D10 Diagnostic transcranial magnetic stimulation (TMS)

Topic

The cerebral cortex can be activated non-invasively via transcranial magnetic stimulation, which induces a focal electric field within the cortex through electromagnetic induction. This focal electric field directly activates cortical neuronal populations, extending the same type of membrane excitability previously used to describe neuronal firing in other models.

Among the activated cortical neuronal populations, the motor threshold is defined as the minimum stimulation intensity that consistently evokes a motor evoked potential in a target muscle; it serves as the fundamental measure of corticospinal excitability. The amplitude of this motor evoked potential does not increase in direct proportion to the stimulation pulse intensity but rather follows a sigmoidal pattern: it rises slowly at first, increases rapidly at intermediate intensities, and finally plateaus at high intensities. The motor evoked potential recorded in the target muscle is formally analogous to the compound muscle action potential used to assess motor units in other diagnostic models.

In addition to the motor threshold, the cerebral cortex can be evaluated using the cortical silent period and paired-pulse stimulation protocols—also known as short-interval intracortical inhibition. Both methods assess inhibitory mechanisms and short-term plasticity within the cortex, extending the concepts of synaptic integration and short-term facilitation and depression previously used to describe synaptic activity in other models.

By combining transcranial magnetic stimulation with simultaneous electroencephalographic recording, it is possible to assess two additional properties of the cerebral cortex: its excitability, measured via the cortical evoked potential recorded after each stimulation pulse; and its capacity for information integration, measured via the perturbational complexity index. This perturbational complexity index is a direct extension of the information integration measure previously defined in the model of levels of consciousness. Overall, it is this very cerebral cortex—activated non-invasively by the focal electric field of transcranial magnetic stimulation—that is characterized by the motor threshold and motor-evoked potential amplitude; by the cortical silent period and paired-pulse stimulation; and by the cortical evoked potential and the perturbational complexity index when transcranial magnetic stimulation is combined with electroencephalography.

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