Anisotropy in the visual cortex investigated by neuronavigated transcranial magnetic stimulation

Responses to transcranial magnetic stimulation (TMS) of the motor cortex depend on the direction of the induced current with an optimum perpendicular to the orientation of the precentral gyrus. Little is known about anisotropy in other cortical regions. We measured phosphene thresholds in the visual cortex using a frameless neuronavigation system. Comparing horizontal and vertical current orientation as well as monophasic and biphasic pulses in 7 subjects, we found lower thresholds with biphasic pulses and a tendency for lower thresholds with horizontal currents. When varying current directions in steps of 45° centered on a hot spot over the occipital cortex, in 10 out of 12 measurements optimal current orientation ran perpendicular to the underlying gyrus (mean deviation 14.6°). Optimal current orientation was determined as the orientation of the second eigenvector from the covariance matrix of the stimulation sites that had been shifted along the respective current direction by the amount of the measured threshold. Individual cortical architecture was obtained by segmentation of a 3d anatomical MR scan, with large interindividual differences among the orientations of the stimulated gyrus. As with the motor system, the optimum threshold with biphasic pulses was flipped about 180° compared to the optimum with monophasic pulses (p < .02) throughout subjects, suggesting both similar anisotropic properties of networks in the visual and motor cortices and the existence of anisotropic behaviour in any cortical region. As a consequence, optimal TMS application should always take into account the individual orientation of the gyrus to be stimulated.