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Although animal research implicates a central role for dopamine in motor skill learning, a direct causal link has yet to be established in neurotypical humans. Here, we tested if a pharmacological manipulation of dopamine alters motor learning, using a paradigm which engaged explicit, goal-directed strategies. Participants (27 females, 11 males, aged 18-29 years) first consumed either 100mg of Levodopa (n=19), a dopamine precursor that increases dopamine availability, or placebo (n=19). Then, during training, participants learnt the explicit strategy of aiming away from presented targets by instructed angles of varying sizes. Targets shifted mid-movement by the instructed aiming angle. Task success was thus contingent upon aiming accuracy. The effect of the dopamine manipulations on skill learning was assessed during training, and at an overnight follow-up. Increasing dopamine availability improved aiming accuracy and lengthened reaction times, particularly for larger, more difficult aiming angles, both at training, and at follow-up. Results support the proposal that dopamine is important in decisions to engage instrumental motivation to optimise performance, particularly when learning to execute goal-directed strategies in motor skill learning.

A key strategic decision one must make in virtually every task context concerns the speed accuracy trade-off (SAT). Experimentally, this ubiquitous phenomenon, whereby response speed and task accuracy are inversely related, is typically studied by explicitly instructing participants to adjust their strategy: by either focusing on speed, or on accuracy. Computational modelling has been applied to deconvolve the latent decision processes involved in the SAT, with considerable evidence suggesting that response caution (the amount of evidence needed for a decision to be reached) is a key variable in the setting of SAT strategy. Neuroimaging has implicated the prefrontal cortex, the pre-supplementary motor area (preSMA), and the striatum in the setting of response caution. In addition, brain stimulation has provided causal evidence for the involvement of the left prefrontal cortex and superior medial frontal cortex (SMFC, which includes the preSMA) in adjustments of response caution following explicit instructions, although stimulation of the two regions has dissociable effects. Here, in a double-blind and preregistered study we investigated the role of these two regions using an incidental manipulation of SAT strategy - via stimulus signal variability - which has previously been shown to influence decision confidence. We again found tDCS applied to both regions modulated response caution, and there was a dissociation: stimulating prefrontal cortex increased, and stimulating SMFC decreased, response caution. These findings provide further support for key, but dissociable, roles of these brain regions in decision strategies whether they are implemented explicitly or incidentally.

Cognitive training and brain stimulation show promise for ameliorating age-related neurocognitive decline. However, evidence for this is controversial. In a Registered Report, we investigated the effects of these interventions, where 133 older adults were allocated to four groups (left prefrontal cortex anodal transcranial direct current stimulation (tDCS) with decision-making training, and three control groups) and trained over 5 days. They completed a task/questionnaire battery pre- and post-training, and at 1- and 3-month follow-ups. COMT and BDNF Val/Met polymorphisms were also assessed. Contrary to work in younger adults, there was evidence against tDCS-induced training enhancement on the decision-making task. Moreover, there was evidence against transfer of training gains to untrained tasks or everyday function measures at any post-intervention time points. As indicated by exploratory work, individual differences may have influenced outcomes. But, overall, the current decision-making training and tDCS protocol appears unlikely to lead to benefits for older adults.

Blinding in non-invasive brain stimulation research is a topic of intense debate, especially regarding the efficacy of sham-controlled methods for transcranial direct current stimulation (tDCS). A common approach to assess blinding success is the inclusion of correct guess rate. However, this method cannot provide insight into the effect of unblinding on observed stimulation outcomes. Thus, the implementation of measures to systematically evaluate subjective expectation regarding stimulation is needed. Previous work evaluated subjective effects in an earlier study which reported a mind-wandering and tDCS data set and concluded that subjective belief drove the pattern of results observed. Here we consider the subjective and objective intervention effects in a key contrast from that data set—2 mA vs. sham—which was not examined in the reanalysis. In addition, we examine another key contrast from a different tDCS mind-wandering study that employed similar methodology. Our findings support objective intervention as the strongest predictor of the observed effects of mind-wandering in both re-analyses, over and above that of subjective intervention. However, it is important to control for and understand the possible inadequacies of sham-controlled methods.

