Abstract
A long-standing but implicit assumption is that words strongly associated with a presented cue are automatically activated in the memory through rapid spread of activation within brain semantic networks. The current study was aimed to provide direct evidence of such rapid access to words’ semantic representations and to investigate its neural sources using magnetoencephalography (MEG) and distributed source localization technique. Thirty-three neurotypical subjects underwent the MEG recording during verb generation task, which was to produce verbs related to the presented noun cues. Brain responses evoked by the noun cues were examined while manipulating the strength of association between the noun and the potential verb responses. The strong vs. weak noun-verb association led to a greater noun-related neural response at 250–400 ms after cue onset, and faster verb production. The cortical sources of the differential response were localized in left temporal pole, previously implicated in semantic access, and left ventrolateral prefrontal cortex (VLPFC), thought to subserve controlled semantic retrieval. The strength of the left VLPFC’s response to the nouns with strong verb associates was positively correlated to the speed of verbs production. Our findings empirically validate the theoretical expectation that in case of a strongly connected noun-verb pair, successful access to target verb representation may occur already at the stage of lexico-semantic analysis of the presented noun. Moreover, the MEG results suggest that contrary to the previous conclusion derived from fMRI studies left VLPFC supports selection of the target verb representations, even if they were retrieved from semantic memory rapidly and effortlessly. The discordance between MEG and fMRI findings in verb generation task may stem from different modes of neural activation captured by phase-locked activity in MEG and slow changes of blood-oxygen-level-dependent (BOLD) signal in fMRI.
Highlights
The retrieval of an intended word from memory storage is a crucial process for speech production
Event-Related Response to the Visual Noun Cue in Verb Generation Task Sensor-level clusters Figure 2B presents a butterfly plot of overlapped evoked responses from all MEG channels averaged across both experimental conditions
Two early components demonstrated typical posterior scalp distributions (Figure 2A) that allowed to identify them as MEG counterparts of P100 and N170 components established in visual word recognition event related potential (ERP) studies (e.g., Hauk and Pulvermüller, 2004; Maurer et al, 2005)
Summary
The retrieval of an intended word from memory storage is a crucial process for speech production. According to conventional models, such high speed of word retrieval is based on well-learned word associations, which become embedded in the structure of semantic memory through frequent co-occurrence in the experience (Ferrer i Cancho and Solé, 2001; Nelson et al, 2004). The key premise for many word production studies lies in that strong links enable quick and automatic retrieval of the related words (e.g., Badre and Wagner, 2002; Whitney et al, 2011). Activation can spread across the strong links from one representation to another automatically, i.e., without additional effort or time delay
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