Abstract
A combination of two factors makes the squid giant axon a favorable preparation for optical measurements: there is a relatively large membrane area and one can obtain good control of membrane potential. The large membrane area makes for relatively large signal-to-noise ratios in the optical measurements and the ability to control the membrane potential allows one to investigate the physiological origins of the signals in some detail. Using giant axons it is relatively straight-forward to determine if a given optical signal depends on changes in membrane potential, or the membrane currents, or the increases in membrane permeability (Cohen et al.,1968, 1972b).