What's the science?

Transcranial magnetic stimulation (TMS) is capable of remodelling how the brain functions and is used to treat some disorders like depression. During TMS, the brain is stimulated via pulses from a magnetic coil resting on the scalp. However, certain techniques used to map the effects of TMS on brain function (magnetic resonance imaging and electroencephalography, for example), do not have sufficient spatial resolution to be able to visualize how TMS induces changes in the brain. This week in PNAS, Kozyrev and colleagues imaged the brain using voltage-sensitive dye to track the functional changes caused by TMS in the brain at high spatial resolution (sub-millimeter range).

How did they do it?

In anesthetized cats, the authors performed TMS of the visual cortex for 30 minutes. Either sham TMS (coil positioned away from head as a control), high frequency (10Hz) or low frequency (1Hz) TMS was applied. Next, passive visual stimulation (a grid with lines oriented in a particular direction) was shown for 30 minutes. Before and after visual stimulation, they used a voltage sensitive dye applied to the visual cortex to optically image which regions of the cortex responded to different line orientations. The optical imaging detects voltage changes using fluorescence. They also compared the visual cortex before and after TMS in order test whether just TMS (before visual stimulation) lead to any changes.

What did they find?

Normally, different parts of the visual cortex are activated by different line orientations. After high frequency TMS (not low frequency or sham) and visual stimulus, however, a larger portion of the visual cortex tissue (18.6% more) was found to be activated by the specific line orientation that had been shown in the visual stimulus. Different line orientations were tested with the same result; the area of cortex that previously corresponded to that particular orientation expanded after high frequency TMS and visual stimuli at that orientation. These new, remodelled maps were stable for up to 6 hours. They observed that regions neighbouring these patches of cortex that were usually activated by slightly different orientations prior to TMS became the patches that responded to orientation of the visual stimulus presented during the experiment. When they looked at the visual cortex after (versus before) TMS, they found that most regions of the visual cortex were less selective across all orientations (orientation tuning was less reproducible across several trials of visual stimulation). Finally, there was a small 'dip' or 'notch' in the upwards phase of the voltage sensitive dye that disappeared after TMS. This suggests that inhibitory processes which could strengthen orientation tuning are erased by TMS, meaning the cortex is more excitable and vulnerable to remodelling.