NOISE MODULATION CAN ENHANCE PERIODICITY ANALYSIS

Single units located in medullar and midbrain acoustical centers of the immobilized frogs usually cease to respond to long tones with small sinusoidal amplitude modulation indexes after several seconds of continuous sounding. The neuronal firing revives after adding some noise components to an envelope waveform. In the presence of such noise the firing could continue almost without attenuation. Moreover the hidden small periodical component could be easily noticed. At some optimal level of the noise the reproduction of sine waveform in firing activity of the units comes up to maximum. The subsequent increase of the noise modulation leads to an increase in firing rate but simultaneously to a decrease in synchronization of firing with sinusoidal component. The modulation noise waveform could have a spectrum included the frequency of the sine modulation or could be located outside of this region both in the lower frequency or higher frequency regions. The result of interaction of two modulations is strongly nonlinear and depends both upon stimulus parameters and upon the neuron's type. This effect reminds the phenomenon of stochastical resonance. It does not observed during presentations of short (< 1s) modulated tone bursts and revealed itself evidently only in adapted state. Similar effects could be observed when a low-frequency sinusoidal modulation had been added to tones amplitude modulated by higher modulation frequency. We provided some psychophysical experiments demonstrated the possibility to improve the detection of sinusoidal modulation in the presence of noise in modulation waveform.