TMS and the brain
TMS operates on Faraday’s principle of electromagnetic induction. Faraday showed that an electrical current passed through one coil could induce a current in a nearby coil. The current in the first coil produces a magnetic field that in turn causes current to flow in the second coil. In TMS that second coil is replaced by brain tissue and the induced electric field elicits neuronal activity. The key features of the technique are that the TMS machine delivers a large current in a short period of time — the current in the TMS coil then produces a magnetic field which, if changing rapidly enough, will induce an electric field sufficient to stimulate neurons or change the resting membrane potentials in the underlying cortex. In short, TMS can be used to induce a transient interruption of normal brain activity in a relatively restricted area of the brain.
The mechanism of interference
Perhaps the most common source of confusion over TMS is exactly how it interferes with cortical information processing to induce such a temporary lesion. As far as neuropsychological studies are concerned, the effect of TMS can be thought of as inducing ‘noise’ into neural processes. If a group of neurons are involved in a given task (for example, identifying a shape or matching a picture to a word), introducing a TMS pulse is highly unlikely to selectively stimulate the same coordinated pattern of neural activity as performance of that task. Rather, TMS induces activity that is random with respect to the goal-state of the area stimulated. In other words, TMS induces disorder rather than order into the information processing system, thereby disrupting task performance.
This ‘neural noise’ concept underpins what has become known as the ‘virtual patient’. By careful application, TMS can be used to transiently recreate the deficits seen in some neuropsychological patients, or can be used to create deficits that are rarely, if ever, obtained in neurological patients.
Spatial resolution
Another important source of confusion is the spatial resolution of TMS. The magnetic field produced by TMS is not spatially focal (in theory it is of infinite extent, like the earth’s gravitational field). However, the distribution of the induced electric field can and has been modelled, and progress has been made in relating the induced currents to specific sites of activation with a gate functions of medial cortex or sub-cortical structures. One should also be aware that stimulating deeper cortical structures (for example, in the sulci) may also stimulate overlying cortex. A potential solution to this problem is to stimulate areas that are accessible in non-human primates but not in human subjects. In cognitive neuroscience the chronometrics of information processing are central to many theories and experiments. For cognitive studies then, an understanding of the temporal resolution of TMS is at least as important as an account of its spatial selectivity. When a TMS pulse is delivered over an area of cortex, the effect is to simultaneously activate many neurons. At the point of maximal activation, the stimulated area will have its lowest signal-to-noise ratio with respect to the task it is trying to perform.
However, as neurons recover, the signal will increase, and whether or not TMS continues to have an effect will depend on the level of signal required for the task. Note that the interaction between the TMS signal and the contribution of an area to a task makes it highly unlikely that the time at which TMS has its maximal effect will correspond with the peak times reported in event-related potential (ERP) or subtractive or magneto-encephalographic (MEG) experiments).
An effectively disruptive pulse will interfere with processes that contribute to the build up of the ERP/MEG signal, so if the signal represents a neural event that is essential to the task, the time of TMS interference will typically precede ERP peaks and is more likely to coincide with single unit data. In other words, where an ERP result reports a peak at, say 300 ms, this may reflect the contribution of more than one neural event with a group maximum at 300 ms. When TMS is applied over the areas that contribute to this signal, it may disrupt processing of the individual components that may be maximal before, at, or after the reported peak at 300 ms.
