Addition of noise enhances neural synchrony to amplitude-modulated sounds in the frog's midbrain

The ability of 109 single units in the midbrain acoustic centre of frogs (Rana ridibunda, Rana temporaria) to reproduce 10%, 20 Hz sinusoidal amplitude modulation of a long-duration characteristic frequency tone was studied. The sinusoidal modulation was presented either in isolation or summed with a low-frequency (0-50 Hz) noise. Recordings were obtained in the adapted state. The magnitude of the 20 Hz periodic response component was estimated by means of the synchronisation coefficient and the amplitude of the sine modulation of the instantaneous spike rate. In many units, addition of noise modulation produced considerable enhancement of both the mean discharge rate and the discharge rate synchronised to the 20 Hz amplitude modulation. This enhancement phenomenon is interpreted in the context of stochastic resonance theory.