Listening to Yourself: Inner Speech Across the Lifespan

Kray, Eber and Lindenberger recently investigated the role of inner speech (i.e., talking to yourself) on executive functioning, the "higher-level processes that organize lower-level processes in order to regulate and verify behavioral activity." These functions are frequently impaired in children and the elderly alike, as measured by tasks of inhibition (how quickly you can stop executing a certain task) as well as by task-switching (how quickly you can go from one task to another). These functions are thought to rely on a critical prefrontal neural network that is among the last to mature in the developing brain, as well as the first to degrade in old age.

To what extent can these executive control functions be influenced by inner speech? Both children and the elderly can be observed to talk to themselves aloud, which according to Kray et al., "can be seen as an attempt to use language as a tool to plan, guide, and monitor goal-directed activity." In Baddeley's model of working memory, the articulatory rehearsal process of the phonological loop represents exactly this pathway, in which information can be actively maintained in a "verbal buffer" of sorts.

One previous study found that relevant inner speech was helpful relative to irrelevant inner speech in a task switching paradigm, but only when the interval between cue (which specifies which task should be performed) and target (when a response should be made) was very long (Goschke, 2000 cited by Kray et al.). Another previous study (Emerson & Miyake, 2003, cited by Kray et al.) found that inner speech most strongly affected global switch costs, i.e. set-selection costs, when external task cues were not present - or in other words, when subjects had to internally maintain the order of tasks, and which task should be performed next.

[Incidentally, the same authors also verified that this global switch cost increase was not simply a result of the dual-tasking, by replacing the articulatory suppression with a tapping task; then the same increase in global switch cost was not observed.]

To further examine the role of inner speech in executive functioning across developmental time, the authors used a task switching paradigm in which 48 subjects (16 young adults, 16 older adults, and 16 children) had to identify whether the target picture was either a fruit or an animal, OR they had to identify whether the target picture was colored or gray. The specific task that a subject had to perform on any given trial was indicated by a cue, presented 1400 ms before the target picture. During this cue-target interval (CTI), subjects had to read aloud either a task compatible word (i.e., if the current task was to judge gray vs. color, the word might be "colored") a task irrelevant word (i.e., the word might be "round"), or a task incompatible word (i.e., the word might be "animal" or "fruit"). Note that in the language used here (German) all of these were 4 letter, one syllable words. Two control conditions were also included, to ensure that any difference in switch costs seen in any of the three above conditions were due to disruption of inner speech, and not simply to the extra cognitive demands of performing a task during the CTI. These control conditions were either to do nothing, or to do a simple motor task.

Leaving aside further methodological and analytical details, the major results of the study are as follows:

• Children and older adults show greater set-selection costs (aka global switch costs) on average than young adults, but there are no age differences in terms of local switch costs.
• It is possible to both positively and negatively prime task-switching with verbalization, even when the response modality is non-verbal - in other words, task-incompatible verbalization hurts task switching, while task-compatible verbalization helps task switching; Furthermore, children "more strongly profit" from task-compatible verbalizations, while the elderly 'suffer' more strongly from task-incompatible verbalizations, than young adults;
• By mature adulthood, the use of inner speech is so well automatized that it does not result in a significant dual-task RT cost - in other words, the older adults were not significantly slower when task-switching while verbalizing as compared to task-switching alone;
• Dual tasking during a task-switching paradigm has its strongest effects on both children and the elderly when the dual-task involves a reponse in the same modality as the primary tasks; however, for young adults the pattern is reversed, such that a secondary verbal task is more disruptive to tasks requiring motor outputs than a motor task.

With regards to the last point, it's also possible that young adults rely on inner speech more strongly for task set selection than children and older adults. It's also possible that, as the authors and many others have argued, children and the elderly have increased difficulty in resolving interference (or inhibiting alternative responses) at output stages.