Diverse ways of holding verbal information in mind revealed with functional Magnetic Resonance Imaging and Transcranial Magnetic Stimulation: Individual differences in left anterior parietal cortex

Cognitive neuroscience has identified brain systems that reliably underpin specific abilities, including verbal short-term memory. However, there is no simple mapping between anatomy and function; moreover, brains are differently organised across people. In two functional magnetic resonance imaging studies, and using transcranial magnetic brain stimulation to test causality, we characterised differences in the neural processes supporting verbal short-term memory in healthy participants who varied in their use of semantic information. In group analyses, left anterior inferior parietal cortex showed the expected pattern for a "phonological buffer" -- this site was responsive to phonological tasks and verbal short-term memory for meaningless items. However, the functions of this site differed across the sample; individuals with the strongest "semantic reliance" in short-term memory (showing higher imageability effects and poorer nonword performance) showed weaker phonological responses. They also showed more activation when maintaining meaningful word sequences in short-term memory, compared with nonwords. In these semantically-reliant participants, left anterior inferior parietal cortex showed stronger functional connectivity to limbic and default mode regions during nonword rehearsal. While inhibitory stimulation to phonologically responsive regions disrupted nonwords more than words in people with good phonological short-term memory, these effects were reversed when verbal short-term memory was more semantically reliant. These findings show that left anterior inferior parietal cortex supports the maintenance of different kinds of verbal information across individuals.