In research on verbal working memory, articulatory rehearsal, a maintenance mechanism for phonological representations, has undergone intensive and excellent study. Possible mechanisms for semantic representation have received less attention. However, several studies have reported a double dissociation in types of memory deficits (semantic memory difficulties vs. phonological memory difficulties). This suggests the separability of two maintenance mechanisms. The present study focused on this separability in individuals with normal memory abilities, using a dual-task interference paradigm. The results indicate a crossover interaction between memory and interference task effects: Preventing articulatory rehearsal more strongly disrupted the phonological memory task, whereas performing a tapping task that interfered with attentional control more strongly disrupted semantic memory. These results suggest that semantic representations are actively maintained by a mechanism other than phonological maintenance.
Models of verbal working memory that incorporate active memory maintenance, long-term memory networks, and attention control have been developed. Current studies suggest that semantic representations of words, evoked via long-term memory networks, are actively maintained until they are needed to fulfill a role. In other words, it is possible that some mechanism actively refreshes semantic representations of words, analogous to but independently from articulatory rehearsal which refreshes phonological representations. One valuable piece of evidence is a double dissociation, observed in a dual task paradigm in which manual tapping disrupted a semantic memory task while articulatory suppression disrupted a phonological memory task. However, in that study, the secondary tasks could have competed not only with the maintenance but also with the encoding activities. Additionally, the study items in the phonological memory tasks were words; hence, the discriminability of the memory tasks is doubtful. The present study, therefore, examined a potential double dissociation in situations where the secondary tasks could not compete with encoding, using a modified phonological memory task. Furthermore, this article discusses a potential mechanism for maintaining semantic representations.
Several neuropsychological studies have reported that patients with memory deficits exhibit a dissociation of effects attributed to semantic and lexical-phonological information in verbal working memory (e.g., Reilly, Martin, & Grossman, 2005; Romani & Martin, 1999). The present study reports on 3 experiments conducted with individuals without memory problems to examine the dissociation between the word imageability effect (WIE) and the word frequency effect (WFE). Experiment 1 used a modified semantic recognition task, in which participants were presented with 10-item lists and selected synonyms of list words from among 5 words. This task led to a significant WIE but no WFE. On the other hand, Experiment 2 used a modified recognition task in which targets were homophones, to promote the usage of phonological information while minimizing the potential impact of semantic information. There was no WIE but a significant WFE in Experiment 2. Experiment 3 replicated the results of Experiment 1 using 5-item lists and a 2-alternative forced choice procedure. Taken together, these results systematically replicate the double dissociation observed in neuropsychological studies. These findings suggest that semantic information can contribute to successful retrieval from verbal working memory without mediation from lexical-phonological information.
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