BackgroundEstablishing adequate blinding for non-invasive brain stimulation research is a topic of extensive debate, especially regarding the efficacy of sham control methods for transcranial direct current stimulation (tDCS) studies. Fassi and Cohen Kadosh [1] assessed the influence of subjective participant belief regarding stimulation type (active or sham) and dosage on behaviour using data from Filmer et al. [2] who applied five stimulation protocols (anodal 1.0mA, cathodal 1.0mA, cathodal 1.5mA, cathodal 2.0mA and sham) to assess the neural substrates of mind wandering. Fassi and Cohen Kadosh [1] concluded that subjective belief drove the pattern of results observed by Filmer et al. [2].ObjectiveFassi and Cohen Kadosh [1] did not assess the key contrast between conditions in Filmer et al. (2019) – 2mA vs sham – rather they examined all stimulation conditions. Here, we consider the relationship between objective and subjective intervention in this key contrast.MethodsWe replicated the analysis and findings of both Filmer et al. [2] and Fassi and Cohen Kadosh [1] before assessing 2mA vs. sham via Bayesian ANOVA on subjective belief regarding stimulation type and dosage.ResultsOur results support objective intervention as the strongest predictor of stimulation effects on mind-wandering when 2mA vs sham was examined, over and above that of subjective intervention.ConclusionsThe conclusions made by Filmer et al. [2] are confirmed. However, it is important to control for and understand the possible effects of subjective beliefs in sham-controlled studies. Best practice to prevent these issues remains the inclusion of active control conditions.

The application of transcranial direct current stimulation (tDCS) to the prefrontal cortex has the potential to improve performance more than cognitive training alone. Such stimulation-induced performance enhancements can generalize beyond trained tasks, leading to benefits for untrained tasks/processes. We have shown evidence that stimulation intensity has non-linear effects on augmenting cognitive training outcomes. However, it is currently unclear how stimulation intensity augments cognitive processing to impact training and transfer effects. Here, we applied decision-making modelling via the linear ballistic accumulator framework to understand what aspects of cognitive processes underlying speeded single-/dual-task decision-making performance change with tDCS intensity. One hundred and twenty-three participants were split into four groups: sham, 0.7 mA, 1.0 mA and 2.0 mA stimulation intensities. Participants completed four training sessions whilst tDCS was delivered. The 0.7 mAamp; 1.0 mA intensities provided the greatest benefit for performance (increased decision-making efficiency as measured by drift rates) on the trained task - more than sham or 2.0 mA stimulation. The latent decision components integrated both accuracy and reaction times to estimate performance more broadly. We see an inverted u-shaped function of stimulation intensity and cognitive performance in the trained-on task, where either no stimulation or too much stimulation is sub-optimal for performance. By contrast, 1.0 mA and 2.0 mA intensities led to increased drift rates in an untrained (transfer) single task. In sum, tDCS intensity non-linearly modulates cognitive processes related to decision-making efficiency.

Applying a weak electrical current to the cortex can have effects on a range of behaviours. Techniques such as transcranial direct current stimulation (tDCS) have been widely used in both research and clinical settings. However, there is significant variability across individuals in terms of their responsiveness to stimulation, which poses practical challenges to the application of tDCS, but also provides a unique opportunity to study the link between the brain and behaviour. Here, we assessed the role of individual differences in cortical morphology - specifically in prefrontal cortical regions of interest - for determining the influence of tDCS on decision-making performance. Specifically, we employed magnetic resonance imaging (MRI) and a previously replicated paradigm in which we modulated learning in a simple decision-making task by applying tDCS to the left prefrontal cortex in human subjects of both sexes. Cortical thickness of the left (but not right) prefrontal cortex accounted for almost 35% of the variance in stimulation efficacy across subjects. This is the first demonstration that variations in cortical architecture are associated with reliable differences in the effects of tDCS on cognition. Our findings have important implications for predicting the likely efficacy of different non-invasive brain stimulation treatments on a case by case basis.

