Enhanced Learning of Sensory-Motor Pairings via Prefrontal Cortex Stimulation

In cognitive processing, response selection is a central, amodal stage at which the most appropriate response to a given stimulus is chosen. When two responses are required at once, they must be processed serially, creating a delay for the second response; this is termed the “central bottleneck”. Recent studies using transcranial direct current stimulation (tDCS) have localised response selection and the central bottleneck to the left posterior lateral prefrontal cortex (pLPFC). There has been extensive investigation into the central bottleneck in the context of training response selection to attenuate dual-task costs. However, the processes involved in initial learning, where stimulus-response mappings are first established, have been overlooked to date. Here, we aimed to investigate the neural underpinnings of this early learning of stimulus-response mappings. Specifically, we examined whether the left pLPFC is causally involved in response selection learning. In the first experiment, we tested three 6 alternate-discrimination tasks where participants learned arbitrary stimulus-response mappings with no prior familiarisation. Results showed a robust learning effect for each task, and also revealed that the tasks varied in difficulty. In the second experiment, a different group of participants completed the same tasks, after receiving either anodal, cathodal, or sham tDCS to the left pLPFC. Collapsed across tasks, there was no effect of tDCS on learning. However, effects of tDCS were found at the task level. For the easiest task, active tDCS (anode and cathode) facilitated learning, relative to sham; however, there were no tDCS effects for more difficult tasks. These findings provide preliminary evidence that the left pLPFC is causally involved in initial learning of stimulus-response mappings. Future directions are discussed, including examining the interaction of tDCS effects with task difficulty under conditions of sufficient statistical power, disentangling task difficulty from stimulus modality, and using brain imaging to further examine how the left pLPFC encodes response selection mappings