responsible for your sense of balance and you feel for a moment as though your body is turning suddenly. At the last instant the hero dives out of the car and hits the ground. We zap the part of your brain responsible for body sensation and you feel a touch as the protagonist makes contact. The car hurtles over the edge and crashes into the canyon. We zap your auditory cortex, and the sound of the crash is augmented by sound signals generated inside your head. The camera zooms in as the car bursts into flame, the inferno filling the screen. At the same time, we zap your visual cortex and the bright image combines with even brighter flashes all over your visual field.
As should be pretty clear from this description, TMS is not going to give us Total Recall. But it can do some things that might be pretty entertaining. Sound like fun? I hereby coin a term for this entertainment medium of the future: Magnovision.
Is Magnovision really practical? Not just yet, but I think it’s quite likely that some kind of electromagnetic brain stimulation for entertainment will become practical in the not-too-distant future. There are some safety concerns and logistical issues that will need to be resolved before this kind of brain stimulation could become available for entertainment. Although single-pulse TMS in the lab is quite safe, it would need to be combined with careful monitoring to be safe in an entertainment setting. It will probably be possible to automate this monitoring, as has been done for defibrillators.
The logistics may prove a bigger challenge. First, for Magnovision, the precise spatial location of the magnet relative to your brain matters. So, in order to join the Magnovision audience, you will need either a comfortable headrest so that you can hold perfectly still through the movie, or a system that can move with your head to keep the magnetic field focused. Popcorn may be tricky. Second, TMS depends on strong magnets, which interfere with cell phones, credit cards, and each other. It may be hard to sit people close together and administer TMS; maybe you will need to check your gadgets and cards outside and be seated alone in a shielded booth. So Magnovision may take us back to the mode of the nickelodeon. The last challenge is perhaps the trickiest. Each of us has a head and a brain that is unique in size and shape. For TMS experiments now, participants usually undergo an MRI before the session to measure their head and brain. The MRI data are used to target the stimulator. This is expensive and time-consuming. It is not necessarily a deal-breaker, though. You need to do it only once, so you could imagine going down to the MRI center at the mall to get fitted for your stimulator settings. Thereafter, whenever you would go to a Magnovision theater, the attendants would load up your data into the machine. Another likely possibility is that we will develop cheaper and faster ways of focusing the stimulator.
Everything I have said so far applies to single magnetic pulses or short trains of a couple of pulses. In clinical and research applications, the same devices are used in another mode called repeated-train TMS. Repeated-train TMS involves sending a continuous stream of pulses to a brain area for up to 10s of minutes. During stimulation, neurons are excited and fire more frequently; the exact effects depend on how strong the current is and the frequency at which it oscillates. After the stimulation is turned off, activity in the area is suppressed or facilitated for a period that lasts at least 15 minutes.[iii] Depending on where the stimulation is applied, it can affect perception and memory, emotional state, and other aspects of functioning. It also can produce changes in the connections of the stimulated neurons that last for days, weeks, or months. These synaptic changes have been exploited for clinical purposes. The most successful example of repeated-train TMS in use is in the treatment of depression. For a long time we have known that particular parts of the brain are misregulated in depression. These include the parts of the temporal lobe, on the bottom of the cerebrum near the middle, including the hippocampus and surrounding tissue. Repeated-train TMS to these areas has proved an effective treatment for difficult cases of depression. The exact mechanism is not known, but it probably involves resetting these areas.[iv] One side effect is a modest memory impairment for events that happened just before and after the treatment. This is a reasonable trade-off in cases of severe depression, but not for a night at the movies. Moreover, who wants to experience an entertainment that wipes itself from memory? Another risk of repeated-train stimulation is the induction of brain seizures. These are rare in cognitive protocols but more common in some of the clinical uses. Repeated-train TMS is a great clinical tool, but these risks probably mean it will never be widely deployed for recreational purposes.
One more plot line—this one from The Matrix. Keanu Reeves lies on a gurney in the rebels’ ship, plugged into their training computer, with his eyes closed but twitching and his neck and arms jerking slightly. Then he quiets, opens his eyes, turns to Morpheus, and says “I know kung fu.” Will TMS allow us to bypass traditional education and just implant skills directly into our brains? No. First, if we could tweak our brains however we wanted, we don’t know what set of tweaks would make someone a kung fu master. Second, even if we did know, TMS is too crude an instrument to do it. It can’t stimulate individual cells, just large areas, and the stimulation is too different from real neural activity to work right.
Much more modestly, there are intriguing data coming out right now suggesting that brain stimulation does have the capacity to enhance traditional learning. One line of research has examined motor learning of the sort that we undergo when we learn to play a piece on the piano or to type on a computer keyboard. In these studies, researchers teach people to perform a motor sequence, usually by presenting a sequence of visual cues and instructing the learners to press a key corresponding to each cue. If you have ever used piano-teaching software, it’s kind of like that. Repeated-train TMS over the motor cortex can improve this kind of learning.[v]
Other studies have used very mild electrical stimulation rather than magnetic stimulation. This technique has even bigger spatial spread than TMS, and it can’t easily be used to produce visual or auditory effects. But it can modulate ongoing patterns of activity, including patterns induced by learning. One set of studies has used both TMS and electrical stimulation in patients
