https://bic.berkeley.edu/sites/default/files/Rossi_09_TMS%20safety%20review.pdf

Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research

The distinct modes of vision offered by feedforward and recurrent processing.

https://www.cell.com/neuron/pdf/S0896-6273(07)00460-6.pdf

Transcranial Magnetic Stimulation: A Primer

Transcranial magnetic stimulation (TMS) is rapidly developing as a powerful, non-invasive tool for studying the human brain. A pulsed magnetic field creates current flow in the brain and can temporarily excite or inhibit specific areas. TMS of motor cortex can produce a muscle twitch or block movement; TMS of occipital cortex can produce visual phosphenes or scotomas. TMS can also alter the functioning of the brain beyond the time of stimulation, offering potential for therapy.

Transcranial magnetic stimulation and the human brain

Transcranial magnetic stimulation has been used to investigate almost all areas of cognitive neuroscience. This article discusses the most important (and least understood) considerations regarding the use of transcranial magnetic stimulation for cognitive neuroscience and outlines advances in the use of this technique for the replication and extension of findings from neuropsychology. We also take a more speculative look forward to the emerging development of strategies for combining transcranial magnetic stimulation with other brain imaging technologies and methods in the cognitive neurosciences.

Transcranial magnetic stimulation and cognitive neuroscience

To determine the timing of visual processing in the early visual cortex, we applied single pulse transcranial magnetic stimulation to the occipital pole of healthy subjects while they were engaged in a forced-choice visual letter-identification task. We found two separate periods of activity, the first ranging from 20 to 60 ms after the onset of the visual stimulus, and the second ranging from 100 to 140 ms after the onset of the visual stimulus. We suggest that these two periods reflect necessary activity in V1, before and after re-entry.

Timing of activity in early visual cortex as revealed by transcranial magnetic stimulation.

Two periods of processing in the (circum)striate visual cortex as revealed by transcranial magnetic stimulation.

To investigate the mechanism of transcranial magnetic stimulation (TMS), we compared the directional effects of two stimulators (Magstim 200 and Magstim Super Rapid). First, stimulating visual cortex and facial nerve with occipital mid-line TMS, we found that, for a particular coil orientation, these two stimulators affected a particular neural structure in opposite hemispheres and that, to affect a particular neural structure in a particular hemisphere, these two stimulators required opposite coil orientations. Second, stimulating a membrane-simulating circuit, we found that, for a particular coil orientation, these two stimulators resulted in a peak induced current of the same polarity but in a peak induced charge accumulation of opposite polarity. We suggest that the critical parameter in TMS is the amplitude of the induced charge accumulation rather than the amplitude of the induced current. Accordingly, TMS would be elicited just before the end of the first (Magstim 200) and second (Magstim Super Rapid) phase of the induced current rather than just after the start of the first phase of the induced current.

Transcranial magnetic stimulation. Which part of the current waveform causes the stimulation

We applied single-pulse transcranial magnetic stimulation (TMS) to the occipital pole of healthy subjects while they performed a forced-choice visual letter-identification task. We found three separate periods when TMS suppressed performance; the first period is best explained by TMS-induced blinking whereas the last two periods are best explained by TMS-induced disruption of letter-processing in the early visual cortex. Unexpectedly, we also found that TMS-induced suppression progressively disappeared during three weeks of repeated TMS experiments. However, it was only suppression during the last two periods that disappeared; suppression during the first period remained undiminished. When subjects were then presented with dimmer letters, suppression reappeared. The most likely explanation is a practice-induced increase in neuronal activity in the early visual cortex.

Plasticity revealed by transcranial magnetic stimulation of early visual cortex.

We recently reported three periods when single-pulse transcranial magnetic stimulation (TMS) of the occipital pole impaired performance on a forced-choice visual letter-identification task. TMS-induced suppression during these periods is best explained by a blink-associated covering of the pupils and by a direct interference with letter-processing neural activity. We now report TMS-induced suppression at times that seem too late for the suppression to be explained by the first mechanism and too early for the suppression to be explained by the second mechanism. The most likely explanation is a blink-associated interference with letter-processing neural activity.

Erik Corthout

Papers

Timing of activity in early visual cortex as revealed by transcranial magnetic stimulation.

Two periods of processing in the (circum)striate visual cortex as revealed by transcranial magnetic stimulation.

Transcranial magnetic stimulation. Which part of the current waveform causes the stimulation

Plasticity revealed by transcranial magnetic stimulation of early visual cortex.

Suppression of vision by transcranial magnetic stimulation: a third mechanism.