Elsevier

Neuroscience

Volume 144, Issue 1, 5 January 2007, Pages 356-367
Neuroscience

Systems neuroscience
Evidence of a nonlinear human magnetic sense

https://doi.org/10.1016/j.neuroscience.2006.08.068Get rights and content

Abstract

Human subjects respond to low-intensity electric and magnetic fields. If the ability to do so were a form of sensory transduction, one would expect that fields could trigger evoked potentials, as do other sensory stimuli. We tested this hypothesis by examining electroencephalograms from 17 subjects for the presence of evoked potentials caused by the onset and by the offset of 2 G, 60 Hz (a field strength comparable to that in the general environment). Both linear (time averaging) and nonlinear (recurrence analysis) methods of data analysis were employed to permit an assessment of the dynamical nature of the stimulus/response relationship. Using the method of recurrence analysis, magnetosensory evoked potentials (MEPs) in the signals from occipital derivations were found in 16 of the subjects (P<0.05 for each subject). The potentials occurred 109–454 ms after stimulus application, depending on the subject, and were triggered by onset of the field, offset of the field, or both. Using the method of time averaging, no MEPs were detected. MEPs in the signals from the central and parietal electrodes were found in most subjects using recurrence analysis, but no MEPs were detected using time averaging. The occurrence of MEPs in response to a weak magnetic field suggested the existence of a human magnetic sense. In distinction to the evoked potentials ordinarily studied, MEPs were nonlinearly related to the stimulus as evidenced by the need to employ a nonlinear method to detect the responses.

Section snippets

Subjects

Seventeen clinically normal subjects were studied: eight males (age range 20–51 years) and nine females (18–50 years). The subjects were informed of the goals, methods, and general design of the investigation, but were not told exactly when or for how long the field would be applied. Written informed consent was obtained from each subject prior to participation in the study. The institutional review board at the Louisiana State University Health Sciences Center approved all experimental

Results

Following application of the magnetic field, changes in the signals from the occipital electrodes were detected by recurrence analysis but not by time averaging; typical results are shown in Fig. 2. V(t) after field onset (t=0.03–1) did not differ from the control (t=5.03–6), as determined by comparing the onset and control epochs, point by point, using the paired t-test (Fig. 2, first column). In contrast, when the determinism in the onset epochs was captured using %R(t)¯, differences in the

Discussion

Strong magnetic fields (∼10,000 G), such as those used for transcutaneous magnetic stimulation, instantaneously activate voltage-sensitive ion channels in axonal membranes. Fields on the order of 1 G cannot do so, and their biophysical mechanism of action is still unknown. Nevertheless, they can produce electrophysiological changes in animals throughout the phylogenetic spectrum (Szabo 1974, Semm and Beason 1990, Dobson et al 2002, Fuller et al 2003). In some species, specialized receptor

Conclusion

In conclusion, human subjects responded to onset and to offset of 2 G, 60 Hz, by exhibiting MEPs with a latency of 109–454 ms (P<0.05 in 16 of 17 subjects). The potentials were nonlinearly related to the stimulus and were observed by sampling the EEG signals at 300 Hz, unfolding in a five-dimensional phase space using a delay time of 5, computing the %R(t) of the signal as a function of time using a window of 30 points, smoothing the computed time series using a window of 30 points, and then

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