Exploring the inner speech process in verbal working memory

Verbal working memory (VWM) is the ability to dynamically preserve and manipulate verbal information for brief periods of time. VWM is maintained through a silent "inner speech" process (Baddeley, 1986; Baddeley & Hitch, 1974). It is well established in the behavioral and neuroimaging literature that VWM can be disrupted by the simultaneous (concurrent) performance of simple speech tasks (e.g. overt concurrent articulation of a word or digit) (Caplan et al., 2000; Larsen & Baddeley, 2003). Our primary goal in these experiments is to test whether VWM and overt concurrent articulation will have one or more overlapping regions of activation in areas commonly associated with speech processing, and to determine whether such regions are active during simple tapping tasks. Due to concerns about overt movement artifacts, we also explore covert version of speech and tapping tasks. Experiment 1 was a behavioral study that examined the effects of overt and covert concurrent articulation and finger tapping on VWM. We found that overtly and covertly concurrently articulating "the" were the most detrimental to subjects' recall ability. These effects could be attributed to dual-task interference effects at the level of inner speech in VWM, thus, indicating a shared set of neural regions for all speech and VWM. At the same time, the effect sizes were different for the overt and covert versions of our tasks, raising questions about the common assumption of shared substrates. Experiment 2 was an imaging study designed to examine whether there were shared neural regions between simple speech tasks and VWM and to further explore differences between overt and covert tasks. The results from this experiment provided only weak evidence implicating two candidate regions as the shared locus of activation: the left cerebellum and left superior temporal gyrus. We also found interesting evidence in support of distinct sets of regions for overt versus covert versions of the tasks