The speed-accuracy trade-off (SAT) is arguably the most robust finding in cognitive psychology. This simple and intuitive effect (the faster subjects respond, the more likely they are to make an error) has been the subject of extensive empirical and modeling work to ascertain the underlying latent process(es). One such process is response caution-the amount of evidence to be acquired before a decision is reached-with debate regarding the involvement of another latent variable, the rate of evidence accumulation. Neuroimaging has implicated two frontal regions as neural substrates of the SAT: the posterior lateral prefrontal cortex and the pre-supplementary motor area (preSMA; part of the superior medial frontal cortex; SMFC). However, there is no causal evidence for these regions' involvement in the SAT, nor is it clear what role each plays in the underlying processes. In a double-blind, preregistered study, we applied cathodal transcranial direct current stimulation (offline) to the prefrontal and SMFC. The SAT was measured using a dot-motion task, with differing response instructions (focus on accuracy, speed, or both). The linear ballistic accumulator model indicated performance modulations were driven by response caution. Moreover, both target regions modulated caution but in opposing directions: Prefrontal stimulation increased, and SMFC stimulation decreased, caution. Discriminability (difference between correct and error evidence accumulation rates) was predominantly affected by stimulation targeting the SMFC and did not vary with response instructions. Overall, the findings indicate that while both the SMFC and the prefrontal cortex are causally involved in the SAT, they play distinct roles in this phenomenon. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Transcranial direct current stimulation (tDCS) has been shown to improve single- and dual-task performance in healthy participants and enhance transferable training gains following multiple sessions of combined stimulation and task practice. However, it has yet to be determined what the optimal stimulation dose is for facilitating such outcomes. We aimed to test the effects of different tDCS intensities, with a commonly used electrode montage, on performance outcomes in a multisession single/dual-task training and transfer protocol. In a preregistered study, 123 participants, who were pseudorandomized across four groups, each completed six sessions (pre- and posttraining sessions and four combined tDCS and training sessions) and received 20 min of prefrontal anodal tDCS at 0.7, 1.0, or 2.0 mA or 15-s sham stimulation. Response time and accuracy were assessed in trained and untrained tasks. The 1.0-mA group showed substantial improvements in single-task reaction time and dual-task accuracy, with additional evidence for improvements in dual-task reaction times, relative to sham performance. This group also showed near transfer to the single-task component of an untrained multitasking paradigm. The 0.7- and 2.0-mA intensities varied in which performance measures they improved on the trained task, but in sum, the effects were less robust than for the 1.0-mA group, and there was no evidence for the transfer of performance. Our study highlights that training performance gains are augmented by tDCS, but their magnitude and nature are not uniform across stimulation intensity.

Mind wandering has been associated with both adaptive outcomes and performance impairment, depending on the context. Recently, non-invasive brain stimulation has been applied in several studies with the aim to investigate the neural region(s) casually involved in mind wandering. However, to date there has been little definitive work assessing whether or not the stimulation of different brain regions leads to distinct mind wandering outcomes. The present preregistered study considered the role of the prefrontal cortex and inferior parietal lobule in mind wandering using two stimulation intensities (1mA and 2mA) and two stimulation polarity montages. One-hundred and fifty subjects were randomly allocated to one of the four active stimulation groups or a sham group. Participants' mind wandering propensity was measured via a task unrelated thought probe dispersed throughout an attention-based task completed directly after stimulation. Anodal stimulation to the prefrontal cortex, and cathodal stimulation to the inferior parietal lobule, increased mind wandering propensity and this effect was relatively unaffected by stimulation dosage. These findings support a causal role for these two regions in mind wandering, one that is polarity specific.

The ability to process multiple sources of information concurrently is particularly impaired as individuals age and such age-related increases in multitasking costs have been linked to impairments in response selection. Previous neuroimaging studies with young adults have implicated the left hemisphere prefrontal cortex (PFC) as a key neural substrate of response selection. In addition, several transcranial direct current stimulation studies (tDCS) have provided causal evidence implicating this region in response selection and multitasking operations. For example, Filmer at al. (2013b) demonstrated that typically observed response selection learning/training gains in young adults were disrupted via offline transcranial direct current stimulation (tDCS) of left, but not right, PFC. Here, considering evidence of functional dedifferentiation in the brains of older adults, we assessed if this pattern of response selection learning disruption via tDCS to the left PFC is observed in older adults, testing if this region remains a key response selection node as individuals age. In a pre-registered study with 58 older adults, we applied anodal, cathodal, and sham stimulation to left and right PFC, and measured performance as participants trained on low- and high-response selection load tasks. Active stimulation did not disrupt training in older adults as compared to younger adults. However, there was evidence of enhanced training gains via tDCS, which scaled with response selection task difficulty. The results highlight age-related differences in the casual neural substrates that subserve response selection and learning.

