Cohen, Falk, Hopp, Tang, Tsau, Wu, and Zecevic: Multi-cell optical recordings in a situation where a large fraction of the relevant neurons are monitored

Yale University School of Medicine

Using voltage-sensitive dyes one can monitor the spike activity of approximately 500 neurons in the Aplysia abdominal ganglion while the ganglion is generating simple behaviors (e.g. the gill-withdrawal reflex). Since the ganglion has only 1,000 neurons, the fraction of monitored neurons is relatively large.

In some ways the results of such a measurement are informative. They show that information about a touch is very widely distributed in the Aplysia nervous system. Approximately of the abdominal ganglion neurons and about of all the animal's central neurons are activated by this very mild and localized mechanical stimulus. Other neurons are no doubt inhibited and yet others must receive large, subthreshold synaptic inputs. It seems that practically every neuron in the ganglion cares about this stimulus.

Other questions which we have asked are the following:

• How consistent is the response to the same stimulus?
• How much correlation is there between the spiking of the individual neurons?
• Are the same neurons used for producing more than one behavior?
• On the other hand, our recordings have not answered the central question. How are the synaptic connections organized to generate the gill withdrawal and the other behaviors that the ganglion controls? Indeed, these recordings raise the possibility that answering such questions may be beyond the present capabilities of neuroscientists.