Transcranial Magnetic Stimulation for Investigating Causal Brain-behavioral Relationships and their Time Course

This article presents a protocol for assessing the causal and temporal involvement of brain regions in cognitive processes using online TMS. This discussion highlights first the critical steps for creating a successful TMS protocol and then the limitations that need to be considered when designing a TMS experiment.

Because TMS protocols have a large number of free parameters, ensuring the optimal stimulation parameters is a critical step in preparing a TMS experiment. Normally, this is achieved through extensive pilot testing in order to determine the stimulation frequency, duration, intensity, inter-trial interval, and coil orientation necessary to produce robust effects. To create an effective “virtual lesion” the frequency must induce a robust effect that covers a sufficiently large time window to encompass the cognitive process of interest. As a result, both frequency and duration vary across studies. Similarly, the “right” stimulation intensity is one that ensures the magnetic field affects neural processing in the target brain region and here the main factor is the distance from the coil to the stimulation site⁵¹. Many studies identify the intensity of stimulation necessary to produce a motor response when stimulating the hand area of primary motor cortex and use this to normalize intensity across participants^52,53-55. This measure, however, is not a reliable index of the optimal intensity for non-motor areas^42,51,56. Another option is to use the same intensity for all participants. The chosen intensity should be effective across all pilot subjects after experimenting with a range of stimulation intensities. Additionally, the coil orientation is an important parameter that requires consideration. The specific coil orientation affects the distribution of the induced electric field within the stimulated neuronal population and therefore may influence behavior. In general, published protocols can provide a starting point that is iteratively modified during pilot testing to suit the specific experiment. Often, however, information about this pilot testing is omitted from the final manuscript, which has the unfortunate effect of hiding some key aspects of the protocol design process.

Choosing a localization procedure is also essential to ensure that stimulation is administered to the optimal site. Although many studies have successfully localized stimulation sites using anatomy-based methods that target a single location across individual participants ^57,58, customizing the stimulation site for each subject individually reduces between-subject variance in behavioral results yielding a more efficient method³¹. Here we presented a TMS-based functional localization procedure that offers advantages over fMRI-based localization. Specifically, it avoids the problem of different spatial biases between fMRI (i.e., draining veins⁵⁹) and TMS (i.e., the orientation of axons within the magnetic field^6,60) that may result in the same neural response being localized to different locations. In addition, it is well known that the specific location of activation “peaks” in fMRI can vary considerably, making them sub-optimal as TMS targets^55,61. Even so, a variety of different localization procedures are demonstrably effective, so the specific choice is less important that ensuring that whichever method is used provides reliable, reproducible effects.

Although the experiment data presented here used reaction times as the dependent measure, there are many other options available. For instance, some studies use accuracy instead^9,12,62. In these cases, normal performance without TMS is already below ceiling levels so the disruption induced by stimulation is reflected in the accuracy scores. Other studies have measured the effects of stimulation on eye movements^63,64. Most cognitive neuroscience experiments with TMS, however, use reaction times as their dependent measure^13,48,65,66. Typically, the effects are on the order of tens of msec, or roughly a 10% change in reaction times⁶⁷. Whatever dependent measure is used should be robust and consistent so that relatively small changes can be easily observed.

Like any experimental technique, TMS has important limitations that need to be considered when choosing this methodology. The most common ones are: i) the spatial resolution of TMS, ii) the non-specific effects associated with stimulation, and iii) safety aspects of the methodology. First, TMS has a limited depth of stimulation because the magnetic field reduces in intensity the further away it is from the coil. Consequently, it is most effective at stimulating brain regions near scalp (~2 – 3 cm)^68,69 and is ineffective in stimulating deep brain structures. As a result, the only regions directly accessible to TMS are limited to the cortical mantle, although different shaped coils are being developed to reach deeper regions such as the basal ganglia⁶⁹. TMS also has a spatial resolution of approximately 0.5 – 1 cm^47,70-72. Thus, the method cannot be used to investigate the functional contributions from fine-grained spatial structures such as cortical columns.

A second limitation of TMS is that stimulation introduces concurrent sensory side effects as a result of the rapidly-changing magnetic field. Most notably, each magnetic pulse is accompanied by an auditory click and a tapping sensation. Therefore TMS may be inappropriate for certain auditory or somatosensory experiments where these side effects may interfere with task performance. Note, however, that online TMS has been used successfully in some auditory experiments^73,74 and is therefore feasible in at least some tasks. Another consideration is that the intensity of the sensory effects differs across head locations. For example, stimulation that is administered to a location close to the ear will sound louder than locations further away. Similarly more ventral locations on the head produce greater muscle contraction than dorsal areas^75,76. Because these site differences can induce experimental confounds, it is important to use either a control site with similar side-effects to the main site such as contralateral homologues⁷⁷ or include control conditions/tasks that do not tap into the process of interest^{24,62,73,78,79}.

Finally, safety considerations must always be taken into account when designing TMS experiments as it can potentially induce syncope and seizures²⁷. To minimize this risk, internationally accepted guidelines for stimulation intensity, frequency, and duration exists, as well as for the total number of pulses and the inter-trial intervals^27,28. Protocols that stay within these guidelines are believed to be safe for neurologically normal participants. It is worth noting, however, that these are as yet incomplete and that often novel TMS protocols are introduced that also prove safe. In general, the evidence suggests that when published guidelines are followed, TMS is a safe procedure with no dangerous side effects. One consequence of these limits, however, is that behavioral protocols will often need to be adjusted before they can be used with TMS. This has implications for several aspects of the design, including the length of the experiment, number of trials, number of conditions and stimulation sites that can be tested. Some of these limitations may be overcome by splitting the experiment into separate sessions such as testing different stimulation sites on different days. In those cases, it is important to ensure that localization and testing of a site are done within the same session. This minimizes experimental variance by maximizing the accuracy of the targeting. When deciding whether to use one or more testing session, the fundamental limitation is the safety of the participant – specifically, the amount of stimulation that is safe in a single session. The total stimulation involves familiarization, practice, localization (if using TMS), and testing, potentially over multiple sites, and critically depends on the number of trials per condition. Where this figure exceeds the guidelines for a single session, it is necessary to break the experiment into multiple sessions, conducted a minimum of 24 hr apart. There are no hard-and-fast rules regarding the minimum number of trials necessary for TMS experiments, but like any experiment, these can be computed using standard power calculations based on the effect size, variance, α-level (typically 0.05) and desired sensitivity. Often reasonable estimates of the effect size and variance are available as a result of the extensive pilot testing done to optimize the experimental protocol.

In summary, TMS has become an important tool with broad applications to cognitive neuroscience. This article provides a basic protocol for online TMS in conjunction with a behavioral task for investigating causal brain-behavioral relationships both in “virtual lesion” mode and also a chronometric tool for exploring the temporal dynamics of regionally-specific neural information processing.