Applying a weak electrical current to the cortex has the potential to modulate neural functioning and behaviour. The most common stimulation technique, transcranial direct current stimulation (tDCS), has been used for causal investigations of brain and cognitive functioning, and to treat psychiatric conditions such as depression. However, the efficacy of tDCS in modulating behaviour varies across individuals. Moreover, despite being associated with different neural effects, the two polarities of electrical stimulation – anodal and cathodal – can result in similar behavioural outcomes. Here we employed a previously replicated behavioural paradigm that has been associated with polarity non-specific disruption of training effects in a simple decision-making task. We then used the linear ballistic accumulator model to quantify latent components of the decision-making task. In addition, magnetic resonance imaging measures were acquired prior to tDCS sessions to quantify cortical morphology and local neurochemical concentrations. Both anodal and cathodal stimulation disrupted learning-related task improvement relative to sham (placebo) stimulation, but the two polarities of stimulation had distinct effects on latent task components. Whereas anodal stimulation tended to affect decision thresholds for the behavioural task, cathodal stimulation altered evidence accumulation rates. Moreover, performance variability with anodal stimulation was related to cortical thickness of the inferior frontal gyrus, whereas performance variability with cathodal stimulation was related to cortical thickness in the inferior precentral sulcus, as well as to prefrontal neurochemical excitability. Our findings demonstrate that both cortical morphology and local neurochemical balance are important determinants of individual differences in behavioural responses to electrical brain stimulation.

Transcranial electrical brain stimulation (tES) techniques have shown substantial promise in research and applied settings. However, over the last few years the technique has courted significant controversy, resulting in scepticism regarding its reported beneficial effects and future potential. In this opinion article, we examine the key points of criticism raised to date, including whether tES has any meaningful effect on the cortex, issues of replicability, and the variability in its efficacy across individuals. For each point, we assess the strength of the evidence for and against the argument and, where relevant, suggest how the field can improve. We conclude that while some of the highlighted shortcomings of research using electrical brain stimulation are justified, on balance the arguments against using such techniques in cognitive neuroscience are often overstated and elevate the risk of the field “throwing the baby out with the bath water”.

The ability to represent the internal thoughts, beliefs and desires of others, and recognise that these might be distinct from one's own, is crucial for adaptive social interaction. Such operations are thought to tap Theory of Mind (ToM), with its importance underscored by the link between ToM impairment and a range of neurodevelopmental disorders (e.g., Autism and Schizophrenia). Extensive investigations into the neural substrates of ToM, when individuals have to make overt/explicit judgments concerning others, have highlighted a link with a network of regions including the temporal parietal junction (TPJ), particularly in the right hemisphere. Recently, evidence has emerged that ToM can also operate implicitly and that this may be particularly impaired in Autism. However, very few studies have examined the neural basis of implicit ToM and none have employed methods allowing casual inferences to be made. Here, using brain stimulation, a Sally-Anne false-belief task, and eye-tracking we show that right TPJ is causally involved in ToM judgments that are made implicitly. These findings have implications for characterising the neural substrates of a key executive function, determining the extent to which implicit and explicit ToM draw on overlapping neural architecture and, potentially, better understanding of disorders tied to ToM impairment.

