Mind what you say—general and specific mechanisms for monitoring in speech production

For most people, speech production is relatively effortless and error-free. Yet it has long been recognized that we need some type of control over what we are currently saying and what we plan to say. Precisely how we monitor our internal and external speech has been a topic of research interest for several decades. The predominant approach in psycholinguistics has assumed monitoring of both is accomplished via systems responsible for comprehending others' speech. This special topic aimed to broaden the field, firstly by examining proposals that speech production might also engage more general systems, such as those involved in action monitoring. A second aim was to examine proposals for a production-specific, internal monitor. Both aims require that we also specify the nature of the representations subject to monitoring. Domain general mechanisms Some of the first evidence to support a proposal of a domain general monitoring or attentional selection mechanism being engaged in speech production was provided by functional magnetic resonance imaging (fMRI) investigations of semantic interference effects in picture-naming paradigms. Those studies identified differential activity in the anterior cingulate cortex (ACC), noting similar activity had been observed in fMRI studies of picture-word (PWI), Stroop and manual interference paradigms (e.g., de Zubicaray et al., 2006). Piai et al. (2013) provide the first confirmatory evidence of a domain general, ACC involvement during performance of Stroop, semantic PWI and manual Simon tasks. Although PWI might be considered a generalization of Stroop-like interference effects (e.g., MacLeod, 1991), the Simon task elicits an interference effect in manual responding by manipulating the spatial location of target stimuli (e.g., a square or triangle) on congruent and incongruent trials. Thus, any overlap in ACC activity across the three tasks can be interpreted as reflecting a domain general mechanism. Electrophysiological studies provide another source of evidence for a domain general fronto-central monitoring mechanism via both stimulus- and response-locked analyses (Ries et al., 2013; Trewartha and Phillips, 2013; Acheson and Hagoort, 2014). Using a phoneme substitution task, Trewartha and Phillips (2013) report an error-related negative potential (ERN) similar to that observed for manual actions. They conclude that speech errors are detected by a general conflict monitoring mechanism supported by the ACC. However, Acheson and Hagoort (2014) tested the domain generality assumption by directly comparing event-related potentials (ERPs) on tongue twister (TT) and Flanker tasks. The TT task required participants to repeatedly and rapidly read sequences of regular non-words. Despite observing similar ERNs, Acheson and Hagoort observed few correlations between behavioral or electrophysiological measures from the two tasks. Together, the results from the above studies suggest that competition among lexical-level representations might engage a domain general mechanism in the ACC, while conflicting sub-lexical (i.e., phoneme) level representations might engage a different mechanism, either in another subdivision of the ACC or elsewhere in superior medial frontal cortex. The working memory requirements of production paradigms might also be crucial for the engagement of domain general mechanisms. Ries et al. (2013) address this issue in patients with lesions of the lateral prefrontal cortex (DLPFC), with results indicating that picture naming might not require mechanisms mediated by this region, unless working memory load is increased, as per the verbal Simon task they employed. Crowther and Martin (2014) demonstrate that working memory is clearly involved in more complex production paradigms that manipulate both semantic context and item repetition, such as blocked cyclic naming, finding significant correlations with verbal memory span. In addition, they report the decreasing slope of naming latencies within cycles derives from a process of narrowing down available responses from the set of items within the block as the trials progress. This indicates participants monitor the item names they produce in order to perform the task. Further, they argue this reflects a strategic, task-specific process rather than a mechanism involved in word production in more naturalistic settings. Intriguingly, Ries et al.'s and Crowther and Martin's converging conclusions regarding the involvement of working memory in production are supported by a recent study by Wirth et al. (2011). The latter authors employed anodal transcranial direct current stimulation (atDCS), an electrical brain stimulation technique that induces more efficient neural processing at the stimulation site. They reported aTDCS applied over the left DLPFC reduced the semantic interference effect observed in the blocked cyclic naming paradigm.