Faraday's law of induction states that changing magnetic fields induce electrical currents in any conducting medium. Induced currents are proportional to the conductivity of the conducting medium and the rate of change of the magnetic field. Thus, time-varying magnetic fields (e.g. the gradients in MRI) will induce circulating eddy currents in conductors in the body and particularly in electrically excitable nerve and muscle cells. Sufficiently large gradient fields, therefore, may affect normal cell function. A well established example of this is the sensation of flashes of light—magnetophosphenes—caused by induced electric currents stimulating the retina. A more serious consequence of electric currents flowing through the body is ventricular fibrillation (though these levels are strictly prevented in MRI). In MR, this induction effect is determined by factors such as pulse duration, wave shape, repetition pattern, and the distribution of current in the body. As a general guide, the faster the imaging or spectroscopy sequence, the greater the rate of change of the gradient fields used, and the resultant current density induced in the tissue is higher.