Transcranial brain stimulation: clinical applications and future directions

Abstract

Noninvasive brain stimulation is a valuable investigative tool and has potential therapeutic applications in cognitive neuroscience, neurophysiology, psychiatry, and neurology. Transcranial magnetic stimulation (TMS) is particularly useful to establish and map causal brain-behavior relations in motor and nonmotor cortical areas. Neuronavigated TMS is able to provide precise information related to the individual's functional anatomy that can be visualized and used during surgical interventions and critically aid in presurgical planning, reducing the need for riskier and more cumbersome intraoperative or invasive mapping procedures. This article reviews methodological aspects, clinical applications, and future directions of TMS-based mapping.

Fig. 1

The homunculus as described by Penfield and Rasmussen. (Adapted from Penfield W, Rasmussen T. The cerebral cortex of man. New York: Macmillan; 1950. p. 44, 56, 214–5.)

Fig. 2

(A) During TMS, a time-pulsed current is discharged through the TMS coil. The resulting time-varying magnetic field is focused onto underlying neural tissue. The eddying currents, produced in the tissue, can affect the neural activity during and after stimulation. The patient is shown wearing a frameless stereotactic device that can be used to predict the location of stimulation relative to the TMS coil, which is tracked via the camera device (inset). (B) A simplified circuit diagram of a single-pulse magnetic stimulator. C, capacitator; D, diode; R, resistor; s, switch; T, thyristor; V, voltage source.

Fig. 3

(A) Head model showing right M1 mapping performed in a healthy individual for upper and lower limb. Each mark represents the hot spot for the muscle mapped. The position of the head of the mark represents the direction of stimulation. The brain is peeled to a depth of 25 mm (ie, the visualized stimulation surface resides at this depth from the scalp). (B) Comparison between navigated (right) and nonnavigated (left) stimulation (1 Hz rTMS) performed in the same individual, on the same target. Note that in the navigated intervention, the dispersion of the stimuli is less and it is more focal than the nonnavigated intervention.

Fig. 4

(A) An nTMS system user interface. (Left panel) The interface allows the user to identify and select stimulation targets based on the patient’s anatomic or functional imaging. It also generates a 3D head model (bottom left) for accurate estimation of the target electric field strength. The targeting system (bottom right) allows the user to stimulate with enhanced spatial resolution. The position feedback indicator provides real-time feedback surface location, roll, pitch, and yaw of the coil, for consistent and reliable targeting. (Right panel) Online EMG recording, along with a modifiable epoch view (right half). A single evoked response is measured as peak-to-peak amplitude and onset latency. ADM, abductor digiti minimi; APB, abductor pollicis brevis; ECR, extensor carpi radialis. (B) Presurgical cortical maps produced in a patient with right parietal lesion. EMG amplitude-based map on the left side can be viewed only in the navigated brain stimulation system. Amplitude-weighted maps on the right side can be viewed in any third-party imaging system capable of reading DICOM (digital imaging and communications in medicine) images.

Fig. 5

Axial slices through head reconstructions showing the hMT/V5+ stimulation site (green sphere) relative to motion-specific activation, representing the location of hMT/V51 (red sphere; calculated using a motion-mapping paradigm; for details see text). The yellow spheres visualize the orientation of the TMS coil. The imagined line through the spheres corresponds to the normal vector originating from the TMS focus (distance between spheres is 1 cm). (Data from Sack AT, Kohler A, Linden DE, et al. The temporal characteristics of motion processing in hMT/V5+: combining fMRI and neuronavigated TMS. Neuroimage 2006;29(4):1326–35.)

Fig. 6

A combined TMS-EEG system. Note that in addition to triggering TMS using criteria based on online EEG recording (such as a specific event-related potential), TMS triggering can be used to control EEG recording as well (such as momentary stoppage of EEG recording immediately after a TMS pulse to prevent saturation of EEG amplifiers because of excessive voltages induced between the leads because of the magnetic field).

Fig.7

Head-model recreationofa comparisonbetween navigated brain stimulation (left) and direct cortical stimulation (right) in a patient with right parietal tumor. Yellow marks on the TMS panel represent the hot spots for hand and lower-limb muscles. Green dots on the SGE grid represent points that resulted in responses from hand and lower limb. SGE, subdural grid electrodes.