Allowing our mind's eye to wander to task unrelated thoughts can impact productivity and, in many everyday settings, safety. However, the drifting of our thoughts from our current task(s)/situation to others is undeniably a common occurrence and has even been associated with adaptive outcomes in terms of creativity. Previous researchers have used non-invasive transcranial direct current stimulation (tDCS), applied to the prefrontal cortex, to modulate mind wandering occurrences. However, little is known about the influence of varying stimulation parameters, such as polarity and intensity, on mind wandering, or even more broadly on executive function performance. Here, in a pre-registered design (n = 150), we investigated the effect of stimulation polarity and intensity on mind wandering while subjects undertook a repetitive cognitive task (sustained attention to response task, SART). We found a linear effect of stimulation dosage on the propensity to have task unrelated thoughts. Specifically, greater intensity cathodal tDCS resulted in increased mind wandering with anodal and cathodal stimulation showed the same pattern of results. These findings confirm a key role for the left prefrontal cortex in mind wandering, and, of import, demonstrate that increased dosage could lead to larger effects on mind wandering behaviours. This is in contrast to some previous reports suggesting that stimulation dosage presents a U-shaped function, highlighting the potential for optimal dosages to vary depending upon the targeted brain region and behaviour.

There is now considerable evidence that applying a small electrical current to the cerebral cortex can have wide ranging effects on cognition and performance, and may provide substantial benefit as a treatment for conditions such as depression. However, there is variability across subjects in the extent to which stimulation modulates behaviour, providing a challenge for the development of applications. Here, we employed an individual differences approach to test if baseline concentrations of the neurochemicals GABA and glutamate are associated with an individual's response to transcranial direct current stimulation (tDCS). Using a previously replicated response selection training paradigm, we applied tDCS to the left prefrontal cortex part-way through the learning of a six-alternative-forced-choice task. Across three sessions, subjects received anodal, cathodal, or sham stimulation. Pre-tDCS baseline measures of GABA and glutamate, acquired using magnetic resonance spectroscopy (MRS), correlated with the extent to which stimulation modulated behaviour. Specifically, relative concentrations of GABA and glutamate (used as an index of neurochemical excitability) in the prefrontal cortex were associated with the degree to which active stimulation disrupted response selection training. This work represents an important step forward in developing models to predict stimulation efficacy, and provides a unique insight into how trait-based properties of the targeted cortex interact with stimulation.

Spatial attention can be used to direct neural processing resources to a subset of task-relevant or otherwise salient items within the environment. Such selective processes are particularly important for resolving competition between multiple stimuli. Deficits in processing single stimuli can arise after damage to parietal, frontal and temporal brain regions, as is typical in patients with contralesional spatial neglect. By contrast, deficits in processing multiple competing stimuli may arise specifically following lesions of the posterior parietal cortex (PPC), as occurs in the disorder of spatial extinction. It remains unclear, however, whether mechanisms involved in selecting single and competing stimuli reflect the same or dissociable neural operations within the PPC. To address this issue, in separate sessions, we applied transcranial direct current stimulation (tDCS) to the left or right PPC and measured the effect on detecting and discriminating single and competing visual stimulus events. Our results revealed reliable tDCS modulations of stimulus processing, specific to the right PPC, as well as a dissociation in the detection of single and competing stimuli. For the right PPC only, single stimuli presented to the left (contralateral) visual field were affected selectively by anodal tDCS, whereas competing stimuli across the two visual fields were affected by both anodal and cathodal tDCS. These contrasting effects of anodal and cathodal tDCS on perception of single and competing stimuli suggest dissociable neural coding properties within the right PPC.

A central assumption of models proposed to explain object substitution masking (OSM) is that the phenomenon arises only when attention is distributed across several possible target locations. However, recent work has questioned the role of attention in OSM, suggesting instead that ceiling effects might explain the apparent interaction between spatial attention and masking. Here the authors report definitive evidence that OSM does not depend upon attention being distributed over space or time. In 2 experiments, they demonstrate reliable OSM for constant, foveal presentations of a single target stimulus. Crucially, in their design participants’ attention was always focused on the target, thus discounting the hypothesis that a key requirement for OSM is distributed attention. The findings challenge how OSM is conceptualized in the broader masking literature and have important implications for theories of visual processing.

