Temporal properties of rehearsal in auditory-verbal short-term memory

Subvocal rehearsal, the use of inner speech for the maintenance of phonological material, is thought to play an important role verbal short-term memory (STM). The importance of rehearsal is based largely on indirect measures, as it is difficult to detect and quantify. To address this issue and investigate rehearsal timing, a novel ‘rehearsal-probe’ task was developed. Individuals silently rehearsed an auditory-verbal sequence, responding after an unpredictable probe (tone) by indicating the item currently being rehearsed. The presentation of probes after variable and repeated delays provides item response proportions over time. The data were analysed using a theory-neutral measure of temporal precision; the circular standard deviations of response distributions.
The methods were established across seven experiments designed to explore whether timing precision is fixed or resource-limited. Experiment 3 showed that timing precision decreases with increased in memory load. Temporal precision was negatively correlated with auditory-verbal STM span in six experiments, including one designed specifically to examine individual differences. Experiments 6 and 7 investigated timing in developmental language disorders, which are characterized by serial ordering deficits. Adults with dyslexia and children with language impairments showed more temporal imprecision compared to matched controls. These results suggest that temporal precision is limited by shared resources and may play a role in language development.
A computational model was also developed to describe the data with four separable temporal properties. The model captured the main characteristics of the data and provided quantitative estimates of each property. In an EEG experiment, event-related responses to item probes were modulated by the contents of rehearsal, and there was increased spectral power at the item rate during sequence presentation and rehearsal, but not baseline, periods. The findings suggest an important role for fine-grained timing information in serial order STM and have broader implications for debates about models of serial order.