Non-invasive brain stimulation (NIBS) is a method for the study of cognitive function that is quickly gaining popularity. It bypasses the correlative approaches of other imaging techniques, making it possible to establish a causal relationship between cognitive processes and the functioning of specific brain areas. Like lesion studies, NIBS can provide information about where a particular process occurs. However, NIBS offers the opportunity to study brain mechanisms beyond process localisation, providing information about when activity in a given brain region is involved in a cognitive process, and even how it is involved. When using NIBS to explore cognitive processes, it is important to understand not only how NIBS functions but also the functioning of the neural structures themselves. We know that NIBS techniques have the potential to transiently influence behaviour by altering neuronal activity, which may have facilitatory or inhibitory behavioural effects, and these alterations can be used to understand how the brain works. Given that NIBS necessarily involves the relatively indiscriminate activation of large numbers of neurons, its impact on a neural system can be easily understood as modulation of neural activity that changes the relation between noise and signal. In this review, we describe the mutual interactions between NIBS and brain activity and provide an updated and precise perspective on the theoretical frameworks of NIBS and their impact on cognitive neuroscience. By transitioning our discussion from one aspect (NIBS) to the other (cognition), we aim to provide insights to guide future research.

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Transcranial alternating current stimulation (tACS) is a promising tool for modulating brain oscillations, as well as a possible therapeutic intervention. However, the lack of conclusive evidence on whether tACS is able to effectively affect cortical activity continues to limit its application. The present study aims to address this issue by exploiting the well-known inhibitory alpha rhythm in the posterior parietal cortex during visual perception and attention orientation. Four groups of healthy volunteers were tested with a Gabor patch detection and discrimination task. All participants were tested at the baseline and selective frequencies of tACS, including Sham, 6 Hz, 10 Hz, and 25 Hz. Stimulation at 6 Hz and 10 Hz over the occipito-parietal area impaired performance in the detection task compared to the baseline. The lack of a retinotopically organised effect and marginal frequency-specificity modulation in the detection task force us to be cautious about the effectiveness of tACS in modulating brain oscillations. Therefore, the present study does not provide significant evidence for tACS reliably inducing direct modulations of brain oscillations that can influence performance in a visual task.

Despite the widespread usage of transcranial magnetic stimulation (TMS) in clinical and basic research, the exact mechanisms of action and interactions with ongoing neural activity remain unclear. However, thanks to recent biophysical studies on electromagnetic induction of neural tissue (Wagner et al., 2009) we now know more about some basic properties of TMS effects. This basic knowledge is important in planning and interpreting TMS studies and in cognitive neuroscience experiments a theoretical framework is also necessary. TMS data have traditionally been interpreted in the ''virtual brain lesion'' framework (Walsh and Cowey, 1998). The terminology was proposed by analogy with neuropsychological and animal lesion studies and TMS is described as inducing a temporary, reversible lesion in the stimulated area, avoiding problems related to cortical plasticity and functional reorganisation (Walsh and Pascual-Leone, 2003). Based on this interpretation, TMS has been utilised to define the putative role of areas during the execution of cognitive tasks, and this approach has been very productive. Semantically, the term ''virtual lesion'' identifies the effect induced by TMS as blocking the function of a population of neurons that are temporarily ''lesioned'' by the TMS pulse. Nevertheless, the TMS pulse induces a depolarisation of a group of neurons that in turn might activate other neurons, and the final behavioural outcome depends on the role of the stimulated area in relation to the network engaged in such a task (Sack and Linden, 2003). The ''virtual lesion'' hypothesis has, however, confused people on the issue of how TMS can possibly lead to enhanced performance (e.g., Harris et al., 2008b; Walsh et al.,