Cognitive training can lead to performance improvements that are specific to the tasks trained. Recent research has suggested that transcranial direct current stimulation (tDCS) applied during training of a simple response-selection paradigm can broaden performance benefits to an untrained task. Here we assessed the impact of combined tDCS and training on multitasking, stimulus-response mapping specificity, response-inhibition, and spatial attention performance in a cohort of healthy adults. Participants trained over four days with concurrent tDCS – anodal, cathodal, or sham – applied to the left prefrontal cortex. Immediately prior to, 1 day after, and 2 weeks after training, performance was assessed on the trained multitasking paradigm, an untrained multitasking paradigm, a go/no-go inhibition task, and a visual search task. Training combined with anodal tDCS, compared with training plus cathodal or sham stimulation, enhanced performance for the untrained multitasking paradigm and visual search tasks. By contrast, there were no training benefits for the go/no-go task. Our findings demonstrate that anodal tDCS combined with multitasking training can extend to untrained multitasking paradigms as well as spatial attention, but with no extension to the domain of response inhibition.

Conscious detection and discrimination of a visual target stimulus can be prevented by the presentation a spatially nonoverlapping, but temporally trailing, visual masking stimulus. This phenomenon, known as object substitution masking (OSM), has long been associated with spatial attention, with diffuse attention seemingly being key for the effect to be observed. Recently, this hypothesis has been questioned. We sought to provide a definitive test of the involvement of spatial attention in OSM by using an eight-alternative forced choice task under a range of mask durations, set sizes, and target/distractor spatial configurations. The results provide very little evidence that set size, and thus the distribution of spatial attention, interacts with masking magnitude. These findings have implications for understanding the mechanisms underlying OSM and the relationship between consciousness and attention.

We have a limited capacity for mapping sensory information onto motor responses. This processing bottleneck is thought to be a key factor in determining our ability to make two decisions simultaneously - i.e., to multitask (Pashler, 1984, 1994; Welford, 1952). Previous functional imaging research (Dux, Ivanoff, Asplund, & Marois, 2006; Dux et al., 2009) has localised this bottleneck to the posterior lateral prefrontal cortex (pLPFC) of the left hemisphere. Currently, however, it is unknown whether this region is causally involved in multitasking performance. We investigated the role of the left pLPFC in multitasking using transcranial direct current stimulation (tDCS). The behavioural paradigm included single- and dual-task trials, each requiring a speeded discrimination of visual stimuli alone, auditory stimuli alone, or both visual and auditory stimuli. Reaction times for single- and dual-task trials were compared before, immediately after, and 20 min after anodal stimulation (excitatory), cathodal stimulation (inhibitory), or sham stimulation. The cost of responding to the two tasks (i.e., the reduction in performance for dual- vs single-task trials) was significantly reduced by cathodal stimulation, but not by anodal or sham stimulation. Overall, the results provide direct evidence that the left pLPFC is a key neural locus of the central bottleneck that limits an individual's ability to make two simple decisions simultaneously.

This study used TMS to examine the role played by striate cortex (V1) in processing the emotional content of visual stimuli. Participants learned to discriminate two sets of body posture images. For half of each set, the posture's emotional significance (threat versus pleasant) provided a redundant cue for the discrimination; the other half were emotionally neutral. An image was briefly presented at a lateral location in the visual field where a TMS pulse produced a phosphene, or at a control location in the opposite hemifield. A TMS pulse 70–140 ms after stimulus presentation at the phosphene location impaired discrimination of neutral stimuli with little effect on discrimination of emotional stimuli; the two classes of stimuli were equally discriminable when presented at the control location. The results are consistent with the proposal that emotionally salient patterns, such as social threat, can be discriminated independently of the geniculo-striate pathway.

Cognitive control is a vital executive process that is involved in selecting, generating, and maintaining appropriate, goal-directed behaviour. One operation that draws heavily on this resource is the mapping of sensory information to appropriate motor responses (i.e., response selection). Recently, a transcranial direct current stimulation (tDCS) study demonstrated that the left posterior lateral prefrontal cortex (pLPFC) is casually involved in response selection and response selection training. Correlational brain imaging evidence has also implicated the superior medial frontal cortex (SMFC) in response selection, and there is causal evidence that this brain region is involved in the proactive modulation of response tendencies when occasional stopping is required (response inhibition). However, to date there is only limited causal evidence that implicates the SMFC in response selection. Here, we investigated the role of SMFC in response selection, response selection training (Experiment 1) and response selection when occasional response inhibition is anticipated (Experiments 2 and 3) by employing anodal, cathodal, and sham tDCS. Cathodal stimulation of the SMFC modulated response selection by increasing reaction times in the context of proactive response inhibition. Our results suggest a context dependent role of the SMFC in response selection and hint that task set can influence the interaction between the brain and behaviour.

