Transcranial magnetic stimulation has been used to investigate almost all areas of cognitive neuroscience. This article discusses the most important (and least understood) considerations regarding the use of transcranial magnetic stimulation for cognitive neuroscience and outlines advances in the use of this technique for the replication and extension of findings from neuropsychology. We also take a more speculative look forward to the emerging development of strategies for combining transcranial magnetic stimulation with other brain imaging technologies and methods in the cognitive neurosciences.

Much is known about the pathways from photoreceptors to higher visual areas in the brain. However, how we become aware of what we see or of having seen at all is a problem that has eluded neuroscience. Recordings from macaque V1 during deactivation of MT+/V5 and psychophysical studies of perceptual integration suggest that feedback from secondary visual areas to V1 is necessary for visual awareness. We used transcranial magnetic stimulation to probe the timing and function of feedback from human area MT+/V5 to V1 and found its action to be early and critical for awareness of visual motion.

Extensive research has demonstrated that several specialized cortical regions respond preferentially to faces. One such region, located in the inferior occipital gyrus, has been dubbed the occipital face area (OFA). The OFA is the first stage in two influential face-processing models, both of which suggest that it constructs an initial representation of a face, but how and when it does so remains unclear. The present study revealed that repetitive transcranial magnetic stimulation (rTMS) targeted at the right OFA (rOFA) disrupted accurate discrimination of face parts but had no effect on the discrimination of spacing between these parts. rTMS to left OFA had no effect. A matched part and spacing discrimination task that used house stimuli showed no impairment. In a second experiment, rTMS to rOFA replicated the face-part impairment but did not produce the same effect in an adjacent area, the lateral occipital cortex. A third experiment delivered double pulses of TMS separated by 40 ms at six periods after stimulus presentation during face-part discrimination. Accuracy dropped when pulses were delivered at 60 and 100 ms only. These findings indicate that the rOFA processes face-part information at an early stage in the face-processing stream.

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Functional magnetic resonance imaging (fMRI) studies have identified spatially distinct face-selective regions in human cortex. These regions have been linked together to form the components of a cortical network specialized for face perception but the cognitive operations performed in each region are not well understood. In this paper, we review the evidence concerning one of these face-selective regions, the occipital face area (OFA), to better understand what cognitive operations it performs in the face perception network. Neuropsychological evidence and transcranial magnetic stimulation (TMS) studies demonstrate the OFA is necessary for accurate face perception. fMRI and TMS studies investigating the functional role of the OFA suggest that it preferentially represents the parts of a face, including the eyes, nose, and mouth and that it does so at an early stage of visual perception. These studies are consistent with the hypothesis that the OFA is the first stage in a hierarchical face perception network in which the OFA represents facial components prior to subsequent processing of increasingly complex facial features in higher face-selective cortical regions.

Neuroscientists have long debated whether focal brain regions perform specific cognitive functions [1-5], and the issue remains central to a current debate about visual object recognition. The distributed view of cortical function suggests that object discrimination depends on dispersed but functionally overlapping representations spread across visual cortex [6-8]. The modular view claims different categories of objects are discriminated in functionally segregated and specialized cortical areas [9-11]. To test these competing theories, we delivered transcranial magnetic stimulation (TMS) over three adjacent functionally localized areas in extrastriate cortex. In three experiments, participants performed discrimination tasks involving faces, bodies, and objects while TMS was delivered over the right occipital face area (rOFA) [12], the right extrastriate body area (rEBA) [13], or the right lateral occipital area (rLO) [14]. All three experiments showed a task selective dissociation with performance impaired only by stimulation at the site selective for that category: TMS over rOFA impaired discrimination of faces but not objects or bodies; TMS over rEBA impaired discrimination of bodies but not faces or objects; TMS over rLO impaired discrimination of objects but not faces or bodies. The results support a modular account in which category-selective areas contribute solely to discrimination of their preferred categories.

SummaryAround 20% of the population exhibits moderate to severe numerical disabilities [1–3], and a further percentage loses its numerical competence during the lifespan as a result of stroke or degenerative diseases [4]. In this work, we investigated the feasibility of using noninvasive stimulation to the parietal lobe during numerical learning to selectively improve numerical abilities. We used transcranial direct current stimulation (TDCS), a method that can selectively inhibit or excitate neuronal populations by modulating GABAergic (anodal stimulation) and glutamatergic (cathodal stimulation) activity [5, 6]. We trained subjects for 6 days with artificial numerical symbols, during which we applied concurrent TDCS to the parietal lobes. The polarity of the brain stimulation specifically enhanced or impaired the acquisition of automatic number processing and the mapping of number into space, both important indices of numerical proficiency [7–9]. The improvement was still present 6 months after the training. Control tasks revealed that the effect of brain stimulation was specific to the representation of artificial numerical symbols. The specificity and longevity of TDCS on numerical abilities establishes TDCS as a realistic tool for intervention in cases of atypical numerical development or loss of numerical abilities because of stroke or degenerative illnesses.

Transcranial magnetic stimulation (TMS) is a tool for inducing transient disruptions of neural activity noninvasively in conscious human volunteers. In recent years, the investigative domain of TMS has expanded and now encompasses causal structure-function relationships across the whole gamut of cognitive functions and associated cortical brain regions. Consequently, the importance of how to determine the target stimulation site has increased and a number of alternative methods have emerged. Comparison across studies is precluded because different studies necessarily use different tasks, sites, TMS conditions, and have different goals. Here, therefore, we systematically compare four commonly used TMS coil positioning approaches by using them to induce behavioral change in a single cognitive study. Specifically, we investigated the behavioral impact of right parietal TMS during a number comparison task, while basing TMS localization either on (i) individual fMRI-guided TMS neuronavigation, (ii) individual MRI-guided TMS neuronavigation, (iii) group functional Talairach coordinates, or (iv) 10-20 EEG position P4. We quantified the exact behavioral effects induced by TMS using each approach, calculated the standardized experimental effect sizes, and conducted a statistical power analysis in order to calculate the optimal sample size required to reveal statistical significance. Our findings revealed a systematic difference between the four approaches, with the individual fMRI-guided TMS neuronavigation yielding the strongest and the P4 stimulation approach yielding the smallest behavioral effect size. Accordingly, power analyses revealed that although in the fMRI-guided neuronavigation approach five participants were sufficient to reveal a significant behavioral effect, the number of necessary participants increased to n = 9 when employing MRI-guided neuronavigation, to n = 13 in case of TMS based on group Talairach coordinates, and to n = 47 when applying TMS over P4. We discuss these graded effect size differences in light of the revealed interindividual variances in the actual target stimulation site within and between approaches.