Optical measurement of action potential activity in invertebrate ganglia.

With a voltage-sensitive dye and an array of silicon photodiodes we monitored the action potential activity of a large fraction of the cells in the Aplysia abdominal ganglion during the gill-withdrawal reflex. This capability allowed a direct determination of the spread of sensory information within the 1,000 neurons of the ganglion. Surprisingly, approximately 30% of the neurons are activated by a light touch to a small area of siphon skin. It is likely that many other neurons also receive large synaptic potentials, either excitatory or inhibitory, and thus even a very mild and restricted stimulus will have widely distributed effects. It seems to us that these results will force a more pessimistic view of the present understanding of the neuronal basis of apparently simple behaviors.