The abilities to select appropriate responses and suppress unwanted actions are key executive functions that enable flexible and goal-directed behavior. However, to date it has been unclear whether these two cognitive operations tap a common action control resource or reflect two distinct processes. In the present study, we used an individual differences approach to examine the underlying relationships across seven paradigms that varied in their response selection and response inhibition requirements: stop-signal, go–no-go, Stroop, flanker, single-response selection, psychological refractory period, and attentional blink tasks. A confirmatory factor analysis suggested that response inhibition and response selection are separable, with stop-signal and go–no-go task performance being related to response inhibition, and performance in the psychological refractory period, Stroop, single-response selection, and attentional blink tasks being related to response selection. These findings provide evidence in support of the hypothesis that response selection and response inhibition reflect two distinct cognitive operations.

In recent years there has been a significant commercial interest in 'brain training' - massed or spaced practice on a small set of tasks to boost cognitive performance. Recently, researchers have combined cognitive training regimes with brain stimulation to try and maximize training benefits, leading to task-specific cognitive enhancement. It remains unclear, however, whether the performance gains afforded by such regimes can transfer to untrained tasks, or how training and stimulation affect the brain's latent information processing dynamics. To examine these issues, we applied transcranial direct current stimulation (tDCS) over the prefrontal cortex while participants undertook decision-making training over several days. Anodal, relative to cathodal/sham tDCS, increased performance gains from training. Critically, these gains were reliable for both trained and untrained tasks. The benefit of anodal tDCS occurred for left, but not right, prefrontal stimulation, and was absent for stimulation delivered without concurrent training. Modeling revealed left anodal stimulation combined with training caused an increase in the brain's rate of evidence accumulation for both tasks. Thus tDCS applied during training has the potential to modulate training gains and give rise to transferable performance benefits for distinct cognitive operations through an increase in the rate at which the brain acquires information.

In recent years there has been an exponential rise in the number of studies employing transcranial direct current stimulation (tDCS) as a means of gaining a systems-level understanding of the cortical substrates underlying behaviour. These advances have allowed inferences to be made regarding the neural operations that shape perception, cognition, and action. Here we summarise how tDCS works, and show how research using this technique is expanding our understanding of the neural basis of cognitive and motor training. We also explain how oscillatory tDCS can elucidate the role of fluctuations in neural activity, in both frequency and phase, in perception, learning, and memory. Finally, we highlight some key methodological issues for tDCS and suggest how these can be addressed.

Humans show large and reliable performance impairments when required to make more than one simple decision simultaneously. Such multitasking costs are thought to largely reflect capacity limits in response selection (Welford, 1952; Pashler, 1984, 1994), the information processing stage at which sensory input is mapped to a motor response. Neuroimaging has implicated the left posterior lateral prefrontal cortex (pLPFC) as a key neural substrate of response selection (Dux et al., 2006, 2009; Ivanoff et al., 2009). For example, activity in left pLPFC tracks improvements in response selection efficiency typically observed following training (Dux et al., 2009). To date, however, there has been no causal evidence that pLPFC contributes directly to sensory-motor training effects, or the operations through which training occurs. Moreover, the left hemisphere lateralization of this operation remains controversial (Jiang and Kanwisher, 2003; Sigman and Dehaene, 2008; Verbruggen et al., 2010). We used anodal (excitatory), cathodal (inhibitory), and sham transcranial direct current stimulation (tDCS) to left and right pLPFC and measured participants' performance on high and low response selection load tasks after different amounts of training. Both anodal and cathodal stimulation of the left pLPFC disrupted training effects for the high load condition relative to sham. No disruption was found for the low load and right pLPFC stimulation conditions. The findings implicate the left pLPFC in both response selection and training effects. They also suggest that training improves response selection efficiency by fine-tuning activity in pLPFC relating to sensory-motor translations.