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Cross-interval and cross-correlation analysis of neuron activity was used to study cell interactions in the cerebellar vermis of conscious rabbits. The cross-correlation method, based on post-synaptic potentials (PSP) from one member of a pair of neurons, supplements the cross-interval analysis used for evaluating the interactions of neurons in terms of action potentials. A total of 16 neuron pairs were analyzed, in which the activity of one neuron was recorded intracellularly and that of the other extra- or intracellularly. Interactions were observed in 10 pairs. Most pairs showed inhibitory interactions. Pairs with inhibitory interactions included those in which the activity of one or both neurons contained complex spikes. Also of interest was the analysis of connections in which influences were excitatory in one direction and inhibitory in the other. In these cases, neuron discharges were invariably simple spikes. Variability and plasticity in the output signals of Purkinje cells were greater than the variability and plasticity of the common inputs to Purkinje cells.

The methods of cross-interval and crosscorrelation analysis were used to study interneuronal connections in cerebellar vermis in awake rabbit. Crosscorrelation, or spike-triggering method, complemented the analysis of cross-intervals which estimated the neuronal interaction comparing spike sequences as point processes. Among 16 neuronal pairs recorded intra- or extracellularly 10 pairs were considered as interconnected. Inhibitory interconnections dominated. The inhibitory neuronal interconnections were revealed in pairs with complex spikes. Neurons which constituted pairs with reciprocal excitatory-inhibitory interaction fired only simple spikes. The variability and plasticity of common inputs to Purkinje cells were seemingly lower than those of direct connections which probably transmitted the output signals of Purkinje cells.