Fig. 3

mTI layout, behavioral paradigm, and simulations in the awake behaving macaque. A Experimental setup. Sketch of the placement of scalp-based stimulation electrodes around the acrylic implant in which the SC recording chamber was embedded. The stimulation electrodes are color coded and connected by colored lines to show the pairs of electrode for mTI. B,C Behavioral tasks. 200 ms of mTI stimulation was embedded either within a visually-guided saccade task where stimulation was either delivered during a period of prolonged visual fixation (Task 1) or around the time of target onset in a visually-guided saccade task (Task 2). We also show the settings of the carrier frequencies for the delivery of mTI stimulation or SHAM stimulation. D Focusing mTI. An intermediate contact on the linear electrode array (shown in red) located within the intermediate SC was selected to be the target for mTI. E Spectrogram of signal—Frequencies used for the stimulation are seen in a time–frequency plot. For this experiment, a 100 Hz envelope was used to evoked activity. F. PSD of signal—Time frequency and periodogram of one of the 200 ms of TI – here only mTI is displayed. The different frequencies applied are visible and a lowpass filter < 100 Hz is enough to remove the artefact of stimulation in the recorded data. The ability to filter stimulation artifacts is an advantage of the TI method, due to the stimulation artifact typically being several thousand Hz above the electrophysiological activity of interest. G Simulations of TI and mTI stimulation. Simulation of the volume of tissue activated following either TI or mTI stimulation. (surface in red showing 6 V/m or higher). The iso-surface from mTI is more focal than that from TI. Furthermore, the cumulative histogram shows the normalized maximum electric field at the target (superior colliculus) vs the rest of the brain. For the mTI (orange), less than 1 percent of the brain receives an electric field higher than the electric field in the colliculus. For the TI (blue), more than 20 percent of the brain receives an electric field higher than the electric field in the colliculus. For context, we see that 40 percent of the brain receives a higher electric field from the high frequency carriers (yellow), which is to be expected as the carrier frequency maximum is closest to the electrodes providing the carrier