Direct current stimulation modulates prefrontal cell activity and behaviour without inducing seizure-like firing

Transcranial direct current stimulation (tDCS) has garnered significant interest for its potential to enhance cognitive functions and as a therapeutic intervention in various cognitive disorders. However, the clinical application of tDCS has been hampered by significant variability in its cognitive outcomes. Furthermore, the widespread use of tDCS has raised concerns regarding its safety and efficacy, particularly in light of our limited understanding of its underlying neural mechanisms at the cellular level. We still do not know 'where', 'when' and 'how' tDCS modulates information encoding by neurons, in order to lead to the observed changes in cognitive functions. Without elucidating these fundamental unknowns, the root causes of its outcome variability and long-term safety remain elusive, challenging the effective application of tDCS in clinical settings. Addressing this gap, our study investigates the effects of tDCS, applied over the dorsolateral prefrontal cortex, on cognitive abilities and individual neuron activity in macaque monkeys performing cognitive tasks. Like humans performing a delayed match-to-sample task, monkeys exhibited practice-related slowing in their responses (within-session behavioural adaptation). Concurrently, there were practice-related changes in simultaneously recorded activity of prefrontal neurons (within-session neuronal adaptation). Anodal tDCS attenuated both these behavioural and neuronal adaptations when compared with sham stimulation. Furthermore, tDCS abolished the correlation between response time of monkeys and neuronal firing rate. At a single-cell level, we also found that following tDCS, neuronal firing rate was more likely to exhibit task-specific modulation than after sham stimulation. These tDCS-induced changes in both behaviour and neuronal activity persisted even after the end of tDCS stimulation. Importantly, multiple applications of tDCS did not alter burst-like firing rates of individual neurons when compared with sham stimulation. This suggests that tDCS modulates neural activity without enhancing susceptibility to epileptiform activity, confirming a potential for safe use in clinical settings. Our research contributes unprecedented insights into the 'where', 'when' and 'how' of tDCS effects on neuronal activity and cognitive functions by showing that modulation of the behaviour of monkeys by the tDCS of the prefrontal cortex is accompanied by alterations in prefrontal cortical cell activity ('where') during distinct trial phases ('when'). Importantly, tDCS led to task-specific and state-dependent alterations in prefrontal cell activities ('how'). Our findings suggest a significant shift from the view that the effects of tDCS are merely attributable to polarity-specific shifts in cortical excitability and instead propose a more complex mechanism of action for tDCS that encompasses various aspects of cortical neuronal activity without increasing burst-like epileptiform susceptibility.