Analysis of single unit activity evoked by tones amplitude-modulated with low-frequency noise in frog medulla

A method of constructing shuffled autocorrelation functions (SACF) was used to characterize single units in the dorsal medullar nucleus of the common frog (Rana temporaria). A continuous characteristic frequency tone modulated by repeating pieces of low-frequency noise was used as a stimulus. SACF was calculated as the correlation between the firing discharges evoked by different repeating pieces of the low-frequency noise. This approach obviates the influence of refractoriness and also considerably increases the sample representativity. Comparison of the SACF with the conventional ACF permits estimating the temporal dynamics of changes in the postspike excitability of the neuron. Analysis of several examples demonstrates the possibility of facilitation just after the period of absolute refractoriness in some tonic units. Neurons exhibiting only a phasic response to the onset of nonmodulated voice-frequency pieces were able to selectively respond to special moments of noise amplitude modulation. These cells demonstrate extremely high synchronism of reaction. We also describe a neuron characterized by an intrinsic periodicity of firing not connected with the dynamics of arriving stimuli.