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Daw-An Wu, Neil Halelamien, Fumiko Hoeft, Shinsuke Shimojo; TMS “instant replay” validated using novel double-blind stimulation technique. Journal of Vision 2007;7(9):275. doi: 10.1167/7.9.275.
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Transcranial magnetic stimulation (TMS) can elicit the re-perception of recently presented visual stimuli, possibly reflecting access to nascent neural representations (Wu et. al. VSS 2001, 2002, 2004, Jolij & Lamme, VSS 2006). However, it is difficult to rule out the contributions of subject bias or confabulation. Here, we employ a custom-built stimulation device to validate the effect under psychologically double-blind conditions.
Our previous “instant replay” experiments employed the commonly used Magstim dual-pulse setup and figure-8 coil. In the modified setup, we take advantage of the fact that the optimal stimulus to stimulate visual cortex involves a lateral-to-medial orientation of the induced electric field. Thus, a horizontally oriented coil placed at the midline can bias the level of stimulation to the left or right hemisphere based on the direction of current flow.
We employ a custom switch and coils validated for their physical, physiological and psychological effects. It was found that subjects and experimenters could not discriminate between the forward and reversed current conditions (Hoeft et. al., in press).
We apply this technique to a natural image replay protocol (see abstract by Halelamien et. al. for details). Subjects (N=4) were presented with mirror-symmetric images and then stimulated with TMS of a polarity determined by the computer. Subjects judged whether the replay percept was more vivid on the left or the right side, rated the level of imbalance on a 10-point analog scale, gave verbal reports and made drawings on the screen.
Ratings, verbal reports and drawings all showed clear and significant differences between trials employing different stimulus polarities. Stronger stimulation to the left hemisphere resulted in more vivid percepts in the right visual hemifield, and vice versa. This provides double-blind validation that cortical representations are being activated directly by TMS, and demonstrated the usefulness of this new device.
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