The joint peristimulus-time scatter diagram is an index of the operational significance of a synapse.

A major goal in neurobiology is to trace the neural circuits responsible for simple behavioural acts, and to try to understand the properties of the behaviour in terms of those of the neural elements and the connexions between them. Once the outlines of a neural circuit have been obtained, the following question frequently arises: What is the functional significance of a particular connexion between neurones, or of a class of connexions among a population of neurones? Statistical techniques have recently been developed to analyse the relations between the neural activity in two or more neurones 2,a,1°,11. These methods, the joint peristimulus-time (PST) scatter diagram and cross-correlation histogram, are usually employed to deduce what connexions may be present or responsible for generating the observed relationsl,5,s,9, az. I report here a new experimental application of these techniques: the joint PST scatter diagram is used to test the effect and significance of known synaptic connexions. The connexions treated in this report are the class of weak electrical synapses between tactile interneurones in the crayfish, Procambarus clarkii. The major input to these interneurones consists of chemically-mediated excitation from tactile afterents17, ~9. When the interneurones are activated by tactile stimuli or by electric shocks to afferent nerves, the summated excitatory postsynaptic potential (EPSP) in an interneurone often includes small rapid unitary EPSPs which are generated by impulses in other interneurones. It has previously been demonstrated ~7 that these potentials are due to weak electrical connexions between some of the interneurones. These unitary EPSPs are always much less than the threshold for generating spikes. What, then, is the utility of these connexions, or are they merely artifacts of a loosely wired nervous system? This question was approached by studying the response patterns of activity in two interneurones which were known to be coupled. I shall describe one typical experiment in detail. The dissection and stimulating and recording techniques are

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