Stochastic Synchronization and Signal Rectification in the Crayfish Caudal Photoreceptor

The crayfish possesses two light‐sensitive neurons in its abdominal 6th ganglion (“caudal photoreceptors”, or CPRs). In addition to being light sensors, the CPRs are secondary neurons in a mechanosensory pathway. These sensory mechanisms, mechanosensitivity and light sensitivity, interact with one another. Pei et al. (1996) demonstrated that light enhances the transduction of weak, periodic hydrodynamic stimuli (measured as an increase in the signal‐to‐noise ratio at the stimulus frequency in the power spectrum of the recorded neural spikes). This has been interpreted as a stochastic resonance effect, in which added light increases the noise intensity of the input to the photoreceptor (possibly through fluctuations in membrane potential), leading to an enhancement of the signal to noise ratio (SNR). We have demonstrated in the crayfish the correlation between a stochastic‐resonance like effect and an increase in stochastic phase synchronization between the neural response and a periodic mechanical stimulus (Bahar et al. 2002). Here, we discuss a novel effect in which light increases the signal‐to‐noise ratio of the second higher harmonic of a periodic input mechanical stimulus, effectively rectifying the input signal. This “second harmonic effect” can be interpreted in terms of stochastic phase synchronization. We hypothesize that the input signal is effectively full‐wave rectified via the summation of two half‐wave rectified mechanosensory signals. This hypothesis can be described by a simple mathematical model which agrees well with our experimental results.