roelofs, 1992, Roelofs, 1997 and Stemberger, 1985). During lexical selection, the speaker retrieves a representation of the word that best matches the concept to be expressed, which is not yet specified as to its form.1 During phonological encoding, the speaker then generates a complete phonological plan for the word. Recent research has utilised this two-stage framework to characterise word production impairments in aphasia (e.g., Badecker, Miozzo, & Zanuttini, 1995; Foygel & Dell, 2000; Laine & Martin, 1996; Lambon Ralph, Sage, & Roberts, 2000; Rapp & Goldrick, 2000; Wilshire & Saffran, 2005). Within this framework, a word production disorder can be attributed to any one of three primary types of impairment. The first is a general impairment activating semantic representations, which will lead to errors in both production and comprehension, particularly semantic errors (e.g., Howard & Orchard-Lisle, 1984; Rapp & Goldrick, 2000 (patient KE)). The second is an impairment involving the lexical selection stage of word production, which will lead to semantic and other whole-word errors in production, but should not affect word comprehension (e.g., Lambon Ralph et al., 2000; Rapp & Goldrick, 2000 (patient PW)). The third is a phonological encoding impairment, which will lead to the production of phonological errors (e.g., Rapp & Goldrick, 2000 (patient CSS); Wilshire & Nespoulous, 2003).
Crucially, within this framework, high rates of semantic paraphasias can occur with both lexical selection and more general semantic processing impairments. These two impairments are distinguished primarily by comparing word production and comprehension. One problem with this method is that production and comprehension tasks are not of equivalent difficulty: comprehension tasks usually involve making selections between alternatives, and can often be done on the basis of quite skeletal information, whereas picture naming requires a full understanding of the item's identity. A mild semantic processing impairment might therefore be evident only in the more difficult task of word production, and could easily be misdiagnosed as a lexical selection impairment. It would therefore be extremely useful to be able to tease apart these two hypothesised processing stages by using some kind of direct experimental manipulation. In this study, we investigate one particular experimental task that may offer potential in this respect, the auditory picture–word interference task. In this task, the participant must name a picture while ignoring an auditory distractor word. It is possible to manipulate not only the nature of the distractor word, but also its precise onset, and in so doing to examine the types of variables that affect performance at different times during the word production process. The task therefore has the potential to tell us a great deal about the nature of the impaired processing stage(s) in aphasic individuals and their temporal dynamics.
