While recording extracellularly from single neurons in the Lateral Geniculate Nucleus (LGN) of cats and monkeys, one often observes, in addition to action potentials, smaller and slower potentials which have been called 'S Potentials' (Bishop et al. 1962). Injection of the sodium channel blocker tetrodotoxin (TTX) into the eye of the cat causes spontaneous and light evoked S potentials to disappear. However, electrical stimulation of ganglion cells axons at the optic chiasm still elicits S potentials. Previous studies have shown that retinal ganglion cell impulses elicit S potentials. Our results prove that ganglion cell impulses are the ONLY source of these potentials; therefore, one can use them as a measure of ganglion cell input to the LGN.