Figure 1
Human sensation evoked by transcranial electric stimulation. The human subject was tested in a double-walled, sound-proof booth, with the light turned off. All stimuli were charge-balanced and delivered through a current source. The sensation threshold was obtained by the method of limits. (a) Electrode montages consisted of eight positions: forehead, top of the head, left and right temple, left and right mastoid, and left and right ear canal (note: only the left temple, left mastoid, and right ear canal electrodes are displayed in the diagram in yellow). The forehead and ear canal electrodes were located closest to the visual and auditory organs: the retina and cochlea, respectively (blue). (b) Distribution of 983 reported sensations by type and other neural and muscular activities (see text for details). (c) Visual, auditory, and somatic sensitivity thresholds in electric current are plotted as a function of stimulus frequency (5 to 10,000 Hz) averaged over all electrode positions. Error bars represent one standard deviation of the mean across all electrode positions. (d) Percentage (y-axis) for each of the three types of sensation (regardless of stimulus frequency and level) for two subsets of electrode configurations: those containing forehead electrode (filled bars in the top panel) vs. non-forehead electrodes (open bars); those containing ear canal electrodes (filled bars in the bottom panel) vs. non-ear-canal electrodes (open bars). The asterisk (*) indicates a significant difference between two occurrences [two-tailed binomial distribution test: z(n1 = 27, n2 = 33) = 2.97, p = 0.003 for the visual sensation between forehead and non-forehead electrodes; z(n1 = 38, n2 = 20) = 3.49, p = 0.0005 for the auditory sensation between ear canal and non-ear canal electrodes. See Procedures in Methods].
