The influence of current direction on phosphene thresholds evoked by transcranial magnetic stimulation

Objectives: To quantify phosphene thresholds evoked by transcranial magneti c stimulation (TMS) in the occipital cortex as a function of induced curren t direction. Methods: Phosphene thresholds were determined in 6 subjects, We compared tw o stimulator types (Medtronic-Dantec and Magstim) with monophasic pulses us ing the standard figure-of-eight coils and systematically varied hemisphere (left and right) and induced current direction (latero-medial and medio-la teral). Each measurement was made 3 times, with a new stimulation site chos en for each repetition. Only those stimulation sites were investigated wher e phosphenes were restricted to one visual hemifield. Coil positions were s tereotactically registered. Functional magnetic resonance imaging (fMRI) of retinotopic areas was performed in 5 subjects to individually characterize the borders of visual areas; TMS stimulation sites were coregistered with respect to visual areas. Results: Despite large interindividual variance we found a consistent patte rn of phosphene thresholds. They were significantly lower if the direction of the induced current was oriented from lateral to medial in the occipital lobe rather than vice versa. No difference with respect to the hemisphere was found. Threshold values normalized to the square root of the stored ene rgy in the stimulators were lower with the Medtronic-Dantec device than wit h the Magstim device. fMRI revealed that stimulation sites generating unila teral phosphenes were situated at V2 and V3, Variability of phosphene thres holds was low within a cortical patch of 2 X 2 cm(2). Stimulation over V1 y ields phosphenes in both visual fields. Conclusions: The excitability of visual cortical areas depends on the direc tion of the induced current with a preference for latero-medial currents. A lthough the coil positions used in this study were centered over visual are as V2 and V3, we cannot rule out the possibility that subcortical structure s or V1 could actually be the main generator for phosphenes. (C) 2001 Elsev ier Science Ireland Ltd. All rights reserved.